64 research outputs found
Combined single-molecule force and fluorescence measurements for biology
Recent advances in single-molecule techniques allow the application of force to an individual biomolecule whilst simultaneously monitoring its response using fluorescent probes. The effects of applied mechanical load on single-enzyme turnovers, biomolecular interactions and conformational changes can now be studied with nanometer precision and millisecond time resolution
UCDs in the Coma Cluster
As part of the HST/ACS Coma Cluster Treasury Survey, we have undertaken a
Keck/LRIS spectroscopic campaign to determine membership for faint dwarf
galaxies. In the process, we discovered a population of Ultra Compact Dwarf
galaxies (UCDs) in the core region of the Coma cluster. At the distance of
Coma, UCDs are expected to have angular sizes 0.01 < R_e < 0.2 arcsec. With ACS
imaging, we can resolve all but the smallest ones with careful fitting.
Candidate UCDs were chosen based on magnitude, color, and degree of resolution.
We spectroscopically confirm 27 objects as bona fide UCD members of the Coma
cluster, a 60% success rate for objects targeted with M_R < -12. We attribute
the high success rate in part to the high resolution of HST data and to an
apparent large population of UCDs in Coma. We find that the UCDs tend to be
strongly clustered around giant galaxies, at least in the core region of the
cluster, and have a distribution and colors that are similar to globular
clusters. These findings suggest that UCDs are not independent galaxies, but
rather have a star cluster origin. This current study provides the dense
environment datapoint necessary for understanding the UCD population.Comment: 6 pages, 9 figures, to appear in the conference proceedings of "A
Universe of Dwarf Galaxies" (Lyon, June 14-18, 2010
Keck/LRIS Spectroscopic Confirmation of Coma Cluster Dwarf Galaxy Membership Assignments
Keck/LRIS multi-object spectroscopy has been carried out on 140 of some of
the lowest and highest surface brightness faint (19 < R < 22) dwarf galaxy
candidates in the core region of the Coma Cluster. These spectra are used to
measure redshifts and establish membership for these faint dwarf populations.
The primary goal of the low surface brightness sample is to test our ability to
use morphological and surface brightness criteria to distinguish between Coma
Cluster members and background galaxies using high resolution HST/ACS images.
Candidates were rated as expected members, uncertain, or expected background.
From 93 spectra, 51 dwarf galaxy members and 20 background galaxies are
identified. Our morphological membership estimation success rate is ~100% for
objects expected to be members and better than ~90% for galaxies expected to be
in the background. We confirm that low surface brightness is a very good
indicator of cluster membership. High surface brightness galaxies are almost
always background with confusion arising only from the cases of the rare
compact elliptical galaxies. The more problematic cases occur at intermediate
surface brightness. Many of these galaxies are given uncertain membership
ratings, and these were found to be members about half of the time. Including
color information will improve membership determination but will fail for some
of the same objects that are already mis-identified when using only surface
brightness and morphology criteria. Compact elliptical galaxies with B-V colors
~0.2 magnitudes redward of the red sequence in particular require spectroscopic
follow-up. In a sample of 47 high surface brightness, UCD candidates, 19
objects have redshifts which place them in the Coma Cluster. Redshift
measurements are presented and the use of indirect means for establishing
cluster membership is discussed.Comment: Accepted for publication in ApJ, 25 pages, 15 figure
The near-infrared luminosity function of cluster galaxies beyond redshift one
We determined the K band luminosity function (LF) of cluster galaxies at
redshift z~1.2, using near-infrared images of three X-ray luminous clusters at
z=1.11,1.24,1.27. The composite LF was derived down to M*+4, by means of
statistical background subtraction, and is well described by a Schechter
function with K*=20.5 AB mag and alpha=-1.
From the K band composite LF we derived the stellar mass function of cluster
galaxies. Using available X-ray mass profiles we determined the M/L ratios of
these three clusters, which tend to be lower than those measured in the local
universe.
With these data, no significant difference can be seen between the shapes of
the cluster galaxies LF and the LF of field galaxies at similar redshift.
We also found no significant evolution out to z ~1.2 in the bright (<M*+4)
part of the cluster galaxies LF probed in this study, apart from a brightening
of ~1.3 mag of the characteristic magnitude of the high redshift LF. We
confirm, and extend to higher redshift, the result from previous work that the
redshift evolution of the characteristic magnitude M* is consistent with
passive evolution of a stellar population formed at z>2.
The results obtained in this work support and extend previous findings that
most of the stars in bright galaxies were formed at high redshift, and that
K-bright (M>10^11 Msun) galaxies were already in place at z ~ 1.2, at least in
the central regions of X-ray luminous clusters. Together with recent results on
the field galaxies stellar mass function, this implies that most of the stellar
mass is already assembled in massive galaxies by z ~ 1, both in low and high
density environments.Comment: 16 pages, 10 figures, to appear in A&
The HST/ACS Coma Cluster Survey. II. Data Description and Source Catalogs
The Coma cluster was the target of a HST-ACS Treasury program designed for
deep imaging in the F475W and F814W passbands. Although our survey was
interrupted by the ACS instrument failure in 2007, the partially completed
survey still covers ~50% of the core high-density region in Coma. Observations
were performed for 25 fields that extend over a wide range of cluster-centric
radii (~1.75 Mpc) with a total coverage area of 274 arcmin^2. The majority of
the fields are located near the core region of Coma (19/25 pointings) with six
additional fields in the south-west region of the cluster. In this paper we
present reprocessed images and SExtractor source catalogs for our survey
fields, including a detailed description of the methodology used for object
detection and photometry, the subtraction of bright galaxies to measure faint
underlying objects, and the use of simulations to assess the photometric
accuracy and completeness of our catalogs. We also use simulations to perform
aperture corrections for the SExtractor Kron magnitudes based only on the
measured source flux and half-light radius. We have performed photometry for
~73,000 unique objects; one-half of our detections are brighter than the
10-sigma point-source detection limit at F814W=25.8 mag (AB). The slight
majority of objects (60%) are unresolved or only marginally resolved by ACS. We
estimate that Coma members are 5-10% of all source detections, which consist of
a large population of unresolved objects (primarily GCs but also UCDs) and a
wide variety of extended galaxies from a cD galaxy to dwarf LSB galaxies. The
red sequence of Coma member galaxies has a constant slope and dispersion across
9 magnitudes (-21<M_F814W<-13). The initial data release for the HST-ACS Coma
Treasury program was made available to the public in 2008 August. The images
and catalogs described in this study relate to our second data release.Comment: Accepted for publication in ApJS. A high-resolution version is
available at http://archdev.stsci.edu/pub/hlsp/coma/release2/PaperII.pd
The HST/ACS Coma Cluster Survey. II. Data Description and Source Catalogs
The Coma cluster, Abell 1656, was the target of a HST-ACS Treasury program designed for deep imaging in the F475W and F814W passbands. Although our survey was interrupted by the ACS instrument failure in early 2007, the partially-completed survey still covers approximately 50% of the core high density region in Coma. Observations were performed for twenty-five fields with a total coverage area of 274 aremin(sup 2), and extend over a wide range of cluster-centric radii (approximately 1.75 Mpe or 1 deg). The majority of the fields are located near the core region of Coma (19/25 pointings) with six additional fields in the south-west region of the cluster. In this paper we present SEXTRACTOR source catalogs generated from the processed images, including a detailed description of the methodology used for object detection and photometry, the subtraction of bright galaxies to measure faint underlying objects, and the use of simulations to assess the photometric accuracy and completeness of our catalogs. We also use simulations to perform aperture corrections for the SEXTRACTOR Kron magnitudes based only on the measured source flux and its half-light radius. We have performed photometry for 76,000 objects that consist of roughly equal numbers of extended galaxies and unresolved objects. Approximately two-thirds of all detections are brighter than F814W=26.5 mag (AB), which corresponds to the 10sigma, point-source detection limit. We estimate that Coma members are 5-10% of the source detections, including a large population of compact objects (primarily GCs, but also cEs and UCDs), and a wide variety of extended galaxies from cD galaxies to dwarf low surface brightness galaxies. The initial data release for the HST-ACS Coma Treasury program was made available to the public in August 2008. The images and catalogs described in this study relate to our second data release
Ultra-Compact Dwarfs in the Coma Cluster
We have undertaken a spectroscopic search for ultra compact dwarf galaxies
(UCDs) in the dense core of the dynamically evolved, massive Coma cluster as
part of the HST/ACS Coma Cluster Treasury Survey. UCD candidates were initially
chosen based on color, magnitude, degree of resolution within the ACS images,
and the known properties of Fornax and Virgo UCDs. Follow-up spectroscopy with
Keck/LRIS confirmed 27 candidates as members of the Coma Cluster, a success
rate > 60% for targeted objects brighter than M_R = -12. Another 14 candidates
may also prove to be Coma members, but low signal-to-noise spectra prevent
definitive conclusions. An investigation of the properties and distribution of
the Coma UCDs finds these objects to be very similar to UCDs discovered in
other environments. The Coma UCDs tend to be clustered around giant galaxies in
the cluster core and have colors/metallicity that correlate with the host
galaxy. With properties and a distribution similar to that of the Coma cluster
globular cluster population, we find strong support for a star cluster origin
for the majority of the Coma UCDs. However, a few UCDs appear to have stellar
population or structural properties which differentiate them from the old star
cluster populations found in the Coma cluster, perhaps indicating that UCDs may
form through multiple formation channels.Comment: 40 pages, 23 figures, accepted for publication in Ap
Galaxy colours in high redshift X-ray selected clusters - I: Blue galaxy fractions in eight clusters
We present initial results from a wide-field, multi-colour imaging project,
designed to study galaxy evolution in X-ray selected clusters at intermediate
(z~0.25) and high redshifts (z~0.5). In this paper we give blue galaxy
fractions from eight X-ray selected clusters, drawn from a combined sample of
three X-ray surveys. We find that all the clusters exhibit excess blue galaxy
populations over the numbers observed in local systems, though a large scatter
is present in the results. We find no significant correlation of blue fraction
with redshift at z>0.2 although the large scatter could mask a positive trend.
We also find no systematic trend of blue fraction with X-ray luminosity. We
show that the blue fraction is a function of (a) radius within a cluster, (b)
absolute magnitude and (c) the passbands used to measure the colour. We find
that our blue fractions (f_b) from galaxy colours close to restframe (U-B),
f_b~0.4, are systematically higher than those from restframe (B-V) colours,
f_b~0.2. We conclude this effect is real, may offer a partial explanation of
the widely differing levels of blue fraction found in previous studies and may
have implications for biases in optical samples selected in different bands.
While the increasing blue fraction with radius can be interpreted as evidence
of cluster infall of field galaxies, the exact physical processes which these
galaxies undergo is unclear. We estimate that, in the cores of the more massive
clusters, galaxies should be experiencing ram--pressure stripping of galactic
gas by the intra--cluster medium. The fact that our low X-ray luminosity
systems show a similar blue fraction as the high luminosity systems, as well as
a significant blue fraction gradient with radius, implies other physical
effects are also important.Comment: Accepted for publication in MNRA
The Alberta moving beyond breast cancer (AMBER) cohort study: a prospective study of physical activity and health-related fitness in breast cancer survivors
- …