Search

Optical emission line diagnostics, which are a common tool to constrain the properties of the interstellar medium (ISM) of galaxies, become progressively inaccessible at higher redshifts for ground-based facilities. Far-infrared (FIR) emission lines, which are redshifted into atmospheric windows accessible by ground-based sub-millimeter facilities, could provide alternative ISM diagnostics to optical emission lines. We investigate FIR line ratios involving [CII]$\lambda 158 \mu$m, [OIII]$\lambda 88 \mu$m, [OIII]$\lambda 52 \mu$m, [NII]$\lambda 122 \mu$m and [NIII$\lambda 57 \mu$m, using synthetic emission lines applied to a high-resolution (m$_{\rm gas}$= 883.4 M$_{\odot}$) cosmological zoom-in simulation, including radiative-transfer post processing with KramsesRT at z = 6.5. We find that the [CII]/[NII]122 ratio is sensitive to the temperature and density of photo-dissociation regions, and thus could be a useful tool to trace the properties of this gas phase in galaxies. We also find that [NII]/[NIII] is a good tracer of the temperature and [OIII]52/[OIII]88 a good tracer of the gas density of HII regions. Emission line ratios containing the [OIII]$\lambda 88 \mu$m line are sensitive to high velocity outflowing gas., Comment: Accepted for publication to A&A. 7 pages, 5 figures

Our knowledge of galaxy formation and evolution has incredibly progressed through multi-wavelength observational constraints of the interstellar medium (ISM) of galaxies at all cosmic epochs. However, little is known about the physical properties of the more diffuse and lower surface brightness reservoir of gas and dust that extends beyond ISM scales and fills dark matter haloes of galaxies up to their virial radii, the circumgalactic medium (CGM). New theoretical studies increasingly stress the relevance of the latter for understanding the feedback and feeding mechanisms that shape galaxies across cosmic times, whose cumulative effects leave clear imprints into the CGM. Recent studies are showing that a -- so far unconstrained -- fraction of the CGM mass may reside in the cold (T < 1e4 K) molecular and atomic phase, especially in high-redshift dense environments. These gas phases, together with the warmer ionised phase, can be studied in galaxies from z ~ 0 to z ~ 10 through bright far-infrared and sub-millimeter emission lines such as [C II] 158${\mu}$m, [O III] 88 ${\mu}$m, [C I] 609${\mu}$m, [C I] 370${\mu}$m, and the rotational transitions of CO. Imaging such hidden cold CGM can lead to a breakthrough in galaxy evolution studies but requires a new facility with the specifications of the proposed Atacama Large Aperture Submillimeter Telescope (AtLAST). In this paper, we use theoretical and empirical arguments to motivate future ambitious CGM observations with AtLAST and describe the technical requirements needed for the telescope and its instrumentation to perform such science., Comment: Submitted to Open Research Europe as part of the AtLAST collection: https://open-research-europe.ec.europa.eu/collections/atlast/about

Radiation from stars and AGN plays an important role in galaxy formation and evolution, and profoundly transforms the IGM, CGM & ISM. On-the-fly RT has started being incorporated in cosmological simulations, but the complex, evolving radiation spectra are often crudely approximated with a small number of broad bands with piece-wise constant intensity and a fixed photo-ionisation cross-section. Such a treatment is unable to capture the changes to the spectrum as light is absorbed while it propagates through a medium with non-zero opacity. This can lead to large errors in photo-ionisation and heating rates. We present a novel approach of discretising the radiation field in narrow bands, located at the edges of the typically used bands, in order to capture the power-law slope of the radiation field. In combination with power-law approximations for the photo-ionisation cross-sections, this model allows us to self-consistently combine radiation from sources with different spectra and accurately follow the ionisation states of primordial and metal species through time. The method is implemented in Gasoline2 in connection with Trevr2. We compare our new piece-wise power-law reconstruction to the piece-wise constant method in calculating the primordial chemistry photo-ionisation and heating rates under an evolving UVB and stellar spectrum, and find that our method reduces errors significantly, up to two orders of magnitude in the case of HeII ionisation. We apply our new spectral reconstruction method in RT post-processing of a cosmological zoom-in simulation, including radiation from stars and a live UVB, and find a significant increase in total neutral hydrogen mass in the ISM and the CGM due to shielding of the UVB and a low escape fraction of the stellar radiation. This demonstrates the importance of RT and an accurate spectral approximation in simulating the CGM-galaxy ecosystem., Comment: Submitted for publication to A&A, 17 pages, 15 figures

We present TREVR2 (Tree-based REVerse Ray Tracing 2), a fast, general algorithm for computing the radiation field, suitable for both particle and mesh codes. It is designed to self-consistently evolve chemistry for zoomed-in astrophysical simulations, such as cosmological galaxies with both internal sources and prescribed background radiation, rather than large periodic volumes. Light is propagated until absorbed, with no imposed speed limit other than those due to opacity changes (e.g. ionization fronts). TREVR2 searches outward from receiving gas in discrete directions set by the HEALPIX algorithm (unlike its slower predecessor TREVR), accumulating optical depth and adding the flux due to sources combined into progressively larger tree cells with distance. We demonstrate $N_\textrm{active}\log_2 N$ execution time with absorption and many sources. This allows multi-band RT costs comparable to tree-based gravity and hydrodynamics, and the usual speed-up when active particles evolve on individual timesteps. Sources embedded in non-homogeneous absorbing material introduce systematic errors. We introduce transmission averaging instead of absorption averaging which dramatically reduces these systematic effects. We outline other ways to address systematics including an explicit complex source model. We demonstrate the overall performance of the method via a set of astrophysical test problems., Comment: Accepted to MNRAS. 15 pages, 13 figures

The presence of cold ($T \lesssim 10^4$ K) gas in the circumgalactic medium (CGM) of galaxies has been confirmed both in observations and high-resolution simulations, but its origin still represents a puzzle. Possible mechanisms are cold accretion from the intergalactic medium (IGM), clumps embedded in outflows and transported from the disk, gas detaching from the hot CGM phase via thermal instabilities. In this work, we aim at characterizing the history of cold CGM gas, in order to identify the dominant origin channels at different evolutionary stages of the main galaxy. To this goal, we track gas particles in different snapshots of the SPH cosmological zoom-in simulation Eris2k. We perform a backward tracking of cold gas, starting from different redshifts, until we identify one of the followings origins for the particle: cold inflow, ejected from the disk, cooling down in-situ or stripped from a satellite. We also perform a forward tracking of gas in different components of the galaxy (such as the disk and outflows). We find a clear transition between two epochs. For $z>2$, most cold gas (up to 80%) in the CGM comes from cold accretion streams as the galaxy is accreting in the "cold mode" from the IGM. At lower $z$, gas either cools down in-situ after several recycles (with 10-20% of the gas cooling in outflow), or it is ejected directly from the disk (up to 30%). Outflows have a major contribution to the cold CGM gas budget at $z<1$, with almost 50% of hot gas cooling in outflow. Finally, we discuss possible mechanisms for CGM cooling, showing that the thermally unstable gas with $t_{\rm cool}/t_{\rm ff}<1$ (precipitation-regulated feedback) is abundant up to $r\sim 100$ kpc and cooling times are shorter than 50 Myr for densities $n>10^{-2}\,{\rm cm}^{-3}$., Comment: 13 pages, 15 figures. Accepted for publication to A&A

The circumgalactic medium (CGM) is a crucial component of galaxy evolution, but thus far its physical properties are highly unconstrained. As of yet, no cosmological simulation has reached convergence when it comes to constraining the cold and dense gas fraction of the CGM. Such components are also challenging to observe, and require sub-millimeter instruments with a high sensitivity to extended, diffuse emission, like the proposed Atacama Large Aperture Sub-millimetre telescope (AtLAST). We present a state-of-the-art theoretical effort at modeling the [CII], [CI](1-0), [CI](2-1), CO(3-2), and [OIII] line emissions of galaxies. We use the high-resolution cosmological zoom-in simulation Ponos, representing a star forming galaxy system at z = 6.5 ($M_*=2\times10^9~M_{\odot}$), undergoing a major merger. We adopt different modeling approaches based on the photoionisation code Cloudy. Our fiducial model uses radiative transfer post-processing with RamsesRT and Krome to create realistic FUV radiation fields, which we compare to sub-grid modeling approaches adopted in the literature. We find significant differences in the luminosity and in the contribution of different gas phases and galaxy components between the different modeling approaches. [CII] is the least model-dependant gas tracer, while [CI](1-0) and CO(3-2) are very model-sensitive. In all models, we find a significant contribution to the emission of [CII] (up to $\sim$10%) and [OIII] (up to $\sim$20%) from the CGM. [CII] and [OIII] trace different regions of the CGM: [CII] arises from an accreting filament and from tidal tails, while [OIII] traces a puffy halo surrounding the main disc, probably linked to SN feedback. We discuss our results in the context of current and future sub-mm observations with ALMA and AtLAST., Comment: Accepted for publication to A&A. 25 pages, 19 figures. Significant changes since version 1. Abstract summarised for arXiv submission

Understanding the numerical dependencies that act on the galactic dynamo is a crucial step in determining what resolution and what conditions are required to properly capture the magnetic fields observed in galaxies. Here, we present an extensive study on the numerical dependencies of the galactic dynamo in isolated spiral galaxies using smoothed particle magnetohydrodynamics (SPMHD). We performed 53 isolated spiral galaxy simulations with different initial setups, feedback, resolution, Jeans floor and dissipation parameters. The results show a strong mean-field dynamo occurring in the spiral-arm region of the disk, likely produced by the classical alpha-omega dynamo or the recently described gravitational instability dynamo. The inclusion of feedback is seen to work in both a destructive and positive fashion for the amplification process. Destructive interference for the amplification occurs due to break down of filament structure in the disk, increase of turbulent diffusion and the ejection of magnetic flux from the central plane to the circumgalactic medium. The positive effect of feedback is the increase in vertical motions and the turbulent fountain flows that develop, showing a high dependence on the small-scale vertical structure and the numerical dissipation within the galaxy. Galaxies with an effective dynamo saturate their magnetic energy density at levels between 10-30% of the thermal energy density. The density averaged numerical Prandtl number is found to be below unity throughout the galaxy for all our simulations, with an increasing value with radius. Assuming a turbulent injection length of 1 kpc, the numerical magnetic Reynolds number are within the range of $Re_{mag}=10-400$, indicating that some regions are below the levels required for the small-scale dynamo ($Re_{mag,crit}=30-2700$) to be active.

We present results from \textsc{GigaEris}, a cosmological, $N$-body hydrodynamical "zoom-in" simulation of the formation of a Milky Way-sized galaxy halo with unprecedented resolution, encompassing of order a billion particles within the refined region. The simulation employs a modern implementation of smoothed-particle hydrodynamics, including metal-line cooling and metal and thermal diffusion. We focus on the early assembly of the galaxy, down to redshift $z=4.4$. The simulated galaxy has properties consistent with extrapolations of the main sequence of star-forming galaxies to higher redshifts and levels off to a star formation rate of $\sim$60$\, M_{\odot}$~yr$^{-1}$ at $z=4.4$. A compact, thin rotating stellar disk with properties analogous to those of low-redshift systems arises already at $z \sim 8$. The galaxy rapidly develops a multi-component structure, and the disk, at least at these early stages, does not grow "upside-down" as often reported in the literature. Rather, at any given time, newly born stars contribute to sustain a thin disk. The kinematics reflect the early, ubiquitous presence of a thin disk, as a stellar disk component with $v_\phi/\sigma_R$ larger than unity is already present at $z \sim 9$--10. Our results suggest that high-resolution spectro-photometric observations of very high-redshift galaxies should find thin rotating disks, consistent with the recent discovery of cold rotating gas disks by ALMA. Finally, we present synthetic images for the JWST NIRCam camera, showing how the early disk would be easily detectable already at those early times., Comment: 19 pages, 14 figures, 1 table, accepted to ApJ

We present results from a high-resolution, cosmological, $\Lambda$CDM simulation of a group of field dwarf galaxies with the "superbubble" model for clustered SN feedback, accounting for thermal conduction and cold gas evaporation. The initial conditions and the galaxy formation physics, other than SN feedback, are the same as in Shen et al. (2014). The simulated luminous galaxies have blue colors, low star formation efficiencies and metallicities, and high cold gas content, reproducing the observed scaling relations of dwarfs in the Local Volume. Bursty star formation histories and superbubble-driven outflows lead to the formation of kpc-size DM cores when stellar masses reaches $M_{*} > 10^6$ $M_{\odot}$, similar to previous findings. However, the superbubble model appears more effective in destroying DM cusps than the previously adopted "blastwave" model, reflecting a higher coupling efficiency of SN energy with the ISM. On larger scale, superbubble-driven outflows have a more moderate impact: galaxies have higher gas content, more extended stellar disks, and a smaller metal-enriched region in the CGM. The two halos with $M_{vir} \sim 10^9$ $M_{\odot}$, which formed ultra-faint dwarf galaxies in Shen et al. (2014), remain dark due to the different impact of metal-enriched galactic winds from two nearby luminous galaxies. The column density distributions of H I, Si II, C IV and O VI are in agreement with recent observations of CGM around isolated dwarfs. While H I is ubiquitous with a covering fraction of unity within the CGM, Si II and C IV are less extended. O VI is more extended, but its mass is only 11% of the total CGM oxygen budget, as the diffuse CGM is highly ionised by the UVB. Superbubble feedback produces C IV and O VI an order of magnitude higher column densities than those with blastwave feedback. The CGM and DM cores are most sensitive probes of feedback mechanisms., Comment: 16 pages, 15 figures

We present results from the "Mint" resolution DC Justice League suite of Milky Way-like zoom-in cosmological simulations, which extend our study of nearby galaxies down into the ultra-faint dwarf (UFD) regime for the first time. The mass resolution of these simulations is the highest ever published for cosmological Milky Way zoom-in simulations run to $z=0$, with initial star (dark matter) particle masses of 994 (17900) M$_\odot$, and a force resolution of 87 pc. We study the surrounding dwarfs and UFDs, and find the simulations match the observed dynamical properties of galaxies with $-3 < M_V < -19$, and reproduce the scatter seen in the size-luminosity plane for $r_h\gtrsim200$ pc. We predict the vast majority of nearby galaxies will be observable by the Vera Rubin Observatory's co-added Legacy Survey of Space and Time (LSST). We additionally show that faint dwarfs with velocity dispersions $\lesssim5$ km/s result from severe tidal stripping of the host halo. We investigate the quenching of UFDs in a hydrodynamical Milky Way context, and find that the majority of UFDs are quenched prior to interactions with the Milky Way, though some of the quenched UFDs retain their gas until infall. Additionally these simulations yield some unique dwarfs that are the first of their kind to be simulated, e.g., an HI-rich field UFD, a late-forming UFD that has structural properties similar to Crater 2, as well as a compact dwarf satellite that has no dark matter at $z=0$., Comment: 33 pages, 17 figures, ApJ accepted version

We present a novel method of magnetohydrodynamics (MHD) within the smoothed particle hydrodynamics scheme using the Geometric Density average force expression (GDSPH). GDSPH has recently been shown to reduce the leading order errors and greatly improve the accuracy near density discontinuities, eliminating surface tension effects. Here, we extend the study to investigate how SPMHD benefits from this method. We implement ideal MHD in the Gasoline2 and Changa codes with both GDSPH and traditional SPH (TSPH) schemes. An constrained hyperbolic divergence cleaning scheme is employed to control the divergence error, and a switch for artificial resistivity with minimized dissipation is used. We test the codes with a large suite of MHD tests, and show that in all problems the results are comparable or improved over previous SPMHD implementations. While both GDSPH and TSPH perform well with relatively smooth or highly supersonic flows, GDSPH shows significant improvements in the presence of strong discontinuities and large dynamic scales. In particular, when applied to an astrophysical problem of the collapse of a magnetized cloud, GDSPH realistically captures the development of a magnetic tower and jet launching in the weak-field regime, and exhibit fast convergence with resolution, while TSPH failed to do so. Our new method shows qualitatively similar results to the ones from the meshless finite mass/volume (MFM/MFV) schemes within the Gizmo code, while remaining computationally less expensive.

The cycling of baryons in and out of galaxies is what ultimately drives galaxy formation and evolution. The circumgalactic medium (CGM) represents the interface between the interstellar medium and the cosmic web, hence its properties are directly shaped by the baryon cycle. Although traditionally the CGM is thought to consist of warm and hot gas, recent breakthroughs are presenting a new scenario according to which an important fraction of its mass may reside in the cold atomic and molecular phase. This would represent fuel that is readily available for star formation, with crucial implications for feeding and feedback processes in galaxies. However, such cold CGM, especially in local galaxies where its projected size on sky is expected to be of several arcminutes, cannot be imaged by ALMA due to interferometric spatial scale filtering of large-scale structures. We show that the only way to probe the multiphase CGM including its coldest component is through a large (e.g. 50-m) single dish (sub-)mm telescope., Comment: Science white paper submitted to the Astro2020 Decadal Survey

Bars are a key factor in the long-term evolution of spiral galaxies, in their unique role in redistributing angular momentum and transporting gas and stars on large scales. The Eris-suite simulations are cosmological zoom-in, N-body, smoothed-particle hydrodynamic simulations built to follow the formation and evolution of a Milky Way-sized galaxy across the build-up of the large scale structure. Here we analyse and describe the outcome of two particular simulations taken from the Eris suite - ErisBH and Eris2k - which mainly differ in the prescriptions employed for gas cooling, star formation, and feedback from supernovae and black holes. Our study shows that the enhanced effective feedback in Eris2k, due to the collective effect of the different micro-physics implementations, results in a galaxy which is less massive than its ErisBH counterpart till z~1. However, when the stellar content is large enough so that global dynamical instabilities can be triggered, the galaxy in Eris2k develops a stronger and more extended bar with respect to ErisBH. We demonstrate that he structural properties and time evolution of the two bars are very different. Our results highlight the importance of accurate sub-grid prescriptions in cosmological zoom-in simulations of the process of galaxy formation and evolution, and the possible use of a statistical sample of barred galaxies to assess the strength of the stellar feedback., Comment: 16 pages, 14 Figure; accepted for publication in MNRAS; added references and figures, more details, results unchanged

We analyze the metal accumulation in dwarf and spiral galaxies by following the history of metal enrichment and outflows in a suite of twenty high-resolution simulated galaxies. These simulations agree with the observed stellar and gas-phase mass-metallicity relation, an agreement that relies on large fractions of the produced metals escaping into the CGM. For instance, in galaxies with Mvir ~ 1e9.5 -- 1e10 solar masses, we find that about ~ 85% of the available metals are outside of the galactic disk at z = 0, although the fraction decreases to a little less than half in Milky Way-mass galaxies. In many cases, these metals are spread far beyond the virial radius. We analyze the metal deficit within the ISM and stars in the context of previous work tracking the inflow and outflow of baryons. Outflows are prevalent across the entire mass range, as is reaccretion. We find that between 40 and 80% of all metals removed from the galactic disk are later reaccreted. The outflows themselves are metal enriched relative to the ISM by a factor of 0.2 dex because of the correspondence between sites of metal enrichment and outflows. As a result, the metal mass loading factor scales as eta_metals \propto v_circ^-0.91, a somewhat shallower scaling than the total mass loading factor. We analyze the simulated galaxies within the context of analytic chemical evolution models by determining their net metal expulsion efficiencies, which encapsulate the rates of metal loss and reaccretion. We discuss these results in light of the inflow and outflow properties necessary for reproducing the mass-metallicity relation., Comment: Under review by ApJ. 21 pages, 15 figures

The dominant feedback mechanism in low mass haloes is usually assumed to take the form of massive stars exploding as supernovae (SNe). We perform very high resolution cosmological zoom-in simulations of five dwarf galaxies to z = 4 with our mechanical SN feedback model. This delivers the correct amount of momentum corresponding to the stage of the SN remnant evolution resolved, and has been shown to lead to realistic dwarf properties in isolated simulations. We find that in 4 out of our 5 simulated cosmological dwarfs, SN feedback has insufficient impact resulting in excessive stellar masses, extremely compact sizes and central super-solar stellar metallicities. The failure of the SN feedback in our dwarfs is physical in nature within our model and is the result of the build up of very dense gas in the early universe due to mergers and cosmic inflows prior to the first SN occurring. We demonstrate that our results are insensitive to resolution (provided that it is high enough), details of the (spatially uniform) UV background and reasonable alterations within our star formation prescription. We therefore conclude that the ability of SNe to regulate dwarf galaxy properties is dependent on other physical processes, such as turbulent pressure support, clustering and runaway of SN progenitors and other sources of stellar feedback., Comment: 18 pages, 13 figures, accepted MNRAS

It has been proposed that close interactions with satellite galaxies can significantly perturb the morphology of the main galaxy. However, the dynamics of an already formed bar following the interaction with the external environment has not been studied in detail in a fully cosmological context. In this work, analysing the cosmological zoom-in simulation Eris2k, we study the effects that a very unequal-mass flyby crossing the stellar disc has on the stability of the pre-existing bar. We characterize the evolution of the bar strength and length showing that the perturbation exerted by the flyby shuffles the orbits of stars for less than one Gyr. After this time, the bar shows a remarkable resilience, reforming with properties comparable to those it had before the interaction. Our work shows that close unequal-mass encounters, the most frequent interactions occurring during the evolution of cosmic structures, have (i) an overall minor impact on the global evolution of the bar in the long term, still (ii) the effect is destructive and (iii) a very weak interaction is sufficient to dismantle a strong bar leading to its "apparent death". As a consequence, due to the non-negligible duration of the bar-less period, a fraction of observed spiral galaxies classified as non-barred could be prone to bar formation., Comment: 6 pages, 3 Figure; Accepted for publication in MNRAS

We present the Feedback Acting on Baryons in Large-scale Environments (FABLE) suite of cosmological hydrodynamical simulations of galaxies, groups and clusters. The simulations use the AREPO moving-mesh code with a set of physical models for galaxy formation based on the successful Illustris simulation, but with updated AGN and supernovae feedback models. This allows us to simultaneously reproduce the observed redshift evolution of the galaxy stellar mass function together with the stellar and gas mass fractions of local groups and clusters across a wide range of halo masses. Focusing on the properties of groups and clusters, we find very good agreement with a range of observed scaling relations, including the X-ray luminosity--total mass and gas mass relations as well as the total mass--temperature and Sunyaev-Zel'dovich flux--mass relations. Careful comparison of our results with scaling relations based on X-ray hydrostatic masses as opposed to weak lensing-derived masses reveals some discrepancies, which hint towards a non-negligible X-ray mass bias in observed samples. We further show that radial profiles of density, pressure and temperature of the simulated intracluster medium are in very good agreement with observations, in particular for $r > 0.3\ r_{500}$. In the innermost regions however we find too large entropy cores, which indicates that a more sophisticated modelling of the physics of AGN feedback may be required to accurately reproduce the observed populations of cool-core and non-cool-core clusters., Comment: 31 pages, 20 figures, 2 appendices; MNRAS accepted

We investigate the contribution of outer HI disks to the observable population of merging black hole binaries. Like dwarf galaxies, the outer HI disks of spirals have low star formation rates and lower metallicities than the inner disks of spirals. Since low-metallicity star formation can produce more detectable compact binaries than typical star formation, the environments in the outskirts of spiral galaxies may be conducive to producing a rich population of massive binary black holes. We consider here both detailed controlled simulations of spirals and cosmological simulations, as well as the current range of observed values for metallicity and star formation in outer disks. We find that outer HI disks contribute at least as much as dwarf galaxies do to the observed LIGO/Virgo detection rates. Identifying the host galaxies of merging massive black holes should provide constraints on cosmological parameters and insights into the formation channels of binary mergers., Comment: accepted to ApJL, 5 pages, 2 figures

While feedback from massive stars exploding as supernovae (SNe) is thought to be one of the key ingredients regulating galaxy formation, theoretically it is still unclear how the available energy couples to the interstellar medium and how galactic scale outflows are launched. We present a novel implementation of six sub-grid SN feedback schemes in the moving-mesh code Arepo, including injections of thermal and/or kinetic energy, two parametrizations of delayed cooling feedback and a `mechanical' feedback scheme that injects the correct amount of momentum depending on the relevant scale of the SN remnant resolved. All schemes make use of individually time-resolved SN events. Adopting isolated disk galaxy setups at different resolutions, with the highest resolution runs reasonably resolving the Sedov-Taylor phase of the SN, we aim to find a physically motivated scheme with as few tunable parameters as possible. As expected, simple injections of energy overcool at all but the highest resolution. Our delayed cooling schemes result in overstrong feedback, destroying the disk. The mechanical feedback scheme is efficient at suppressing star formation, agrees well with the Kennicutt-Schmidt relation and leads to converged star formation rates and galaxy morphologies with increasing resolution without fine tuning any parameters. However, we find it difficult to produce outflows with high enough mass loading factors at all but the highest resolution, indicating either that we have oversimplified the evolution of unresolved SN remnants, require other stellar feedback processes to be included, require a better star formation prescription or most likely some combination of these issues., Comment: 32 pages, 22 figures. Accepted for publication in MNRAS

We use high-resolution cosmological hydrodynamical simulations of Milky Way-sized galaxies with varying supernovae feedback strengths and merger histories to investigate the formation of their gaseous halos and especially their hot ($>10^6$~K) X-ray luminous coronae. Our simulations predict the presence of significant hot gas in the halos as early as $z=3-4$, well before the halos ought to be able to sustain hot mode accretion in the conventional picture. The nascent coronae grow inside-out and initially do so primarily as a result of outflows from the central galaxies powered by merger-induced shock heating and strong supernovae feedback, both of which are elemental features of today's successful galaxy formation models. Furthermore, the outflows and the forming coronae also accelerate the transition from cold to hot mode accretion by contributing to the conditions for sustaining stable accretion shocks. They also disrupt the filamentary streams funneling cold gas onto the central galaxies by causing their mouths to fray into a broad delta, detach from the galaxies, and be pushed away to larger radii. And even though at early times the filaments repeatedly re-form, the hot gas and the outflows act to weaken the filaments and accelerate their ultimate disruption. Although galactic outflows are generally thought of as ejective feedback, we find that their action on the filaments suggests a preventive role as well., Comment: accepted for a publication at ApJ

We investigate the angular momentum evolution of four disk galaxies residing in Milky Way-sized halos formed in cosmological zoom-in simulations with various sub-grid physics and merging histories. We decompose these galaxies kinematically and photometrically, into their disk and bulge components. The simulated galaxies and their components lie on the observed sequences in the $j_*$--$M_*$ diagram relating the specific angular momentum and mass of the stellar component. We find that galaxies in low-density environments follow the relation $j_* \propto M_*^{\alpha}$ past major mergers, with $\alpha \sim 0.6$ in the case of strong feedback, when bulge-to-disk ratios are relatively constant, and $\alpha \sim 1.4$ in the other cases, when secular processes operate on shorter timescales. We compute the retention factors (i.e. the ratio of the specific angular momenta of stars and dark matter) for both disks and bulges and show that they vary relatively slowly after averaging over numerous but brief fluctuations. For disks, the retention factors are usually close to unity, while for bulges, they are a few times smaller. Our simulations therefore indicate that galaxies and their halos grow in a quasi-homologous way., Comment: accepted by ApJ

Using the Eris zoom-in cosmological simulation of assembly of a Milky Way analog, we study chemical enrichment of stars due to accretion of metal-enriched gas from the interstellar medium during the Galaxy's development. We consider metal-poor and old stars in both Galactic halo and bulge and make use of stellar orbits, gas density and metallicity distributions in Eris. Assuming spherically symmetric Bondi-Hoyle accretion, we find that halo and bulge stars accrete metals at the rate of about 10^-24 solar mass per year and 10^-22 solar mass per year, respectively, at redshifts z < 3, but this accretion rate increases hundred-fold to about 10^-20 solar mass per year at higher redshifts due to increased gas density. Bulge and halo stars accrete similar amounts of metals at high redshifts as kinematically distinct bulge and halo are not yet developed at these redshifts and both sets of stars encounter similar metal distribution in the ISM on average. Accretion alone can enrich main-sequence stars up to [Fe/H] -2 in extreme cases.Median enrichment level due to accretion in these stars is about [Fe/H]~-6 to -5.Because accretion mostly takes place at high redshifts, it is alpha-enriched to [alpha/Fe]~0.5. We find that accretive metal enrichment is significant enough to affect the predicted metallicity distribution function of halo stars at [Fe/H] < -5.This suggests that attempts to infer the natal chemical environment of the most metal-poor stars from their observed enrichment today can be hindered due to metal accretion. Peculiar enrichment patterns such as those predicted to arise from pair-instability supernovae could help in disentangling natal and accreted metal content of stars., Comment: 11 pages, 6 figures, accepted for publication on MNRAS

We investigate the simultaneous triggering of active galactic nuclei (AGN) in merging galaxies, using a large suite of high-resolution hydrodynamical simulations. We compute dual-AGN observability time-scales using bolometric, X-ray, and Eddington-ratio thresholds, confirming that dual activity from supermassive black holes (BHs) is generally higher at late pericentric passages, before a merger remnant has formed, especially at high luminosities. For typical minor and major mergers, dual activity lasts ~20-70 and ~100-160 Myr, respectively. We also explore the effects of X-ray obscuration from gas, finding that the dual-AGN time decreases at most by a factor of ~2, and of contamination from star formation. Using projected separations and velocity differences rather than three-dimensional quantities can decrease the dual-AGN time-scales by up to ~4, and we apply filters which mimic current observational-resolution limitations. In agreement with observations, we find that, for a sample of major and minor mergers hosting at least one AGN, the fraction harbouring dual AGN is ~20-30 and ~1-10 per cent, respectively. We quantify the effects of merger mass ratio (0.1 to 1), geometry (coplanar, prograde, retrograde, and inclined), disc gas fraction, and BH properties, finding that the mass ratio is the most important factor, with the difference between minor and major mergers varying between factors of a few to orders of magnitude, depending on the luminosity and filter used. We also find that a shallow imaging survey will require very high angular resolution, whereas a deep imaging survey will be less resolution-dependent., Comment: Accepted for publication in MNRAS; added online-only material

Using an isolated Milky Way-mass galaxy simulation, we compare results from 9 state-of-the-art gravito-hydrodynamics codes widely used in the numerical community. We utilize the infrastructure we have built for the AGORA High-resolution Galaxy Simulations Comparison Project. This includes the common disk initial conditions, common physics models (e.g., radiative cooling and UV background by the standardized package Grackle) and common analysis toolkit yt, all of which are publicly available. Subgrid physics models such as Jeans pressure floor, star formation, supernova feedback energy, and metal production are carefully constrained across code platforms. With numerical accuracy that resolves the disk scale height, we find that the codes overall agree well with one another in many dimensions including: gas and stellar surface densities, rotation curves, velocity dispersions, density and temperature distribution functions, disk vertical heights, stellar clumps, star formation rates, and Kennicutt-Schmidt relations. Quantities such as velocity dispersions are very robust (agreement within a few tens of percent at all radii) while measures like newly-formed stellar clump mass functions show more significant variation (difference by up to a factor of ~3). Systematic differences exist, for example, between mesh-based and particle-based codes in the low density region, and between more diffusive and less diffusive schemes in the high density tail of the density distribution. Yet intrinsic code differences are generally small compared to the variations in numerical implementations of the common subgrid physics such as supernova feedback. Our experiment reassures that, if adequately designed in accordance with our proposed common parameters, results of a modern high-resolution galaxy formation simulation are more sensitive to input physics than to intrinsic differences in numerical schemes., Comment: 28 pages, 35 figures, Accepted for publication in the Astrophysical Journal, Image resolution greatly reduced, High-resolution version of this article is available at http://www.jihoonkim.org/agora/AGORA_Paper4_draft.pdf, The first paper of the AGORA Initiative is at http://adsabs.harvard.edu/abs/2014ApJS..210...14K, More information on AGORA is at http://www.AGORAsimulations.org/

Cosmological simulations of galaxy formation can produce present-day galaxies with a large range of assembly and star formation histories. A detailed study of the metallicity evolution and star formation history of such simulations can assist in predicting LIGO-detectable compact object binary mergers. Recent simulations of compact binary evolution suggest the compact object merger rate depends sensitively on the progenitor's metallicity. Rare low-metallicity star formation during galaxy assembly can produce more detected compact binaries than typical star formation. Using detailed simulations of galaxy and chemical evolution, we determine how sensitively the compact binary populations of galaxies with similar present-day appearance depend on the details of their assembly. We also demonstrate by concrete example the extent to which dwarf galaxies overabundantly produce compact binary mergers, particularly binary black holes, relative to more massive galaxies. We discuss the implications for transient multimessenger astronomy with compact binary sources., Comment: 9 pages, re-submitted to MNRAS after responding to referee comments

We confirm that the object DDO216-A1 is a substantial globular cluster at the center of Local Group galaxy DDO216 (the Pegasus dwarf irregular), using Hubble Space Telescope ACS imaging. By fitting isochrones, we find the cluster metallicity to be [M/H] = -1.6 +/-0.2, for reddening E(B-V) = 0.16 +/-0.02; the best-fit age is 12.3 +/-0.8 Gyr. There are ~30 RR Lyrae variables in the cluster; the magnitude of the fundamental mode pulsators gives a distance modulus of 24.77 +/-0.08 - identical to the host galaxy. The ratio of overtone to fundamental mode variables and their mean periods make DDO216-A1 an Oosterhoff Type I cluster. We find an I-band central surface brightness 20.85 +/-0.17 F814W mag per square arcsecond, a half-light radius of 3.1 arcsec (13.4 pc), and an absolute magnitude M814 = -7.90 +/-0.16 (approximately 10^5 solar masses). King models fit to the cluster give the core radius and concentration index, r_c = 2.1" +/-0.9" and c = 1.24 +/-0.39. The cluster is an "extended" cluster somewhat typical of some dwarf galaxies and the outer halo of the Milky Way. The cluster is projected <30 pc south of the center of DDO216, unusually central compared to most dwarf galaxy globular clusters. Analytical models of dynamical friction and tidal destruction suggest that it probably formed at a larger distance, up to ~1 kpc, and migrated inward. DDO216 has an unexceptional cluster specific frequency, S_N = 10. DDO216 is the lowest-luminosity Local Group galaxy to host a 10^5 solar mass globular cluster, and the only transition-type (dSph/dIrr) in the Local Group with a globular., Comment: Accepted by ApJ on 2017-01-31 ; 13 pages, 8 figures, 2 tables. 3 additional figures and some clarified discussion compared to previous version

The r-process nuclei are robustly synthesized in the material ejected during a neutron star binary merger (NSBM), as tidal torques transport angular momentum and energy through the outer Lagrange point in the form of a vast tidal tail. If NSBM are indeed solely responsible for the solar system r- process abundances, a galaxy like our own would require to host a few NSBM per million years, with each event ejecting, on average, about 5x10^{-2} M_sun of r-process material. Because the ejecta velocities in the tidal tail are significantly larger than in ordinary supernovae, NSBM deposit a comparable amount of energy into the interstellar medium (ISM). In contrast to extensive efforts studying spherical models for supernova remnant evolution, calculations quantifying the impact of NSBM ejecta in the ISM have been lacking. To better understand their evolution in a cosmological context, we perform a suite of three-dimensional hydrodynamic simulations with optically-thin radiative cooling of isolated NSBM ejecta expanding in environments with conditions adopted from Milky Way-like galaxy simulations. Although the remnant morphology is highly complex at early times, the subsequent radiative evolution that results from thermal instability in atomic gas is remarkably similar to that of a standard supernova blast wave. This implies that sub-resolution supernova feedback models can be used in galaxy-scale simulations that are unable to resolve the key evolutionary phases of NSBM blast waves. Among other quantities, we examine the radius, time, mass and kinetic energy content of the NSBM remnant at shell formation as well as the momentum injected to the ISM. We find that the shell formation epoch is attained when the swept-up mass is about 10^3 M_sun, at this point the mass fraction of r-process material is drastically enhanced up to two orders of magnitude in relation to a solar metallicity ISM., Comment: 8 pages, 6 figures, 2 table. Submitted to ApJ

We investigate the properties of halo gas using three cosmological `zoom-in' simulations of realistic Milky Way-galaxy analogs with varying sub-grid physics. In all three cases, the mass of hot ($T > 10^6$ K) halo gas is $\sim 1\%$ of the host's virial mass. The X-ray luminosity of two of the runs is consistent with observations of the Milky Way, while the third simulation is X-ray bright and resembles more closely a very massive, star-forming spiral. Hot halos extend to 140 kpc from the galactic center and are surrounded by a bubble of warm-hot ($T = 10^5 - 10^6$K) gas that extends to the virial radius. Simulated halos agree well outside 20-30 kpc with the $\beta$-model of Miller & Bregman (2013) based on OVII absorption and OVIII emission measurements. Warm--hot and hot gas contribute up to $80\%$ of the total gas reservoir, and contain nearly the same amount of baryons as the stellar component. The mass of warm-hot and hot components falls into the range estimated for $L^*$ galaxies. With key observational constraints on the density of the Milky Way corona being satisfied, we show that concealing of the ubiquitous warm-hot baryons, along with the ejection of just $20-30 \%$ of the diffuse gas out of the potential wells by supernova-driven outflows, can solve the "missing baryon problem". The recovered baryon fraction within 3 virial radii is $90\%$ of the universal value. With a characteristic density of $\sim 10^{-4}$ cm$^{-3}$ at $50-80$ kpc, diffuse coronae meet the requirement for fast and complete ram--pressure stripping of the gas reservoirs in dwarf galaxy satellites., Comment: accepted for publication in ApJ, 5 figures

We study the effect of sub-grid physics, galaxy mass, structural parameters and resolution on the fragmentation of gas-rich galaxy discs into massive star forming clumps. The initial conditions are set up with the aid of the ARGO cosmological hydrodynamical simulation. Blast-wave feedback does not suppress fragmentation, but reduces both the number of clumps and the duration of the unstable phase. Once formed, bound clumps cannot be destroyed by our feedback model. Widespread fragmentation is promoted by high gas fractions and low halo concentrations. Yet giant clumps $M > 10^8 M_{\odot}$ lasting several hundred Myr are rare and mainly produced by clump-clump mergers. They occur in massive discs with maximum rotational velocities $V_{max} > 250$ km/s at $z \sim 2$, at the high mass end of the observed galaxy population at those redshifts. The typical gaseous and stellar masses of clumps in all runs are in the range $\sim 10^7-10^8 M_{\odot}$ for galaxies with disc mass in the range $10^{10}-8\times 10^{10} M_{\odot}$. Clumps sizes are usually in the range $100-400$ pc, in agreement with recent clump observations in lensed high-z galaxies. \\ We argue that many of the giant clumps identified in observations are not due to in-situ fragmetation, or are the result of blending of smaller structures owing to insufficient resolution. Using an analytical model describing local collapse inside spiral arms, we can predict the characteristic gaseous masses of clumps at the onset of fragmentation ($\sim 3-5 \times 10^7 M_{\odot}$) quite accurately, while the conventional Toomre mass overestimates them. Due to their moderate masses, clumps which migrate to the centre have marginal effect on bulge growth., Comment: 27 pages, 25 figures and two tables. Accepted by MNRAS

We present novel 3D multi-scale SPH simulations of gas-rich galaxy mergers between the most massive galaxies at $z \sim 8 - 10$, designed to scrutinize the direct collapse formation scenario for massive black hole seeds proposed in \citet{mayer+10}. The simulations achieve a resolution of 0.1 pc, and include both metallicity-dependent optically-thin cooling and a model for thermal balance at high optical depth. We consider different formulations of the SPH hydrodynamical equations, including thermal and metal diffusion. When the two merging galaxy cores collide, gas infall produces a compact, optically thick nuclear disk with densities exceeding $10^{-10}$ g cm$^3$. The disk rapidly accretes higher angular momentum gas from its surroundings reaching $\sim 5$ pc and a mass of $\gtrsim 10^9$ $M_{\odot}$ in only a few $10^4$ yr. Outside $\gtrsim 2$ pc it fragments into massive clumps. Instead, supersonic turbulence prevents fragmentation in the inner parsec region, which remains warm ($\sim 3000-6000$ K) and develops strong non-axisymmetric modes that cause prominent radial gas inflows ($> 10^4$ $M_{\odot}$ yr$^{-1}$), forming an ultra-dense massive disky core. Angular momentum transport by non-axisymmetric modes should continue below our spatial resolution limit, quickly turning the disky core into a supermassive protostar which can collapse directly into a massive black hole of mass $10^8-10^9$ $M_{\odot}$ via the relativistic radial instability. Such a "cold direct collapse"' explains naturally the early emergence of high-z QSOs. Its telltale signature would be a burst of gravitational waves in the frequency range $10^{-4} - 10^{-1}$ Hz, possibly detectable by the planned eLISA interferometer., Comment: 14 pages, 10 figures, submitted to ApJ, comments are welcome

We examine gas accretion and subsequent star formation in representative galaxies from the McMaster Unbiased Galaxy Simulations (Stinson et al. 2010). Accreted gas is bimodal with a natural temperature division at $10^5$ K, near the peak of the cooling curve. Cold-mode accretion dominates inflows at early times, creating a peak in total accretion at redshift z=2-4 and declining exponentially below z$\sim$2. Hot-mode accretion peaks near z=1-2 and declines gradually. Hot-mode exceeds cold-mode accretion at z$\sim$1.8 for all four galaxies rather than when the galaxy reaches a characteristic mass. Cold-mode accretion can fuel immediate star formation, while hot-mode accretion preferentially builds a large, hot gas reservoir in the halo. Late-time star formation relies on reservoir gas accreted 2-8 Gyr prior. Thus, the reservoir allows the star formation rate to surpass the current overall gas accretion rate. Stellar feedback cycles gas from the interstellar medium back into the hot reservoir. Stronger feedback results in more gas cycling, gas removal in a galactic outflow and less star formation overall, enabling simulations to match the observed star formation history. For lower mass galaxies in particular, strong feedback can delay the star formation peak to z=1-2 from the accretion peak at z=2-4., Comment: 10 pages, 7 figures. Accepted for publication in MNRAS

We investigate the production sites and the enrichment history of $r$-process elements in the Galaxy, as traced by the [Eu/Fe] ratio, using the high resolution, cosmological zoom-in simulation `Eris'. At $z=0$, Eris represents a close analog to the Milky Way, making it the ideal laboratory to understand the chemical evolution of our Galaxy. Eris formally traces the production of oxygen and iron due to Type-Ia and Type-II supernovae. We include in post-processing the production of $r$-process elements from compact binary mergers. Unlike previous studies, we find that the nucleosynthetic products from compact binary mergers can be incorporated into stars of very low metallicity and at early times, even with a minimum delay time of 100 Myr. This conclusion is relatively insensitive to modest variations in the merger rate, minimum delay time, and the delay time distribution. By implementing a first-order prescription for metal-mixing, we can further improve the agreement between our model and the data for the chemical evolution of both [$\alpha$/Fe] and [Eu/Fe].We argue that compact binary mergers could be the dominant source of $r$-process nucleosynthesis in the Galaxy., Comment: 10 pages, 8 figures. Accepted for publication in ApJ

We use high resolution Hydro$+$N-Body cosmological simulations to compare the assembly and evolution of a small field dwarf (stellar mass ~ 10$^{6-7}$ M$\odot$, total mass 10$^{10}$ M$\odot$ in $\Lambda$ dominated CDM and 2keV WDM cosmologies. We find that star formation (SF) in the WDM model is reduced and delayed by 1-2 Gyr relative to the CDM model, independently of the details of SF and feedback. Independent of the DM model, but proportionally to the SF efficiency, gas outflows lower the central mass density through `dynamical heating', such that all realizations have circular velocities $<$ 20kms at 500$~$pc, in agreement with local kinematic constraints. As a result of dynamical heating, older stars are less centrally concentrated than younger stars, similar to stellar population gradients observed in nearby dwarf galaxies. Introducing an important diagnostic of SF and feedback models, we translate our simulations into artificial color-magnitude diagrams and star formation histories in order to directly compare to available observations. The simulated galaxies formed most of their stars in many $\sim$10 Myr long bursts. The CDM galaxy has a global SFH, HI abundance and Fe/H and alpha-elements distribution well matched to current observations of dwarf galaxies. These results highlight the importance of directly including `baryon physics' in simulations when 1) comparing predictions of galaxy formation models with the kinematics and number density of local dwarf galaxies and 2) differentiating between CDM and non-standard models with different DM or power spectra., Comment: 13 pages including Appendix on Color Magnitude Diagrams. Accepted by MNRAS. Added one plot and details on ChaNGa implementation. Reduced number of citations after editorial request

We present more results from a fully cosmological LCDM simulation of a group of isolated dwarf galaxies that has been shown to reproduce the observed stellar mass and cold gas content, resolved star formation histories, and metallicities of dwarfs in the Local Volume. Here we investigate the energetics and timetable of the cusp-core transformation. As suggested by previous work, supernova-driven gas outflows remove dark matter (DM) cusps and create kpc-size cores in all systems having a stellar mass above 1e6 Msun. The "DM core mass removal efficiency" -- dark mass ejected per unit stellar mass -- ranges today from a few to a dozen, and increases with decreasing host mass. Because dwarfs form the bulk of their stars prior to redshift 1 and the amount of work required for DM heating and core formation scales approximately as Mvir^{5/3}, the unbinding of the DM cusp starts early and the formation of cored profiles is not as energetically onerous as previously claimed. DM particles in the cusp typically migrate to 2-3 core radii after absorbing a few percent of the energy released by supernovae. The present-day slopes of the inner dark matter mass profiles, Gamma=d log M/dlog R=2.5-3, of the simulated "Bashful" and "Doc" dwarfs are similar to those measured in the luminous Fornax and Sculptor dwarf spheroidals. None of the simulated galaxies has a circular velocity profile exceeding 20 km/s in the inner 1 kpc, implying that supernova feedback is key to solve the "too-big-to-fail" problem for Milky Way subhalos., Comment: 7 pages, 4 figures, accepted for publication in the Astrophysical Journal Letters

We examine the pressure of the star-forming interstellar medium (ISM) of Milky-Way sized disk galaxies using fully cosmological SPH+N-body, high resolution simulations. These simulations include explicit treatment of metal-line cooling in addition to dust and self-shielding, $\mathrm{H_{2}}$ based star formation. The 4 simulated halos have masses ranging from a few times $10^{10}$ to nearly $10^{12}$ solar masses. Using a kinematic decomposition of these galaxies into present-day bulge and disk components, we find that the typical pressure of the star-forming ISM in the present-day bulge is higher than that in the present-day disk by an order of magnitude. We also find that pressure of the star-forming ISM at high redshift is on average, higher than ISM pressures at low redshift. This explains the why the bulge forms at higher pressures: the disk assembles at lower redshift, when the ISM is lower pressure and the bulge forms at high redshift, when the ISM is at higher pressure. If ISM pressure and IMF variation are tied together as suggested in studies like \cite{Conroy2012}, these results could indicate a time-dependent IMF in Milky-Way like systems, as well as a different IMF in the bulge and the disk., Comment: Accepted 12/3/2013 to ApJL

We present results from a fully cosmological, very high-resolution, LCDM "zoom-in" simulation of a group of seven field dwarf galaxies with present-day virial masses in the range M_vir=4.4e8-3.6e10 Msun. The simulation includes a blastwave scheme for supernova feedback, a star formation recipe based on a high gas density threshold, metal-dependent radiative cooling, a scheme for the turbulent diffusion of metals and thermal energy, and a uniform UV background that modifies the ionization and excitation state of the gas. The properties of the simulated dwarfs are strongly modulated by the depth of the gravitational potential well. All three halos with M_vir < 1e9 Msun are devoid of stars, as they never reach the density threshold for star formation of 100 atoms/cc. The other four, M_vir > 1e9 Msun dwarfs have blue colors, low star formation efficiencies, high cold gas to stellar mass ratios, and low stellar metallicities. Their bursty star formation histories are characterized by peak specific star formation rates in excess of 50-100 1/Gyr, far outside the realm of normal, more massive galaxies, and in agreement with observations of extreme emission-line starbursting dwarfs by the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey. Metal-enriched galactic outflows produce sub-solar effective yields and pollute with heavy elements a Mpc-size region of the intergalactic medium, but are not sufficient to completely quench star formation activity and are not ubiquitous in our dwarfs. Within the limited size of the sample, our simulations appear to simultaneously reproduce the observed stellar mass and cold gas content, resolved star formation histories, stellar kinematics, and metallicities of field dwarfs in the Local Volume., Comment: 15 pages, 10 figures, version accepted for publication in The Astrophysical Journal

We introduce the AGORA project, a comprehensive numerical study of well-resolved galaxies within the LCDM cosmology. Cosmological hydrodynamic simulations with force resolutions of ~100 proper pc or better will be run with a variety of code platforms to follow the hierarchical growth, star formation history, morphological transformation, and the cycle of baryons in and out of 8 galaxies with halo masses M_vir ~= 1e10, 1e11, 1e12, and 1e13 Msun at z=0 and two different ("violent" and "quiescent") assembly histories. The numerical techniques and implementations used in this project include the smoothed particle hydrodynamics codes GADGET and GASOLINE, and the adaptive mesh refinement codes ART, ENZO, and RAMSES. The codes will share common initial conditions and common astrophysics packages including UV background, metal-dependent radiative cooling, metal and energy yields of supernovae, and stellar initial mass function. These are described in detail in the present paper. Subgrid star formation and feedback prescriptions will be tuned to provide a realistic interstellar and circumgalactic medium using a non-cosmological disk galaxy simulation. Cosmological runs will be systematically compared with each other using a common analysis toolkit, and validated against observations to verify that the solutions are robust - i.e., that the astrophysical assumptions are responsible for any success, rather than artifacts of particular implementations. The goals of the AGORA project are, broadly speaking, to raise the realism and predictive power of galaxy simulations and the understanding of the feedback processes that regulate galaxy "metabolism." The proof-of-concept dark matter-only test of the formation of a galactic halo with a z=0 mass of M_vir ~= 1.7e11 Msun by 9 different versions of the participating codes is also presented to validate the infrastructure of the project., Comment: 21 pages, 6 figures, Accepted for publication in the Astrophysical Journal Supplement, Image resolution greatly reduced, High-resolution version of this article and more information about the AGORA Project including the science goals of the 13 Working Groups are available at http://www.AGORAsimulations.org/ and at http://sites.google.com/site/santacruzcomparisonproject/details/

We compare the central mass distribution of galaxies simulated with three different models of the interstellar medium (ISM) with increasing complexity: primordial (H+He) cooling down to 10^4K, additional cooling via metal lines and to lower temperatures, and molecular hydrogen (H_2) with shielding of atomic and molecular hydrogen, in addition to metal line cooling. In order to analyze the effect of these models, we follow the evolution of four field galaxies with V_peak < 120 km/s to a redshift of zero using high-resolution Smoothed Particle Hydrodynamic simulations in a fully cosmological LCDM context. The spiral galaxies produced in simulations with either primordial cooling or H_2 physics have realistic, rising rotation curves. In contrast, the simulations with metal line cooling and otherwise similar feedback and star formation produced galaxies with the peaked rotation curves typical of most previous LCDM simulations of spiral galaxies. The less-massive bulges and non-peaked rotation curves in the galaxies simulated with primordial cooling or H_2 are linked to changes in the angular momentum distribution of the baryons. These galaxies had smaller amounts of low-angular momentum baryons because of increased gas loss from stellar feedback. When there is only primordial cooling, the star forming gas is hotter and the feedback-heated gas cools more slowly than when metal line cooling is included and so requires less energy to be expelled. When H_2 is included, the accompanying shielding produces large amounts of clumpy, cold gas where H_2 forms. Star formation in clumpy gas results in more concentrated supernova feedback and greater efficiency of mass loss. The higher feedback efficiency causes a decrease of low-angular momentum material. (abridged), Comment: 19 pages, 14 figures, submitted to MNRAS

Using high resolution cosmological hydrodynamical simulations of Milky Way-massed disk galaxies, we demonstrate that supernovae feedback and tidal stripping lower the central masses of bright (-15 < M_V < -8) satellite galaxies. These simulations resolve high density regions, comparable to giant molecular clouds, where stars form. This resolution allows us to adopt a prescription for H_2 formation and destruction that ties star formation to the presence of shielded, molecular gas. Before infall, supernova feedback from the clumpy, bursty star formation captured by this physically motivated model leads to reduced dark matter (DM) densities and shallower inner density profiles in the massive satellite progenitors (Mvir > 10^9 Msun, Mstar > 10^7 Msun) compared to DM-only simulations. The progenitors of the lower mass satellites are unable to maintain bursty star formation histories, due to both heating at reionization and gas loss from initial star forming events, preserving the steep inner density profile predicted by DM-only simulations. After infall, tidal stripping acts to further reduce the central densities of the luminous satellites, particularly those that enter with cored dark matter halos, increasing the discrepancy in the central masses predicted by baryon+DM and DM-only simulations. We show that DM-only simulations, which neglect the baryonic effects described in this work, produce denser satellites with larger central velocities. We provide a simple correction to the central DM mass predicted for satellites by DM-only simulations. We conclude that DM-only simulations should be used with great caution when interpreting kinematic observations of the Milky Way's dwarf satellites., Comment: Version accepted for publication in ApJ. Now includes an appendix with a discussion of numerical convergence

Motivated by the observed connection between molecular hydrogen (H2) and star formation, we present a method for tracking the non-equilibrium abundance and cooling processes of H2 and H2-based star formation in Smoothed Particle Hydrodynamic simulations. The local abundances of H2 are calculated by integrating over the hydrogen chemical network. This calculation includes the gas-phase and dust grain formation of H2, shielding of HI and H2, and photodissociation of H2 by Lyman-Werner radiation from nearby stellar populations. Because this model does not assume equilibrium abundances, it is particularly well suited for simulations that model low-metallicity environments, such as dwarf galaxies and the early Universe. We further introduce an explicit link between star formation and local H2 abundance. This link limits star formation to "star-forming regions," represented by areas with abundant H2. With this implementation, we determine the effect of H2 on star formation in a cosmological simulation of a dwarf galaxy. This simulation is the first cosmological simulation with non-equilibrium H2 abundances to be integrated to a redshift of zero or to include efficient SN feedback. We find that our simulations are consistent with the observed Tully-Fisher, global Kennicutt-Schmidt, and resolved Kennicutt-Schmidt relations. We find that the inclusion of shielding of both the atomic and molecular hydrogen and, to a lesser extent, the additional cooling from H2 at temperatures between 200 and 5000 K increases the amount of cold gas in the galaxies. The changes to the ISM result in an increased amount of cold, dense gas in the disk of the galaxy and the formation of a clumpier interstellar media (ISM). The explicit link between star formation and H2 and the clumpier ISM results in a bluer galaxy with a greater spatial distribution of star formation at a redshift of zero. (abridged), Comment: 20 pages, 15 figures, resubmitted to MNRAS following referee report

We present new results on the kinematics, thermal and ionization state, and spatial distribution of metal-enriched gas in the circumgalactic medium (CGM) of massive galaxies at redshift 3, using the "Eris" suite of cosmological "zoom-in" simulations. The reference run adopts a blastwave scheme for supernova feedback that produces galactic outflows, a star formation recipe based on a high gas density threshold, metal-dependent radiative cooling, and a model for the diffusion of metals and thermal energy. Synthetic spectra through the multiphase CGM produce interstellar absorption line strengths of Lya, CII, CIV, SiII, and SiIV as a function of galactocentric impact parameter (scaled to the virial radius) that are in broad agreement with those observed at high-redshift by Steidel et al. (2010). Only about one third of all the gas within R_vir is outflowing. The fraction of sightlines within one virial radius that intercept optically thick material is 27%, in agreement with recent observations by Rudie et al. (2012). Such optically thick absorption is shown to trace inflowing "cold" streams that penetrate deep inside the virial radius. The streams, enriched to metallicities above 0.01 solar, give origin to strong (log N > 13) CII absorption with a covering factor of 22% within R_vir and 10% within 2 R_vir. Galactic outflows do not cause any substantial suppression of the cold accretion mode. The central galaxy is surrounded by a large OVI halo, with a typical column density log N>14 and a near unity covering factor maintained all the way out to 150 kpc. This matches the trends recently observed in star-forming galaxies at low redshift by Tumlinson et al. (2011). Our zoom-in simulations of this single system appear to reproduce quantitatively the complex baryonic processes that determine the exchange of matter, energy, and metals between galaxies and their surroundings. (Abridged), Comment: 16 pages, 13 figures, accepted for publication in The Astrophysical Journal

We explore the circumgalactic medium (CGM) of two simulated star-forming galaxies with luminosities L ~ 0.1 and 1 L* generated using the smooth particle hydrodynamic code GASOLINE. These simulations are part of the Making Galaxies In a Cosmological Context (MAGICC) program in which the stellar feedback is tuned to match the stellar mass-halo mass relationship. For comparison, each galaxy was also simulated using a 'lower feedback' (LF) model which has strength comparable to other implementations in the literature. The 'MAGICC feedback' (MF) model has a higher incidence of massive stars and an approximately two times higher energy input per supernova. Apart from the low-mass halo using LF, each galaxy exhibits a metal-enriched CGM that extends to approximately the virial radius. A significant fraction of this gas has been heated in supernova explosions in the disc and subsequently ejected into the CGM where it is predicted to give rise to substantial O VI absorption. The simulations do not yet address the question of what happens to the O VI when the galaxies stop forming stars. Our models also predict a reservoir of cool H I clouds that show strong Ly\alpha absorption to several hundred kpc. Comparing these models to recent surveys with the Hubble Space Telescope, we find that only the MF models have sufficient O VI and H I gas in the CGM to reproduce the observed distributions. In separate analyses, these same MF models also show better agreement with other galaxy observables (e.g. rotation curves, surface brightness profiles and H I gas distribution). We infer that the CGM is the dominant reservoir of baryons for galaxy haloes., Comment: 9 pages, 7 figures, submitted MNRAS, comments welcome

A clear prediction of the Cold Dark Matter model is the existence of cuspy dark matter halo density profiles on all mass scales. This is not in agreement with the observed rotation curves of spiral galaxies, challenging on small scales the otherwise successful CDM paradigm. In this work we employ high resolution cosmological hydro-dynamical simulations to study the effects of dissipative processes on the inner distribution of dark matter in Milky-Way like objects (M~1e12 Msun). Our simulations include supernova feedback, and the effects of the radiation pressure of massive stars before they explode as supernovae. The increased stellar feedback results in the expansion of the dark matter halo instead of contraction with respect to N-body simulations. Baryons are able to erase the dark matter cuspy distribution creating a flat, cored, dark matter density profile in the central several kpc of a massive Milky-Way like halo. The profile is well fit by a Burkert profile, with fitting parameters consistent with the observations. In addition, we obtain flat rotation curves as well as extended, exponential stellar disk profiles. While the stellar disk we obtain is still partially too thick to resemble the MW thin disk, this pilot study shows that there is enough energy available in the baryonic component to alter the dark matter distribution even in massive disc galaxies, providing a possible solution to the long standing problem of cusps vs. cores., Comment: 5 pages, 4 figures, accepted for publication in ApJ Letter

We present a detailed study of the metal-enriched circum-galactic medium of a massive galaxy at z=3 using "ErisMC", a new cosmological hydrodynamic "zoom-in" simulations of a disk galaxy with mass comparable to the Milky Way. The run adopts a blastwave scheme for supernova feedback that generates galactic outflows without explicit wind particles, a star formation recipe based on a high gas density threshold, and high temperature metal cooling. ErisMC's main progenitor at z=3 resembles a "Lyman break" galaxy of mass M_vir=2.4e11 M_sun, virial radius R_vir=48 kpc, and star formation rate 18 M_sun/yr, and its metal-enriched CGM extends to 200 (physical) kpc from its center. Approximately 41, 9, and 50 percent of all gas-phase metals at z=3 are locked in a hot (T> 3e5 K), warm (3e5 >T> 3e4 K), and cold (T< 3e4 K) medium, respectively. We identify three sources of heavy elements: 1) the main host, responsible for 60% of all the metals found within 3R_vir; 2) its satellite progenitors, responsible for 28% of all the metals within 3R_vir, and for only 5% of those beyond 3R_vir; and nearby dwarfs, which give origin to 12% of all the metals within 3R_vir and 95% of those beyond 3R_vir. Late (z<5) galactic "superwinds" account for only 9% of all the metals observed beyond 2R_vir, the bulk having been released at redshifts 5< z < 8 by early star formation and outflows. In the CGM, lower overdensities are typically enriched by `older', colder metals. Heavy elements are accreted onto Eris along filaments via low-metallicity cold inflows, and are ejected hot via galactic outflows at a few hundred km/s. The outflow mass-loading factor is of order unity for the main halo, but can exceed 10 for nearby dwarfs. We stress that our "zoom-in" simulation focuses on the CGM of a single massive system and cannot describe the enrichment history of the IGM as a whole. (abridged), Comment: 13 pages, 16 figures, accepted for publication in ApJ

We investigate the formation of the first massive black holes in high redshift galaxies, with the goal of providing insights to which galaxies do or do not host massive black holes. We adopt a novel approach to forming seed black holes in galaxy halos in cosmological SPH+N-body simulations. The formation of massive black hole seeds is dictated directly by the local gas density, temperature, and metallicity, and motivated by physical models of massive black hole formation. We explore seed black hole populations as a function of halo mass and redshift, and examine how varying the efficiency of massive black hole seed formation affects the relationship between black holes and their hosts. Seed black holes tend to form in halos with mass between 10^7 and 10^9 Msun, and the formation rate is suppressed around z = 5 due to the diffusion of metals throughout the intergalactic medium. We find that the time of massive black hole formation and the occupation fraction of black holes are a function of the host halo mass. By z = 5, halos with mass M_halo > 3 x 10^9 Msun host massive black holes regardless of the efficiency of seed formation, while the occupation fraction for smaller halos increases with black hole formation efficiency. Our simulations explain why massive black holes are found in some bulgeless and dwarf galaxies, but we also predict that their occurrence becomes rarer and rarer in low-mass systems., Comment: Replaced with ApJ accepted version. 7 pages, 3 figures

We perform SPH+N-body cosmological simulations of massive disk galaxies, including a formalism for black hole seed formation and growth, and find that satellite galaxies containing supermassive black hole seeds are often stripped as they merge with the primary galaxy. These events naturally create a population of "wandering" black holes that are the remnants of stripped satellite cores; galaxies like the Milky Way may host 5 -- 15 of these objects within their halos. The satellites that harbor black hole seeds are comparable to Local Group dwarf galaxies such as the Small and Large Magellanic Clouds; these galaxies are promising candidates to host nearby intermediate mass black holes. Provided that these wandering black holes retain a gaseous accretion disk from their host dwarf galaxy, they give a physical explanation for the origin and observed properties of some recently discovered off-nuclear ultraluminous X-ray sources such as HLX-1., Comment: Accepted for publication in ApJ Letters