3,117 research outputs found
A Characteristic Mass Scale in the Mass-Metallicity Relation of Galaxies
We study the shape of the gas-phase mass-metallicity relation (MZR) of a
combined sample of present-day dwarf and high-mass star-forming galaxies using
IZI, a Bayesian formalism for measuring chemical abundances presented in Blanc
et al. 2015. We observe a characteristic stellar mass scale at M, above which the ISM undergoes a sharp increase in its
level of chemical enrichment. In the M range the MZR
follows a shallow power-law () with slope
. At approaching M the MZR
steepens significantly, showing a slope of in the
M range, and a flattening towards a constant
metallicity at higher stellar masses. This behavior is qualitatively different
from results in the literature that show a single power-law MZR towards the low
mass end. We thoroughly explore systematic uncertainties in our measurement,
and show that the shape of the MZR is not induced by sample selection, aperture
effects, a changing N/O abundance, the adopted methodology used to construct
the MZR, secondary dependencies on star formation activity, nor diffuse ionized
gas (DIG) contamination, but rather on differences in the method used to
measure abundances. High resolution hydrodynamical simulations can
qualitatively reproduce our result, and suggest a transition in the ability of
galaxies to retain their metals for stellar masses above this threshold. The
MZR characteristic mass scale also coincides with a transition in the scale
height and clumpiness of cold gas disks, and a typical gas fraction below which
the efficiency of star formation feedback for driving outflows is expected to
decrease sharply.Comment: 24 pages, 11 figures, 4 tables, accepted for publication in Ap
The importance of preventive feedback: inference from observations of the stellar masses and metallicities of Milky Way dwarf galaxies
Dwarf galaxies are known to have remarkably low star formation efficiency due
to strong feedback. Adopting the dwarf galaxies of the Milky Way as a
laboratory, we explore a flexible semi-analytic galaxy formation model to
understand how the feedback processes shape the satellite galaxies of the Milky
Way. Using Markov-Chain Monte-Carlo, we exhaustively search a large parameter
space of the model and rigorously show that the general wisdom of strong
outflows as the primary feedback mechanism cannot simultaneously explain the
stellar mass function and the mass--metallicity relation of the Milky Way
satellites. An extended model that assumes that a fraction of baryons is
prevented from collapsing into low-mass halos in the first place can be
accurately constrained to simultaneously reproduce those observations. The
inference suggests that two different physical mechanisms are needed to explain
the two different data sets. In particular, moderate outflows with weak halo
mass dependence are needed to explain the mass--metallicity relation, and
prevention of baryons falling into shallow gravitational potentials of low-mass
halos (e.g. "pre-heating") is needed to explain the low stellar mass fraction
for a given subhalo mass.Comment: 14 pages, 4 figures, accepted for publication in Ap
The connection between the host halo and the satellite galaxies of the Milky Way
Many properties of the Milky Way's dark matter halo, including its mass
assembly history, concentration, and subhalo population, remain poorly
constrained. We explore the connection between these properties of the Milky
Way and its satellite galaxy population, especially the implication of the
presence of the Magellanic Clouds for the properties of the Milky Way halo.
Using a suite of high-resolution -body simulations of Milky Way-mass halos
with a fixed final Mvir ~ 10^{12.1}Msun, we find that the presence of
Magellanic Cloud-like satellites strongly correlates with the assembly history,
concentration, and subhalo population of the host halo, such that Milky
Way-mass systems with Magellanic Clouds have lower concentration, more rapid
recent accretion, and more massive subhalos than typical halos of the same
mass. Using a flexible semi-analytic galaxy formation model that is tuned to
reproduce the stellar mass function of the classical dwarf galaxies of the
Milky Way with Markov-Chain Monte-Carlo, we show that adopting host halos with
different mass-assembly histories and concentrations can lead to different
best-fit models for galaxy-formation physics, especially for the strength of
feedback. These biases arise because the presence of the Magellanic Clouds
boosts the overall population of high-mass subhalos, thus requiring a different
stellar-mass-to-halo-mass ratio to match the data. These biases also lead to
significant differences in the mass--metallicity relation, the kinematics of
low-mass satellites, the number counts of small satellites associated with the
Magellanic Clouds, and the stellar mass of Milky Way itself. Observations of
these galaxy properties can thus provide useful constraints on the properties
of the Milky Way halo.Comment: 20 pages, 12 figures, accepted for publication in ApJ. A new section
on the effect of host halo mass-assembly history on the central galaxy
stellar mass is adde
Identification and characterization of Etv4/5 target genes during ureteric bud branching morphogenesis
Every mapping class group is generated by 6 involutions
Let Mod_{g,b} denote the mapping class group of a surface of genus g with b
punctures. Feng Luo asked in a recent preprint if there is a universal upper
bound, independent of genus, for the number of torsion elements needed to
generate Mod_{g,b}. We answer Luo's question by proving that 3 torsion elements
suffice to generate Mod_{g,0}. We also prove the more delicate result that
there is an upper bound, independent of genus, not only for the number of
torsion elements needed to generate Mod_{g,b} but also for the order of those
elements. In particular, our main result is that 6 involutions (i.e.
orientation-preserving diffeomorphisms of order two) suffice to generate
Mod_{g,b} for every genus g >= 3, b = 0, and g >= 4, b = 1.Comment: 15 pages, 7 figures; slightly improved main result; minor revisions.
to appear in J. Al
Modelling solute transport in soil columns using advective-dispersive equations with fractional spatial derivatives
Solute transport in soils is commonly simulated with the advective–dispersive equation, or ADE. It has been reported that this model cannot take into account several important features of solute movement through soil. Recently, a new model has been suggested that results in a solute transport equation with fractional spatial derivatives, or FADE. We have assembled a database on published solute transport experiments in soil columns to test the new model. The FADE appears to be a useful generalization of the ADE. The order of the fractional differentiation reflects differences in physical conditions of the solute transport in soi
A Bayesian approach to the semi-analytic model of galaxy formation: methodology
We believe that a wide range of physical processes conspire to shape the
observed galaxy population but we remain unsure of their detailed interactions.
The semi-analytic model (SAM) of galaxy formation uses multi-dimensional
parameterisations of the physical processes of galaxy formation and provides a
tool to constrain these underlying physical interactions. Because of the high
dimensionality, the parametric problem of galaxy formation may be profitably
tackled with a Bayesian-inference based approach, which allows one to constrain
theory with data in a statistically rigorous way. In this paper we develop a
SAM in the framework of Bayesian inference. We show that, with a parallel
implementation of an advanced Markov-Chain Monte-Carlo algorithm, it is now
possible to rigorously sample the posterior distribution of the
high-dimensional parameter space of typical SAMs. As an example, we
characterise galaxy formation in the current CDM cosmology using the
stellar mass function of galaxies as an observational constraint. We find that
the posterior probability distribution is both topologically complex and
degenerate in some important model parameters, suggesting that thorough
explorations of the parameter space are needed to understand the models. We
also demonstrate that because of the model degeneracy, adopting a narrow prior
strongly restricts the model. Therefore, the inferences based on SAMs are
conditional to the model adopted. Using synthetic data to mimic systematic
errors in the stellar mass function, we demonstrate that an accurate
observational error model is essential to meaningful inference.Comment: revised version to match published article published in MNRA
Galaxy formation in the Planck cosmology - I. Matching the observed evolution of star formation rates, colours and stellar masses
We have updated the Munich galaxy formation model to the Planck first-year cosmology, while modifying the treatment of baryonic processes to reproduce recent data on the abundance and passive fractions of galaxies from z = 3 down to z = 0. Matching these more extensive and more precise observational results requires us to delay the reincorporation of wind ejecta, to lower the surface density threshold for turning cold gas into stars, to eliminate ram-pressure stripping in haloes less massive than ∼1014 M⊙, and to modify our model for radio mode feedback. These changes cure the most obvious failings of our previous models, namely the overly early formation of low-mass galaxies and the overly large fraction of them that are passive at late times. The new model is calibrated to reproduce the observed evolution both of the stellar mass function and of the distribution of star formation rate at each stellar mass. Massive galaxies (log M⋆/M⊙ ≥ 11.0) assemble most of their mass before z = 1 and are predominantly old and passive at z = 0, while lower mass galaxies assemble later and, for log M⋆/M⊙ ≤ 9.5, are still predominantly blue and star forming at z = 0. This phenomenological but physically based model allows the observations to be interpreted in terms of the efficiency of the various processes that control the formation and evolution of galaxies as a function of their stellar mass, gas content, environment and time
Convergence of Galaxy Properties with Merger Tree Temporal Resolution
Dark matter halo merger trees are now routinely extracted from cosmological
simulations of structure formation. These trees are frequently used as inputs
to semi-analytic models of galaxy formation to provide the backbone within
which galaxy formation takes place. By necessity, these merger trees are
constructed from a finite set of discrete "snapshots" of the N-body simulation
and so have a limited temporal resolution. To date, there has been little
consideration of how this temporal resolution affects the properties of
galaxies formed within these trees. In particular, the question of how many
snapshots are needed to achieve convergence in galaxy properties has not be
answered. Therefore, we study the convergence in the stellar and total baryonic
masses of galaxies, distribution of merger times, stellar mass functions and
star formation rates in the Galacticus model of galaxy formation as a function
of the number of "snapshot" times used to represent dark matter halo merger
trees. When utilizing snapshots between z=20 and z=0, we find that at least 128
snapshots are required to achieve convergence to within 5% for galaxy masses.
This convergence is obtained for mean quantities averaged over large samples of
galaxies - significant variance for individual galaxies remains even when using
very large numbers of snapshots. We find only weak dependence of the rate of
convergence on the distribution of snapshots in time - snapshots spaced
uniformly in the expansion factor, uniformly in the logarithm of expansion
factor or uniformly in the logarithm of critical overdensity for collapse work
equally well in almost all cases. We provide input parameters to Galacticus
which allow this type of convergence study to be tuned to other simulations and
to be carried out for other galaxy properties.Comment: 15 pages, 12 figures, submitted to MNRAS. Parameter files and
plotting scripts available at
http://www.ctcp.caltech.edu/galacticus/parameters/dmTreeConvergence.tar.bz2
and in the anciliary data stored with this arXiv submissio
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