542 research outputs found
Halpha surface photometry of galaxies in the Virgo cluster. IV: the current star formation in nearby clusters of galaxies
Halpha+[NII] imaging observations of 369 late-type galaxies in the Virgo
cluster and in the Coma/A1367 supercluster are analyzed. They constitute an
optically selected sample (m_p<16.0) observed with 60% c.a. completeness.These
observations provide us with the current (T<10^7 yrs) star formation properties
of galaxies. The expected decrease of the star formation rate (SFR),as traced
by the Halpha E.W., with decreasing clustercentric projected distance is found
only when galaxies brighter than M_p=-19.5 are considered. We also include in
our analysis Near Infrared data, providing us with informations on the old
(T>10^9yrs) stars. Put together, the young and the old stellar indicators give
the ratio of currently formed stars over the stars formed in the past, or
"birthrate" parameter b. We also determine the "global gas content" combining
HI with CO observations. We define the "gas deficiency" parameter as the
logarithmic difference between the gas content of isolated galaxies of a given
Hubble type and the measured gas content.For the isolated objects we find that
b decreases with increasing NIR luminosity. The gas-deficient objects,
primarily members to the Virgo cluster, have their birthrate significantly
lower than the isolated objects with normal gas content and of similar NIR
luminosity. This indicates that the current star formation is regulated by the
gaseous content of spirals.Whatever mechanism (most plausibly ram-pressure
stripping) is responsible for the pattern of gas deficiency observed in spiral
galaxies members to rich clusters, it also produces the observed quenching of
the current star formation.Comment: 22 pages,14 figures,3 figures available in jpeg format.To be
published in A&
Charting the evolution of the ages and metallicities of massive galaxies since z=0.7
The stellar populations of intermediate-redshift galaxies can shed light onto
the growth of massive galaxies in the last 8 billion years. We perform deep,
multi-object rest-frame optical spectroscopy with IMACS/Magellan of ~70
galaxies in the E-CDFS with redshift 0.6522.7 and
stellar mass >10^{10}Msun. Following the Bayesian approach adopted for previous
low-redshift studies, we constrain the stellar mass, mean stellar age and
stellar metallicity of individual galaxies from stellar absorption features. We
characterize for the first time the dependence of stellar metallicity and age
on stellar mass at z~0.7 for all galaxies and for quiescent and star-forming
galaxies separately. These relations for the whole sample have a similar shape
as the z=0.1 SDSS analog, but are shifted by -0.28 dex in age and by -0.13 dex
in metallicity, at odds with simple passive evolution. We find that no
additional star formation and chemical enrichment are required for z=0.7
quiescent galaxies to evolve into the present-day quiescent population.
However, this must be accompanied by the quenching of a fraction of z=0.7
Mstar>10^{11}Msun star-forming galaxies with metallicities comparable to those
of quiescent galaxies, thus increasing the scatter in age without affecting the
metallicity distribution. However rapid quenching of the entire population of
massive star-forming galaxies at z=0.7 would be inconsistent with the
age/metallicity--mass relation for the population as a whole and with the
metallicity distribution of star-forming galaxies only, which are on average
0.12 dex less metal-rich than their local counterparts. This indicates chemical
enrichment until the present in at least a fraction of the z=0.7 massive
star-forming galaxies.[abridged]Comment: accepted for publication on ApJ, 26 pages, 13 figure
The ages and metallicities of galaxies in the local universe
We derive stellar metallicities, light-weighted ages and stellar masses for a
magnitude-limited sample of 175,128 galaxies drawn from the Sloan Digital Sky
Survey Data Release Two (SDSS DR2). We compute median-likelihood estimates of
these parameters using a large library of model spectra at medium-high
resolution, covering a comprehensive range of star formation histories. The
constraints we derive are set by the simultaneous fit of five spectral
absorption features, which are well reproduced by our population synthesis
models. By design, these constraints depend only weakly on the alpha/Fe element
abundance ratio. Our sample includes galaxies of all types spanning the full
range in star formation activity, from dormant early-type to actively
star-forming galaxies. We show that, in the mean, galaxies follow a sequence of
increasing stellar metallicity, age and stellar mass at increasing 4000AA-break
strength (D4000). For galaxies of intermediate mass, stronger Balmer absorption
at fixed D4000 is associated with higher metallicity and younger age. We
investigate how stellar metallicity and age depend on total galaxy stellar
mass. Low-mass galaxies are typically young and metal-poor, massive galaxies
old and metal-rich, with a rapid transition between these regimes over the
stellar mass range 3x10^9<M/Msun<3x10^10. Both high- and low-concentration
galaxies follow these relations, but there is a large dispersion in stellar
metallicity at fixed stellar mass, especially for low-concentration galaxies of
intermediate mass. Despite the large scatter, the relation between stellar
metallicity and stellar mass is similar to the correlation between gas-phase
oxygen abundance and stellar mass for star-forming galaxies. [abriged]Comment: 22 pages, 14 figures, accepted for publication on MNRAS, data
available at http://www.mpa-garching.mpg.de/SDSS
Gravitational lens candidates in the E-CDFS
We report ten lens candidates in the E-CDFS from the GEMS survey. Nine of the
systems are new detections and only one of the candidates is a known lens
system. For the most promising five systems including the known lens system, we
present results from preliminary lens mass modelling, which tests if the
candidates are plausible lens systems. Photometric redshifts of the candidate
lens galaxies are obtained from the COMBO-17 galaxy catalog. Stellar masses of
the candidate lens galaxies within the Einstein radius are obtained by using
the -band luminosity and the color-based stellar mass-to-light ratios.
As expected, the lensing masses are found to be larger than the stellar masses
of the candidate lens galaxies. These candidates have similar dark matter
fractions as compared to lenses in SLACS and COSMOS. They also roughly follow
the halo mass-stellar mass relation predicted by the subhalo abundance matching
technique. One of the candidate lens galaxies qualifies as a LIRG and may not
be a true lens because the arc-like feature in the system is likely to be an
active region of star formation in the candidate lens galaxy. Amongst the five
best candidates, one is a confirmed lens system, one is a likely lens system,
two are less likely to be lenses and the status of one of the candidates is
ambiguous. Spectroscopic follow-up of these systems is still required to
confirm lensing and/or for more accurate determination of the lens masses and
mass density profiles.Comment: 12 pages, 5 figures, 3 tables, ApJ accepte
Abundance patterns in early-type galaxies: is there a 'knee' in the [Fe/H] vs. [alpha/Fe] relation?
Early-type galaxies (ETGs) are known to be enhanced in alpha elements, in
accordance with their old ages and short formation timescales. In this
contribution we aim to resolve the enrichment histories of ETGs. This means we
study the abundance of Fe ([Fe/H]) and the alpha-element groups ([alpha/Fe])
separately for stars older than 9.5 Gyr ([Fe/H]o, [alpha/Fe]o) and for stars
between 1.5 and 9.5 Gyr ([Fe/H]i, [alpha/Fe]i). Through extensive simulation we
show that we can indeed recover the enrichment history per galaxy. We then
analyze a spectroscopic sample of 2286 early-type galaxies from the SDSS
selected to be ETGs. We separate out those galaxies for which the abundance of
iron in stars grows throughout the lifetime of the galaxy, i.e. in which
[Fe/H]o < [Fe/H]i. We confirm earlier work where the [Fe/H] and [alpha/Fe]
parameters are correlated with the mass and velocity dispersion of ETGs. We
emphasize that the strongest relation is between [alpha/Fe] and age. This
relation falls into two regimes, one with a steep slope for old galaxies and
one with a shallow slope for younger ETGs. The vast majority of ETGs in our
sample do not show the 'knee' in the plot of [Fe/H] vs. [alpha/Fe] commonly
observed in local group galaxies. This implies that for the vast majority of
ETGs, the stars younger than 9.5 Gyrs are likely to have been accreted or
formed from accreted gas. The properties of the intermediate-age stars in
accretion-dominated ETGs indicate that mass growth through late (minor) mergers
in ETGs is dominated by galaxies with low [Fe/H] and low [alpha/Fe]. The method
of reconstructing the stellar enrichment histories of ETGs introduced in this
paper promises to constrain the star formation and mass assembly histories of
large samples of galaxies in a unique way.Comment: 22 pages, 25 figures, accepted for publication by A&
SPIDER X - Environmental effects in central and satellite early-type galaxies through the stellar fossil record
A detailed analysis of how environment affects the star formation history of
early-type galaxies (ETGs) is undertaken via high signal to noise ratio stacked
spectra obtained from a sample of 20,977 ETGs (morphologically selected) from
the SDSS-based SPIDER survey. Two major parameters are considered for the
study: the central velocity dispersion (sigma), which relates to local drivers
of star formation, and the mass of the host halo, which relates to
environment-related effects. In addition, we separate the sample between
centrals (the most massive galaxy in a halo) and satellites. We derive trends
of age, metallicity, and [alpha/Fe] enhancement, with sigma. We confirm that
the major driver of stellar population properties in ETGs is velocity
dispersion, with a second-order effect associated to the central/satellite
nature of the galaxy. No environmental dependence is detected for satellite
ETGs, except at low sigma - where satellites in groups or in the outskirts of
clusters tend to be younger than those in the central regions of clusters. In
contrast, the trends for centrals show a significant dependence on halo mass.
Central ETGs in groups (i.e. with a halo mass >10^12.5 M_Sun) have younger
ages, lower [alpha/Fe], and higher internal reddening, than "isolated" systems
(i.e. centrals residing in low-mass, <10^12.5 M_Sun, halos). Our findings imply
that central ETGs in groups formed their stellar component over longer time
scales than "isolated" centrals, mainly because of gas-rich interactions with
their companion galaxies.Comment: 22 pages, 19 figures, accepted for publication in MNRA
The large molecular gas fraction of post-starburst galaxies at z > 1
Post-starburst galaxies are sources that had the last major episode of star
formation about 1 Gyr before the epoch of the observations and are on their way
to quiescence. It is important to study such galaxies at redshift z > 1, during
their main quenching phase, and estimate their molecular gas content to
constrain the processes responsible for the cessation of star formation. We
present CO(3-2) ALMA observations of two massive (Mstar ~ 5 x 10^10 Msun)
post-starburst galaxies at z > 1. We measure their molecular gas fraction to be
f_H2 = M_H2/Mstar ~ 8% - 16%, consistent with z < 1 post-starburst galaxies
from the literature. The star formation efficiency of our targets is ~ 10x
lower than that of star-forming galaxies at similar redshift, and they are
outliers of the f_H2 - specific star formation rate (sSFR) relation of
star-forming galaxies, as they have larger f_H2 than expected given their sSFR.
The gas fraction of post-starbursts from our sample and the literature
correlates with the Dn4000 spectral index, a proxy of the stellar population
age. This suggests that their gas content decreases after the last major burst
of star formation. Finally, one of our targets is undergoing a major merger
phase with two highly star-forming companions. This hints at a picture where a
perturber event (e.g., major merger) quenches star formation without completely
removing the molecular gas.Comment: Accepted for publication in MNRA
Applicability of the Decision Matrix of North Eastern Atlantic, Mediterranean and connected seas Tsunami Warning System to the Italian tsunamis
After the 2004 Indian Ocean tsunami catastrophe, UNESCO through the IOC (Intergovernmental Oceanographic Commission) sponsored the establishment of Intergovernmental Coordination Groups (ICG) with the aim to devise and implement Tsunami Warning Systems (TWSs) in all the oceans exposed to tsunamis, in addition to the one already in operation in the Pacific (PTWS). In this context, since 2005 efforts started for the establishment of TWSs in the Indian Ocean (IOTWS), in the Caribbean area (CARIBE EWS) and in the North Eastern Atlantic, the Mediterranean and Connected Seas (NEAMTWS).
In this paper we focus on a specific tool that was first introduced in the PTWS routine operations, i.e. the Decision Matrix (DM). This is an easy-to-use table establishing a link between the main parameters of an earthquake and the possible ensuing tsunami in order to make quick decision on the type of alert bulletins that a Tsunami Warning Center launches to its recipients. In the process of implementation of a regional TWS for the NEAM area, two distinct DMs were recently proposed by the ICG/NEAMTWS, one for the Atlantic and the other for the entire Mediterranean area.
This work applies the Mediterranean NEAMTWS DM to the earthquakes recorded in Italy and compares the action predicted by the DM vs. the action that should be appropriate in view of the observed tsunami characteristics with the aim to establish how good the performance of the Italian TWS will be when it uses the DM for future events. To this purpose, we make use of the parametric catalogue of the Italian earthquakes (CPTI04) compiled in 2004 and the most recent compilation of the Italian tsunami, based on the Italian Tsunami Catalogue of 2004 and the subsequent revisions. In order to better compare the TWS actions we have identified four different kinds of action coding them from 0 to 3 according to the tsunami severity and have further considered three different distance ranges where these actions apply, that is local, regional and basin-wide, that refer to the distance of the message recipients from the tsunami source. The result of our analysis is that the actions prescribed by the DM are adequate only in 45%-55% of the cases, overestimations are about 37% and underestimations are the rest. As a whole, the predictive ability of the DM is not satisfactory, which implies that recipients have the difficult task to manage bulletins carrying a great deal of uncertainty and on the other hand also suggests that strategies to improve the DM or to go beyond the DM need to be found
Differential stellar population models: how to reliably measure [Fe/H] and [alpha/Fe] in galaxies
We present differential stellar population models, which allow improved
determinations of the ages, iron and alpha-element abundances of old stellar
populations from spectral fitting. These new models are calibrated at solar
abundances using the predictions from classical, semi-empirical stellar
population models. We then use the predictive power of fully synthetic models
to compute predictions for different [Fe/H] and [alpha/Fe]. We show that these
new differential models provide remarkably accurate fits to the integrated
optical spectra of the bulge globular clusters NGC6528 and NGC6553, and that
the inferred [Fe/H] and [alpha/Fe] agree with values derived elsewhere from
stellar photometry and spectroscopy. The analysis of a small sample of SDSS
early-type galaxies further confirms that our alpha-enhanced models provide a
better fit to the spectra of massive ellipticals than the solar-scaled ones.
Our approach opens new opportunities for precision measurements of abundance
ratios in galaxies.Comment: 5 pages, 5 figures, MNRAS in pres
Microbiological and serological monitoring in hooded crow (Corvus corone cornix) in the Region Lombardia, Italy
The health status of 276 hooded crows (Corvus corone cornix) from various provinces of Lombardy was monitored for three years. Bacteriological examination detected E. coli (76%), Campylobacter jejuni (17%), Salmonella typhimurium (11.6%), Yersinia spp. (6.5%), Clamydophila abortus and C. psittaci (2.6%); from six birds showing severe prostration Pasteurella multocida was isolated. Virological and serological tests were negative for Avian Influenza virus (AIV), West Nile virus (WNV) and only three samples were positive for Newcastle disease virus (NDV) but only at serology (titre 1:16)
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