338 research outputs found
Carbon-Enhanced Metal-Poor Stars, the Cosmic Microwave Background, and the Stellar IMF in the Early Universe
The characteristic mass of stars at early times may have been higher than
today owing to the cosmic microwave background (CMB). This study proposes that
(1) the testable predictions of this "CMB-IMF" hypothesis are an increase in
the fraction of carbon-enhanced metal-poor (CEMP) stars with declining
metallicity and an increase from younger to older populations at a single
metallicity (e.g. disk to halo), and (2) these signatures are already seen in
recent samples of CEMP stars and can be better tested with anticipated data.
The expected spatial variation may explain discrepancies of CEMP frequency
among published surveys. The ubiquity and time dependence of the CMB will
substantially alter the reconstruction of star formation histories in the Local
Group and early Universe.Comment: 7 pages emulateapj format, three figures, accepted for ApJ Letter
Redefining the Missing Satellites Problem
Numerical simulations of Milky-Way size Cold Dark Matter (CDM) halos predict
a steeply rising mass function of small dark matter subhalos and a substructure
count that greatly outnumbers the observed satellites of the Milky Way. Several
proposed explanations exist, but detailed comparison between theory and
observation in terms of the maximum circular velocity (Vmax) of the subhalos is
hampered by the fact that Vmax for satellite halos is poorly constrained. We
present comprehensive mass models for the well-known Milky Way dwarf
satellites, and derive likelihood functions to show that their masses within
0.6 kpc (M_0.6) are strongly constrained by the present data. We show that the
M_0.6 mass function of luminous satellite halos is flat between ~ 10^7 and 10^8
M_\odot. We use the ``Via Lactea'' N-body simulation to show that the M_0.6
mass function of CDM subhalos is steeply rising over this range. We rule out
the hypothesis that the 11 well-known satellites of the Milky Way are hosted by
the 11 most massive subhalos. We show that models where the brightest
satellites correspond to the earliest forming subhalos or the most massive
accreted objects both reproduce the observed mass function. A similar analysis
with the newly-discovered dwarf satellites will further test these scenarios
and provide powerful constraints on the CDM small-scale power spectrum and warm
dark matter models.Comment: 8 pages, 6 figure
Dynamical Instability of Self-Tuning Solution with Antisymmetric Tensor Field
We consider the dynamical stability of a static brane model that incorporates
a three-index antisymmetric tensor field and has recently been proposed as a
possible solution to the cosmological constant problem. Ultimately, we are able
to establish the existence of time-dependent, purely gravitational
perturbations. As a consequence, the static solution of interest is
``dangerously'' located at an unstable saddle point. This outcome is suggestive
of a hidden fine tuning in what is an otherwise self-tuning model.Comment: 16 Pages, Latex; Discussion added but conclusions unchange
The galaxy halo formation in the absence of violent relaxation and a universal density profile of the halo center
While N-body simulations testify for a cuspy profile of the central region of
the dark matter haloes, observations favor a shallow, cored density profile of
the central region of, at least, some spiral galaxies and dwarf spheroidals. We
show that a central profile, very close to the observed one, inevitably forms
in the center of dark matter haloes if we make a supposition about a moderate
energy relaxation of the system during the halo formation. If we assume the
energy exchange between dark matter particles during the halo collapse to be
not too intensive, the profile is universal: it depends almost not at all on
the properties of the initial perturbation and is very akin, but not identical,
to the Einasto profile with small Einasto index . We estimate the
size of the 'central core' of the distribution, i.e., the extent of the very
central region with a respectively gentle profile, and show that the cusp
formation is unlikely, even if the dark matter is cold. The obtained profile is
in a good agreement with observational data for, at least, some types of
galaxies, but clearly disagrees with N-body simulations.Comment: 8 pages, 4 figure
Two Body Relaxation in Simulated Cosmological Haloes
This paper aims at quantifying discreetness effects, born of finite particle
number, on the dynamics of dark matter haloes forming in the context of
cosmological simulations. By generalising the standard calculation of two body
relaxation to the case when the size and mass distribution are variable, and
parametrising the time evolution using established empirical relations, we find
that the dynamics of a million particle halo is noise-dominated within the
inner percent of the final virial radius. Far larger particle numbers (~ 10^8)
are required for the RMS perturbations to the velocity to drop to the 10 %
level there. The radial scaling of the relaxation time is simple and strong:
t_relax ~ r^2, implying that numbers >> 10^8 are required to faithfully model
the very inner regions; artificial relaxation may thus constitute an important
factor, contributing to the contradictory claims concerning the persistence of
a power law density cusp to the very centre. The cores of substructure haloes
can be many relaxation times old. Since relaxation first causes their expansion
before recontraction occurs, it may render them either more difficult or easier
to disrupt, depending on their orbital parameters. It may thus modify the
characteristics of the subhalo distribution and effects of interactions with
the parent. We derive simple closed form formulas for the characteristic
relaxation times, as well as for the weak N-scaling reported by Diemand et al.
when the main contribution comes from relaxing subhaloes (abridged).Comment: 11 Pages, 7 figs, Monthly Notices styl
The Via Lactea INCITE Simulation: Galactic Dark Matter Substructure at High Resolution
It is a clear unique prediction of the cold dark matter paradigm of
cosmological structure formation that galaxies form hierarchically and are
embedded in massive, extended dark halos teeming with self-bound substructure
or "subhalos". The amount and spatial distribution of subhalos around their
host provide unique information and clues on the galaxy assembly process and
the nature of the dark matter. Here we present results from the Via Lactea
INCITE simulation, a one billion particle, one million cpu-hour simulation of
the formation and evolution of a Galactic dark matter halo and its substructure
population.Comment: 10 pages, Proceedings of the SciDAC 2008 conference, (Seattle, July
13-17, 2008
Quasars at z=6: the survival of the fittest
The Sloan Digital Sky survey detected luminous quasars at very high redshift,
z>6. Follow-up observations indicated that at least some of these quasars are
powered by supermassive black holes (SMBHs) with masses in excess of billion
solar masses. SMBHs, therefore, seem to have already existed when the Universe
was less than 1 Gyr old, and the bulk of galaxy formation still has to take
place. We investigate in this paper to which extent accretion and dynamical
processes influence the early growth of SMBHs. We assess the impact of (i)
black hole mergers, (ii) the influence of the merging efficiency and (iii) the
negative contribution due to dynamical effects which can kick black holes out
of their host halos (gravitational recoil). We find that if accretion is always
limited by the Eddington rate via a thin disc, the maximum radiative efficiency
allowed to reproduce the LF at z=6 is of order 12%, when the adverse effect of
the gravitational recoil is taken into consideration. Dynamical effects cannot
be neglected in studies of high-redshift SMBHs. If black holes can accrete at
super-critical rate during an early phase, reproducing the observed SMBH mass
values is not an issue, even in the case that the recoil velocity is in the
upper limits range, as the mass ratios of merging binaries are skewed towards
low values, where the gravitational recoil effect is very mild. We propose that
SMBH growth at early times is very selective, and efficient only for black
holes hosted in high density peak halos.Comment: Accepted for publication in the ApJ. 9 pages, 6 b/w figure
Density Profiles of Cold Dark Matter Substructure: Implications for the Missing Satellites Problem
The structural evolution of substructure in cold dark matter (CDM) models is
investigated combining ``low-resolution'' satellites from cosmological N-body
simulations of parent halos with N=10^7 particles with high-resolution
individual subhalos orbiting within a static host potential. We show that, as a
result of mass loss, convergence in the central density profiles requires the
initial satellites to be resolved with N=10^7 particles and parsec-scale force
resolution. We find that the density profiles of substructure halos can be well
fitted with a power-law central slope that is unmodified by tidal forces even
after the tidal stripping of over 99% of the initial mass and an exponential
cutoff in the outer parts. The solution to the missing-satellites problem
advocated by Stoehr et al. in 2002 relied on the flattening of the dark matter
(DM) halo central density cusps by gravitational tides, enabling the observed
satellites to be embedded within DM halos with maximum circular velocities as
large as 60 km/s. In contrast, our results suggest that tidal interactions do
not provide the mechanism for associating the dwarf spheroidal satellites
(dSphs) of the Milky Way with the most massive substructure halos expected in a
CDM universe. We compare the predicted velocity dispersion profiles of Fornax
and Draco to observations, assuming that they are embedded in CDM halos. Models
with isotropic and tangentially anisotropic velocity distributions for the
stellar component fit the data only if the surrounding DM halos have maximum
circular velocities in the range 20-35 km/s. If the dSphs are embedded within
halos this large then the overabundance of satellites within the concordance
LCDM cosmological model is significantly alleviated, but this still does not
provide the entire solution.Comment: Accepted for publication in ApJ, 17 pages, 9 figures, LaTeX (uses
emulateapj5.sty
Non-Gaussian velocity distributions - The effect on virial mass estimates of galaxy groups
We present a study of 9 galaxy groups with evidence for non-Gaussianity in
their velocity distributions out to 4R200. This sample is taken from 57 groups
selected from the 2PIGG catalog of galaxy groups. Statistical analysis
indicates that non-Gaussian groups have masses significantly higher than
Gaussian groups. We also have found that all non-Gaussian systems seem to be
composed of multiple velocity modes. Besides, our results indicate that
multimodal groups should be considered as a set of individual units with their
own properties. In particular, we have found that the mass distribution of such
units are similar to that of Gaussian groups. Our results reinforce the idea of
non-Gaussian systems as complex structures in the phase space, likely
corresponding to secondary infall aggregations at a stage before virialization.
The understanding of these objects is relevant for cosmological studies using
groups and clusters through the mass function evolution.Comment: 5 pages, 4 figures and 2 tables. Accepted for publication in the
MNRA
The nature of dark matter and the density profile and central behavior of relaxed halos
We show that the two basic assumptions of the model recently proposed by
Manrique and coworkers for the universal density profile of cold dark matter
(CDM) halos, namely that these objects grow inside out in periods of smooth
accretion and that their mass profile and its radial derivatives are all
continuous functions, are both well understood in terms of the very nature of
CDM. Those two assumptions allow one to derive the typical density profile of
halos of a given mass from the accretion rate characteristic of the particular
cosmology. This profile was shown by Manrique and coworkers to recover the
results of numerical simulations. In the present paper, we investigate its
behavior beyond the ranges covered by present-day N-body simulations. We find
that the central asymptotic logarithmic slope depends crucially on the shape of
the power spectrum of density perturbations: it is equal to a constant negative
value for power-law spectra and has central cores for the standard CDM power
spectrum. The predicted density profile in the CDM case is well fitted by the
3D S\'ersic profile over at least 10 decades in halo mass. The values of the
S\'ersic parameters depend on the mass of the structure considered. A practical
procedure is provided that allows one to infer the typical values of the best
NFW or S\'ersic fitting law parameters for halos of any mass and redshift in
any given standard CDM cosmology.Comment: 9 pages, 6 figures, to appear in the ApJ vol. 647, september 20,
2007. Minor changes to match the published versio
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