'Observations Meet Theory: Merger Rates, Star Formation History, and Bulgeless Galaxies'
  
Hierarchical LCDM models provide a successful paradigm for the growth of dark matter on large scales, but they face important challenges in predicting how the baryonic components of galaxies evolve. I present constraints on two aspects of this evolution. (1) The merger and star formation history of galaxies since z<1: From an HST ACS study of ~4500 galaxies in the GEMS survey, we  find that  over the interval z~0.24--0.80,  9% to 12% of high mass galaxies in every Gyr bin  show evidence of strong distortions indicative of  high mass ratio mergers or interactions.  We compare the results with the minor+major merger fraction predicted by a suite of LCDM simulations. Using UV and Spitzer data, we find  that at z<1,the average SFR of strongly disturbed or interacting systems is only modestly enhanced with respect to undisturbed galaxies. In fact, strongly disturbed  systems only account for a small fraction of the cosmic SFR density at z<1. This suggests that the decline in the cosmic SFR density since z~1, is driven primarily by a shutdown in the star formation of undisturbed galaxies.   (2) The problem of bulgeless galaxies: From the 2D structural bulge+disk+bar decomposition of NIR images for a complete sample of nearby luminous galaxies, we present the distribution of bulge-to-total (B/T) ratio for  luminous galaxies. We find that a large fraction of local galaxies with B/T < 0.2, in stark contrast to LCDM cosmogonies which severely underpredict the frequency of such bulgeless or low B/T systems.




Comments
There are a large number of  co-authors from UT Austin and the GEMS collaboration on this abstract and they will be included when the article is written up. The full author list is 

Shardha  Jogee (UT Austin),  Sarah Miller (UT Austin),  Kyle Penner (UT Austin), Tim Weinzirl (UT Austin), Chris Conselice (Nottingham), Eric Bell (MPIA), Xian Zhong Zheng (PMO), Casey Papovich (Arizona), Rosalind E. Skelton (MPIA), Rachel Somerville (MPIA),Hans-Walter Rix (MPIA), Fabio Barazza (EPFL), Marco Barden (Innsbruck), Andrea Borch (MPIA), Steve Beckwith (JHU),John Caldwell (UT McDonald), Boris Haeussler (Nottingham), Catherine Heymans (UBC IAP), Knud Jahnke (MPIA), Daniel McIntosh (UMass),Klaus Meisenheimer (MPIA), Chien Peng (NRC HIA STScI), Aday Robaina (MPIA), Sebastian Sanchez (CAHA), Lutz Wisotzki (AIP), Christian Wolf (Oxford)}