3,250 research outputs found
A Survey of CN and CH Variations in Galactic Globular Clusters from SDSS Spectroscopy
We present a homogeneous survey of the CN and CH bandstrengths in eight
Galactic globular clusters observed during the course of the Sloan Extension
for Galactic Understanding and Exploration (SEGUE) sub-survey of the SDSS. We
confirm the existence of a bimodal CN distribution among RGB stars in all of
the clusters with metallicity greater than [Fe/H] = -1.7; the lowest
metallicity cluster with an observed CN bimodality is M53, with [Fe/H] ~ -2.1.
There is also some evidence for individual CN groups on the subgiant branches
of M92, M2, and M13, and on the red giant branches of M92 and NGC 5053.
Finally, we quantify the correlation between overall cluster metallicity and
the slope of the CN bandstrength-luminosity plot as a means of further
demonstrating the level of CN-enrichment in cluster giants. Our results agree
well with previous studies reported in the literature.Comment: AJ submitted; 80 pages, 22 figure
Discovery of A Very Bright, Strongly-Lensed z=2 Galaxy in the SDSS DR5
We report on the discovery of a very bright z = 2.00 star-forming galaxy that
is strongly lensed by a foreground z=0.422 luminous red galaxy (LRG). This
system was found in a systematic search for bright arcs lensed by LRGs and
brightest cluster galaxies in the Sloan Digital Sky Survey Data Release 5
sample. Follow-up observations on the Subaru 8.2m telescope on Mauna Kea and
the Astrophysical Research Consortium 3.5m telescope at Apache Point
Observatory confirmed the lensing nature of this system. A simple lens model
for the system, assuming a singular isothermal ellipsoid mass distribution,
yields an Einstein radius of 3.82 +/- 0.03 arcsec or 14.8 +/- 0.1 kpc/h at the
lens redshift. The total projected mass enclosed within the Einstein radius is
2.10 +/- 0.03 x 10^12 M_sun/h, and the magnification factor for the source
galaxy is 27 +/- 1. Combining the lens model with our gVriz photometry, we find
an (unlensed) star formation rate for the source galaxy of 32 M_sun/h / yr,
adopting a fiducial constant star formation rate model with an age of 100 Myr
and E(B-V) = 0.25. With an apparent magnitude of r = 19.9, this system is among
the very brightest lensed z >= 2 galaxies, and provides an excellent
opportunity to pursue detailed studies of the physical properties of an
individual high-redshift star-forming galaxy.Comment: 31 pages, 12 figures, 4 tables, submitted to Ap
The Mechanical Properties of PCNA: Implications for the Loading and Function of a DNA Sliding Clamp
Sliding clamps are toroidal proteins that encircle DNA and act as mobile platforms for DNA replication and repair machinery. To be loaded onto DNA, the eukaryotic sliding clamp Proliferating Cell Nuclear Antigen (PCNA) must be splayed open at one of the subunit-subunit interfaces by the ATP-dependent clamp loader, Replication Factor C, whose clamp-interacting sites form a right-handed spiral. Earlier molecular dynamics (MD) studies suggested that when PCNA opens, it preferentially adopts a right-handed spiral to match the spiral of the clamp loader. Here, analysis of considerably longer MD simulations shows that although the opened form of PCNA can achieve conformations matching the helical pitch of Replication Factor C, it is not biased toward a right-handed spiral structure. A coarse-grained elastic model was also built; its strong correspondence to the all-atom MD simulations of PCNA suggests that the behavior of the open clamp is primarily due to elastic deformation governed by the topology of the clamp domains. The elastic model was further used to construct the energy landscape of the opened PCNA clamp, including conformations that would allow binding to the clamp loader and loading onto double-stranded DNA. A picture of PCNA emerges of a rather flexible protein that, once opened, is mechanically compliant in the clamp opening process
The star formation histories of red and blue low surface brightness disk galaxies
We study the star formation histories (SFH) and stellar populations of 213
red and 226 blue nearly face-on low surface brightness disk galaxies (LSBGs),
which are selected from the main galaxy sample of Sloan Digital Sky Survey
(SDSS) Data Release Seven (DR7). We also want to compare the stellar
populations and SFH between the two groups. The sample of both red and blue
LSBGs have sufficient signal-to-noise ratio in the spectral continua. We obtain
their absorption-line indices (e.g. Mg_2, H\delta_A), D_n(4000) and stellar
masses from the MPA/JHU catalogs to study their stellar populations and SFH.
Moreover we fit their optical spectra (stellar absorption lines and continua)
by using the spectral synthesis code STARLIGHT on the basis of the templates of
Simple Stellar Populations (SSPs). We find that red LSBGs tend to be relatively
older, higher metallicity, more massive and have higher surface mass density
than blue LSBGs. The D_n(4000)-H\delta_A plane shows that perhaps red and blue
LSBGs have different SFH: blue LSBGs are more likely to be experiencing a
sporadic star formation events at the present day, whereas red LSBGs are more
likely to form stars continuously over the past 1-2 Gyr. Moreover, the fraction
of galaxies that experienced recent sporadic formation events decreases with
increasing stellar mass. Furthermore, two sub-samples are defined for both red
and blue LSBGs: the sub-sample within the same stellar mass range of 9.5 <=
log(M_\star/M_\odot) <= 10.3, and the surface brightness limiting sub-sample
with \mu_0(R) <= 20.7 mag arcsec^{-2}. They show consistent results with the
total sample in the corresponding relationships, which confirm that our results
to compare the blue and red LSBGs are robust.Comment: 9 pages, 7 figures, 2 tables, Accepted for publication in A&
Galaxy Zoo: Dust in Spirals
We investigate the effect of dust on spiral galaxies by measuring the
inclination-dependence of optical colours for 24,276 well-resolved SDSS
galaxies visually classified in Galaxy Zoo. We find clear trends of reddening
with inclination which imply a total extinction from face-on to edge-on of 0.7,
0.6, 0.5 and 0.4 magnitudes for the ugri passbands. We split the sample into
"bulgy" (early-type) and "disky" (late-type) spirals using the SDSS fracdeV (or
f_DeV) parameter and show that the average face-on colour of "bulgy" spirals is
redder than the average edge-on colour of "disky" spirals. This shows that the
observed optical colour of a spiral galaxy is determined almost equally by the
spiral type (via the bulge-disk ratio and stellar populations), and reddening
due to dust. We find that both luminosity and spiral type affect the total
amount of extinction, with "disky" spirals at M_r ~ -21.5 mags having the most
reddening. This decrease of reddening for the most luminous spirals has not
been observed before and may be related to their lower levels of recent star
formation. We compare our results with the latest dust attenuation models of
Tuffs et al. We find that the model reproduces the observed trends reasonably
well but overpredicts the amount of u-band attenuation in edge-on galaxies. We
end by discussing the effects of dust on large galaxy surveys and emphasize
that these effects will become important as we push to higher precision
measurements of galaxy properties and their clustering.Comment: MNRAS in press. 25 pages, 22 figures (including an abstract comparing
GZ classifications with common automated methods for selecting disk/early
type galaxies in SDSS data). v2 corrects typos found in proof
Nearby early-type galaxies with ionized gas. The UV emission from GALEX observations
We present GALEX far-ultraviolet (FUV, =1538 \AA) and
near-ultraviolet (NUV, =2316 \AA) surface photometry of 40
early-type galaxies (ETGs) selected from a wider sample of 65 nearby ETGs
showing emission lines in their optical spectra. We derive FUV and NUV surface
brightness profiles, (FUV-NUV) colour profiles and D integrated
magnitudes. We extend the photometric study to the optical {\it r} band from
SDSS imaging for 14 of these ETGs. In general, the (FUV-NUV) radial colour
profiles become redder with galactocentric distance in both rejuvenated ( Gyr) and old ETGs. Colour profiles of NGC 1533, NGC 2962, NGC 2974, NGC
3489, and IC 5063 show rings and/or arm-like structures, bluer than the body of
the galaxy, suggesting the presence of recent star formation. Although seven of
our ETGs show shell systems in their optical image, only NGC 7135 displays
shells in the UV bands. We characterize the UV and optical surface brightness
profiles, along the major axis, using a Sersic law. The Sersic law exponent,
, varies from 1 to 16 in the UV bands. S0 galaxies tend to have lower values
of (). The Sersic law exponent seems to be a watershed: ETGs
with tend to have [/Fe] greater than 0.15, implying a short
star-formation time scale. We find a significant correlation between the
FUVNUV colour and central velocity dispersions , with the UV colours
getting bluer at larger . This trend is likely driven by a combined
effect of `downsizing' and of the mass-metallicity relation.Comment: Accepted for publication in MNRAS, 33 pages, 7 figure
Structural properties of disk galaxies. II. Intrinsic shape of bulges
(Abridged) The structural parameters of a magnitude-limited sample of 148
unbarred S0-Sb galaxies were analyzed to derive the intrinsic shape of their
bulges. We developed a new method to derive the intrinsic shape of bulges based
on the geometrical relationships between the apparent and intrinsic shapes of
bulges and disks. The equatorial ellipticity and intrinsic flattening of bulges
were obtained from the length of the apparent major and minor semi-axes of the
bulge, twist angle between the apparent major axis of the bulge and the galaxy
line of nodes, and galaxy inclination. We found that the intrinsic shape is
well constrained for a subsample of 115 bulges with favorable viewing angles. A
large fraction of them is characterized by an elliptical section (B/A<0.9).
This fraction is 33%, 55%, and 43% if using their maximum, mean, or median
equatorial ellipticity, respectively. Most are flattened along their polar axis
(C<(A+B)/2). The distribution of triaxiality is strongly bimodal. This
bimodality is driven by bulges with Sersic index n>2, or equivalently, by the
bulges of galaxies with a bulge-to-total ratio B/T>0.3. In particular, bulges
with n\leq2 and with B/T\leq0.3 show a larger fraction of oblate axisymmetric
(or nearly axisymmetric) bulges, a smaller fraction of triaxial bulges, and
fewer prolate axisymmetric (or nearly axisymmetric) bulges with respect to
bulges with n>2 and with B/T>0.3, respectively. According to predictions of the
numerical simulations of bulge formation, bulges with n\leq2, which show a high
fraction of oblate axisymmetric (or nearly axisymmetric) shapes and have
B/T\leq0.3, could be the result of dissipational minor mergers. Both major
dissipational and dissipationless mergers seem to be required to explain the
variety of shapes found for bulges with n>2 and B/T>0.3.Comment: 16 pages, 12 figures; accepted for publication in A&
The VIMOS VLT Deep Survey: Tracing the galaxy stellar mass assembly history over the last 8Gyr
We selected a mass-limited sample of 4048 objects from the VIMOS VLT Deep
Survey in the redshift interval 0.5<z<1.3. We used the amplitude of the 4000
Balmer break (Dn4000) to separate the galaxy population and the EW[OII]3727
line as proxy for the star formation activity. We discuss to what extent
stellar mass drives galaxy evolution, showing for the first time the interplay
between stellar ages and stellar masses over the past 8Gyr. Low-mass galaxies
have small Dn4000 and at increasing stellar mass, the galaxy distribution moves
to higher Dn4000 values as observed in the local Universe. As cosmic time goes
by, we witness an increasing abundance of massive spectroscopically ET systems
at the expense of the LT systems. This spectral transformation is a process
started at early epochs and continuing efficiently down to the local Universe.
This is confirmed by the evolution of our type-dependent stellar mass function.
The underlying stellar ages of LT galaxies apparently do not show evolution,
likely as a result of a continuous formation of new stars. All star formation
activity indicators consistently point towards a star formation history peaked
in the past for massive galaxies, with little or no residual star formation
taking place in the most recent epochs. The activity and efficiency of forming
stars are mechanisms that depend on stellar mass, and the mass assembly becomes
progressively less efficient in massive systems as time elapses. The concepts
of star formation downsizing and mass assembly downsizing describe a single
scenario that has a top-down evolutionary pattern. The role of (dry) merging
events seems to be only marginal at z<1.3, as our estimated efficiency in
stellar mass assembly can possibly account for the progressive accumulation of
passively evolving galaxies.Comment: Accepted for pubblication in A&A, 14 pages, 5 figure
Weighing the Giants - I. Weak-lensing masses for 51 massive galaxy clusters: project overview, data analysis methods and cluster images
This is the first in a series of papers in which we measure accurate
weak-lensing masses for 51 of the most X-ray luminous galaxy clusters known at
redshifts 0.15<z<0.7, in order to calibrate X-ray and other mass proxies for
cosmological cluster experiments. The primary aim is to improve the absolute
mass calibration of cluster observables, currently the dominant systematic
uncertainty for cluster count experiments. Key elements of this work are the
rigorous quantification of systematic uncertainties, high-quality data
reduction and photometric calibration, and the "blind" nature of the analysis
to avoid confirmation bias. Our target clusters are drawn from RASS X-ray
catalogs, and provide a versatile calibration sample for many aspects of
cluster cosmology. We have acquired wide-field, high-quality imaging using the
Subaru and CFHT telescopes for all 51 clusters, in at least three bands per
cluster. For a subset of 27 clusters, we have data in at least five bands,
allowing accurate photo-z estimates of lensed galaxies. In this paper, we
describe the cluster sample and observations, and detail the processing of the
SuprimeCam data to yield high-quality images suitable for robust weak-lensing
shape measurements and precision photometry. For each cluster, we present
wide-field color optical images and maps of the weak-lensing mass distribution,
the optical light distribution, and the X-ray emission, providing insights into
the large-scale structure in which the clusters are embedded. We measure the
offsets between X-ray centroids and Brightest Cluster Galaxies in the clusters,
finding these to be small in general, with a median of 20kpc. For offsets
<100kpc, weak-lensing mass measurements centered on the BCGs agree well with
values determined relative to the X-ray centroids; miscentering is therefore
not a significant source of systematic uncertainty for our mass measurements.
[abridged]Comment: 26 pages, 19 figures (Appendix C not included). Accepted after minor
revisio
The Seventh Data Release of the Sloan Digital Sky Survey
This paper describes the Seventh Data Release of the Sloan Digital Sky Survey
(SDSS), marking the completion of the original goals of the SDSS and the end of
the phase known as SDSS-II. It includes 11663 deg^2 of imaging data, with most
of the roughly 2000 deg^2 increment over the previous data release lying in
regions of low Galactic latitude. The catalog contains five-band photometry for
357 million distinct objects. The survey also includes repeat photometry over
250 deg^2 along the Celestial Equator in the Southern Galactic Cap. A
coaddition of these data goes roughly two magnitudes fainter than the main
survey. The spectroscopy is now complete over a contiguous area of 7500 deg^2
in the Northern Galactic Cap, closing the gap that was present in previous data
releases. There are over 1.6 million spectra in total, including 930,000
galaxies, 120,000 quasars, and 460,000 stars. The data release includes
improved stellar photometry at low Galactic latitude. The astrometry has all
been recalibrated with the second version of the USNO CCD Astrograph Catalog
(UCAC-2), reducing the rms statistical errors at the bright end to 45
milli-arcseconds per coordinate. A systematic error in bright galaxy photometr
is less severe than previously reported for the majority of galaxies. Finally,
we describe a series of improvements to the spectroscopic reductions, including
better flat-fielding and improved wavelength calibration at the blue end,
better processing of objects with extremely strong narrow emission lines, and
an improved determination of stellar metallicities. (Abridged)Comment: 20 pages, 10 embedded figures. Accepted to ApJS after minor
correction
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