The frequency of very young galaxies in the local Universe: II. The view from SDSS spectra

TL;DR

This study estimates the fraction of very young galaxies in the local universe using SDSS spectra, revealing a decline with galaxy mass and providing insights into galaxy formation models and star formation histories.

Contribution

It introduces a non-parametric method to estimate VYG fractions from SDSS spectra and compares results with galaxy formation models, highlighting discrepancies and constraints.

Findings

VYG fractions decrease from ~50% at 10^8 M_sun to ~0.1% at 10^{11.5} M_sun

VYGs are more common in lower-mass galaxies and less so in massive ones

Results align better with hydrodynamical simulations than with analytical models.

Abstract

Only a handful of galaxies in the local Universe appear to be very young. We estimate the fraction of very young galaxies (VYGs), defined as those with more than half their stellar masses formed within the last Gyr. We fit non-parametric star formation histories (SFHs) to ~280 000 galaxy spectra from a flux- and volume-limited subsample of the Main Galaxy Sample (MGS) of the SDSS, which is also complete in mass-to-light ratio, thus properly accounting for passive galaxies of a given mass. The VYG fractions decrease with increasing galaxy stellar mass, from ~50% at $m = 10^8\,\rm M_\odot$ to ~0.1% at $m = 10^{11.5}\,\rm M_\odot$, with differences of up to 1 dex between the different spectral models used to estimate the SFH and on how we treat aperture effects. But old stellar populations may hide in our VYGs despite our conservative VYG sample built with galaxies that are globally bluer than within the region viewed by the SDSS fibre. The VYG fractions versus mass decrease more gradually compared to the Tweed et al. predictions using analytical and semi-analytical models of galaxy formation, but agree better with the SIMBA hydrodynamical simulation. These discrepancies highlight the usefulness of VYGs in constraining the strong uncertainties in both galaxy formation models and spectral modelling of galaxy SFHs. Given the lognormal cosmic SFH, these mean VYG fractions suggest that galaxies with $m > 10^8\,\rm M_\odot$ undergo at most 4 major starbursts on average.