Source counts at 15 microns from the AKARI NEP survey

¹ Space Science and Technology Department, CCLRC Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, UK e-mail: chris.pearson@stfc.ac.uk
² Department of Physics, University of Lethbridge, 4401 University Drive, Lethbridge, Alberta T1J 1B1, Canada
³ Astrophysics Group, Department of Physics, The Open University, Milton Keynes, MK7 6AA, UK
⁴ Institute of Space and Astronautical Science, Yoshinodai 3-1-1, Sagamihara, Kanagawa 229 8510, Japan
⁵ Department of Physics and Astronomy, Seoul National University, Shillim-Dong, Kwanak-Gu, Seoul 151-742, Korea
⁶ Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI, 96822, USA
⁷ National Astronomical Observatory, 2-21-1 Osawa, Mitaka, Tokyo, 181-8588, Japan
⁸ Academia Sinica, Institute of Astronomy and Astrophysics, Taiwan

Received: 30 September 2009
Accepted: 3 March 2010

Abstract

We present galaxy counts at 15 microns using the Japanese AKARI satelitte's NEP-deep and NEP-wide legacy surveys at the north ecliptic pole. The total number of sources detected are approximately 6700 and 10 700 down to limiting fluxes of 117 and 250 microJy (5 sigma) for the NEP-deep and NEP-wide survey respectively. We construct the Euclidean normalized differential source counts for both data sets (assuming 80 percent completeness levels of 200 and 270 microJy respectively) to produce the widest and deepest contiguous survey at 15 microns to date covering the entire flux range from the deepest to shallowest surveys made with the infrared space observatory (ISO) over areas sufficiently significant to overcome cosmic variance, detecting six times as many sources as the largest survey carried out with ISO. We compare the results from AKARI with the previous surveys with ISO at the same wavelength and the Spitzer observations at 16 microns using the peek-up camera on its IRS instrument. The AKARI source counts are consistent with other results to date reproducing the steep evolutionary rise at fluxes less than a milliJansky and super-Euclidean slopes. We find the the AKARI source counts show a slight excess at fluxes fainter than 200 microJanskys which is not predicted by previous source count models at 15 microns. However, we caution that at this level we may be suffering from the effects of source confusion in our data. At brighter fluxes greater than a milliJansky, the NEP-wide survey source counts agree with the Northern ISO-ELAIS field results, resolving the discrepancy of the bright end calibration in the ISO 15 micron source counts.