Red Nugget Watch

from Saracco, Longhetti, & Andreon, http://arxiv.org/abs/0810.2795

This paper presents another analysis of the sizes and surface densities of early-type galaxies at z=1-2; here they use a sample of 32 spectroscopically-confirmed galaxies with a mean redshift of 1.45 from several different surveys. Masses and ages are determined via SED fitting to the photometry. As has been reported before, these galaxies lie well off the z=0 size-luminosity relation. By evolving the galaxies (assuming pure luminosity evolution) from their measured  redshifts to the present, the authors find that some galaxies would actually evolve to the z=0 relation in this manner, while some would not. These galaxies appear to be "young" and "old" respectively, and the authors appear to claim a bimodal age distribution in ETGs at this redshift, with typical ages ~1 Gyr and 3.5 Gyr. Young ETGs follow the local size-mass relation, old ETGs do not.

The authors conclude that the young objects have more or less completed their evolution (except for luminosity evolution), while the old galaxies still need some process to increase their effective radii. Dry merging cannot do this because it would create too many high-mass galaxies, so some other process must be at work. Much of this rests on the assumption that the relative ages can be accurately determined through photometry, of course.

The evolution of stellar mass and the implied star formation history

Figure 2 in arXiv:0801.1594, Wilkins et al.
Caption: The authors compile a number of measurements of the stellar mass density as a function of redshift and convert this to a star formation history. They then compare this to measurements of the instantaneous star formation history.

The plot shows the results. The dark solid line is a best fit to the star formation history inferred from the mass density history and the grey shaded areas denote the 1 and 3 sigma uncertainty regions. The various symbols are the measurements of the instantaneous star formation with different symbols denoting different methods of measuring the instantaneous SFR.

For redshifts lower than 0.7 the two methods of determining the star formation history agree quite well, but for larger redshifts there is a clear discrepancy between both methods. Suggested causes for this discrepancy include overestimation of the extinction correction and evolution of the IMF with time.

Post by Ernst Kuiper