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تسجيل مستخدم جديدRadio Galaxies in Cooling Cores: Insights from a Complete Sample
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We have observed a new, complete, cooling-core sample with the VLA, in order to understand how the massive black hole in the central galaxy interacts with the local cluster plasma. We find that every cooling core is currently being energized by an active radio jet, which has probably been destabilized by its interaction with the cooling core. We argue that current models of cooling-core radio galaxies need to be improved before they can be used to determine the rate at which the jet is heating the cooling core. We also argue that the extended radio haloes we see in many cooling-core clusters need extended, in situ re-energization, which cannot be supplied solely by the central galaxy.
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A currently active radio galaxy sits at the center of almost every strong cooling core. What effect does it have on the cooling core? Could its effect be strong enough to offset the radiative cooling which should be occuring in these cores? In order
to answer these questions we need to know how much energy the radio jet carries to the cooling core; but we have no way to measure the jet power directly. We therefore need to understand how the radio source evolves with time, and how it radiates, in order to use the data to determine the jet power. When some simple models are compared to the data, we learn that cluster-center radio galaxies probably are energetically important -- but not necessarily dominant -- in cooling cores.
Almost every strong cooling core contains an active radio galaxy. Combined radio and X-ray images reveal the dramatic interaction which is taking place between the radio jet and the central cluster plasma. At least two important questions can in prin
ciple be answered by comparing the new data to theoretical models. The first is how the radio jet propagates, and disrupts, in the cooling core environment: why are these cluster-center radio sources unusual? The second is the effect the radio jet has on the cooling core: is it energetically important to the core? Thanks to the new data we are beginning to be able to answer these questions.
One hundred seven ultraluminous X-ray (ULX) sources with 0.3-10.0 keV luminosities in excess of 1e39 erg/s are identified in a complete sample of 127 nearby galaxies. The sample includes all galaxies within 14.5 Mpc above the completeness limits of b
oth the Uppsala Galaxy Catalog and the Infrared Astronomical Satellite survey. The galaxy sample spans all Hubble types, a four decade range in mass and in star-formation rate. ULXs are detected in this sample at rates of one per 3.2e10 solar mass, one per 0.5 solar mass/year star-formation rate, and one per 57 cubic Mpc corresponding to a luminosity density of ~2e37 erg/s/Mpc3. At these rates we estimate as many as 19 additional ULXs remain undetected in fainter dwarf galaxies within the survey volume. An estimated 14 or 13%, of the 107 ULX candidates are expected to be background sources. The differential ULX luminosity function shows a power law slope of -1.2 to -2.0 with an exponential cutoff at 2e40 erg/s with precise values depending on the model and on whether the ULX luminosities are estimated from their observed numbers of counts or, for a subset of candidates, from their spectral shapes. Extrapolating the observed luminosity function predicts at most one very luminous ULX, L~1e41 erg/s, within a distance as small as 100 Mpc. The luminosity distribution of ULXs within the local universe cannot account for the recent claims of luminosities in excess of 2e41 erg/s requiring a new population class to explain these extreme objects.
The existence of a correlation between observed radio spectral index (alpha) and redshift (z) has long been used as a method for locating high-z radio galaxies. We use 9 highly spectroscopically complete radio samples, selected at different frequenci
es and flux limits, to determine the efficiency of this method, and compare consistently observed correlations between alpha, luminosity, linear size, and redshift. We observe a weak correlation between z and alpha which remains even when Malmquist bias is removed. The strength of this correlation depends on both the k-correction and sample selection frequency, in addition to the frequency at which alpha is measured, and consistent results for both high and low frequency selected samples are only seen if analysis is restricted to just extended radio galaxies. Many of the highest redshift radio galaxies are very compact and often display a negatively curved or peaked spectrum, and therefore the low-frequency radio spectrum as a whole should be studied; this is something for which the LOFAR will be crucial. We quantify both the efficiency and the completeness of various techniques used to select high-z radio galaxies. A steep-spectrum cut applied to low-frequency selected samples can more than double the fraction of high-z sources, but at a cost of excluding over half of the high-z sources present in the original sample. An angular size cut is an almost as equally effective method as a steep-spectrum cut, and works for both high and low frequency selected samples. In multi-wavelength data, selection first of infrared-faint radio sources remains by far the most efficient method of selecting high-z sources. We present a simple method for selecting high-z radio galaxies, based purely on combining their radio properties of alpha and angular size, with the addition of the K-band magnitude if available.[abridged]
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his sample. These observations, together with similar data of the remaining eight sources, are combined with data from the WENSS, NVSS and GB6 surveys to study the radio properties of the lobes of these sources at arcminute resolution. We investigate radio source asymmetries, equipartition energy densities in the lobes, the presence of lobe pressure evolution with redshift, the spectral age and the density of the environments of these sources. We find that the armlength asymmetries of GRGs are slightly larger than those of smaller sized 3CR radio galaxies and that these are difficult to explain as arising from orientation effects only. We also find indications that the lobes of the GRGs, despite their large sizes, are still overpressured with respect to their environment. Further, we argue that any evolution of lobe pressure with redshift in these large sources is due to selection effects. A spectral ageing analysis suggests that the GRGs in our sample are the oldest members of the group of relatively high power radio sources whose radio powers have evolved to their currently observed lower values.
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