Diverging UV and Halpha fluxes of star forming galaxies predicted by the IGIMF theory

TL;DR

This paper predicts that in star-forming galaxies, the Halpha flux declines faster than UV flux at low star formation rates due to the IGIMF theory, impacting galaxy luminosity interpretations.

Contribution

It introduces galaxy-wide IMF relations based on the IGIMF theory, highlighting how Halpha and UV fluxes diverge at low SFRs, a novel prediction for dwarf galaxies.

Findings

Halpha luminosity decreases faster than UV at low SFRs.

Turn-down of Halpha-UV flux ratio below SFR of 10^-2 M_sun/yr.

IGIMF effects are significant in low-mass galaxies.

Abstract

Although the stellar initial mass function (IMF) has only been directly determined in star clusters it has been manifoldly applied on galaxy-wide scales. But taking the clustered nature of star formation into account the galaxy-wide IMF is constructed by adding all IMFs of all young star clusters leading to an integrated galactic initial mass function (IGIMF). The IGIMF is top-light compared to the canonical IMF in star clusters and steepens with decreasing total star formation rate (SFR). This discrepancy is marginal for large disk galaxies but becomes significant for SMC-type galaxies and less massive ones. We here construct IGIMF-based relations between the total FUV and NUV luminosities of galaxies and the underlying SFR. We make the prediction that the Halpha luminosity of star forming dwarf galaxies decreases faster with decreasing SFR than the UV luminosity. This turn-down of the Halpha-UV flux ratio should be evident below total SFRs of 10^-2 M_sun/yr.