The European Large Area ISO Survey - II. Mid-infrared extragalactic sourcecounts

We present preliminary source counts at 6.7 and 15 mu m from the preliminar y analysis of the European Large Area ISO Survey, with limiting flux densit ies of similar to 2 mJy at 15 mu m and similar to 1 mJy at 6.7 mu m. We sep arate the stellar contribution from the extragalactic using identifications with automated plate measurement sources made with the likelihood ratio te chnique. We quantify the completeness and reliability of our source extract ion using (a) repeated observations over small areas, (b) cross-identificat ions with stars of known spectral type, (c) detections of the point spread function wings around bright sources and (d) comparison with independent al gorithms. Flux calibration at 15 mu m was performed using stellar identific ations; the calibration does not agree with the pre-flight estimates, proba bly due to effects of detector hysteresis and photometric aperture correcti on. The 6.7-mu m extragalactic counts are broadly reproduced in the Pearson & Rowan-Robinson model, but the Franceschini et al. model underpredicts th e observed source density by similar to 0.5-1 dex, although the photometry at 6.7 mu m is still preliminary. At 15 mu m the extragalactic counts are i n excellent agreement with the predictions of the Pearson & Rowan-Robinson, Franceschini et al., Guiderdoni et al. models and the evolving models of X u et al., over seven orders of magnitude in 15-mu m flux density. The count s agree with other estimates from the ISOCAM instrument at overlapping flux densities, provided a consistent flux calibration is used. Luminosity evol ution at a rate of (1+z)(3), incorporating mid-infrared spectral features, provides a better fit to the 15-mu m differential counts than (1+z)(4) dens ity evolution. No-evolution models are excluded, implying that below around 10 mJy at 15 mu m the source counts become dominated by an evolving cosmol ogical population of dust-shrouded starbursts and/or active galaxies.