Galaxies in the act of quenching star formation

Detecting galaxies when their star formation is being quenched is crucial to understand the mechanisms driving their evolution. We identify for the first time a sample of quenching galaxies selected just after the interruption of their star formation by exploiting the [O III] λ5007/H α ratio and searching for galaxies with undetected [O III]. Using a sample of ˜174000 star-forming galaxies extracted from the SDSS-DR8 at 0.04 ≤ z< 0.21, weidentify the ˜300 quenching galaxy best candidates with low [O III]/H α, out of ˜26000 galaxies without [O III] emission. They have masses between 10^{9.7} and 10^{10.8} M_{⊙}, consistently with the corresponding growth of the quiescent population at these redshifts. Their main properties (i.e. star-formation rate, colours, and metallicities) are comparable to those of the star-forming population, coherently with the hypothesis of recent quenching, but preferably reside in higher-density environments. Most candidates have morphologies similar to star-forming galaxies, suggesting that no morphological transformation has occurred yet. From a survival analysis we find a low fraction of candidates (˜0.58 per cent of the star-forming population), leading to a short quenching time-scale of tQ ˜ 50 Myr and an e-folding time for the quenching history of τQ ˜ 90 Myr, and their upper limits of tQ < 0.76 Gyr and τQ <1.5 Gyr, assuming as quenching galaxies 50 per cent of objects without [O III] (˜ 7.5 per cent). Ourresults are compatible with a 'rapid' quenching scenario of satellites galaxies due to the final phase of strangulation or ram-pressure stripping. This approach represents a robust alternative to methods used so far to select quenched galaxies (e.g. colours, specific star-formation rate, or post-starburst spectra).