Near and mid-infrared colours of star-forming galaxies in ELAIS fields

Abstract

We present J and K-band near-infrared photometry of a sample of mid-infrared sources detected by the Infrared Space Observatory (ISO) as part of the European Large Area ISO-Survey (ELAIS) and study their classification and star-forming properties. We have used the Preliminary ELAIS Catalogue for the 6.7 micron (LW2) and 15 micron (LW3) fluxes. All of the high-reliability LW2 sources and 80 per cent of the LW3 sources are identified in the near-IR survey reaching K = 17.5 mag. The near- to mid-IR flux ratios can effectively be used to separate stars from galaxies in mid-IR surveys. The stars detected in our survey region are used to derive a new accurate calibration for the ELAIS ISOCAM data in both the LW2 and LW3 filters. We show that near to mid-IR colour-colour diagrams can be used to further classify galaxies, as well as study star-formation. The ISOCAM ELAIS survey is found to mostly detect strongly star-forming late-type galaxies, possibly starburst powered galaxies, and it also picks out obscured AGN. The ELAIS galaxies yield an average mid-IR flux ratio LW2/LW3 = 0.67 +/- 0.27. We discuss this [6.7/15] ratio as a star formation tracer using ISO and IRAS data of a local comparison sample. We find that the [K/15] ratio is also agood indicator of activity level in galaxies and conclude that the drop in the [6.7/15] ratio seen in strongly star-forming galaxies is a result of both an increase of 15 mic emission and an apparent depletion of 6.7 mic emission. Near-IR data together with the mid-IR give the possibility to estimate the relative amount of interstellar matter in the galaxies.

18 pages, 15 figures; accepted for publication in MNRAS

References in corpus (7)

• The relationship between star formation rates and mid-infrared emission in galactic disks
• The obscured mid-infrared continuum of NGC 4418: a dust- and ice-enshrouded AGN
• Quasars in the 2MASS Second Incremental Data Release
• An atlas of mid-infrared dust emission in spiral galaxies
• A New Method for ISOCAM Data Reduction - I. Application to the European Large Area ISO Survey Southern Field: Method and Results
• The impact of bars on the mid-infrared dust emission of spiral galaxies: global and circumnuclear properties
• The European Large Area ISO Survey VI - Discovery of a new hyperluminous infrared galaxy

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• Final Analysis of ELAIS 15 $\mu$m Observations : Method, Reduction and Catalogue
• The Brera Multi-scale Wavelet Chandra Survey. I. Serendipitous source catalogue
• AKARI/IRC source catalogues and source counts for the IRAC Dark Field, ELAIS North and the AKARI Deep Field South
• ISOCAM observations of the L1551 star formation region
• The AKARI FU-HYU galaxy evolution program: First results from the GOODS-N field
• A population of extreme mid-to-near-infrared sources: obscured AGN and dusty starbursts
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