600 research outputs found
Building Late-Type Spiral Galaxies by In-Situ and Ex-Situ Star Formation
We analyze the formation and evolution of the stellar components in "Eris", a
120 pc-resolution cosmological hydrodynamic simulation of a late-type spiral
galaxy. The simulation includes the effects of a uniform UV background, a
delayed-radiative-cooling scheme for supernova feedback, and a star formation
recipe based on a high gas density threshold. It allows a detailed study of the
relative contributions of "in-situ" (within the main host) and "ex-situ"
(within satellite galaxies) star formation to each major Galactic component in
a close Milky Way analog. We investigate these two star-formation channels as a
function of galactocentric distance, along different lines of sight above and
along the disk plane, and as a function of cosmic time. We find that: 1)
approximately 70 percent of today's stars formed in-situ; 2) more than two
thirds of the ex-situ stars formed within satellites after infall; 3) the
majority of ex-situ stars are found today in the disk and in the bulge; 4) the
stellar halo is dominated by ex-situ stars, whereas in-situ stars dominate the
mass profile at distances < 5 kpc from the center at high latitudes; and 5)
approximately 25% of the inner, r < 20 kpc, halo is composed of in-situ stars
that have been displaced from their original birth sites during Eris' early
assembly history.Comment: 12 pages, 8 figures; submitted to Ap
Shape of Dark Matter Haloes in the Illustris Simulation: Effects of Baryons
We study the effect of baryonic processes on the shapes of dark matter (DM)
haloes from Illustris, a suite of hydrodynamical (Illustris) and DM-only
(Illustris-Dark) cosmological simulations performed with the moving-mesh code
{\sc arepo}. DM halo shapes are determined using an iterative method based on
the inertia tensor for a wide range of masses (). Convergence tests shows that the local DM
shape profiles are converged only for , being the
Plummer-equivalent softening length, larger than expected. Haloes from
non-radiative simulations (i.e. neglecting radiative processes, star formation,
and feedback) exhibit no alteration in shapes from their DM-only counterparts:
thus moving-mesh hydrodynamics alone is insufficient to cause differences in DM
shapes. With the full galaxy-physics implementation, condensation of baryons
results in significantly rounder and more oblate haloes, with the median
minor-to-major axis ratio \left \approx 0.7, almost
invariant throughout the halo and across halo masses. This somewhat improves
the agreement between simulation predictions and observational estimates of the
Milky Way halo shape. Consistently, the velocity anisotropy of DM is also
reduced in Illustris, across halo masses and radii. Within the inner halo
(), both and (intermediate-to-major axis ratio) exhibit
non-monotonicity with galaxy mass, peaking at , which we find is due to the strong dependence of inner halo shape
with galaxy formation efficiency. Baryons in Illustris affect the correlation
of halo shape with halo properties, leading to a positive correlation of
sphericity of MW-mass haloes with halo formation time and concentration, the
latter being mildly more pronounced than in Illustris-Dark.Comment: 18 pages, 14 figure
Galaxies with Shells in the Illustris Simulation: Metallicity Signatures
Stellar shells are low surface brightness arcs of overdense stellar regions,
extending to large galactocentric distances. In a companion study, we
identified 39 shell galaxies in a sample of 220 massive ellipticals
() from the
Illustris cosmological simulation. We used stellar history catalogs to trace
the history of each individual star particle inside the shell substructures,
and we found that shells in high-mass galaxies form through mergers with
massive satellites (stellar mass ratios ).
Using the same sample of shell galaxies, the current study extends the stellar
history catalogs in order to investigate the metallicity of stellar shells
around massive galaxies. Our results indicate that outer shells are often times
more metal-rich than the surrounding stellar material in a galaxy's halo. For a
galaxy with two different satellites forming shells, we find a
significant difference in the metallicity of the shells produced by each
progenitor. We also find that shell galaxies have higher mass-weighted
logarithmic metallicities ([Z/H]) at -
compared to galaxies without shells. Our results indicate that observations
comparing the metallicities of stars in tidal features, such as shells, to the
average metallicities in the stellar halo can provide information about the
assembly histories of galaxies.Comment: 15 pages, 5 figures. Article published in a special issue of MDPI
Galaxies after the conference "On the Origin (and Evolution) of Baryonic
Galaxy Halos", Galapagos Islands, 201
Formation and Incidence of Shell Galaxies in the Illustris Simulation
Shells are low surface brightness tidal debris that appear as interleaved
caustics with large opening angles, often situated on both sides of the galaxy
center. In this paper, we study the incidence and formation processes of shell
galaxies in the cosmological gravity+hydrodynamics Illustris simulation. We
identify shells at redshift z=0 using stellar surface density maps, and we use
stellar history catalogs to trace the birth, trajectory and progenitors of each
individual star particle contributing to the tidal feature. Out of a sample of
the 220 most massive galaxies in Illustris
(),
of the galaxies exhibit shells. This fraction increases with
increasing mass cut: higher mass galaxies are more likely to have stellar
shells. Furthermore, the fraction of massive galaxies that exhibit shells
decreases with increasing redshift. We find that shell galaxies observed at
redshift form preferentially through relatively major mergers
(1:10 in stellar mass ratio). Progenitors are accreted on low angular
momentum orbits, in a preferred time-window between 4 and 8 Gyrs ago. Our
study indicates that, due to dynamical friction, more massive satellites are
allowed to probe a wider range of impact parameters at accretion time, while
small companions need almost purely radial infall trajectories in order to
produce shells. We also find a number of special cases, as a consequence of the
additional complexity introduced by the cosmological setting. These include
galaxies with multiple shell-forming progenitors, satellite-of-satellites also
forming shells, or satellites that fail to produce shells due to multiple major
mergers happening in quick succession.Comment: 27 pages, 18 figures. Accepted for publication in MNRAS (new figures
3 and D1 + additional minor changes to match accepted version
Can we really measure fnl from the galaxy power spectrum?
The scale-dependent galaxy bias generated by primordial non-Gaussianity (PNG)
can be used to detect and constrain deviations from standard single-field
inflation. The strongest signal is expected in the local model for PNG, where
the amplitude of non-Gaussianity can be expressed by a set of parameters (fnl,
gnl, ...). Current observational constraints from galaxy clustering on fnl and
gnl assume that the others PNG parameters are vanishing. Using two sets of
cosmological N-body simulations where both fnl and gnl are non-zero, we show
that this strong assumption generally leads to biased estimates and spurious
redshift dependencies of the parameters. Additionally, if the signs of fnl and
gnl are opposite, the amplitude of the scale-dependent bias is reduced,
possibly leading to a false null detection. Finally we show that model
selection techniques like the Bayesian evidence can (and should) be used to
determine if more than one PNG parameter is required by the data.Comment: 5 pages, 3 figures. Accepted for publication in MNRAS Letters. Minor
changes to previous versio
Formation and Incidence of Shell Galaxies in the Illustris Simulation
Shells are low surface brightness tidal debris that appear as interleaved
caustics with large opening angles, often situated on both sides of the galaxy
center. In this paper, we study the incidence and formation processes of shell
galaxies in the cosmological gravity+hydrodynamics Illustris simulation. We
identify shells at redshift z=0 using stellar surface density maps, and we use
stellar history catalogs to trace the birth, trajectory and progenitors of each
individual star particle contributing to the tidal feature. Out of a sample of
the 220 most massive galaxies in Illustris
(),
of the galaxies exhibit shells. This fraction increases with
increasing mass cut: higher mass galaxies are more likely to have stellar
shells. Furthermore, the fraction of massive galaxies that exhibit shells
decreases with increasing redshift. We find that shell galaxies observed at
redshift form preferentially through relatively major mergers
(1:10 in stellar mass ratio). Progenitors are accreted on low angular
momentum orbits, in a preferred time-window between 4 and 8 Gyrs ago. Our
study indicates that, due to dynamical friction, more massive satellites are
allowed to probe a wider range of impact parameters at accretion time, while
small companions need almost purely radial infall trajectories in order to
produce shells. We also find a number of special cases, as a consequence of the
additional complexity introduced by the cosmological setting. These include
galaxies with multiple shell-forming progenitors, satellite-of-satellites also
forming shells, or satellites that fail to produce shells due to multiple major
mergers happening in quick succession.Comment: 27 pages, 18 figures. Accepted for publication in MNRAS (new figures
3 and D1 + additional minor changes to match accepted version
Zooming in on accretion - I. The structure of halo gas
We study the properties of gas in and around 10^12 solar mass halos at z=2
using a suite of high-resolution cosmological hydrodynamic 'zoom' simulations.
We quantify the thermal and dynamical structure of these gaseous reservoirs in
terms of their mean radial distributions and angular variability along
different sightlines. With each halo simulated at three levels of increasing
resolution, the highest reaching a baryon mass resolution of ~10,000 solar
masses, we study the interaction of filamentary inflow and the quasi-static hot
halo atmosphere. We highlight the discrepancy between the spatial resolution
available in the halo gas as opposed to within the galaxy itself, and find that
stream morphologies become increasingly complex at higher resolution, with
large coherent flows revealing density and temperature structure at
progressively smaller scales. Moreover, multiple gas components co-exist at the
same radius within the halo, making radially averaged analyses misleading. This
is particularly true where the hot, quasi-static, high entropy halo atmosphere
interacts with cold, rapidly inflowing, low entropy accretion. We investigate
the process of gas virialization and identify different regimes for the heating
of gas as it accretes from the intergalactic medium. Haloes at this mass have a
well-defined virial shock, associated with a sharp jump in temperature and
entropy at ~1.25 r_vir. The presence, radius, and radial width of this boundary
feature, however, vary not only from halo to halo, but also as a function of
angular direction, covering roughly ~85% of the 4pi sphere. Our findings are
relevant for the proper interpretation of observations pertaining to the
circumgalactic medium, including evidence for large amounts of cold gas
surrounding massive haloes at intermediate redshifts.Comment: High-res PDF and simulation movies available at
http://www.cfa.harvard.edu/~dnelson/#research (MNRAS submitted, comments
welcome
The bispectrum of redshifted 21-cm fluctuations from the dark ages
Brightness-temperature fluctuations in the redshifted 21-cm background from
the cosmic dark ages are generated by irregularities in the gas-density
distribution and can then be used to determine the statistical properties of
density fluctuations in the early Universe. We first derive the most general
expansion of brightness-temperature fluctuations up to second order in terms of
all the possible sources of spatial fluctuations. We then focus on the
three-point statistics and compute the angular bispectrum of
brightness-temperature fluctuations generated prior to the epoch of hydrogen
reionization. For simplicity, we neglect redshift-space distortions. We find
that low-frequency radio experiments with arcmin angular resolution can easily
detect non-Gaussianity produced by non-linear gravity with high signal-to-noise
ratio. The bispectrum thus provides a unique test of the gravitational
instability scenario for structure formation, and can be used to measure the
cosmological parameters. Detecting the signature of primordial non-Gaussianity
produced during or right after an inflationary period is more challenging but
still possible. An ideal experiment limited by cosmic variance only and with an
angular resolution of a few arcsec has the potential to detect primordial
non-Gaussianity with a non-linearity parameter of f_NL ~ 1. Additional sources
of error as weak lensing and an imperfect foreground subtraction could severely
hamper the detection of primordial non-Gaussianity which will benefit from the
use of optimal estimators combined with tomographic techniques.Comment: 15 pages, 4 figures, revised version accepted for publication in ApJ
(contains an improved discussion of gas temperature fluctuations
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