17 research outputs found
The X-Ray Derived Cosmological Star Formation History and the Galaxy X-Ray Luminosity Functions in the Chandra Deep Fields North and South
The cosmological star formation rate in the combined Chandra Deep Fields
North and South is derived from our X-Ray Luminosity Function for Galaxies in
these Deep Fields. Mild evolution is seen up to redshift order unity with SFR ~
(1 + z)^{2.7}. This is the first directly observed normal star-forming galaxy
X-ray luminosity function (XLF) at cosmologically interesting redshifts (z>0).
This provides the most direct measure yet of the X-ray derived cosmic
star-formation history of the Universe. We make use of Bayesian statistical
methods to classify the galaxies and the two types of AGN, finding the most
useful discriminators to be the X-ray luminosity, X-ray hardness ratio, and
X-ray to optical flux ratio. There is some residual AGN contamination in the
sample at the bright end of the luminosity function. Incompleteness slightly
flattens the XLF at the faint end of the luminosity function. The XLF has a
lognormal distribution and agrees well with the radio and infrared luminosity
functions. However, the XLF does not agree with the Schechter luminosity
function for the H-alpha LF indicating that additional and different physical
processes may be involved in the establishment of the lognormal form of the
XLF. The agreement of our star formation history points with the other star
formation determinations in different wavebands (IR, Radio, H-alpha) gives an
interesting constraint on the IMF, and X-rays may be measuring directly the
binary star formation history of the Universe. X-ray studies will continue to
be useful for probing the star formation history of the universe by avoiding
problems of obscuration. Star formation may therefore be measured in more
detail by deep surveys with future x-ray missions.Comment: Accepted for publication in ApJ. 19 pages with 10 figures formatted
with emulateapj. Version with B/W only figures available at
http://www.pha.jhu.edu/~ptak/paper
The Gaia mission
Gaia is a cornerstone mission in the science programme of the EuropeanSpace Agency (ESA). The spacecraft construction was approved in 2006, following a study in which the original interferometric concept was changed to a direct-imaging approach. Both the spacecraft and the payload were built by European industry. The involvement of the scientific community focusses on data processing for which the international Gaia Data Processing and Analysis Consortium (DPAC) was selected in 2007. Gaia was launched on 19 December 2013 and arrived at its operating point, the second Lagrange point of the Sun-Earth-Moon system, a few weeks later. The commissioning of the spacecraft and payload was completed on 19 July 2014. The nominal five-year mission started with four weeks of special, ecliptic-pole scanning and subsequently transferred into full-sky scanning mode. We recall the scientific goals of Gaia and give a description of the as-built spacecraft that is currently (mid-2016) being operated to achieve these goals. We pay special attention to the payload module, the performance of which is closely related to the scientific performance of the mission. We provide a summary of the commissioning activities and findings, followed by a description of the routine operational mode. We summarise scientific performance estimates on the basis of in-orbit operations. Several intermediate Gaia data releases are planned and the data can be retrieved from the Gaia Archive, which is available through the Gaia home page. http://www.cosmos.esa.int/gai
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