The First Source Counts at 18 microns from the AKARI NEP Survey

TL;DR

This paper presents the first 18-micron galaxy counts from the AKARI NEP survey, revealing evolutionary features of infrared galaxies and resolving over half of the cosmic infrared background at this wavelength.

Contribution

It introduces a new source extraction method for 18-micron data and provides the first comprehensive galaxy counts at this wavelength from AKARI.

Findings

Source counts show an upturn at around 1 mJy indicating galaxy evolution.

The counts are consistent with models of strongly evolving luminous infrared galaxies.

AKARI resolves approximately 55% of the 18-micron cosmic infrared background.

Abstract

We present the first galaxy counts at 18 microns using the Japanese AKARI satellite's survey at the North Ecliptic Pole (NEP), produced from the images from the NEP-Deep and NEP-Wide surveys covering 0.6 and 5.8 square degrees respectively. We describe a procedure using a point source filtering algorithm to remove background structure and a minimum variance method for our source extraction and photometry that delivers the optimum signal to noise for our extracted sources, confirming this by comparison with standard photometry methods. The final source counts are complete and reliable over three orders of magnitude in flux density, resulting in sensitivities (80 percent completeness) of 0.15mJy and 0.3mJy for the NEP-Deep and NEP-Wide surveys respectively, a factor of 1.3 deeper than previous catalogues constructed from this field. The differential source counts exhibit a characteristic upturn from Euclidean expectations at around a milliJansky and a corresponding evolutionary bump between 0.2-0.4 mJy consistent with previous mid-infrared surveys with ISO and Spitzer at 15 and 24 microns. We compare our results with galaxy evolution models confirming the striking divergence from the non-evolving scenario. The models and observations are in broad agreement implying that the source counts are consistent with a strongly evolving population of luminous infrared galaxies at redshifts higher than unity. Integrating our source counts down to the limit of the NEP survey at the 150 microJy level we calculate that AKARI has resolved approximately 55 percent of the 18 micron cosmic infrared background relative to the predictions of contemporary source count models.