Friday, 19 December 2008

Regulation of Black Hole Growth in Low Redshift Galaxies

This paper by Kauffmann & Heckman discusses the accretion onto central black holes in SDSS. The authors use the OIII line luminosity as a proxy AGN accretion rate, and they infer the black hole mass from the stellar velocity dispersion; thus L[OIII]/M_bh becomes a measure of the Eddington ratio. The upper panel in this figure shows the distribution of Eddington ratios for SDSS galaxies. In the lower panels, the galaxies are split up by the strength of their 4000AA break, which is a measure of galaxy age. It appears that young galaxies have a broad log-normal distribution which is independent of age (it is also independent of M_bh, but that isn't visible in this figure). But older galaxies have an approximate power-law distribution which does depend on age (and also on M_bh).

The authors interpret the log-normal distribution as a reflecting black hole self-regulation, with a negative feedback effect that kicks in at the peak of the distribution (~1% Eddington). But since the distribution doesn't depend on black hole mass or on the star formation in the rest of the galaxy, the feedback must operate only in the immediate vicinity of the black hole. On the other hand, the power-law distribution for the older galaxies does not suggest self-regulation. The authors show that the accretion rate onto the the black hole is roughly proportional to the bulge stellar mass, which is consistent with a scenario in which the black hole is fed by stellar mass loss... however this is a rather speculative conclusion.

Thursday, 11 December 2008

The impact of TP-AGB stars on hierarchical galaxy formation models

From: arXiv:0812.1225 [ps, pdf, other]
Title: The impact of TP-AGB stars on hierarchical galaxy formation models
Authors: Chiara Tonini (1), Claudia Maraston (1), Julien Devriendt (2), Daniel Thomas (1), Joseph Silk (2) ((1) Institute of Cosmology and Gravitation, University of Portsmouth, UK; (2) University of Oxford, UK)
Comments: 5 pages, 4 figures. Submitted to MNRAS Letters
Subjects: Astrophysics (astro-ph)
The authors paint galaxy magnitudes on a semi-analytic model of galaxy formation, comparing two population synthesis packages: Maraston 05 (with 'proper' treatment of TP-AGB stars) and Pegase (without those beasts). here they show the V-K,V color magnitude relation at 4 different redshifts for disks and spheroids. The TP-AGBs are particularly relevant at an age of 1 Gyr in the K band. Mass tot K-band light ratios differ by a factor of ~3, for 1 Gyr SSPs, and less for other wavelength bands and ages (V for example is almost indistinghuishable). This may have big consequences for the fitting of stellar masses on the basis of rest-frame K band photometry, as is often done.

Friday, 5 December 2008

Merger rates at z~3

from Bluck et al.,

The authors use pair counts and galaxy morphologies (CAS) to estimate
merger rates in the GOODS North and South fields. The plot above
shows the merger fraction for galaxies above log(M)=11. At z>3 the
merger rate continues to increase, so the peak of merger activity for
these galaxies must be at higher redshifts. This is in contrast to
lower-mass galaxies (10^10), where the merger peak is seen around
z=2. Thus, the authors conclude that high-mass galaxies undergo
major mergers at higher redshifts than lower-mass galaxies.