657 research outputs found
Double barred galaxies at intermediate redshifts: A feasibility study
Despite the increasing number of studies of barred galaxies at intermediate
and high redshifts, double-barred (S2B) systems have only been identified in
the nearby (z<0.04) universe thus far. In this feasibility study we demonstrate
that the detection and analysis of S2Bs is possible at intermediate redshifts
(0.1 < z < 0.5) with the exquisite resolution of the Hubble Space Telescope
Advanced Camera for Surveys (HST/ACS). We identify barred galaxies in the
HST/ACS data of the Great Observatories Origins Deep Survey (GOODS) using a
novel method. The radial profile of the Gini coefficient -- a model-independent
structure parameter -- is able to detect bars in early-type galaxies that are
large enough that they might host an inner bar of sufficient angular size.
Using this method and subsequent examination with unsharp masks and ellipse
fits we identified the two most distant S2Bs currently known (at redshifts
z=0.103 and z=0.148). We investigate the underlying stellar populations of
these two galaxies through a detailed colour analysis, in order to demonstrate
the analysis that could be performed on a future sample of
intermediate-redshift S2Bs. We also identify two S2Bs and five S2B candidates
in the HST/ACS data of the Cosmic Evolution Survey (COSMOS). Our detections of
distant S2Bs show that deep surveys like GOODS and COSMOS have the potential to
push the limit for S2B detection and analysis out by a factor of ten in
redshift and lookback time (z=0.5, t=5Gyr) compared to the previously known
S2Bs. This in turn would provide new insight into the formation of these
objects.Comment: 9 pages + 10 figures. Accepted for publication in MNRAS. Main change
from version 1 is an extension of the introduction/motivation and discussion
section. A full resolution version including colour figures is available at
http://www.astro.unibas.ch/~tlisker/papers/lisker2006_s2b.pd
How young are early-type cluster galaxies ? Quantifying the young stellar component in a rich cluster at z=0.41
We present a new method of quantifying the mass fraction of young stars in
galaxies by analyzing near-ultraviolet (NUV)-optical colors. We focus our
attention on early-type cluster galaxies, whose star formation history is at
present undetermined. Rest-frame NUV (F300W) and optical (F702W) images of
cluster Abell 851 (z=0.41) using HST/WFPC2 allow us to determine a NUV-optical
color-magnitude relation, whose slope is incompatible with a monolithic
scenario for star formation at high redshift. A degeneracy between a young
stellar component and its fractional mass contribution to the galaxy is found,
and a photometric analysis comparing the data with the predictions for a simple
two-stage star formation history is presented. The analysis shows that some of
the early-type galaxies may have fractions higher than 10% of the total mass
content in stars formed at z~0.5. An increased scatter is found in the
color-magnitude relation at the faint end, resulting in a significant fraction
of faint blue early-type systems. This would imply that less massive galaxies
undergo more recent episodes of star formation, and this can be explained in
terms of a positive correlation between star formation efficiency and
luminosity.Comment: Accepted for publication in ApJ Letters. Uses emulateapj.sty. 5 pages
with 3 embedded EPS figure
Finding Double-Barred Galaxies with HST
We show that the detection of double-barred (S2B) galaxies beyond the nearby universe is possible out to redshifts 0.1 z 0.5 with the resolution of the HST Advanced Camera for Surveys. We present the most distant S2B currently known, at z = 0.14
Light versus dark in strong-lens galaxies: Dark matter haloes that are rounder than their stars
We measure the projected density profile, shape and alignment of the stellar
and dark matter mass distribution in 11 strong-lens galaxies. We find that the
projected dark matter density profile - under the assumption of a Chabrier
stellar initial mass function - shows significant variation from galaxy to
galaxy. Those with an outermost image beyond kpc are very well fit by
a projected NFW profile; those with images within 10 kpc appear to be more
concentrated than NFW, as expected if their dark haloes contract due to
baryonic cooling. We find that over several half-light radii, the dark matter
haloes of these lenses are rounder than their stellar mass distributions. While
the haloes are never more elliptical than , their stars can
extend to . Galaxies with high dark matter ellipticity and weak
external shear show strong alignment between light and dark; those with strong
shear () can be highly misaligned. This is reassuring since
isolated misaligned galaxies are expected to be unstable. Our results provide a
new constraint on galaxy formation models. For a given cosmology, these must
explain the origin of both very round dark matter haloes and misaligned
strong-lens systems.Comment: 16 pages, 7 figures, 4 tables. Accepted for publication by MNRA
Double-barred galaxies at intermediate redshifts: a feasibility study
Despite the increasing number of studies of barred galaxies at intermediate and high redshifts, double-barred (S2B) systems have only been identified in the nearby (z†0.04) universe thus far. In this feasibility study, we demonstrate that the detection and analysis of S2Bs is possible at intermediate redshifts (0.1 âČzâČ 0.5) with the exquisite resolution of the Hubble Space Telescope Advanced Camera for Surveys (HST/ACS). We identify barred galaxies in the HST/ACS data of the Great Observatories Origins Deep Survey (GOODS) using a novel method. The radial profile of the Gini coefficient - a model-independent structure parameter - is able to detect bars in early-type galaxies that are large enough that they might host an inner bar of sufficient angular size. Using this method and subsequent examination with unsharp masks and ellipse fits, we identified the two most distant S2Bs currently known (at redshifts z= 0.103 and 0.148). We investigate the underlying stellar populations of these two galaxies through a detailed colour analysis, in order to demonstrate the analysis that could be performed on a future sample of intermediate-redshift S2Bs. We also identify two S2Bs and five S2B candidates in the HST/ACS data of the Cosmic Evolution Survey (COSMOS). Our detections of distant S2Bs show that deep surveys like GOODS and COSMOS have the potential to push the limit for S2B detection and analysis out by a factor of 10 in redshift and look-back time (zâ 0.5, Îtâ 5-Gyr) compared to the previously known S2Bs. This in turn would provide new insight into the formation of these object
A panchromatic analysis of starburst galaxy M82: Probing the dust properties
(Abridged) We combine NUV, optical and IR imaging of the nearby starburst
galaxy M82 to explore the properties of the dust both in the interstellar
medium of the galaxy and the dust entrained in the superwind. The three NUV
filters of Swift/UVOT enable us to probe in detail the properties of the
extinction curve in the region around the 2175A bump. The NUV colour-colour
diagram strongly rules out a Calzetti-type law, which can either reflect
intrinsic changes in the dust properties or in the star formation history
compared to starbursts well represented by such an attenuation law. We
emphasize that it is mainly in the NUV region where a standard Milky-Way-type
law is preferred over a Calzetti law. The age and dust distribution of the
stellar populations is consistent with the scenario of an encounter with M81 in
the recent 400 Myr. The radial gradients of the NUV and optical colours in the
superwind region support the hypothesis that the emission in the wind cone is
driven by scattering from dust grains entrained in the ejecta. The observed
wavelength dependence reveals either a grain size distribution , where is the size of the grain, or a flatter distribution with a
maximum size cutoff, suggesting that only small grains are entrained in the
supernovae-driven wind.Comment: 12 pages, 12 figures, 3 tables, MNRAS, in pres
Towards a Solution for the Ca II Triplet Puzzle : Results from Dwarf Elliptical Galaxies
We present new estimates of ages and metallicities, based on FORS/VLT optical
(4400-5500A) spectroscopy, of 16 dwarf elliptical galaxies (dE's) in the Fornax
Cluster and in Southern Groups. These dE's are more metal-rich and younger than
previous estimates based on narrow-band photometry and low-resolution
spectroscopy. For our sample we find a mean metallicity [Z/H] = -0.33 dex and
mean age 3.5 Gyr, consistent with similar samples of dE's in other environments
(Local Group, Virgo). Three dE's in our sample show emission lines and very
young ages. This suggests that some dE's formed stars until a very recent epoch
and were self-enriched by a long star formation history. Previous observations
of large near-infrared (~8500A) Ca II absorption strengths in these dE's are in
good agreement with the new metallicity estimates, solving part of the
so-called Calcium puzzle.Comment: ApJ Letters accepted, 5 pages emulateapj, 2 figure
Non-gaussianity of optical emission lines in SDSS star-forming galaxies and its implications on galactic outflows
The shape of emission lines in the optical spectra of star-forming galaxies
reveals the kinematics of the diffuse gaseous component. We analyse the shape
of prominent emission lines in a sample of ~53,000 star-forming galaxies from
the Sloan Digital Sky Survey, focusing on departures from gaussianity.
Departures from a single gaussian profile allow us to probe the motion of gas
and to assess the role of outflows. The sample is divided into groups according
to their stellar velocity dispersion and star formation rate. The spectra
within each group are stacked to improve the signal-to-noise ratio of the
emission lines, to remove individual signatures, and to enhance the effect of
star formation rate on the shapes of the emission lines. The moments of the
emission lines, including kurtosis and skewness, are determined. We find that
most of the emission lines in strong star-forming systems unequivocally feature
negative kurtosis. This signature is present in H, H, [N II] and
[S II] in massive galaxies with high star formation rates. We attribute it as
evidence of radial outflows of ionised gas driven by the star formation of the
galaxies. Also, most of the emission lines in low-mass systems with high star
formation rates feature negative skewness, and we interpret it as evidence of
dust obscuration in the galactic disk. These signatures are however absent in
the [O III] line, which is believed to trace a different gas component. The
observed trend is significantly stronger in face-on galaxies, indicating that
star formation drives the outflows along the galactic rotation axis, presumably
the path of least resistance. The data suggest that outflows driven by star
formation exert accumulated impacts on the interstellar medium, and the outflow
signature is more evident in older galaxies as they have experienced a longer
total duration of star formation.Comment: 16 pages, 13 figures, Accepted for publication in PAS
Stellar and total mass in early-type lensing galaxies
For 18 well-observed gravitationally lensed QSOs, we compare new
non-parametric mass profiles for the lensing galaxies with stellar-population
models derived from published HST photometry. The large volume of parameter
space searched -- with respect to the possible star formation histories --
allows us to infer robust estimates and uncertainties for the stellar masses.
The most interesting results are: (1) the transition from little or no dark
matter in the inner regions (<r_e) to dark matter dominating on the ~5r_e scale
(~20 kpc) is clearly seen in massive ellipticals; (2) Such a trend is not seen
in lower-mass galaxies, so that the stellar content dominates the mass budget
out to ~5r_e; (3) the radial gradient in the dark-matter fraction for these
intermediate redshift galaxies agrees with published data on nearby galaxies.
This result can help reconcile the discrepancies found in recent estimates of
dark matter in elliptical galaxies using different techniques (e.g. Planetary
nebulae versus X-ray). The observed trend suggests the stellar component in
massive galaxies extends further out in terms of the dark matter scale radius.Comment: 4 pages, 4 figures, uses emulateapj. Accepted for publication in The
Astrophysical Journal Letter
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