Friday, 26 September 2008

Time to go into finance?

Conroy, Gunn, & White,

This paper explores the effects of stellar evolution uncertainties
(particularly the properties of thermally pulsating AGB stars, but
also metallicity and the IMF) on quantities derived through stellar
population modeling like age, mass, and star-formation rate. From
computing expected colors of LMC star clusters the authors conclude
that the temperature and luminosity of the TP-AGB phase could vary by
as much as +-0.2 dex and +-0.4 dex respectively, and so they allow
these parameters to vary in their stellar population fitting. The
plot above shows 68% and 95% likelihood contours for the derived
properties for a bright z~2 quiescent galaxy, with probability
distributions (blue line: AGB star uncertainties included; black: not
included) in the top panel. Interestingly, the degeneracies between
the AGB parameters and derived quantities are weak at best, and the
uncertainties don't seem to increase much (blue vs. black curves).

Friday, 12 September 2008

Now you see it, now you don't

Barbary et al (2008, have discovered an (apparently) new class of optical transient objects. The spectrum is a mystery, the source brightened by >5-6 magnitudes over 100 days and no host galaxy can be seen. Might be Galactic, might be extra-galactic. If the latter, the most reasonable estimate of redshift gives a peak luminosity of -22.1 - close to the brightest SNe seen.

Do Sub-mm Galaxies Really Trace The Most Massive Dark Matter Halos?

Chapman et al.

This paper presents evidence of a strong over-density of sub-mm galaxies at z=1.99 in GOODS-N. The interesting thing about it is that there is also an over-density of the more typical blue star-forming galaxies at the same redshift, but that the density contrast for the blue galaxies is much weaker than for the sub-mm galaxies.

The authors suggest that this is a cluster in the first stages of formation. The strength of the over-density of sub-mm galaxies is due to the numerous ongoing mergers, and thus it is not representative of the overall matter over-density, which would be much weaker. The fact that the masses and SFRs of the blue star-forming galaxies in the redshift spike is similar to the values for galaxies outside of the spike supports this argument, since you would expect most galaxies in a cluster to be older and more massive (even at these high redshifts).

The biggest caveat that the authors note is that there may be a large population of quiescent galaxies in this redshift spike that have not been observed spectroscopically. If this were the case, the true matter over-density would be closer to the over-density of sub-mm galaxies, and the merger argument wouldn't be necessary. The same caveat applies for blue star-forming galaxies that don't have spectroscopic redshifts. So without having a good idea of the statistical significance of these results, I would say that the main conclusion is a bit of a stretch... although not any more so than some other recent claims in the literature.

Obscured Star Formation in Abell 901/902

The authors investigate the amount of obscured star-formation as a function of environment. They find that ~40% of the star forming galaxies has red optical colors at intermediate and high densities. This suggests that environmental interactions trigger a phase of obscured star formation before complete quenching.

Friday, 5 September 2008

DOG fight

from Pope et al., arXiv:0808.2816. Dust-obscured galaxies (DOGs)
have been recently defined as objects with extremely red R-[24um]
colors. Another recent paper by Fiore et al. studied the X-ray
properties of a similarly-selected set of galaxies and found that
they are consistent with being Compton-thick AGN. The DOG people
claimed that these things were primarily starbursts, and so they
launch a counteroffensive with the above figure. While most bright
DOGs show a strong 8um excess (and are therefore likely powered by
AGN), most of the overall sample seems to be dominated by star formation.