1,986 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
Ca II and Na I Quasar Absorption-Line Systems in an Emission-Selected Sample of SDSS DR7 Galaxy/Quasar Projections: I. Sample Selection
The aim of this project is to identify low-redshift host galaxies of quasar
absorption-line systems by selecting galaxies which are seen in projection onto
quasar sightlines. To this end, we use the Seventh Data Release of the Sloan
Digital Sky Survey (SDSS-DR7) to construct a parent sample of 97489
galaxy/quasar projections at impact parameters of up to 100 kpc to the
foreground galaxy. We then search the quasar spectra for absorption line
systems of Ca II and Na I within +- 500 km/s of the galaxy's velocity. This
yields 92 Ca II and 16 Na I absorption systems. We find that most of the Ca II
and Na I systems are sightlines through the Galactic disk, through High
Velocity Cloud complexes in our halo, or Virgo cluster sightlines. Placing
constraints on the absorption line rest equivalent width significance (>=3.0
sigma), the Local Standard of Rest velocity along the sightline (>= 345 km/s),
and the ratio of the impact parameter to the galaxy optical radius (<=5.0), we
identify 4 absorption line systems that are associated with low-redshift
galaxies at high confidence, consisting of two Ca II systems (one of which also
shows Na I), and two Na I systems. These 4 systems arise in blue, L_r^*
galaxies. Tables of the 108 absorption systems are provided to facilitate
future follow up.Comment: 13 pages, 11 figures, 6 tables; online data included in electronic
version as 1 FITS table and 2 machine readable tables; to be published in The
Astronomical Journa
Cosmic homogeneity demonstrated with luminous red galaxies
We test the homogeneity of the Universe at with the Luminous Red
Galaxy (LRG) spectroscopic sample of the Sloan Digital Sky Survey. First, the
mean number of LRGs within completely surveyed LRG-centered spheres of
comoving radius is shown to be proportional to at radii greater than
. The test has the virtue that it does not rely
on the assumption that the LRG sample has a finite mean density; its results
show, however, that there \emph{is} such a mean density. Secondly, the survey
sky area is divided into 10 disjoint solid angular regions and the fractional
rms density variations of the LRG sample in the redshift range
among these () regions is found to be 7
percent of the mean density. This variance is consistent with typical biased
\lcdm models and puts very strong constraints on the quality of SDSS
photometric calibration.Comment: submitted to Ap
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