2009/03/21 by S. J. Kannappan, J. M. Guie, A. J. Baker · 1 citation
Physics and Astronomy · #astro-ph.CO
paper · pdf · doi:10.1088/0004-6256/138/2/579
published as Astron.J.138:579,2009 · AJ, submitted, revised, 21 pages with 15 figures (one in two parts, one color); full resolution version available at http://www.physics.unc.edu/~sheila/kgb.pdf
arxiv created 2009/03/21 · arxiv updated 2010/11/02
We identify a population of morphologically defined E/S0 galaxies lying on the blue sequence at the present epoch. Using three samples, we analyze blue-sequence E/S0s with stellar masses >108 Msun, arguing that individual objects may be evolving either up toward the red sequence or down into the blue sequence. Blue-sequence E/S0 galaxies become more common with decreasing stellar mass, comprising <2% of E/S0s near the "shutdown mass" Ms ~ 1-2 x 1011 Msun, increasing to >5% near the "bimodality mass" Mb ~ 3 x 1010 Msun, and sharply rising to >20-30% below the "threshold mass" Mt ~ 4-6 x 109 Msun. The strong emergence of blue-sequence E/S0s below Mt coincides with a previously reported global increase in mean atomic gas fractions below Mt for galaxies of all types on both sequences, suggesting that the availability of cold gas may be basic to blue-sequence E/S0s' existence. Environmental analysis reveals that many sub-Mb blue-sequence E/S0s reside in low to intermediate density environments. In mass-radius and mass-sigma scaling relations, blue-sequence E/S0s are more similar to red-sequence E/S0s than to late-type galaxies, but they represent a transitional class. While some of them, especially in the high-mass range from Mb to Ms, resemble major-merger remnants that will likely fade onto the red sequence, most blue-sequence E/S0s below Mb show signs of disk and/or pseudobulge building, which may be enhanced by companion interactions. We argue that sub-Mb blue-sequence E/S0s occupy a "sweet spot" in stellar mass and concentration, with both abundant gas and optimally efficient star formation, which may enable the formation of large spiral disks. [abridged]