Beecroft Institute for Particle Astrophysics and Cosmology

Abstract:

MCG-6-30-15 is the archetypal example of a type I active galaxy showing broad 'red-wing' emission in its X-ray spectrum at energies below the 6.4 keV Fe Kα emission line and a continuum excess above 20 keV. Miller et al. showed that these spectral features could be caused by clumpy absorbing material, but Reynolds et al. have argued that the observed Fe Kα line luminosity is inconsistent with this explanation unless the global covering factor of the absorber(s) is very low. However, the Reynolds et al. calculation effectively considers the only source of opacity to be the Fe K bound-free transition and neglects the opacity at the line energy: correction to realistic opacity decreases the predicted line flux by a large factor. We also discuss the interpretation of the covering factor and the possible effect of occultation by the accretion disc. Finally, we consider a model for MCG-6-30-15 dominated by clumpy absorption, which is consistent with a global covering factor of 0.45, although models that include the effects of Compton scattering are required to reach a full understanding. Variations in covering fraction may dominate the observed X-ray spectral variability. © 2009 The Authors. Journal compilation © 2009 RAS.

Abstract:

In a recent paper we presented the first semi-analytic model of galaxy formation in which the Thermally-Pulsing Asymptotic Giant Branch phase of stellar evolution has been fully implemented. Here we address the comparison with observations, and show how the TP-AGB recipe affects the performance of the model in reproducing the colours and near-IR luminosities of high-redshift galaxies. We find that the semi-analytic model with the TP-AGB better matches the colour-magnitude and colour-colour relations at z ~ 2, both for nearly-passive and for star-forming galaxies. The model with TP-AGB produces star-forming galaxies with red V-K colours, thus revising the unique interpretation of high-redshift red objects as 'red & dead'. We also show that without the TP-AGB the semi-analytic model fails at reproducing the observed colours, a situation that cannot be corrected by dust reddening. We also explore the effect of nebular emission on the predicted colour-magnitude relation of star-forming galaxies, to conclude that it does not play a significant role in reddening their colours, at least in the range of star-formation rates covered by the model. Finally, the rest-frame K-band luminosity function at z ~ 2.5 is more luminous by almost 1 magnitude. This indicates that the AGN feedback recipe that is adopted to regulate the high-mass end of the luminosity function should be sophisticated to take the effect of the stellar populations into account at high redshifts.