1,632 research outputs found
Active galactic nucleus feedback in clusters of galaxies
Observations made during the last ten years with the Chandra X-ray
Observatory have shed much light on the cooling gas in the centers of clusters
of galaxies and the role of active galactic nucleus (AGN) heating. Cooling of
the hot intracluster medium in cluster centers can feed the supermassive black
holes found in the nuclei of the dominant cluster galaxies leading to AGN
outbursts which can reheat the gas, suppressing cooling and large amounts of
star formation. AGN heating can come in the form of shocks, buoyantly rising
bubbles that have been inflated by radio lobes, and the dissipation of sound
waves.Comment: Refereed review article published in Chandra's First Decade of
Discovery Special Feature edition of the Proceedings of the National Academy
of Science
The Rise of Massive Red Galaxies: the color-magnitude and color-stellar mass diagrams for z < ~2 from the MUltiwavelength Survey by Yale-Chile (MUSYC)
We present the color-magnitude and color-stellar mass diagrams for galaxies
with z_phot < ~2, based on a K < 22 (AB) catalog of the Extended Chandra Deep
Field South (ECDFS) from the MUltiwavelength Survey by Yale-Chile (MUSYC). Our
main sample of 7840 galaxies contains 1297 M_* > 10^11 M_Sol galaxies in the
range 0.2 < z_phot < 1.8. We show empirically that this catalog is
approximately complete for M_* > 10^11 M_Sol galaxies for z_phot < 1.8. For
this mass-limited sample, we show that the locus of the red sequence
color-stellar mass relation evolves as Del(u-r) ~ (-0.44+/-0.02) z_phot for
z_phot ~1.3, however, we are no longer able to reliably
distinguish red and blue subpopulations from the observed color distribution;
we show that this would require much deeper near infrared data. At 1.5 < z_phot
10^11 M_Sol galaxies is ~50% of the
local value, with a red fraction of ~33%. Making a parametric fit to the
observed evolution, we find n_tot(z) ~ (1+z_phot)^(-0.52+/-0.12(+/-0.20)). We
find stronger evolution in the red fraction: f_red(z) ~
(1+z_phot)^(-1.17+/-0.18(+/-0.21)). Through a series of sensitivity analyses,
we show that the most important sources of systematic error are: 1. systematic
differences in the analysis of the z~0 and z>>0 samples; 2. systematic effects
associated with details of the photometric redshift calculation; and 3.
uncertainties in the photometric calibration. With this in mind, we show that
our results based on photometric redshifts are consistent with a completely
independent analysis which does not require redshift information for individual
galaxies. Our results suggest that, at most, 1/5 of local red sequence galaxies
with M_* >10^11 M_Sol were already in place at z ~ 2.Comment: Accepted for publication in ApJ. 31 pages in emulateapj format; 18
figues (14 in main text). Additional online data available through
http://www.strw.leidenuniv.nl/~ent
Influences of physical oceanographic processes on chlorophyll distributions in coastal and estuarine waters of the South Atlantic Bight
Coastal and estuarine waters of the South Atlantic Bight are highly productive, with primary production of 600-700 gC/m2/y. While controls and fate of this production are conceptually well understood, the importance of meteorology and physical circulation processes on phytoplankton has not received equivalent attention. Here, we describe the effects of wind stress and tidal currents on temporal and spatial distributions of phytoplankton biomass represented as chlorophyll a (chl a). Moored instruments were deployed and shipboard sampling was conducted in the North Edisto estuary (South Carolina) and adjacent inner shelf waters during four, two-week field studies in May and August 1993, and June and September 1994. Local wind regimes induced upwelling- and downwelling-favorable conditions which strengthened or reduced vertical density stratification in the coastal frontal zone, respectively, and shifted the location of the front. Chl a in shelf waters was more or less homogenous independent of the wind regime, while chl a on the estuary delta was generally vertically stratified. Within the estuary, chl a concentrations were positively correlated with the alongshore component of wind stress; chl a was not correlated with the weaker cross-shelf component of wind stress. Highest chl a occurred during strong downwelling-favorable events. The quick response time to wind forcing (6-12 hrs) implied a direct effect on chl a distributions and not a stimulation of growth processes. The source of the elevated chl a in response to wind forcing was apparently resuspension of settled and epibenthic algal cells. Tidal currents also influenced the vertical distribution and concentration of chl a. Time series sampling on the estuary delta showed that, with increasing velocity of ebb and flood tide currents, the relative contributions of pennate and centric diatoms with attached detritus and sand grains also increased, indicating that tidal resuspension of settled and epibenthic microalgae also occurred. Vertical stratification of chl a (highest concentrations near the bottom) began to degrade upon mixing by tidal currents with velocities as low as 10 cm/sec. Homogenization of 5-7 m water columns was fully achieved at velocities of 20-30 cm/sec. The data document the direct and comparatively immediate (timescales of minuteshours) impact of tidal and wind energy on concentrations and distribution patterns of phytoplankton in coastal and estuarine waters of the South Atlantic Bight
On viscosity, conduction and sound waves in the intracluster medium
Recent X-ray and optical observations of the Perseus cluster indicate that
the viscous and conductive dissipation of sound waves is the mechanism
responsible for heating the intracluster medium and thus balancing radiative
cooling of cluster cores. We discuss this mechanism more generally and show how
the specific heating and cooling rates vary with temperature and radius. It
appears that the heating mechanism is most effective above 10^7K, which allows
for radiative cooling to proceed within normal galaxy formation but will stifle
the growth of very massive galaxies. The scaling of the wavelength of sound
waves with cluster temperature and feedback in the system are investigated.Comment: 5 pages, 4 figures, MNRAS accepte
A Study of Nine High-Redshift Clusters of Galaxies: IV. Photometry and Sp ectra of Clusters 1324+3011 and 1604+4321
New photometric and spectroscopic observations of galaxies in the directions
of three distant clusters are presented as part of our on-going high-redshift
cluster survey. The clusters are CL1324+3011 at z = 0.76, CL1604+4304 at z =
0.90, and CL1604+4321 at z = 0.92. The observed x-ray luminosities in these
clusters are at least a factor of 3 smaller than those observed in clusters
with similar velocity dispersions at z <= 0.4. These clusters contain a
significant population of elliptical-like galaxies, although these galaxies are
not nearly as dominant as in massive clusters at z <= 0.5. We also find a large
population of blue cluster members. Defining an active galaxy as one in which
the rest equivalent width of [OII] is greater than 15 Angstroms, the fraction
of active cluster galaxies, within the central 1.0 Mpc, is 45%. In the field
population, we find that 65% of the galaxies with redshifts between z = 0.40
and z = 0.85 are active, while the fraction is 79% for field galaxies at z >
0.85. The star formation rate normalized by the rest AB B-band magnitude, SFRN,
increases as the redshift increases at a given evolving luminosity. At a given
redshift, however, SFRN decreases linearly with increasing luminosity
indicating a remarkable insensitivity of the star formation rate to the
intrinsic luminosity of the galaxy over the range -18 >= ABB >= -22. Cluster
galaxies in the central 1 Mpc regions exhibit depressed star formation rates.
We are able to measure significant evolution in the B-band luminosity function
over the range 0.1 <= z <= 1. The characteristic luminosity increases by a
factor of 3 with increasing redshift over this range.Comment: 64 pages, 18 figures, accepted for publication in the Astronomical
Journal on May 25, 2001. Scheduled to appear in Sept 2001 issu
Vegetative succession in a restored urban wetland: If we build it will they come?
Urban wetlands can provide many important ecosystem services to society. However, they are often severely degraded by runoff, pollution, and invasion by aggressive non-native plant species. This Creative Inquiry project is establishing a long-term monitoring program to document vegetation composition in a restored wetland in the Hunnicutt Creek watershed for the purposes of inventory, assessment of environmental conditions, and adaptive management. Using a standard ecological observation unit, a 10 x 10 meter sampling plot originally developed by The Carolina Vegetation Survey (CVS), we are evaluating the performance of re-introduced and naturally regenerated native woody species, and documenting the recruitment of non-natives from adjacent unrestored areas. A total of eight CVS plots have been established and will be resampled twice annually for the next several years
Chandra Observation of the Central Region of the Cooling Flow Cluster Abell 262: A Radio Source that is a Shadow of its Former Self?
We present a Chandra observation of the cooling flow cluster Abell 262.
Spectral fits show that the intracluster medium (ICM) in A262 cools by a factor
of three from 2.7 keV to 0.9 keV at the cluster center. A mass deposition rate
of Mdot = 19 +6/-5 Msun/yr is measured. Complex structure is found in the very
inner regions of the cluster, including knots of emission and a clear deficit
of emission to the east of the cluster center. The bright X-ray structures are
located in the same regions as optical line emission, indicating that cooling
to low temperatures has occurred in these regions. The X-ray deficit is
spatially coincident with the eastern radio lobe associated with the active
galactic nucleus hosted by the central cD galaxy. The region surrounding the
X-ray hole is cool, and shows no evidence that it has been strongly shocked.
This joins the ranks of other cooling flow clusters with Chandra-detected
bubbles blown by central radio sources. This source is different than the other
well-known cases, in that the radio source is orders of magnitude less luminous
and has produced a much smaller bubble. Comparing the energy output of the
radio source with the luminosity of the cooling gas shows that energy
transferred to the ICM from the radio source is insufficient to offset the
cooling flow unless the radio source is currently experiencing a less powerful
than average outburst, and was more powerful in the past.Comment: Astrophysical Journal, in press; 9 pages, 8 figure
Are galaxies with AGN a transition population?
We present the results of an analysis of a well-selected sample of galaxies
with active and inactive galactic nuclei from the Sloan Digital Sky Survey, in
the range 0.01 < z < 0.16. The SDSS galaxy catalogue was split into two classes
of active galaxies, Type~2 AGN and composites, and one set of inactive,
star-forming/passive galaxies. For each active galaxy, two inactive control
galaxies were selected by matching redshift, absolute magnitude, inclination,
and radius. The sample of inactive galaxies naturally divides into a red and a
blue sequence, while the vast majority of AGN hosts occur along the red
sequence. In terms of H-alpha equivalent width, the population of composite
galaxies peaks in the valley between the two modes, suggesting a transition
population. However, this effect is not observed in other properties such as
colour-magnitude space, or colour-concentration plane. Active galaxies are seen
to be generally bulge-dominated systems, but with enhanced H-alpha emission
compared to inactive red-sequence galaxies. AGN and composites also occur in
less dense environments than inactive red-sequence galaxies, implying that the
fuelling of AGN is more restricted in high-density environments. These results
are therefore inconsistent with theories in which AGN host galaxies are a
`transition' population. We also introduce a systematic 3D spectroscopic
imaging survey, to quantify and compare the gaseous and stellar kinematics of a
well-selected, distance-limited sample of up to 20 nearby Seyfert galaxies, and
20 inactive control galaxies with well-matched optical properties. The survey
aims to search for dynamical triggers of nuclear activity and address
outstanding controversies in optical/IR imaging surveys.Comment: 12 pages, 8 figures, accepted by MNRA
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