Galaxy Counts at 24 Microns in the SWIRE Fields

TL;DR

This study analyzes galaxy counts at 24 microns across six SWIRE fields, revealing a steep rise in counts, field-to-field variations, and insights into galaxy redshift distributions and types contributing at different flux levels.

Contribution

It provides updated galaxy source counts at 24 microns, compares them across fields and models, and combines multi-band data to analyze galaxy redshift distributions and types.

Findings

Steep rise in counts between 2 mJy and 0.3 mJy.

Field variations show sample variance effects.

Majority of bright counts are low-redshift galaxies.

Abstract

This paper presents galaxy source counts at 24 microns in the six Spitzer Wide-field InfraRed Extragalactic (SWIRE) fields. The source counts are compared to counts in other fields, and to model predictions that have been updated since the launch of Spitzer. This analysis confirms a very steep rise in the Euclidean-normalized differential number counts between 2 mJy and 0.3 mJy. Variations in the counts between fields show the effects of sample variance in the flux range 0.5-10 mJy, up to 100% larger than Poisson errors. Nonetheless, a "shoulder" in the normalized counts persists at around 3 mJy. The peak of the normalized counts at 0.3 mJy is higher and narrower than most models predict. In the ELAIS N1 field, the 24 micron data are combined with Spitzer-IRAC data and five-band optical imaging, and these bandmerged data are fit with photometric redshift templates. Above 1 mJy the counts are dominated by galaxies at z less than 0.3. By 300 microJy, about 25% are between z ~ 0.3-0.8, and a significant fraction are at z ~ 1.3-2. At low redshifts the counts are dominated by spirals, and starbursts rise in number density to outnumber the spirals' contribution to the counts below 1 mJy.