Converting Halpha luminosities into SFRs

TL;DR

This paper revises the relation between Halpha luminosity and star formation rate by incorporating the integrated galactic initial stellar mass function (IGIMF), revealing non-linearities especially in dwarf galaxies and impacting cosmological star formation estimates.

Contribution

It introduces the IGIMF framework to correct the linear L_Halpha-SFR relation, accounting for galaxy SFR dependence and improving accuracy in dwarf galaxy star formation measurements.

Findings

The L_Halpha-SFR relation is non-linear and flattens at low SFRs.

Dwarf galaxies' SFRs can be underestimated by up to three orders of magnitude.

A constant gas depletion time scale of a few Gyrs is supported across different galaxy masses.

Abstract

Star-formation rates (SFRs) of galaxies are commonly calculated by converting the measured Halpha luminosities (L_Halpha) into current SFRs. This conversion is based on a constant initial mass function (IMF) independent of the total SFR. As recently recognised the maximum stellar mass in a star cluster is limited by the embedded total cluster mass and, in addition, the maximum embedded star cluster mass is constrained by the current SFR. The combination of these two relations leads to an integrated galaxial initial stellar mass function (IGIMF, the IMF for the whole galaxy) which is steeper in the high mass regime than the constant canonical IMF, and is dependent on the SFR of the galaxy. Consequently, the L_Halpha-SFR relation becomes non-linear and flattens for low SFRs. Especially for dwarf galaxies the SFRs can be underestimated by up to three orders of magnitude. We revise the existing linear L_Halpha-SFR relations using our IGIMF notion. These are likely to lead to a revision of the cosmological star formation histories. We also demonstrate that in the case of the Sculptor dwarf irregular galaxies the IGIMF-formalism implies a linear dependence of the total SFR on the total galaxy gas mass. A constant gas depletion time scale of a few Gyrs results independently of the galaxy gas mass with a reduced scatter compared to the conventional results. Our findings are qualitatively independent of the explicit choice of the IGIMF details and challenges current star formation theory in dwarf galaxies.