195 research outputs found
CIV Absorption From Galaxies in the Process of Formation
We investigate the heavy element QSO absorption systems caused by gas
condensations at high redshift which evolve into galaxies with circular
velocity of 100 to 200 km/s at the present epoch. Artificial QSO spectra were
generated for a variety of lines-of-sight through regions of the universe
simulated with a hydrodynamics code. The CIV and HI absorption features in
these spectra closely resemble observed CIV and HI absorption systems over a
wide range in column density. CIV absorption complexes with multiple-component
structure and velocity spreads up to about 600 km/s are found. The broadest
systems are caused by lines-of-sight passing through groups of protogalactic
clumps with individual velocity dispersions of less than 150 km/s aligned along
filamentary structures. The temperature of most of the gas does not take the
photoionization equilibrium value. This invalidates density and size estimates
derived from thermal equilibrium models. Consequences for metal abundance
determinations are briefly discussed. We predict occasional exceptionally large
ratios of CIV to HI column density (up to a third) for lines-of-sight passing
through compact halos of hot gas with temperature close to 3 10^5 K. Our model
may be able to explain both high-ionization multi-component heavy-element
absorbers and damped Lyman alpha systems as groups of small protogalactic
clumps.Comment: 13 pages, uuencoded postscript file, 4 figures included submitted to
ApJ (Letters); complete version also available at
http://www.mpa-garching.mpg.de/Galaxien/prep.htm
Galaxy-Induced Transformation of Dark Matter Halos
We use N-body/gasdynamical LambdaCDM cosmological simulations to examine the
effect of the assembly of a central galaxy on the shape and mass profile of its
dark halo. Two series of simulations are compared; one that follows only the
evolution of the dark matter component and a second one where a baryonic
component is added. These simulations include radiative cooling but neglect
star formation and feedback, leading most baryons to collect at the halo center
in a disk which is too small and too massive when compared with typical spiral.
This unrealistic model allows us, nevertheless, to gauge the maximum effect
that galaxies may have in transforming their dark halos. We find that the shape
of the halo becomes more axisymmetric: halos are transformed from triaxial into
essentially oblate systems, with well-aligned isopotential contours of roughly
constant flattening (c/a ~ 0.85). Halos always contract as a result of galaxy
assembly, but the effect is substantially less pronounced than predicted by the
"adiabatic contraction" hypothesis. The reduced contraction helps to reconcile
LambdaCDM halos with constraints on the dark matter content inside the solar
circle and should alleviate the long-standing difficulty of matching
simultaneously the scaling properties of galaxy disks and the luminosity
function. The halo contraction is also less pronounced than found in earlier
simulations, a disagreement that suggests that halo contraction is not solely a
function of the initial and final distribution of baryons. Not only how much
baryonic mass has been deposited at the center of a halo matters, but also the
mode of its deposition. It might prove impossible to predict the halo response
without a detailed understanding of a galaxy's assembly history. (Abriged)Comment: 11 pages and 9 figure
Damped Lyman alpha absorbers at high redshift -- large disks or galactic building blocks?
We investigate the nature of the physical structures giving rise to damped
Lyman alpha absorption systems (DLAS) at high redshift. In particular, we
examine the suggestion that rapidly rotating large disks are the only viable
explanation for the characteristic observed asymmetric profiles of low
ionization absorption lines. We demonstrate using hydrodynamic simulations of
galaxy formation in a cosmological context that irregular protogalactic clumps
can reproduce the observed velocity width distribution and asymmetries of the
absorption profiles equally well. The velocity broadening in the simulated
clumps is due to a mixture of rotation, random motions, infall and merging. The
observed velocity width correlates with the virial velocity of the dark matter
halo of the forming protogalactic clump (v_{wid} ~ 0.6 times v_{vir} for the
median values with a large scatter of order a factor two between different
lines-of-sight). The typical virial velocity of the halos required to give rise
to the DLAS population is about 100 km/s and most standard hierarchical
structure formation scenarios can easily account even for the largest observed
velocity widths. We conclude that the evidence that DLAS at high redshift are
related to large rapidly rotating disks with v_circ >= 200 km/s is not
compelling.Comment: 24 pages, LaTeX , 10 postscript figures included; submitted to ApJ.
The paper can also be retrieved at http://www.mpa-garching/~haehnel
Star formation and chemical evolution of damped Lyman systems
In this paper, we investigate the star formation and chemical evolution of
damped Lyman- systems (DLAs) based on the disc galaxy formation model
which is developed by Mo, Mao & White. We propose that the DLAs are the central
galaxies of less massive dark haloes present at redshifts , and they
should inhabit haloes of moderately low circular velocity. The empirical
Schmidt law of star formation rates, and closed box model of chemical evolution
that an approximation known as instantaneous recycling is assumed, are adopted.
In our models, when the predicted distribution of metallicity for DLAs is
calculated, two cases are considered. One is that, using the closed box model,
empirical Schmidt law and star formation time, the distribution of metallicity
can be directly calculated. The other is that, when the simple gravitational
instability of a thin isothermal gas disc as first discussed by Toomre is
considered, the star formation occurs only in the region where the surface
density of gas satisfies the critical value, not everywhere of a gas disc. We
assume that star formation in each DLA lasts for a period of 1 Gyr from
redshifts . There is only one output parameter in our models, i.e. the
stellar yield, which relates to the time of star formation history and is
obtained by normalizing the predicted distribution of metallicity to the mean
value of 1/13 as presented by Pettini et al. The predicted
metallicity distribution is consistent with the current (rather limited)
observational data. A random distribution of galactic discs is taken into
account.Comment: 7 pages, 6 figures, will appear in MNRAS, 2001 (21 April), 322,
927-93
The Angular Momentum Problem in Cosmological Simulations of Disk Galaxy Formation
We conduct a systematic study of the angular momentum problem in numerical
simulations of disk galaxy formation. We investigate the role of numerical
resolution using a semi-cosmological setup which combines an efficient use of
the number of particles in an isolated halo while preserving the hierarchical
build-up of the disk through the merging of clumps. We perform the same
simulation varying the resolution over 4 orders of magnitude. Independent on
the level of resolution, the loss of angular momentum stays the same and can be
tied to dynamical friction during the build-up phase. This is confirmed in a
cosmological simulation. We also perform simulations including star formation
and star formation and supernova feedback. While the former has no influence on
the angular momentum problem, the latter reduces the loss to a level
potentially in agreement with observations. This is achieved through a
suppression of early star formation and therefore the formation of a large,
slowly rotating bulge. We conclude that feedback is a critical component to
achieve realistic disk galaxies in cosmological simulations. Numerical
resolution is important, but by itself not capable of solving the angular
momentum problem.Comment: 13 pages, 13 figures, submitted to MNRA
Angular momentum and clustering properties of early dark matter halos
In this paper we study the angular momentum properties of simulated dark
matter halos at high redshift that likely host the first stars in the Universe.
Calculating the spin distributions of these 10^6 - 10^7 \Msun halos in
redshift slices from , we find that they are well fit by a
log-normal distribution as is found for lower redshift and more massive halos
in earlier work. We find that both the mean value of the spin and dispersion
are largely unchanged with redshift for all halos. Our key result is that
subsamples of low and high spin 10^6 \Msun and 10^7 \Msun halos show
difference in clustering strength. In both mass bins, higher spin halos are
more strongly clustered in concordance with a tidal torquing picture for the
growth of angular momentum in dark matter halos in the CDM paradigm.Comment: 6 pages, 3 figures. Accepted MNRA
The Formation of a Disk Galaxy within a Growing Dark Halo
We present a dynamical model for the formation and evolution of a massive
disk galaxy, within a growing dark halo whose mass evolves according to
cosmological simulations of structure formation. The galactic evolution is
simulated with a new 3D chemo-dynamical code, including dark matter, stars and
a multi-phase ISM. The simulations start at redshift z=4.85 with a small dark
halo in a LCDM universe and we follow the evolution until the present epoch.
The energy release by massive stars and SNe prevents a rapid collapse of the
baryonic matter and delays the maximum star formation until z=1. The galaxy
forms radially from inside-out and vertically from halo to disk. The first
galactic component that forms is the halo, followed by the bulge, the disk-halo
transition region, and the disk. At z=1, a bar begins to form which later turns
into a triaxial bulge. There is a pronounced deficiency of low-metallicity disk
stars due to pre-enrichment of the disk ISM with metal-rich gas from the bulge
and inner disk (G-dwarf problem). The mean rotation and the distribution of
orbital eccentricities for all stars as a function of metallicity are not very
different from those observed in the solar neighbourhood, showing that
homogeneous collapse models are oversimplified. The approach presented here
provides a detailed description of the formation and evolution of an isolated
disk galaxy in a LCDM universe, yielding new information about the kinematical
and chemical history of the stars and the ISM, but also about the evolution of
the luminosity, the colours and the morphology of disk galaxies.Comment: 23 pages, LaTeX, 18 figures, A&A accepted, a high resolution version
of the paper can be found at http://www.astro.unibas.ch/leute/ms.shtm
Populating a cluster of galaxies - I. Results at z=0
We simulate the assembly of a massive rich cluster and the formation of its
constituent galaxies in a flat, low-density universe. Our most accurate model
follows the collapse, the star-formation history and the orbital motion of all
galaxies more luminous than the Fornax dwarf spheroidal, while dark halo
structure is tracked consistently throughout the cluster for all galaxies more
luminous than the SMC. Within its virial radius this model contains about 2.0e7
dark matter particles and almost 5000 distinct dynamically resolved galaxies.
Simulations of this same cluster at a variety of resolutions allow us to check
explicitly for numerical convergence both of the dark matter structures
produced by our new parallel N-body and substructure identification codes, and
of the galaxy populations produced by the phenomenological models we use to
follow cooling, star formation, feedback and stellar aging. This baryonic
modelling is tuned so that our simulations reproduce the observed properties of
isolated spirals outside clusters. Without further parameter adjustment our
simulations then produce a luminosity function, a mass-to-light ratio,
luminosity, number and velocity dispersion profiles, and a morphology-radius
relation which are similar to those observed in real clusters. In particular,
since our simulations follow galaxy merging explicitly, we can demonstrate that
it accounts quantitatively for the observed cluster population of bulges and
elliptical galaxies.Comment: 28 pages, submitted to MNRA
The Peaks Formalism and the Formation of Cold Dark Matter Haloes
We use two cosmological simulations of structure formation to study the
conditions under which dark matter haloes emerge from the linear density field.
Our analysis focuses on matching sites of halo collapse to local density
maxima, or "peaks", in the initial conditions of the simulations and provides a
crucial test of the central ansatz of the peaks formalism. By identifying peaks
on a variety of smoothed, linearly extrapolated density fields we demonstrate
that as many as ~70% of well-resolved dark matter haloes form preferentially
near peaks whose characteristic masses are similar to that of the halo, with
more massive haloes showing a stronger tendency to reside near peaks initially.
We identify a small but significant fraction of haloes that appear to evolve
from peaks of substantially lower mass than that of the halo itself. We refer
to these as "peakless haloes" for convenience. By contrasting directly the
properties of these objects with the bulk of the proto-halo population we find
two clear differences: 1) their initial shapes are significantly flatter and
more elongated than the predominantly triaxial majority, and 2) they are, on
average, more strongly compressed by tidal forces associated with their
surrounding large scale structure. Using the two-point correlation function we
show that peakless haloes tend to emerge from highly clustered regions of the
initial density field implying that, at fixed mass, the accretion geometry and
mass accretion histories of haloes in highly clustered environments differ
significantly from those in the field. This may have important implications for
understanding the origin of the halo assembly bias, of galaxy properties in
dense environments and how environment affects the morphological transformation
of galaxies near groups and rich galaxy clusters.Comment: 13 pages, 11 figures, published in MNRA
Preheating by Previrialization and its Impact on Galaxy Formation
We use recent observations of the HI-mass function to constrain galaxy
formation. The data conflicts with the standard model where most of the gas in
a low-mass dark matter halo is assumed to settle into a disk of cold gas that
is depleted by star formation and supernova-driven outflows until the disk
becomes gravitationally stable. A consistent model can be found if low-mass
haloes are embedded in a preheated medium, with a specific gas entropy ~ 10Kev
cm^2. Such a model simultaneously matches the faint-end slope of the galaxy
luminosity function. We propose a preheating model where the medium around
low-mass haloes is preheated by gravitational pancaking. Since gravitational
tidal fields suppress the formation of low-mass haloes while promoting that of
pancakes, the formation of massive pancakes precedes that of the low-mass
haloes within them. We demonstrate that the progenitors of present-day dark
matter haloes with M<10^{12}h^{-1}\msun were embedded in pancakes of masses
~5x10^{12}h^{-1}\msun at z~2. The formation of such pancakes heats the gas to
a temperature of 5x10^5K and compresses it to an overdensity of ~10. Such gas
has a cooling time that exceeds the age of the Universe at z~2, and has a
specific entropy of ~15Kev cm^2, almost exactly the amount required to explain
the stellar and HI mass functions. (Abridged)Comment: 13 pages, 3 figures. Accepted for publication in MNRA
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