62 research outputs found
Faint dwarfs as a test of DM models: WDM vs. CDM
We use high resolution HydroN-Body cosmological simulations to compare the
assembly and evolution of a small field dwarf (stellar mass ~ 10
M, total mass 10 M in dominated CDM and 2keV WDM
cosmologies. We find that star formation (SF) in the WDM model is reduced and
delayed by 1-2 Gyr relative to the CDM model, independently of the details of
SF and feedback. Independent of the DM model, but proportionally to the SF
efficiency, gas outflows lower the central mass density through `dynamical
heating', such that all realizations have circular velocities 20kms at
500pc, in agreement with local kinematic constraints. As a result of
dynamical heating, older stars are less centrally concentrated than younger
stars, similar to stellar population gradients observed in nearby dwarf
galaxies. Introducing an important diagnostic of SF and feedback models, we
translate our simulations into artificial color-magnitude diagrams and star
formation histories in order to directly compare to available observations. The
simulated galaxies formed most of their stars in many 10 Myr long bursts.
The CDM galaxy has a global SFH, HI abundance and Fe/H and alpha-elements
distribution well matched to current observations of dwarf galaxies. These
results highlight the importance of directly including `baryon physics' in
simulations when 1) comparing predictions of galaxy formation models with the
kinematics and number density of local dwarf galaxies and 2) differentiating
between CDM and non-standard models with different DM or power spectra.Comment: 13 pages including Appendix on Color Magnitude Diagrams. Accepted by
MNRAS. Added one plot and details on ChaNGa implementation. Reduced number of
citations after editorial reques
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
Inflationary perturbations in anisotropic backgrounds and their imprint on the CMB
We extend the standard theory of cosmological perturbations to homogeneous
but anisotropic universes. We present an exhaustive computation for the case of
a Bianchi I model, with a residual isotropy between two spatial dimensions,
which is undergoing complete isotropization at the onset of inflation; we also
show how the computation can be further extended to more general backgrounds.
In presence of a single inflaton field, there are three physical perturbations
(precisely as in the isotropic case), which are obtained (i) by removing gauge
and nondynamical degrees of freedom, and (ii) by finding the combinations of
the remaining modes in terms of which the quadratic action of the perturbations
is canonical. The three perturbations, which later in the isotropic regime
become a scalar mode and two tensor polarizations (gravitational wave), are
coupled to each other already at the linearized level during the anisotropic
phase. This generates nonvanishing correlations between different modes of the
CMB anisotropies, which can be particularly relevant at large scales (and,
potentially, be related to the large scale anomalies in the WMAP data). As an
example, we compute the spectrum of the perturbations in this Bianchi I
geometry, assuming that the inflaton is in a slow roll regime also in the
anisotropic phase. For this simple set-up, fixing the initial conditions for
the perturbations appears more difficult than in the standard case, and
additional assumptions seem to be needed to provide predictions for the CMB
anisotropies.Comment: 31 pages, 3 figure
Linearization of homogeneous, nearly-isotropic cosmological models
Homogeneous, nearly-isotropic Bianchi cosmological models are considered.
Their time evolution is expressed as a complete set of non-interacting linear
modes on top of a Friedmann-Robertson-Walker background model. This connects
the extensive literature on Bianchi models with the more commonly-adopted
perturbation approach to general relativistic cosmological evolution.
Expressions for the relevant metric perturbations in familiar coordinate
systems can be extracted straightforwardly. Amongst other possibilities, this
allows for future analysis of anisotropic matter sources in a more general
geometry than usually attempted.
We discuss the geometric mechanisms by which maximal symmetry is broken in
the context of these models, shedding light on the origin of different Bianchi
types. When all relevant length-scales are super-horizon, the simplest Bianchi
I models emerge (in which anisotropic quantities appear parallel transported).
Finally we highlight the existence of arbitrarily long near-isotropic epochs
in models of general Bianchi type (including those without an exact isotropic
limit).Comment: 31 pages, 2 figures. Submitted to CQ
Faint dwarfs as a test of DM models: WDM versus CDM
We use high-resolution Hydro+N-Body cosmological simulations to compare the assembly and evolution of a small field dwarf (stellar mass ∼106−7M⊙, total mass 1010M⊙) in Λ-dominated cold dark matter (CDM) and 2keV warm dark matter (WDM) cosmologies. We find that star formation (SF) in the WDM model is reduced and delayed by 1-2Gyr relative to the CDM model, independently of the details of SF and feedback. Independent of the dark matter (DM) model, but proportionally to the SF efficiency, gas outflows lower the central mass density through ‘dynamical heating', such that all realizations have circular velocities <20 km s−1 at 500pc, in agreement with local kinematic constraints. As a result of dynamical heating, older stars are less centrally concentrated than younger stars, similar to stellar population gradients observed in nearby dwarf galaxies. Introducing an important diagnostic of SF and feedback models, we translate our simulations into artificial colour-magnitude diagrams and star formation histories (SFHs) in order to directly compare to available observations. The simulated galaxies formed most of their stars in many ∼10Myr long bursts. The CDM galaxy has a global SFH, H i abundance and Fe/H and alpha-elements distribution well matched to current observations of dwarf galaxies. These results highlight the importance of directly including ‘baryon physics' in simulations when (1) comparing predictions of galaxy formation models with the kinematics and number density of local dwarf galaxies and (2) differentiating between CDM and non-standard models with different DM or power spectr
Extreme Emission Line Galaxies in CANDELS: Broad-Band Selected, Star-Bursting Dwarf Galaxies at z>1
We identify an abundant population of extreme emission line galaxies (EELGs)
at redshift z~1.7 in the Cosmic Assembly Near-IR Deep Extragalactic Legacy
Survey (CANDELS) imaging from Hubble Space Telescope/Wide Field Camera 3
(HST/WFC3). 69 EELG candidates are selected by the large contribution of
exceptionally bright emission lines to their near-infrared broad-band
magnitudes. Supported by spectroscopic confirmation of strong [OIII] emission
lines -- with rest-frame equivalent widths ~1000\AA -- in the four candidates
that have HST/WFC3 grism observations, we conclude that these objects are
galaxies with 10^8 Msol in stellar mass, undergoing an enormous starburst phase
with M_*/(dM_*/dt) of only ~15 Myr. These bursts may cause outflows that are
strong enough to produce cored dark matter profiles in low-mass galaxies. The
individual star formation rates and the co-moving number density (3.7x10^-4
Mpc^-3) can produce in ~4 Gyr much of the stellar mass density that is
presently contained in 10^8-10^9 Msol dwarf galaxies. Therefore, our
observations provide a strong indication that many or even most of the stars in
present-day dwarf galaxies formed in strong, short-lived bursts, mostly at z>1.Comment: accepted for publication in ApJ; 10 pages; 6 figures; 1 tabl
Improved Modeling of the Mass Distribution of Disk Galaxies by the Einasto Halo Model
(Abridged) The analysis of the rotation curves (RCs) of spiral galaxies
provides an efficient diagnostic for studying the properties of dark matter
halos and their relations with the baryonic material. We have modeled the RCs
of galaxies from The HI Nearby Galaxy Survey (THINGS) with the Einasto halo
model, which has emerged as the best-fitting model of the halos arising in
dissipationless cosmological N-body simulations. We find that the RCs are
significantly better fit with the Einasto halo than with either a
pseudo-isothermal sphere (Iso) or Navarro-Frenk-White (NFW) halo models. In our
best-fit models, the radius of density slope -2 and the density at this radius
are highly correlated. The Einasto index, which controls the overall shape of
the density profile, is near unity on average for intermediate and low mass
halos. This is not in agreement with the predictions from LCDM simulations. The
indices of the most massive halos are in rough agreement with those of
cosmological simulations and appear correlated with the halo virial mass. We
find that a typical Einasto density profile declines more strongly in its
outermost parts than any of the Iso or NFW models whereas it is relatively
shallow in its innermost regions. The core nature of those regions of halos
thus extends the cusp-core controversy found for the NFW model with low surface
density galaxies to the Einasto halo with more massive galaxies like those of
THINGS. We thus find that the Einasto halo model provides, so far, the best
match to the observed RCs, and can therefore be considered as a new standard
model for dark matter halos.Comment: 15 pages, 14 figures, The Astronomical Journal, in press, Volume 4,
2011 Octobe
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
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
Evolutionary Conserved Function of Barley and Arabidopsis 3-KETOACYL-CoA SYNTHASES in Providing Wax Signals for Germination of Powdery Mildew Fungi
- …