200 research outputs found
The star formation history of early-type galaxies in the fornax cluster
We have measured central line strengths and line-strength gradients for a complete sample of early-type galaxies in the Fornax cluster, comprising 11 elliptical and 11 lenticular galaxies, more luminous than M(_B) = -17. We find that the centres of Fornax ellipticals follow the locus of galaxies of fixed age in Worthey's models and have metallicities varying roughly from half solar to 2.5 times solar. Line-strength gradients indicate that elliptical galaxies do not show age gradients with radius but do exhibit a decrease of ~ 0.4 dex in [Fe/H] between the centre and one effective radius. The centres of lenticular galaxies however exhibit a substantial spread to younger luminosity weighted ages indicating a more extended star formation history. Metallicity gradients are generally shallower than for ellipticals. Five of the faint S0s have experienced a central starburst on top of an underlying older stellar population. Our conclusions are based on several age/metallicity diagnostic diagrams in the Lick/IDS system comprising established indices such as [MgFe] and Hβ as well as new and more sensitive indices such as C4668, Fe3 and Hγ(_A). The inferred difference in the age distribution between lenticular and elliptical galaxies is a robust conclusion as the models generate consistent relative ages using different age and metallicity indicators even though the absolute ages remain uncertain. The absolute age uncertainty is caused by the effects of non-solar abundance ratios which are not accounted for in the stellar population models. We find that Es are generally overabundant in magnesium where the most luminous galaxies show stronger overabundances. The luminosity weighted stellar populations of young S0s are consistent with solar abundance ratios, however the bulges of the two large S0s in our sample have [Mg/Fe] > 0. We have analysed in detail the sources of scatter in the Mg-crg relation by investigating the effects of age, metallicity and [Mg/Fe] variations. We find that young stellar populations are responsible for most of the scatter towards weak Mg-absorption. However, for the roughly coeval ellipticals the scatter at a given ctq is correlated with [Mg/Fe variations; metallicity and age effects seem to be less important. The young luminosity weighted ages of the faint S0s in the Fornax cluster are consistent with the recent discovery that the fraction of S0 galaxies in intermediate redshift clusters is a factor of 2-3 lower than found locally, and suggests that a fraction of the cluster spiral galaxy population has evolved into these faint S0s in the 5 Gyrs interval from z = 0.5 to the present. One of the proposed transformation mechanisms is the galaxy harassment picture. The properties of young S0s with large bulge to disk ratios in our sample are remarkably similar to the proposed end-products of galaxy harassment. However, we note that there are young disky S0s which are unlikely to be the result of harassment. Two of the faintest lenticular galaxies in our sample have blue continua and extremely strong Balmer-line absorption suggesting starbursts < 2 Gyrs ago
Secular evolution in action: central values and radial trends in the stellar populations of boxy bulges
We determine central values and radial trends in the stellar populations of
the bulges of a sample of 28 edge-on S0-Sb disk galaxies, 22 of which are
boxy/peanut-shaped (and therefore barred). Our principal findings are the
following. (1) At a given velocity dispersion, the central stellar populations
of galaxies with boxy/peanut-shaped bulges are indistinguishable from those of
early-type (elliptical and S0) galaxies. Either secular evolution affects
stellar populations no differently to monolithic collapse or mergers, or
secular evolution is not important in the central regions of these galaxies,
despite the fact that they are barred. (2) The radial metallicity gradients of
boxy/peanut-shaped bulges are uncorrelated with velocity dispersion and are, on
average, shallower than those of unbarred early-type galaxies. This is
qualitatively consistent with chemodynamical models of bar formation, in which
radial inflow and outflow smears out pre-existing gradients.Comment: MNRAS Letters accepted. 5 page
Spatially Resolved Spectroscopy of Starburst and Post-Starburst Galaxies in The Rich z~0.55 Cluster CL0016+16
We have used the Low Resolution Imaging Spectrograph (LRIS) on the W.M. Keck
I telescope to obtain spatially resolved spectroscopy of a small sample of six
post-starburst and three dusty-starburst galaxies in the rich cluster CL0016+16
at z=0.55. We use this to measure radial profiles of the Hdelta and OII3727
lines which are diagnostic probes of the mechanisms that give rise to the
abrupt changes in star-formation rates in these galaxies. In the post-starburst
sample we are unable to detect any radial gradients in the Hdelta line
equivalent width - although one galaxy exhibits a gradient from one side of the
galaxy to the other. The absence of Hdelta gradients in these galaxies is
consistent with their production via interaction with the intra-cluster medium,
however, our limited spatial sampling prevents us from drawing robust
conclusions. All members of the sample have early type morphologies, typical of
post-starburst galaxies in general, but lack the high incidence of tidal tails
and disturbances seen in local field samples. This argues against a merger
origin and adds weight to a scenario where truncation by the intra-cluster
medium is at work. The post-starburst spectral signature is consistent over the
radial extent probed with no evidence of OII3727 emission and strong Hdelta
absorption at all radii i.e. the post-starburst classification is not an
aperture effect. In contrast the dusty-starburst sample shows a tendency for a
central concentration of OII3727 emission. This is most straightforwardly
interpreted as the consequence of a central starburst. However, other
possibilities exist such as a non-uniform dust distribution (which is expected
in such galaxies) and/or a non-uniform starburst age distribution. The sample
exhibit late type and irregular morphologies.Comment: accepted for publication in PAS
The Slitless Spectroscopy Data Extraction Software aXe
The methods and techniques for the slitless spectroscopy software aXe, which
was designed to reduce data from the various slitless spectroscopy modes of
Hubble Space Telescope instruments, are described. aXe can treat slitless
spectra from different instruments such as ACS, NICMOS and WFC3 through the use
of a configuration file which contains all the instrument dependent parameters.
The basis of the spectral extraction within aXe are the position, morphology
and photometry of the objects on a companion direct image. Several aspects of
slitless spectroscopy, such as the overlap of spectra, an extraction dependent
on object shape and the provision of flat-field cubes, motivate a dedicated
software package, and the solutions offered within aXe are discussed in detail.
The effect of the mutual contamination of spectra can be quantitatively
assessed in aXe, using spectral and morphological information from the
companion direct image(s). A new method named 'aXedrizzle' for 2D rebinning and
co-adding spectral data, taken with small shifts or dithers, is described. The
extraction of slitless spectra with optimal weighting is outlined and the
correction of spectra for detector fringing for the ACS CCD's is presented.
Auxiliary software for simulating slitless data and for visualizing the results
of an aXe extraction is outlined.Comment: 18 pages, 10 figures, accepted for publication in PASP. A high
resolution version is available at
http://www.stecf.org/software/slitless_software/axe/axe_PASP.pd
The Ages and Metallicities of Early Type Galaxies in the Fornax Cluster
We have measured central line strengths for a complete sample of early type
galaxies in the Fornax cluster, comprising 11 elliptical and 11 lenticular
galaxies, more luminous than M_B=-17. In contrast to the elliptical galaxies in
the sample studied by Gonzalez (and recently revisited by Trager) we find that
the Fornax ellipticals follow the locus of galaxies of fixed age in Worthey's
models and have metallicities varying from roughly solar to three times solar.
The lenticular galaxies however exhibit a substantial spread to younger
luminosity weighted ages indicating a more extended star formation history. We
present measurements of the more sensitive indices: C4668 and Hgamma_A; these
confirm and re-enforce the conclusions that the elliptical galaxies are coeval
and that only the lenticular galaxies show symptoms of late star-formation. The
inferred difference in the age distribution between lenticular and elliptical
galaxies is a robust conclusion as the models generate consistent relative ages
using different age and metallicity indicators even though the absolute ages
remain uncertain. The young luminosity weighted ages of the S0s in the Fornax
cluster are consistent with the recent discovery that the fraction of S0
galaxies in intermediate redshift clusters is a factor of 2-3 lower than found
locally and suggests that a fraction of the cluster spiral galaxy population
has evolved to quiescence in the 5 Gyr interval from z=0.5 to the present. Two
of the faintest lenticular galaxies in our sample have blue continua and strong
Balmer-line absorption suggesting starbursts \la2 Gyrs ago. These may be the
low redshift analogues of the starburst or post-starburst galaxies seen in
clusters at z=0.3, similar to the Hdelta strong galaxies in the Coma cluster.Comment: 6 pages and 3 postscript figures. Accepted for publication in MNRAS
(letter
A way to deal with the fringe-like pattern in VIMOS-IFU data
The use of integral field units is now commonplace at all major observatories
offering efficient means of obtaining spectral as well as imaging information
at the same time. IFU instrument designs are complex and spectral images have
typically highly condensed formats, therefore presenting challenges for the IFU
data reduction pipelines. In the case of the VLT VIMOS-IFU, a fringe-like
pattern affecting the spectra well into the optical and blue wavelength regime
as well as artificial intensity variations, require additional reduction steps
beyond standard pipeline processing. In this research note we propose an
empirical method for the removal of the fringe-like pattern in the spectral
domain and the intensity variations in the imaging domain. We also demonstrate
the potential consequences for data analysis if the effects are not corrected.
Here we use the example of deriving stellar velocity, velocity dispersion and
absorption line-strength maps for early-type galaxies. We derive for each
spectrum, reduced by the ESO standard VIMOS pipeline, a correction-spectrum by
using the median of the eight surrounding spectra as a proxy for the
unaffected, underlying spectrum. This method relies on the fact that our
science targets (nearby ETGs) cover the complete FoV of the VIMOS-IFU with
slowly varying spectral properties and that the exact shape of the fringe-like
pattern is nearly independent and highly variable between neighboring spatial
positions. We find that the proposed correction methods for the removal of the
fringe-like pattern and the intensity variations in VIMOS-IFU data-cubes are
suitable to allow for meaningful data analysis in our sample of nearby
early-type galaxies. Since the method relies on the scientific target
properties it is not suitable for general implementation in the pipeline
software for VIMOS.Comment: 7 pages, 10 figures, accepted for publication in A&A, new version
after comments from language edito
- …