18 research outputs found
The Dual Origin of Stellar Halos. II. Chemical Abundances as Tracers of Formation History
This paper is a computational study of the formation of the stellar halos of Milky Way-massed galaxies, and the fingerprint of the formation history imprinted on the chemical abundances of those stars. --author-supplied descriptio
The Dual Origin of Stellar Halos II: Chemical Abundances as Tracers of Formation History
Fully cosmological, high resolution N-Body + SPH simulations are used to
investigate the chemical abundance trends of stars in simulated stellar halos
as a function of their origin. These simulations employ a physically motivated
supernova feedback recipe, as well as metal enrichment, metal cooling and metal
diffusion. As presented in an earlier paper, the simulated galaxies in this
study are surrounded by stellar halos whose inner regions contain both stars
accreted from satellite galaxies and stars formed in situ in the central
regions of the main galaxies and later displaced by mergers into their inner
halos. The abundance patterns ([Fe/H] and [O/Fe]) of halo stars located within
10 kpc of a solar-like observer are analyzed. We find that for galaxies which
have not experienced a recent major merger, in situ stars at the high [Fe/H]
end of the metallicity distribution function are more [alpha/Fe]-rich than
accreted stars at similar [Fe/H]. This dichotomy in the [O/Fe] of halo stars at
a given [Fe/H] results from the different potential wells within which in situ
and accreted halo stars form. These results qualitatively match recent
observations of local Milky Way halo stars. It may thus be possible for
observers to uncover the relative contribution of different physical processes
to the formation of stellar halos by observing such trends in the halo
populations of the Milky Way, and other local L* galaxies.Comment: Version accepted for publication in ApJ Part 1. This version of the
paper has been extended to include a detailed discussion of numerical issue
Compaction and Quenching of High-z Galaxies in Cosmological Simulations: Blue and Red Nuggets
We use cosmological simulations to study a characteristic evolution pattern
of high redshift galaxies. Early, stream-fed, highly perturbed, gas-rich discs
undergo phases of dissipative contraction into compact, star-forming systems
(blue nuggets) at z~4-2. The peak of gas compaction marks the onset of central
gas depletion and inside-out quenching into compact ellipticals (red nuggets)
by z~2. These are sometimes surrounded by gas rings or grow extended dry
stellar envelopes. The compaction occurs at a roughly constant specific
star-formation rate (SFR), and the quenching occurs at a constant stellar
surface density within the inner kpc (). Massive galaxies quench
earlier, faster, and at a higher than lower-mass galaxies, which
compactify and attempt to quench more than once. This evolution pattern is
consistent with the way galaxies populate the SFR-radius-mass space, and with
gradients and scatter across the main sequence. The compaction is triggered by
an intense inflow episode, involving (mostly minor) mergers, counter-rotating
streams or recycled gas, and is commonly associated with violent disc
instability. The contraction is dissipative, with the inflow rate >SFR, and the
maximum anti-correlated with the initial spin parameter, as
predicted by Dekel & Burkert (2014). The central quenching is triggered by the
high SFR and stellar/supernova feedback (possibly also AGN feedback) due to the
high central gas density, while the central inflow weakens as the disc
vanishes. Suppression of fresh gas supply by a hot halo allows the long-term
maintenance of quenching once above a threshold halo mass, inducing the
quenching downsizing.Comment: Resubmitted to MNRAS after responding to referee's comments; Updated
and added two figure
The accretion origin of the Milky Way's stellar halo
We have used data from the Sloan Digital Sky Survey (SDSS) Data Release 5 to
explore the overall structure and substructure of the stellar halo of the Milky
Way using about 4 million color-selected main sequence turn-off stars. We fit
oblate and triaxial broken power-law models to the data, and found a `best-fit'
oblateness of the stellar halo 0.5<c/a<0.8, and halo stellar masses between
Galactocentric radii of 1 and 40kpc of (3.7+/-1.2)x10^8 M_sun. The density
profile of the stellar halo is approximately r^{-3}; it is possible that the
power law slope is shallower inside 20kpc and steeper outside that radius. Yet,
we found that all smooth and symmetric models were very poor fits to the
distribution of stellar halo stars because the data exhibit a great deal of
spatial substructure. We quantified deviations from a smooth oblate/triaxial
model using the RMS of the data around the model profile on scales >~100pc,
after accounting for the (known) contribution of Poisson uncertainties. The
fractional RMS deviation of the actual stellar distribution from any smooth,
parameterized halo model is >~40%: hence, the stellar halo is highly
structured. We compared the observations with simulations of galactic stellar
halos formed entirely from the accretion of satellites in a cosmological
context by analysing the simulations in the same way as the data. While the
masses, overall profiles, and degree of substructure in the simulated stellar
halos show considerable scatter, the properties and degree of substructure in
the Milky Way's halo match well the properties of a `typical' stellar halo
built exclusively out of the debris from disrupted satellite galaxies. Our
results therefore point towards a picture in which an important fraction of the
Milky Way's stellar halo has been accreted from satellite galaxies.Comment: Submitted to the Astrophysical Journal. 14 pages; 11 figure
The Dual Origin of Stellar Halos
We investigate the formation of the stellar halos of four simulated disk
galaxies using high resolution, cosmological SPH + N-Body simulations. These
simulations include a self-consistent treatment of all the major physical
processes involved in galaxy formation. The simulated galaxies presented here
each have a total mass of ~10^12 M_sun, but span a range of merger histories.
These simulations allow us to study the competing importance of in-situ star
formation (stars formed in the primary galaxy) and accretion of stars from
subhalos in the building of stellar halos in a LambdaCDM universe. All four
simulated galaxies are surrounded by a stellar halo, whose inner regions (r <
20 kpc) contain both accreted stars, and an in-situ stellar population. The
outer regions of the galaxies' halos were assembled through pure accretion and
disruption of satellites. Most of the in-situ halo stars formed at high
redshift out of smoothly accreted cold gas in the inner 1 kpc of the galaxies'
potential wells, possibly as part of their primordial disks. These stars were
displaced from their central locations into the halos through a succession of
major mergers. We find that the two galaxies with recently quiescent merger
histories have a higher fraction of in-situ stars (~20-50%) in their inner
halos than the two galaxies with many recent mergers (~5-10% in-situ fraction).
Observational studies concentrating on stellar populations in the inner halo of
the Milky Way will be the most affected by the presence of in-situ stars with
halo kinematics, as we find that their existence in the inner few tens of kpc
is a generic feature of galaxy formation.Comment: Version accepted to ApJ. Content is unchanged from previous version,
but paper has been restructured for clarit
Understanding Dwarf Galaxies in order to Understand Dark Matter
Much progress has been made in recent years by the galaxy simulation
community in making realistic galaxies, mostly by more accurately capturing the
effects of baryons on the structural evolution of dark matter halos at high
resolutions. This progress has altered theoretical expectations for galaxy
evolution within a Cold Dark Matter (CDM) model, reconciling many earlier
discrepancies between theory and observations. Despite this reconciliation, CDM
may not be an accurate model for our Universe. Much more work must be done to
understand the predictions for galaxy formation within alternative dark matter
models.Comment: Refereed contribution to the Proceedings of the Simons Symposium on
Illuminating Dark Matter, to be published by Springe
Keck-I MOSFIRE Spectroscopy of Compact Star-Forming Galaxies at \u3cem\u3ez\u3c/em\u3e ≳ 2: High Velocity Dispersions in Progenitors of Compact Quiescent Galaxies
We present Keck-I MOSFIRE near-infrared spectroscopy for a sample of 13 compact star-forming galaxies (SFGs) at redshift 2 ≤ z ≤ 2.5 with star formation rates of SFR ~ 100 M ☉ yr–1 and masses of log(M/M ☉) ~10.8. . . .
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http://dx.doi.org/10.1088/0004-637X/795/2/14