Mid-IR emission of galaxies in the Virgo cluster and in the Coma supercluster.IV. The nature of the dust heating sources

TL;DR

This study investigates the sources of dust heating in late-type galaxies by analyzing mid-IR emission and its relation to other star formation indicators, revealing that mid-IR emission is mainly driven by evolved stars rather than direct star formation activity.

Contribution

It demonstrates that mid-IR emission correlates more with far-IR than with direct star formation tracers, highlighting the role of evolved stars in dust heating in normal galaxies.

Findings

Mid-IR correlates better with far-IR than Hα or UV luminosity.

Mid-IR emission includes diffuse components not linked to HII regions.

Metallicity influences the scatter in mid-IR and star formation relations.

Abstract

We study the relationship between the mid-IR (5-18 $\mu$m) emission of late-type galaxies and various other star formation tracers in order to investigate the nature of the dust heating sources in this spectral domain. The analysis is carried out using a sample of 123 normal, late-type, nearby galaxies with available data at several frequencies. The mid-IR luminosity (normalized to the H-band luminosity) correlates better with the far-IR luminosity than with more direct tracers of the young stellar population such as the H$\alpha$ and the UV luminosity. The comparison of resolved images reveals a remarkable similarity in the H$\alpha$ and mid-IR morphologies, with prominent HII regions at both frequencies. The mid-IR images, however, show in addition a diffuse emission not associated with HII regions nor with the diffuse H$\alpha$ emission. This evidence indicates that the stellar population responsible for the heating of dust emitting in the mid-IR is similar to that heating big grains emitting in the far-IR, including relatively evolved stars responsible for the non-ionizing radiation. The scatter in the mid-IR vs. H$\alpha$, UV and far-IR luminosity relation is mostly due to metallicity effects, with metal-poor objects having a lower mid-IR emission per unit star formation rate than metal-rich galaxies. Our analysis indicates that the mid-IR luminosity is not an optimal star formation tracer in normal, late-type galaxies.