Under the Firelight: Stellar Tracers of the Local Dark Matter Velocity Distribution in the Milky Way

The Gaia era opens new possibilities for discovering the remnants of disrupted satellite galaxies in the Solar neighborhood. If the population of local accreted stars is correlated with the dark matter sourced by the same mergers, one can then map the dark matter distribution directly. Using two cosmological zoom-in hydrodynamic simulations of Milky Way-mass galaxies from the Latte suite of Fire-2 simulations, we find a strong correlation between the velocity distribution of stars and dark matter at the solar circle that were accreted from luminous satellites. This correspondence holds for dark matter that is either relaxed or in kinematic substructure called debris flow, and is consistent between two simulated hosts with different merger histories. The correspondence is more problematic for streams because of possible spatial offsets between the dark matter and stars. We demonstrate how to reconstruct the dark matter velocity distribution from the observed properties of the accreted stellar population by properly accounting for the ratio of stars to dark matter contributed by individual mergers. After demonstrating this method using the Fire-2 simulations, we apply it to the Milky Way and use it to recover the dark matter velocity distribution associated with the recently discovered stellar debris field in the Solar neighborhood. Based on results from Gaia, we estimate that $42 ^{+26}_{-22}\%$ of the local dark matter that is accreted from luminous mergers is in debris flow.Comment: 18+5 pages, 12+5 figures. Supplementary Data can be found here https://linoush.github.io/DM_Velocity_Distribution

Reconciling observed and simulated stellar halo masses

We use cosmological hydrodynamical simulations of Milky-Way-mass galaxies from the FIRE project to evaluate various strategies for estimating the mass of a galaxy's stellar halo from deep, integrated-light images. We find good agreement with integrated-light observations if we mimic observational methods to measure the mass of the stellar halo by selecting regions of an image via projected radius relative to the disk scale length or by their surface density in stellar mass . However, these observational methods systematically underestimate the accreted stellar component, defined in our (and most) simulations as the mass of stars formed outside of the host galaxy, by up to a factor of ten, since the accreted component is centrally concentrated and therefore substantially obscured by the galactic disk. Furthermore, these observational methods introduce spurious dependencies of the estimated accreted stellar component on the stellar mass and size of galaxies that can obscure the trends in accreted stellar mass predicted by cosmological simulations, since we find that in our simulations the size and shape of the central galaxy is not strongly correlated with the assembly history of the accreted stellar halo. This effect persists whether galaxies are viewed edge-on or face-on. We show that metallicity or color information may provide a way to more cleanly delineate in observations the regions dominated by accreted stars. Absent additional data, we caution that estimates of the mass of the accreted stellar component from single-band images alone should be taken as lower limits.Comment: Version accepted by Ap

Simulating galaxies in the reionization era with FIRE-2: morphologies and sizes

We study the morphologies and sizes of galaxies at z>5 using high-resolution cosmological zoom-in simulations from the Feedback In Realistic Environments project. The galaxies show a variety of morphologies, from compact to clumpy to irregular. The simulated galaxies have more extended morphologies and larger sizes when measured using rest-frame optical B-band light than rest-frame UV light; sizes measured from stellar mass surface density are even larger. The UV morphologies are usually dominated by several small, bright young stellar clumps that are not always associated with significant stellar mass. The B-band light traces stellar mass better than the UV, but it can also be biased by the bright clumps. At all redshifts, galaxy size correlates with stellar mass/luminosity with large scatter. The half-light radii range from 0.01 to 0.2 arcsec (0.05-1 kpc physical) at fixed magnitude. At z>5, the size of galaxies at fixed stellar mass/luminosity evolves as (1+z)^{-m}, with m~1-2. For galaxies less massive than M_star~10^8 M_sun, the ratio of the half-mass radius to the halo virial radius is ~10% and does not evolve significantly at z=5-10; this ratio is typically 1-5% for more massive galaxies. A galaxy's "observed" size decreases dramatically at shallower surface brightness limits. This effect may account for the extremely small sizes of z>5 galaxies measured in the Hubble Frontier Fields. We provide predictions for the cumulative light distribution as a function of surface brightness for typical galaxies at z=6.Comment: 11 pages, 11 figures, resubmitted to MNRAS after revision for referee's comment

Formation of Globular Cluster Candidates in Merging Proto-galaxies at High Redshift: A View from the FIRE Cosmological Simulations

Using a state-of-the-art cosmological simulation of merging proto-galaxies at high redshift from the FIRE project, with explicit treatments of star formation and stellar feedback in the interstellar medium, we investigate the formation of star clusters and examine one of the formation hypothesis of present-day metal-poor globular clusters. We find that frequent mergers in high-redshift proto-galaxies could provide a fertile environment to produce long-lasting bound star clusters. The violent merger event disturbs the gravitational potential and pushes a large gas mass of ~> 1e5-6 Msun collectively to high density, at which point it rapidly turns into stars before stellar feedback can stop star formation. The high dynamic range of the reported simulation is critical in realizing such dense star-forming clouds with a small dynamical timescale, t_ff <~ 3 Myr, shorter than most stellar feedback timescales. Our simulation then allows us to trace how clusters could become virialized and tightly-bound to survive for up to ~420 Myr till the end of the simulation. Because the cluster's tightly-bound core was formed in one short burst, and the nearby older stars originally grouped with the cluster tend to be preferentially removed, at the end of the simulation the cluster has a small age spread.Comment: 14 pages, 14 figures, Accepted for publication in the Monthly Notices of the Royal Astronomical Society, High-resolution version of this article also available at http://www.jihoonkim.org/index/research.html#g

Star formation histories of dwarf galaxies in the FIRE simulations: dependence on mass and Local Group environment

We study star formation histories (SFHs) of $\simeq500$ dwarf galaxies (stellar mass $M_\ast = 10^5 - 10^9\,M_\odot$) from FIRE-2 cosmological zoom-in simulations. We compare dwarfs around individual Milky Way (MW)-mass galaxies, dwarfs in Local Group (LG)-like environments, and true field (i.e. isolated) dwarf galaxies. We reproduce observed trends wherein higher-mass dwarfs quench later (if at all), regardless of environment. We also identify differences between the environments, both in terms of "satellite vs. central" and "LG vs. individual MWvs. isolated dwarf central." Around the individual MW-mass hosts, we recover the result expected from environmental quenching: central galaxies in the "near field" have more extended SFHs than their satellite counterparts, with the former more closely resemble isolated ("true field") dwarfs (though near-field centrals are still somewhat earlier forming). However, this difference is muted in the LG-like environments, where both near-field centrals and satellites have similar SFHs, which resemble satellites of single MW-mass hosts. This distinction is strongest for $M_\ast = 10^6 - 10^7\,M_\odot$ but exists at other masses. Our results suggest that the paired halo nature of the LG may regulate star formation in dwarf galaxies even beyond the virial radii of the MW and Andromeda. Caution is needed when comparing zoom-in simulations targeting isolated dwarf galaxies against observed dwarf galaxies in the LG.Comment: Main text: 11 pages, 8 figures; appendices: 4 pages, 4 figures. Submitted to MNRAS; comments welcom

A profile in FIRE: resolving the radial distributions of satellite galaxies in the Local Group with simulations

While many tensions between Local Group (LG) satellite galaxies and LCDM cosmology have been alleviated through recent cosmological simulations, the spatial distribution of satellites remains an important test of physical models and physical versus numerical disruption in simulations. Using the FIRE-2 cosmological zoom-in baryonic simulations, we examine the radial distributions of satellites with Mstar > 10^5 Msun around 8 isolated Milky Way- (MW) mass host galaxies and 4 hosts in LG-like pairs. We demonstrate that these simulations resolve the survival and physical destruction of satellites with Mstar >~ 10^5 Msun. The simulations broadly agree with LG observations, spanning the radial profiles around the MW and M31. This agreement does not depend strongly on satellite mass, even at distances <~ 100 kpc. Host-to-host variation dominates the scatter in satellite counts within 300 kpc of the hosts, while time variation dominates scatter within 50 kpc. More massive host galaxies within our sample have fewer satellites at small distances, likely because of enhanced tidal destruction of satellites via the baryonic disks of host galaxies. Furthermore, we quantify and provide fits to the tidal depletion of subhalos in baryonic relative to dark matter-only simulations as a function of distance. Our simulated profiles imply observational incompleteness in the LG even at Mstar >~ 10^5 Msun: we predict 2-10 such satellites to be discovered around the MW and possibly 6-9 around M31. To provide cosmological context, we compare our results with the radial profiles of satellites around MW analogs in the SAGA survey, finding that our simulations are broadly consistent with most SAGA systems.Comment: 18 pages, 10 figures, plus appendices. Main results in figures 2, 3, and 4. Accepted versio