Local Volume HI Survey: the far-infrared radio correlation

TL;DR

This study confirms a tight linear far-infrared radio correlation in local gas-rich galaxies, including dwarfs, and explores the underlying factors maintaining this relation despite individual deficiencies.

Contribution

It provides the first detailed analysis of the FIR-radio correlation in a volume-limited sample of local galaxies, including dwarfs, and investigates the physical factors behind the correlation's robustness.

Findings

FIR-radio correlation holds over four orders of magnitude.

Dwarf galaxies are deficient in FIR and radio emissions relative to star formation.

Multiple galaxy properties cancel out, maintaining the FIR-radio ratio.

Abstract

In this paper we measure the far-infrared (FIR) and radio flux densities of a sample of 82 local gas-rich galaxies, including 70 "dwarf" galaxies ($M_* < 10^9 M_\odot$), from the Local Volume HI Survey (LVHIS), which is close to volume limited. It is found that LVHIS galaxies hold a tight linear FIR-radio correlation (FRC) over four orders of magnitude ($F_{1.4GHz} \propto F_{FIR}^{1.00\pm0.08}$). However, for detected galaxies only, a trend of larger FIR-to-radio ratio with decreasing flux density is observed. We estimate the star formation rate by combining UV and mid-IR data using empirical calibration. It is confirmed that both FIR and radio emission are strongly connected with star formation but with significant non-linearity. Dwarf galaxies are found radiation deficient in both bands, when normalized by star formation rate. It urges a "conspiracy" to keep the FIR-to-radio ratio generally constant for dwarf galaxies. By using partial correlation coefficient in Pearson definition, we identify the key galaxy properties associated with the FIR and radio deficiency. Some major factors, such as stellar mass surface density, will cancel out when taking the ratio between FIR and radio fluxes. The remaining factors, such as HI-to-stellar mass ratio and galaxy size, are expected to cancel each other due to the distribution of galaxies in the parameter space. Such cancellation is probably responsible for the "conspiracy" to keep the FRC alive.