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We use cosmological hydrodynamical galaxy formation simulations from the NIHAO project to investigate the response of cold dark matter (CDM) haloes to baryonic processes. Previous work has shown that the halo response is primarily a function of the ratio between galaxy stellar mass and total virial mass, and the density threshold above which gas is eligible to form stars, $n [{\rm cm}^{-3}]$. At low $n$ all simulations in the literature agree that dwarf galaxy haloes are cuspy, but at high $n\ge 100$ there is no consensus. We trace halo contraction in dwarf galaxies with $n\ge 100$ reported in some previous simulations to insufficient spatial resolution. Provided the adopted star formation threshold is appropriate for the resolution of the simulation, we show that the halo response is remarkably stable for $n\ge 5$, up to the highest star formation threshold that we test, $n=500$. This free parameter can be calibrated using the observed clustering of young stars. Simulations with low thresholds $n\le 1$ predict clustering that is too weak, while simulations with high star formation thresholds $n\ge 5$, are consistent with the observed clustering. Finally, we test the CDM predictions against the circular velocities of nearby dwarf galaxies. Low thresholds predict velocities that are too high, while simulations with $n\sim 10$ provide a good match to the observations. We thus conclude that the CDM model provides a good description of the structure of galaxies on kpc scales provided the effects of baryons are properly captured., Comment: 15 pages, 10 figures, published in MNRAS

We investigate how the NIHAO galaxies match the observed star formation main sequence (SFMS) and what the origin of its scatter is. The NIHAO galaxies reproduce the SFMS and generally agree with observations, but the slope is about unity and thus significantly larger than observed values. This is because observed galaxies at large stellar masses, although still being part of the SFMS, are already influenced by quenching. This partial suppression of star formation by AGN feedback leads to lower star formation rates and therefore to lower observed slopes. We confirm that including the effects of AGN in our galaxies leads to slopes in agreement with observations. We find the deviation of a galaxy from the SFMS is correlated with its $z=0$ dark matter halo concentration and thus with its halo formation time. This means galaxies with a higher-than-average star formation rate (SFR) form later and vice versa. We explain this apparent correlation with the SFR by re-interpreting galaxies that lie above the SFMS (higher-than-average SFR) as lying to the left of the SFMS (lower-than-average stellar mass) and vice versa. Thus later forming haloes have a lower-than-average stellar mass, this is simply because they have had less-than-average time to form stars, and vice versa. It is thus the nature, i.e. how and when these galaxies form, that sets the path of a galaxy in the SFR versus stellar mass plane., Comment: 8 pages, 11 figures, accepted by MNRAS

We present a comprehensive test of the relation between stellar and total mass in galaxies as predicted by popular models based on abundance matching (AM) techniques. We use the ``Spectroscopy and H-band Imaging of Virgo cluster galaxies'' (SHIVir) survey with photometric and dynamical profiles for 190 Virgo cluster galaxies to establish a relation between the stellar and dynamical masses measured within the isophotal radius $r_{23.5}$. Various dark matter and galaxy scaling relations are combined with results from the NIHAO suite of hydrodynamical simulations to recast AM predictions in terms of these observed quantities. Our results are quite insensitive to the exact choice of dark matter profile and halo response to baryon collapse. We find that theoretical models reproduce the slope and normalization of the observed stellar-to-halo mass relation (SHMR) over more than three orders of magnitude in stellar mass $(10^8 < M_*/M_{\odot} < 2 \times 10^{11})$. However, the scatter of the observed SHMR exceeds that of AM predictions by a factor $\sim$5. For systems with stellar masses exceeding $5 \times 10^{10}~M_{\odot}$, AM overpredicts the observed stellar masses for a given dynamical mass. The latter offset may support previous indications of a different stellar initial mass function in these massive galaxies. Overall our results support the validity of AM predictions on a wide dynamical range., Comment: 5 pages, 4 figures, accepted for publication in MNRAS Letters

We explore a function with two shape parameters for the dark-matter halo density profile subject to baryonic effects, which is a special case of the general Zhao family of models applied to simulated dark matter haloes by Dekel et al. This profile has variable inner slope and concentration parameter, and analytic expressions for the gravitational potential, velocity dispersion, and lensing properties. Using the NIHAO cosmological simulations, we find that it provides better fits than the Einasto profile and the generalized NFW profile with variable inner slope, in particular towards the halo centers. We show that the profile parameters are correlated with the stellar-to-halo mass ratio $M_{\rm star}/M_{\rm vir}$. This defines a mass-dependent density profile describing the average dark matter profiles in all galaxies, which can be directly applied to observed rotation curves of galaxies, gravitational lenses, and semi-analytic models of galaxy formation or satellite-galaxy evolution. The effect of baryons manifests itself by a significant flattening of the inner density slope and a 20\% decrease of the concentration parameter for $M_{\rm star}/M_{\rm vir} = 10^{-3.5}$ to $10^{-2}$, corresponding to $M_{\rm star} \sim 10^{7-10} M_\odot$. The accuracy by which this profile fits simulated galaxies is similar to certain multi-parameter, mass-dependent profiles, but its fewer parameters and analytic nature make it most desirable for many purposes., Comment: Accepted by MNRAS, 22 pages (main) + 15 pages (appendices)

This paper is one in a series that explores the importance of radius as a second parameter in galaxy evolution. The topic investigated here is the relationship between star formation rate (SFR) and galaxy radius ($R_{\rm e}$) for main-sequence star-forming galaxies. The key observational result is that, over a wide range of stellar mass and redshift in both CANDELS and SDSS, there is little trend between SFR and $R_{\rm e}$ at fixed stellar mass. The Kennicutt-Schmidt law, or any similar density-related star formation law, then implies that smaller galaxies must have lower gas fractions than larger galaxies (at fixed $M_{\ast}$), and this is supported by observations of local star-forming galaxies. We investigate the implication by adopting the equilibrium "bathtub" model: the ISM gas mass is assumed to be constant over time and the net star formation rate is the difference between the accretion rate of gas onto the galaxy from the halo and the outflow rate due to winds. To match the observed null correlation between SFR and radius, the bathtub model requires that smaller galaxies at fixed mass have weaker galactic winds. Our hypothesis is that galaxies are a 2-dimensional family whose properties are set mainly by halo mass and concentration. Galaxy radius and accretion rate plausibly both depend on halo concentration, which predicts how wind strength should vary with $R_{\rm e}$ and SFR., Comment: 21 pages, 9 figures, ApJ accepted

Simulating thin and extended galactic disks has long been a challenge in computational astrophysics. We introduce the NIHAO-UHD suite of cosmological hydrodynamical simulations of Milky Way mass galaxies and study stellar disk properties such as stellar mass, size and rotation velocity which agree well with observations of the Milky Way and local galaxies. In particular, the simulations reproduce the age-velocity dispersion relation and a multi-component stellar disk as observed for the Milky Way. Half of our galaxies show a double exponential vertical profile, while the others are well described by a single exponential model which we link to the disk merger history. In all cases, mono-age populations follow a single exponential whose scale height varies monotonically with stellar age and radius. The scale length decreases with stellar age while the scale height increases. The general structure of the stellar disks is already set at time of birth as a result of the inside-out and upside-down formation. Subsequent evolution modifies this structure by increasing both the scale length and height of all mono-age populations. Thus, our results put tight constraints on how much dynamical memory stellar disks can retain over cosmological timescales. Our simulations demonstrate that it is possible to form thin galactic disks in cosmological simulations provided there are no significant stellar mergers at low redshifts. Most of the stellar mass is formed in-situ with only a few percent ($\lesssim5\%$) brought in by merging satellites at early times. Redshift zero snapshots and halo catalogues are publicly available., Comment: simulations publicly available at http://www2.mpia-hd.mpg.de/~buck/#sim_data , 19 pages (14 main text, 5 appendix), 14 figures (plus 4 appendix), accepted by MNRAS

We implement an optically thin approximation for the effects of the local radiation field from stars and hot gas on the gas heating and cooling in the N-body SPH code GASOLINE2. We resimulate three galaxies from the NIHAO project: one dwarf, one Milky Way-like and one massive spiral, and study what are the local radiation field effects on various galaxy properties. We also study the effects of varying the Ultra Violet Background (UVB) model, by running the same galaxies with two different UVBs. Galaxy properties at $z=0$ like stellar mass, stellar effective mass radius, HI mass, and radial extent of the HI disc, show significant changes between the models with and without the local radiation field, and smaller differences between the two UVB models. The intrinsic effect of the local radiation field through cosmic time is to increase the equilibrium temperature at the interface between the galaxies and their circumgalactic media (CGM), moving this boundary inwards, while leaving relatively unchanged the gas inflow rate. Consequently, the temperature of the inflow increases when considering the local radiation sources. This temperature increase is a function of total galaxy mass, with a median CGM temperature difference of one order of magnitude for the massive spiral. The local radiation field suppresses the stellar mass growth by $\sim$20 per cent by $z=0$ for all three galaxies, while the HI mass is roughly halfed. The differences in the gas phase diagrams, significantly impact the HI column densities, shifting their peaks in the distributions towards lower $N_{\rm HI}$., Comment: resubmitted to MNRAS after minor revision

We present a simple model for the response of a dissipationless spherical system to an instantaneous mass change at its center, describing the formation of flat cores in dark matter haloes and ultra-diffuse galaxies (UDGs) from feedback-driven outflow episodes in a specific mass range. This model generalizes an earlier simplified analysis of an isolated shell into a system with continuous density, velocity and potential profiles. The response is divided into an instantaneous change of potential at constant velocities due to a given mass loss or gain, followed by energy-conserving relaxation to a new Jeans equilibrium. The halo profile is modeled by a two-parameter function with a variable inner slope and an analytic potential profile (Dekel et al. 2017), which enables determining the associated kinetic energy at equilibrium. The model is tested against NIHAO cosmological zoom-in simulations, where it successfully predicts the evolution of the inner dark-matter profile between successive snapshots in about 75% of the cases, failing mainly in merger situations. This model provides a simple understanding of the formation of dark-matter halo cores and UDGs by supernova-driven outflows, and a useful analytic tool for studying such processes., Comment: 27 pages, 22 figures, accepted by MNRAS

We introduce algorithms for black hole physics, i.e., black hole formation, accretion and feedback, into the NIHAO (Numerical Investigation of a Hundred Astrophysical Objects) project of galaxy simulations. This enables us to study high mass, elliptical galaxies, where feedback from the central black hole is generally thought to have a significant effect on their evolution. We furthermore extend the NIHAO suite by 45 simulations that encompass $z=0$ halo masses from $1 \times 10^{12}$ to $4 \times 10^{13}\,\mathrm{M}_{\odot}$, and resimulate five galaxies from the original NIHAO sample with black hole physics, which have $z=0$ halo masses from $8 \times 10^{11}$ to $3 \times 10^{12}\,\mathrm{M}_{\odot}$. Now NIHAO contains 144 different galaxies and thus has the largest sample of zoom-in simulations of galaxies, spanning $z=0$ halo masses from $9 \times 10^{8}$ to $4 \times 10^{13}\,\mathrm{M}_{\odot}$. In this paper we focus on testing the algorithms and calibrating their free parameters against the stellar mass versus halo mass relation and the black hole mass versus stellar mass relation. We also investigate the scatter of these relations, which we find is a decreasing function with time and thus in agreement with observations. For our fiducial choice of parameters we successfully quench star formation in objects above a $z=0$ halo mass of $10^{12}\,\mathrm{M}_{\odot}$, thus transforming them into red and dead galaxies., Comment: 14 pages, 11 figures, accepted by MNRAS

We use thirty-eight high-resolution simulations of galaxy formation between redshift 10 and 5 to study the impact of a 3 keV warm dark matter (WDM) candidate on the high-redshift Universe. We focus our attention on the stellar mass function and the global star formation rate and consider the consequences for reionization, namely the neutral hydrogen fraction evolution and the electron scattering optical depth. We find that three different effects contribute to differentiate warm and cold dark matter (CDM) predictions: WDM suppresses the number of haloes with mass less than few $10^9$ M$_{\odot}$; at a fixed halo mass, WDM produces fewer stars than CDM; and finally at halo masses below $10^9$ M$_{\odot}$, WDM has a larger fraction of dark haloes than CDM post-reionization. These three effects combine to produce a lower stellar mass function in WDM for galaxies with stellar masses at and below $\sim 10^7$ M$_{\odot}$. For $z > 7$, the global star formation density is lower by a factor of two in the WDM scenario, and for a fixed escape fraction, the fraction of neutral hydrogen is higher by 0.3 at $z \sim 6$. This latter quantity can be partially reconciled with CDM and observations only by increasing the escape fraction from 23 per cent to 34 per cent. Overall, our study shows that galaxy formation simulations at high redshift are a key tool to differentiate between dark matter candidates given a model for baryonic physics., Comment: 11 pages, 8 figures, submitted to MNRAS

We derive the mass weighted total density slopes within the effective (half-light) radius, $\gamma'$, for more than 2000 nearby galaxies from the SDSS-IV MaNGA survey using Jeans-anisotropic-models applied to IFU observations. Our galaxies span a wide range of the stellar mass ($10^9$ $M_{\rm \odot}< M_* < 10^{12}$ M$_{\odot}$) and the velocity dispersion (30 km/s $< \sigma_v <$ 300 km/s). We find that for galaxies with velocity dispersion $\sigma_v>100$ km/s, the density slope has a mean value $\langle \gamma^{\prime} \rangle = 2.24$ and a dispersion $\sigma_{\gamma}=0.22$, almost independent of velocity dispersion. A clear turn over in the $\gamma'-\sigma_v$ relation is present at $\sigma\sim 100$ km/s, below which the density slope decreases rapidly with $\sigma_v$. Our analysis shows that a large fraction of dwarf galaxies (below $M_* = 10^{10}$ M$_{\odot}$) have total density slopes shallower than 1, which implies that they may reside in cold dark matter halos with shallow density slopes. We compare our results with that of galaxies in hydrodynamical simulations of EAGLE, Illustris and IllustrisTNG projects, and find all simulations predict shallower density slopes for massive galaxies with high $\sigma_v$. Finally, we explore the dependence of $\gamma'$ on the positions of galaxies in halos, namely centrals vs. satellites, and find that for the same velocity dispersion, the amplitude of $\gamma'$ is higher for satellite galaxies by about 0.1., Comment: 11 pages, 10 figures, submitted to MNRAS

We present catalogs of calibrated photometry and spectroscopic redshifts in the Extended Groth Strip, intended for studies of photometric redshifts (photo-z's). The data includes ugriz photometry from CFHTLS and Y-band photometry from the Subaru Suprime camera, as well as spectroscopic redshifts from the DEEP2, DEEP3 and 3D-HST surveys. These catalogs incorporate corrections to produce effectively matched-aperture photometry across all bands, based upon object size information available in the catalog and Moffat profile point spread function fits. We test this catalog with a simple machine learning-based photometric redshift algorithm based upon Random Forest regression, and find that the corrected aperture photometry leads to significant improvement in photo-z accuracy compared to the original SExtractor catalogs from CFHTLS and Subaru. The deep ugrizY photometry and spectroscopic redshifts are well-suited for empirical tests of photometric redshift algorithms for LSST. The resulting catalogs are publicly available. We include a basic summary of the strategy of the DEEP3 Galaxy Redshift Survey to accompany the recent public release of DEEP3 data., Comment: 20 pages, 9 figures; published in MNRAS; for associated catalogs, see http://d-scholarship.pitt.edu/36064/

The phenomenological basis for Modified Newtonian Dynamics (MOND) is the radial-acceleration-relation (RAR) between the observed acceleration, $a=V^2_{rot}(r)/r$, and the acceleration accounted for by the observed baryons (stars and cold gas), $a_{bar}=V_{bar}^2(r)/r$. We show that the RAR arises naturally in the NIHAO sample of 89 high-resolution LCDM cosmological galaxy formation simulations. The overall scatter from NIHAO is just 0.079 dex, consistent with observational constraints. However, we show that the scatter depends on stellar mass. At high masses ($10^9

We have used the NIHAO simulations to explore how supernovae (SNe) affect star formation in galaxies. We find that SN feedback operates on all scales from the interstellar medium (ISM) to several virial radii. SNe regulate star formation by preventing condensation of HI into H$_2$ and by moving cold neutral gas to the hot HII phase. The first effect explains why the cold neutral gas in dwarf galaxies forms stars inefficiently. The second maintains the hot ISM of massive galaxies (HII vents out at lower masses). At $v_{\rm vir}\simeq 67{\rm\,km\,s}^{-1}$, the outflow rate follows the relation: $\dot{M}_{\rm out}=23\,(v_{\rm vir}/67{\rm\,km\,s}^{-1})^{-4.6}\,{\rm SFR}$. $20\%$ to $70\%$ of the gas expelled from galaxies escapes from the halo (ejective feedback) but outflows are dominated by cold swept-up gas, most of which falls back onto the galaxy on a $\sim 1\,$Gyr timescale. This `fountain feedback' reduces the masses of galaxies by a factor of two to four, since gas spends half to three quarter of its time in the fountain. Less than $10\%$ of the ejected gas mixes with the hot circumgalactic medium and this gas is usually not reaccreted. On scales as large as $6r_{\rm vir}$, galactic winds divert the incoming gas from cosmic filaments and prevent if from accreting onto galaxies (pre-emptive feedback). This process is the main reason for the low baryon content of ultradwarves., Comment: Submitted for publication in MNRAS

In this third paper of the series, we investigate the effects of warm dark matter with a particle mass of $m_\mathrm{WDM}=3\,\mathrm{keV}$ on the smallest galaxies in our Universe. We present a sample of 21 hydrodynamical cosmological simulations of dwarf galaxies and 20 simulations of satellite-host galaxy interaction that we performed both in a Cold Dark Matter (CDM) and Warm Dark Matter (WDM) scenario. In the WDM simulations, we observe a higher critical mass for the onset of star formation. Structure growth is delayed in WDM, as a result WDM haloes have a stellar population on average two Gyrs younger than their CDM counterparts. Nevertheless, despite this delayed star formation, CDM and WDM galaxies are both able to reproduce the observed scaling relations for velocity dispersion, stellar mass, size, and metallicity at $z=0$. WDM satellite haloes in a Milky Way mass host are more susceptible to tidal stripping due to their lower concentrations, but their galaxies can even survive longer than the CDM counterparts if they live in a dark matter halo with a steeper central slope. In agreement with our previous CDM satellite study we observe a steepening of the WDM satellites' central dark matter density slope due to stripping. The difference in the average stellar age for satellite galaxies, between CDM and WDM, could be used in the future for disentangling these two models., Comment: 10 pages, 11 figures, accepted for publication on MNRAS

The existence of galaxies with a surface brightness $\mu$ lower than the night sky has been known since three decades. Yet, their formation mechanism and emergence within a $\rm\Lambda CDM$ universe has remained largely undetermined. For the first time, we investigated the origin of Low Surface Brightness (LSB) galaxies with M$_{\star}$$\sim$10$^{9.5-10}$M$_{\odot}$, which we are able to reproduce within hydrodynamical cosmological simulations from the NIHAO suite. The simulated and observed LSBs share similar properties, having large HI reservoir, extended star formation histories and effective radii, low S\'{e}rsic index and slowly rising rotation curves. The formation mechanism of these objects is explored: simulated LSBs form as a result of co-planar co-rotating mergers and aligned accretion of gas at early times, while perpendicular mergers and mis-aligned gas accretion result in higher $\mu$ galaxies by $z$=0. The larger the merger, the stronger the correlation between merger orbital configuration and final $\mu$. While the halo spin parameter is consistently high in simulated LSB galaxies, the impact of halo concentration, feedback-driven gas outflows and merger time only plays a minor-to-no role in determining $\mu$. Interestingly, the formation scenario of such `classical' LSBs differs from the one of less massive, M$_{\star}$$\sim$10$^{7-9}$M$_{\odot}$, Ultra-Diffuse Galaxies, the latter resulting from the effects of SNae driven gas outflows: a M$_{\star}$ of $\sim$10$^9$M$_{\odot}$ thus represents the transition regime between a feedback-dominated to an angular momentum-dominated formation scenario in the LSB realm. Observational predictions are offered regarding spatially resolved star formation rates through LSB discs: these, together with upcoming surveys, can be used to verify the proposed emergence scenario of LSB galaxies., Comment: MNRAS Accepted 2019 April 02; in original form 2019 January 24. 15 pages,17 figures. main results in Fig 12 and 14. Added appendix A

State-of-the-art cosmological hydrodynamical simulations of galaxy formation have reached the point at which their outcomes result in galaxies with ever more realism. Still, the employed sub-grid models include several free parameters such as the density threshold, $n$, to localize the star-forming gas. In this work, we investigate the possibilities to utilize the observed clustered nature of star formation (SF) in order to refine SF prescriptions and constrain the density threshold parameter. To this end, we measure the clustering strength, correlation length and power-law index of the two-point correlation function of young ($\tau<50$ Myr) stellar particles and compare our results to observations from the HST Legacy Extragalactic UV Survey (LEGUS). Our simulations reveal a clear trend of larger clustering signal and power-law index and lower correlation length as the SF threshold increases with only mild dependence on galaxy properties such as stellar mass or specific star formation rate. In conclusion, we find that the observed clustering of SF is inconsistent with a low threshold for SF ($n<1$ cm$^{-3}$) and strongly favours a high value for the density threshold of SF ($n>10$ cm$^{-3}$), as for example employed in the NIHAO project., Comment: accepted by MNRAS, 7 pages, 5 figures, key figure: figure 5

We study ultra-diffuse galaxies (UDGs) in zoom in cosmological simulations, seeking the origin of UDGs in the field versus galaxy groups. We find that while field UDGs arise from dwarfs in a characteristic mass range by multiple episodes of supernova feedback (Di Cintio et al. 2017), group UDGs may also form by tidal puffing up and they become quiescent by ram-pressure stripping. The field and group UDGs share similar properties, independent of distance from the group centre. Their dark-matter haloes have ordinary spin parameters and centrally dominant dark-matter cores. Their stellar components tend to have a prolate shape with a Sersic index n~1 but no significant rotation. Ram pressure removes the gas from the group UDGs when they are at pericentre, quenching star formation in them and making them redder. This generates a colour/star-formation-rate gradient with distance from the centre, as observed in clusters. We find that ~20 per cent of the field UDGs that fall into a massive halo survive as satellite UDGs. In addition, normal field dwarfs on highly eccentric orbits can become UDGs near pericentre due to tidal puffing up, contributing about half of the group-UDG population. We interpret our findings using simple toy models, showing that gas stripping is mostly due to ram pressure rather than tides. We estimate that the energy deposited by tides in the bound component of a satellite over one orbit can cause significant puffing up provided that the orbit is sufficiently eccentric., Comment: 20 pages, 13 figures, submitted to MNRAS

We use cosmological hydrodynamical galaxy formation simulations from the NIHAO project to investigate the impact of the threshold for star formation on the response of the dark matter (DM) halo to baryonic processes. The fiducial NIHAO threshold, $n=10\, {\rm cm}^{-3}$, results in strong expansion of the DM halo in galaxies with stellar masses in the range $10^{7.5} < M_{star} < 10^{9.5} M_{\odot}$. We find that lower thresholds such as $n=0.1$ (as employed by the EAGLE/APOSTLE and Illustris/AURIGA projects) do not result in significant halo expansion at any mass scale. Halo expansion driven by supernova feedback requires significant fluctuations in the local gas fraction on sub-dynamical times (i.e., < 50 Myr at galaxy half-light radii), which are themselves caused by variability in the star formation rate. At one per cent of the virial radius, simulations with $n=10$ have gas fractions of $\simeq 0.2$ and variations of $\simeq 0.1$, while $n=0.1$ simulations have order of magnitude lower gas fractions and hence do not expand the halo. The observed DM circular velocities of nearby dwarf galaxies are inconsistent with CDM simulations with $n=0.1$ and $n=1$, but in reasonable agreement with $n=10$. Star formation rates are more variable for higher $n$, lower galaxy masses, and when star formation is measured on shorter time scales. For example, simulations with $n=10$ have up to 0.4 dex higher scatter in specific star formation rates than simulations with $n=0.1$. Thus observationally constraining the sub-grid model for star formation, and hence the nature of DM, should be possible in the near future., Comment: 18 pages, 13 figures, accepted to MNRAS

The NIHAO cosmological simulations form a collection of a hundred high-resolution galaxies. We used these simulations to test the impact of stellar feedback on the morphology of the HI distribution in galaxies. We ran a subsample of twenty of the galaxies with different parameterizations of stellar feedback, looking for differences in the HI spatial distribution and morphology. We found that different feedback models do leave a signature in HI, and can potentially be compared with current and future observations. These findings can help inform future modeling efforts in the parameterization of stellar feedback in cosmological simulations of galaxy formation and evolution., Comment: Conference proceedings presented at the First Sharjah International Conference on Particle Physics, Astrophysics, and Cosmology

We used high resolution dark matter only cosmological simulations to investigate the structural properties of Lambda Cold Dark Matter ($\Lambda$CDM) haloes over cosmic time. The haloes in our study range in mass from $\sim 10^{10}$ to $\sim 10^{12} \mathrm{M}_\odot$, and are resolved with $10^5$ to $10^7$ particles. We fit the spherically averaged density profiles of DM haloes with the three parameter Einasto function. For our sample of haloes, the Einasto shape parameter, $\alpha$, is uncorrelated with the concentration, $c$, at fixed halo mass, and at all redshifts. Previous reports of an anti-correlation are traced to fitting degeneracies, which our fits are less sensitive to due to our higher spatial resolution. However, for individual haloes the evolution in $\alpha$ and $c$ is anti-correlated: at redshift $z=7$, $\alpha \simeq 0.4$ and decreases with time, while $c\simeq 3$ and increases with time. The evolution in structure is primarily due to accretion of mass at larger radii. We suggest that $\alpha$ traces the evolutionary state of the halo, with dynamically young haloes having high $\alpha$ (closer to a top-hat: $\alpha^{-1}=0$), and dynamically relaxed haloes having low $\alpha$ (closer to isothermal: $\alpha=0$). Such an evolutionary dependence reconciles the increase of $\alpha$ vs peak height, $\nu$, with the dependence on the slope of the power spectrum of initial density fluctuations found by previous studies., Comment: accepted for publication in MNRAS

We study the origin of bulge stars and their angular momentum (AM) evolution in 10 spiral galaxies with baryonic masses above $10^{10}$M$_\odot$ in the NIHAO galaxy formation simulations. The simulated galaxies are in good agreement with observations of the relation between specific AM and mass of the baryonic component and the stellar bulge-to-total ratio ($B/T$). We divide the star particles at $z=0$ into disc and bulge components using a hybrid photometric/kinematic decomposition method that identifies all central mass above an exponential disc profile as the `bulge'. By tracking the bulge star particles back in time, we find that on average 95\% of the bulge stars formed {\it in situ}, 3\% formed {\it ex situ} in satellites of the same halo, and only 2\% formed {\it ex situ} in external galaxies. The evolution of the AM distribution of the bulge stars paints an interesting picture: the higher the final $B/T$ ratio, the more the specific AM remains preserved during the bulge formation. In all cases, bulge stars migrate significantly towards the central region, reducing their average galactocentric radius by roughly a factor 2, independently of the final $B/T$ value. However, in the higher $B/T$ ($\gtrsim0.2$) objects, the velocity of the bulge stars increases and the AM of the bulge is almost conserved, whereas at lower $B/T$ values, the velocity of the bulge stars decreases and the AM of bulge reduces. The correlation between the evolution of the AM and $B/T$ suggests that bulge and disc formation are closely linked and cannot be treated as independent processes., Comment: 17 pages, 16 Figures, 1 table; accepted for publication in MNRAS

We analyze 88 independent high-resolution cosmological zoom-in simulations of disk galaxies in the NIHAO simulations suite to explore the connection between the atomic gas fraction and angular momentum of baryons throughout cosmic time. The study is motivated by the analytic model of \citet{obreschkow16}, which predicts a relation between the atomic gas fraction $f_{\rm atm}$ and the global atomic stability parameter $q \equiv j\sigma / (GM)$, where $M$ and $j$ are the mass and specific angular momentum of the galaxy (stars+cold gas) and $\sigma$ is the velocity dispersion of the atomic gas. We show that the simulated galaxies follow this relation from their formation ($z\simeq4$) to present within $\sim 0.5$ dex. To explain this behavior, we explore the evolution of the local Toomre stability and find that $90\%$--$100\%$ of the atomic gas in all simulated galaxies is stable at any time. In other words, throughout the entire epoch of peak star formation until today, the timescale for accretion is longer than the timescale to reach equilibrium, thus resulting in a quasi-static equilibrium of atomic gas at any time. Hence, the evolution of $f_{\rm atm}$ depends on the complex hierarchical growth history primarily via the evolution of $q$. An exception are galaxies subject to strong environmental effects., Comment: 12 pages, 7 figures; accepted to ApJ

We use 85 pairs of high resolution LCDM cosmological simulations from the NIHAO project to investigate why in dwarf galaxies neutral hydrogen (HI) linewidths measured at 50% of the peak flux W_50 /2 (from the hydrodynamical simulations) tend to underpredict the maximum circular velocity VmaxDMO (from the corresponding dark matter only simulations). There are two main contributing processes. 1) Lower mass galaxies are less rotationally supported. This is confirmed observationally from the skewness of linewidths in bins of HI mass in both ALFALFA and HIPASS observations. 2) The HI distributions are less extended (relative to the dark matter halo) in dwarf galaxies. Coupled to the lower baryon-to-halo ratio results in rotation curves that are still rising at the last measured data point, in agreement with observations from SPARC. Combining these two effects, in both simulations and observations lower mass galaxies have on average lower W_50 / W_20. Additionally, mass loss driven by supernovae and projection effects (dwarf galaxies are in general not thin disks) further reduce the linewidths. The implied HI linewidth velocity function from NIHAO is in good agreement with observations in the nearby Universe of dwarf galaxies: 10 < W_50 /2 < 80 km/s. The dark matter only slope of -2.9 is reduced to -1.0 in the hydro simulations. Future radio observations of unbiased samples with higher spatial resolution will enable stricter tests of the simulations, and thus of the LCDM model., Comment: 21 pages, 17 figures, accepted to MNRAS

The stellar populations in the inner kiloparsecs of the Milky Way (MW) show complex kinematical and chemical structures. The origin and evolution of these structures is still under debate. Here we study the central region of a fully cosmological hydrodynamical simulation of a disk galaxy that reproduces key properties of the inner kiloparsecs of the MW: it has a boxy morphology and shows an overall rotation and dispersion profile in agreement with observations. We use a clustering algorithm on stellar kinematics to identify a number of discrete kinematic components: a high- and low-spin disk, a stellar halo and two bulge components; one fast rotating and one slow-rotating. We focus on the two bulge components and show that the slow rotating one is spherically symmetric while the fast rotating component shows a boxy/peanut morphology. Although the two bulge components are kinematically discrete populations at present-day, they are both mostly formed over similar time scales, from disk material. We find that stellar particles with lower initial birth angular momentum (most likely thick disc stars) end up in the slow-rotating low-spin bulge, while stars with higher birth angular momentum (most likely thin disc stars) are found in the high-spin bulge. This has the important consequence that a bulge population with a spheroidal morphology does not necessarily indicate a merger origin. In fact, we do find that only $\sim2.3$\% of the stars in the bulge components are ex-situ stars brought in by accreted dwarf galaxies early on. We identify these ex-situ stars as the oldest and most metal-poor stars on highly radial orbits with large vertical excursions from the disk., Comment: accepted by ApJ, 14 pages (plus 3 pages appendix), 13 + 8 figures

The similarity between the distributions of spins for galaxies ($\lambda_{\rm g}$) and for dark-matter haloes ($\lambda_{\rm h}$), indicated both by simulations and observations, is naively interpreted as a one-to-one correlation between the spins of a galaxy and its host halo. This is used to predict galaxy sizes in semi-analytic models via $R_{\rm e}\simeq\lambda_{\rm h} R_{\rm v}$, with $R_{\rm e}$ the half-mass radius of the galaxy and $R_{\rm v}$ the halo radius. Utilizing two different suites of zoom-in cosmological simulations, we find that $\lambda_{\rm g}$ and $\lambda_{\rm h}$ are in fact only barely correlated, especially at $z\geq 1$. A general smearing of this correlation is expected based on the different spin histories, where the more recently accreted baryons through streams gain and then lose significant angular momentum compared to the gradually accumulated dark matter. Expecting the spins of baryons and dark matter to be correlated at accretion into $R_{\rm v}$, the null correlation at the end reflects an anti-correlation between $\lambda_{\rm g}/\lambda_{\rm h}$ and $\lambda_{\rm h}$, which can partly arise from mergers and a compact star-forming phase that many galaxies undergo. On the other hand, the halo and galaxy spin vectors tend to be aligned, with a median $\cos\theta=0.6$-0.7 between galaxy and halo, consistent with instreaming within a preferred plane. The galaxy spin is better correlated with the spin of the inner halo, but this largely reflects the effect of the baryons on the halo. Following the null spin correlation, $\lambda_{\rm h}$ is not a useful proxy for $R_{\rm e}$. While our simulations reproduce a general relation of the sort $R_{\rm e}=AR_{\rm vir}$, in agreement with observational estimates, the relation becomes tighter with $A=0.02(c/10)^{-0.7}$, where $c$ is the halo concentration, which in turn introduces a dependence on mass and redshift., Comment: 20 pages, 16 figures, submitted to MNRAS

We use 25 simulated galaxies from the NIHAO project to define and characterize a variety of kinematic stellar structures: thin and thick discs, large scale single discs, classical and pseudo bulges, spheroids, inner discs, and stellar haloes. These structures have masses, spins, shapes and rotational support in good agreement with theoretical expectations and observational data. Above a dark matter halo mass of $2.5\times10^{\rm~11}M_{\rm\odot}$, all galaxies have a classical bulge and 70\% have a thin and thick disc. The kinematic (thin) discs follow a power-law relation between angular momentum and stellar mass $J_{\rm *}=3.4M_{\rm *}^{\rm1.26\pm0.06}$, in very good agreement with the prediction based on the empirical stellar-to-halo mass relation in the same mass range, and show a strong correlation between maximum `observed' rotation velocity and dark matter halo circular velocity $v_{\rm c}=6.4v_{\rm max}^{0.64\pm0.04}$. Tracing back in time these structures' progenitors, we find all to lose a fraction $1-f_j$ of their maximum angular momentum. Thin discs are significantly better at retaining their high-redshift spins ($f_j\sim0.70$) than thick ones ($f_j\sim0.40$). Stellar haloes have their progenitor baryons assembled the latest ($z_{\rm~1/2}\sim1.1$) and over the longest timescales ($\tau\sim6.2$~Gyr), and have the smallest fraction of stars born in-situ ($f_{\rm in-situ}=0.35\pm0.14$). All other structures have $1.5\lesssim z_{\rm1/2}\lesssim3$, $\tau=4\pm2$~Gyr and $f_{\rm in-situ}\gtrsim0.9$., Comment: Accepted by MNRAS. First application of the methods described in arXiv:1804.05576

We present the first results of applying Gaussian Mixture Models in the stellar kinematic space of normalized angular momentum and binding energy on NIHAO high resolution galaxies to separate the stars into multiple components. We exemplify this method using a simulated Milky Way analogue, whose stellar component hosts: thin and thick discs, classical and pseudo bulges, and a stellar halo. The properties of these stellar structures are in good agreement with observational expectations in terms of sizes, shapes and rotational support. Interestingly, the two kinematic discs show surface mass density profiles more centrally concentrated than exponentials, while the bulges and the stellar halo are purely exponential. We trace back in time the Lagrangian mass of each component separately to study their formation history. Between z~3 and the end of halo virialization, z~1.3, all components lose a fraction of their angular momentum. The classical bulge loses the most (~95%) and the thin disc the least (~60%). Both bulges formed their stars in-situ at high redshift, while the thin disc formed ~98% in-situ, but with a constant SFR~1.5M$_{\rm\odot}$/yr$^{\rm-1}$ over the last ~11 Gyr. Accreted stars (6% of total stellar mass) are mainly incorporated to the thick disc or the stellar halo, which formed ex-situ 8% and 45% of their respective masses. Our analysis pipeline is freely available at https://github.com/aobr/gsf., Comment: MNRAS accepted. First results described in arXiv:1804.06635

We study the impact of the host on dwarf galaxy properties using four new Milky Way-like, ultra high-resolution simulations, ($N_{\rm part} >10^7$) from the NIHAO project. We split our sample in satellite ($R2.5 R_{\rm 200}$) galaxies. Simulated galaxies from all three groups are in excellent agreement with Local Group dwarf galaxies in terms of: stellar mass-velocity dispersion, stellar mass-metallicity relation, star formation histories, and stellar mass functions. Satellites and nearby galaxies show lower velocity dispersions and gas fractions compared to field galaxies. While field galaxies follow global abundance matching relations, satellites and nearby galaxies deviate from them, showing lower dark matter masses for given stellar mass. The reason for this deficit in dark matter mass is substantial mass loss experienced by satellites and $\sim80$\% of the nearby galaxies, while orbiting inside $R_{200}$ at earlier times. However, both satellites and nearby objects fall back onto the relation for field galaxies if we use the maximum of their virial mass instead of the present-day value. This allows us to provide estimates for the peak masses of observed Local Group galaxies. Finally, using radial velocities, distances, and the velocity dispersion-stellar mass relation from our simulations, we derive a metric to distinguish between galaxies harassed by the central object and unaffected ones. Applying this metric to observed objects we find that even far away dwarf galaxies like Eri II ($D\approx$ 370 kpc) have a strong probability ($\approx 83$\%) of having been affected by the Milky Way in the past. This naturally explains the lack of gas and recent star formation seen in Eri II., Comment: accepted by MNRAS, 31 pages, 17 pages main text, 14 pages appendix with tables of dwarf galaxy properties and convergence tests, 17 main figures

Galaxy formation entails the hierarchical assembly of mass, along with the condensation of baryons and the ensuing, self-regulating star formation. The stars form a collisionless system whose orbit distribution retains dynamical memory that can constrain a galaxy's formation history. The ordered-rotation dominated orbits with near maximum circularity $\lambda_z \simeq1$ and the random-motion dominated orbits with low circularity $\lambda_z \simeq0$ are called kinematically cold and kinematically hot, respectively. The fraction of stars on `cold' orbits, compared to the fraction of stars on `hot' orbits, speaks directly to the quiescence or violence of the galaxies' formation histories. Here we present such orbit distributions, derived from stellar kinematic maps via orbit-based modelling for a well defined, large sample of 300 nearby galaxies. The sample, drawn from the CALIFA survey, includes the main morphological galaxy types and spans the total stellar mass range from $10^{8.7}$ to $10^{11.9}$ solar masses. Our analysis derives the orbit-circularity distribution as a function of galaxy mass, $p(\lambda_z~|~M_\star)$, and its volume-averaged total distribution, $p(\lambda_z)$. We find that across most of the considered mass range and across morphological types, there are more stars on `warm' orbits defined as $0.25\le \lambda_z \le 0.8$ than on either `cold' or `hot' orbits. This orbit-based "Hubble diagram" provides a benchmark for galaxy formation simulations in a cosmological context., Comment: Published in Nature Astronomy, 1 January 2018; doi:10.1038/s41550-017-0348-1; 22pages, 8 figures, 2 tables

We show for the first time, that a fully cosmological hydrodynamical simulation can reproduce key properties of the innermost region of the Milky Way. Our high resolution simulation matches the profile and kinematics of the Milky Way's boxy/peanut-shaped bulge, and hence we can use it to reconstruct and understand the bulge assembly. In particular, the age dependence of the X-shape morphology of the simulated bulge parallels the observed metallicity dependent split in the red clump stars of the inner Galaxy. We use this feature to derive an observational metric that allows us to quantify when the bulge formed from the disk. The metric we propose can be employed with upcoming survey data to constrain the age of the Milky Way bar. From the split in stellar counts we estimate the formation of the 4~kpc scale bar in the simulation to have happened $t^{\rm bar}_{\rm form}\sim8^{+2}_{-2}$ Gyr ago, in good agreement with conventional methods to measure bar formation in simulations. We test the prospects for observationally differentiating the stars that belong to the bulge/bar compared to the surrounding disk, and find that the inner disk and bulge are practically indistinguishable in both chemistry and ages., Comment: 14+2 pages, 12+2 figures, accepted by ApJ

In an effort to search for Ly$\alpha$ emission from circum- and intergalactic gas on scales of hundreds of kpc around $z\sim3$ quasars, and thus characterise the physical properties of the gas in emission, we have initiated an extensive fast-survey with the Multi Unit Spectroscopic Explorer (MUSE): Quasar Snapshot Observations with MUse: Search for Extended Ultraviolet eMission (QSO MUSEUM). In this work, we report the discovery of an enormous Ly$\alpha$ nebula (ELAN) around the quasar SDSS~J102009.99+104002.7 at $z=3.164$, which we followed-up with deeper MUSE observations. This ELAN spans $\sim297$ projected kpc, has an average Ly$\alpha$ surface brightness ${\rm SB}_{\rm Ly\alpha}\sim 6.04\times10^{-18}$ erg s$^{-1}$ cm$^{-2}$ arcsec$^{-2}$ (within the $2\sigma$ isophote), and is associated with an additional four, previously unknown embedded sources: two Ly$\alpha$ emitters and two faint active galactic nuclei (one Type-1 and one Type-2 quasar). By mapping at high significance the line-of-sight velocity in the entirety of the observed structure, we unveiled a large-scale coherent rotation-like pattern spanning $\sim300$ km s$^{-1}$ with a velocity dispersion of $<270$ km s$^{-1}$, which we interpret as a signature of the inspiraling accretion of substructures within the quasar's host halo. Future multiwavelength data will complement our MUSE observations, and are definitely needed to fully characterise such a complex system. None the less, our observations reveal the potential of new sensitive integral-field spectrographs to characterise the dynamical state of diffuse gas on large scales in the young Universe, and thereby witness the assembly of galaxies., Comment: 39 pages with 27 figures and 5 appendices. Accepted to MNRAS

In the first paper we presented 27 hydrodynamical cosmological simulations of galaxies with total masses between $5 \times 10^8$ and $10^{10}\,\mathrm{M}_\odot$. In this second paper we use a subset of these cosmological simulations as initial conditions (ICs) for more than forty hydrodynamical simulations of satellite and host galaxy interaction. Our cosmological ICs seem to suggest that galaxies on these mass scales have very little rotational support and are velocity dispersion ($\sigma$) dominated. Accretion and environmental effects increase the scatter in the galaxy scaling relations (e.g. size - velocity dispersion) in very good agreement with observations. Star formation is substantially quenched after accretion. Mass removal due to tidal forces has several effects: it creates a very flat stellar velocity dispersion profiles, and it reduces the dark matter content at all scales (even in the centre), which in turn lowers the stellar velocity on scales around 0.5 kpc even when the galaxy does not lose stellar mass. Satellites that start with a cored dark matter profile are more prone to either be destroyed or to end up in a very dark matter poor galaxy. Finally, we found that tidal effects always increase the "cuspyness" of the dark matter profile, even for haloes that infall with a core., Comment: 13 pages, 15 figures, accepted to be published in MNRAS (with minor changes to the published version). This is the second paper of a series of two. The first paper can be found here arXiv:1707.01106

The satellites of the Milky Way and Andromeda represent the smallest galaxies we can observe in our Universe. In this series of papers we aim to shed light on their formation and evolution using cosmological hydrodynamical simulations. In this first paper we focus on the galaxy properties before accretion, by simulating twenty seven haloes with masses between $5\times 10^8$ and $10^{10} M_\odot$. Out of this set nineteen haloes successfully form stars, while eight remain dark. The simulated galaxies match quite well present day observed scaling relations between stellar mass, size and metallicity, showing that such relations are in place before accretion. Our galaxies show a large variety of star formation histories, from extended star formation periods to single bursts. As in more massive galaxies, large star formation bursts are connected with major mergers events, which greatly contribute to the overall stellar mass build up. The intrinsic stochasticity of mergers induces a large scatter in the stellar mass halo mass relation, up to two orders of magnitude. Despite the bursty star formation history, on these mass scales baryons are very ineffective in modifying the dark matter profiles, and galaxies with a stellar mass below $\approx 10^6 M_\odot$ retain their cuspy central dark matter distribution, very similar to results from pure N-body simulations., Comment: 12 pages, 15 figures. Two new figures added, discussion improved, several new references, conclusions unchanged. Accepted to be published in MNRAS. the second paper can found at arXiv:1707.01102

The significant diversity of rotation curve (RC) shapes in dwarf galaxies has recently emerged as a challenge to LCDM: in dark matter (DM) only simulations, DM halos have a universal cuspy density profile that results in self-similar RC shapes. We compare RC shapes of simulated galaxies from the NIHAO project with observed galaxies from the homogeneous SPARC dataset. The DM halos of the NIHAO galaxies can expand to form cores, with the degree of expansion depending on their stellar-to-halo mass ratio. By means of the $V_{\rm 2kpc}-V_{\rm Rlast}$ relation (where $V_{\rm Rlast}$ is the outermost measured rotation velocity), we show that both the average trend and the scatter in RC shapes of NIHAO galaxies are in reasonable agreement with SPARC: this represents a significant improvement compared to simulations that do not result in DM core formation, suggesting that halo expansion is a key process in matching the diversity of dwarf galaxy RCs. Note that NIHAO galaxies can reproduce even the extremely slowly rising RCs of IC 2574 and UGC 5750. Revealingly, the range where observed galaxies show the highest diversity corresponds to the range where core formation is most efficient in NIHAO simulations, 50$

We present here parameter distributions and fundamental scaling relations for 190 galaxies as part of the Spectroscopy and H-bang Imaging of Virgo cluster galaxies (SHIVir) survey. We find the distribution of galaxy velocities to be bimodal about $V_{\rm circ} \sim 125$ km ${\rm s^{-1}}$, hinting at the existence of dynamically unstable modes in the inner regions of galaxies. An analysis of the Tully-Fisher relation (TFR) of late-type galaxies (LTGs) and fundamental plane (FP) of early-type galaxies (ETGs) is also presented, yielding a compendium of galaxy scaling relations. The slope and zero-point of the Virgo TFR match those of field galaxies, while scatter differences likely reflect distinct evolutionary histories. The velocities minimizing scatter for the TFR and FP are measured at large apertures where the baryonic fraction becomes subdominant. While TFR residuals remain independent of any galaxy parameters, FP residuals (i.e. the FP "tilt") correlate strongly with the dynamical-to-stellar mass ratio, yielding stringent galaxy formation constraints. Furthermore, we construct a stellar-to-total mass relation (STMR) for ETGs and LTGs and find linear but distinct trends over the range $M_{*} = 10^{8-11} M_{\odot}$. Stellar-to-halo mass relations (SHMRs), which probe the extended dark matter halo, can be scaled down to masses estimated within the optical radius, showing a tight match with the Virgo STMR at low masses; however, possibly inadequate halo abundance matching prescriptions and broad radial scalings complicate this comparison at all masses. While ETGs appear to be more compact than LTGs of the same stellar mass in projected space, their mass-size relations in physical space are identical. The trends reported here call for validation through well-resolved numerical simulations., Comment: Accepted to ApJ. 28 pages, 16 figures

We perform Jeans anisotropic modeling (JAM) on elliptical and spiral galaxies from the MaNGA DR13 sample. By comparing the stellar mass-to-light ratios estimated from stellar population synthesis (SPS) and from JAM, we find a similar systematic variation of the initial mass function (IMF) as in the earlier $\rm ATLAS^{3D}$ results. Early type galaxies (elliptical and lenticular) with lower velocity dispersions within one effective radius are consistent with a Chabrier-like IMF while galaxies with higher velocity dispersions are consistent with a more bottom heavy IMF such as the Salpeter IMF. Spiral galaxies have similar systematic IMF variations, but with slightly different slopes and larger scatters, due to the uncertainties caused by higher gas fractions and extinctions for these galaxies. Furthermore, we examine the effects of stellar mass-to-light ratio gradients on our JAM modeling, and find that the trends from our results becomes stronger after considering the gradients., Comment: Accepted for publication in ApJ. 9 pages, 9 figures

The decrease in star formation (SF) and the morphological change necessary to produce the $z=0$ elliptical galaxy population are commonly ascribed to a sudden quenching event, which is able to rid the central galaxy of its cold gas reservoir in a short time. Following this event, the galaxy is able to prevent further SF and stay quiescent via a maintenance mode. We test whether such a quenching event is truly necessary using a simple model of quiescence. In this model, hot gas (all gas above a temperature threshold) in a $\sim10^{12} M_{\odot}$ halo mass galaxy at redshift $z\sim3$ is prevented from cooling. The cool gas continues to form stars at a decreasing rate and the galaxy stellar mass, morphology, velocity dispersion and position on the color magnitude diagram (CMD) proceed to evolve. By $z=0$, the halo mass has grown to $10^{13} M_{\odot}$ and the galaxy has attained characteristics typical of an observed $z=0$ elliptical galaxy. Our model is run in the framework of a cosmological, smooth particle hydrodynamic code which includes SF, early stellar feedback, supernova feedback, metal cooling and metal diffusion. Additionally, we post-process our simulations with a radiative transfer code to create a mock CMD. In contrast to previous assumptions that a pure "fade away" model evolves too slowly to account for the sparsity of galaxies in the "green valley", we demonstrate crossing times of $<$1 Gyr. We conclude that no sudden quenching event is necessary to produce such rapid colour transitions., Comment: 12 pages, 11 figures, accepted to be published in MNRAS

Many massive star forming disc galaxies in the redshift range 3 to 0.5 are observed to have a clumpy morphology showing giant clumps of size $\sim$1 kpc and masses of about $10^7M_{\odot}$ to $10^{10} M_{\odot}$. The nature and fate of these giant clumps is still under debate. In this work we use 19 high-resolution simulations of disc galaxies from the NIHAO sample to study the formation and the evolution of clumps in the discs of high redshift galaxies. We use mock HST - CANDELS observations created with the radiative transfer code GRASIL-3D to carry out, for the first time, a quantitative comparison of the observed fraction of clumpy galaxies and its evolution with redshift with simulations. We find a good agreement between the observed clumpy fraction and the one of the NIHAO galaxies. We find that dust attenuation can suppress intrinsically bright clumps and enhance less luminous ones. In our galaxy sample we only find clumps in light (u-band) from young stars but not in stellar mass surface density maps. This means that the NIHAO sample does not show clumpy stellar discs but rather a clumpy light distribution originating from clumpy star formation events. The clumps found in the NIHAO sample match observed age/color gradients as a function of distance from the galaxy center but they show no sign of inward migration. Clumps in our simulations disperse on timescales of a about a hundred Myr and their contribution to bulge growth is negligible., Comment: 17 pages, 18 figures, accepted by MNRAS

We use a sample of 83 high-resolution cosmological zoom-in simulations and a semi-analytic model (SAM) to study the stochasticity of galaxy formation in haloes ranging from dwarf to Milky Way masses. Our simulated galaxies reproduce the observed inefficiency of galaxy formation as expressed through the stellar, gas and baryonic Tully-Fisher relations. For HI velocities in the range (70 < V < 220 km/s), the scatter is just 0.08 to 0.14 dex, consistent with the observed intrinsic scatter at these scales. At low velocities (20 < V < 70 km/s), the simulated scatter is 0.2-0.25 dex, which could be tested with future observations. The scatter in the stellar mass versus dark halo velocity relation is constant for 30 < V < 180 km/s, and smaller (~0.17 dex) when using the maximum circular velocity of the dark matter only simulation, Vmax, compared to the virial velocity ($V_{200}$ or $V_{200}^{DMO}$). The scatter in stellar mass is correlated with halo concentration, and is minimized when using a circular velocity at a fixed fraction of the virial radius $\simeq 0.4 R_{200}$ or with $V_{\alpha}=V_{200}^{DMO} (V_{max}^{DMO} / V_{200}^{DMO})^\alpha$ with \alpha ~ 0.7, consistent with constraints from halo clustering. Using the SAM we show the correlation between halo formation time and concentration is essential in order to reproduce this result. This uniformity in galaxy formation efficiency we see in our hydrodynamical simulations and a semi-analytic model proves the simplicity and self-regulating nature of galaxy formation in a \Lambda Cold Dark Matter universe., Comment: 16 pages, 13 figures, accepted to MNRAS

We present useful functions for the profiles of dark-matter (DM) haloes with a free inner slope, from cusps to cores, where the profiles of density, mass-velocity and potential are simple analytic expressions. Analytic velocity is obtained by expressing the mean density as a simple functional form, and deriving the local density by differentiation. The function involves four shape parameters, with only two or three free: a concentration parameter $c$, inner and outer asymptotic slopes $\alpha$ and $\bar{\gamma}$, and a middle shape parameter $\beta$. Analytic expressions for the potential and velocity dispersion exist for $\bar{\gamma}=3$ and for $\beta$ a natural number. We match the models to the DM haloes in cosmological simulations, with and without baryons, ranging from steep cusps to flat cores. Excellent fits are obtained with three free parameters ($c$, $\alpha$, $\bar{\gamma}$) and $\beta=2$. For an analytic potential, similar fits are obtained for $\bar{\gamma}=3$ and $\beta=2$ with only two free parameters ($c$, $\alpha$); this is our favorite model. A linear combination of two such profiles, with an additional free concentration parameter, provides excellent fits also for $\beta=1$, where the expressions are simpler. The fit quality is comparable to non-analytic popular models. An analytic potential is useful for modeling the inner-halo evolution due to gas inflows and outflows, studying environmental effects on the outer halo, and generating halo potentials or initial conditions for simulations. The analytic velocity can quantify simulated and observed rotation curves without numerical integrations., Comment: Minor changes, accepted to MNRAS, 21 pages, 10 figures

We investigate the matter distribution of a spiral galaxy with a counter-rotating stellar core, SDSS J1331+3628 (J1331), independently with gravitational lensing and stellar dynamical modelling. By fitting a gravitational potential model to a quadruplet of lensing images around J1331's bulge, we tightly constrain the mass inside the Einstein radius R_ein = (0.91 +/- 0.02)'' (~= 1.83 +/- 0.04 kpc) to within 4%: M_ein = (7.8 +/- 0.3) x 10^10 M_Sun. We model observed long-slit major axis stellar kinematics in J1331's central regions by finding Multi-Gaussian Expansion (MGE) models for the stellar and dark matter distribution that solve the axisymmetric Jeans equations. The lens and dynamical model are independently derived, but in very good agreement with each other around ~R_ein. We find that J1331's center requires a steep total mass-to-light ratio gradient. A dynamical model including an NFW halo (with virial velocity v_200 ~= 240 +/- 40 km/s and concentration c_200 ~= 8 +/- 2) and moderate tangential velocity anisotropy (beta_z ~= -0.4 +/- 0.1) can reproduce the signatures of J1331's counter-rotating core and predict the stellar and gas rotation curve at larger radii. However, our models do not agree with the observed velocity dispersion at large radii. We speculate that the reason could be a non-trivial change in structure and kinematics due to a possible merger event in J1331's recent past., Comment: 20 pages, 11 figures, 7 tables, accepted for publication in MNRAS

We address the origin of Ultra-Diffuse Galaxies (UDGs), which have stellar masses typical of dwarf galaxies but effective radii of Milky Way-sized objects. Their formation mechanism, and whether they are failed $\rm L_{\star}$ galaxies or diffuse dwarfs, are challenging issues. Using zoom-in cosmological simulations from the NIHAO project, we show that UDG analogues form naturally in medium-mass haloes due to episodes of gas outflows associated with star formation. The simulated UDGs live in isolated haloes of masses $10^{10-11}\rm M_{\odot}$, have stellar masses of $10^{7-8.5}\rm M_{\odot}$, effective radii larger than 1 kpc and dark matter cores. They show a broad range of colors, an average S\'ersic index of 0.83, a typical distribution of halo spin and concentration, and a non-negligible HI gas mass of $10^{7-9}\rm M_{\odot}$, which correlates with the extent of the galaxy. Gas availability is crucial to the internal processes that form UDGs: feedback driven gas outflows, and subsequent dark matter and stellar expansion, are the key to reproduce faint, yet unusually extended, galaxies. This scenario implies that UDGs represent a dwarf population of low surface brightness galaxies and should exist in the field. The largest isolated UDGs should contain more HI gas than less extended dwarfs of similar $\rm M_{\star}$., Comment: matches accepted version, MNRAS Letter 2016-10-12

We used 87 high resolution hydrodynamical cosmological simulations from the NIHAO suite to investigate the relation between the maximum circular velocity (Vmax) of a dark matter halo in a collisionless simulation and the velocity width of the HI gas in the same halo in the hydrodynamical simulation. These two quantities are normally used to compare theoretical and observational velocity functions and have led to a possible discrepancy between observations and predictions based on the Cold Dark Matter (CDM) model. We show that below 100 km/s, there is clear bias between HI based velocities and Vmax, that leads to an underestimation of the actual circular velocity of the halo. When this bias is taken into account the CDM model has no trouble in reproducing the observed velocity function and no lack of low velocity galaxies is actually present. Our simulations also reproduce the linewidth - stellar mass (Tully-Fisher) relation and HI sizes, indicating that the HI gas in our simulations is as extended as observed. The physical reason for the lower than expected linewidths is that, in contrast to high mass galaxies, low mass galaxies no longer have extended thin HI rotating disks, as is commonly assumed., Comment: 7 pages, 4 figures, minor changes to match the version accepted on MNRAS Letters

We present a useful function for describing the profiles of dark-matter haloes with a varying asymptotic inner slope -\alpha, ranging from a cusp to a core, where the profiles of density, mass-velocity and potential are simple analytic expressions for any \alpha. The idea is to express the mean-density profile as a simple functional form, and obtain the local density by derivative. The model involves a concentration parameter c analogous to the NFW profile. More flexibility is provided by a sum of two such functions, with concentrations c_1 and c_2. An optional additional parameter is the asymptotic outer slope -\gamma, which allows more flexibility in the outskirts of haloes. For \gamma different than 3, there are analytic expressions for the profiles of density and mass-velocity but not for the potential. We match the proposed function, in different variants, to the dark-matter profiles of haloes in cosmological simulations, with and without baryons, spanning inner profiles that range from steep cusps to flat cores, and find excellent fits. The analytic potential profile is useful in modeling the evolution of the inner halo due to episodes of gas inflow and outflow. The analytic profile, especially with a free \gamma, is useful for a study of environmental effects in the outer halo. In general, these analytic profiles can serve for straightforwardly quantifying the shapes of simulated and observed rotation curves, without the need for numerical integrations., Comment: This paper has been withdrawn by the author because it became obsolete

We use ~100 cosmological galaxy formation zoom-in simulations using the smoothed particle hydrodynamics code {\sc gasoline} to study the effect of baryonic processes on the mass profiles of cold dark matter haloes. The haloes in our study range from dwarf (M_{200}~10^{10}Msun) to Milky Way (M_{200}~10^{12}Msun) masses. Our simulations exhibit a wide range of halo responses, primarily varying with mass, from expansion to contraction, with up to factor ~10 changes in the enclosed dark matter mass at one per cent of the virial radius. Confirming previous studies, the halo response is correlated with the integrated efficiency of star formation: e_SF=(M_{star}/M_{200})/(\Omega_b/\Omega_m). In addition we report a new correlation with the compactness of the stellar system: e_R=r_{1/2}/R_{200}. We provide an analytic formula depending on e_SF and e_R for the response of cold dark matter haloes to baryonic processes. An observationally testable prediction is that, at fixed mass, larger galaxies experience more halo expansion, while the smaller galaxies more halo contraction. This diversity of dark halo response is captured by a toy model consisting of cycles of adiabatic inflow (causing contraction) and impulsive gas outflow (causing expansion). For net outflow, or equal inflow and outflow fractions, f, the overall effect is expansion, with more expansion with larger f. For net inflow, contraction occurs for small f (large radii), while expansion occurs for large f (small radii), recovering the phenomenology seen in our simulations. These regularities in the galaxy formation process provide a step towards a fully predictive model for the structure of cold dark matter haloes., Comment: 20 pages, 15 figures, accepted to MNRAS

We study the hot and cold circum-galactic medium (CGM) of 86 galaxies of the cosmological, hydrodynamical simulation suite NIHAO. NIHAO allows a study of how the $z=0$ CGM varies across 5 orders of magnitude of stellar mass using OVI and HI as proxies for hot and cold gas. The cool HI covering fraction and column density profiles match observations well, particularly in the inner CGM. OVI shows increasing column densities with mass, a trend seemingly echoed in the observations. As in multiple previous simulations, the OVI column densities in simulations are lower than observed and optically thick HI does not extend as far out as in observations. We take a look at the collisional ionisation fraction of OVI as a function of halo mass. We make observable predictions of the bipolarity of outflows and their effect on the general shape of the CGM. Bipolar outflows can be seen out to around 40 kpc in intermediate and low mass halos ($M_{\mathrm{halo}}<10^{11}M_{\mathrm{sun}}$), but outside that radius, the CGM is too well mixed to detect an elongated shape. Larger halos have extended gas discs beyond the stellar disc that dominate the shape of the inner CGM. The simulated CGM is remarkably spherical even in low mass simulations. The chemical enrichment of both halo and disc gas follow expected increasing trends as a function of halo mass that are well fit with power laws. These relations can be used in non-hydrodynamic models, such as semi-analytic models., Comment: 23 pages, 19 figures, accepted for publication in MNRAS

We use the NIHAO galaxy formation simulations to make predictions for the baryonic budget in present day galaxies ranging from dwarf to Milky Way masses. The sample is made of 88 independent high resolution cosmological zoom-in simulations. NIHAO galaxies reproduce key properties of observed galaxies, such as the stellar mass vs halo mass and cold gas vs stellar mass relations. Thus they make plausible predictions for the baryon budget. We present the mass fractions of stars, cold gas ($T<10^4$K), cool gas ($10^4 < T < 10^5$K), warm-hot gas ($10^5 < T < 5\times10^6$K), and hot gas (T$> 5\times10^6$K) inside the virial radius, $R_{200}$. Compared to the predicted baryon mass, using the dark halo mass and the universal baryon fraction, $f_{\rm b}\equiv \Omega_{\rm b}/\Omega_{\rm m}=0.15$, we find that all of our haloes are missing baryons. The missing mass has been relocated past 2 virial radii, and cool gas dominates the corona at low mass (M$_{200} < 3 \times 10^{11} M_\odot$) while the warm-hot gas dominates at high mass (M$_{200} > 3 \times 10^{11} M_\odot$). Haloes of mass $\sim 10^{10} M_\odot$ are missing $\sim 90\%$ of their baryons. More massive haloes ($\sim 10^{12} M_\odot$) retain a higher fraction of their baryons, with $\sim 30\%$ missing, consistent with recent observational estimates. Moreover, these more massive haloes reproduce the observed fraction of cold, warm-hot and hot gas. The fraction of cool gas we predict ($0.11\pm0.06$) is significantly lower than the observation from COS-HALOs (0.3-0.47), but agrees with the alternative analysis of Stern et al. 2016., Comment: 11 pages, 8 figures, 1 table accepted for publication by MNRAS

We compare the half-light circular velocities, V_{1/2}, of dwarf galaxies in the Local Group to the predicted circular velocity curves of galaxies in the NIHAO suite of LCDM simulations. We use a subset of 34 simulations in which the central galaxy has a stellar luminosity in the range 0.5 x 10^5 < L_V < 2 x 10^8 L_{sun}. The NIHAO galaxy simulations reproduce the relation between stellar mass and halo mass from abundance matching, as well as the observed half-light size vs luminosity relation. The corresponding dissipationless simulations over-predict the V_{1/2}, recovering the problem known as too big to fail (TBTF). By contrast, the NIHAO simulations have expanded dark matter haloes, and provide an excellent match to the distribution of V_{1/2} for galaxies with L_V > 2 x 10^6 L_{sun}. For lower luminosities our simulations predict very little halo response, and tend to over predict the observed circular velocities. In the context of LCDM, this could signal the increased stochasticity of star formation in haloes below M_{halo} \sim 10^{10} M_{sun}, or the role of environmental effects. Thus, haloes that are "too big to fail", do not fail LCDM, but haloes that are "too small to pass" (the galaxy formation threshold) provide a future test of LCDM., Comment: 6 pages, 3 figures, accepted to MNRAS letters

The radial profiles of stars in disc galaxies are observed to be either purely exponential (Type-I), truncated (Type-II) or anti-truncated (Type-III) exponentials. Controlled formation simulations of isolated galaxies can reproduce all of these profile types by varying a single parameter, the initial halo spin. In this paper we examine these simulations in more detail in an effort to identify the physical mechanism that leads to the formation of Type-III profiles. The stars in the anti-truncated outskirts of such discs are now on eccentric orbits, but were born on near-circular orbits at much smaller radii. We show that, and explain how, they were driven to the outskirts via non-linear interactions with a strong and long-lived central bar, which greatly boosted their semi-major axis but also their eccentricity. While bars have been known to cause radial heating and outward migration to stellar orbits, we link this effect to the formation of Type-III profiles. This predicts that the anti-truncated parts of galaxies have unusual kinematics for disc-like stellar configurations: high radial velocity dispersions and slow net rotation. Whether such discs exist in nature, can be tested by future observations., Comment: 16 pages, 15 figures, accepted for publication in MNRAS major changes from submitted version, central conclusion slightly modified, initial conditions code pyICs publicly available on https://github.com/jakobherpich/pyICs