7,281 research outputs found
Color bimodality: Implications for galaxy evolution
We use a sample of 69726 galaxies from the SDSS to study the variation of the
bimodal color-magnitude (CM) distribution with environment. Dividing the galaxy
population by environment (Sigma_5) and luminosity (-23<M_r<-17), the u-r color
functions are modeled using double-Gaussian functions. This enables a
deconvolution of the CM distributions into two populations: red and blue
sequences. The changes with increasing environmental density can be separated
into two effects: a large increase in the fraction of galaxies in the red
distribution, and a small color shift in the CM relations of each distribution.
The average color shifts are 0.05+-0.01 and 0.11+-0.02 for the red and blue
distributions, respectively, over a factor of 100 in projected neighbor
density. The red fraction varies between about 0% and 70% for low-luminosity
galaxies and between about 50% and 90% for high-luminosity galaxies. This
difference is also shown by the variation of the luminosity functions with
environment. We demonstrate that the effects of environment and luminosity can
be unified. A combined quantity, Sigma_mod = Sigma_5/Mpc^{-2} + L_r/L_{-20.2},
predicts the fraction of red galaxies, which may be related to the probability
of transformation events. Our results are consistent with major interactions
(mergers and/or harassment) causing galaxies to transform from the blue to the
red distribution. We discuss this and other implications for galaxy evolution
from earlier results and model the effect of slow transformations on the color
functions.Comment: 14 pages, 8 figures, in AIP Conf. Proc., The New Cosmology, eds. R.
E. Allen et al. (aka. The Mitchell Symposium), see
http://proceedings.aip.org/proceedings/confproceed/743.jsp ; v2: replaced
Figure 5 which was incomplete in original submissio
The First Detailed X-ray Observations of High-Redshift, Optically-Selected Clusters: XMM-Newton Results for Cl 1324+3011 at z = 0.76 and Cl 1604+4304 at z = 0.90
We present the first detailed X-ray observations of optically-selected
clusters at high redshift. Two clusters, Cl 1324+3011 at z = 0.76 and Cl
1604+4304 at z = 0.90, were observed with XMM-Newton. The optical center of
each cluster is coincident with an extended X-ray source whose emission is
detected out to a radius of 0.5 Mpc. The emission from each cluster appears
reasonably circular, with some indication of asymmetries and more complex
morphologies. Similarly to other optically-selected clusters at redshifts of z
> 0.4, both clusters are modest X-ray emitters with bolometric luminosities of
only Lx = 1.4 - 2.0 x 10^(44) erg/s. We measure gas temperatures of T = 2.88
(+0.71/-0.49) keV for Cl 1324+3011 and 2.51 (+1.05/-0.69) keV for Cl 1604+4304.
The X-ray properties of both clusters are consistent with the high-redshift
Lx-T relation measured from X-ray-selected samples at z > 0.5. However, based
on the local relations, their X-ray luminosities and temperatures are low for
their measured velocity dispersions (sigma). The clusters are cooler by a
factor of 2 - 9 compared to the local sigma-T relation. We briefly discuss the
possible explanations for these results.Comment: 14 pages, 4 figures; accepted for publication in Astrophysical
Journal Letters; version with full resolution figures available at
http://bubba.ucdavis.edu/~lubin/xmm.pd
What is the Hidden Depolarization Mechanism in Low Luminosity AGN?
Millimeter wavelength polarimetry of accreting black hole systems can provide
a tomographic probe of the accretion flow on a wide range of linear scales. We
searched for linear polarization in two low luminosity active galactic nuclei
(LLAGN), M81 and M84, using the Combined Array for Millimeter Astronomy (CARMA)
and the Submillimeter Array (SMA). We find upper limits of
averaging over the full bandwidth and with a rotation measure (RM) synthesis
technique. These low polarization fractions, along with similar low values for
LLAGN M87 and 3C84, suggest that LLAGN have qualitatively different
polarization properties than radio-loud sources and Sgr A*. If the sources are
intrinsically polarized and then depolarized by Faraday rotation then we place
lower limits on the RM of a few times for the full
bandwidth case and for the RM synthesis
analysis. These limits are inconsistent with or marginally consistent with
expected accretion flow properties. Alternatively, the sources may be
depolarized by cold electrons within a few Schwarzschild radii from the black
hole, as suggested by numerical models.Comment: Accepted for publication in ApJ
Colors, magnitudes and velocity dispersions in early-type galaxies: Implications for galaxy ages and metallicities
We present an analysis of the color-magnitude-velocity dispersion relation
for a sample of 39320 early-type galaxies within the Sloan Digital Sky Survey.
We demonstrate that the color-magnitude relation is entirely a consequence of
the fact that both the luminosities and colors of these galaxies are correlated
with stellar velocity dispersions. Previous studies of the color-magnitude
relation over a range of redshifts suggest that the luminosity of an early-type
galaxy is an indicator of its metallicity, whereas residuals in color from the
relation are indicators of the luminosity-weighted age of its stars. We show
that this, when combined with our finding that velocity dispersion plays a
crucial role, has a number of interesting implications. First, galaxies with
large velocity dispersions tend to be older (i.e., they scatter redward of the
color-magnitude relation). Similarly, galaxies with large dynamical mass
estimates also tend to be older. In addition, at fixed luminosity, galaxies
which are smaller, or have larger velocity dispersions, or are more massive,
tend to be older. Second, models in which galaxies with the largest velocity
dispersions are also the most metal poor are difficult to reconcile with our
data. However, at fixed velocity dispersion, galaxies have a range of ages and
metallicities: the older galaxies have smaller metallicities, and vice-versa.
Finally, a plot of velocity dispersion versus luminosity can be used as an age
indicator: lines of constant age run parallel to the correlation between
velocity dispersion and luminosity.Comment: 12 pages, 9 figures. Accepted by A
Detection of Circular Polarization in the Galactic Center Black Hole Candidate Sagittarius A*
We report here the detection of circular polarization in the Galactic Center
black hole candidate, Sagittarius A*. The detection was made at 4.8 GHz and 8.4
GHz with the Very Large Array. We find that the fractional circular
polarization at 4.8 GHz is and that the spectral index of
the circular polarization is (). The systematic error in is less than 0.04% at both
frequencies. In light of our recent lower limits on the linear polarization in
Sgr A*, this detection is difficult to interpret with standard models. We
consider briefly whether scattering mechanisms could produce the observed
polarization. Detailed modeling of the source and the scattering medium is
necessary. We propose a simple model in which low energy electrons reduce
linear polarization through Faraday depolarization and convert linear
polarization into circular polarization. Circular polarization may represent a
significant new parameter for studying the obscured centimeter wavelength radio
source in Sgr A*.Comment: ApJL accepted, 11 pages including 1 figur
Signatures of Interstellar-Intracluster Medium Interactions: Spiral Galaxy Rotation Curves in Abell 2029
We investigate the rich cluster Abell 2029 (z~0.08) using optical imaging and
long-slit spectral observations of 52 disk galaxies distributed throughout the
cluster field. No strong emission-line galaxies are present within ~400 kpc of
the cluster center, a region largely dominated by the similarly-shaped X-ray
and low surface brightness optical envelopes centered on the giant cD galaxy.
However, two-thirds of the galaxies observed outside the cluster core exhibit
line emission. H-alpha rotation curves of 14 cluster members are used in
conjunction with a deep I band image to study the environmental dependence of
the Tully-Fisher relation. The Tully-Fisher zero-point of Abell 2029 matches
that of clusters at lower redshifts, although we do observe a relatively larger
scatter about the Tully-Fisher relation. We do not observe any systematic
variation in the data with projected distance to the cluster center: we see no
environmental dependence of Tully-Fisher residuals, R-I color, H-alpha
equivalent width, and the shape and extent of the rotation curves.Comment: 22 pages, 6 figures, 3 tables; to appear in the August 2000
Astronomical Journa
The Linear Polarization of Sagittarius A* I. VLA Spectro-polarimetry at 4.8 and 8.4 GHz
Synchrotron radiation from active galactic nuclei (AGN) is often highly
polarized. We present a search for linear polarization with the Very Large
Array (VLA) at 4.8 GHz and 8.4 GHz from the nearest AGN, Sagittarius A*. As a
part of this study we used spectro-polarimetric data that were sensitive to a
rotation measure (RM) as large as 3.5 x 10^6 rad m^-2 at 4.8 GHz and 1.5 x 10^7
rad m^-2 at 8.4 GHz. The upper limit to the linear polarization of Sgr A* over
a broad range of RM is 0.2% at both frequencies. We also present continuum
observations with the VLA at 4.8 GHz which give an upper limit of 0.1% for RMs
less than 10^4 rad m^-2. We conclude that depolarization is unlikely to occur
in the Galacter Center scattering medium. However, it is possible for
depolarization to occur in the accretion region of Sgr A* if the outer scale of
turbulence is small enough. We also consider the implications of a very low
intrinsic polarization for Sgr A*.Comment: 16 pages, 3 figures, accepted for publication in the Astrophysical
Journal, August 20, 1999, Vol 521 #
Towards a Holistic View of the Heating and Cooling of the Intracluster Medium
(Abridged) X-ray clusters are conventionally divided into two classes: "cool
core" (CC) clusters and "non-cool core" (NCC) clusters. Yet relatively little
attention has been given to the origins of this dichotomy and, in particular,
to the energetics and thermal histories of the two classes. We develop a model
for the entropy profiles of clusters starting from the configuration
established by gravitational shock heating and radiative cooling. At large
radii, gravitational heating accounts for the observed profiles and their
scalings well. However, at small and intermediate radii, radiative cooling and
gravitational heating cannot be combined to explain the observed profiles of
either type of cluster. The inferred entropy profiles of NCC clusters require
that material is preheated prior to cluster collapse in order to explain the
absence of low entropy (cool) material in these systems. We show that a similar
modification is also required in CC clusters in order to match their properties
at intermediate radii. In CC clusters, this modification is unstable, and an
additional process is required to prevent cooling below a temperature of a few
keV. We show that this can be achieved by adding a self-consistent AGN feedback
loop in which the lowest-entropy, most rapidly cooling material is heated so
that it rises buoyantly to mix with material at larger radii. The resulting
model does not require fine tuning and is in excellent agreement with a wide
variety of observational data. Some of the other implications of this model are
briefly discussed.Comment: 27 pages, 13 figures, MNRAS accepted. Discussion of cluster heating
energetics extended, results unchange
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