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We use a sample of 73 simulated satellite and central dwarf galaxies spanning a stellar mass range of $10^{5.3}-10^{9.1} M_\odot$ to investigate the origin of their stellar age gradients. We find that dwarf galaxies often form their stars "inside-out," i.e., the stars form at successively larger radii over time. However, the oldest stars get reshuffled beyond the star forming radius by fluctuations in the gravitational potential well caused by stellar feedback (the same mechanisms that cause dwarfs to form dark matter cores). The result is that many dwarfs appear to have an "outside-in" age gradient at $z=0$, with younger stellar populations more centrally concentrated. However, for the reshuffled galaxies with the most extended star formation, young stars can form out to the large radii to which the old stars have been reshuffled, erasing the age gradient. We find that major mergers do not play a significant role in setting the age gradients of dwarfs. We find similar age gradient trends in satellites and field dwarfs, suggesting environment plays only a minor role, if any. Finally, we find that the age gradient trends are imprinted on the galaxies at later times, suggesting that the stellar reshuffling dominates after the galaxies have formed 50% of their stellar mass. The later reshuffling is at odds with results from the FIRE-2 simulations. Hence, age gradients offer a test of current star formation and feedback models that can be probed via observations of resolved stellar populations., Comment: 25 pages, 9 figures

The AGORA Cosmorun (arXiv:2106.09738) is a set of hydrodynamical cosmological zoom-in simulations carried out within the AGORA High-resolution Galaxy Simulations Comparison Project (arXiv:1308.2669,arXiv:1610.03066). These simulations show the formation and evolution of a Milky Way-sized galaxy using eight of the most widely used numerical codes in the community (Art-I, Enzo, Ramses, Changa, Gadget-3, Gear, Gizmo, and Arepo). In this short report, we describe the public release of the raw output data from all of these simulations at z = 8, 7, 6, 5, 4, 3, 2 (plus at z=1, 0 when available), and several metadata files containing the halo centers, virial quantities, and merger trees. The data from even thinner timesteps will be released as soon as the upcoming collaboration papers (VII-IX) are submitted and accepted., Comment: ArXiv Data Release from the AGORA Collaboration. arXiv admin note: text overlap with arXiv:2001.04354

The formation and evolution of supermassive black holes (SMBHs) remains an open question in the field of modern cosmology. The detection of nanohertz (n-Hz) gravitational waves via pulsar timing arrays (PTAs) in the form of individual events and the stochastic gravitational wave background (SGWB) offers a promising avenue for studying SMBH evolution across cosmic time, with SGWB signal being the immediately detectable signal with the currently accessible telescope sensitivities. By connecting the galaxy properties in the large scale (Gpc scale) cosmological simulation such as \texttt{MICECAT} with the small scale ($\sim$ Mpc scale) galaxy simulations from \texttt{ROMULUS}, we show that different scenarios of galaxy-SMBH evolution with redshift leads to a frequency-dependent spatial anisotropy in the SGWB signal. The presence of slow evolution of the SMBHs in the Universe leads to a pronounced blue anisotropic spectrum of the SGWB. In contrast, if SMBHs grow faster in the Universe in lighter galaxies, the frequency-dependent spatial anisotropy exhibits a more flattened anisotropic spectrum. This additional aspect of the SGWB signal on top of the monopole SGWB signal, can give insight on how the SMBHs form in the high redshift Universe and its interplay with the galaxy formation from future measurements., Comment: 17 pages, 11 figures

The presence of dual active galactic nuclei (AGN) on scales of a few tens of kpc can be used to study merger-induced accretion on supermassive black holes (SMBHs) and offer insights about SMBH mergers, using dual AGNs as merger precursors. This study uses the {\sc Romulus25} cosmological simulation to investigate the properties and evolution of dual AGNs. We first analyze the properties of AGNs ($L_{bol} > 10^{43} \rm $ erg s$^{-1}$) and their neighboring SMBHs (any SMBHs closer than 30 pkpc to an AGN) at $z \leq 2$. This is our underlying population. We then applied the luminosity threshold of $L_{bol} > 10^{43} $ erg s$^{-1}$ to the neighboring SMBHs thereby identifying dual and multiple AGNs. Our findings indicate an increase in the number of both single and dual AGNs from lower to higher redshifts. We also find that the number of dual AGNs with separations of 0.5-4 kpc is twice the number of duals with separations of 4-30 kpc. All dual AGNs in our sample resulted from major mergers. Compared to single AGNs, duals have a lower black hole-to-halo mass ratio. We found that the properties of dual AGN host halos, including halo mass, stellar mass, star formation rate (SFR), and gas mass, are generally consistent with those of single AGN halos, albeit tending towards the higher end of their respective property ranges. Our analysis uncovered a diverse array of evolutionary patterns among dual AGNs, including rapidly evolving systems, slower ones, and instances where SMBH mergers are ineffective.

In this fourth paper from the AGORA Collaboration, we study the evolution down to redshift $z=2$ and below of a set of cosmological zoom-in simulations of a Milky Way mass galaxy by eight of the leading hydrodynamic simulation codes. We also compare this CosmoRun suite of simulations with dark matter-only simulations by the same eight codes. We analyze general properties of the halo and galaxy at $z=4$ and 3, and before the last major merger, focusing on the formation of well-defined rotationally-supported disks, the mass-metallicity relation, the specific star formation rate, the gas metallicity gradients, and the non-axisymmetric structures in the stellar disks. Codes generally converge well to the stellar-to-halo mass ratios predicted by semi-analytic models at $z\sim$2. We see that almost all the hydro codes develop rotationally-supported structures at low redshifts. Most agree within 0.5 dex with the observed MZR at high and intermediate redshifts, and reproduce the gas metallicity gradients obtained from analytical models and low-redshift observations. We confirm that the inter-code differences in the halo assembly history reported in the first paper of the collaboration also exist in CosmoRun, making the code-to-code comparison more difficult. We show that such differences are mainly due to variations in code-dependent parameters that control the time-stepping strategy of the gravity solver. We find that variations in the early stellar feedback can also result in differences in the timing of the low-redshift mergers. All the simulation data down to $z=2$ and the auxiliary data will be made publicly available., Comment: Submitted to ApJ. Visit the AGORA Collaboration website (www.agorasimulations.org ) for more information

We analyze and compare the satellite halo populations at $z\sim2$ in the high-resolution cosmological zoom-in simulations of a $10^{12}\,{\rm M}_{\odot}$ target halo ($z=0$ mass) carried out on eight widely-used astrophysical simulation codes ({\sc Art-I}, {\sc Enzo}, {\sc Ramses}, {\sc Changa}, {\sc Gadget-3}, {\sc Gear}, {\sc Arepo-t}, and {\sc Gizmo}) for the {\it AGORA} High-resolution Galaxy Simulations Comparison Project. We use slightly different redshift epochs near $z=2$ for each code (hereafter ``$z\sim2$') at which the eight simulations are in the same stage in the target halo's merger history. After identifying the matched pairs of halos between the {\it CosmoRun} simulations and the DMO simulations, we discover that each {\it CosmoRun} halo tends to be less massive than its DMO counterpart. When we consider only the halos containing stellar particles at $z\sim2$, the number of satellite {\it galaxies} is significantly fewer than that of dark matter halos in all participating {\it AGORA} simulations, and is comparable to the number of present-day satellites near the Milky Way or M31. The so-called ``missing satellite problem' is fully resolved across all participating codes simply by implementing the common baryonic physics adopted in {\it AGORA} and the stellar feedback prescription commonly used in each code, with sufficient numerical resolution ($\lesssim100$ proper pc at $z=2$). We also compare other properties such as the stellar mass$-$halo mass relation and the mass$-$metallicity relation. Our work highlights the value of comparison studies such as {\it AGORA}, where outstanding problems in galaxy formation theory are studied simultaneously on multiple numerical platforms., Comment: Accepted for publication to the ApJ, 19 pages, 10 figures. Visit the AGORA Collaboration website (www.agorasimulations.org) for more information. For summary video, please see https://www.youtube.com/watch?v=HShRJYvPe1k

We analyze the circumgalactic medium (CGM) for eight commonly-used cosmological codes in the AGORA collaboration. The codes are calibrated to use identical initial conditions, cosmology, heating and cooling, and star formation thresholds, but each evolves with its own unique code architecture and stellar feedback implementation. Here, we analyze the results of these simulations in terms of the structure, composition, and phase dynamics of the CGM. We show properties such as metal distribution, ionization levels, and kinematics are effective tracers of the effects of the different code feedback and implementation methods, and as such they can be highly divergent between simulations. This is merely a fiducial set of models, against which we will in the future compare multiple feedback recipes for each code. Nevertheless, we find that the large parameter space these simulations establish can help disentangle the different variables that affect observable quantities in the CGM, e.g., showing that abundances for ions with higher ionization energy are more strongly determined by the simulation's metallicity, while abundances for ions with lower ionization energy are more strongly determined by the gas density and temperature., Comment: 32 pages, 17 figures. Published in ApJ

The cold ($\sim 10^{4}\,{\rm K}$) component of the circumgalactic medium (CGM) accounts for a significant fraction of all galactic baryons. However, using current galaxy-scale simulations to determine the origin and evolution of cold CGM gas poses a significant challenge, since it is computationally infeasible to directly simulate a galactic halo alongside the sub-pc scales that are crucial for understanding the interactions between cold CGM gas and the surrounding ''hot'' medium. In this work, we introduce a new approach: the Cold Gas Subgrid Model (CGSM), which models unresolved cold gas as a second fluid in addition to the standard ''normal'' gas fluid. The CGSM tracks the total mass density and bulk momentum of unresolved cold gas, deriving the properties of its unresolved cloudlets from the resolved gas phase. The interactions between the subgrid cold fluid and the resolved fluid are modeled by prescriptions from high-resolution simulations of ''cloud crushing'' and thermal instability. Through a series of idealized tests, we demonstrate the CGSM's ability to overcome the resolution limitations of traditional hydrodynamics simulations, successfully capturing the correct cold gas mass, its spatial distribution, and the timescales for cloud destruction and growth. We discuss the implications of using this model in cosmological simulations to more accurately represent the microphysics that govern the galactic baryon cycle., Comment: Submitted to MNRAS

We are entering an era in which we will be able to detect and characterize hundreds of dwarf galaxies within the Local Volume. It is already known that a strong dichotomy exists in the gas content and star formation properties of field dwarf galaxies versus satellite dwarfs of larger galaxies. In this work, we study the more subtle differences that may be detectable in galaxies as a function of distance from a massive galaxy, such as the Milky Way. We compare smoothed particle hydrodynamic simulations of dwarf galaxies formed in a Local Volume-like environment (several Mpc away from a massive galaxy) to those formed nearer to Milky Way-mass halos. We find that the impact of environment on dwarf galaxies extends even beyond the immediate region surrounding Milky Way-mass halos. Even before being accreted as satellites, dwarf galaxies near a Milky Way-mass halo tend to have higher stellar masses for their halo mass than more isolated galaxies. Dwarf galaxies in high-density environments also tend to grow faster and form their stars earlier. We show observational predictions that demonstrate how these trends manifest in lower quenching rates, higher HI fractions, and bluer colors for more isolated dwarf galaxies., Comment: 26 pages, 14 figures, submitted to ApJ

The formation of supermassive black holes (SMBHs) in the Universe and its role in the properties of the galaxies is one of the open questions in astrophysics and cosmology. Though, traditionally, electromagnetic waves have been instrumental in direct measurements of SMBHs, significantly influencing our comprehension of galaxy formation, gravitational waves (GW) bring an independent avenue to detect numerous binary SMBHs in the observable Universe in the nano-Hertz range using the pulsar timing array observation. This brings a new way to understand the connection between the formation of binary SMBHs and galaxy formation if we can connect theoretical models with multi-messenger observations namely GW data and galaxy surveys. Along these lines, we present here the first paper on this series based on {\sc Romulus25} cosmological simulation on the properties of the host galaxies of SMBHs and propose on how this can be used to connect with observations of nano-Hertz GW signal and galaxy surveys. We show that the most dominant contribution to the background will arise from sources with high chirp masses which are likely to reside in low redshift early-type galaxies with high stellar mass, largely old stellar population, and low star formation rate, and that reside at centers of galaxy groups and manifest evidence of recent mergers. The masses of the sources show a correlation with the halo mass and stellar mass of the host galaxies. This theoretical study will help in understanding the host properties of the GW sources and can help in establishing a connection with observations., Comment: 17 pages, 12 figures. Accepted for publication in MNRAS

Formation models in which terrestrial bodies grow via the pairwise accretion of planetesimals have been reasonably successful at reproducing the general properties of the solar system, including small body populations. However, planetesimal accretion has not yet been fully explored in the context of the wide variety of recently discovered extrasolar planetary systems, particularly those that host short-period terrestrial planets. In this work, we use direct N-body simulations to explore and understand the growth of planetary embryos from planetesimals in disks extending down to ~1 day orbital periods. We show that planetesimal accretion becomes nearly 100 percent efficient at short orbital periods, leading to embryo masses that are much larger than the classical isolation mass. For rocky bodies, the physical size of the object begins to occupy a significant fraction of its Hill sphere towards the inner edge of the disk. In this regime, most close encounters result in collisions, rather than scattering, and the system does not develop a bimodal population of dynamically hot planetesimals and dynamically cold oligarchs, like is seen in previous studies. The highly efficient accretion seen at short orbital periods implies that systems of tightly-packed inner planets should be almost completely devoid of any residual small bodies. We demonstrate the robustness of our results to assumptions about the initial disk model, and also investigate the effects that our simplified collision model has on the emergence of this non-oligarchic growth mode in a planet forming disk., Comment: 22 pages, 15 figures

The occupation fraction of massive black holes (MBHs) in dwarf galaxies offers interesting insights into initial black hole seeding mechanisms and their mass assembly history, though disentangling these two effects remains challenging. Using the {\sc Romulus} cosmological simulations we examine the impact of environment on the occupation fraction of MBHs in low mass galaxies. Unlike most modern cosmological simulations, {\sc Romulus} seeds MBHs based on local gas properties, selecting dense ($n>3$ cm$^{-3}$), pristine ($Z<3e-4Z_{\odot}$), and rapidly collapsing regions in the early Universe as sites to host MBHs without assuming anything about MBH occupation as a function of galaxy stellar mass, or halo mass, {\it a priori}. The simulations predict that dwarf galaxies with M$_{\star}<10^9$ M$_{\odot}$ in cluster environments are $\sim2$ times more likely to host a MBH compared to those in the field. The predicted occupation fractions are remarkably consistent with those of nuclear star clusters. Across cluster and field environments, dwarf galaxies with earlier formation times are more likely to host a MBH. While the MBH occupation function is similar between cluster and field environments at high redshift ($z>3$), a difference arises as late-forming dwarfs -- which do not exist in the cluster environment -- begin to dominate in the field and pull the MBH occupation fraction down for low mass galaxies. Additionally, prior to in-fall some cluster dwarfs are similar to progenitors of massive, isolated galaxies, indicating that they might have grown to higher masses had they not been impeded by the cluster environment. While the population of MBHs in dwarf galaxies is already widely understood to be important for understanding MBH formation, this work demonstrates that environmental dependence is important to consider as future observations search for low mass black holes in dwarf galaxies., Comment: 16 pages, 7 figures, Accepted to the Open Journal of Astrophysics

Using high-resolution {\sc Romulus} simulations, we explore the origin and evolution of the circumgalactic medium (CGM) in the region 0.1 $\leq \mathrm{R}/\mathrm{R}_\mathrm{500} \leq$ 1 around massive central galaxies in group-scale halos. We find that the CGM is multiphase and highly dynamic. Investigating the dynamics, we identify seven patterns of evolution. We show that these are robust and detected consistently across various conditions. The gas cools via two pathways: (1) filamentary cooling inflows and (2) condensations forming from rapidly cooling density perturbations. In our cosmological simulations, the perturbations are mainly seeded by orbiting substructures. The condensations can form even when the median $t_\mathrm{cool} / t_\mathrm{ff}$ of the X-ray emitting gas is above 10 or 20. Strong amplitude perturbations can provoke runaway cooling regardless of the state of the background gas. We also find perturbations whose local $t_\mathrm{cool} / t_\mathrm{ff}$ ratios drop below the threshold but which do not condense. Rather, the ratios fall to some minimum value and then bounce. These are weak perturbations that are temporarily swept up in satellite wakes and carried to larger radii. Their $t_\mathrm{cool} / t_\mathrm{ff}$ ratios decrease because $t_\mathrm{ff}$ is increasing, not because $t_\mathrm{cool}$ is decreasing. For structures forming hierarchically, our study highlights the challenge of using a simple threshold argument to infer the CGM's evolution. It also highlights that the median hot gas properties are suboptimal determinants of the CGM's state and dynamics. Realistic CGM models must incorporate the impact of mergers and orbiting satellites, along with the CGM's heating and cooling cycles., Comment: 25 pages, 12 figures

We study satellite counts and quenched fractions for satellites of Milky Way analogs in Romulus25, a large-volume cosmological hydrodynamic simulation. Depending on the definition of a Milky Way analog, we have between 66 and 97 Milky Way analogs in Romulus25, a 25 Mpc per-side uniform volume simulation. We use these analogs to quantify the effect of environment and host properties on satellite populations. We find that the number of satellites hosted by a Milky Way analog increases predominantly with host stellar mass, while environment, as measured by the distance to a Milky Way-mass or larger halo, may have a notable impact in high isolation. Similarly, we find that the satellite quenched fraction for our analogs also increases with host stellar mass, and potentially in higher-density environments. These results are robust for analogs within 3 Mpc of another Milky Way-mass or larger halo, the environmental parameter space where the bulk of our sample resides. We place these results in the context of observations through comparisons to the Exploration of Local VolumE Satellites and Satellites Around Galactic Analogs surveys. Our results are robust to changes in Milky Way analog selection criteria, including those that mimic observations. Finally, as our samples naturally include Milky Way-Andromeda pairs, we examine quenched fractions in pairs vs isolated systems. We find potential evidence, though not conclusive, that pairs, defined as being within 1 Mpc of another Milky Way-mass or larger halo, may have higher satellite quenched fractions.

Nuclear star clusters (NSCs) are massive star clusters found in the innermost region of most galaxies. While recent studies suggest that low-mass NSCs in dwarf galaxies form largely out of the merger of globular clusters and NSCs in massive galaxies accumulate mass primarily through central star formation, the formation channel of the Milky Way's NSC is still uncertain. In this work, we use GigaEris, a high resolution N-body, hydrodynamical, cosmological ``zoom-in'' simulation, to investigate a possible formation path of the NSC in the progenitor of a Milky Way-sized galaxy, as well as its relation to the assembly and evolution of the galactic nuclear region. We study the possibility that bound, young, gas-rich, stellar clusters within a radius of 1.5 kpc of the main galaxy's centre at z>4 are the predecessors of the old, metal-poor stellar population of the Milky Way's NSC. We identify 47 systems which satisfy our criteria, with a total stellar mass of $10^{7.5}$ M$_{\odot}$. We demonstrate that both stellar cluster accretion and in-situ star formation will contribute to the formation of the NSC, providing evidence for a hybrid formation scenario for the first time in an N-body, hydrodynamical, cosmological ``zoom-in'' simulation. Additionally, we find that the gas required for in-situ star formation can originate from two pathways: gas-rich stellar clusters and gas influx driven by large-scale non-axisymmetric structures within the galaxy. This is partly supported by the presence of a stellar ring, resulting from gas dynamics, with properties similar to those of the Milky Way's nuclear stellar disc., Comment: 13 pages, 10 figures, Accepted by MNRAS

In Paper I we showed that clumps in high-redshift galaxies, having a high star formation rate density (\Sigma_SFR), produce disks with two tracks in the [Fe/H]-[\alpha/Fe] chemical space, similar to that of the Milky Way's (MW's) thin + thick disks. Here we investigate the effect of clumps on the bulge's chemistry. The chemistry of the MW's bulge is comprised of a single track with two density peaks separated by a trough. We show that the bulge chemistry of an N-body + smoothed particle hydrodynamics clumpy simulation also has a single track. Star formation within the bulge is itself in the high-\Sigma_SFR clumpy mode, which ensures that the bulge's chemical track follows that of the thick disk at low [Fe/H] and then extends to high [Fe/H], where it peaks. The peak at low metallicity instead is comprised of a mixture of in-situ stars and stars accreted via clumps. As a result, the trough between the peaks occurs at the end of the thick disk track. We find that the high-metallicity peak dominates near the mid-plane and declines in relative importance with height, as in the MW. The bulge is already rapidly rotating by the end of the clump epoch, with higher rotation at low [\alpha/Fe]. Thus clumpy star formation is able to simultaneously explain the chemodynamic trends of the MW's bulge, thin + thick disks and the Splash., Comment: 25 pages, in press at ApJ

We examine the quenching characteristics of $328$ isolated dwarf galaxies $\left(10^{8} < M_{\rm star}/M_\odot < 10^{10} \right)$ within the \Rom{} cosmological hydrodynamic simulation. Using mock observation methods, we identify isolated dwarf galaxies with quenched star formation and make direct comparisons to the quenched fraction in the NASA Sloan Atlas (NSA). Similar to other cosmological simulations, we find a population of quenched, isolated dwarf galaxies below $M_{\rm star} < 10^{9} M_\odot$ not detected within the NSA. We find that the presence of massive black holes (MBHs) in \Rom{} is largely responsible for the quenched, isolated dwarfs, while isolated dwarfs without an MBH are consistent with quiescent fractions observed in the field. Quenching occurs between $z=0.5-1$, during which the available supply of star-forming gas is heated or evacuated by MBH feedback. Mergers or interactions seem to play little to no role in triggering the MBH feedback. At low stellar masses, $M_{\rm star} \lesssim 10^{9.3} M_\odot$, quenching proceeds across several Gyr as the MBH slowly heats up gas in the central regions. At higher stellar masses, $M_{\rm star} \gtrsim 10^{9.3} M_\odot$, quenching occurs rapidly within $1$ Gyr, with the MBH evacuating gas from the central few kpc of the galaxy and driving it to the outskirts of the halo. Our results indicate the possibility of substantial star formation suppression via MBH feedback within dwarf galaxies in the field. On the other hand, the apparent over-quenching of dwarf galaxies due to MBH suggests higher resolution and/or better modeling is required for MBHs in dwarfs, and quenched fractions offer the opportunity to constrain current models., Comment: 23 pages, 11 figures; submitted to ApJ

The formation history of globular clusters (GCs) at redshift $z > 4$ remains an unsolved problem. In this work, we use the cosmological, $N$-body hydrodynamical ``zoom-in'' simulation GigaEris to study the properties and formation of proto-GC candidates in the region surrounding the progenitor of a Milky Way-sized galaxy. The simulation employs a modern implementation of smoothed-particle hydrodynamics, including metal-line cooling and metal and thermal diffusion and allows to resolve systems at the scale of star clusters. We define proto-GC candidate systems as gravitationally bound stellar systems with baryonic mass fraction $F_{\rm b} \geq 0.75$ and stellar velocity dispersion $\sigma_{\star} < 20$ km s$^{-1}$. At $z=4.4$ we identify 9 systems which satisfy our criteria, all of which form between 10 kpc to 30 kpc from the centre of the main host. Their baryonic masses are in the range $10^5$- $10^7$ M$_{\odot}$. By the end of the simulation, they still have a relatively low stellar mass ($M_{\star} \sim 10^4$--$10^5$ M$_{\odot}$) and a metallicity ($-1.8 \lesssim {\rm [Fe/H]} \lesssim -0.8$) similar to the blue Galactic GCs. All of the identified systems except one appear to be associated with gas filaments accreting onto the main galaxy in the circum-galactic region, and formed at $z=5-4$. The exception is the oldest object, which appears to be a stripped compact dwarf galaxy that has interacted with the main halo between $z = 5.8$ and $z=5.2$ and has lost its entire dark matter content due to tidal mass loss., Comment: 11 pages, 8 figures, accepted for publication in MNRAS

We explore the characteristics of actively accreting MBHs within dwarf galaxies in the \textsc{Romulus25} cosmological hydrodynamic simulation. We examine the MBH occupation fraction, x-ray active fractions, and AGN scaling relations within dwarf galaxies of stellar mass $10^{8} < M_{\rm star} < 10^{10} M_\odot$ out to redshift $z=2$. In the local universe, the MBH occupation fraction is consistent with observed constraints, dropping below unity at $M_{\rm star} < 3\times10^{10} M_{\odot}$, $M_{\rm 200} < 3\times10^{11} M_\odot$. Local dwarf AGN in \textsc{Romulus25} follow observed scaling relations between AGN x-ray luminosity, stellar mass, and star formation rate, though they exhibit slightly higher active fractions and number densities than comparable x-ray observations. Since $z=2$, the MBH occupation fraction has decreased, the population of dwarf AGN has become overall less luminous, and as a result, the overall number density of dwarf AGN has diminished. We predict the existence of a large population of MBHs in the local universe with low x-ray luminosities and high contamination from x-ray binaries and the hot interstellar medium that are undetectable by current x-ray surveys. These hidden MBHs make up $76\%$ of all MBHs in local dwarf galaxies, and include many MBHs that are undermassive relative to their host galaxy's stellar mass. Their detection relies not only on greater instrument sensitivity but on better modeling of x-ray contaminants or multi-wavelength surveys. Our results indicate dwarf AGN were substantially more active in the past despite being low-luminosity today, and indicate future deep x-ray surveys may uncover many hidden MBHs in dwarf galaxies out to at least $z=2$.

Contrary to many stereotypes about massive galaxies, observed brightest group galaxies (BGGs) are diverse in their star formation rates, kinematic properties, and morphologies. Studying how they evolve into and express such diverse characteristics is an important piece of the galaxy formation puzzle. We use a high-resolution cosmological suite of simulations Romulus and compare simulated central galaxies in group-scale halos at $z=0$ to observed BGGs. The comparison encompasses the stellar mass-halo mass relation, various kinematic properties and scaling relations, morphologies, and the star formation rates. Generally, we find that Romulus reproduces the full spectrum of diversity in the properties of the BGGs very well, albeit with a tendency toward lower than the observed fraction of quenched BGGs. We find both early-type S0 and elliptical galaxies as well as late-type disk galaxies; we find Romulus galaxies that are fast-rotators as well as slow-rotators; and we observe galaxies transforming from late-type to early-type following strong dynamical interactions with satellites. We also carry out case studies of selected Romulus galaxies to explore the link between their properties, and the recent evolution of the stellar system as well as the surrounding intragroup/circumgalactic medium. In general, mergers/strong interactions quench star-forming activity and disrupt the stellar disk structure. Sometimes, however, such interactions can also trigger star-formation and galaxy rejuvenation. Black hole feedback can also lead to a decline of the star formation rate but by itself, it does not typically lead to complete quenching of the star formation activity in the BGGs., Comment: 27 pages, 18 figures, Accepted to MNRAS

We use hydrodynamical simulations of two Milky Way-mass galaxies to demonstrate the impact of cosmic-ray pressure on the kinematics of cool and warm circumgalactic gas. Consistent with previous studies, we find that cosmic-ray pressure can dominate over thermal pressure in the inner 50 kpc of the circumgalactic medium (CGM), creating an overall cooler CGM than that of similar galaxy simulations run without cosmic rays. We generate synthetic sightlines of the simulated galaxies' CGM and use Voigt profile fitting methods to extract ion column densities, Doppler-b parameters, and velocity centroids of individual absorbers. We directly compare these synthetic spectral line fits with HST/COS CGM absorption-line data analyses, which tend to show that metallic species with a wide range of ionization potential energies are often kinematically aligned. Compared to the Milky-Way simulation run without cosmic rays, the presence of cosmic-ray pressure in the inner CGM creates narrower OVI absorption features and broader SiIII absorption features, a quality which is more consistent with observational data. Additionally, because the cool gas is buoyant due to nonthermal cosmic-ray pressure support, the velocity centroids of both cool and warm gas tend to align in the simulated Milky Way with feedback from cosmic rays. Our study demonstrates that detailed, direct comparisons between simulations and observations, focused on gas kinematics, have the potential to reveal the dominant physical mechanisms that shape the CGM., Comment: 26 pages including 19 figures and appendix. Accepted by ApJ. Updated version includes additional figures, tests, and expanded discussion. Supplemental figures at irynabutsky.me/data

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 develop a hybrid model of galactic chemical evolution that combines a multi-ring computation of chemical enrichment with a prescription for stellar migration and the vertical distribution of stellar populations informed by a cosmological hydrodynamic disc galaxy simulation. Our fiducial model adopts empirically motivated forms of the star formation law and star formation history, with a gradient in outflow mass loading tuned to reproduce the observed metallicity gradient. With this approach, the model reproduces many of the striking qualitative features of the Milky Way disc's abundance structure: (i) the dependence of the [O/Fe]-[Fe/H] distribution on radius $R_\text{gal}$ and midplane distance $|z|$; (ii) the changing shapes of the [O/H] and [Fe/H] distributions with $R_\text{gal}$ and $|z|$; (iii) a broad distribution of [O/Fe] at sub-solar metallicity and changes in the [O/Fe] distribution with $R_\text{gal}$, $|z|$, and [Fe/H]; (iv) a tight correlation between [O/Fe] and stellar age for [O/Fe] $>$ 0.1; (v) a population of young and intermediate-age $\alpha$-enhanced stars caused by migration-induced variability in the Type Ia supernova rate; (vi) non-monotonic age-[O/H] and age-[Fe/H] relations, with large scatter and a median age of $\sim$4 Gyr near solar metallicity. Observationally motivated models with an enhanced star formation rate $\sim$2 Gyr ago improve agreement with the observed age-[Fe/H] and age-[O/H] relations, but worsen agreement with the observed age-[O/Fe] relation. None of our models predict an [O/Fe] distribution with the distinct bimodality seen in the observations, suggesting that more dramatic evolutionary pathways are required. All code and tables used for our models are publicly available through the Versatile Integrator for Chemical Evolution (VICE; https://pypi.org/project/vice)., Comment: 24 pages; 18 figures; to be submitted to MNRAS; comments welcome

In circumstellar discs, collisional grinding of planetesimals produces second-generation dust. While it remains unclear whether this ever becomes a major component of the total dust content, the presence of such dust, and potentially the substructure within, it can be used to explore a disc's physical conditions. A perturbing planet produces nonaxisymmetric structures and gaps in the dust, regardless of its origin. The dynamics of planetesimals, however, will be very different than that of small dust grains due to weaker gas interactions. Therefore, planetesimal collisions could create dusty disc structures that would not exist otherwise. In this work, we use N-body simulations to investigate the collision rate profile of planetesimals near mean-motion resonances. We find that a distinct bump or dip feature is produced in the collision profile, the presence of which depends on the libration width of the resonance and the separation between the peri- and apocenter distances of the edges of the resonance. The presence of one of these two features depends on the mass and eccentricity of the planet. Assuming that the radial dust emission traces the planetesimal collision profile, the presence of a bump or dip feature in the dust emission at the 2:1 mean-motion resonance can constrain the orbital properties of the perturbing planet. This assumption is valid, so long as radial drift does not play a significant role during the collisional cascade process. Under this assumption, these features in the dust emission should be marginally observable in nearby protoplanetary disks with ALMA., Comment: 17 pages, 12 figures, accepted for publication in MNRAS

Massive black holes often exist within dwarf galaxies, and both simulations and observations have shown that a substantial fraction of these may be off-center with respect to their hosts. We trace the evolution of off-center massive black holes (MBHs) in dwarf galaxies using cosmological hydrodynamical simulations, and show that the reason for off-center locations is mainly due to galaxy-galaxy mergers. We calculate dynamical timescales and show that off-center MBHs are unlikely to sink to their galaxys' centers within a Hubble time, due to the shape of the hosts' potential wells and low stellar densities. These wandering MBHs are unlikely to be detected electromagnetically, nor is there a measurable dynamical effect on the galaxy's stellar population. We conclude that off-center MBHs may be common in dwarfs, especially if the mass of the MBH is small or the stellar mass of the host galaxy is large. However detecting them is extremely challenging, because their accretion luminosities are very low and they do not measurably alter the dynamics of their host galaxies., Comment: updated to reflect MNRAS accepted version. Includes carbon footprint estimate due to supercomputer use

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

Large reservoirs of cold (~ 10^4 K) gas exist out to and beyond the virial radius in the circumgalactic medium (CGM) of all types of galaxies. Photoionization modeling suggests that cold CGM gas has significantly lower densities than expected by theoretical predictions based on thermal pressure equilibrium with hot CGM gas. In this work, we investigate the impact of cosmic ray physics on the formation of cold gas via thermal instability. We use idealized three-dimensional magnetohydrodynamic simulations to follow the evolution of thermally unstable gas in a gravitationally stratified medium. We find that cosmic ray pressure lowers the density and increases the size of cold gas clouds formed through thermal instability. We develop a simple model for how the cold cloud sizes and the relative densities of cold and hot gas depend on cosmic ray pressure. Cosmic ray pressure can help counteract gravity to keep cold gas in the CGM for longer, thereby increasing the predicted cold mass fraction and decreasing the predicted cold gas inflow rates. Efficient cosmic ray transport, by streaming or diffusion, redistributes cosmic ray pressure from the cold gas to the background medium, resulting in cold gas properties that are in-between those predicted by simulations with inefficient transport and simulations without cosmic rays. We show that cosmic rays can significantly reduce galactic accretion rates and resolve the tension between theoretical models and observational constraints on the properties of cold CGM gas., Comment: Accepted for publication in ApJ; 28 pages, 15 figures. Simulation movies can be found at https://www.irynabutsky.me/movies

The orbital decay of a perturber within a larger system plays a key role in the dynamics of many astrophysical systems -- from nuclear star clusters or globular clusters in galaxies, to massive black holes in galactic nuclei, to dwarf galaxy satellites within the dark matter halos of more massive galaxies. For many decades, there have been various attempts to determine the underlying physics and time-scales of the drag mechanism, ranging from the local dynamical friction approach to descriptions based on the back-reaction of global modes induced in the background system. We present ultra-high-resolution $N$-body simulations of massive satellites orbiting a Milky Way-like galaxy (with $> 10^8$ particles), that appear to capture both the local "wake" and the global "mode" induced in the primary halo. We address directly the mechanism of orbital decay from the combined action of local and global perturbations and specifically analyze where the bulk of the torque originates., Comment: 18 pages, 11 figures, 3 tables, accepted by ApJ

Kinematic studies of disk galaxies, using individual stars in the Milky Way or statistical studies of global disk kinematics over time, provide insight into how disks form and evolve. We use a high-resolution, cosmological zoom-simulation of a Milky Way-mass disk galaxy h277 to tie together local disk kinematics and the evolution of the disk over time. The present-day stellar age-velocity relationship (AVR) of h277 is nearly identical to that of the analogous solar-neighborhood measurement in the Milky Way. A crucial element of this success is the simulation's dynamically cold multi-phase ISM, which allows young stars to form with a low velocity dispersion ($\sigma_{\mathrm{birth}}$$\sim 6 - 8\ \mathrm{km}\, \mathrm{s}^{-1}$) at late times. Older stars are born kinematically hotter (i.e., the disk settles over time in an "upside-down" formation scenario), and are subsequently heated after birth. The disk also grows "inside-out", and many of the older stars in the solar neighborhood at $z=0$ are present because of radial mixing. We demonstrate that the evolution of $\sigma_{\mathrm{birth}}$ in h277 can be explained by the same model used to describe the general decrease in velocity dispersion observed in disk galaxies from $z\sim 2-3$ to the present-day, in which the disk evolves in quasi-stable equilibrium and the ISM velocity dispersion decreases over time due to a decreasing gas fraction. Thus, our results tie together local observations of the Milky Way's AVR with observed kinematics of high $z$ disk galaxies., Comment: Submitting to MNRAS. Comments welcomed. This simulation is publicly available. Contact Alyson Brooks for details

We use the \textsc{Romulus25} cosmological simulation volume to identify the largest-ever simulated sample of {\it field} ultra-diffuse galaxies (UDGs). At $z=0$, we find that isolated UDGs have average star formation rates, colors, and virial masses for their stellar masses and environment. UDGs have moderately elevated HI masses, being 70\% (300\%) more HI-rich than typical isolated dwarf galaxies at luminosities brighter (fainter) than M$_\mathrm{B}$=-14. However, UDGs are consistent with the general isolated dwarf galaxy population and make up $\sim$20\% of all field galaxies with 10$^7$

Using cosmological hydrodynamic simulations with physically motivated models of super-massive black hole (SMBH) formation and growth, we compare the assembly of Milky Way-mass ( $M_{\mathrm{vir}} \approx 7 \times 10^{11}$ $M_{\odot}$ at $z = 0$) galaxies in cold dark matter (CDM) and self-interacting dark matter (SIDM) models. Our SIDM model adopts a constant cross-section of 1 cm$^2$/g. We find that SMBH formation is suppressed in the early universe due to SIDM coring. SMBH-SMBH mergers are also suppressed in SIDM as a consequence of the lower number of SMBHs formed. Lack of initial merger-driven SMBH growth in turn delays SMBH growth by billions of years in SIDM compared to CDM. Further, we find that this delayed growth suppresses SMBH accretion in the largest progenitors of the main SIDM galaxies during the first 5 Gyrs of their evolution. Nonetheless, by $z = 0.8$ the CDM and SIDM SMBH masses differ only by around 0.2 dex, so that both remain compatible with the $M_{BH}-M_{*}$ relation. We show that the reduced accretion causes the SIDM SMBHs to less aggressively regulate star formation in their host galaxies than their CDM counterparts, resulting in a factor of 3 or more stars being produced over the lifetime of the SIDM galaxies compared to the CDM galaxies. Our results highlight a new way in which SIDM can affect the growth and merger history of SMBHs and ultimately give rise to very different galaxy evolution compared to the classic CDM model.

We report on high-resolution simulations that explore the orbital decay of massive black hole (MBH) pairs with masses between $10^5$ and $10^7 M_{\odot}$ embedded in a circumnuclear gas disk (CND). An adiabatic equation of state is adopted, with a range of adiabatic indices, which maintains a smooth flow. Mergers between MBHs in this mass range would be detectable by the upcoming Laser Inteferometer Space Antenna (LISA). The orbital evolution is followed from the CND scale ($100$~pc) down to separations of $0.1$--$0.01$~pc at which a circumbinary disk (CBD) could form. The decay is erratic and strongly dependent on the gas flow within the disk, that ultimately determines the net torques experienced by the sinking MBH. Overall, we can identify three different evolutionary stages: (i) an initially slow decay that leads to no significant change in the orbital angular momentum, resulting in some circularization; (ii) a fast migration phase in which the orbital angular momentum decreases rapidly; and (iii) a final, very slow decay phase, in which orbital angular momentum can even increase, and a CBD can form. The fast migration phase owes to disk-driven torques originating primarily from the co-orbital region of the secondary MBH, at a distance of 1--3 Hill radii. We find strong analogies with fast Type III migration for massive planets in protoplanetary disks. The CBD forms only when the decay rate becomes small enough to allow it enough time to carve a cavity around the primary MBH, at scales $\lesssim 1$~pc; when this happens, the MBH separation nearly stalls in our higher-resolution run. We suggest an empirically modified gap-opening criterion that takes into account such timescale effects as well as other deviations from standard assumptions made in the literature. [Abriged], Comment: 28 pages, 16 figures, 1 table, submitted to ApJ

The evolution of degenerate matter out of equilibrium is a topic of interest in fields such as condensed matter, nuclear and atomic physics, and increasingly cosmology, including inflaton physics prior to reheating. This follow-up paper extends the recent paper on the super-de Broglie dynamics of pure condensates of non-relativistic identical particles subject to non-local two-body interactions to the dynamics of mixed states. It is found that the two-body correlation function plays an increasingly dynamical role in these systems, driving the development of condensates and distributed phases alike. Examples of distribution and correlation evolution are presented, including instances of collapse, bound and unbound states, and phonons in the bulk. Potential applications are also discussed., Comment: 6 pages, 3 figures

As part of the AGORA High-resolution Galaxy Simulations Comparison Project (Kim et al. 2014, 2016) we have generated a suite of isolated Milky Way-mass galaxy simulations using 9 state-of-the-art gravito-hydrodynamics codes widely used in the numerical galaxy formation community. In these simulations we adopted identical galactic disk initial conditions, and common physics models (e.g., radiative cooling and ultraviolet background by a standardized package). Subgrid physics models such as Jeans pressure floor, star formation, supernova feedback energy, and metal production were carefully constrained. Here we release the simulation data to be freely used by the community. In this release we include the disk snapshots at 0 and 500Myr of evolution per each code as used in Kim et al. (2016), from simulations with and without star formation and feedback. We encourage any member of the numerical galaxy formation community to make use of these resources for their research - for example, compare their own simulations with the AGORA galaxies, with the common analysis yt scripts used to obtain the plots shown in our papers, also available in this release., Comment: AGORA links: http://www.AGORAsimulations.org/ and http://sites.google.com/site/santacruzcomparisonproject/blogs/quicklinks/

A fraction of dwarf galaxies in the Virgo cluster contain disk features like bars and spiral arms. Using $N$-body simulations, we investigate the effects of tidal forces on the formation of such disk features in disk dwarf galaxies resembling VCC856. We consider 8 Cluster-Galaxy models in which disk dwarf galaxies with differing pericenter distance and spin orientation experience the tidal gravitational force of a Virgo-like NFW halo, and additional 8 Galaxy-Galaxy models in which two dwarf galaxies undergo tidal interactions with different strength. We find that the cluster tidal effect is moderate due to the small galaxy size, making the bars form earlier by $\sim1$--$1.5\Gyr$ compared to the cases in isolation. While the galactic halos significantly lose their mass within the virial radius due to the cluster tidal force, the mass of the stellar disks is nearly unchanged, suggesting that the inner regions of a disk-halo system is secured from the tidal force. The tidal forcing from either the cluster potential or a companion galaxy triggers the formation of two-armed spirals at early time before a bar develops. The tidally-driven arms decay and wind with time, suggesting that they are kinematic density waves. In terms of the strength and pitch angle, the faint arms in VCC856 are best matched with the arms in a marginally unstable galaxy produced by a distant tidal encounter with its neighbor $\sim0.85\Gyr$ ago., Comment: Accepted for publication in the ApJ

We study the origins of 122 ultra-diffuse galaxies (UDGs) in the {\sc RomulusC} zoom-in cosmological simulation of a galaxy cluster (M$_{200} = 1.15\times10^{14}$ M$_{\odot}$), one of the only such simulations capable of resolving the evolution and structure of dwarf galaxies (M$_{\star} < 10^9$ M$_{\odot}$). We find broad agreement with observed cluster UDGs and predict that they are not separate from the overall cluster dwarf population. UDGs in cluster environments form primarily from dwarf galaxies that experienced early cluster in-fall and subsequent quenching due to ram pressure. The ensuing dimming of these dwarf galaxies due to passive stellar evolution results in a population of very low surface brightness galaxies that are otherwise typical dwarfs. UDGs and non-UDGs alike are affected by tidal interactions with the cluster potential. Tidal stripping of dark matter, as well as mass loss from stellar evolution, results in the adiabatic expansion of stars, particularly in the lowest mass dwarfs. High mass dwarf galaxies show signatures of tidal heating while low mass dwarfs that survive until $z=0$ typically have not experienced such impulsive interactions. There is little difference between UDGs and non-UDGs in terms of their dark matter halos, stellar morphology, colors, and location within the cluster. In most respects cluster UDG and non-UDGs alike are similar to isolated dwarf galaxies, except for the fact that they are typically quenched., Comment: 26 Pages, 22 Figures. Accepted for publication in MNRAS

We describe a software package called VPLanet that simulates fundamental aspects of planetary system evolution over Gyr timescales, with a focus on investigating habitable worlds. In this initial release, eleven physics modules are included that model internal, atmospheric, rotational, orbital, stellar, and galactic processes. Many of these modules can be coupled simultaneously to simulate the evolution of terrestrial planets, gaseous planets, and stars. The code is validated by reproducing a selection of observations and past results. VPLanet is written in C and designed so that the user can choose the physics modules to apply to an individual object at runtime without recompiling, i.e., a single executable can simulate the diverse phenomena that are relevant to a wide range of planetary and stellar systems. This feature is enabled by matrices and vectors of function pointers that are dynamically allocated and populated based on user input. The speed and modularity of VPLanet enables large parameter sweeps and the versatility to add/remove physical phenomena to assess their importance. VPLanet is publicly available from a repository that contains extensive documentation, numerous examples, Python scripts for plotting and data management, and infrastructure for community input and future development., Comment: 75 pages, 34 figures, 10 tables, accepted to the Proceedings of the Astronomical Society of the Pacific. Source code, documentation, and examples available at https://github.com/VirtualPlanetaryLaboratory/vplanet

The first paper in this series showed that QCD axion dark matter, as a highly-correlated Bose fluid, contains extra-classical physics on cosmological scales. The nature of the derived extra-classical physics is exchange-correlation interactions induced by the constraints of symmetric particle exchange and inter-axion correlations from self-gravitation. The paper also showed that the impact of extra-classical physics on early structure formation is marginal, as the exchange-correlation interaction is inherently non-linear. This paper continues the study of axion structure formation into the non-linear regime by now considering the case of full collapse and virialization. The N-body method is chosen to solve the collapse problem, and its algorithms are derived for a condensed Bose fluid. Simulations of isolated gravitational collapse are performed for both Bose and cold dark matter fluids using a prototype N-body code. Unique Bose structures are found to survive even the most violent collapses. Bose post-collapse features include dynamical changes to global structures, creation of new broad substructures, violations of classical binding energy conditions, and new fine structures. Effective models of the novel structures are constructed and possibilities for their discovery are discussed., Comment: 29 pages, 27 figures

Quasar absorption-line studies in the ultraviolet (UV) can uniquely probe the nature of the multiphase cool-warm (10^4 < T < 10^6 K) gas in and around galaxy clusters, promising to provide unprecedented insights into 1) interactions between the circumgalactic medium (CGM) associated with infalling galaxies and the hot (T > 10^6 K) X-ray emitting intracluster medium (ICM), 2) the stripping of metal-rich gas from the CGM, and 3) a multiphase structure of the ICM with a wide range of temperatures and metallicities. In this work, we present results from a high-resolution simulation of a ~10^14 solar mass galaxy cluster to study the physical properties and observable signatures of this cool-warm gas in galaxy clusters. We show that the ICM becomes increasingly multiphased at large radii, with the cool-warm gas becoming dominant in cluster outskirts. The diffuse cool-warm gas also exhibits a wider range of metallicity than the hot X-ray emitting gas. We make predictions for the covering fractions of key absorption-line tracers, both in the ICM and in the CGM of cluster galaxies, typically observed with the Cosmic Origins Spectrograph aboard the Hubble Space Telescope (HST). We further extract synthetic spectra to demonstrate the feasibility of detecting and characterizing the thermal, kinematic, and chemical composition of the cool-warm gas using H I, O VI, and C IV lines, and we predict an enhanced population of broad Ly-alpha absorbers tracing the warm gas. Lastly, we discuss future prospects of probing the multiphase structure of the ICM beyond HST., Comment: 15 pages, 12 figures

The disc structure of the Milky Way is marked by a chemical dichotomy, with high-alpha and low-alpha abundance sequences, traditionally identified with the geometric thick and thin discs. This identification is aided by the old ages of the high-alpha stars, and lower average ages of the low-alpha ones. Recent large scale surveys such as APOGEE have provided a wealth of data on this chemical structure, including showing that an identification of chemical and geometric thick discs is not exact, but the origin of the chemical dichotomy has remained unclear. Here we demonstrate that a dichotomy arises naturally if the early gas-rich disc fragments, leading to some fraction of the star formation occurring in clumps of the type observed in high-redshift galaxies. These clumps have high star formation rate density. They, therefore, enrich rapidly, moving from the low-alpha to the high-alpha sequence, while more distributed star formation produces the low-alpha sequence. We demonstrate that this model produces a chemically-defined thick disc that has many of the properties of the Milky Way's thick disc. Because clump formation is common in high redshift galaxies, we predict that chemical bimodalities are common in massive galaxies., Comment: Accepted to MNRAS

Compact ellipticals (cEs) are outliers from the scaling relations of early-type galaxies, particularly the mass-metallicity relation which is an important outcome of feedback. The formation of such low-mass, but metal-rich and compact, objects is a long-standing puzzle. Using a pair of high-resolution N-body+gas simulations, we investigate the evolution of a gas-rich low-mass galaxy on a highly radial orbit around a massive host galaxy. As the infalling low-mass galaxy passes through the host's corona at supersonic speeds, its diffuse gas outskirts are stripped by ram pressure, as expected. However, the compactness increases rapidly because of bursty star formation in the gas tidally driven to the centre. The metal-rich gas produced by supernovae and stellar winds is confined by the ram pressure from the surrounding environment, leading to subsequent generations of stars being more metal-rich. After the gas is depleted, tidal interactions enhance the metallicity further via the stripping of weakly bound, old, and metal-poor stars, while the size of the satellite is changed only modestly. The outcome is a metal-rich cE that is consistent with observations. These results argue that classical cEs are neither the stripped remnants of much more massive galaxies nor the merger remnants of normal dwarfs. We present observable predictions that can be used to test our model., Comment: 14 pages, 11 figures; accepted by ApJ. Minor change

Cosmological simulations are reaching the resolution necessary to study ultra-faint dwarf galaxies. Observations indicate that in small populations, the stellar initial mass function (IMF) is not fully populated; rather, stars are sampled in a way that can be approximated as coming from an underlying probability density function. To ensure the accuracy of cosmological simulations in the ultra-faint regime, we present an improved treatment of the IMF. We implement a self-consistent, stochastically populated IMF in cosmological hydrodynamic simulations. We test our method using high-resolution simulations of a Milky Way halo, run to $z=6$, yielding a sample of nearly 100 galaxies. We also use an isolated dwarf galaxy to investigate the resulting systematic differences in galaxy properties. We find that a stochastic IMF in simulations makes feedback burstier, strengthening feedback, and quenching star formation earlier in small dwarf galaxies. For galaxies in halos with mass $\lesssim10^{8.5}$ M$_\odot$, a stochastic IMF typically leads to lower stellar mass compared to a continuous IMF, sometimes by more than an order of magnitude. We show that existing methods of ensuring discrete supernovae incorrectly determine the mass of the star particle and its associated feedback. This leads to overcooling of surrounding gas, with at least ${\sim}$10 per cent higher star formation and ${\sim}$30 per cent higher cold gas content. Going forward, to accurately model dwarf galaxies and compare to observations, it will be necessary to incorporate a stochastically populated IMF that samples the full spectrum of stellar masses., Comment: 14 pages, 11 figures, Updated to reflect accepted version, MNRAS

The existence of ultra-faint dwarf (UFD) galaxies highlights the need to push our theoretical understanding of galaxies to extremely low mass. We examine the formation of UFDs by twice running a fully cosmological simulations of dwarf galaxies, but varying star formation. One run uses a temperature-density threshold for star formation, while the other uses an H$_{2}$-based sub-grid star formation model. The total number of dwarf galaxies that forms is different by a factor of 2 between the two runs, but most of these are satellites, leading to a factor of 5 difference in the number of luminous UFD companions around more massive, isolated dwarfs. The first run yields a 47\% chance of finding a satellite around a M$_{halo}$ $\sim 10^{10}$ M$_{\odot}$ host, while the H$_2$ run predicts only a 16\% chance. Metallicity is the primary physical parameter that creates this difference. As metallicity decreases, the formation of H$_2$ is slowed and relegated to higher-density material. Thus, our H$_2$ run is unable to form many (and often, any) stars before reionization removes gas. These results emphasize that predictions for UFD properties made using hydrodynamic simulations, in particular regarding the frequency of satellites around dwarf galaxies, the slope of the stellar mass function at low masses, as well as the properties of ultra-faint galaxies occupying the smallest halos, are extremely sensitive to the subgrid physics of star formation contained within the simulation. However, upcoming discoveries of ultra-faint dwarfs will provide invaluable constraining power on the physics of the first star formation., Comment: 18 pages, 7 Figures. Accepted version, ApJ

Axions as dark matter is an increasingly important subject in astrophysics and cosmology. Experimental and observational searches are mounting across the mass spectrum of axion-like particles, many of which require detailed knowledge of axion structure over a wide range of scales. Current understanding of axion structures is far from complete, however, due largely to controversy in modeling the candidate's highly-degenerate state. The series Axion Structure Formation seeks to develop a consistent model of QCD axion dark matter dynamics that follows their highly-degenerate nature to the present using novel modeling techniques and sophisticated simulations. This inaugural paper presents the problem of describing many non-relativistic axions with minimal degrees of freedom and constructs a theory of axion infall for the limit of complete condensation. The derived model is shown to contain axion-specific dynamics not unlike the exchange-correlation influences experienced by identical fermions. Perturbative calculations are performed to explore the potential for imprints in early universe structures., Comment: 15 pages, 4 figures, submitted to MNRAS

Systems of identical particles possessing non-local interactions are capable of exhibiting extra-classical properties beyond the characteristic quantum length scales. This letter derives the dynamics of such systems in the non-relativistic and degenerate limit, showing the effect of exchange symmetry and correlations on structure both in and out of equilibrium. Such descriptions may be crucial to understanding systems ranging from nuclei to dark matter. Appropriate limits for restoring the mean-field description are also discussed., Comment: 4 pages, 1 figure, submitted to PRD Rapid Communications

We present the first results from RomulusC, the highest resolution cosmological hydrodynamic simulation of a galaxy cluster run to date. RomulusC, a zoom-in simulation of a halo with $z=0$ mass $10^{14}$ M$_{\odot}$, is run with the same sub-grid physics and resolution as Romulus25 (Tremmel et al. 2017). With unprecedented mass and spatial resolution, RomulusC represents a unique opportunity to study the evolution of galaxies in dense environments down to dwarf masses. We demonstrate that RomulusC results in an intracluster medium (ICM) consistent with observations. The star formation history and stellar mass of the brightest cluster galaxy (BCG) is consistent with observations and abundance matching results, indicating that our sub-grid models, optimized only to reproduce observations of field dwarf and Milky Way mass galaxies, are able to produce reasonable galaxy masses and star formation histories in much higher mass systems. Feedback from supermassive black holes (SMBHs) regulates star formation by driving large-scale, collimated outflows that coexist with a low entropy core. We find that non-BCG cluster member galaxies are substantially quenched compared to the field down to dwarf galaxy masses and, at low masses, quenching is seen to have no dependence on mass or distance from the cluster center. This enhanced quenched population extends beyond $R_{200}$ and is in place at high redshift. Similarly, we predict that SMBH activity is significantly suppressed within clusters outside of the BCG, but show how the effect could be lost when only focusing on the brightest AGN in the most massive galaxies., Comment: Submitted to MNRAS, 29 pages, 19 figures, Accepted to MNRAS

Using an energy balance model with ice sheets, we examine the climate response of an Earth-like planet orbiting a G dwarf star and experiencing large orbital and obliquity variations. We find that ice caps couple strongly to the orbital forcing, leading to extreme ice ages. In contrast with previous studies, we find that such exo-Milankovitch cycles tend to impair habitability by inducing snowball states within the habitable zone. The large amplitude changes in obliquity and eccentricity cause the ice edge, the lowest latitude extent of the ice caps, to become unstable and grow to the equator. We apply an analytical theory of the ice edge latitude to show that obliquity is the primary driver of the instability. The thermal inertia of the ice sheets and the spectral energy distribution of the G dwarf star increase the sensitivity of the model to triggering runaway glaciation. Finally, we apply a machine learning algorithm to demonstrate how this technique can be used to extend the power of climate models. This work illustrates the importance of orbital evolution for habitability in dynamically rich planetary systems. We emphasize that as potentially habitable planets are discovered around G dwarfs, we need to consider orbital dynamics., Comment: 37 pages, 26 figures, accepted at the Astronomical Journal

We outline a mechanism that explains the observed lack of circumbinary planets (CBPs) via coupled stellar-tidal evolution of isolated binary stars. Tidal forces between low-mass, short-period binary stars on the pre-main sequence slow the stellar rotations, transferring rotational angular momentum to the orbit as the stars approach the tidally locked state. This transfer increases the binary orbital period, expanding the region of dynamical instability around the binary, and destabilizing CBPs that tend to preferentially orbit just beyond the initial dynamical stability limit. After the stars tidally lock, we find that angular momentum loss due to magnetic braking can significantly shrink the binary orbit, and hence the region of dynamical stability, over time impacting where surviving CBPs are observed relative to the boundary. We perform simulations over a wide range of parameter space and find that the expansion of the instability region occurs for most plausible initial conditions and that in some cases, the stability semi-major axis doubles from its initial value. We examine the dynamical and observable consequences of a CBP falling within the dynamical instability limit by running N-body simulations of circumbinary planetary systems and find that typically, at least one planet is ejected from the system. We apply our theory to the shortest period {\it Kepler} binary that possesses a CBP, Kepler-47, and find that its existence is consistent with our model. Under conservative assumptions, we find that coupled stellar-tidal evolution of pre-main sequence binary stars removes at least one close-in CBP in $87\%$ of multi-planet circumbinary systems., Comment: 26 pages, 16 figures, accepted, ApJ

The majority of galactic baryons reside outside of the galactic disk in the diffuse gas known as the circumgalactic medium (CGM). While state-of-the art simulations excel at reproducing galactic disk properties, many struggle to drive strong galactic winds or to match the observed ionization structure of the CGM using only thermal supernova feedback. To remedy this, recent studies have invoked non-thermal cosmic ray (CR) stellar feedback prescriptions. However, numerical schemes of CR transport are still poorly constrained. We explore how the choice of CR transport affects the multiphase structure of the simulated CGM. We implement anisotropic CR physics in the astrophysical simulation code, {\sc Enzo} and simulate a suite of isolated disk galaxies with varying prescriptions for CR transport: isotropic diffusion, anisotropic diffusion, and streaming. We find that all three transport mechanisms result in strong, metal-rich outflows but differ in the temperature and ionization structure of their CGM. Isotropic diffusion results in a spatially uniform, warm CGM that underpredicts the column densities of low-ions. Anisotropic diffusion develops a reservoir of cool gas that extends further from the galactic center, but disperses rapidly with distance. CR streaming projects cool gas out to radii of 200 kpc, supporting a truly multiphase medium. In addition, we find that streaming is less sensitive to changes in constant parameter values like the CR injection fraction, transport velocity, and resolution than diffusion. We conclude that CR streaming is a more robust implementation of CR transport and motivate the need for detailed parameter studies of CR transport., Comment: 21 pages, 15 figures. Version 2: updated text, 1 new figure; matches manuscript version accepted to ApJ

We present a self-consistent prediction from a large-scale cosmological simulation for the population of `wandering' supermassive black holes (SMBHs) of mass greater than $10^6$ M$_{\odot}$ on long-lived, kpc-scale orbits within Milky Way (MW)-mass galaxies. We extract a sample of MW-mass halos from the Romulus25 cosmological simulation (Tremmel et al. 2017), which is uniquely able to capture the orbital evolution of SMBHs during and following galaxy mergers. We predict that such halos, regardless of recent merger history or morphology, host an average of $5.1 \pm 3.3$ SMBHs, including their central black hole, within 10 kpc from the galactic center and an average of $12.2 \pm 8.4$ SMBHs total within their virial radius, not counting those in satellite halos. Wandering SMBHs exist within their host galaxies for several Gyrs, often accreted by their host halo in the early Universe. We find, with $>4\sigma$ significance, that wandering SMBHs are preferentially found outside of galactic disks., Comment: 7 pages, 5 figures, accepted for publication in ApJ Letters