The relationship between star formation rates and mid-infrared emission in galactic disks

TL;DR

This study demonstrates that mid-infrared emission at 7 and 15 microns effectively traces star formation in galactic disks, revealing a consistent origin linked to aromatic infrared bands and clarifying the relation with Halpha and far-infrared emissions.

Contribution

The paper establishes a new calibration method linking mid-infrared fluxes to star formation rates, based on observed correlations with Halpha emission in spiral galaxies.

Findings

Mid-infrared emission correlates with Halpha in galactic disks.

The 15/7 micron flux ratio remains nearly constant across different conditions.

Circumnuclear dust emission affects far-infrared and Halpha flux relations.

Abstract

The Halpha and mid-infrared mean disk surface brightnesses are compared in a sample of nearby spirals observed by ISOCAM. This shows that, in spiral disks, dust emission at 7 and 15 microns provides a reasonable star formation tracer. The fact that the 15 to 7 micron flux ratio is nearly constant in various global exciting conditions indicates a common origin, namely the aromatic infrared band carriers, and implies that at these wavelengths, dust emission from the disks of normal galaxies is dominated by photodissociation regions and not by HII regions themselves. We use this newly-found correlation between the mid-infrared and the Halpha line to investigate the nature of the link between the far-infrared (60 and 100 microns) and Halpha. Although the separation of the central regions from the disk is impossible to achieve in the far-infrared, we show that a circumnuclear contribution to the dust emission, having no equivalent counterpart in Halpha, is most likely responsible for the well-known non-linearity between far-infrared and Halpha fluxes in spiral galaxies. We derive a calibration of 7 and 15 micron fluxes in terms of star formation rates from a primary calibration of Halpha in the literature, and also outline the applicability limits of the proposed conversion, which should not be blindly extrapolated to objects whose nature is unknown.