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 the blue sequence to the red sequence. We propose a simple technique for constraining this mass flux using the volume-corrected number density in the extinction-corrected UV-optical color-magnitude distribution, the stellar age indexes Hδ_A and D_n(4000), and a simple prescription for spectral evolution using a quenched star formation history. We exploit the excellent separation of red and blue sequences in the NUV − r band Hess function. The final value we measure, ρ_T = 0.033 M_⊙ yr^(−1) Mpc^(−3), is strictly speaking an upper limit due to the possible contributions of bursting, composite, and extincted galaxies. However, it compares favorably with estimates of the average mass flux that we make based on the red luminosity function evolution derived from the DEEP2 and COMBO-17 surveys, ρ_R = +0.034 M_⊙ yr^(−1) Mpc^(−3). We find that the blue sequence mass has remained roughly constant since z = 1 (ρ_B ≃ 0.01 M_⊙ yr^(−1) Mpc^(−3)), but the average on-going star formation of ρ_(SF) ≃ 0.037 M_⊙ yr^(−1) Mpc^(−3) over 0 < z < 1 is balanced by mass flux off the blue sequence. We explore the nature of the galaxies in the transition zone with particular attention to the frequency and impact of active galactic nuclei (AGNs). The AGN fraction peaks in the transition zone. We find circumstantial, albeit weak evidence that the quench rates are higher in higher luminosity AGNs.