1,856 research outputs found
The StarScan plate measuring machine: overview and calibrations
The StarScan machine at the U.S. Naval Observatory (USNO) completed measuring
photographic astrograph plates to allow determination of proper motions for the
USNO CCD Astrograph Catalog (UCAC) program. All applicable 1940 AGK2 plates,
about 2200 Hamburg Zone Astrograph plates, 900 Black Birch (USNO Twin
Astrograph) plates, and 300 Lick Astrograph plates have been measured. StarScan
comprises of a CCD camera, telecentric lens, air-bearing granite table, stepper
motor screws, and Heidenhain scales to operate in a step-stare mode. The
repeatability of StarScan measures is about 0.2 micrometer. The CCD mapping as
well as the global table coordinate system has been calibrated using a special
dot calibration plate and the overall accuracy of StarScan x,y data is derived
to be 0.5 micrometer. Application to real photographic plate data shows that
position information of at least 0.65 micrometer accuracy can be extracted from
course grain 103a-type emulsion astrometric plates. Transformations between
"direct" and "reverse" measures of fine grain emulsion plate measures are
obtained on the 0.3 micrometer level per well exposed stellar image and
coordinate, which is at the limit of the StarScan machine.Comment: 24 pages, 8 figures, accepted for PAS
Real time response on dS_3: the Topological AdS Black Hole and the Bubble
We study real time correlators in strongly coupled N=4 supersymmetric
Yang-Mills theory on dS_3 x S^1, with antiperiodic boundary conditions for
fermions on the circle. When the circle radius is larger than a critical value,
the dual geometry is the so-called "topological AdS_5 black hole". Applying the
Son- Starinets recipe in this background we compute retarded glueball
propagators which exhibit an infinite set of poles yielding the quasinormal
frequencies of the topological black hole. The imaginary parts of the
propagators exhibit thermal effects associated with the Gibbons-Hawking
temperature due to the cosmological horizon of the de Sitter boundary. We also
obtain R-current correlators and find that after accounting for a small
subtlety, the Son-Starinets prescription yields the retarded Green's functions.
The correlators do not display diffusive behaviour at late times. Below the
critical value of the circle radius, the topological black hole decays to the
AdS_5 "bubble of nothing". Using a high frequency WKB approximation, we show
that glueball correlators in this phase exhibit poles on the real axis. The
tunnelling from the black hole to the bubble is interpreted as a hadronization
transition.Comment: 52 pages, 11 figures, typos corrected, references adde
The Feedback-Regulated Growth of Black Holes and Bulges through Gas Accretion and Starbursts in Cluster Central Dominant Galaxies
We present an analysis of the growth of black holes through accretion and
bulges through star formation in 33 galaxies at the centers of cooling flows.
Most of these systems show evidence of cavities in the intracluster medium
(ICM) inflated by radio jets emanating from their active galactic nuclei (AGN).
We present a new and extensive analysis of X-ray cavities in these systems. We
find that AGN are energetically able to balance radiative losses (cooling) from
the ICM in more than half of our sample. Using a subsample of 17 systems, we
examine the relationship between cooling and star formation. We find that the
star formation rates are approaching or are comparable to X-ray and far UV
limits on the rates of gas condensation onto the central galaxy. The remaining
radiative losses could be offset by AGN feedback. The vast gulf between
radiative losses and the sink of cooling material, which has been the primary
objection to cooling flows, has narrowed and, in some cases, is no longer a
serious issue. Using the cavity (jet) powers, we place strong lower limits on
the rate of growth of supermassive black holes in central galaxies, and we find
that they are growing at an average rate of ~ 0.1 solar masses per year, with
some systems growing as quickly as ~ 1 solar mass per year. We find a trend
between bulge growth (star formation) and black hole growth that is
approximately in accordance with the slope of the local (Magorrian) relation
between black hole and bulge mass. However, the large scatter in the trend
suggests that bulges and black holes do not always grow in lock step.
(Abridged)Comment: 17 pages, 6 figures, accepted to ApJ. Minor changes to text and
figure
First LOFAR results on galaxy clusters
Deep radio observations of galaxy clusters have revealed the existence of
diffuse radio sources related to the presence of relativistic electrons and
weak magnetic fields in the intracluster volume. The role played by this
non-thermal intracluster component on the thermodynamical evolution of galaxy
clusters is debated, with important implications for cosmological and
astrophysical studies of the largest gravitationally bound structures of the
Universe. The low surface brightness and steep spectra of diffuse cluster radio
sources make them more easily detectable at low-frequencies. LOFAR is the first
instrument able to detect diffuse radio emission in hundreds of massive galaxy
clusters up to their formation epoch. We present the first observations of
clusters imaged by LOFAR and the huge perspectives opened by this instrument
for non-thermal cluster studies.Comment: Proceedings of the 2012 week of the French Society of Astronomy and
Astrophysics (SF2A) held in Nice, June 5th-8t
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