7 research outputs found
The Formation Times and Building Blocks of Milky Way-mass Galaxies in the FIRE Simulations
Surveys of the Milky Way (MW) and M31 enable detailed studies of stellar
populations across ages and metallicities, with the goal of reconstructing
formation histories across cosmic time. These surveys motivate key questions
for galactic archaeology in a cosmological context: when did the main
progenitor of a MW/M31-mass galaxy form, and what were the galactic building
blocks that formed it? We investigate the formation times and progenitor
galaxies of MW/M31-mass galaxies using the FIRE-2 cosmological simulations,
including 6 isolated MW/M31-mass galaxies and 6 galaxies in Local Group
(LG)-like pairs at z = 0. We examine main progenitor "formation" based on two
metrics: (1) transition from primarily ex-situ to in-situ stellar mass growth
and (2) mass dominance compared to other progenitors. We find that the main
progenitor of a MW/M31-mass galaxy emerged typically at z ~ 3-4 (11.6-12.2 Gyr
ago), while stars in the bulge region (inner 2 kpc) at z = 0 formed primarily
in a single main progenitor at z < 5 (< 12.6 Gyr ago). Compared with isolated
hosts, the main progenitors of LG-like paired hosts emerged significantly
earlier (\Delta z ~ 2, \Delta t ~ 1.6 Gyr), with ~ 4x higher stellar mass at
all z > 4 (> 12.2 Gyr ago). This highlights the importance of environment in
MW/M31-mass galaxy formation, especially at early times. Overall, about 100
galaxies with M_star > 10^5 M_sun formed a typical MW/M31-mass system. Thus,
surviving satellites represent a highly incomplete census (by ~ 5x) of the
progenitor population.Comment: 20 pages, 8 figures, accepted for publication in MNRA
The origin of metal-poor stars on prograde disk orbits in FIRE simulations of Milky Way-mass galaxies
The formation times and building blocks of Milky Way-mass galaxies in the FIRE simulations
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The origin of metal-poor stars on prograde disk orbits in FIRE simulations of Milky Way-mass galaxies
Abstract
In hierarchical structure formation, metal-poor stars in and around the Milky Way (MW) originate primarily from mergers of lower-mass galaxies. A common expectation is therefore that metal-poor stars should have isotropic, dispersion-dominated orbits that do not correlate strongly with the MW disk. However, recent observations of stars in the MW show that metal-poor ([Fe/H]≲ −2) stars are preferentially on prograde orbits with respect to the disk. Using the FIRE-2 suite of cosmological zoom-in simulations of MW/M31-mass galaxies, we investigate the prevalence and origin of prograde metal-poor stars. Almost all (11 of 12) of our simulations have metal-poor stars on preferentially prograde orbits today and throughout most of their history: we thus predict that this is a generic feature of MW/M31-mass galaxies. The typical prograde-to-retrograde ratio is ∼2 : 1, which depends weakly on stellar metallicity at [Fe/H]≲ −1. These trends predicted by our simulations agree well with MW observations. Prograde metal-poor stars originate largely from a single LMC/SMC-mass gas-rich merger 7 − 12.5Gyr ago, which deposited existing metal-poor stars and significant gas on an orbital vector that sparked the formation of and/or shaped the orientation of a long-lived stellar disk, giving rise to a prograde bias for all low-metallicity stars. We find sub-dominant contributions from in-situ stars formed in the host galaxy before this merger, and in some cases, additional massive mergers. We find few clear correlations between any properties of our MW/M31-mass galaxies at z = 0 and the degree of this prograde bias as a result of diverse merger scenarios
Recommended from our members
The formation times and building blocks of Milky Way-mass galaxies in the FIRE simulations
ABSTRACT
Surveys of the Milky Way (MW) and M31 enable detailed studies of stellar populations across ages and metallicities, with the goal of reconstructing formation histories across cosmic time. These surveys motivate key questions for galactic archaeology in a cosmological context: When did the main progenitor of an MW/M31-mass galaxy form, and what were the galactic building blocks that formed it? We investigate the formation times and progenitor galaxies of MW/M31-mass galaxies using the Feedback In Realistic Environments-2 cosmological simulations, including six isolated MW/M31-mass galaxies and six galaxies in Local Group (LG)-like pairs at z = 0. We examine main progenitor ‘formation’ based on two metrics: (1) transition from primarily ex-situ to in-situ stellar mass growth and (2) mass dominance compared to other progenitors. We find that the main progenitor of an MW/M31-mass galaxy emerged typically at z ∼ 3–4 ( ago), while stars in the bulge region (inner 2 kpc) at z = 0 formed primarily in a single main progenitor at z ≲ 5 ( ago). Compared with isolated hosts, the main progenitors of LG-like paired hosts emerged significantly earlier (Δz ∼ 2, ), with ∼4× higher stellar mass at all z ≳ 4 ( ago). This highlights the importance of environment in MW/M31-mass galaxy formation, especially at early times. On average, about 100 galaxies with \rm {\it{ M}}_\rm {star}\!\gtrsim \!10^5\, \rm {M}_\odot went into building a typical MW/M31-mass system. Thus, surviving satellites represent a highly incomplete census (by ∼5×) of the progenitor population
Streams on FIRE: Populations of Detectable Stellar Streams in the Milky Way and FIRE
We present the first detailed comparison of populations of dwarf galaxy stellar streams in cosmological simulations and the Milky Way. In particular, we compare streams identified around 13 Milky Way analogs in the FIRE-2 simulations to streams observed by the Southern Stellar Stream Spectroscopic Survey ( S ^5 ). For an accurate comparison, we produce mock Dark Energy Survey (DES) observations of the FIRE streams and estimate the detectability of their tidal tails and progenitors. The number and stellar mass distributions of detectable stellar streams is consistent between observations and simulations. However, there are discrepancies in the distributions of pericenters and apocenters, with the detectable FIRE streams, on average, forming at larger pericenters (out to >110 kpc) and surviving only at larger apocenters (≳40 kpc) than those observed in the Milky Way. We find that the population of high-stellar-mass dwarf galaxy streams in the Milky Way is incomplete. Interestingly, a large fraction of the FIRE streams would only be detected as intact satellites in DES-like observations, since their tidal tails have too low surface brightness to be detectable. We thus predict a population of yet-undetected tidal tails around Milky Way satellites, as well as a population of fully undetected low-surface-brightness stellar streams, and estimate their detectability with the Rubin Observatory. Finally, we discuss the causes and implications of the discrepancies between the stream populations in FIRE and the Milky Way, and explore future avenues for tests of satellite disruption in cosmological simulations