69 research outputs found
The Coarse Geometry of Merger Trees in \Lambda CDM
We introduce the contour process to describe the geometrical properties of
merger trees. The contour process produces a one-dimensional object, the
contour walk, which is a translation of the merger tree. We portray the contour
walk through its length and action. The length is proportional to to the number
of progenitors in the tree, and the action can be interpreted as a proxy of the
mean length of a branch in a merger tree.
We obtain the contour walk for merger trees extracted from the public
database of the Millennium Run and also for merger trees constructed with a
public Monte-Carlo code which implements a Markovian algorithm. The trees
correspond to halos of final masses between 10^{11} h^{-1} M_sol and 10^{14}
h^{-1} M_sol. We study how the length and action of the walks evolve with the
mass of the final halo. In all the cases, except for the action measured from
Markovian trees, we find a transitional scale around 3 \times 10^{12} h^{-1}
M_sol. As a general trend the length and action measured from the Markovian
trees show a large scatter in comparison with the case of the Millennium Run
trees.Comment: 7 pages, 5 figures, submitted to MNRA
Conditional Mass Functions and Merger Rates of Dark Matter Halos in the Ellipsoidal Collapse Model
Analytic models based on spherical and ellipsoidal gravitational collapse
have been used to derive the mass functions of dark matter halos and their
progenitors (the conditional mass function). The ellipsoidal model generally
provides a better match to simulation results, but there has been no simple
analytic expression in this model for the conditional mass function that is
accurate for small time steps, a limit that is important for generating halo
merger trees and computing halo merger rates. We remedy the situation by
deriving accurate analytic formulae for the first-crossing distribution, the
conditional mass function, and the halo merger rate in the ellipsoidal collapse
model in the limit of small look-back times. We show that our formulae provide
a closer match to the Millennium simulation results than those in the spherical
collapse model and the ellipsoidal model of Sheth & Tormen (2002).Comment: 5 pages, 3 figures, accepted by MNRAS letter
Accretion of gas onto nearby spiral galaxies
We present evidence for cosmological gas accretion onto spiral galaxies in
the local universe. The accretion is seen through its effects on the dynamics
of the extra-planar neutral gas. The accretion rates that we estimate for two
nearby spiral galaxies are of the order of their star formation rates. Our
model shows that most of the extra-planar gas is produced by supernova feedback
(galactic fountain) and only 10-20 % comes from accretion. The accreting
material must have low specific angular momentum about the disc's spin axis,
although the magnitude of the specific angular-momentum vector can be higher.
We also explore the effects of a hot corona on the dynamics of the extra-planar
gas and find that it is unlikely to be responsible for the observed kinematical
pattern and the source of accreted gas. However, the interaction with the
fountain flow should profoundly affect the hydrodynamics of the corona.Comment: 11 pages, 6 figures, accepted for publication in MNRA
Cold streams in early massive hot haloes as the main mode of galaxy formation
The massive galaxies in the young universe, ten billion years ago, formed
stars at surprising intensities. Although this is commonly attributed to
violent mergers, the properties of many of these galaxies are incompatible with
such events, showing gas-rich, clumpy, extended rotating disks not dominated by
spheroids (Genzel et al. 2006, 2008). Cosmological simulations and clustering
theory are used to explore how these galaxies acquired their gas. Here we
report that they are stream-fed galaxies, formed from steady, narrow, cold gas
streams that penetrate the shock-heated media of massive dark matter haloes
(Dekel & Birnboim 2006; Keres et al. 2005). A comparison with the observed
abundance of star-forming galaxies implies that most of the input gas must
rapidly convert to stars. One-third of the stream mass is in gas clumps leading
to mergers of mass ratio greater than 1:10, and the rest is in smoother flows.
With a merger duy cycle of 0.1, three-quarters of the galaxies forming stars at
a given rate are fed by smooth streams. The rarer, submillimetre galaxies that
form stars even more intensely are largely merger-induced starbursts. Unlike
destructive mergers, the streams are likely to keep the rotating disk
configuration intact, although turbulent and broken into giant star-forming
clumps that merge into a central spheroid (Noguchi 1999; Genzel et al. 2008,
Elmegreen, Bournaud & Elmegreen 2008, Dekel, Sari & Ceverino 2009). This
stream-driven scenario for the formation of disks and spheroids is an
alternative to the merger picture.Comment: Improved version, 25 pages, 13 figures, Letter to Nature with
Supplementary Informatio
The Excursion Set Theory of Halo Mass Functions, Halo Clustering, and Halo Growth
I review the excursion set theory (EST) of dark matter halo formation and
clustering. I recount the Press-Schechter argument for the mass function of
bound objects and review the derivation of the Press-Schechter mass function in
EST. The EST formalism is powerful and can be applied to numerous problems. I
review the EST of halo bias and the properties of void regions. I spend
considerable time reviewing halo growth in the EST. This section culminates
with descriptions of two Monte Carlo methods for generating halo mass accretion
histories. In the final section, I emphasize that the standard EST approach is
the result of several simplifying assumptions. Dropping these assumptions can
lead to more faithful predictions and a more versatile formalism. One such
assumption is the constant height of the barrier for nonlinear collapse. I
review implementations of the excursion set approach with arbitrary barrier
shapes. An application of this is the now well-known improvement to standard
EST that follows from the ellipsoidal-collapse barrier. Additionally, I
emphasize that the statement that halo accretion histories are independent of
halo environments is a simplifying assumption, rather than a prediction of the
theory. I review the method for constructing correlated random walks of the
density field in more general cases. I construct a simple toy model with
correlated walks and I show that excursion set theory makes a qualitatively
simple and general prediction for the relation between halo accretion histories
and halo environments: regions of high density preferentially contain
late-forming halos and conversely for regions of low density. I conclude with a
brief discussion of this prediction in the context of recent numerical studies
of the environmental dependence of halo properties. (Abridged)Comment: 62 pages, 19 figures. Review article based on lectures given at the
Sixth Summer School of the Helmholtz Institute for Supercomputational
Physics. Accepted for Publication in IJMPD. Comments Welcom
Ram pressure profiles in galaxy groups and clusters
Using a hybrid method which combines non-radiative hydrodynamical simulations
with a semi-analytic model of galaxy formation, we determine the ram pressure
as a function of halocentric distance experienced by galaxies in haloes with
virial masses 12.5 <= log (M_200 h/M_Sun) < 15.35, for redshifts 0 <= z <= 3.
The ram pressure is calculated with a self-consistent method which uses the
simulation gas particles to obtain the properties of the intergalactic medium.
The ram pressure profiles obtained can be well described by beta profile
models, with parameters that depend on redshift and halo virial mass in a
simple fashion. The fitting formulae provided here will prove useful to include
ram pressure effects into semi-analytic models based on methods which lack gas
physics, such as dark matter-only simulations or the Press-Schechter formalism.Comment: 7 pages, 6 figures. Accepted for publication in MNRA
The dynamics of S0 galaxies and their Tully-Fisher relation
This paper investigates the detailed dynamical properties of a relatively
homogeneous sample of disc-dominated S0 galaxies, with a view to understanding
their formation, evolution and structure. By using high signal-to-noise ratio
long-slit spectra of edge-on systems, we have been able to reconstruct the
complete line-of-sight velocity distributions of stars along the galaxies'
major axes. From these data, we have derived both model distribution functions
(the phase density of their stars) and the approximate form of their
gravitational potentials. The derived distribution functions are all consistent
with these galaxies being simple disc systems, with no evidence for a complex
formation history. Essentially no correlation is found between the
characteristic mass scale-lengths and the photometric scale-lengths in these
galaxies, suggesting that they are dark-matter dominated even in their inner
parts. Similarly, no correlation is found between the mass scale-lengths and
asymptotic rotation speed, implying a wide range of dark matter halo
properties. By comparing their asymptotic rotation speeds with their absolute
magnitudes, we find that these S0 galaxies are systematically offset from the
Tully-Fisher relation for later-type galaxies. The offset in luminosity is what
one would expect if star formation had been suddenly switched off a few Gyrs
ago, consistent with a simple picture in which these S0s were created from
ordinary later-type spirals which were stripped of their star-forming ISM when
they encountered a dense cluster environment.Comment: 8 pages, 16 figures, accepted by MNRA
Structure of the Galaxies in the NGC 80 Group
BV-bands photometric data obtained at the 6-m telescope of the Special
Astrophysical Observatory are used to analyze the structure of 13 large disk
galaxies in the NGC 80 group. Nine of the 13 galaxies under consideration are
classified by us as lenticular galaxies. The stellar populations in the
galaxies are very different, from old ones with ages of T>10 Gyrs (IC 1541) to
relatively young, with the ages of T<2-3 Gyr (IC 1548, NGC 85). In one case,
current star formation is known (UCM 0018+2216). In most of the galaxies, more
precisely in all of them more luminous than M(B) -18, two-tiered
(`antitruncated') stellar disks are detected, whose radial surface brightness
profiles can be fitted by two exponential segments with different scalelengths
-- shorter near the center and longer at the periphery. All dwarf S0 galaxies
with single-scalelength exponential disks are close companions to giant
galaxies. Except for this fact, no dependence of the properties of S0 galaxies
on distance from the center of the group is found. Morphological traces of
minor merger are found in the lenticular galaxy NGC 85. Basing on the last two
points, we conclude that the most probable mechanisms for the transformation of
spirals into lenticular galaxies in groups are gravitational ones, namely,
minor mergers and tidal interactions.Comment: 24 pages, 9 figures, slightly improved version of the paper published
in the December, 2009, issue of the Astronomy Report
The Millennium Run Observatory: First Light
Simulations of galaxy evolution aim to capture our current understanding as
well as to make predictions for testing by future experiments. Simulations and
observations are often compared in an indirect fashion: physical quantities are
estimated from the data and compared to models. However, many applications can
benefit from a more direct approach, where the observing process is also
simulated and the models are seen fully from the observer's perspective. To
facilitate this, we have developed the Millennium Run Observatory (MRObs), a
theoretical virtual observatory which uses virtual telescopes to `observe'
semi-analytic galaxy formation models based on the suite of Millennium Run dark
matter simulations. The MRObs produces data that can be processed and analyzed
using the standard software packages developed for real observations. At
present, we produce images in forty filters from the rest-frame UV to IR for
two stellar population synthesis models, three different models of IGM
absorption, and two cosmologies (WMAP1/7). Galaxy distributions for a large
number of mock lightcones can be `observed' using models of major ground- and
space-based telescopes. The data include lightcone catalogues linked to
structural properties of galaxies, pre-observation model images, mock telescope
images, and Source Extractor products that can all be traced back to the higher
level dark matter, semi-analytic galaxy, and lightcone catalogues available in
the Millennium database. Here, we describe our methods and announce a first
public release of simulated surveys (e.g., SDSS, CFHT-LS, GOODS, GOODS/ERS,
CANDELS, and HUDF). The MRObs browser, an online tool, further facilitates
exploration of the simulated data. We demonstrate the benefits of a direct
approach through a number of example applications (galaxy number counts in
CANDELS, clusters, morphologies, and dropout selections).Comment: MNRAS, in press. Millennium Run Observatory data products, online
tools, and more available through http://galformod.mpa-garching.mpg.de/mrobs
Gravitational stability and dynamical overheating of stellar disks of galaxies
We use the marginal stability condition for galactic disks and the stellar
velocity dispersion data published by different authors to place upper limits
on the disk local surface density at two radial scalelengths .
Extrapolating these estimates, we constrain the total mass of the disks and
compare these estimates to those based on the photometry and color of stellar
populations. The comparison reveals that the stellar disks of most of spiral
galaxies in our sample cannot be substantially overheated and are therefore
unlikely to have experienced a significant merging event in their history. The
same conclusion applies to some, but not all of the S0 galaxies we consider.
However, a substantial part of the early type galaxies do show the stellar
velocity dispersion well in excess of the gravitational stability threshold
suggesting a major merger event in the past. We find dynamically overheated
disks among both seemingly isolated galaxies and those forming pairs. The ratio
of the marginal stability disk mass estimate to the total galaxy mass within
four radial scalelengths remains within a range of 0.4---0.8. We see no
evidence for a noticeable running of this ratio with either the morphological
type or color index.Comment: 25 pages, 5 figures, accepted to Astronomy Letter
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