[Paper Review] ISOCAM observations in the Lockman Hole - II The 14.3 micron deep survey: data reduction, catalogue and source counts

Laura Bisigello, C. Lari+4 more|ArXiv.org|Jul 30, 2004

Astronomy and Astrophysical Research19 citations

TL;DR

This paper presents a deep 14.3 µm ISOCAM survey of the Lockman Hole, reducing data to produce a 90% complete catalogue of 283 extragalactic sources down to 1 mJy with 1.5 arcsec positional accuracy. The study confirms strong evolution in the infrared galaxy population, with 85% of sources having optical counterparts and only 6% classified as stars, supporting models of evolving dusty star-forming galaxies as major contributors to the cosmic infrared background.

ABSTRACT

We present a new analysis of the ISOCAM 14.3 micron deep survey in a 20x20 square arcmins area in the Lockman Hole. This survey is intermediate between the ultra-deep surveys and the shallow surveys in the ELAIS fields. The data have been analyzed with the method presented by Lari et al. (2001). We have produced a catalogue of 283 sources detected above the 5-sigma threshold, with fluxes in the interval 0.1-8 mJy. The catalogue is 90% complete at 1 mJy. The positional accuracy, estimated from the cross-correlation of infrared and optical sources, is around 1.5 arcsec. The search for the optical counterparts of the sources in the survey is performed on a medium-deep r' band optical image (5-sigma depth of r'=25), making use of the radio detections when available. The photometry has been checked through simulations and by comparing the data with those presented in a shallower and more extended ISOCAM survey in the Lockman Hole, that we have presented in a companion paper. Only 15% of the 14.3 micron sources do not have an optical counterpart down to r'=25 mag. We use the 6.7/14.3 micron colour as a star/galaxy separator, together with a visual inspection of the optical image and an analysis of the observed Spectral Energy Distribution of the ISOCAM sources. The stars in the sample turn out to be only 6% of the sample. We discuss the 14.3 micron counts of extragalactic sources, combining our catalogue with that obtained from the shallower ISOCAM survey. The data in the two surveys are consistent, and our results fully support the claims in previous works for the existence of an evolving population of infrared galaxies, confirming the evident departure from non-evolutionary model predictions.

Motivation & Objective

• To produce a deep, high-cadence 14.3 µm survey in the Lockman Hole using ISOCAM data to probe faint infrared sources.
• To reduce and calibrate ISOCAM data with improved techniques to ensure flux accuracy and completeness.
• To create a reliable source catalogue with accurate positions and fluxes for extragalactic source counts.
• To identify optical counterparts and classify sources as stars or galaxies using multi-wavelength data and SED analysis.
• To measure 14.3 µm source counts and test for cosmic evolution in the infrared galaxy population.

Proposed method

• Applied the data reduction method of Lari et al. (2001) to ISOCAM 14.3 µm observations in a 20×20 arcmin region of the Lockman Hole.
• Used a 5-σ detection threshold to identify 283 sources with fluxes between 0.1 and 8 mJy.
• Calibrated fluxes using the ISO cookbook factor, correcting for a ~10% discrepancy between ISO and IRAS zero points.
• Performed optical cross-identification using a medium-deep r’-band image (5σ depth r’=25) and radio data when available.
• Classified sources via 6.7/14.3 µm colour, visual inspection of optical images, and SED analysis to separate stars from galaxies.
• Combined data from this deep survey with a shallower ISOCAM survey (Paper I) to compute and analyze 14.3 µm source counts.

Experimental results

Research questions

• RQ1 What is the completeness and flux accuracy of the ISOCAM 14.3 µm deep survey in the Lockman Hole?
• RQ2 How many of the 14.3 µm sources have optical counterparts, and what fraction are stars versus galaxies?
• RQ3 Do the 14.3 µm source counts show evidence of cosmic evolution in the infrared galaxy population?
• RQ4 How do the results compare with non-evolving and evolving models of galaxy evolution?
• RQ5 To what extent do the flux and positional measurements align with independent calibration and cross-identification?

Key findings

• The survey produced a 90% complete catalogue of 283 sources at 1 mJy, with fluxes ranging from 0.1 to 8 mJy.
• Positional accuracy was estimated at 1.5 arcsec via cross-correlation with optical sources.
• Only 15% of 14.3 µm sources lacked an optical counterpart down to r’=25 mag.
• Stars made up only 6% of the sample, confirmed via 6.7/14.3 µm colour, visual inspection, and SED analysis.
• The 14.3 µm source counts from the deep and shallow surveys were consistent and showed a clear departure from non-evolving models.
• The results strongly support the existence of an evolving population of infrared galaxies, consistent with models predicting luminous, dusty starbursts at moderate redshifts (z ≈ 0.5–1.3).