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Abstract:

In colour-magnitude diagrams most galaxies fall in either the ``blue cloud'' or the ``red sequence'', with the red sequence extending to significantly brighter magnitudes than the blue cloud. The bright-end of the red sequence comprises elliptical galaxies (Es) with boxy isophotes and luminosity profiles with shallow central cores, while fainter Es have disky isophotes and power-law inner surface-brightness (SB) profiles. An analysis of published data reveals that the centres of galaxies with power-law central SB profiles have younger stellar populations than the centres of cored galaxies. We argue that thermal evaporation of cold gas by virial-temperature gas plays an important role in determining these phenomena. In less massive galaxies, thermal evaporation is not very efficient, so significant amounts of cold gas can reach the galaxy centre and fill a central core with newly formed stars, consistent with the young stellar ages of the cusps of Es with power-law SB profiles. In more massive galaxies, cold gas is evaporated within a dynamical time, so star formation is inhibited, and a core in the stellar density profile produced by dissipationless dynamics cannot be refilled. The different observed properties of AGN in higher-mass and lower-mass ellipticals are also explained because in the former the central black holes invariably accrete hot gas, while in the latter they typically accrete cold gas. An important consequence of our results is that at the present time there cannot be blue, star-forming galaxies in the most massive galactic halos, consistent with the observed truncation of the blue cloud at L*. [abridged]

Abstract:

We analyzed the feedback of AGN jets on cooling flow clusters using three-dimensional AMR hydrodynamic simulations. We studied the interaction of the jet with the intracluster medium and creation of low X-ray emission cavities (Bubbles) in cluster plasma. The distribution of energy input by the jet into the system was quantified in its different forms, i.e. internal, kinetic and potential. We find that the energy associated with the bubbles, (pV + gamma pV/(gamma-1)), accounts for less than 10 percent of the jet energy.

Abstract:

Black hole (BH) masses predicted from the Ṁ-σ relationship conflict with predictions from the Ṁ-L relationship for high-luminosity galaxies, such as brightest cluster galaxies (BCGs). The Ṁ-L relationship predicts that some BCGs may harbor BHs with M approaching 10¹⁰ M⊙, while the Ṁ- σ relationship always predicts Ṁ < 3 × 10 ⁹ M⊙. We argue that the Ṁ-L relationship is a plausible description for galaxies of high luminosity. If the cores in central stellar density are formed by binary BHs, the inner core cusp radius, rγ, may be an independent witness of Ṁ. Using central structural parameters derived from a large sample of early-type galaxies observed by HST, we argue that L is superior to σ as an indicator of rγ. Further, the rγ-Ṁ relationship for 11 core galaxies with measured Ṁ appears to be consistent with the Ṁ-L relationship for BCGs. BCGs have large cores appropriate for their large luminosities that may be difficult to generate with the more modest BH masses inferred from the Ṁ-σ relationship. Ṁ ∼ M would be expected for BCGs, if they were formed in dissipationless mergers, which should preserve the ratio of BH to stellar mass, M. This scenario appears to be consistent with the slow increase in a with L and the more rapid increase in effective radii with L seen in BCGs as compared to less luminous galaxies. If BCGs have large BHs commensurate with their luminosities, then the local BH mass function for Ṁ > 3 × 10⁹ M⊙ ould be nearly an order of magnitude richer than that inferred from the Ṁ-σ relationship. The volume density of the most luminous QSOs may favor the Ṁ-L relationship. © 2007. The American Astronomical Society. All rights reserved.