The Evolution of the Interstellar Medium in Post-starburst Galaxies

Abstract

We derive dust masses (M-dust) from the spectral energy distributions of 58 post-starburst galaxies (PSBs). There is an anticorrelation between specific dust mass (M-dust/M-star) and the time elapsed since the starburst ended, indicating that dust was either destroyed, expelled, or rendered undetectable over the similar to 1 Gyr after the burst. The M-dust/M-star depletion timescale, 205(-37)(+58) Myr, is consistent with that of the CO-traced M-H2/M-star, suggesting that dust and gas are altered via the same process. Extrapolating these trends leads to the M-dust/M-star and M-H2/M-star values of early-type galaxies (ETGs) within 1-2 Gyr, a timescale consistent with the evolution of other PSB properties into ETGs. Comparing M-dust and M-H2 for PSBs yields a calibration, log M-H2 = 0.45 log M-dust + 6.02, that allows us to place 33 PSBs on the Kennicutt-Schmidt (KS) plane, Sigma SFR-Sigma M-H2. Over the first similar to 200-300 Myr, the PSBs evolve down and off of the KS relation, as their star formation rate (SFR) decreases more rapidly than M-H2. Afterwards, M-H2 continues to decline whereas the SFR levels off. These trends suggest that the star formation efficiency bottoms out at 10-11 yr(-1) and will rise to ETG levels within 0.5-1.1 Gyr afterwards. The SFR decline after the burst is likely due to the absence of gas denser than the CO-traced H-2. The mechanism of the M-dust/M-star and M-H2/M-star decline, whose timescale suggests active galactic nucleus/low-ionization nuclear emission-line region feedback, may also be preventing the large CO-traced molecular gas reservoirs from collapsing and forming denser star-forming clouds.