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Register a new userInteraction of the AGN and X-ray Emitting Gas
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Paul Nulsen
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The process that prevents the deposition of cooled gas in cooling flows must rely on feedback in order to maintain gas with short cooling times, while preventing the bulk of the gas from cooling to low temperatures. The primary candidate for the feedback mechanism is the accretion of cooled and cooling gas by an active galactic nucleus (AGN). Despite some difficulties with this model, the high incidence of central radio sources in cooling flows and the common occurrence of radio lobe cavities, together, support this view. The Bondi accretion rate for the intracluster gas onto the AGN depends on the gas properties only through its specific entropy and that is governed directly by competition between heating and cooling. This provides a viable link for the feedback process. It is argued that the mass accreted between outbursts by the central AGN is only sensitive to the mass of the black hole and the gas temperature. Bondi accretion by an AGN leads to a simple expression for outburst energy that can be tested against observations.
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Recent observations of the interactions between radio sources and the X-ray-emitting gas in cooling flows in the cores of clusters of galaxies are reviewed. The radio sources inflate bubbles in the X-ray gas, which then rise buoyantly outward in the clusters transporting energy to the intracluster medium (ICM). The bright rims of gas around the radio bubbles are cool, rather than hot, and do not show signs of being strongly shocked. Energy deposited into the ICM over the lifetime of a cluster through several outbursts of a radio source helps to account for at least some of the gas that is missing in cooling flows at low temperatures.
We present Chandra X-ray Observations of the Hydra A cluster of galaxies, and we report the discovery of structure in the central 80 kpc of the clusters X-ray-emitting gas. The most remarkable structures are depressions in the X-ray surface brightness, $sim 25-35$ kpc diameter, that are coincident with Hydra As radio lobes. The depressions are nearly devoid of X-ray-emitting gas, and there is no evidence for shock-heated gas surrounding the radio lobes. We suggest the gas within the surface brightness depressions was displaced as the radio lobes expanded subsonically, leaving cavities in the hot atmosphere. The gas temperature declines from 4 keV at 70 kpc to 3 keV in the inner 20 kpc of the brightest cluster galaxy (BCG), and the cooling time of the gas is $sim 600$ Myr in the inner 10 kpc. These properties are consistent with the presence of a $sim 34 msunyr$ cooling flow within a 70 kpc radius. Bright X-ray emission is present in the BCG surrounding a recently-accreted disk of nebular emission and young stars. The star formation rate is commensurate with the cooling rate of the hot gas within the volume of the disk, although the sink for the material cooling at larger radii remains elusive.
I describe the different classes of Active Galactic Nuclei (AGN) and the basic tenets of unified schemes. I then review the properties of the extragalactic sources detected in the GeV and TeV bands, showing that the vast majority of them belong to the very rare blazar class. I further discuss the kind of AGN GLAST is likely to detect, making some predictions going from the obvious to the likely, all the way to the less probable.
Using archived data from the Chandra X-ray telescope, we have extracted the diffuse X-ray emission from 49 equal-mass interacting/merging galaxy pairs in a merger sequence, from widely separated pairs to merger remnants. After removal of contributions from unresolved point sources, we compared the diffuse thermal X-ray luminosity from hot gas (L(X)(gas)) with the global star formation rate (SFR). After correction for absorption within the target galaxy, we do not see strong trend of L(X)(gas)/SFR with SFR or merger stage for galaxies with SFR > 1 M(sun) yr^-1. For these galaxies, the median L(X)(gas)/SFR is 5.5 X 10^39 ((erg s^-1)/M(sun) yr^-1)), similar to that of normal spiral galaxies. These results suggest that stellar feedback in star forming galaxies reaches an approximately steady state condition, in which a relatively constant fraction of about 2% of the total energy output from supernovae and stellar winds is converted into X-ray flux. Three late-stage merger remnants with low SFRs and high K band luminosities (L(K)) have enhanced L(X)(gas)/SFR; their UV/IR/optical colors suggest that they are post-starburst galaxies, perhaps in the process of becoming ellipticals. Systems with L(K) < 10^10 L(sun) have lower L(X)(gas)/SFR ratios than the other galaxies in our sample, perhaps due to lower gravitational fields or lower metallicities. We see no relation between L(X)(gas)/SFR and Seyfert activity in this sample, suggesting that feedback from active galactic nuclei is not a major contributor to the hot gas in our sample galaxies.
We have analyzed X-ray spectra from six galaxy clusters which contain cooling flows: A85, A478, A1795, A2142, A2147, & A2199. The X-ray spectra were taken with the HEAO1-A2 Medium and High Energy Detectors and the Einstein Solid State Spectrometer. For each cluster, we simultaneously fit the spectra from these three detectors with models incorporating one or more emission components comprised of either thermal or cooling flow models. Five of the clusters (all but A2142) are better fit by a multi-component model (a cooling flow plus one or two thermal components or a two thermal component model) than by isothermal models. In four of the clusters (A85, A1795, A2147, & A2199), we find evidence for cool gas outside of the canonical cooling flow region. These latter four clusters can be characterized by three temperature components: a temperature inversion in the central region, a hotter region with an emission-weighted temperature which is higher than that of an isothermal model fit to the entire cluster, and a cooler region with an emission-weighted temperature of ~1 keV. The cool component outside the cooling flow region has a large minimum emission measure which we attribute, in part, to diffuse cool gas in the outer cluster atmosphere. If at least some of the cool exterior gas is virialized, this would imply a radially decreasing temperature profile. Together with the density profiles we have found, this leads to a baryon fraction in gas which increases with radius and is larger than that for an isothermal cluster atmosphere. Consequently, if clusters of galaxies trace the mass distribution in the Universe, the gas mass fraction we have calculated for an isothermal gas (which is ~15%) together with the nominal galaxy contribution (~5%) gives a baryon fraction of ~20%. Using the upper limit to the baryon density derived from Big
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