The UV‐Optical Galaxy Color‐Magnitude Diagram. III. Constraints on Evolution from the Blue to the Red Sequence

Abstract

We introduce a new quantity, the mass flux density of galaxies evolving from\nthe blue sequence to the red sequence. We propose a simple technique for\nconstraining this mass flux using the volume corrected number density in the\nextinction-corrected UV-optical color magnitude distribution, the stellar age\nindexes H-delta-a and D_n(4000), and a simple prescription for spectral\nevolution using a quenched star formation history. We exploit the excellent\nseparation of red and blue sequences in the NUV-r band color-magnitude diagram.\nThe final value we measure, 0.033 M_sun yr^-1 Mpc^-3, is strictly speaking an\nupper limit due to the possible contributions of bursting, composite, and\nextincted galaxies. However, it compares favorably with estimates of the\naverage mass flux that we make based on the red luminosity function evolution\nderived from the DEEPII and COMBO-17 surveys (Bell et al 2004; Faber et al.\n2005), 0.034 M_sun yr^-1 Mpc^-3. We find that the blue sequence mass has\nremained roughly constant since z=1 (mass flux 0.01 M_sun yr^-1 Mpc^-3) but the\naverage on-going star formation of 0.037$ M_sun yr^-1 Mpc^-3 over 0<z<1 is\nbalanced by mass flux off the blue sequence. We explore the nature of the\ngalaxies in the transition zone with particular attention to the frequency and\nimpact of AGNs. The AGN fraction peaks in the transition zone. We find\ncircumstantial, albeit weak evidence that the quench rates are higher in higher\nluminosity AGNs.\n