No Arabic abstract
Extended, old, and round stellar halos appear to be ubiquitous around high-mass dwarf galaxies ($10^{8.5}<M_star/M_odot<10^{9.6}$) in the observed universe. However, it is unlikely that these dwarfs have undergone a sufficient number of minor mergers to form stellar halos that are composed of predominantly accreted stars. Here, we demonstrate that FIRE-2 (Feedback in Realistic Environments) cosmological zoom-in simulations are capable of producing dwarf galaxies with realistic structure, including both a thick disk and round stellar halo. Crucially, these stellar halos are formed in-situ, largely via the outward migration of disk stars. However, there also exists a large population of non-disky dwarfs in FIRE that lack a well-defined disk/halo and do not resemble the observed dwarf population. These non-disky dwarfs tend to be either more gas poor or to have burstier recent star formation histories than the disky dwarfs, suggesting that star formation feedback may be preventing disk formation. Both classes of dwarfs underscore the power of a galaxys intrinsic shape -- which is a direct quantification of the distribution of the galaxys stellar content -- to interrogate the feedback implementation in simulated galaxies.
We use a particle tracking analysis to study the origins of the circumgalactic medium (CGM), separating it into (1) accretion from the intergalactic medium (IGM), (2) wind from the central galaxy, and (3) gas ejected from other galaxies. Our sample consists of 21 FIRE-2 simulations, spanning the halo mass range log(Mh/Msun) ~ 10-12 , and we focus on z=0.25 and z=2. Owing to strong stellar feedback, only ~L* halos retain a baryon mass >~50% of their cosmic budget. Metals are more efficiently retained by halos, with a retention fraction >~50%. Across all masses and redshifts analyzed >~60% of the CGM mass originates as IGM accretion (some of which is associated with infalling halos). Overall, the second most important contribution is wind from the central galaxy, though gas ejected or stripped from satellites can contribute a comparable mass in ~L* halos. Gas can persist in the CGM for billions of years, resulting in well-mixed halo gas. Sight lines through the CGM are therefore likely to intersect gas of multiple origins. For low-redshift ~L* halos, cool gas (T<10^4.7 K) is distributed on average preferentially along the galaxy plane, however with strong halo-to-halo variability. The metallicity of IGM accretion is systematically lower than the metallicity of winds (typically by >~1 dex), although CGM and IGM metallicities depend significantly on the treatment of subgrid metal diffusion. Our results highlight the multiple physical mechanisms that contribute to the CGM and will inform observational efforts to develop a cohesive picture.
We explore the stellar mass density and colour profiles of 118 low redshift, massive, central galaxies, selected to have assembled 90 percent of their stellar mass 6 Gyr ago, finding evidence of the minor merger activity expected to be the driver behind the size growth of quiescent galaxies. We use imaging data in the $g, r, i, z, y$ bands from the Subaru Hyper Suprime-Cam survey and perform SED fitting to construct spatially well-resolved radial profiles in colour and stellar mass surface density. Our visual morphological classification reveals that $sim 42$ percent of our sample displays tidal features, similar to previous studies, $sim 43$ percent of the remaining sample display a diffuse stellar halo and only $sim 14$ percent display no features, down to a limiting $mu_{rmathrm{-band}}$ $sim$ 28 mag arcsec$^{-2}$. We find good agreement between the stacked colour profiles of our sample to those derived from previous studies and an expected smooth, declining stellar mass surface density profile in the central regions (< 3 R$_{mathrm{e}}$). However, we also see a flattening of the profile ($Sigma_* sim 10^{7.5}$ M$_odot$ kpc$^{-2}$) in the outskirts (up to 10 R$_{mathrm{e}}$), which is revealed by our method of specifically targeting tidal/accretion features. We find similar levels of tidal features and behaviour in the stellar mass surface density profiles in a younger comparison sample, however a lack of diffuse haloes. We also apply stacking techniques, similar to those in previous studies, finding such procedures wash out tidal features and thereby produces smooth declining profiles. The stellar material in the outskirts contributes on average $sim 10^{10}$ M$_odot$ or a few percent of the total stellar mass and has similar colours to SDSS satellites of similar stellar mass.
The galaxy size-stellar mass and central surface density-stellar mass relationships are observational constraints on galaxy formation models. However, inferring the physical size of a galaxy from observed stellar emission is non-trivial due to various observational effects. Consequently, forward-modeling light-based sizes from simulations is desirable. In this work, we use the {skirt} dust radiative transfer code to generate synthetic observations of massive galaxies ($M_{*}sim10^{11},rm{M_{odot}}$ at $z=2$, hosted by haloes of mass $M_{rm{halo}}sim10^{12.5},rm{M_{odot}}$) from high-resolution cosmological zoom-in simulations that form part of the Feedback In Realistic Environments (FIRE) project. The simulations used in this paper include explicit stellar feedback but no active galactic nucleus (AGN) feedback. From each mock observation, we infer the effective radius ($R_e$), as well as the stellar mass surface density within this radius and within $1,rm{kpc}$ ($Sigma_e$ and $Sigma_1$, respectively). We first investigate how well the intrinsic half-mass radius and stellar mass surface density can be inferred from observables. The predicted sizes and surface densities are within a factor of two of the intrinsic values. We then compare our predictions to the observed size-mass relationship and the $Sigma_1-M_star$ and $Sigma_e-M_star$ relationships. At $zgtrsim2$, the simulated massive galaxies are in general agreement with observational scaling relations. At $zlesssim2$, they evolve to become too compact but still star-forming, in the stellar mass and redshift regime where many of them should be quenched. Our results suggest that some additional source of feedback, such as AGN driven outflows, is necessary in order to decrease the central densities of the simulated massive galaxies to bring them into agreement with observations at $zlesssim2$.
Stellar migration, whether due to trapping by transient spirals (churning), or to scattering by non-axisymmetric perturbations, has been proposed to explain the presence of stars in outer disks. After a review of the basic theory, we present compelling, but not yet conclusive, evidence that churning has been important in the outer disks of galaxies with type II (down-bending) profiles, while scattering has produced the outer disks of type III (up-bending) galaxies. In contrast, field galaxies with type I (pure exponential) profiles appear to not have experienced substantial migration. We conclude by suggesting work that would improve our understanding of the origin of outer disks.
We present a suite of cosmological zoom-in simulations at z>5 from the Feedback In Realistic Environments project, spanning a halo mass range M_halo~10^8-10^12 M_sun at z=5. We predict the stellar mass-halo mass relation, stellar mass function, and luminosity function in several bands from z=5-12. The median stellar mass-halo mass relation does not evolve strongly at z=5-12. The faint-end slope of the luminosity function steepens with increasing redshift, as inherited from the halo mass function at these redshifts. Below z~6, the stellar mass function and ultraviolet (UV) luminosity function slightly flatten below M_star~10^4.5 M_sun (fainter than M_1500~-12), owing to the fact that star formation in low-mass halos is suppressed by the ionizing background by the end of reionization. Such flattening does not appear at higher redshifts. We provide redshift-dependent fitting functions for the SFR-M_halo, SFR-M_star, and broad-band magnitude-stellar mass relations. We derive the star formation rate density and stellar mass density at z=5-12 and show that the contribution from very faint galaxies becomes more important at z>8. Furthermore, we find that the decline in the z~6 UV luminosity function brighter than M_1500~-20 is largely due to dust attenuation. Approximately 37% (54%) of the UV luminosity from galaxies brighter than M_1500=-13 (-17) is obscured by dust at z~6. Our results broadly agree with current data and can be tested by future observations.