Identification of galaxies that experienced a recent major drop of star formation

International audienceVariations of star formation activity may happen on a large range of timescales and some of them are expected to be short, that is, a few hundred million years. The study of the physical processes linked to these rapid variations requires large statistical samples to pinpoint galaxies undergoing such transformations. Building upon a previous study, we define a method to blindly identify galaxies that have undergone, and may still be undergoing, a fast downfall of their star formation activity, that is, a more than 80% drop in star formation rate (SFR) occurring in less than 500 Myr. Modeling galaxies’ spectral energy distribution (SED) with a delayed-τ star formation history, with and without allowing an instantaneous SFR drop within the last hundred million years, we isolate 102 candidates out of a subsample of 6680 galaxies classified as “star forming” from the UVJ criterion in the ZFOURGE catalogs. These galaxies are mostly located in the lower part of the SFR-M* main sequence (MS) and extend up to a factor 100 below it. They also lie close to the limit between the passive and active regions on the UVJ diagram, indicating that they are in a transition phase. We show that the selected candidates have different physical properties compared to galaxies with similar UVJ colors, namely, lower SFRs and different stellar masses. The morphology of the candidates shows no preference for a particular type. Among the 102 candidates, only 4 show signs of a active galactic nucleus (AGN) activity (from X-ray luminosity or ultraviolet–infrared (UV–IR) SED fitting decomposition). This low fraction of AGNs among the candidates implies that AGN activity may not be the main driver of the recent downfall, although timescale differences and duty cycle must be taken into account. We finally attempt to recover the past position of these galaxies on the SFR-M* plane, before the downfall of their star formation and show that some of them were in the starburst region before, and are now back on the MS. These candidates constitute a promising sample that needs more investigation in order to understand the different mechanisms at the origin of the star formation decrease of the Universe since z ~ 2