Friday, 12 March 2010
The mid-IR luminosities of normal galaxies
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.