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Register a new userGas infall and radial transport in cosmological simulations of Milky Way-mass disks
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Observations indicate that a continuous supply of gas is needed to maintain observed star formation rates in large, disky galaxies. To fuel star formation, gas must reach the inner regions of such galaxies. Despite its crucial importance for galaxy evolution, how and where gas joins galaxies is poorly constrained observationally and is rarely explored in fully cosmological simulations. To investigate gas accretion in the vicinity of galaxies, we analyze the FIRE-2 cosmological zoom-in simulations for 4 Milky Way mass galaxies (M_halo ~ 10E12 solar masses), focusing on simulations with cosmic ray physics. We find that at z~0, gas approaches the disk with angular momentum similar to the gaseous disk edge and low radial velocities, piling-up near the edge and settling into full rotational support. Accreting gas moves predominantly parallel to the disk with small but nonzero vertical velocity components, and joins the disk largely in the outskirts as opposed to raining down onto the disk. Once in the disk, gas trajectories are complex, being dominated by spiral arm induced oscillations and feedback. However, time and azimuthal averages show clear but slow net radial infall with transport speeds of 1-3 km/s and net mass fluxes through the disk on the order of one solar mass per year, comparable to the star formation rates of the galaxies and decreasing towards galactic center as gas is sunk into star formation. These rates are slightly higher in simulations without cosmic rays (1-7 km/s, ~4-5 solar masses per year). We find overall consistency of our results with observational constraints and discuss prospects of future observations of gas flows in and around galaxies.
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We apply a semi-analytic galaxy formation model to two high resolution cosmological N-body simulations to investigate analogues of the Milky Way system. We select these according to observed properties of the Milky Way rather than by halo mass as in most previous work. For disk-dominated central galaxies with stellar mass (5--7) x 10d10Msun, the median host halo mass is 1.4 x 10d12Msun, with 1 sigma dispersion in the range [0.86, 3.1] x 10d12Msun, consistent with dynamical measurements of the Milky Way halo mass. For any given halo mass, the probability of hosting a Milky Way system is low, with a maximum of ~20% in haloes of mass ~10d12Msun. The model reproduces the V-band luminosity function and radial profile of the bright (MV < -9) Milky Way satellites. Galaxy formation in low mass haloes is found to be highly stochastic, resulting in an extremely large scatter in the relation between MV (or stellar mass) for satellites and the depth of the subhalo potential well in which they live, as measured by the maximum of the rotation curve, Vmax. We conclude that the too big to fail problem is an artifact of selecting satellites in N-body simulations according to subhalo properties: in 10% of cases we find that three or fewer of the brightest (or most massive) satellites have Vmax > 30 km/s. Our model predicts that around half of the dark matter subhaloes with Vmax > 20 km/s host satellites fainter than MV = -9 and so may be missing from existing surveys.
With Gaia Data Release 2, the astronomical community is entering a new era of multidimensional surveys of the Milky Way. This new phase-space view of our Galaxy demands new tools for comparing observations to simulations of Milky-Way-mass galaxies in a cosmological context, to test the physics of both dark matter and galaxy formation. We present ananke, a framework for generating synthetic phase-space surveys from high-resolution baryonic simulations, and use it to generate a suite of synthetic surveys resembling Gaia DR2 in data structure, magnitude limits, and observational errors. We use three cosmological simulations of Milky-Way-mass galaxies from the Latte suite of the Feedback In Realistic Environments (FIRE) project, which feature self-consistent clustering of star formation in dense molecular clouds and thin stellar/gaseous disks in live cosmological halos with satellite dwarf galaxies and stellar halos. We select three solar viewpoints from each simulation to generate nine synthetic Gaia-like surveys. We sample synthetic stars by assuming each star particle (of mass 7070 $M_{odot}$) represents a single stellar population. At each viewpoint, we compute dust extinction from the simulated gas metallicity distribution and apply a simple error model to produce a synthetic Gaia-like survey that includes both observational properties and a pointer to the generating star particle. We provide the complete simulation snapshot at $z = 0$ for each simulated galaxy. We describe data access points, the data model, and plans for future upgrades. These synthetic surveys provide a tool for the scientific community to test analysis methods and interpret Gaia data.
We present the first measurement of the lifetimes of Giant Molecular Clouds (GMCs) in cosmological simulations at $z = 0$, using the Latte suite of FIRE-2 simulations of Milky Way-mass galaxies. We track GMCs with total gas mass $gtrsim 10^5$ M$_odot$ at high spatial ($sim1$ pc), mass ($7100$ M$_{odot}$), and temporal (1 Myr) resolution. Our simulated GMCs are consistent with the distribution of masses for massive GMCs in the Milky Way and nearby galaxies. We find GMC lifetimes of $5-7$ Myr, or 1-2 freefall times, on average, with less than 2$%$ of clouds living longer than 20 Myr. We find decreasing GMC lifetimes with increasing virial parameter, and weakly increasing GMC lifetimes with galactocentric radius, implying that environment affects the evolutionary cycle of GMCs. However, our GMC lifetimes show no systematic dependence on GMC mass or amount of star formation. These results are broadly consistent with inferences from the literature and provide an initial investigation into ultimately understanding the physical processes that govern GMC lifetimes in a cosmological setting.
Stellar streams record the accretion history of their host galaxy. We present a set of simulated streams from disrupted dwarf galaxies in 13 cosmological simulations of Milky Way (MW)-mass galaxies from the FIRE-2 suite at $z=0$, including 7 isolated Milky Way-mass systems and 6 hosts resembling the MW-M31 pair (full dataset at: https://flathub.flatironinstitute.org/sapfire). In total, we identify 106 simulated stellar streams, with no significant differences in the number of streams and masses of their progenitors between the isolated and paired environments. We resolve simulated streams with stellar masses ranging from $sim 5times10^5$ up to $sim 10^{9} M_odot$, similar to the mass range between the Orphan and Sagittarius streams in the MW. We confirm that present-day simulated satellite galaxies are good proxies for stellar stream progenitors, with similar properties including their stellar mass function, velocity dispersion, [Fe/H] and [$alpha$/H] evolution tracks, and orbital distribution with respect to the galactic disk plane. Each progenitors lifetime is marked by several important timescales: its infall, star-formation quenching, and stream-formation times. We show that the ordering of these timescales is different between progenitors with stellar masses higher and lower than $sim 2times10^6 M_odot$. Finally, we show that the main factor controlling the rate of phase-mixing, and therefore fading, of tidal streams from satellite galaxies in MW-mass hosts is non-adiabatic evolution of the host potential. Other factors commonly used to predict phase-mixing timescales, such as progenitor mass and orbital circularity, show virtually no correlation with the number of dynamical times required for a stream to become phase-mixed.
We quantify the stellar abundances of neutron-rich r-process nuclei in cosmological zoom-in simulations of a Milky Way-mass galaxy from the Feedback In Realistic Environments project. The galaxy is enriched with r-process elements by binary neutron star (NS) mergers and with iron and other metals by supernovae. These calculations include key hydrodynamic mixing processes not present in standard semi-analytic chemical evolution models, such as galactic winds and hydrodynamic flows associated with structure formation. We explore a range of models for the rate and delay time of NS mergers, intended to roughly bracket the wide range of models consistent with current observational constraints. We show that NS mergers can produce [r-process/Fe] abundance ratios and scatter that appear reasonably consistent with observational constraints. At low metallicity, [Fe/H]<-2, we predict there is a wide range of stellar r-process abundance ratios, with both supersolar and subsolar abundances. Low-metallicity stars or stars that are outliers in their r-process abundance ratios are, on average, formed at high redshift and located at large galactocentric radius. Because NS mergers are rare, our results are not fully converged with respect to resolution, particularly at low metallicity. However, the uncertain rate and delay time distribution of NS mergers introduces an uncertainty in the r-process abundances comparable to that due to finite numerical resolution. Overall, our results are consistent with NS mergers being the source of most of the r-process nuclei in the Universe.
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