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