Open AccessNear‐ and mid‐infrared colours of star‐forming galaxies in European Large Area ISO Survey fields
Author(s) -
Väisänen P.,
Morel T.,
RowanRobinson M.,
Serjeant S.,
Oliver S.,
Sumner T.,
Crockett H.,
Gruppioni C.,
Tollestrup E. V.
Publication year - 2002
Publication title -
monthly notices of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.058
H-Index - 383
eISSN - 1365-2966
pISSN - 0035-8711
DOI - 10.1046/j.1365-8711.2002.06006.x
Subject(s) - physics , astrophysics , galaxy , star formation , stars , photometry (optics) , luminous infrared galaxy , infrared , flux (metallurgy) , astronomy , active galactic nucleus , materials science , metallurgy
We present J ‐ and K ‐band near‐infrared (near‐IR) photometry of a sample of mid‐infrared (mid‐IR) 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‐μm (LW2) and 15‐μm (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‐IR/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 classify galaxies further, as well as to study star formation. The ELAIS ISOCAM survey is found mostly to detect strongly star‐forming late‐type galaxies, possibly starburst‐powered galaxies, and it also picks out obscured active galactic nuclei. The ELAIS galaxies yield an average mid‐IR flux ratio LW2/LW3 = 0.67 ± 0.27 . We discuss the f ν (6.7 μ m)/ f ν (15 μ m) ratio as a star formation tracer using ISO and IRAS data of a local comparison sample. We find that the f ν (2.2 μ m)/ f ν (15 μ m) ratio is also a good indicator of activity level in galaxies and conclude that the drop in the f ν (6.7 μ m)/ f ν (15 μ m) ratio seen in strongly star‐forming galaxies is a result of both an increase of 15‐μm emission and an apparent depletion of 6.7‐μm emission. Near‐IR together with the mid‐IR data make it possible to estimate the relative amount of interstellar matter in the galaxies.