Global correlations between the radio continuum, infrared, and CO emissions in dwarf galaxies

Abstract

Correlations between the radio continuum, infrared and CO emission are known to exist for several types of galaxies and across several orders of magnitude. However, the low-mass, low-luminosity and low-metallicity regime of these correlations is not well known. A sample of metal-rich and metal-poor dwarf galaxies from the literature has been assembled to explore this extreme regime. The results demonstrate that the properties of dwarf galaxies are not simple extensions of those of more massive galaxies; the different correlations reflect different star-forming conditions and different coupling between the star formation and the various quantities. It is found that dwarfs show increasingly weaker CO and infrared emission for their luminosity, as expected for galaxies with a low dust content, slower reaction rates, and a hard ionizing radiation field. In the higher-luminosity dwarf regime (L 1.4 GHz 10 27 W, where L 1.4 GHz ≃ 10 29 W for a Milky Way star formation rate of ≃1 M ⊙ yr −1 ), the total and nonthermal radio continuum emission appear to adequately trace the star formation rate. A breakdown of the dependence of the (Hα-based) thermal, non-thermal, and, hence, total radio continuum emission on star formation rate occurs below L 1.4 GHz ≃ 10 27 W, resulting in a steepening or downturn of the relations at extreme low luminosity. Below L FIR ≃ 10 36 W ≃ 3 × 10 9 L ⊙ , the infrared emission ceases to adequately trace the star formation rate. A lack of a correlation between the magnetic field strength and the star formation rate in low star formation rate dwarfs suggests a breakdown of the equipartition assumption. As extremely metal-poor dwarfs mostly populate the low star formation rate and low luminosity regime, they stand out in their infrared, radio continuum and CO properties.

Radio jets from AGNs in dwarf galaxies in the COSMOS survey: mechanical feedback out to redshift ∼3.4

“…To estimate the radio emission from star formation processes we use the correlation between SFR and non-thermal 1.4 GHz luminosity of Filho et al (2019):…”

“…The thermal fraction in spiral galaxies is typically of 10%, while in dwarf galaxies it is generally no more than 30% (e.g. Condon 1992;Roychowdhury & Chengalur 2012;Filho et al 2019). To estimate the contribution of thermal emission to the 1.4 GHz radio luminosity (L thermal 1.4GHz ) we use the nearly one-to-one ratio between thermal and non-thermal emission of Filho et al (2019): log L non−thermal 1.4GHz,Filho ∝ 0.92 × log L thermal 1.4GHz .…”

“…Top. Radio luminosity at 1.4 GHz for the parent sample of 43 dwarf galaxies versus non-thermal 1.4 GHz radio luminosity expected from star formation derived using the correlation for dwarf galaxies of Filho et al (2019). Bottom.…”

Revisiting the Equipartition Assumption in Star-Forming Galaxies

“…Equipartition is not valid in starburst regions or in ultraluminous infrared galaxies. The equipartition condition may also fail in dwarf galaxies [63].…”

“…The equipartition condition probably fails also in dwarf galaxies with a low star-formation rate (SFR) and hence a discontinuous supply of CRs [62]. Below SFR 0.01 M o /yr, radio synchrotron emission does no longer trace the SFR and the equipartition field strength is no longer correlated with SFR [63].…”