Friday, 18 April 2008
This is from the paper "The Halpha Galaxy Survey V. The star formation history of late type galaxies", by Phil James et al. astro-ph/0804.2167
They have SFRs from Halpha, stellar masses from K and R band photometry (all with the 1m Kapteyn Telescope) and gas masses from Westerbork neutral hydrogen observations for local late type field galaxies. Here they plot the star formation timescale (Mstar/SFR) and gas depletion timescale (Mgas/SFR) for galaxies as a function of mass and type. The dashed lines indicate the age of the universe. The fact that the star formation timescale for low mass/late type galaxies is similar to the age of the universe and their gas depletion time is much longer, whereas it is reversed for the high mass/earlier type galaxies is used as an argument that the star formation history of very late type galaxies is constant over the age of the universe and the stellar mass gradually builds up, an that for more bulgy galaxies the bulk of the star formation happens in short bursts of high SFR.
The possibility of having a higher SFR in the past is not mentioned...
The authors use a CDM simulation which includes IGM metal enrichment from superwinds to predict the galaxy-OVI absorber cross-correlation at low redshifts; or, as in the figure above, the fraction of galaxies with an OVI absorber within a given distance, at different galaxy luminosities, absorber strengths, and projected absorber-galaxy separations. They find that the correlation length depends strongly on galaxy luminosity (with faint, low-mass galaxies having more nearby absorbers on average) but not on absorber strength. Only ~15% of OVI absorbers come from >L* galaxies, implying that IGM enrichment may be predominantly due to many faint sources rather than a few bright ones.
They note also that these results are somewhat preliminary and their simulation resolution may cause problems for the lowest-mass galaxies, but these will be a valuable starting point for comparison to upcoming large COS surveys.
Friday, 11 April 2008
This plot shows the average number of galaxy companions as a function of redshift for different pair separations. With increasing separations, the evolution of the pair fraction decreases. Assuming these pairs represent early-stage mergers, this may imply that the infall/merging timescales are changing with redshift: at high redshift it takes longer for a galaxy to finish merging (i.e. go from 20 kpc to 0 kpc) than at low redshift, relative to the inital (r=150 to 50 kpc) infall. The authors suggest that such a change in timescale may be due to dark matter halos at lower redshift being more concentrated: since the density is lower in the outskirts of highly-concentrated halos, the dynamical friction timescale at large radii is longer, and therefore one might expect merging galaxies to spend more time at larger radii (thus increasing the large-separation pair fraction) at lower redshifts.
Figure 2 from Woo et al., astro-ph 0804.0235
The M_BH-sigma relation of active galaxies.
Left panel: local Seyferts with sigma from Greene & Ho (2006) and our own M_BH estimates consistently calibrated with our estimates for distant samples (black circles); local Seyferts with M_BH, measured via reverberation mapping (Onken et al. 2004; magenta circles)
Right panel: new measurements at z=0.57 (red stars); Seyfert galaxies at z=0.36 from our earlier work (blue circles). The local relationship of quiescent galaxies (Tremaine et al. 2002; black points) are shown for comparison as a solid (Tremaine et al. 2002) and dashed (Ferrarese & Ford 2005) line.