Infrared Properties of a Complete Sample of Star-Forming Dwarf Galaxies

TL;DR

This study analyzes mid-infrared properties of star-forming dwarf galaxies, revealing their diverse characteristics, a deficit in 8.0 μm PAH emission, and their significant contribution to the local universe's 8.0 μm luminosity density.

Contribution

It provides a comprehensive analysis of a large, complete sample of dwarf galaxies, highlighting new correlations and the role of these galaxies in the local infrared luminosity function.

Findings

Dwarf galaxies show a deficit in 8.0 μm PAH emission.

There are more extremely red [3.6]-[8.0] galaxies than in normal samples.

Nearly all 8.0 μm luminosity density below 10^9 L_sun comes from dwarf galaxies with strong Hα emission.

Abstract

We present a study of a large, statistically complete sample of star-forming dwarf galaxies using mid-infrared observations from the {\it Spitzer Space Telescope}. The relationships between metallicity, star formation rate (SFR) and mid-infrared color in these systems show that the galaxies span a wide range of properties. However, the galaxies do show a deficit of 8.0 \um\ polycyclic aromatic hydrocarbon emission as is apparent from the median 8.0 \um\ luminosity which is only 0.004 \lstarf\ while the median $B$-band luminosity is 0.05 \lstarb. Despite many of the galaxies being 8.0 \um\ deficient, there is about a factor of 4 more extremely red galaxies in the [3.6] $-$ [8.0] color than for a sample of normal galaxies with similar optical colors. We show correlations between the [3.6] $-$ [8.0] color and luminosity, metallicity, and to a lesser extent SFRs that were not evident in the original, smaller sample studied previously. The luminosity--metallicity relation has a flatter slope for dwarf galaxies as has been indicated by previous work. We also show a relationship between the 8.0 \um\ luminosity and the metallicity of the galaxy which is not expected given the competing effects (stellar mass, stellar population age, and the hardness of the radiation field) that influence the 8.0 \um\ emission. This larger sample plus a well-defined selection function also allows us to compute the 8.0 \um\ luminosity function and compare it with the one for the local galaxy population. Our results show that below 10$^{9}$ $L$\solar, nearly all the 8.0 \um\ luminosity density of the local universe arises from dwarf galaxies that exhibit strong \ha\ emission -- i.e., 8.0 \um\ and \ha\ selection identify similar galaxy populations despite the deficit of 8.0 \um\ emission observed in these dwarfs.