The Evolution of the Interstellar Medium in Post-starburst Galaxies

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 ) and the time elapsed since the starburst ended, indicating that dust was either destroyed, expelled, or rendered undetectable over the ~1 Gyr after the burst. The M_(dust)/M depletion timescale, 205^(+58)_(-37) Myr, is consistent with that of the CO-traced M_(H2)/M⋆, suggesting that dust and gas are altered via the same process. Extrapolating these trends leads to the M_(dust)/M and M_(H2)/M⋆ 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, ΣSFR-ΣM_(H2). Over the first ~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 and M_(H2)/M⋆ 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.