Anomalously low PAH emission from low-luminosity galaxies

TL;DR

This study investigates the relationship between PAH emission and galaxy properties using Spitzer and SDSS data, revealing that low-luminosity, star-forming galaxies exhibit unexpectedly weak PAH emission, possibly due to metallicity effects.

Contribution

It provides new insights into PAH emission behavior in low-luminosity galaxies and its relation to star formation and metallicity, using combined infrared and optical data.

Findings

Red galaxies show low PAH-to-star ratios due to dust attenuation.

Blue, star-forming galaxies have high PAH-to-star ratios, except for low-luminosity cases.

Low-luminosity star-forming galaxies exhibit anomalously low PAH emission, possibly linked to metallicity.

Abstract

The Spitzer Space Telescope First Look Survey Infrared Array Camera (IRAC) near and mid-infrared imaging data partially overlaps the Sloan Digital Sky Survey (SDSS), with 313 visually selected (r<17.6 mag) SDSS Main Sample galaxies in the overlap region. The 3.5 and 7.8 um properties of the galaxies are investigated in the context of their visual properties, where the IRAC [3.5] magnitude primarily measures starlight, and the [7.8] magnitude primarily measures PAH emission from the interstellar medium. As expected, we find a strong inverse correlation between [3.5]-[7.8] and visual color; galaxies red in visual colors (`red galaxies') tend to show very little dust and molecular emission (low `PAH-to-star' ratios), and galaxies blue in visual colors (`blue galaxies,' ie, star-forming galaxies) tend to show large PAH-to-star ratios. Red galaxies with high PAH-to-star ratios tend to be edge-on disks reddened by dust lanes. Simple, visually inferred attenuation corrections bring the visual colors of these galaxies in line with those of face-on disks; ie, PAH emission is closely related to attenuation-corrected, optically inferred star-formation rates. Blue galaxies with anomalously low PAH-to-star ratios are all low-luminosity star-forming galaxies. There is some weak evidence in this sample that the deficiency in PAH emission for these low-luminosity galaxies may be related to emission-line metallicity.