1,325 research outputs found
The Origin of [OII] Emission in Recently Quenched AGN Hosts
We have employed emission-line diagnostics derived from DEIMOS and NIRSPEC
spectroscopy to determine the origin of the [OII] emission line observed in six
AGN hosts at z~0.9. These galaxies are a subsample of AGN hosts detected in the
Cl1604 supercluster that exhibit strong Balmer absorption lines in their
spectra and appear to be in a post-starburst or post-quenched phase, if not for
their [OII] emission. Examining the flux ratio of the [NII] to Halpha lines, we
find that in five of the six hosts the dominant source of ionizing flux is AGN
continuum emission. Furthermore, we find that four of the six galaxies have
over twice the [OII] line luminosity that could be generated by star formation
processes alone given their Halpha line luminosities. This strongly suggests
that AGN-excited narrow-line emission is contaminating the [OII] line flux. A
comparison of star formation rates calculated from extinction-corrected [OII]
and Halpha line luminosities indicates that the former yields a five-fold
overestimate of current activity in these galaxies. Our findings reveal the
[OII] line to be a poor indicator of star formation activity in a majority of
these moderate-luminosity Seyferts. This result bolsters our previous findings
that an increased fraction of AGN at high redshifts are hosted by galaxies in a
post-starburst phase. The relatively high fraction of AGN hosts in the Cl1604
supercluster that show signs of recently truncated star formation activity
suggest AGN feedback may play an increasingly important role in suppressing
ongoing activity in large-scale structures at high redshift.Comment: 5 Pages, 4 Figures, submitted to ApJ
The Determination of the Star Formation Rate in Galaxies
A spectrophotometric model able to compute the integrated spectrum of a
galaxy, including the contribution both of the stellar populations and of the
ionized interstellar gas of the HII regions powered by young hot stars, has
been used to study several spectral features and photometric quantities in
order to derive calibrations of the star formation history of late type
galaxies. Attention has been paid to analyze the emission of the Balmer lines
and the [OII]3727 line to test their attitude at providing estimates
of the present star formation rate in galaxies. Other features, like D
and the equivalent width of the H line, influenced by the presence
of intermediate age stars, have been considered. Several ways of estimating the
star formation rates in normal galaxies are discussed and some considerations
concerning the applicability of the models are presented. Criteria have been
also studied for ascertaining the presence of a burst, current or ended not
long ago. Bursts usually hinder the determination of the past star formation
rate.Comment: 21 pages, needs A&A l-aa.sty, accepted for publication in Astronomy &
Astrophysic
The evolution of the colour-magnitude relation and of the star formation activity in galaxy clusters since z~0.8
We present recent results on the evolution of the colour-magnitude relation
and of the star formation activity in galaxy clusters since z~0.8. Results are
based on the ESO Distant Cluster Survey (EDisCS) - an ESO large programme aimed
at the study of cluster structure and cluster galaxy evolution over a
significant fraction of cosmic time - and are discussed in the framework of the
current standard paradigm of structure formation.Comment: 8 pages, 5 figure, to appear in ASP Conference Series (Proceedings of
the 1st Subaru International Conference "Panoramic Views of Galaxy Formation
and Evolution", held in Japan, 10-15 December 2007
The connection between galaxy structure and quenching efficiency
Using data from the SDSS-DR7, including structural measurements from 2D
surface brightness fits with GIM2D, we show how the fraction of quiescent
galaxies depends on galaxy stellar mass , effective radius , fraction
of band light in the bulge, , and their status as a central or
satellite galaxy at . For central galaxies we confirm that the
quiescent fraction depends not only on stellar mass, but also on . The
dependence is particularly strong as a function of , with
. This appears to be driven by a simple dependence on
over the mass range , and is qualitatively
similar even if galaxies with are excluded. For satellite galaxies,
the quiescent fraction is always larger than that of central galaxies, for any
combination of , and . The quenching efficiency is not
constant, but reaches a maximum of for galaxies with and kpc. This is the same region of parameter
space in which the satellite fraction itself reaches its maximum value,
suggesting that the transformation from an active central galaxy to a quiescent
satellite is associated with a reduction in due to an increase in
dominance of a bulge component.Comment: 17 pages, 28 figures, accepted to MNRAS. Catalog available at
http://quixote.uwaterloo.ca/~mbalogh/downloads/Omand_published.fit
Optical Spectral Signatures of Dusty Starburst Galaxies
We analyse the optical spectral properties of the complete sample of Very
Luminous Infrared Galaxies presented by Wu et al. (1998a,b) and we find a high
fraction (~50 %) of spectra showing both a strong H_delta line in absorption
and relatively modest [OII] emission (e(a) spectra). The e(a) signature has
been proposed as an efficient method to identify dusty starburst galaxies and
we study the star formation activity and the nature of these galaxies, as well
as the effects of dust on their observed properties. We examine their emission
line characteristics, in particular their [OII]/H_alpha ratio, and we find this
to be greatly affected by reddening. A search for AGN spectral signatures
reveals that the e(a)'s are typically HII/LINER galaxies. We compare the star
formation rates derived from the FIR luminosities with the estimates based on
the H_alpha line and find that the values obtained from the optical emission
lines are a factor of 10-70 (H_alpha) and 20-140 ([OII]) lower than the FIR
estimates (50-300 M_sun yr^-1). We then study the morphological properties of
the e(a) galaxies, looking for a near companion or signs of a
merger/interaction. In order to explore the evolution of the e(a) population,
we present an overview of the available observations of e(a)'s in different
environments both at low and high redshift. Finally, we discuss the role of
dust in determining the e(a) spectral properties and we propose a scenario of
selective obscuration in which the extinction decreases with the stellar age.Comment: 26 pages, Latex, including 7 postscript figures, accepted for
publication in the Astrophysical Journa
The role of E+A and post-starburst galaxies – II. Spectral energy distributions and comparison with observations
‘The definitive version is available at www3.interscience.wiley.com '. Copyright Royal Astronomical Society. DOI: 10.1111/j.1365-2966.2009.15036.xIn a previous paper, we have shown that the classical definition of E+A galaxies excludes a significant number of post-starburst galaxies. We suggested that analysing broad-band spectral energy distributions (SEDs) is a more comprehensive method to select and distinguish post-starburst galaxies than the classical definition of measuring equivalent widths of (Hδ) and [O ii] lines. In this paper, we will carefully investigate this new method and evaluate it by comparing our model grid of post-starburst galaxies to observed E+A galaxies from the MORPHS catalogue. In the first part, we investigate the UV-optical-NIR (near-infrared) SEDs of a large variety in terms of progenitor galaxies, burst strengths and time-scales of post-starburst models and compare them to undisturbed spiral, S0 and E galaxies as well as to galaxies in their starburst phase. In the second part, we compare our post-starburst models with the observed E+A galaxies in terms of Lick indices, luminosities and colours. We then use the new method of comparing the model SEDs with SEDs of the observed E+A galaxies. We find that the post-starburst models can be distinguished from undisturbed spiral, S0 and E galaxies and galaxies in their starburst phase on the basis of their SEDs. It is even possible to distinguish most of the different post-starbursts by their SEDs. From the comparison with observations, we find that all observed E+A galaxies from the MORPHS catalogue can be matched by our models. However, only models with short decline time-scales for the star formation rate are possible scenarios for the observed E+A galaxies in agreement with our results from the first paper.Peer reviewe
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