Friday, 26 March 2010

In 1003.4018 Schawinski et al take a sample of morphologically selected Early types from SDSS stripe 82 (deeper imaging), and by eye investigate whether they show signs of major mergers. The fraction of mergers is plotted against three measures of time: u-r colour, Emiisionline classification and post-starburst age.

They want to conclude from this seqence that mergers drive AGN, with a delay time of ~500 Myr and say this is one of very few observational indications of the theoretical understanding of AGN feeding by mergers.

The FIR SEDs of z~2 galaxies: evidence for scaled up cool galaxies

Last week we talked about a paper that calibrated the relationship between the observed 24um (= 8um in the restframe) flux and SFR for z~2 galaxies. This paper by Muzzin et al. looks at a similar issue, but takes a slightly different approach.

The Chary & Elbaz templates describe the infrared SEDs of local galaxies. These templates have a luminosity dependence, such that IR-luminous galaxies have templates that peak at shorter wavelengths, due to increased dust temperatures.

So one method of estimating the SFR of a galaxy is to see which template corresponds to the observed restframe 8um luminosity, assume that the template accurately describes the SED throughout the rest of the IR regime, and to convert the total IR luminosity of the template into an SFR.

The red dashed curves in the SEDs shown above illustrate the procedure. But Muzzin et al. have compared the templates to the observed fluxes at different IR wavelengths for these two galaxies, and obviously the templates don't work very well. But the solid curves, which are templates for low-luminosity galaxies that have been scaled up to match the data points, do provide good fits. This shows that the correlation between IR luminosity and dust temperature that we see in the local universe doesn't work at higher redshifts, and that even IR-luminous galaxies at z~2 can have relatively cool dust. Previous studies that relied on this correlation may have overestimated the SFR by a factor of several.

Muzzin et al. go on to show general agreement between the SFRs as derived from the IR luminosity and from the dust-corrected Halpha luminosity (where the extinction in Halpha is taken to be roughly twice the extinction to the stellar continuum, which in turn is estimated using stellar population synthesis modeling).

Friday, 12 March 2010

The mid-IR luminosities of normal galaxies

In the last few years there has been a lot of discussion about the effect of TP-AGB stars on stellar mass estimates for high redshift galaxies. Such stars produce a large fraction of the rest-frame NIR light from youngish (~1 Gyr old) stellar populations, so if observations at these wavelengths are used when fitting stellar population models, it is important to take these stars into account. However, given all of the modeling and observational uncertainties associated with our understanding of TP-AGB stars, we are a long way away from being able to robustly incorporate them into stellar population synthesis (SPS) models.

This letter by Dan Keslon and Brad Holden looks at another important effect of TP-AGB stars: their contribution to the MIR luminosity, which is frequently used as an indicator of the star formation rate. The authors estimate contribution to the MIR luminosity by adding the observed K-MIR colors for Galactic TP-AGB M and C stars to the expected contribution to the K-band luminosity that comes from Maraston's SPS model. This figure shows some of the results. The upper blue lines show the maximum contribution to the MIR, which occurs when the TP-AGB C stars dominate (although this comparison must depend somewhat on the star formation history assumed in the models). It thus appears that the MIR luminosity of galaxies is consistent with being entirely due to the TP-AGB stars.

There's a lot of stuff in this (very long) letter, but what is most striking is their figure 2d, which I haven't shown here. That figure shows that the authors are able to reproduce the observed correlation between 24um luminosity and star formation rate from Chary & Elbaz (2001) very well, but only when the SFR is averaged over the last 1.5 Gyr (which is when the TP-AGB stars are important). But I do find this result a little weird, since presumably that correlation was made against SFR indicators that are sensitive only to more recent SF. Also, it would suggest that very little of the MIR luminosity comes from dust that is not immediately surrounding TP-AGB stars.

Friday, 5 March 2010

The Merger-Driven Evolution of Massive Galaxies

Robaina et al. estimate the contribution of major mergers to the growth of the red sequence at M>10^11 Msun from z~1 to z~0.  To do this, they estimate the merger rate of all galaxies (i.e. both red and blue) more massive then 5x10^10 Msun using the fraction of galaxies in close pairs in COSMOS and COMBO-17, and make the assumption that the remnants of all such mergers lie on the red sequence (or that they move onto the RS very quickly).  The data points in this figure show the observed evolution in the number density of M>10^11 Msun RS galaxies, and the curve shows the predicted evolution due to their major merger estimates.

The agreement is impressively good.  But given the uncertainties (in the number density evolution, which is uncertain in part because of the uncertainty in the M/L evolution, as well as the uncertainties involved in getting a merger rate from a correlation function), it doesn't seem like you can draw very strong conclusions.  Additionally, as the authors mention, they somewhat underestimate the growth due to mergers because they would not count a merger between e.g. an 6x10^10 Msun galaxy and an 4x10^10 Msun galaxy.

The authors also find that a present day M>10^11 Msun galaxy has undergone 0.5 major mergers between M>5x10^10 Msun galaxies since z=0.6, and 0.7 such mergers since z=1.2.

Anyway, this is a nice work, and points to the importance in major mergers at the massive end.  I am somewhat curious about what the growth due to more minor mergers is, especially given that minor mergers are supposed to be what drives the size evolution of the RS galaxies.