Detection of young (≤20 Myr) stellar populations in apparently quenched low-mass galaxies using red spectral line indices

Abstract

We report on the detection of a small contribution (around and below 1% in mass) from young stellar components with ages ≤20 Myr in low-mass galaxies purposely selected from the MaNGA survey to be already-quenched systems. Among the sample of 28 galaxies, eight of them show signatures of having suffered a very recent burst of star formation. The detection has been done through the analysis of line-strength indices in the red spectral range [5700,8800] Å. The increasing contribution of red supergiants to this red regime is responsible for a deviation of the index measurements with respect to their position within the model grids in the standard spectral range [3600,5700] Å. We demonstrate that a combination of red indices, as well as a qualitative assessment of the mean luminosity-weighted underlying stellar population, is required in order to distinguish between a true superyoung population and other possible causes of this deviation, such as abundance ratio variations. Our result implies that many presumably quenched low-mass galaxies actually contain gas that is triggering some level of star formation. They have, therefore, either accreted external gas, internally recycled enough gas from stellar evolution to trigger new star formation, or they kept a gas reservoir after the harassment or stripping process that quenched them in first place. Internal processes are favoured since we find no particular trends between our non-quenched galaxies and their environment, although more work is needed to fully discard an external influence.

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Do galaxies die? Different views from simulations and observations in the Local Universe

“…Unfortunately, accurately measuring any level of star formation is always challenging, and intrinsic differences between observational tracers have been reported in the literature (Salim et al 2007;Kennicutt & Evans 2012;Davies et al 2016;Katsianis et al 2020). This fact becomes critical at the low-SFR regime, both for low-mass galaxies with faint fluxes as well as massive systems with young stellar fractions of the order of one percent or below (Salvador-Rusiñol et al 2020;de Lorenzo-Cáceres et al 2020). Besides, selection effects at different wavelengths may affect the statistical distribution of galaxies if not properly accounted for (Eales et al 2018;Davies et al 2019).…”