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

TL;DR

This study detects very young stellar populations in low-mass galaxies previously thought to be quenched, revealing ongoing or recent star formation possibly fueled by internal gas reservoirs, challenging assumptions about galaxy quenching.

Contribution

It introduces a novel method using red spectral line indices to identify minor young stellar components in quenched low-mass galaxies, highlighting their potential for residual star formation.

Findings

Detected young stellar components in 8 out of 28 galaxies.

Red spectral indices reveal recent star formation activity.

Internal gas processes likely sustain residual star formation.

Abstract

We report on the detection of a small contribution (around and below 1% in mass) from young stellar components with ages $\leq$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] \AA. 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] \AA. 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.