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We analyze four epochs of HST imaging over 18 years for the Draco dwarf spheroidal galaxy. We measure precise proper motions (PMs) for hundreds of stars and combine these with existing line-of-sight (LOS) velocities. This provides the first radially-resolved 3D velocity dispersion profiles for any dwarf galaxy. These constrain the intrinsic velocity anisotropy and resolve the mass-anisotropy degeneracy. We solve the Jeans equations in oblate axisymmetric geometry to infer the mass profile. We find the velocity dispersion to be radially anisotropic along the symmetry axis and tangentially anisotropic in the equatorial plane, with a globally-averaged value $\overline{\beta_{\mathrm B}}=-0.20^{+ 0.28}_{- 0.53}$, (where $1 - \beta_{\mathrm B} \equiv \langle v_{\mathrm{ tan}}^2 \rangle / \langle v_{\mathrm{ rad}}^2 \rangle$ in 3D). The logarithmic dark matter (DM) density slope over the observed radial range, $\Gamma_{\mathrm{ dark}}$, is $-0.83^{+ 0.32}_{- 0.37}$, consistent with the inner cusp predicted in $\Lambda$CDM cosmology. As expected given Draco's low mass and ancient star formation history, it does not appear to have been dissolved by baryonic processes. We rule out cores larger than 487, 717, 942 pc at respective 1-, 2-, 3-$\sigma$ confidence, thus imposing important constraints on the self-interacting DM cross-section. Spherical models yield biased estimates for both the velocity anisotropy and the inferred slope. The circular velocity at our outermost data point (900 pc) is $24.19^{+ 6.31}_{- 2.97} \ \mathrm{km~s^{-1}}s$. We infer a dynamical distance of $75.37^{+ 4.73}_{- 4.00}$ kpc, and show that Draco has a modest LOS rotation, with $\left = 0.22 \pm 0.09$. Our results provide a new stringent test of the so-called `cusp-core' problem that can be readily extended to other dwarfs., Comment: 35 pages, 18 figures, 3 Tables. Accepted for publication in ApJ. Journal version has better readability. Data is available at Zenodo: https://zenodo.org/records/11111113

The properties of satellite dwarf galaxies pose important empirical constraints to verify cosmological models on galaxy scales. Their phase-space correlations, in particular, offer interesting insights into various models. Next to the planes-of-satellites phenomenon, the lopsided distribution of satellites relative to their host galaxy has been studied observationally and in cosmological simulations. It is still unclear how observed lopsidedness aligns with expectations from simulations. We measure lopsidedness in observed isolated satellite systems using six different metrics. We study 47 systems from the MATLAS survey beyond the Local Volume (LV) as well as 21 LV satellite systems from the ELVES survey. We find that the so-called wedge metric, counting the number of dwarfs in wedges with varying opening angles, is best suited to capture a system's overall lopsidedness. Under this metric, our analysis reveals that ~16 percent of the tested satellite systems exhibit a statistically significant degree of lopsidedness when compared to systems with randomly generated satellite position angles. This presents a notable excess over the expected 5% (2 sigma level) of significantly lopsided systems in a sample with no overall inherent lopsidedness. However, using multiple metrics provides a more complete picture. Combining all tested metrics, the number of significantly lopsided systems increases to ~21 percent. Contrary to recent results from the literature, we find more lopsided systems among the red early-type galaxies in the MATLAS survey compared to the mostly blue late-type hosts in ELVES. We further find that satellite galaxies at larger distances from the host, potentially recently accreted, are likely the primary contributors to the reported excess of lopsidedness. Our results enable comparisons with similar systems in simulations to assess consistency with the standard model., Comment: 23 pages, 9 figures. Accepted for publication in A&A. Abstract abridged

The galaxy system around NGC4490 was recently highlighted to display a flattened, kinematically correlated structure reminiscent of satellite galaxy planes around other hosts. Since known satellite planes are in tension with $\Lambda$CDM expectations from cosmological simulations, we quantitatively assess for the first time the tension posed by the NGC4490 system. We measure the on-sky flattening as the major-to-minor axis ratio b/a of the satellite distribution and their line-of-sight kinematic correlation. Analogs are selected in the IllustrisTNG-50 simulation and their flattening and correlation are similarly measured. We confirm the strong kinematic coherence of all 12 observed objects with available line-of-sight velocities (of 14 in total): the northern ones approach and the southern ones recede relative to the host. The spatial distribution of all 14 objects is substantially flattened with b/a=0.38 (0.26 considering only the 12 objects with velocities). Such extreme arrangements are rare in the simulation at 0.21-0.35%. This would drop further if at least one of the two satellite objects without velocities is confirmed to follow the kinematic trend, and would become zero if both are rejected as non-members. We also identify a likely galaxy pair in the observed system, and find a similar pair in the best-matching simulated analog. Our findings establish NGC4490 as another strong example of a satellite plane in the Local Volume. This emphasizes that planes of satellites are a more general issue faced by $\Lambda$CDM also beyond the Local Group. The tension with typical systems in simulations suggests that the observed one requires a specific formation scenario, potentially connected to the larger-scale galaxy alignment in its vicinity. The presence of galaxy pairs in the observed and a simulated system hints at the importance such groupings may have to understand satellite planes., Comment: 9 pages, 6 figures, 1 satellite galaxy plane. Accepted for publication in A&A. Abstract slightly abridged

Dwarf galaxy abundances can serve as discernment tests for models of structure formation. Previous small-scale tensions between observations and dark matter-only cosmological simulations may have been resolved with the inclusion of baryonic processes; however, these successes have been largely concentrated on the Local Group dwarfs the feedback models were initially calibrated on. We investigate whether the $\Lambda$CDM model can reliably reproduce dwarf abundances in the MATLAS low-to-moderate density fields that are centred upon early-type host galaxies beyond the Local Volume. We carried out mock observations of MATLAS-like fields with the high-resolution hydrodynamic simulation IllustrisTNG-50. We used matching selection criteria and compared the properties of dwarfs contained within them with their MATLAS analogues. Although simulated MATLAS-like dwarfs demonstrate photometric properties that are consistent with the observed galaxy population and follow the same scaling relations, TNG50 underestimates the number of dwarf galaxies in isolated MATLAS fields at the 6 sigma level. This significance is maintained within crowded fields containing more than a single bright host. Our 55-62% estimate of the fraction of background galaxies is in agreement with estimates by MATLAS, but is wholly insufficient to alleviate this discrepancy in dwarf abundances. Any incompleteness in the observed fields further exacerbates this tension. We identified a "too-many-satellites" problem in $\Lambda$CDM, emphasising the need for the continued testing and refining of current models of galaxy formation in environments beyond the Local Group., Comment: 10 pages, 6 figures, accepted in Astronomy & Astrophysics

The problem of missing satellites still remains one of the well-known problems of the Lambda-CDM cosmological theory. Despite significant progress in cosmological modeling achieved in recent years, the search for new dwarf galaxies-satellites of nearby giant galaxies remains extremely important. In this series of papers we report the first results of an on-going systematic survey of faint dwarf spheroidal galaxies in the vicinity of the bright late-type spiral NGC 253 galaxy, the brightest member of the Sculptor filament located at a distance of 3.7 Mpc. We performed a new NGC 253 satellite search by means of visual inspection using co-added image cutouts reprocessed in the DESI Legacy image surveys, reaching a very low surface brightness regime (28.0--29.0 mag arcsec-2). Five new dwarf galaxies have been discovered in the vicinity of NGC 253, that we named them Do V, Do VI, Do VII, Do VIII and Do IX. Assuming they are associated to NGC 253, their total absolute V-magnitudes fall in the -7 to -9 mag range, which is typical for dwarf satellites in the local Universe. The central surface brightness tend to be extremely low for all the discovered dwarfs and fall roughly in the range of 25--26 mag arcsec-2 in g-band. We present a new list of galaxies located around the giant spiral NGC 253. With the inclusion of these additional satellites, the overall spatial distribution of the system becomes less flattened and is now broadly consistent with analogs drawn from Lambda-CDM expectations. Interestingly, the distribution appears to be rather lopsided. Yet, firm conclusions on the presence of absence of a correlated satellite structure are hampered since distance information is lacking, the census of observed dwarfs in the system remains far from complete, and spectroscopic velocities are not even available for most known satellites., Comment: 10 pages, 6 figures, 4 Tables, submitted to Astronomy and Astrophysics

Context. Within $\Lambda$ Cold Dark Matter ($\Lambda$CDM) simulations, Milky Way-like galaxies accrete some of their satellite galaxies in groups of 3-5 members rather than individually. It was also suggested that this might be the reason behind the origin of satellite planes. Objects accreted in groups are expected to share similar specific total energy and angular momentum, and also identical orbital planes and directions. Aims. Looking at observations of Milky Way satellites, the dwarf galaxies Leo II, IV, V, Crater II, and the star cluster Crater 1 were proposed to be a vestige of group infall. The suggested "Crater-Leo group" shows a coherent distance gradient and all these objects align along a great circle on the sky. We use proper motion data to investigate whether the phase-space distribution of the members of the proposed group are indeed consistent with group infall. Methods. To further investigate this possibility, we use Gaia Data Release 3 (DR3) and new Hubble Space Telescope (HST) proper motions, $(\mu_{\alpha*}, \mu_\delta) = (-0.1921 \pm 0.0514, -0.0686 \pm 0.0523)$ mas yr$^{-1}$ for Leo IV and $(\mu_{\alpha*}, \mu_\delta) = (0.1186 \pm 0.1943, -0.1183 \pm 0.1704)$ mas yr$^{-1}$ for Leo V, to derive accurate orbital properties for the proposed group objects. In addition, we explore other possible members of this putative association. Results. Leo II, Leo IV, and Crater 1 show orbital properties consistent with those we predict from assuming group infall. However, our results suggest that Crater II was not accreted with the rest of the objects. If confirmed with increasingly accurate proper motions in the future, the Crater-Leo objects appear to constitute the first identified case of a cosmologically expected, typical group infall event, as opposed to the highly hierarchical Magellanic Cloud system., Comment: Accepted for publication in A&A on 23-04-2024. 20 pages, 18 figures, 5 tables

Dwarf galaxies in groups of galaxies provide excellent test cases for models of structure formation. This led to a so-called small-scale crisis, including the famous missing satellite and too-big-to-fail problems. It was suggested that these two problems are solved by the introduction of baryonic physics in cosmological simulations. We test for the nearby grand spiral M83 - a Milky Way sibling - whether its number of dwarf galaxy companions is compatible with today's $\Lambda$ + Cold Dark Matter model using two methods: with cosmological simulations that include baryons, as well as with theoretical predictions from the sub-halo mass function. By employing distance measurements we recover a list of confirmed dwarf galaxies within 330 kpc around M83 down to a magnitude of $M_V =-10$. We found that both the state-of-the-art hydrodynamical cosmological simulation Illustris-TNG50 and theoretical predictions agree with the number of confirmed satellites around M83 at the bright end of the luminosity function (>10$^8$ solar masses) but underestimate it at the faint end (down to 10$^6$ solar masses) at more than 3$\sigma$ and 5$\sigma$ levels, respectively. This indicates a too-many satellites problem in $\Lambda$CDM for M83. The actual degree of tension to cosmological models is underestimated, because the number of observed satellites is incomplete due to the high contamination of spurious stars and galactic cirrus., Comment: Accepted for publication in Astronomy & Astrophysics Letters, 8 pages, 6 figures

The spatial distribution of dwarf galaxies around their host galaxies is a critical test for the standard model of cosmology because it probes the dynamics of dark matter halos and is independent of the internal baryonic processes of galaxies. Co-moving planes-of-satellites have been found around the Milky Way, the Andromeda galaxy, and the nearby CenA galaxy, which seem to be at odds with the standard model of galaxy formation. Another nearby galaxy group, with a putative flattened distribution of dwarfs, is the M81 group. We present a quantitative analysis of the distribution of the M81 satellites using a Hough transform to detect linear structures. We confirm a flattened distribution of dwarf galaxies. Depending on the morphological type, we find a minor-to-major axis ratio of the satellite distribution to be 0.5 (all types) or 0.3 (dSph), which is in line with previous results for the M81 group. Comparing the orientation of this flattened structure in 3D with the surrounding large-scale matter distribution, we find a strong alignment with the local sheet and the planes-of-satellites around the Andromeda galaxy and Cen A. Employing velocities for a sub-sample of the dwarfs, we find no signal of co-rotation. Comparing the flattening and motion of the M81 dwarf galaxy system with IllustrisTNG50 we find good agreement between observations and simulations, but caution that i) velocity information of half of the satellites is missing, ii) velocities are coming mainly from dwarf irregulars clustered around NGC3077, which may hint towards an infall of a dwarf galaxy group and iii) some of the dwarfs may actually be tidal dwarf galaxies. From the missing velocities, we predict that the observed frequency within TNG may range between 2 to 29 per cent. Any conclusions about the agreement/disagreement with cosmological models needs to wait for a more complete picture of the dwarf galaxy system., Comment: 11 pages, 7 figures, 1 table, accepted for publication in Astronomy & Astrophysics

We report the discovery of 40 new satellite dwarf galaxy candidates in the sphere of influence of the Sombrero galaxy (M104) the most luminous galaxy in the Local Volume. Using the Subaru Hyper Suprime-Cam, we surveyed 14.4 square degrees of its surroundings, extending to the virial radius. Visual inspection of the deep images and GALFIT modelling yielded a galaxy sample highly complete down to $M_{g}\sim-9$ ($L_{g}\sim3\times 10^{5}\,L_\odot$) and spanning magnitudes $-16.4 < M_g < -8$ and half-light radii $50\,pc\ <\ r_e\ <\ 1600\,pc$ assuming the distance of M104. These 40 new, out of which 27 are group members with high confidence, double the number of potential satellites of M104 within the virial radius, placing it among the richest hosts in the Local Volume. Using a Principle Component Analysis (PCA), we find that the entire sample of candidates consistent with an almost circular on-sky distribution, more circular than any comparable environment found in the Illustris TNG100-1 simulation. However the distribution of the high probability sample is more oblate and consistent with the simulation. The cumulative satellite luminosity function is broadly consistent with analogues from the simulation, albeit it contains no bright satellite with $M_{g}<-16.4$ ($L_{g}\sim3 \times 10^{8}\,L_\odot$), a $2.3\,\sigma$ occurrence. Follow-up spectroscopy to confirm group membership will begin to demonstrate how these systems can act as probes of the structure and formation history of the halo of M104., Comment: 16 pages, 11 figures, accepted to MNRAS

I briefly discuss the current state of the Planes of Satellite Galaxies Problem in light of some new observational data for satellite galaxies of the Milky Way, Andromeda, Centaurus A, and other systems beyond the Local Group. In particular, I present how a new proper motion measurment for Leo I enhances the overall orbital coherence among the MW's classical satellites and thus its tension with cosmological expectations., Comment: 5 pages, 2 figures. Version after addressing referee comments. To appear in Memorie della Societ\`a Astronomica Italiana, in the proceedings of EAS 2022 Symposium S3

Most Milky Way dwarf galaxies are much less bound to their host than are relics of Gaia-Sausage-Enceladus and Sgr. These dwarfs are expected to have fallen into the Galactic halo less than 3 Gyr ago, and will therefore have undergone no more than one full orbit. Here, we have performed hydrodynamical simulations of this process, assuming that their progenitors are gas-rich, rotation-supported dwarfs. We follow their transformation through interactions with the hot corona and gravitational field of the Galaxy. Our dedicated simulations reproduce the structural properties of three dwarf galaxies: Sculptor, Antlia II and, with somewhat a lower accuracy, Crater II. This includes reproducing their large velocity dispersions, which are caused by ram-pressure stripping and Galactic tidal shocks. Differences between dwarfs can be interpreted as due to different orbital paths, as well as to different initial conditions for their progenitor gas and stellar contents. However, we failed to suppress in a single orbit the rotational support of our Sculptor analog if it is fully dark-matter dominated. In addition, we have found that classical dwarf galaxies like Sculptor may have stellar cores sufficiently dense to survive the pericenter passage through adiabatic contraction. On the contrary, our Antlia II and Crater II analogs are tidally stripped, explaining their large sizes, extremely low surface brightnesses, and velocity dispersion. This modeling explains differences between dwarf galaxies by reproducing them as being at different stages of out-of-equilibrium stellar systems., Comment: Paper III, 15 pages, 9 Figures, 2 Tables, accepted by MNRAS 23rd November, see also Paper II (arXiv:2311.05677), please see the video of a dwarf (Sculptor) transformation after its first infall in the MW halo at: https://www.youtube.com/watch?v=SwxSdmfQis4

We study how structural properties of globular clusters and dwarf galaxies are linked to their orbits in the Milky Way halo. From the inner to the outer halo, orbital energy increases and stellar-systems gradually move out of internal equilibrium: in the inner halo, high-surface brightness globular clusters are at pseudo-equilibrium, while further away, low-surface brightness clusters and dwarfs appear more tidally disturbed. Dwarf galaxies are the latest to arrive into the halo as indicated by their large orbital energies and pericenters, and have no time for more than one orbit. Their (gas-rich) progenitors likely lost their gas during their recent arrival in the Galactic halo. If dwarfs are at equilibrium with their dark matter (DM) content, the DM density should anti-correlate with pericenter. However, the transformation of DM dominated dwarfs from gas-rich rotation-supported into gas-poor dispersion-supported systems is unlikely accomplished during a single orbit. We suggest instead that the above anti-correlation is brought by the combination of ram-pressure stripping and of Galactic tidal shocks. Recent gas removal leads to an expansion of their stellar content caused by the associated gravity loss, making them sufficiently fragile to be transformed near pericenter passage. Out of equilibrium dwarfs would explain the observed anti-correlation of kinematics-based DM density with pericenter without invoking DM density itself, questioning its previous estimates. Ram-pressure stripping and tidal shocks may contribute to the dwarf velocity dispersion excess. It predicts the presence of numerous stars in their outskirts and a few young stars in their cores., Comment: Paper II, 18 pages, 15 Figures, MNRAS 527, 2178-2733 (2024), see also paper III with simulations (arXiv:2311.05687), as well as a video showing the transformation of a dwarf recently falling into the MW halo at: https://www.youtube.com/watch?v=SwxSdmfQis4

Tidal dwarf galaxies (TDGs) are low-mass objects that form within tidal and/or collisional debris ejected from more massive interacting galaxies. We use CO($1-0$) observations from ALMA and integral-field spectroscopy from MUSE to study molecular and ionized gas in three TDGs: two around the collisional galaxy NGC 5291 and one in the late-stage merger NGC 7252. The CO and H$\alpha$ emission is more compact than the HI emission and displaced from the HI dynamical center, so these gas phases cannot be used to study the internal dynamics of TDGs. We use CO, HI, and H$\alpha$ data to measure the surface densities of molecular gas ($\Sigma_{\rm mol}$), atomic gas ($\Sigma_{\rm atom}$) and star-formation rate ($\Sigma_{\rm SFR}$), respectively. We confirm that TDGs follow the same spatially integrated $\Sigma_{\rm SFR}-\Sigma_{\rm gas}$ relation of regular galaxies, where $\Sigma_{\rm gas} = \Sigma_{\rm mol} + \Sigma_{\rm atom}$, even though they are HI dominated. We find a more complex behaviour in terms of the spatially resolved $\Sigma_{\rm SFR}-\Sigma_{\rm mol}$ relation on sub-kpc scales. The majority ($\sim$60$\%$) of SF regions in TDGs lie on the same $\Sigma_{\rm SFR}-\Sigma_{\rm mol}$ relation of normal spiral galaxies but show a higher dispersion around the mean. The remaining fraction of SF regions ($\sim$40$\%$) lie in the starburst region and are associated with the formation of massive super star clusters, as shown by Hubble Space Telescope images. We conclude that the local SF activity in TDGs proceeds in a hybrid fashion, with some regions comparable to normal spiral galaxies and others to extreme starbursts., Comment: 11 pages, 6 figures, 3 tables, Accepted for publication in MNRAS

Flattened and kinematically correlated planes of dwarf satellite galaxies have been observed in the Local Volume. The slinging out of satellites during host galaxy mergers has been suggested as a formation mechanism for these peculiar structures. We statistically examined the impact of major mergers on present-time satellite systems for the first time in a full cosmological context using the IllustrisTNG suite of hydrodynamic simulations. Mergers with mass ratios above 1/3 generally have a negligible or adverse impact on the phase-space correlation of observationally motivated satellites. Even high-angular momentum mergers are inefficient at slinging satellites outward due to the extended nature of simulated satellite distributions. Furthermore, any potential merger imprint is partially washed out by post-merger accretion of satellites, while satellites bound to the merging haloes since the merger's beginning are disrupted and stripped of mass - minimising the merger's influence on the present-time distribution of the most massive satellites after 2-5 Gyr. Constraining our sample to satellites bound to their host throughout the full duration of their system's last merger, we recover no particular improvement in their phase-space correlation. Instead, such participant satellites experience a contraction of their radial distribution during and after the merger, resulting in smaller absolute plane heights (but comparable axis ratios). Overall, major mergers do not appear to form correlated planes in a statistical sample. Mergers that efficiently transfer their angular momentum to satellite distributions can marginally enhance their phase-space correlation, but cannot form highly flattened and orbitally coherent configurations as observed in our local Universe., Comment: 16 pages, 13 figures, 4 tables, accepted for publication in MNRAS

We probe the dynamical mass profiles of 10 galaxy clusters from the HIghest X-ray FLUx Galaxy Cluster Sample (HIFLUGCS) using galaxy kinematics. We numerically solve the spherical Jeans equation, and parameterize the dynamical mass profile and the galaxy velocity anisotropy profile using two general functions to ensure that our results are not biased towards any specific model. The mass-velocity anisotropy degeneracy is ameliorated by using two "virial shape parameters" that depend on the fourth moment of velocity distribution. The resulting velocity anisotropy estimates consistently show a nearly isotropic distribution in the inner regions, with an increasing radial anisotropy towards large radii. We compare our derived dynamical masses with those calculated from X-ray gas data assuming hydrostatic equilibrium, finding that massive and rich relaxed clusters generally present consistent mass measurements, while unrelaxed or low-richness clusters have systematically larger total mass than hydrostatic mass by an average of 50\%. This might help alleviate current tensions in the measurement of $\sigma_8$, but it also leads to cluster baryon fractions below the cosmic value. Finally, our approach probes accelerations as low as $10^{-11}$ m s$^{-2}$, comparable to the outskirts of individual late-type galaxies. We confirm that galaxy clusters deviate from the radial acceleration relation defined by galaxies., Comment: 16 pages, 8 figures, 1 table. A&A accepted version

The flattened, possibly co-rotating plane of satellite galaxies around Centaurus A, if more than a fortuitous alignment, adds to the pre-existing tension between the well-studied Milky Way and M31 planes and the $\Lambda$CDM model of structure formation. It was recently reported that the Centaurus A satellite plane (CASP) may be rotationally supported, but a further understanding of the system's kinematics is elusive in the absence of full three-dimensional velocities. We constrain the transverse velocities of 27 satellites that would rotationally stabilise the Centaurus A plane, and classify the satellites by whether their possible orbits are consistent with the CASP. Five satellites are identified to be unlikely to participate in the plane, two of which are clearly non-members. Despite their previously reported line-of-sight velocity trend suggestive of a common co-rotating motion, 17 out of 22 potential CASP members are consistent with either orbital direction within both the full range of possible kinematics as well as when limiting orbits to those within the plane. On the other hand, disregarding the 5 off-plane satellites found to be inconsistent with CASP membership enhances the significance of the CASP's line-of-sight velocity trend fivefold. Our results are robust with different mass estimates of the Centaurus A halo, and the adoption of either spherical or triaxial NFW potentials., Comment: 15 pages, 8 figures, 3 tables, accepted for publication in MNRAS

Halo inhabitants are individual stars, stellar streams, star and globular clusters, and dwarf galaxies. Here we compare the two last categories that include objects of similar stellar mass, which are often studied as self-dynamical equilibrium systems. We discover that the half-light radius of globular clusters depends on their orbital pericenter and total energy, and that Milky Way (MW) tides may explain the observed correlation. We also suggest that the accretion epoch of stellar systems in the MW halo can be calibrated by the total orbital energy, and that such a relation is due to both the mass growth of the MW and dynamical friction affecting mostly satellites with numerous orbits. This calibration starts from the bulge, to Kraken, Gaia Sausage Enceladus, Sagittarius stellar systems, and finally to the new coming dwarfs, either or not linked to the vast-polar structure. The most eccentric globular clusters and dwarfs have their half-light radius scaling as the inverse of their binding energy, and this over more than two decades. This means that earlier arriving satellites are smaller due to the tidal effects of the MW. Therefore, most halo inhabitants appear to have their structural parameters shaped by MW tides and also by ram-pressure for the most recent arrivals, the dwarf galaxies. The correlations found in this study can be used as tools to further investigate the origin of globular clusters and dwarfs, as well as the assembly history of our Galaxy., Comment: 18 pages, 8 Figures, 2 Tables with orbital properties of globular clusters, accepted in MNRAS, December the 12, 2022, version similar to the printed version

The condensation of baryons within a dark matter (DM) halo during galaxy formation should result in some contraction of the halo as the combined system settles into equilibrium. We quantify this effect on the cuspy primordial halos predicted by DM-only simulations for the baryon distributions observed in the galaxies of the SPARC database. We find that the DM halos of high surface brightness galaxies (with $\Sigma_{\rm eff}\gtrsim100$ $L_\odot$ pc$^{-2}$ at 3.6 $\mu$m) experience strong contraction. Halos become more cuspy as a result of compression: the inner DM density slope increases with the baryonic surface mass density. We iteratively fit rotation curves to find the balance between initial halo parameters (constrained by abundance matching), compression, and stellar mass-to-light ratio. The resulting fits often require lower stellar masses than expected for stellar populations, particularly in galaxies with bulges: stellar mass must be reduced to make room for the DM it compresses. This trade off between dark and luminous mass is reminiscent of the cusp-core problem in dwarf galaxies, but occurs in more massive systems: the present-epoch DM halos cannot follow from cuspy primordial halos unless (1) the stellar mass-to-light ratios are systematically smaller than expected from standard stellar population synthesis models, and/or (2) there is a net outward mass redistribution from the initial cusp, even in massive galaxies widely considered to be immune from such effects., Comment: 13 pages, 9 figures, 1 table. Accepted for publication in A&A

Observational studies of pairs of galaxies have uncovered that their differential line-of-sight velocities indicate the presence of a peak in their three-dimensional intervelocity distribution at 130-150 km/s. It had been argued that galaxy pairs in the standard model of cosmology, $\Lambda$CDM, should not exhibit such an intervelocity peak, while Modified Newtonian Dynamics (MOND) predicts such a preferred intervelocity for paired galaxies. However, no direct comparison with $\Lambda$CDM applying the same selection criteria and methodology as the observational studies has been performed yet, placing the comparison on unsure footing. To rectify this, we investigate this potential challenge for $\Lambda$CDM by determining whether an analog of the observed intervelocity peak is present in galaxy pairs within the IllustrisTNG-300 cosmological simulation. We identify galaxy pairs following the observational study's selection criteria, measure their projected velocity difference, and analyse both the de-projected as well as the full velocity difference for this galaxy pair sample in the simulation. We recover a deprojected intervelocity peak at ~130 km/s for galaxy pairs selected from the simulation. The full three-dimensional velocity information available for the pairs in the simulation also reveals a clear preference for this intervelocity. The intervelocity peak among galaxy pairs does not appear to be a feature unique to MOND, but is also present in $\Lambda$CDM. It can thus not be claimed as a unique success of either theory over the other. Developing the galaxy pair intervelocity into a test of gravity in the low acceleration regime will require more detailed studies to identify measurable differences in the models., Comment: 9 pages, 8 figures. Accepted for publication in A&A Letters

The existence of mutually correlated thin and rotating planes of satellite galaxies around both the Milky Way (MW) and Andromeda (M31) calls for an explanation. Previous work in Milgromian dynamics (MOND) indicated that a past MW-M31 encounter might have led to the formation of these satellite planes. We perform the first-ever hydrodynamical MOND simulation of the Local Group using Phantom of RAMSES. We show that an MW-M31 encounter at $z \approx 1$, with a perigalactic distance of about 80 kpc, can yield two disc galaxies at $z=0$ oriented similarly to the observed galactic discs and separated similarly to the observed M31 distance. Importantly, the tidal debris are distributed in phase space similarly to the observed MW and M31 satellite planes, with the correct preferred orbital pole for both. The MW-M31 orbital geometry is consistent with the presently observed M31 proper motion despite this not being considered as a constraint when exploring the parameter space. The mass of the tidal debris around the MW and M31 at $z=0$ compare well with the mass observed in their satellite systems. The remnant discs of the two galaxies have realistic radial scale lengths and velocity dispersions, and the simulation naturally produces a much hotter stellar disc in M31 than in the MW. However, reconciling this scenario with the ages of stellar populations in satellite galaxies would require that a higher fraction of stars previously formed in the outskirts of the progenitors ended up within the tidal debris, or that the MW-M31 interaction occurred at $z>1$., Comment: 33 pages, 18 figures, 10 tables. Published in the Monthly Notices of the Royal Astronomical Society in this form

The nearby elliptical galaxy Cen A is surrounded by a flattened system of dwarf satellite galaxies with coherent motions. Using a novel Bayesian approach, we measure the mean rotation velocity $v_{\rm rot}$ and velocity dispersion $\sigma_{\rm int}$ of the satellite system. We find $v_{\rm rot}/\sigma_{\rm int} \simeq 0.7$ indicating that the satellite system has non-negligible rotational support. Using Jeans' equations, we measure a circular velocity of 258 km s$^{-1}$ and a dynamical mass of $1.2\times 10^{13}$ M$_\odot$ within 800 kpc. In a $\Lambda$CDM cosmological context, we find that the Cen A group has a baryon fraction $M_{\rm b}/M_{200}\simeq0.035$ and is missing $\sim$77$\%$ of the cosmologically available baryons. Consequently, Cen A should have a hot intergalactic medium with a mass of $\sim$8$\times$10$^{11}$ M$_\odot$, which is more than $\sim$20 times larger than current X-ray estimates. Intriguingly, The whole Cen A group lies on the baryonic Tully-Fisher relation defined by individual rotationally supported galaxies, as expected in Milgromian dynamics (MOND) with no need of missing baryons., Comment: Accepted for publication in A&A, 7 pages, 3 figures, 1 table

The reflex motion and distortion of the Milky Way (MW) halo caused by the infall of a massive Large Magellanic Cloud (LMC) has been demonstrated to result in an excess of orbital poles of dark matter halo particles towards the LMC orbital pole. This was suggested to help explain the observed preference of MW satellite galaxies to co-orbit along the Vast Polar Structure (VPOS). We test this idea by correcting the positions and velocities of the MW satellites for the Galactocentric-distance-dependent shifts inferred from a LMC-infall simulation. While this should substantially reduce the observed clustering of orbital poles if it were mainly caused by the LMC, we instead find that the strong clustering remains preserved. We confirm the initial study's main result with our simulation of an MW-LMC-like interaction, and use it to identify two reasons why this scenario is unable to explain the VPOS: (1) the orbital pole density enhancement in our simulation is very mild (~10% within 50-250 kpc) compared to the observed enhancement (~220-300%), and (2) it is very sensitive to the specific angular momenta (AM) of the simulation particles, with higher AM particles being affected the least. Particles in simulated dark matter halos tend to follow more radial orbits (lower AM), so their orbital poles are more easily affected by small offsets in position and velocity caused by an LMC infall than objects with more tangential velocity (higher AM), such as the observed dwarf galaxies surrounding the MW. The origin of the VPOS thus remains unexplained., Comment: 13 pages, 8 figures. Accepted for publication in ApJ

Half of the satellite galaxies of Andromeda form a narrow plane termed the Great Plane of Andromeda (GPoA), and their line-of-sight velocities display correlation reminiscent of a rotating structure. Recently reported first proper motion measurements for the on-plane satellites NGC 147 and NGC 185 indicate that they indeed co-orbit along the GPoA. This provides a novel opportunity to compare the M31 satellite system to $\Lambda$CDM expectations. We perform the first detailed comparison of the orbital alignment of two satellite galaxies beyond the Milky Way with several hydrodynamical and dark-matter-only cosmological simulations (Illustris TNG-50, TNG-100, ELVIS, PhatELVIS), in the context of the Planes of Satellite Galaxies Problem. In line with previous works, we find that the spatial flattening and line-of-sight velocity correlation alone is already in substantial tension with $\Lambda$CDM, with none of the simulated analogs simultaneously reproducing both parameters. Almost none (3 to 4\%) of the simulated systems contain two satellites with orbital poles as well aligned with their satellite plane as indicated by the most-likely proper motions of NGC 147 and NGC 185. However, within current measurement uncertainties it is common (~70%) that the two best-aligned satellites of simulated systems are consistent with the orbital alignment. Yet, the chance that any two simulated on-plane satellites have as well aligned orbital poles as observed is low (~4%). We conclude that confirmation of the tight orbital alignment for these two objects via improved measurements, or the discovery of similar alignments for additional GPoA members, hold the potential to further raise the tension with $\Lambda$CDM expectations., Comment: 27 pages, 13 figures, 5 tables, accepted for publication in ApJ

Driven by increasingly complete observational knowledge of systems of satellite galaxies, mutual spatial alignments and relations in velocities among satellites belonging to a common host have become a productive field of research. Numerous studies have investigated different types of such phase-space correlations, and were met with varying degrees of attention by the community. The Planes of Satellite Galaxies issue is maybe the best-known example, with a rich field of research literature and an ongoing, controversial debate on how much of a challenge it poses to the LCDM model of cosmology. Another type of correlation, the apparent excess of close pairs of dwarf galaxies, has received considerably less attention despite its reported tension with LCDM expectations. With the expansion of proper motion measurements, largely driven by Gaia, other peculiar phase-space correlations were uncovered among the Milky Way satellites. Examples are the tangential velocity excess of satellites compared to cosmological expectations, and the unexpected preference of satellites to be close to pericenter. At the same time, other kinds of correlations have been found to be more in line with cosmological expectations, specifically lopsided satellite galaxy systems and the accretion of groups of satellite galaxies. The latter has mostly been studied in cosmological simulations thus far, but offers the potential to address some of the other issues by providing a way to produce correlations among the orbits of a group's satellite galaxy members. This review is the first to provide an introduction to the highly active field of phase-space correlations among satellite galaxy systems. It summarizes existing, recent research and highlights interdependencies between the different, currently almost exclusively individually considered types of correlations. Future prospects are also briefly discussed. (abridged), Comment: Invited review, 33 pages, 10 figures, 2 tables, 155 references. Accepted for publication in the special issue Challenges in Our Understanding of Dwarf Galaxies, edited by Yanbin Yang and Behnam Javanmardi, in Galaxies

In the last years, a new generation of large-scale imaging surveys have probed wide field regions around some nearby galaxies at unprecedented low surface brightness regime (~28.0-29.0 mag arcsec^-2). This offers a chance of discovering very faint dwarf satellites by means of visual inspection of these public deep images. We report the first results of a systematic survey of faint dwarf spheroidal galaxies in the vicinity of the bright late-type spiral NGC 253 galaxy by means of a visual inspection of the images taken by the Dark Energy Survey. Three new dwarf galaxies have been discovered in the vicinity of the brightest member of the Sculptor filament, the late-type spiral NGC 253. Assuming they are companions of NGC 253, their total absolute V-magnitudes fall in the -7 to -9 mag range, which is typical for dwarf satellites in the local Universe. The central surface brightness tend to be extremely low for all the discovered dwarfs and fall roughly in the range of 25-26 mag arcsec^-2 in g-band. Using known data on distances and velocities of galaxies, we estimate the total virial mass of the NGC 253 group to be 8 x 10^11 Mo, which gives the virial radius R_200 = 186 kpc and the turn-around radius of 706 kpc. We also discuss the possible existence of a spatially flattened and velocity-correlated satellite system around NGC 253. This large-scale structure is orientated almost edge-on to line of sight. The possible plane of satellites is only 31 kpc thick with the minor-to-major axis ratio of 0.14. Four out of five galaxies with measured velocities follow a common velocity trend similar to those observed in the planes of satellites around the Andromeda and Centaurus A galaxies. However, the small number of galaxies with known velocities prevents to reach a definitive conclusion about the formation scenario of the structure and its possible relation to the surrounding cosmic web., Comment: 12 pages, 9 figures, 3 tables, accepted for publication in Astronomy & Astrophysics

The dwarf galaxy NGC 3109 is receding 105 km/s faster than expected in a $\Lambda$CDM timing argument analysis of the Local Group and external galaxy groups within 8 Mpc (Banik \& Zhao 2018). If this few-body model accurately represents long-range interactions in $\Lambda$CDM, this high velocity suggests that NGC 3109 is a backsplash galaxy that was once within the virial radius of the Milky Way and was slingshot out of it. Here, we use the Illustris TNG300 cosmological hydrodynamical simulation and its merger tree to identify backsplash galaxies. We find that backsplashers as massive ($\geq 4.0 \times 10^{10} M_\odot$) and distant ($\geq 1.2$ Mpc) as NGC 3109 are extremely rare, with none having also gained energy during the interaction with their previous host. This is likely due to dynamical friction. Since we identified 13225 host galaxies similar to the Milky Way or M31, we conclude that postulating NGC 3109 is a backsplash galaxy causes $>3.96\sigma$ tension with the expected distribution of backsplashers in $\Lambda$CDM. We show that the dark matter only version of TNG300 yields much the same result, demonstrating its robustness to how the baryonic physics is modelled. If instead NGC 3109 is not a backsplasher, consistency with $\Lambda$CDM would require the 3D timing argument analysis to be off by 105 km/s for this rather isolated dwarf, which we argue is unlikely. We discuss a possible alternative scenario for NGC 3109 and the Local Group satellite planes in the context of MOND, where the Milky Way and M31 had a past close flyby $7-10$ Gyr ago., Comment: 19 pages, 15 figures, 3 tables, 2 MB. Published in the Monthly Notices of the Royal Astronomical Society in this form

Satellites are not randomly distributed around their central galaxies but show polar and planar structures. In this paper, we investigate the axis-asymmetry or lopsidedness of satellite galaxy distributions around isolated galaxies in a hydrodynamic cosmological simulation. We find a statistically significant lopsided signal by studying the angular distribution of the satellite galaxies' projected positions around isolated central galaxies in a two-dimensional plane. The signal is dependent on galaxy mass, color and large-scale environment. Satellites that inhabit low-mass blue hosts, or located further from the hosts show the most lopsided signal. Galaxy systems with massive neighbors exhibit stronger lopsidedness. This satellite axis-asymmetry signal also decreases as the universe evolves. Our findings are in agreement with recent observational results, and they provide a useful perspective for studying galaxy evolution, especially on the satellite accretion, internal evolution and interaction with the cosmic large-scale structure., Comment: 13 pages, 8 figures, accepted by ApJ

Based on Gaia Early Data Release 3 (EDR3), we estimate the proper motions for 46 dwarf spheroidal galaxies (dSphs) of the Milky Way. The uncertainties in proper motions, determined by combining both statistical and systematic errors, are smaller by a factor 2.5, when compared with Gaia Data Release 2. We have derived orbits in four Milky Way potential models that are consistent with the MW rotation curve, with total mass ranging from $2.8\times10^{11}$ $M_{\odot}$ to $15\times10^{11}$ $M_{\odot}$. Although the type of orbit (ellipse or hyperbola) are very dependent on the potential model, the pericenter values are firmly determined, largely independent of the adopted MW mass model. By analyzing the orbital phases, we found that the dSphs are highly concentrated close to their pericenter, rather than to their apocenter as expected from Kepler's law. This may challenge the fact that most dSphs are Milky Way satellites, or alternatively indicates an unexpected large number of undiscovered dSphs lying very close to their apocenters. Between half and two thirds of the satellites have orbital poles that indicate them to orbit along the Vast Polar Structure (VPOS), with the vast majority of these co-orbiting in a common direction also shared by the Magellanic Clouds, which is indicative of a real structure of dSphs., Comment: 3rd version: 26 pages, 5 Figures, 6 Tables, version consistent with that to be published in the Astrophysical Journal

The plane-of-satellites problem is one of the most severe small-scale challenges for the standard $\Lambda$CDM cosmological model: several dwarf galaxies around the Milky Way and Andromeda co-orbit in thin, planar structures. A similar case has been identified around the nearby elliptical galaxy Centaurus A (Cen A). In this Letter, we study the satellite system of Cen A adding twelve new galaxies with line-of-sight velocities from VLT/MUSE observations. We find 21 out of 28 dwarf galaxies with measured velocities share a coherent motion. Similarly flattened and coherently moving structures are found only in 0.2% of Cen A analogs in the Illustris-TNG100 cosmological simulation, independently of whether we use its dark-matter-only or hydrodynamical run. These analogs are not co-orbiting, and arise only by chance projection, thus they are short-lived structures in such simulations. Our findings indicate that the observed co-rotating planes of satellites are a persistent challenge for $\Lambda$CDM, which is largely independent from baryon physics., Comment: 7 pages, 4 figures, 1 table, published in Astronomy & Astrophysics Letters

Dwarf spheroidal galaxies (dSphs) have been extensively investigated in the Local Group, but their low luminosity and surface brightness make similar work in more distant galaxy groups challenging. Modern instrumentation unlocks the possibility of scrutinizing these faint systems in other environments, expanding the parameter space of group properties. We use MUSE spectroscopy to study the properties of 14 known or suspected dSph satellites of Cen A. Twelve targets are confirmed to be group members based on their radial velocities. Two targets are background galaxies at 50 Mpc: KK 198 is a face-on spiral galaxy, and dw1315-45 is an ultra-diffuse galaxy with an effective radius of 2300 pc. The 12 confirmed dSph members of the Cen A group have old and metal-poor stellar populations and follow the stellar metallicity-luminosity relation defined by the dwarf galaxies in the Local Group. In the three brightest dwarf galaxies (KK 197, KKs 55, and KKs 58), we identify globular clusters, as well as a planetary nebula in KK 197, although its association with this galaxy and/or the extended halo of Cen A is uncertain. Using four discrete tracers, we measure the velocity dispersion and dynamical mass of KK 197. This dSph appears dark matter dominated and lies on the radial acceleration relation of star-forming galaxies within the uncertainties. It also is consistent with predictions stemming from modified Newtonian dynamics (MOND). Surprisingly, in the dwarf KK 203 we find an extended Halpha ring. Careful examination of Hubble Space Telescope photometry reveals a very low level of star formation at ages between 30-300 Myr. The Halpha emission is most likely linked to a 40 Myr old supernova remnant, although other possibilities for its origin cannot be entirely ruled out., Comment: 16 pages, 13 figures, accepted for publication in A&A

We present a detailed characterization of planes of satellite galaxies in the Milky Way (MW) and M31. For a positional analysis, we introduce an extension to the `4-galaxy-normal density plot' method \citep[][P13]{Pawlowski13}. It finds the normal directions to the predominant planar configurations of satellites of a system, yielding for each a \textit{collection} of planes of increasing member satellites. This allows to quantify the quality of planes in terms of population ($N_{\rm sat}$) and spatial flattening ($c/a$). We apply this method to the latest data for confirmed MW and M31 satellite samples, with 46 and 34 satellites, respectively. New MW satellites form part of planes previously identified from the sample with $N_{\rm sat}=27$ studied in P13: we identify a new plane with $N_{\rm sat}=39$ as thin as the VPOS-3 ($c/a\sim 0.2$), and with roughly the same normal direction; so far the most populated plane that thin reported in the Local Group. We introduce a new method to determine, using kinematic data, the axis of maximum co-orbitation of MW satellites. Interestingly, this axis approximately coincides with the normal to the former plane: $\geq45\pm5\%$ of satellites co-orbit. In M31 we discover a plane with $N_{\rm sat}=18$ and $c/a\sim0.15$, i.e., quality comparable to the GPoA, and perpendicular to it. This structure is viewed face-on from the Sun making it susceptible to M31 satellite distance uncertainties. An estimation of the perpendicular velocity dispersion suggests it is dynamically unstable. Finally we find that mass is not a property determining a satellite's membership to good quality planes., Comment: 20 pages. 13 figures. 5 tables. Accepted for publication in MNRAS. Supercedes arXiv:1908.02298. Please cite the NEW version. This updated and extended version of the paper considers up-to-date samples of confirmed MW and M31 satellite galaxies and studies the co-orbitation of MW satellites

We present the Hestia simulation suite: High-resolutions Environmental Simulations of The Immediate Area, a set of cosmological simulations of the Local Group. Initial conditions constrained by the observed peculiar velocity of nearby galaxies are employed to accurately simulate the local cosmography. Halo pairs that resemble the Local Group are found in low resolutions constrained, dark matter only simulations, and selected for higher resolution magneto hydrodynamic simulation using the Arepo code. Baryonic physics follows the Auriga model of galaxy formation. The simulations contain a high resolution region of 3-5 Mpc in radius from the Local Group midpoint embedded in the correct cosmographic landscape. Within this region a simulated Local Group consisting of a Milky Way and Andromeda like galaxy forms, whose description is in excellent agreement with observations. The simulated Local Group galaxies resemble the Milky Way and Andromeda in terms of their halo mass, mass ratio, stellar disc mass, morphology separation, relative velocity, rotation curves, bulge-disc morphology, satellite galaxy stellar mass function, satellite radial distribution and in some cases, the presence of a Magellanic cloud like object. Because these simulations properly model the Local Group in their cosmographic context, they provide a testing ground for questions where environment is thought to play an important role., Comment: 20 pages, 13 figures, submitted to MNRAS

Galaxies, as well as their satellites, are known to form within the cosmic web: the large, multi-scale distribution of matter in the universe. It is known that the surrounding large scale structure (LSS) can impact and influence the formation of galaxies, e.g. the spin and shape of haloes or galaxies are correlated with the LSS and the correlation depends on halo mass or galaxy morphology. In this work, we use group and filament catalogues constructed from the SDSS DR12 to investigate the correlation between satellite systems and the large scale filaments they are located in. We find that the distribution of satellites is significantly correlated with filaments, namely the major axis of the satellite systems are preferentially aligned with the spine of the closest filament. Stronger alignment signals are found for the cases where the system away from the filament spine, while systems close to the filament spine show significantly weaker alignment. Our results suggest that satellites are accreted along filaments, which agrees with previous works. The case of which away from the filament spine may help us to understand how the filament forms as well as the peculiar satellite distribution in the Local Universe., Comment: 10 pages, 7 figures, accepted by ApJ

Studies of nucleated dwarf galaxies can constrain the scenarios for the formation and evolution of nuclear star clusters (NSC) in low-mass galaxies and give us insights on the origin of ultra compact dwarf galaxies (UCDs). We report the discovery of a NSC in the dwarf galaxy KKs58 and investigate its properties together with those of another NSC in KK197. Both NSCs are hosted by dwarf elliptical galaxies of the Centaurus group. Combining ESO VLT MUSE data with photometry from VLT FORS2, CTIO Blanco DECam, and HST ACS, as well as high-resolution spectroscopy from VLT UVES, we analyse the photometric, kinematic and stellar population properties of the NSCs and their host galaxies. We confirm membership of the NSCs based on their radial velocities and location close to the galaxy centres. We also confirm the membership of two globular clusters (GCs) and detect oblate rotation in the main body of KK197. Based on high signal-to-noise spectra taken with MUSE of the NSCs of both KKs58 and KK197 we measure low metallicities, [Fe/H] = $-1.75 \pm 0.06$ dex and [Fe/H] = $-1.84 \pm 0.05$ dex, and stellar masses of $7.3 \times 10^5 M_\odot$ and $1.0 \times 10^6 M_\odot$, respectively. Both NSCs are more metal-poor than their hosts that have metallicities of $-1.35 \pm 0.23$ dex (KKs58) and $-0.84 \pm 0.12$ dex (KK197). This can be interpreted as NSC formation via the in-spiral of GCs. The masses, sizes and metallicities of the two NSCs place them among other NSCs, but also among the known UCDs of the Centaurus group. This indicates that NSCs might constitute the progenitors of a part of the low-mass UCDs, although their properties are almost indistinguishable from typical GCs., Comment: 13 pages, 8 figures, accepted for publication by A&A

With the release of Gaia DR2, it is now possible to measure the proper motions (PMs) of the lowest mass, ultra-faint satellite galaxies in the Milky Way's (MW) halo for the first time. Many of these faint satellites are posited to have been accreted as satellites of the Magellanic Clouds (MCs). Using their 6-dimensional phase space information, we calculate the orbital histories of 13 ultra-faint satellites and five classical dwarf spheroidals in a combined MW+LMC+SMC potential to determine which galaxies are dynamically associated with the MCs. These 18 galaxies are separated into four classes: i.) long-term Magellanic satellites that have been bound to the MCs for at least the last two consecutive orbits around the MCs (Carina 2, Carina 3, Horologium 1, Hydrus 1); ii.) Magellanic satellites that were recently captured by the MCs $<$ 1 Gyr ago (Reticulum 2, Phoenix 2); iii.) MW satellites that have interacted with the MCs (Sculptor 1, Tucana 3, Segue 1); and iv.) MW satellites (Aquarius 2, Canes Venatici 2, Crater 2, Draco 1, Draco 2, Hydra 2, Carina, Fornax, Ursa Minor). Results are reported for a range of MW and LMC masses. Contrary to previous work, we find no dynamical association between Carina, Fornax, and the MCs. Finally, we determine that the addition of the SMC's gravitational potential affects the longevity of satellites as members of the Magellanic system (long-term versus recently captured), but it does not change the total number of Magellanic satellites., Comment: 32 pages, 11 figures. Accepted to ApJ

We study the correlation of orbital poles of the 11 classical satellite galaxies of the Milky Way, comparing results from previous proper motions with the independent data by Gaia DR2. Previous results on the degree of correlation and its significance are confirmed by the new data. A majority of the satellites co-orbit along the Vast Polar Structure, the plane (or disk) of satellite galaxies defined by their positions. The orbital planes of eight satellites align to $<20^\circ$ with a common direction, seven even orbit in the same sense. Most also share similar specific angular momenta, though their wide distribution on the sky does not support a recent group infall or satellites-of-satellites origin. The orbital pole concentration has continuously increased as more precise proper motions were measured, as expected if the underlying distribution shows true correlation that is washed out by observational uncertainties. The orbital poles of the up to seven most correlated satellites are in fact almost as concentrated as expected for the best-possible orbital alignment achievable given the satellite positions. Combining the best-available proper motions substantially increases the tension with $\Lambda$CDM cosmological expectations: <0.1 per cent of simulated satellite systems in IllustrisTNG contain seven orbital poles as closely aligned as observed. Simulated systems that simultaneously reproduce the concentration of orbital poles and the flattening of the satellite distribution have a frequency of <0.1 per cent for any number of k > 3 combined orbital poles, indicating that these results are not affected by a look-elsewhere effect. This compounds the Planes of Satellite Galaxies Problem., Comment: 20 pages, 10 figures, 4 tables. 11 classical satellites, 8 correlated, of which 7 co-orbit. Accepted for publication in MNRAS

We present an empirical method to measure the halo mass function (HMF) of galaxies. We determine the relation between the \hi\ line-width from single-dish observations and the dark matter halo mass ($M_{200}$) inferred from rotation curve fits in the SPARC database, then we apply this relation to galaxies from the \hi\ Parkes All Sky Survey (HIPASS) to derive the HMF. This empirical HMF is well fit by a Schecther function, and matches that expected in $\Lambda$CDM over the range $10^{10.5} < M_{200} < 10^{12}\;\mathrm{M}_{\odot}$. More massive halos must be poor in neutral gas to maintain consistency with the power law predicted by $\Lambda$CDM. We detect no discrepancy at low masses. The lowest halo mass probed by HIPASS, however, is just greater than the mass scale where the Local Group missing satellite problem sets in. The integrated mass density associated with the dark matter halos of \hi-detected galaxies sums to $\Omega_{\rm m,gal} \approx 0.03$ over the probed mass range., Comment: 7 pages, 3 figures, 1 table. Accepted for publication in ApJ Letters

To date at least 10 highly flattened planes of dwarf galaxies are claimed to have been discovered in the Local Universe. The origin of these planes of galaxies remains unknown. One suggestion is that they are related to the large-scale structure of the cosmic web. A recent study found that the normal of a number of these dwarf galaxy planes are very closely aligned with the eigenvector of the shear tensor corresponding to the direction of greatest collapse obtained by reconstructing the full velocity field in the linear regime. Here we extend that work by both considering an additional 5 planes beyond the 5 examined previously and by examining the alignment with respect to the quasi-linear field, a more sophisticated reconstruction, which is a better approximation on smaller (quasi-linear) scales. Our analysis recovers the previous result while not finding a significantly tight alignment with the additional 5 planes. However, the additional 5 planes normals also do not appear to be randomly oriented. We conclude that this could either be due to the normals of the new planes being poorly defined and described; the quasi-linear field at those locations being poorly constrained; or different formation mechanisms for the orientation of planes of dwarf galaxies., Comment: 9 pages, 3 figures, 1 table

We present a detailed characterization of planes of satellites in the Milky Way (MW) and M31 systems. To this end we introduce an extension to the '4-galaxy-normal density plot' method \citep{Pawlowski13}, by which plot over-densities signal the normal direction to predominant planes of satellites within a given sample. For a given over-density, the extension provides a \textit{collection} of planes, each including a different number of objects $N_{\rm sat}$. We apply this method to the position data of confirmed MW and M31 satellites and quantify the quality of planes through the outputs of a Tensor of Inertia plane-fitting technique. Plane quality is quantified in terms of population ($N_{\rm sat}$) and flattening (the short-to-long axis ratio $c/a$ or the rms thickness normal to the plane). Therefore, planes with the same population or flattening can be compared with each other allowing us to single-out best-quality planes. For the first time, we study the second-most predominant planar configuration of satellites in M31, singling out a plane with 18 satellite members that shows a quality comparable to the Great Plane of Andromeda (GPoA, with $N_{\rm sat}=19$) despite it being more affected by distance uncertainties. This structure is viewed nearly face-on from the MW and is approximately normal to the GPoA. Overall, we find planes of satellites around the MW and M31 with higher qualities than those previously reported with a given $N_{\rm sat}$. We also show that mass plays no role in determining a satellite's membership or not to the respective best-quality planes., Comment: 10 pages, 6 figures, submitted to MNRAS

Dwarf galaxy satellite systems are essential probes to test models of structure formation, making it necessary to establish a census of dwarf galaxies outside of our own Local Group. We present deep FORS2 VI band images from the ESO Very Large Telescope for 15 dwarf galaxy candidates in the Centaurus group of galaxies. We confirm nine dwarfs to be members of CenA by measuring their distances using a Bayesian approach to determine the tip of the red giant branch luminosity. We have also fitted theoretical isochrones to measure their mean metallicities. The properties of the new dwarfs are similar to those in the Local Group in terms of their sizes, luminosities, and mean metallicities. Within our photometric precision there is no evidence of a metallicity spread, but we do observe possible extended star formation in several galaxies, as evidenced by a population of asymptotic giant branch stars brighter than the red giant branch tip. The new dwarfs do not show any signs of tidal disruption. Together with the recently reported dwarf galaxies by the complementary PISCeS survey, we study the luminosity function and 3D structure of the group. By comparing the observed luminosity function to the high-resolution cosmological simulation IllustrisTNG, we find agreement within a 90% confidence interval. However, CenA seems to be missing its brightest satellites and has an overabundance of the faintest dwarfs in comparison to its simulated analogs. In terms of the overall 3D distribution of the observed satellites, we find that the whole structure is flattened along the line-of-sight, with an root-mean-square (rms) height of 130 kpc and a rms semi major axis length of 330 kpc. Future distance measurements of the remaining dwarf galaxy candidates are needed to complete the census of dwarf galaxies in the Centaurus group., Comment: 21 pages, 17 figures, 4 tables, accepted for publication in A&A, version 2: fixed some typos in the coordinate list in Table 3

Observations of low-mass satellite galaxies in the nearby Universe point towards a strong dichotomy in their star-forming properties relative to systems with similar mass in the field. Specifically, satellite galaxies are preferentially gas poor and no longer forming stars, while their field counterparts are largely gas rich and actively forming stars. Much of the recent work to understand this dichotomy has been statistical in nature, determining not just that environmental processes are most likely responsible for quenching these low-mass systems but also that they must operate very quickly after infall onto the host system, with quenching timescales $\lesssim 2~ {\rm Gyr}$ at ${M}_{\star} \lesssim 10^{8}~{\rm M}_{\odot}$. This work utilizes the newly-available $Gaia$ DR2 proper motion measurements along with the Phat ELVIS suite of high-resolution, cosmological, zoom-in simulations to study low-mass satellite quenching around the Milky Way on an object-by-object basis. We derive constraints on the infall times for $37$ of the known low-mass satellite galaxies of the Milky Way, finding that $\gtrsim~70\%$ of the `classical' satellites of the Milky Way are consistent with the very short quenching timescales inferred from the total population in previous works. The remaining classical Milky Way satellites have quenching timescales noticeably longer, with $\tau_{\rm quench} \sim 6 - 8~{\rm Gyr}$, highlighting how detailed orbital modeling is likely necessary to understand the specifics of environmental quenching for individual satellite galaxies. Additionally, we find that the $6$ ultra-faint dwarf galaxies with publicly available $HST$-based star-formation histories are all consistent with having their star formation shut down prior to infall onto the Milky Way -- which, combined with their very early quenching times, strongly favors quenching driven by reionization., Comment: 13 pages, 7 figures, submitted to MNRAS

(Abridged) The Maunakea Spectroscopic Explorer (MSE) is an end-to-end science platform for the design, execution and scientific exploitation of spectroscopic surveys. It will unveil the composition and dynamics of the faint Universe and impact nearly every field of astrophysics across all spatial scales, from individual stars to the largest scale structures in the Universe. Major pillars in the science program for MSE include (i) the ultimate Gaia follow-up facility for understanding the chemistry and dynamics of the distant Milky Way, including the outer disk and faint stellar halo at high spectral resolution (ii) galaxy formation and evolution at cosmic noon, via the type of revolutionary surveys that have occurred in the nearby Universe, but now conducted at the peak of the star formation history of the Universe (iii) derivation of the mass of the neutrino and insights into inflationary physics through a cosmological redshift survey that probes a large volume of the Universe with a high galaxy density. MSE is positioned to become a critical hub in the emerging international network of front-line astronomical facilities, with scientific capabilities that naturally complement and extend the scientific power of Gaia, the Large Synoptic Survey Telescope, the Square Kilometer Array, Euclid, WFIRST, the 30m telescopes and many more., Comment: 9 chapters, 301 pages, 100 figures. This version of the DSC is a comprehensive update of the original version, released in 2016, which can be downloaded at arXiv:1606.00043. A detailed summary of the design of MSE is available in the MSE Book 2018, available at arXiv:1810.08695

The Milky Way, the Andromeda galaxy, and Centaurus A host flattened distributions of satellite galaxies which exhibits coherent velocity trends indicative of rotation. Comparably extreme satellite structures are very rare in cosmological LCDM simulations, giving rise to the `satellite plane problem'. As a possible explanation it has been suggested that earlier-forming, higher concentration host halos contain more flattened and kinematically coherent satellite planes. We have tested for such a proposed correlation between the satellite plane and host halo properties in the ELVIS suite of simulations. We find evidence neither for a correlation of plane flattening with halo concentration or formation time, nor for a correlation of kinematic coherence with concentration. The height of the thinnest sub-halo planes does correlate with the host virial radius and with the radial extent of the sub-halo system. This can be understood as an effect of not accounting for differences in the radial distribution of sub-halos, and selecting them from different volumes than covered by the actual observations. Being part of a halo pair like the Local Group does not result in more narrow or more correlated satellite planes either. Additionally, using the PhatELVIS simulations we show that the presence of a central galaxy potential does not favor more narrow or more correlated satellite planes, it rather leads to slightly wider planes. Such a central potential is a good approximation of the dominant effect baryonic physics in cosmological simulations has on a sub-halo population. This suggests that, in contrast to other small-scale problems, the planes of satellite galaxies issue is made worse by accounting for baryonic effects., Comment: 23 pages, 11 figures, 4 tables. Accepted for publication in The Astrophysical Journal. Short answer: no

We discuss how astrophysical observations with the Maunakea Spectroscopic Explorer (MSE), a high-multiplexity (about 4300 fibers), wide field-of-view (1.5 square degree), large telescope aperture (11.25 m) facility, can probe the particle nature of dark matter. MSE will conduct a suite of surveys that will provide critical input for determinations of the mass function, phase-space distribution, and internal density profiles of dark matter halos across all mass scales. N-body and hydrodynamical simulations of cold, warm, fuzzy and self-interacting dark matter suggest that non-trivial dynamics in the dark sector could have left an imprint on structure formation. Analysed within these frameworks, the extensive and unprecedented datasets produced by MSE will be used to search for deviations away from cold and collisionless dark matter model. MSE will provide an improved estimate of the local density of dark matter, critical for direct detection experiments, and will improve estimates of the J-factor for indirect searches through self-annihilation or decay into Standard Model particles. MSE will determine the impact of low mass substructures on the dynamics of Milky Way stellar streams in velocity space, and will allow for estimates of the density profiles of the dark matter halos of Milky Way dwarf galaxies using more than an order of magnitude more tracers. In the low redshift Universe, MSE will provide critical redshifts to pin down the luminosity functions of vast numbers of satellite systems, and MSE will be an essential component of future strong lensing measurements to constrain the halo mass function. Across nearly all mass scales, the improvements offered by MSE, in comparison to other facilities, are such that the relevant analyses are limited by systematics rather than statistics., Comment: 44 pages, 19 figures. To appear as a chapter for "The Detailed Science Case for the Maunakea Spectroscopic Explorer, 2019"

The Magellanic Clouds (MCs) are the most massive gas-bearing systems falling into the Galaxy at the present epoch. They show clear signs of interaction, manifested in particular by the Magellanic Stream, a spectacular gaseous wake that trails from the MCs extending more than 150 degree across the sky. Ahead of the MCs is the "Leading Arm" usually interpreted as the tidal counterpart of the Magellanic Stream, an assumption we now call into question. We revisit the formation of these gaseous features in a first-infall scenario, including for the first time a Galactic model with a weakly magnetised, spinning hot corona. In agreement with previous studies, we broadly recover the location and the extension of the Stream on the sky. In contrast, we find that the formation of the Leading Arm -- that is otherwise present in models without a corona -- is inhibited by the hydrodynamic interaction with the hot component. These results hold with or without coronal rotation or a weak, ambient magnetic field. Since the existence of the hot corona is well established, we are led to two possible interpretations: (i) the Leading Arm survives because the coronal density beyond 20 kpc is a factor of 10 lower than required by conventional spheroidal coronal x-ray models, consistent with recent claims of rapid coronal rotation; or (ii) the `Leading Arm' is cool gas trailing from a frontrunner, a satellite moving ahead of the MCs, consistent with its higher metallicity compared to the trailing stream. Both scenarios raise issues that we discuss., Comment: MNRAS accepted. Matches accepted version

We introduce an extension of the ELVIS project to account for the effects of the Milky Way galaxy on its subhalo population. Our simulation suite, Phat ELVIS, consists of twelve high-resolution cosmological dark matter-only (DMO) zoom simulations of Milky Way-size $\Lambda$CDM~ haloes ($M_{\rm v} = 0.7-2 \times 10^{12} \,\mathrm{M}_\odot$) along with twelve re-runs with embedded galaxy potentials grown to match the observed Milky Way disk and bulge today. The central galaxy potential destroys subhalos on orbits with small pericenters in every halo, regardless of the ratio of galaxy mass to halo mass. This has several important implications. 1) Most of the $\mathtt{Disk}$ runs have no subhaloes larger than $V_{\rm max} = 4.5$ km s$^{-1}$ within $20$ kpc and a significant lack of substructure going back $\sim 8$ Gyr, suggesting that local stream-heating signals from dark substructure will be rare. 2) The pericenter distributions of Milky Way satellites derived from $\mathit{Gaia}$ data are remarkably similar to the pericenter distributions of subhaloes in the $\mathtt{Disk}$ runs, while the DMO runs drastically over-predict galaxies with pericenters smaller than 20 kpc. 3) The enhanced destruction produces a tension opposite to that of the classic `missing satellites' problem: in order to account for ultra-faint galaxies known within $30$ kpc of the Galaxy, we must populate haloes with $V_\mathrm{peak} \simeq 7$ km s$^{-1}$ ($M \simeq 3 \times 10^{7} \,\mathrm{M}_\odot$ at infall), well below the atomic cooling limit of $V_\mathrm{peak} \simeq 16$ km s$^{-1}$ ($M \simeq 5 \times 10^{8} \,\mathrm{M}_\odot$ at infall). 4) If such tiny haloes do host ultra-faint dwarfs, this implies the existence of $\sim 1000$ satellite galaxies within 300 kpc of the Milky Way., Comment: 16 pages, 13 figures, 2 tables. Halo catalogs are available at http://localgroup.ps.uci.edu/phat-elvis/

Satellite galaxies of the Milky Way and of the Andromeda galaxy have been found to preferentially align in significantly flattened planes of satellite galaxies, and available velocity measurements are indicative of a preference of satellites in those structures to co-orbit. There is increasing evidence that such kinematically correlated satellite planes are also present around more distant hosts. Detailed comparisons show that similarly anisotropic phase-space distributions of sub-halos are exceedingly rare in cosmological simulations based on the $\Lambda$CDM paradigm. Analogs to the observed systems have frequencies of $\leq 0.5$ per cent in such simulations. In contrast to other small-scale problems, the satellite planes issue is not strongly affected by baryonic processes because the distribution of sub-halos on scales of hundreds of kpc is dominated by gravitational effects. This makes the satellite planes one of the most serious small-scale problem for $\Lambda$CDM. This review summarizes the observational evidence for planes of satellite galaxies in the Local Group and beyond, and provides an overview of how they compare to cosmological simulations. It also discusses scenarios which aim at explaining the coherence of satellite positions and orbits, and why they all are currently unable to satisfactorily resolve the issue., Comment: Invited review for MPLA, accepted for publication. 26 pages, 3 figures

The Milky Way and Andromeda galaxy are each surrounded by a thin plane of satellite galaxies that may be corotating. Cosmological simulations predict that most satellite galaxy systems are close to isotropic with random motions, so those two well-studied systems are often interpreted as rare statistical outliers. We test this assumption using the kinematics of satellite galaxies around the Centaurus A galaxy. Our statistical analysis reveals evidence for corotation in a narrow plane: of the 16 Centaurus A's satellites with kinematic data, 14 follow a coherent velocity pattern aligned with the long axis of their spatial distribution. In standard cosmology simulations, < 0.5% of Centaurus A-like systems show such behavior. Corotating satellite systems may be common in the Universe, challenging small-scale structure formation in the prevailing cosmological paradigm., Comment: Published in the first Feb 2018 issue of Science (M\"uller et al., Science 359, 534, 2018). This preprint corresponds to the accepted and language edited version of the manuscript. 32 pages, 5 figures

The radial acceleration measured in bright galaxies tightly correlates with that generated by the observed distribution of baryons, a phenomenon known as the radial acceleration relation (RAR). Dwarf spheroidal satellite galaxies have been recently found to depart from the extrapolation of the RAR measured for more massive objects but with a substantially larger scatter. If confirmed by new data, this result provides a powerful test of the theory of gravity at low accelerations that requires robust theoretical predictions. By using high-resolution hydrodynamical simulations, we show that, within the standard model of cosmology ($\Lambda$CDM), satellite galaxies are expected to follow the same RAR as brighter systems but with a much larger scatter which does not correlate with the physical properties of the galaxies. In the simulations, the RAR evolves mildly with redshift. Moreover, the acceleration due to the gravitational field of the host has no effect on the RAR. This is in contrast with the External Field Effect in Modified Newtonian Dynamics (MOND) which causes galaxies in strong external fields to deviate from the RAR. This difference between $\Lambda$CDM and MOND offers a possible way to discriminate between them., Comment: 5 pages, 4 figures, accepted for publication in PRL