59 research outputs found
Recovering the nonlinear density field from the galaxy distribution with a Poisson-Lognormal filter
We present a general expression for a lognormal filter given an arbitrary
nonlinear galaxy bias. We derive this filter as the maximum a posteriori
solution assuming a lognormal prior distribution for the matter field with a
given mean field and modeling the observed galaxy distribution by a Poissonian
process. We have performed a three-dimensional implementation of this filter
with a very efficient Newton-Krylov inversion scheme. Furthermore, we have
tested it with a dark matter N-body simulation assuming a unit galaxy bias
relation and compared the results with previous density field estimators like
the inverse weighting scheme and Wiener filtering. Our results show good
agreement with the underlying dark matter field for overdensities even above
delta~1000 which exceeds by one order of magnitude the regime in which the
lognormal is expected to be valid. The reason is that for our filter the
lognormal assumption enters as a prior distribution function, but the maximum a
posteriori solution is also conditioned on the data. We find that the lognormal
filter is superior to the previous filtering schemes in terms of higher
correlation coefficients and smaller Euclidean distances to the underlying
matter field. We also show how it is able to recover the positive tail of the
matter density field distribution for a unit bias relation down to scales of
about >~2 Mpc/h.Comment: 17 pages, 9 figures, 1 tabl
Galaxies in a Simulated CDM Universe II: Observable Properties and Constraints on Feedback
We compare the properties of galaxies that form in a cosmological simulation
without strong feedback to observations at z=0. We confirm previous findings
that models without strong feedback overproduce the observed galaxy baryonic
mass function, especially at the low and high mass extremes. Through
post-processing we investigate what kinds of feedback would be required to
reproduce observed galaxy masses and star formation rates. To mimic an extreme
form of "preventive" feedback (e.g., AGN radio mode) we remove all baryonic
mass that was originally accreted via "hot mode" from shock-heated gas. This
does not bring the high mass end of the galaxy mass function into agreement
with observations because much of the stellar mass in these systems formed at
high redshift from baryons that originally accreted via "cold mode" onto lower
mass progenitors. An efficient "ejective" feedback mechanism, such as supernova
driven winds, must reduce the masses of these progenitors. Feedback must also
reduce the masses of lower mass z=0 galaxies, which assemble at lower redshifts
and have much lower star formation rates. If we monotonically re-map galaxy
masses to reproduce the observed mass function, but retain the simulation's
predicted star formation rates, we obtain fairly good agreement with the
observed sequence of star-forming galaxies but fail to recover the observed
population of passive, low star formation rate galaxies. Suppressing all hot
mode accretion improves agreement for high mass galaxies but worsens the
agreement at intermediate masses. Reproducing these z=0 observations requires a
feedback mechanism that dramatically suppresses star formation in a fraction of
galaxies, increasing with mass, while leaving star formation rates of other
galaxies essentially unchanged.Comment: MNRAS in press. 15 pages, 5 figures, minimal changes from the first
versio
The effects of a hot gaseous halo on disc thickening in galaxy minor mergers
We employ hydrodynamical simulations to study the effects of dissipational
gas physics on the vertical heating and thickening of disc galaxies during
minor mergers. For the first time we present a suite of simulations that
includes a diffuse, rotating, cooling, hot gaseous halo, as predicted by
cosmological hydrodynamical simulations as well as models of galaxy formation.
We study the effect of this new gaseous component on the vertical structure of
a Milky Way-like stellar disc during 1:10 and 1:5 mergers. For 1:10 mergers we
find no increased final thin disc scale height compared to the isolated
simulation, leading to the conclusion that thin discs can be present even after
a 1:10 merger if a reasonable amount of hot gas is present. The reason for this
is the accretion of new cold gas, leading to the formation of a massive new
thin stellar disc that dominates the surface brightness profile. In a previous
study, in which we included only cold gas in the disk, we showed that the
presence of cold gas decreased the thickening by a minor merger relative to the
no-gas case. Here, we show that the evolution of the scale height in the
presences of a cooling hot halo is dominated by the formation of the new
stellar disc. In this scenario, the thick disc is the old stellar disc that has
been thickened in a minor merger at z>1, while the thin disc is the new stellar
disc that reforms after this merger. In addition, we study the evolution of the
scale height during a 1:5 merger and find that a thin disc can be present even
after this merger, provided enough hot gas is available. The final scale height
in our simulations depends on the mass of the hot gaseous halo, the efficiency
of the winds and the merger mass ratio. We find post-merger values in the range
0.5<z0<1.0 kpc in good agreement with observational constraints by local
galaxies.Comment: 14 pages, 10 figures, 2 tables, submitted to MNRA
Star formation rate and metallicity of damped Lyman-alpha absorbers in cosmological SPH simulations
We study the distribution of the star formation rate and metallicity of
damped Lyman-alpha absorbers using cosmological SPH simulations of the Lambda
cold dark matter model in the redshift range z=0-4.5. Our approach includes a
phenomenological model of galactic wind. We find that there is a positive
correlation between the projected stellar mass density and the neutral hydrogen
column density (NHI) of DLAs for high NHI systems, and that there is a good
correspondence in the spatial distribution of stars and DLAs in the
simulations. The evolution of typical star-to-gas mass ratios in DLAs can be
characterised by an increase from about 2 at z=4.5 to 3 at z=3, to 10 at z=1,
and finally to 20 at z=0. We also find that the projected SFR density in DLAs
follows the Kennicutt law well at all redshifts, and the simulated values are
consistent with the recent observational estimates of this quantity by Wolfe et
al. (2003a,b). The rate of evolution in the mean metallicity of simulated DLAs
as a function of redshift is mild, and is consistent with the rate estimated
from observations. The predicted metallicity of DLAs is generally sub-solar in
our simulations, and there is a significant scatter in the distribution of DLA
metallicity for a given NHI. However, we find that the median metallicity of
simulated DLAs is close to that of Lyman-break galaxies, which is higher than
the values typically observed for DLAs by nearly an order of magnitude. This
discrepancy with observations could be due to an inadequate treatment of SN
feedback in our current simulations, perhaps indicating that metals are not
expelled efficiently enough from DLAs by outflows. Alternatively, the current
observations might be missing the majority of the high metallicity DLAs due to
selection effects. (abridged)Comment: 18 pages, 15 figures. Accepted to MNRAS. More visual presentations
and the version with high resolution figures are available at
http://cfa-www.harvard.edu/~knagamine/DLA-pics
The GalMer database: Galaxy Mergers in the Virtual Observatory
We present the GalMer database, a library of galaxy merger simulations, made
available to users through tools compatible with the Virtual Observatory (VO)
standards adapted specially for this theoretical database. To investigate the
physics of galaxy formation through hierarchical merging, it is necessary to
simulate galaxy interactions varying a large number of parameters:
morphological types, mass ratios, orbital configurations, etc. On one side,
these simulations have to be run in a cosmological context, able to provide a
large number of galaxy pairs, with boundary conditions given by the large-scale
simulations, on the other side the resolution has to be high enough at galaxy
scales, to provide realistic physics. The GalMer database is a library of
thousands simulations of galaxy mergers at moderate spatial resolution and it
is a compromise between the diversity of initial conditions and the details of
underlying physics. We provide all coordinates and data of simulated particles
in FITS binary tables. The main advantages of the database are VO access
interfaces and value-added services which allow users to compare the results of
the simulations directly to observations: stellar population modelling, dust
extinction, spectra, images, visualisation using dedicated VO tools. The GalMer
value-added services can be used as virtual telescope producing broadband
images, 1D spectra, 3D spectral datacubes, thus making our database oriented
towards the usage by observers. We present several examples of the GalMer
database scientific usage obtained from the analysis of simulations and
modelling their stellar population properties, including: (1) studies of the
star formation efficiency in interactions; (2) creation of old counter-rotating
components; (3) reshaping metallicity profiles in elliptical galaxies; (4)
orbital to internal angular momentum transfer; (5) reproducing observed colour
bimodality of galaxies.Comment: 15 pages, 11 figures, 10 tables accepted to A&A. Visualisation of
GalMer simulations, access to snapshot files and value-added tools described
in the paper are available at http://galmer.obspm.fr
The Baryonic Assembly of Dark Matter Halos
We use a suite of cosmological hydrodynamic simulations to quantify the
accretion rates of baryons into dark matter halos and the resulting baryon mass
fractions, as a function of halo mass, redshift, and baryon type (including
cold and hot gas). We find that the net baryonic accretion rates through the
virial radius are sensitive to galactic outflows and explore a range of outflow
parameters to illustrate the effects. We show that the cold gas accretion rate
is in general not a simple universal factor of the dark matter accretion rate,
and that galactic winds can cause star formation rates to deviate significantly
from the external gas accretion rates, both via gas ejection and re-accretion.
Furthermore, galactic winds can inject enough energy and momentum in the
surrounding medium to slow down accretion altogether, especially in low-mass
halos and at low redshift. By resolving the accretion rates versus radius from
the halo centers, we show how cold streams penetrate the hot atmospheres of
massive halos at z>2, but gradually disappear at lower redshift. The total
baryon mass fraction is also strongly suppressed by outflows in low-mass halos,
but is nearly universal in the absence of feedback in halos above the UV
background suppression scale. The transition halo mass, at which the gas mass
in halos is equal for the cold and hot components, is roughly constant at
~10^11.5 Msun and does not depend sensitively on the wind prescription. We
provide simple fitting formulae for the cold gas accretion rate into halos in
the no-wind case. Finally, we show that cold accretion is broadly consistent
with driving the bulk of the highly star-forming galaxies observed at z~2, but
that the more intense star formers likely sample the high end of the accretion
rate distribution, and may be additionally fueled by a combination of gas
recycling, gas re-accretion, hot mode cooling, and mergers.Comment: 20 pages, 10 figures. MNRAS, in pres
The formation of disc galaxies in a LCDM universe
We study the formation of disc galaxies in a fully cosmological framework
using adaptive mesh refinement simulations. We perform an extensive parameter
study of the main subgrid processes that control how gas is converted into
stars and the coupled effect of supernovae feedback. We argue that previous
attempts to form disc galaxies have been unsuccessful because of the universal
adoption of strong feedback combined with high star formation efficiencies.
Unless extreme amounts of energy are injected into the interstellar medium
during supernovae events, these star formation parameters result in bulge
dominated S0/Sa galaxies as star formation is too efficient at z~3. We show
that a low efficiency of star-formation more closely models the subparsec
physical processes, especially at high redshift. We highlight the successful
formation of extended disc galaxies with scale lengths r_d=4-5 kpc, flat
rotation curves and bulge to disc ratios of B/D~1/4. Not only do we resolve the
formation of a Milky Way-like spiral galaxy, we also observe the secular
evolution of the disc as it forms a pseudo-bulge. The disc properties agree
well with observations and are compatible with the photometric and baryonic
Tully-Fisher relations, the Kennicutt-Schmidt relation and the observed angular
momentum content of spiral galaxies. We conclude that underlying small-scale
star formation physics plays a larger role than previously considered in
simulations of galaxy formation.Comment: Published in MNRA
Parallel TreeSPH
We describe PTreeSPH, a gravity treecode combined with an SPH hydrodynamics
code designed for massively parallel supercomputers having distributed memory.
Our computational algorithm is based on the popular TreeSPH code of Hernquist &
Katz (1989). PTreeSPH utilizes a domain decomposition procedure and a
synchronous hypercube communication paradigm to build self-contained subvolumes
of the simulation on each processor at every timestep. Computations then
proceed in a manner analogous to a serial code. We use the Message Passing
Interface (MPI) communications package, making our code easily portable to a
variety of parallel systems. PTreeSPH uses individual smoothing lengths and
timesteps, with a communication algorithm designed to minimize exchange of
information while still providing all information required to accurately
perform SPH computations. We have additionally incorporated cosmology, periodic
boundary conditions with forces calculated using a quadrupole Ewald summation
method, and radiative cooling and heating from a parameterized ionizing
background following Katz, Weinberg & Hernquist (1996). The addition of other
physical processes, such as star formation, is straightforward. A cosmological
simulation from z=49 to z=2 with 64^3 gas particles and 64^3 dark matter
particles requires ~6000 node-hours on a Cray T3D, with a communications
overhead of ~10% and is load balanced to a ~90% level. When used on the new
Cray T3E, this code will be capable of performing cosmological hydrodynamical
simulations down to z=0 with ~2x10^6 particles, or to z=2 with ~10^7 particles,
in a reasonable amount of time. Even larger simulations will be practical in
situations where the matter is not highly clustered or when periodic boundaries
are not required.Comment: 30 pages, 6 Postscript figures, Submitted to New Astronom
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