Friday, 28 March 2008

From The author postulates that a natural explanation for the observed metallicity gradient in DLAs can be explained via dense, metal poor disks and less dense metal rich outflows.

Star formation in BCGs

(Figure 10 of O'Dea et al.,

Upper limits on the mass deposition rate (derived from X-ray emission measurements) vs. star formation rate for brightest cluster galaxies. Filled circles denote the maximum mass deposition rate (assuming no gas heating), while open circles are the rates derived from detailed cooling-flow model fits.

This paper discusses results from a Spitzer survey of 62 brightest cluster galaxies that exhibit optical line emission, specifically the ~50% of those galaxies that show excess infrared emission. While four of these appear to be dominated by AGN, they claim that the remainder show evidence for star formation (from L_IR, CO measurements, and Halpha luminosity). The above figure then appears to show a strong correlation between mass deposition from the ICM and star formation, and therefore such cooling may be the fuel source for the SF activity in these galaxies. Furthermore, the MDR exceeds the SFR, indicating that some reheating mechanism may be at work. It should be noted that all the points in this figure are formally upper limits, so in fact the cooling rate could be entirely consistent with the SFR.

Friday, 14 March 2008

arXiv:0803.1489 [ps, pdf, other]
Title: The contribution of star formation and merging to stellar mass buildup in galaxies
Authors: Niv Drory (1), Marcelo Alvarez (2) ((1) MPE, Garching, Germany, (2) KIPAC-Stanford, USA)
Comments: Accepted for publication in ApJ
Subjects: Astrophysics (astro-ph)
The authors take data from the FORS Deep Field (FDF), 5500 galaxies with redshifts between 0 and 5. They obtain stellar masses and SFRs as well as photo-z's. By integrating the SFR over time they calculate, as a function of mass, the change in number density of galaxies of that mass due to star formation. They compare that to the actual change of the mass function to obtain the contribution that is from anything other than star formation, being mainly merging and accretion (assuming that tidal stripping and so on are negligable). They call this the assembly rate and that is what is shown in the figure (the y-axis normalized to have the relative change), as a function of z and mass (in both plots). See the paper for selection effects...

Monday, 10 March 2008

Are SNIa a two-component family?

In astro-ph/0803.1130, Dahlen, Strolger & Riess report on their HST survey for high redshift SN Ia. Their results strengthen the argument that the SNIa rate drops at high redshift. The immediate interpretation of this is that SN Ia only start after some long delay. Their favourite model has a delay time of 3.7 Gyr and is shown as the solid line in the Figure. The solid points in the Figure shows the observed SN Ia rate.

Now the problem is that a number of groups have recently argued, fairly convincingly, that SN Ia at lower redshift clearly fall into two categories: A group of delayed explosions and a group of "prompt" explosions - ala SN II. Three such models are shown as non-solid lines in the Figure and the match to the observations fails to excite :) What gives? The authors argue that dust obscured star formation might explain the differences, but it is not settled.

Friday, 7 March 2008

The evolution of submillimetre galaxies: two populations and a redshift cut-off

Wall, Pope, & Scott 2008, MNRAS, 383, 435

These authors find reasonably secure counterparts for 35/38 SCUBA sub-mm sources in GOODS-N. This plot shows the relationship between redshift (combination of spectroscopic and photometric redshifts) and rest-frame 850 micron luminosity (which is also illustrated by the size of the circular plotting symbols). The black symbols are those below the median luminosity, and the blue symbols are above.  The curves show the detection limits; since the noise varies strongly over the SCUBA map, a different completeness curve is shown for the location on the map of each detected galaxy.  The fact that these curves flatten at high redshift is due to the negative K-correction.

One notes an apparent lack of high-luminosity sources at z<~1.5 and of low-luminosity sources at z>~2.5.  This is interpreted as an aspect of the generic downsizing trend of star-formation.  The authors split the sample in half by luminosity, and model the evolution of the luminosity function of the two samples independently as a function of redshift.  But the qualitative conclusions remain the same, and based on the data you'd have to be pretty brave to conclude very much more than that.  I'll post a few more thoughts in the comments, but those will probably only be relevant to those who have actually taken a look at this paper.

WMAP 5 year results!

WMAP 5 year results are out in a series of 7, ~50 page papers. The main, and probably most important, difference is in the value of sigma-8. This is higher than in the 3 year results, but still lower than in the first release.

This table is in
arXiv:0803.0586 [ps, pdf, other]
Title: Five-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Likelihoods and Parameters from the WMAP data
Comments: 57 pages, 21 figures, submitted to ApJS. For higher quality figs, see version on this http URL
Subjects: Astrophysics (astro-ph)