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Register a new userLive Fast, Die Young: GMC lifetimes in the FIRE cosmological simulations of Milky Way-mass galaxies
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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.
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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.
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 examine the prevalence, longevity, and causes of planes of satellite dwarf galaxies, as observed in the Local Group. We use 14 Milky Way/Andromeda-(MW/M31) mass host galaxies from the FIRE-2 simulations. We select the 14 most massive satellites by stellar mass within 300 kpc of each host and correct for incompleteness from the foreground galactic disc when comparing to the MW. We find that MW-like planes as spatially thin and/or kinematically coherent as observed are uncommon, but they do exist in our simulations. Spatially thin planes occur in 1-2 per cent of snapshots during $z=0-0.2$, and kinematically coherent planes occur in 5 per cent of snapshots. These planes are generally transient, surviving for less than 500 Myr. However, if we select hosts with an LMC-like satellite near first pericentre, the fraction of snapshots with MW-like planes increases dramatically to 7-16 per cent, with lifetimes of 0.7-1 Gyr, likely because of group accretion of satellites. We find that M31s satellite distribution is much more common: M31s satellites lie within about 1 sigma of the simulation median for every plane metric we consider. We find no significant difference in average satellite planarity for isolated hosts versus hosts in LG-like pairs. Baryonic and dark matter-only simulations exhibit similar levels of planarity, even though baryonic subhaloes are less centrally concentrated within their host haloes. We conclude that planes of satellites are not a strong challenge to LCDM cosmology.
We introduce the ARTEMIS simulations, a new set of 42 zoomed-in, high-resolution (baryon particle mass of ~ 2x10^4 Msun/h), hydrodynamical simulations of galaxies residing in haloes of Milky Way mass, simulated with the EAGLE galaxy formation code with re-calibrated stellar feedback. In this study, we analyse the structure of stellar haloes, specifically the mass density, surface brightness, metallicity, colour and age radial profiles, finding generally very good agreement with recent observations of local galaxies. The stellar density profiles are well fitted by broken power laws, with inner slopes of ~ -3, outer slopes of ~ -4 and break radii that are typically ~ 20-40 kpc. The break radii generally mark the transition between in situ formation and accretion-driven formation of the halo. The metallicity, colour and age profiles show mild large-scale gradients, particularly when spherically-averaged or viewed along the major axes. Along the minor axes, however, the profiles are nearly flat, in agreement with observations. Overall, the structural properties can be understood by two factors: that in situ stars dominate the inner regions and that they reside in a spatially-flattened distribution that is aligned with the disc. Observations targeting both the major and minor axes of galaxies are thus required to obtain a complete picture of stellar haloes.
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.
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