72 research outputs found
Angular momentum, accretion and radial flows in chemodynamical models of spiral galaxies
Gas accretion and radial flows are key ingredients of the chemical evolution
of spiral galaxies. They are also tightly linked to each other (accretion
drives radial flows, due to angular momentum conservation) and should therefore
be modelled simultaneously. We summarise an algorithm that can be used to
consistently compute accretion profiles, radial flows and abundance gradients
under quite general conditions and we describe illustrative applications to the
Milky Way. We find that gas-phase abundance gradients strongly depend on the
angular momentum of the accreting material and, in the outer regions, they are
significantly affected by the choice of boundary conditions.Comment: 4 pages, 2 figures. Proceedings of the 592 WE-Heraeus Seminar. To
appear in Astronomische Nachricthen, special issue "Reconstructing the Milky
Way's history: spectroscopic surveys, asteroseismology and chemodynamical
models", Guest Editors C. Chiappini, J. Montalban and M. Steffe
Implications of a spatially resolved main sequence for the size evolution of star-forming galaxies
Two currently debated problems in galaxy evolution, the fundamentally local
or global nature of the main sequence of star formation and the evolution of
the mass-size relation of star forming galaxies (SFGs), are shown to be
intimately related to each other. As a preliminary step, a growth function
is defined, which quantifies the differential change in half-mass radius per
unit increase in stellar mass () due to star
formation. A general derivation shows that , meaning
that is proportional to the relative difference in specific star formation
rate between the outer and inner half of a galaxy, with a dimensionless
structural factor for which handy expressions are provided. As an application,
it is shown that galaxies obeying a fundamentally local main sequence also
obey, to a good approximation, , where is the slope
of the normalized local main sequence ()
and the Sersic index. An exact expression is also provided. Quantitatively,
a fundamentally local main sequence is consistent with SFGs growing along a
stationary mass-size relation, but inconsistent with the continuation at
of evolutionary laws derived at higher . This demonstrates that either the
main sequence is not fundamentally local, or the mass-size relation of SFGs has
converged to an equilibrium state some finite time in the past, or both.Comment: Accepted for publication in MNRA
The density distribution of accreting cosmic filaments as shaped by Kelvin-Helmholtz instability
Cosmic filaments play a crucial role in galaxy evolution transporting gas
from the intergalactic medium into galaxies. However, little is known about the
efficiency of this process and whether the gas is accreted in a homogenous or
clumpy way. Recent observations suggest the presence of broad gas density
distributions in the circumgalactic medium which could be related to the
accretion of filaments. By means of high-resolution hydrodynamical simulations,
we explore here the evolution of cold accreting filaments flowing through the
hot circumgalactic medium (CGM) of high-z galaxies. In particular, we examine
the nonlinear effects of Kelvin-Helmholtz instability (KHI) on the development
of broad gas density distributions and on the formation of cold, dense clumps.
We explore a large parameter space in filament and perturbation properties,
such as, filament Mach number, initial perturbation wavelength, and thickness
of the interface between the filament and the halo. We find that the time
averaged density distribution of the cold gas is qualitatively consistent with
a skewed log-normal probability distribution function (PDF) plus an additional
component in form of a high density tail for high Mach-numbers. Our results
suggest a tight correlation between the accreting velocity and the maximum
densities developing in the filament which is consistent with the variance-Mach
number relation for turbulence. Therefore, cosmological accretion could be a
viable mechanism to produce turbulence and broad gas density distributions
within the CGM.Comment: 12 pages, 14 figures, submitted to MNRAS on April 3rd 201
A high baryon fraction in massive haloes at z~3
We investigate the baryon content of the circumgalactic medium (CGM) within
the virial radius of haloes at z ~ 3, by
modelling the surface brightness profile of the giant Ly nebulae
recently discovered by MUSE around bright QSOs at this redshift. We initially
assume fluorescent emission from cold photo-ionized gas confined by the
pressure of a hot halo. Acceptable CGM baryon fractions (equal or smaller than
the cosmological value) require that the cold gas occupies 1% of the
volume, but is about as massive as the hot gas. CGM baryon fractions as low as
30% of the cosmic value, as predicted by some strongly ejective feedback models
at this redshift, are not easy to reconcile with observations, under our
assumptions, unless both the QSO-hosting haloes at are more massive
than recent BOSS estimates based on clustering and the photo-ionized gas is
colder than expected in a standard QSO ionizing radiation field. We also
consider the option that the emission is dominated by photons scattered from
the QSO broad line region. In this scenario, a very stringent lower limit to
the baryon fraction can be obtained under the extreme assumption of optically
thin scattering. We infer in this case a baryon fraction of at least 70% of the
cosmic value, for fiducial parameters. Lower values require halo masses or gas
temperatures different than expected, or that some mechanism keeps the cold gas
systematically over-pressured with respect to the ambient medium.Comment: Accepted for publication in MNRAS. 22 pages, 10 Figure
Angular Momentum Accretion onto Disc Galaxies
Throughout the Hubble time, gas makes its way from the intergalactic medium into galaxies fuelling their star formation and promoting their growth. One of the key properties of the accreting gas is its angular momentum, which has profound implications for the evolution of, in particular, disc galaxies. Here, we discuss how to infer the angular momentum of the accreting gas using observations of present-day galaxy discs. We first summarize evidence for ongoing inside-out growth of star forming discs. We then focus on the chemistry of the discs and show how the observed metallicity gradients can be explained if gas accretes onto a disc rotating with a velocity 20 - 30% lower than the local circular speed. We also show that these gradients are incompatible with accretion occurring at the edge of the discs and flowing radially inward. Finally, we investigate gas accretion from a hot corona with a cosmological angular momentum distribution and describe how simple models of rotating coronae guarantee the inside-out growth of disc galaxies
The angular momentum-mass relation: a fundamental law from dwarf irregulars to massive spirals
In a CDM Universe, the specific stellar angular momentum ()
and stellar mass () of a galaxy are correlated as a consequence of the
scaling existing for dark matter haloes ().
The shape of this law is crucial to test galaxy formation models, which are
currently discrepant especially at the lowest masses, allowing to constrain
fundamental parameters, e.g. the retained fraction of angular momentum. In this
study, we accurately determine the empirical relation (Fall
relation) for 92 nearby spiral galaxies (from S0 to Irr) selected from the
Spitzer Photometry and Accurate Rotation Curves (SPARC) sample in the
unprecedented mass range . We
significantly improve all previous estimates of the Fall relation by
determining profiles homogeneously for all galaxies, using extended HI
rotation curves, and selecting only galaxies for which a robust could
be measured (converged radial profile). We find the relation to be
well described by a single, unbroken power-law
over the entire mass range, with and orthogonal intrinsic
scatter of dex. We finally discuss some implications for galaxy
formation models of this fundamental scaling law and, in particular, the fact
that it excludes models in which discs of all masses retain the same fraction
of the halo angular momentum.Comment: A&A Letters, accepte
"Zombie" or active? An alternative explanation to the properties of star-forming galaxies at high redshift
Star-forming galaxies at high redshift show anomalous values of infrared
excess, which can be described only by extremizing the existing relations
between the shape of their ultraviolet continuum emission and their
infrared-to-ultraviolet luminosity ratio, or by constructing \textit{ad-hoc}
models of star formation and dust distribution. We present an alternative
explanation, based on unveiled AGN activity, to the existence of such galaxies.
In fact, the presence of a weak AGN configures as a natural scenario in order
to explain the observed spectral properties of such high- objects in terms
of a continuum slope distribution rather than altered infrared excesses, due to
the different shape of the AGN continuum emission with respect to quiescent
galaxies. To this aim, we directly compare the infrared-to-ultraviolet
properties of high-redshift galaxies to those of known categories of AGN
(quasars and Seyferts). We also infer the characteristics of their possible
X-ray emission. We find a strong similarity between the spectral shapes and
luminosity ratios of AGN with the corresponding properties of such galaxies. In
addition, we derive expected X-ray fluxes that are compatible with energetics
from AGN activity. We conclude that a moderate AGN contribution to the UV
emission of such high- objects is a valid alternative to explain their
spectral properties. Even the presence of an active nucleus in each source
would not violate the expected quasar statistics. Furthermore, we suggest that
the observed similarities between anomalous star-forming galaxies and quasars
may provide a benchmark for future theoretical and observational studies on the
galaxy population in the early Universe.Comment: 13 pages, 7 figures, 4 tables, accepted for publication in A&
The instantaneous radial growth rate of stellar discs
We present a new and simple method to measure the instantaneous mass and
radial growth rates of the stellar discs of spiral galaxies, based on their
star formation rate surface density (SFRD) profiles. Under the hypothesis that
discs are exponential with time-varying scalelengths, we derive a universal
theoretical profile for the SFRD, with a linear dependence on two parameters:
the specific mass growth rate and
the specific radial growth rate
of the disc. We test our theory on a sample of 35 nearby spiral galaxies, for
which we derive a measurement of and . 32/35
galaxies show the signature of ongoing inside-out growth (). The typical derived e-folding timescales for mass and radial growth in our
sample are ~ 10 Gyr and ~ 30 Gyr, respectively, with some systematic
uncertainties. More massive discs have a larger scatter in and
, biased towards a slower growth, both in mass and size. We
find a linear relation between the two growth rates, indicating that our galaxy
discs grow in size at ~ 0.35 times the rate at which they grow in mass; this
ratio is largely unaffected by systematics. Our results are in very good
agreement with theoretical expectations if known scaling relations of disc
galaxies are not evolving with time.Comment: MNRAS, accepted. 14 pages, 4 figures, 3 tables. Additional material
(Atlas.pdf) available at
http://www.filippofraternali.com/downloads/index.htm
Most of the cool CGM of star-forming galaxies is not produced by supernova feedback
The characterization of the large amount of gas residing in the galaxy halos,
the so called circumgalactic medium (CGM), is crucial to understand galaxy
evolution across cosmic time. We focus here on the the cool ( K)
phase of this medium around star-forming galaxies in the local universe, whose
properties and dynamics are poorly understood. We developed semi-analytical
parametric models to describe the cool CGM as an outflow of gas clouds from the
central galaxy, as a result of supernova explosions in the disc (galactic
wind). The cloud motion is driven by the galaxy gravitational pull and by the
interactions with the hot ( K) coronal gas. Through a bayesian
analysis, we compare the predictions of our models with the data of the
COS-Halos and COS-GASS surveys, which provide accurate kinematic information of
the cool CGM around more than 40 low-redshift star-forming galaxies, probing
distances up to the galaxy virial radii. Our findings clearly show that a
supernova-driven outflow model is not suitable to describe the dynamics of the
cool circumgalactic gas. Indeed, to reproduce the data, we need extreme
scenarios, with initial outflow velocities and mass loading factors that would
lead to unphysically high energy coupling from the supernovae to the gas and
with supernova efficiencies largely exceeding unity. This strongly suggests
that, since the outflows cannot reproduce most of the cool gas absorbers, the
latter are likely the result of cosmological inflow in the outer galaxy halos,
in analogy to what we have previously found for early-type galaxies.Comment: Accepted for publication on MNRA
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