252 research outputs found
What triggers black-hole growth? Insights from star formation rates
We present a new semi-analytic model for the common growth of black holes
(BHs) and galaxies within a hierarchical Universe. The model is tuned to match
the mass function of BHs at z=0 and the luminosity functions of active galactic
nuclei (AGNs) at z<4. We use a new observational constraint, which relates the
luminosity of AGNs to the star-formation rate (SFR) of their host galaxies. We
show that this new constraint is important in various aspects: a) it indicates
that BH accretion events are episodic; b) it favours a scenario in which BH
accretion is triggered by merger events of all mass ratios; c) it constrains
the duration of both merger-induced star-bursts and BH accretion events. The
model reproduces the observations once we assume that only 4 per cent of the
merger events trigger BH accretion; BHs accretion is not related to secular
evolution; and only a few per cent of the mass made in bursts goes into the BH.
We find that AGNs with low or intermediate luminosity are mostly being
triggered by minor merger events, in broad agreement with observations. Our
model matches various observed properties of galaxies, such as the stellar mass
function at z<4 and the clustering of galaxies at redshift zero. This allows us
to use galaxies as a reliable backbone for BH growth, with reasonable estimates
for the frequency of merger events. Other modes of BH accretion, such as
disk-instability events, were not considered here, and should be further
examined in the future.Comment: accepted to MNRAS, minor changes from version
Magnetically Regulated Gas Accretion in High-Redshift Galactic Disks
Disk galaxies are in hydrostatic equilibrium along their vertical axis. The
pressure allowing for this configuration consists of thermal, turbulent,
magnetic and cosmic ray components. For the Milky Way(MW) the thermal pressure
contributes ~10% of the total pressure near the plane, with this fraction
dropping towards higher altitudes. Out of the rest, magnetic fields contribute
~1/3 of the pressure to distances of ~3kpc above the disk plane. In this letter
we attempt to extrapolate these local values to high redshift, rapidly
accreting, rapidly star forming disk galaxies and study the effect of the extra
pressure sources on the accretion of gas onto the galaxies. In particular,
magnetic field tension may convert a smooth cold-flow accretion to clumpy,
irregular star formation regions and rates. The infalling gas accumulates on
the edge of the magnetic fields, supported by magnetic tension. When the mass
of the infalling gas exceeds some threshold mass, its gravitational force
cannot be balanced by magnetic tension anymore, and it falls toward the disk's
plane, rapidly making stars. Simplified estimations of this threshold mass are
consistent with clumpy star formation observed in SINS, UDF, GOODS and GEMS
surveys. We discuss the shortcomings of pure hydrodynamic codes in simulating
the accretion of cold flows into galaxies, and emphasize the need for
magneto-hydrodynamic simulationsComment: 12 pages, 3 figures, accepted to ApJ
Constructing Merger Trees that Mimic N-Body Simulations
We present a simple and efficient empirical algorithm for constructing
dark-matter halo merger trees that reproduce the distribution of trees in the
Millennium cosmological -body simulation. The generated trees are
significantly better than EPS trees. The algorithm is Markovian, and it
therefore fails to reproduce the non-Markov features of trees across short time
steps, except for an accurate fit to the evolution of the average main
progenitor. However, it properly recovers the full main progenitor distribution
and the joint distributions of all the progenitors over long-enough time steps,
, where is the
self-similar time variable and refers to the linear growth of density
fluctuations. We find that the main progenitor distribution is log-normal in
the variable , the variance of linear density fluctuations in a
sphere encompassing mass . The secondary progenitors are successfully drawn
one by one from the remaining mass using a similar distribution function. These
empirical findings may be clues to the underlying physics of merger-tree
statistics. As a byproduct, we provide useful, accurate analytic time-invariant
approximations for the main progenitor accretion history and for halo merger
rates.Comment: 13 pages, 9 figures. Accepted for MNRAS. Minor changes from version
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
An Analytic Model for the Evolution of the Stellar, Gas, and Metal Content of Galaxies
We present an analytic formalism that describes the evolution of the stellar,
gas, and metal content of galaxies. It is based on the idea, inspired by
hydrodynamic simulations, that galaxies live in a slowly-evolving equilibrium
between inflow, outflow, and star formation. We argue that this formalism
broadly captures the behavior of galaxy properties evolving in simulations. The
resulting equilibrium equations for the star formation rate, gas fraction, and
metallicity depend on three key free parameters that represent ejective
feedback, preventive feedback, and re-accretion of ejected material. We
schematically describe how these parameters are constrained by models and
observations. Galaxies perturbed off the equilibrium relations owing to inflow
stochasticity tend to be driven back towards equilibrium, such that deviations
in star formation rate at a given mass are correlated with gas fraction and
anti-correlated with metallicity. After an early gas accumulation epoch,
quiescently star-forming galaxies are expected to be in equilibrium over most
of cosmic time. The equilibrium model provides a simple intuitive framework for
understanding the cosmic evolution of galaxy properties, and centrally features
the cycle of baryons between galaxies and surrounding gas as the driver of
galaxy growth.Comment: 11 pages, MNRAS, accepte
How do dwarf galaxies acquire their mass & when do they form their stars?
We apply a simple, one-equation, galaxy formation model on top of the halos
and subhalos of a high-resolution dark matter cosmological simulation to study
how dwarf galaxies acquire their mass and, for better mass resolution, on over
10^5 halo merger trees, to predict when they form their stars. With the first
approach, we show that the large majority of galaxies within group- and
cluster-mass halos have acquired the bulk of their stellar mass through gas
accretion and not via galaxy mergers. We deduce that most dwarf ellipticals are
not built up by galaxy mergers. With the second approach, we constrain the star
formation histories of dwarfs by requiring that star formation must occur
within halos of a minimum circular velocity set by the evolution of the
temperature of the IGM, starting before the epoch of reionization. We
qualitatively reproduce the downsizing trend of greater ages at greater masses
and predict an upsizing trend of greater ages as one proceeds to masses lower
than m_crit. We find that the fraction of galaxies with very young stellar
populations (more than half the mass formed within the last 1.5 Gyr) is a
function of present-day mass in stars and cold gas, which peaks at 0.5% at
m_crit=10^6-8 M_Sun, corresponding to blue compact dwarfs such as I Zw 18. We
predict that the baryonic mass function of galaxies should not show a maximum
at masses above 10^5.5, M_Sun, and we speculate on the nature of the lowest
mass galaxies.Comment: 6 pages, to appear in "A Universe of Dwarf Galaxies: Observations,
Theories, Simulations", ed. M. Koleva, P. Prugniel & I. Vauglin, EAS Series
(Paris: EDP
The specific star formation rate of high redshift galaxies: the case for two modes of star formation
We study the specific star formation rate (SSFR) and its evolution at
z\gtsim 4, in models of galaxy formation, where the star formation is driven
by cold accretion flows. We show that constant star formation and feedback
efficiencies cannot reproduce the observed trend of SSFR with stellar mass and
its observed lack of evolution at . Model galaxies with \log(M_*) \ltsim
9.5 M show systematically lower specific star formation rates by
orders of magnitudes, while massive galaxies with M_* \gtsim 5 \times 10^{10}
M have up to an order of magnitude larger SSFRs, compared to recent
observations by Stark et al.. To recover these observations we apply an
empirical star formation efficiency in galaxies that scales with the host halo
velocity dispersion as during galaxy mergers. We find that
this modification needs to be of stochastic nature to reproduce the
observations, i.e. only applied during mergers and not during accretion driven
star formation phases. Our choice of star formation efficiency during mergers
allows us to capture both, the boost in star formation at low masses and the
quenching at high masses, and at the same time produce a constant SSFR-stellar
mass relation at z\gtsim 4 under the assumption that most of the observed
galaxies are in a merger triggered star formation phase. Our results suggest
that observed high-z low mass galaxies with high SSFRs are likely to be
frequently interacting systems, which experienced bursts in their star
formation rate and efficiency (mode 1), in contrast to low redshift z \ltsim
3 galaxies which are cold accretion-regulated star forming systems with lower
star formation efficiencies (mode 2).Comment: 5 pages, accepted to MNRAS, replaced by version with including
referees comment
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