4,379 research outputs found
The early-type dwarf galaxy population of the Centaurus cluster
We present a photometric study of the early-type dwarf galaxy population of
the Centaurus cluster, aiming at investigating the galaxy luminosity function
(LF) and galaxy scaling relations down to the regime of galaxies with M_V~-10
mag. On deep VLT/FORS1 V- and I-band images of the central part of the cluster,
we identify cluster dwarf-galaxy candidates using both morphological and
surface brightness selection criteria. Photometric and structural parameters of
the candidates are derived from analysis of their surface brightness profiles.
Fundamental scaling relations, such as the colour-magnitude and the
magnitude-surface brightness relation, are used to distinguish the cluster from
the background. We find a flat LF with a slope of \alpha = -1.14 \pm 0.12 for
M_V>-14 mag, when fitting a power law to the completeness-corrected galaxy
number counts. When plotting the central surface brightness of a Sersic model
vs. the galaxy magnitude, we find a continuous relation for magnitudes
-20<M_V<-10 mag, with only the brightest core galaxies deviating from this
relation, in agreement with previous studies of other clusters. In a
size-luminosity diagram of early-type galaxies from a range of environments, we
observe that R_eff slowly decreases with decreasing luminosity for -21<M_V<-13
mag and decreases more rapidly at fainter magnitudes. This trend continues to
the ultra-faint Local Group dwarf galaxies (M_V~-4 mag). The continuous central
surface brightness vs. absolute magnitude relation and the smooth relation in
the size-luminosity diagram over a wide range of magnitudes are consistent with
the interpretation of dwarf galaxies and more massive elliptical galaxies being
one family of objects with gradually changing structural properties. The most
massive core galaxies and the rare cE galaxies are the only exceptions.Comment: 14 pages, 10 figures, 4 tables, accepted for publication in A&
Spectroscopy of clusters in the ESO Distant Cluster Survey (EDisCS)
We present spectroscopic observations of galaxies in 4 clusters at z =
0.7-0.8 and in one cluster at z~0.5 obtained with the FORS2 spectrograph on the
VLT as part of the ESO Distant Cluster Survey (EDisCS), a photometric and
spectroscopic survey of 20 intermediate to high redshift clusters. We describe
our target selection, mask design, observation and data reduction procedures,
using these first 5 clusters to demonstrate how our strategies maximise the
number of cluster members for which we obtain spectroscopy. We present
catalogues containing positions, I-band magnitudes and spectroscopic redshifts
for galaxies in the fields of our 5 clusters. These contain 236 cluster
members, with the number of members per cluster ranging from 30 to 67. Our
spectroscopic success rate, i.e. the fraction of spectroscopic targets which
are cluster members, averages 50% and ranges from 30% to 75%. We use a robust
biweight estimator to measure cluster velocity dispersions from our
spectroscopic redshift samples. We also make a first assessment of substructure
within our clusters. The velocity dispersions range from 400 to 1100 km s-1.
Some of the redshift distributions are significantly non-Gaussian and we find
evidence for significant substructure in two clusters, one at z~0.79 and the
other at z~0.54. Both have velocity dispersions exceeding 1000 km s-1 but are
clearly not fully virialised; their velocity dispersions may thus be a poor
indicator of their masses. The properties of these first 5 EDisCS clusters span
a wide range in redshift, velocity dispersion, richness and substructure, but
are representative of the sample as a whole. Spectroscopy for the full dataset
will allow a comprehensive study of galaxy evolution as a function of cluster
environment and redshift.Comment: 18 pages, 27 figures, accepted for publication in A&A, Table 4 is
available ahead of journal publication by downloading the source files for
this astro-ph submission or from first author on request
([email protected]
STAGES: the Space Telescope A901/2 Galaxy Evolution Survey
We present an overview of the Space Telescope A901/2 Galaxy Evolution Survey
(STAGES). STAGES is a multiwavelength project designed to probe physical
drivers of galaxy evolution across a wide range of environments and luminosity.
A complex multi-cluster system at z~0.165 has been the subject of an 80-orbit
F606W HST/ACS mosaic covering the full 0.5x0.5 (~5x5 Mpc^2) span of the
supercluster. Extensive multiwavelength observations with XMM-Newton, GALEX,
Spitzer, 2dF, GMRT, and the 17-band COMBO-17 photometric redshift survey
complement the HST imaging. Our survey goals include simultaneously linking
galaxy morphology with other observables such as age, star-formation rate,
nuclear activity, and stellar mass. In addition, with the multiwavelength
dataset and new high resolution mass maps from gravitational lensing, we are
able to disentangle the large-scale structure of the system. By examining all
aspects of environment we will be able to evaluate the relative importance of
the dark matter halos, the local galaxy density, and the hot X-ray gas in
driving galaxy transformation. This paper describes the HST imaging, data
reduction, and creation of a master catalogue. We perform Sersic fitting on the
HST images and conduct associated simulations to quantify completeness. In
addition, we present the COMBO-17 photometric redshift catalogue and estimates
of stellar masses and star-formation rates for this field. We define galaxy and
cluster sample selection criteria which will be the basis for forthcoming
science analyses, and present a compilation of notable objects in the field.
Finally, we describe the further multiwavelength observations and announce
public access to the data and catalogues.Comment: 29 pages, 22 figures; accepted to MNRAS. Full data release available
at http://www.nottingham.ac.uk/astronomy/stage
Stops and Stares: Street Stops, Surveillance, and Race in the New Policing
The use of proactive tactics to disrupt criminal activities, such as Terry street stops and concentrated misdemeanor arrests, are essential to the ânew policing.â This model applies complex metrics, strong management, and aggressive enforcement and surveillance to focus policing on high crime risk persons and places. The tactics endemic to the ânew policingâ gave rise in the 1990s to popular, legal, political and social science concerns about disparate treatment of minority groups in their everyday encounters with law enforcement. Empirical evidence showed that minorities were indeed stopped and arrested more frequently than similarly situated whites, even when controlling for local social and crime conditions. In this article, we examine racial disparities under a unique configuration of the street stop prong of the ânew policingâ â the inclusion of non-contact observations (or surveillances) in the field interrogation (or investigative stop) activity of Boston Police Department officers. We show that Boston Police officers focus significant portions of their field investigation activity in two areas: suspected and actual gang members, and the cityâs high crime areas. Minority neighborhoods experience higher levels of field interrogation and surveillance activity net of crime and other social factors. Relative to white suspects, Black suspects are more likely to be observed, interrogated, and frisked or searched controlling for gang membership and prior arrest history. Moreover, relative to their black counterparts, white police officers conduct high numbers of field investigations and are more likely to frisk/search subjects of all races. We distinguish between preference-based and statistical discrimination by comparing stops by officer-suspect racial pairs. If officer activity is independent of officer race, we would infer that disproportionate stops of minorities reflect statistical discrimination. We show instead that officers seem more likely to investigate and frisk or search a minority suspect if officer and suspect race differ. We locate these results in the broader tensions of racial profiling that pose recurring social and constitutional concerns in the ânew policing.â
Special Libraries, September 1969
Volume 60, Issue 7https://scholarworks.sjsu.edu/sla_sl_1969/1006/thumbnail.jp
Clusters at Half Hubble Time: Galaxy Structure and Colors in RXJ0152.7-1357 and MS1054-03
We study the photometric and structural properties of spectroscopically
confirmed members in the two massive X-ray--selected z=0.83 galaxy clusters
MS1054-03 and RXJ0152-1357 using three-band mosaic imaging with the Hubble
Space Telescope Advanced Camera for Surveys. The samples include 105 and 140
members of MS1054-03 and RXJ0152-1357, respectively, with ACS F775W magnitude <
24.0. We develop a promising new structural classification method, based on a
combination of the best-fit Sersic indices and the normalized root-mean-square
residuals from the fits; the resulting classes agree well with the visual ones,
but are less affected by galaxy orientation. We examine the color--magnitude
relations in detail and find that the color residuals correlate with the local
mass density measured from our weak lensing maps; we identify a threshold
density of , in units of the critical density, above which
the star formation appears to cease. For RXJ0152-1357, we also find a trend in
the color residuals with velocity, resulting from an offset of about 980 km/s
in the mean redshifts of the early- and late-type galaxies. Analysis of the
color--color diagrams indicates that a range of star formation time-scales are
needed to reproduce the loci of the galaxy colors. We also identify some
cluster galaxies whose colors can only be explained by large amounts, mag, of internal dust extinction. [Abstract shortened]Comment: 30 pages, emulateapj format; 23 figures, many in color. Accepted by
ApJ; scheduled for the 10 June 2006 issue. Some figures degraded; for a
higher resolution version, see: http://astro.wsu.edu/blakeslee/z1clusters
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