83 research outputs found
Scaling Relations and the Fundamental Line of the Local Group Dwarf Galaxies
We study the scaling relations between global properties of dwarf galaxies in
the Local Group. In addition to quantifying the correlations between pairs of
variables, we explore the "shape" of the distribution of galaxies in log
parameter space using standardised Principal Component Analysis (PCA), the
analysis is performed first in the 3-D structural parameter space of stellar
mass M*, internal velocity V and characteristic radius R* (or surface
brightness mu*). It is then extended to a 4-D space that includes a
stellar-population parameter such as metallicity Z or star formation rate SFR.
We find that the Local-Group dwarfs basically define a one-parameter
"Fundamental Line" (FL), primarily driven by stellar mass, M*. A more detailed
inspection reveals differences between the star-formation properties of dwarf
irregulars (dI's) and dwarf ellipticals (dE's), beyond the tendency of the
latter to be more massive. In particular, the metallicities of dI's are
typically lower by a factor of 3 at a given M* and they grow faster with
increasing M*, showing a tighter FL in the 4-D space for the dE's. The
structural scaling relations of dI's resemble those of the more massive
spirals, but the dI's have lower star formation rates for a given M* which also
grow faster with increasing M*. On the other hand, the FL of the dE's departs
from the fundamental plane of bigger ellipticals. While the one-parameter
nature of the FL and the associated slopes of the scaling relations are
consistent with the general predictions of supernova feedback (Dekel & Woo
2003), the differences between the FL's of the dE's and the dI's remain a
challenge and should serve as a guide for the secondary physical processes
responsible for these two types.Comment: 18 pages, 17 figures. Accepted for publication in the MNRAS. This
latest version includes a revised Fig 5 and new reference
New insight into the relation between star formation activity and dust content in galaxies
(Abridged) We assemble a sample of 3258 low-redshift galaxies from the SDSS
DR6 with complementary photometric observations by GALEX, 2MASS and IRAS at
far-ultraviolet and infrared wavelengths. We use a recent, simple but
physically motivated model to interpret the observed spectral energy
distributions of the galaxies in this sample in terms of statistical
constraints on physical parameters describing the star formation history and
dust content. The focus on a subsample of 1658 galaxies with highest S/N
observations enables us to investigate most clearly several strong correlations
between various derived physical properties of galaxies. We find that the
typical dust mass of a star-forming correlates remarkably well with the star
formation rate (SFR). We also find that the dust-to-stellar mass ratio, the
ratio of dust mass to star formation rate and the fraction of dust luminosity
contributed by the diffuse interstellar medium all correlate strongly with
specific SFR. A comparison with recent models of chemical and dust evolution of
galaxies suggests that these correlations could arise, at least in part, from
an evolutionary sequence. As galaxies form stars, their ISM becomes enriched in
dust, while the drop in gas supply makes the specific SFR decrease.
Interestingly, as a result, a young, actively star-forming galaxy with low
dust-to-gas ratio may still be highly dusty because it contains large amounts
of interstellar gas. This may be important for the interpretation of the
infrared emission from young, gas-rich star-forming galaxies at high redshift.
Our study provides a useful local reference for future statistical studies of
the star formation and dust properties of galaxies at high redshifts.Comment: 15 pages, 10 figures, accepted for publication in MNRAS.
Full-resolution figures available from
http://users.physics.uoc.gr/~dacunha/paper_dacunha.pd
An Investigation of Sloan Digital Sky Survey Imaging Data and Multi-Band Scaling Relations of Spiral Galaxies (with Dynamical Information)
We have compiled a sample of 3041 spiral galaxies with multi-band gri imaging
from the Sloan Digital Sky Survey (SDSS) Data Release 7 and available galaxy
rotational velocities derived from HI line widths. We compare the data products
provided through the SDSS imaging pipeline with our own photometry of the SDSS
images, and use the velocities (V) as an independent metric to determine ideal
galaxy sizes (R) and luminosities (L). Our radial and luminosity parameters
improve upon the SDSS DR7 Petrosian radii and luminosities through the use of
isophotal fits to the galaxy images. This improvement is gauged via VL and RV
relations whose respective scatters are reduced by ~8% and ~30% compared to
similar relations built with SDSS parameters. The tightest VRL relations are
obtained with the i-band radius, R235i, measured at 23.5 mag/arcsec^-2, and the
luminosity L235i, measured within R235i. Our VRL scaling relations compare
well, both in scatter and slope, with similar studies (such comparisons however
depend sensitively on the nature and size of the compared samples). The typical
slopes, b, and observed scatters, sigma, of the i-band VL, RL and RV relations
are bVL=0.27+/-0.01, bRL=0.41+/-0.01, bRV=1.52+/-0.07, and sigmaVL=0.074,
sigmaRL=0.071, sigmaRV=0.154 dex. Similar results for the SDSS g and r bands
are also provided. Smaller scatters may be achieved for more pruned samples. We
also compute scaling relations in terms of the baryonic mass (stars + gas),
Mbar, ranging from 10^8.7 Msol to 10^11.6 Msol. Our baryonic velocity-mass (VM)
relation has slope 0.29+/-0.01 and a measured scatter sigma_meas = 0.076 dex.
While the observed VL and VM relations have comparable scatter, the stellar and
baryonic VM relations may be intrinsically tighter, and thus potentially more
fundamental, than other VL relations of spiral galaxies.Comment: Submitted to MNRAS, comments welcom
Post-starburst galaxies: more than just an interesting curiosity
From the VIMOS VLT DEEP Survey (VVDS) we select a sample of 16 galaxies with
spectra which identify them as having recently undergone a strong starburst and
subsequent fast quenching of star formation. These post-starburst galaxies lie
in the redshift range 0.510^9.75Msun. They have a number
density of 1x10^-4 per Mpc^3, almost two orders of magnitude sparser than the
full galaxy population with the same mass limit. We compare with simulations to
show that the galaxies are consistent with being the descendants of gas rich
major mergers. Starburst mass fractions must be larger than ~5-10% and decay
times shorter than ~10^8 years for post-starburst spectral signatures to be
observed in the simulations. We find that the presence of black hole feedback
does not greatly affect the evolution of the simulated merger remnants through
the post-starburst phase. The multiwavelength spectral energy distributions of
the post-starburst galaxies show that 5/16 have completely ceased the formation
of new stars. These 5 galaxies correspond to a mass flux entering the
red-sequence of rhodot(A->Q, PSB) = 0.0038Msun/Mpc^3/yr, assuming the defining
spectroscopic features are detectable for 0.35Gyr. If the galaxies subsequently
remain on the red sequence, this accounts for 38(+4/-11)% of the growth rate of
the red sequence. Finally, we compare our high redshift results with a sample
of galaxies with 0.05<z<0.1 observed in the SDSS and UKIDSS surveys. We find a
very strong redshift evolution: the mass density of strong post-starburst
galaxies is 230 times lower at z~0.07 than at z~0.7.Comment: 18 pages, 12 figures, to match version accepted to MNRAS. Minor
reordering of text in places and Sec 2.2 on SPH simulation comparisons
expande
A census of metals and baryons in stars in the local Universe
We combine stellar metallicity and stellar mass estimates for a large sample
of galaxies drawn from the SDSS DR2 spanning wide ranges in physical
properties, in order to derive an inventory of the total mass of metals and
baryons locked up in stars today. Physical parameter estimates are derived from
galaxy spectra with high S/N (>20). Coadded spectra of galaxies with similar
velocity dispersions, absolute r-band magnitudes and 4000\AA-break values are
used for those regions of parameter space where individual spectra have lower
S/N. We estimate the total density of metals and of baryons in stars and, from
these two quantities, we obtain a mass- and volume-averaged stellar metallicity
of =1.04+-0.14 Z_sun, i.e. consistent with solar. We also study how
metals are distributed in galaxies according to their mass, morphology and age,
and we then compare these distributions with the corresponding distributions of
stellar mass. We find that the bulk of metals locked up in stars in the local
Universe reside in massive, bulge-dominated galaxies, with red colours and high
4000\AA-break values corresponding to old stellar populations. Bulge-dominated
and disc-dominated galaxies contribute similar amounts to the total stellar
mass density, but have different fractional contributions to the mass density
of metals in stars, in agreement with the mass-metallicity relation.
Bulge-dominated galaxies contain roughly 40% of the total amount of metals in
stars, while disc-dominated galaxies less than 25%. Finally, at a given galaxy
stellar mass, we define two characteristic ages as the median of the
distributions of mass and metals as a function of age. These characteristic
ages decrease progressively from high-mass to low-mass galaxies, consistent
with the high formation epochs of stars in massive galaxies.Comment: replaced with accepted version, minor changes, references adde
Stellar Population and Kinematic Profiles in Spiral Bulges & Disks: Population Synthesis of Integrated Spectra
We present a detailed study of the stellar populations (SPs) and kinematics
of the bulge and inner disk regions of eight nearby spiral galaxies (Sa-Sd)
based on deep Gemini/GMOS data. The long-slit spectra extend to 1-2 disk scale
lengths with S/N/Ang>=50. Several different model fitting techniques involving
absorption-line indices and full spectrum fitting are explored and found to
weigh age, metallicity, and abundance ratios differently. The SPs of spiral
galaxies are not well matched by single episodes of star formation;
representative SPs must involve average SP values integrated over the star
formation history (SFH) of the galaxy. Our "full population synthesis" method
is an optimised linear combination of model templates to the full spectrum with
masking of regions poorly represented by the models. Our spiral bulges follow
the same correlations of increasing light-weighted age and metallicity with
central velocity dispersion as those of elliptical galaxies and early-type
bulges found in other studies, but when SFHs more complex and realistic than a
single burst are invoked, the trend with age is shallower and the scatter much
reduced. In a mass-weighted context, all bulges are predominantly composed of
old and metal-rich SPs. Bulge formation appears to dominated by early processes
that are common to all spheroids, whether they currently reside in disks or
not. While monolithic collapse cannot be ruled out in some cases, merging must
be invoked to explain the SP gradients in most bulges. Further bulge growth via
secular processes or "rejuvenated" star formation generally contributes
minimally to the stellar mass budget. (Abridged)Comment: 38 pages, 19 figures, accepted for publication in MNRA
A simple model to interpret the ultraviolet, optical and infrared emission from galaxies
We present a simple, largely empirical but physically motivated model to
interpret the mid- and far-infrared spectral energy distributions of galaxies
consistently with the emission at ultraviolet, optical and near-infrared
wavelengths. Our model relies on an existing angle-averaged prescription to
compute the absorption of starlight by dust in stellar birth clouds and in the
ambient ISM in galaxies. We compute the spectral energy distribution of the
power reradiated by dust in stellar birth clouds as the sum of three
components: a component of PAHs; a mid-IR continuum characterising the emission
from hot grains; and a component of warm grains in thermal equilibrium with
adjustable temperature. In the ambient ISM, we fix for simplicity the relative
proportions of these three components to reproduce the spectral shape of
diffuse cirrus emission in the Milky Way, and we include a component of cold
grains in thermal equilibrium with adjustable temperature. Our model can be
used to derive statistical constraints on the star formation histories and dust
contents of large samples of galaxies using UV, optical and IR observations. We
illustrate this by deriving median-likelihood estimates of the star formation
rates, stellar masses, effective dust optical depths, dust masses, and relative
strengths of different dust components of 66 well-studied nearby star- forming
galaxies from the Spitzer Infrared Nearby Galaxy Survey (SINGS). From this
analysis, we conclude that the mid- and far-IR colours of galaxies correlate
strongly with the specific star formation rate, as well as with other
galaxy-wide quantities connected to this parameter. Our model can be
straightforwardly applied to interpret UV, optical and IR spectral energy
distributions from any galaxy sample. [abridged]Comment: 27 pages, 15 figures, accepted for publication in MNRAS.
Full-resolution figures available from
ftp://ftp.iap.fr/pub/from_users/dacunha/fullres_figure
Tracers of stellar mass-loss. I. Optical and near-IR colours and surface brightness fluctuations
We present optical and IR integrated colours and SBF magnitudes, computed
from stellar population synthesis models that include emission from the dusty
envelopes surrounding TP-AGB stars undergoing mass-loss. We explore the effects
of varying the mass-loss rate by one order of magnitude around the fiducial
value, modifying accordingly both the stellar parameters and the output spectra
of the TP-AGB stars plus their dusty envelopes. The models are single burst,
and range in age from a few Myr to 14 Gyr, and in metallicity between =
0.0001 and = 0.07; they combine new calculations for the evolution of stars
in the TP-AGB phase, with star plus envelope SEDs produced with the radiative
transfer code DUSTY. We compare these models to optical and near-IR data of
single AGB stars and Magellanic star clusters. This comparison validates the
current understanding of the role of mass-loss in determining stellar
parameters and spectra in the TP-AGB. However, neither broad-band colours nor
SBF measurements in the optical or the near-IR can discern global changes in
the mass-loss rate of a stellar population. We predict that mid-IR SBF
measurements can pick out such changes, and actually resolve whether a relation
between metallicity and mass-loss exists.Comment: 31 pages, 17 figures, accepted for publication in MNRA
Ages and metallicities of early-type galaxies in the SDSS: new insight into the physical origin of the colour-magnitude and the Mg2-sigmaV relations
We exploit recent constraints on the ages and metallicities of early-type
galaxies in the Sloan Digital Sky Survey (SDSS) to gain new insight into the
physical origin of two fundamental relations obeyed by these galaxies: the
colour-magnitude and the Mg2-sigmaV relations. Our sample consists of 26,003
galaxies selected from the SDSS DR2 on the basis of their concentrated light
profiles, for which we have previously derived median-likelihood estimates of
stellar metallicity, light-weighted age and stellar mass. Our analysis provides
the most unambiguous demonstration to date of the fact that both relations are
primarily sequences in stellar mass and that total stellar metallicity,
alpha-elements-to-iron abundance ratio and light-weighted age all increase with
mass along the two relations. For high-mass ellipticals, the dispersion in age
is small and consistent with the error. At the low-mass end, there is a tail
towards younger ages, which dominates the scatter in colour and index strength
at fixed mass. A small, but detectable, intrinsic scatter in the
mass-metallicity relation also contributes to the scatter in the two
observational scaling relations, even at high masses. Our results suggest that
the chemical composition of an early-type galaxy is more tightly related to its
dynamical mass (including stars and dark matter) than to its stellar mass. The
ratio between stellar mass and dynamical mass appears to decrease from the
least massive to the most massive galaxies in our sample.Comment: 22 pages, 19 figures, accepted for publication in MNRAS, couple of
references added and minor changes after proof correctio
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