A new method for ISOCAM data reduction - II. Mid-infrared extragalactic source counts in the ELAIS Southern field

Abstract

We present the 15‐μm extragalactic source counts from the Final Analysis Catalogue of the European Large Area ISO Survey (ELAIS) Southern hemisphere field S1, extracted using the Lari method. The large number of extragalactic sources (∼350) detected over this area between about 0.5 and 100 mJy guarantee a high statistical significance of the source counts in the previously poorly covered flux density range between IRAS and the Deep ISOCAM Surveys. The bright counts in S1 (S15 μm≳ 2 mJy) are significantly lower than other published ISOCAM counts in the same flux range and are consistent with a flat, Euclidean slope, suggesting the dominance of a non‐evolving population. In contrast, at fainter fluxes (S15 μm≲ 2 mJy) our counts show a strong departure from no‐evolution models, with a very steep super‐Euclidean slope down to our flux limit (∼0.5 mJy). Strong luminosity and density evolutions of the order of, respectively, L∝ (1 +z)3.0 and ρ∝ (1 +z)3.5 are needed at least for the population of star‐forming galaxies in order to fit the counts and the redshift distributions observed at different fluxes. A luminosity break around 1010.8L⊙ must be introduced in the local luminosity function of starburst galaxies in order to reproduce our sharp increase of the counts below 2 mJy and the redshift distributions observed for 15‐μm sources at different flux levels. The contribution of the strongly evolving starburst population (down to 50 μJy) to the 15‐μm cosmic background is estimated to be ∼2.2 nW m−2 sr−1, which corresponds to ∼67 per cent of the total mid‐infrared background estimate.

Modeling the evolution of infrared galaxies: a parametric backward evolution model

“…At 15 μm, the Elbaz et al (1999) counts from different fields are incompatible with each other, but our counts coincide with in the cloud of points. The Gruppioni et al (2002) counts are significantly lower than our model and other works. We marginally agree with the Pearson et al (2010) counts.…”

“…Number counts at 15 μm based on Infrared Space Observatory (ISO) data (Elbaz et al 1999;Gruppioni et al 2002) and the AKARI data (Pearson et al 2010;Hopwood et al 2010) differ by a factor of about 2, and their errors do not include the effects of cosmic variance. The results of these papers were not used for the fit.…”

ISOCAM observations in the Lockman Hole

“…This is a very important point since these catalogues are used to compute deep 14.3 µm extra-galactic counts. The counts of Gruppioni et al (2002) assume, for instance, a slightly different calibration based on a relationship between near-IR magnitudes and IRAS fluxes.…”

AGN counts at 15$\mu{\rm m}$

“…It covers an area of ∼4 deg 2 centered at α(2000) = 00 h 34 m 44.4 s , δ(2000) = −43 • 28 12 and includes 462 mid-IR sources down to a flux limit of 0.5 mJy. Mid-IR source counts based on this catalogue have been presented and discussed by Gruppioni et al (2002). La Franca et al (2004) presents the optical and spectroscopic identifications and classification of a more reliable subsample of 406 sources from the Lari et al (2001) catalogue.…”

“…MIR surveys, carried out with the ISOCAM instrument on board the Infrared Space Observatory (ISO), indicates that the MIR galaxies have evolved significantly faster than deduced from optical surveys (e.g. Elbaz et al 1999;Gruppioni et al 2002;Metcalfe et al 2003;Rodighiero et al 2004;Pozzi et al 2004). These results are now confirmed and extended by the data coming from the Spitzer Space Telescope (e.g.…”