354 research outputs found
A Broad Search for Counterrotating Gas and Stars: Evidence for Mergers and Accretion
We measure the frequency of bulk gas-stellar counterrotation in a sample of
67 galaxies drawn from the Nearby Field Galaxy Survey, a broadly representative
survey of the local galaxy population down to M_B-15. We detect 4
counterrotators among 17 E/S0's with extended gas emission (24% +8 -6). In
contrast, we find no clear examples of bulk counterrotation among 38 Sa-Sbc
spirals, although one Sa does show peculiar gas kinematics. This result implies
that, at 95% confidence, no more than 8% of Sa-Sbc spirals are bulk
counterrotators. Among types Sc and later, we identify only one possible
counterrotator, a Magellanic irregular. We use these results together with the
physical properties of the counterrotators to constrain possible origins for
this phenomenon.Comment: 19 pages, 4 figures, AJ, accepte
A High Merger Fraction in the Rich Cluster MS1054-03 at z=0.83: Direct Evidence for Hierarchical Formation of Massive Galaxies
We present a morphological study of the galaxy population of the luminous
X-ray cluster MS1054-03 at z=0.83. The sample consists of 81 spectroscopically
confirmed cluster members in a 3 x 2 Mpc area imaged in F606W and F814W with
WFPC2. We find thirteen ongoing mergers in MS1054-03, comprising 17% of the L >
L* cluster population. Most of these mergers will likely evolve into luminous
(\sim 2 L*) elliptical galaxies, and some may evolve into S0 galaxies. Assuming
the galaxy population in MS1054-03 is typical for its redshift it is estimated
that \sim 50% of present-day cluster ellipticals experienced a major merger at
z < 1. The mergers are preferentially found in the outskirts of the cluster,
and probably occur in small infalling clumps. Morphologies, spectra, and colors
of the mergers show that their progenitors were typically E/S0s or early-type
spirals with mean stellar formation redshifts z* \gtrsim 1.7. The red colors of
the merger remnants are consistent with the low scatter in the color-magnitude
relation in rich clusters at lower redshift. The discovery of a high fraction
of mergers in this young cluster is direct evidence against formation of
ellipticals in a single ``monolithic'' collapse at high redshift, and in
qualitative agreement with predictions of hierarchical models for structure
formation.Comment: Added GIF version of Figure 1. At
http://www.astro.rug.nl/~dokkum/preprints/merger_fig1.eps.gz the PS file is
available. Accepted for publication in ApJ Letter
The Cryogenic Refractive Indices of S-FTM16, a Unique Optical Glass for Near-Infrared Instruments
The Ohara glass S-FTM16 is of considerable interest for near-infrared optical
designs because it transmits well through the K band and because negative
S-FTM16 elements can be used to accurately achromatize positive calcium
fluoride elements in refractive collimators and cameras. Glass manufacturers
have sophisticated equipment to measure the refractive index at room
temperature, but cannot typically measure the refractive index at cryogenic
temperatures. Near-infrared optics, however, are operated at cryogenic
temperatures to reduce thermal background. Thus we need to know the temperature
dependence of S-FTM16's refractive index. We report here our measurements of
the thermal dependence of S-FTM16's refractive index between room temperature
and ~77 K. Within our measurement errors we find no evidence for a wavelength
dependence or a nonlinear temperature term so our series of measurements can be
reduced to a single number. We find that Delta n_{abs} / Delta T = -2.4x10^{-6}
K^{-1} between 298 K and ~77 K and in the wavelength range 0.6 micron to 2.6
micron. We estimate that the systematic error (which dominates the measurement
error) in our measurement is 10%, sufficiently low for most purposes. We also
find the integrated linear thermal expansion of S-FTM16 between 298 K and 77 K
is -0.00167 m m^{-1}.Comment: 8 pages, including 9 figures. Uses emulateapj.cls. Accepted for
publication in PAS
The Chandra XBootes Survey - III: Optical and Near-IR Counterparts
The XBootes Survey is a 5-ks Chandra survey of the Bootes Field of the NOAO
Deep Wide-Field Survey (NDWFS). This survey is unique in that it is the largest
(9.3 deg^2), contiguous region imaged in X-ray with complementary deep optical
and near-IR observations. We present a catalog of the optical counterparts to
the 3,213 X-ray point sources detected in the XBootes survey. Using a Bayesian
identification scheme, we successfully identified optical counterparts for 98%
of the X-ray point sources. The optical colors suggest that the optically
detected galaxies are a combination of z<1 massive early-type galaxies and
bluer star-forming galaxies whose optical AGN emission is faint or obscured,
whereas the majority of the optically detected point sources are likely quasars
over a large redshift range. Our large area, X-ray bright, optically deep
survey enables us to select a large sub-sample of sources (773) with high X-ray
to optical flux ratios (f_x/f_o>10). These objects are likely high redshift
and/or dust obscured AGN. These sources have generally harder X-ray spectra
than sources with 0.1<f_x/f_o<10. Of the 73 X-ray sources with no optical
counterpart in the NDWFS catalog, 47 are truly optically blank down to R~25.5
(the average 50% completeness limit of the NDWFS R-band catalogs). These
sources are also likely to be high redshift and/or dust obscured AGN.Comment: 19 pages, 13 figures, ApJ accepted. Catalog can be found at:
http://www.noao.edu/noao/noaodeep or
ftp://archive.noao.edu/pub/catalogs/xbootes
The Color-Magnitude Relation in CL 1358+62 at z=0.33: Evidence for Significant Evolution in the S0 Population
We use a large mosaic of HST WFPC2 images to measure the colors and
morphologies of 194 spectroscopically confirmed members of the rich galaxy
cluster CL1358+62 at z=0.33. We study the color-magnitude (CM) relation as a
function of radius in the cluster. The intrinsic scatter in the restframe B-V
CM relation of the elliptical galaxies is very small: ~0.022 magnitudes. The CM
relation of the ellipticals does not depend significantly on the distance from
the cluster center. In contrast, the CM relation for the S0 galaxies does
depend on radius: the S0s in the core follow a CM relation similar to the
ellipticals, but at large radii (R>0.7Mpc) the S0s are systematically bluer and
the scatter in the CM relation approximately doubles to ~0.043 magnitudes. The
blueing of the S0s is significant at the 95% confidence level. These results
imply that the S0 galaxies in the outer parts of the cluster have formed stars
more recently than the S0s in the inner parts. A likely explanation is that
clusters at z=0.33 continue to accrete galaxies and groups from the field and
that infall extinguishes star formation. The apparent homogeneity of the
elliptical galaxy population implies that star formation in recently accreted
ellipticals was terminated well before accretion occurred. We have constructed
models to explore the constraints that these observations place on the star
formation history of cluster galaxies. We conclude that the population of S0s
in clusters is likely to evolve as star forming galaxies are converted into
passively evolving galaxies. Assuming a constant accretion rate after z=0.33,
we estimate ~15% of the present day early-type galaxy population in rich
clusters was accreted between z=0.33 and z=0. The ellipticals (and the
brightest S0s) are probably a more stable population, at least since z=0.6.Comment: Accepted for publication in the ApJ. 20 pages, 12 figures. Full
version and plates available at http://www.astro.rug.nl/~dokkum/papers.htm
An analysis of spectra in the Red Rectangle nebula
This paper presents an analysis of a series of spectra in the Red Rectangle
nebula. Only the reddest part of the spectra can safely be attributed to light
from the nebula, and indicates Rayleigh scattering by the gas, in conformity
with the large angles of scattering involved and the proximity of the star. In
the blue, light from HD44179, refracted or scattered in the atmosphere,
dominates the spectra. This paper questions the reliability of ground-based
observations of extended objects in the blue.Comment: 25 figure
V and R-band Galaxy Luminosity Functions and Low Surface Brightness Galaxies in the Century Survey
We use 64 square degrees of deep V and R CCD images to measure the local V
and R band luminosity function of galaxies. The V_0<16.7 and R_0<16.2 redshift
samples contain 1250 galaxies and are 98% complete. We apply k-corrections
before the magnitude selection so that the completeness is to the same depth
for all spectral types. The V and R faint end slopes are surprisingly
identical: alpha = -1.07+-0.09. Representative Schechter function parameters
for H_0=100 are: M^*_R = -20.88+-0.09, phi^*_R = 0.016+-0.003 Mpc^-3 and M^*_V
= -20.23+-0.09, phi^*_V = 0.020+-0.003 Mpc^-3. The V and R local luminosity
densities, j_R = (1.9+-0.6)x10^8 and j_V = (2.2+-0.7)x10^8 L_sun, are in
essential agreement with the recent 2dF and SDSS determinations.
All low surface brightness (LSB) galaxies fall in the large scale structure
delineated by high surface brightness galaxies. The properties and surface
number density of our LSB galaxies are consistent with the LSB galaxy catalog
of O'Neil, Bothun & Cornell, suggesting that our samples are complete for LSB
galaxies to the magnitude limits. We measure colors, surface brightnesses, and
luminosities for our samples, and find strong correlations among these galaxy
properties.Comment: 20 pages, includes 20 figures, accepted for publication in the
Astronomical Journa
The Contribution of Experiential Avoidance and Social Cognitions in the Prediction of Social Anxiety
BACKGROUND: Cognitive models propose that social anxiety arises from specific dysfunctional cognitions about the likelihood and severity of embarrassment. Relational frame theory (RFT), on the other hand, posits that social anxiety arises from the unwillingness to endure unpleasant internal experiences (i.e. experiential avoidance [EA]). Although cognitive models have garnered empirical support, it may be that newer models such as RFT can improve our ability to predict and treat social anxiety.
AIMS: We aimed to elucidate the relationship between dysfunctional cognitions and EA, as well as their independent and relative contributions to the prediction of social anxiety symptoms. We hypothesized that dysfunctional cognitions and EA would each be associated with social anxiety, as well as with each other. We also predicted that both EA and dysfunctional cognitions would remain independent predictors of social anxiety symptoms after controlling for each other and general distress.
METHOD: Undergraduates high (n = 173) and low (n = 233) in social anxiety completed measures of social anxiety, dysfunctional cognitions, EA, and general distress. The overall sample was 66.3% female; mean age = 20.01 years (SD = 2.06).
RESULTS: Correlational analyses revealed that EA, dysfunctional cognitions, and social anxiety symptoms were moderately correlated with one another. Additionally, hierarchical regression analyses revealed that dysfunctional cognitions predicted social anxiety symptoms even after controlling for EA; the reverse was not found.
CONCLUSIONS: RESULTS suggest that EA and social anxiety specific cognitive distortions overlap to a moderate extent. EA does not add to the prediction of social anxiety symptoms above and beyond dysfunctional cognitions. Additional theoretical and treatment implications of the results are discussed
Formation of a galaxy with a central black hole in the Lemaitre-Tolman model
We construct two models of the formation a galaxy with a central black hole,
starting from a small initial fluctuation at recombination. This is an
application of previously developed methods to find a Lemaitre-Tolman model
that evolves from a given initial density or velocity profile to a given final
density profile. We show that the black hole itself could be either a collapsed
object, or a non-vacuum generalisation of a full Schwarzschild-Kruskal-Szekeres
wormhole. Particular attention is paid to the black hole's apparent and event
horizons.Comment: REVTeX, 22 pages including 11 figures (25 figure files). Replacement
has minor changes in response to the referee, and editorial corrections. To
appear in PR
The entropy and energy of intergalactic gas in galaxy clusters
Studies of the X-ray surface brightness profiles of clusters, coupled with
theoretical considerations, suggest that the breaking of self-similarity in the
hot gas results from an `entropy floor', established by some heating process,
which affects the structure of the intracluster gas strongly in lower mass
systems. Fitting analytical models for the radial variation in gas density and
temperature to X-ray spectral images from the ROSAT PSPC and ASCA GIS, we
derive gas entropy profiles for 20 galaxy clusters and groups. Scaling these
profiles to coincide in the self-similar case, the lowest mass systems are
found to have higher scaled entropy profiles than more massive systems. This
appears to be due to a baseline entropy of 70-140 h50^-1/3 keV cm^2, depending
on the extent to which shocks have been suppressed in low mass systems. The
extra entropy may be present in all systems, but is detectable only in poor
clusters, compared to the entropy generated by gravitational collapse. This
excess entropy appears to be distributed uniformly with radius outside the
central cooling regions. We determine the energy associated with this entropy
floor, by studying the net reduction in binding energy of the gas in low mass
systems, and find that it corresponds to a preheating temperature of ~0.3 keV.
Since the relationship between entropy and energy injection depends upon gas
density, we can combine the excesses of 70-140 keV cm^2 and 0.3 keV to derive
the typical electron density of the gas into which the energy was injected. The
resulting value of 1-3x10^-4 h50^1/2 cm-3, implies that the heating must have
happened prior to cluster collapse but after a redshift z~7-10. The energy
requirement is well matched to the energy from supernova explosions responsible
for the metals which now pollute the intracluster gas.Comment: 15 pages, 10 figures, accepted for publication in MNRA
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