EDP Sciences
Free access
Issue
A&A
Volume 427, Number 3, December I 2004
Page(s) 795 - 801
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20041616


A&A 427, 795-801 (2004)
DOI: 10.1051/0004-6361:20041616

A population of extreme mid-to-near-infrared sources: Obscured AGN and dusty starbursts

P. H. Johansson1, 2, P. Väisänen3, 4 and M. Vaccari5

1  Institute of Astronomy, Madingley Road, Cambridge, CB3 0HA, UK
    e-mail: phjohans@ast.cam.ac.uk
2  Observatory, PO Box 14, 00014 University of Helsinki, Finland
3  European Southern Observatory, Casilla 19001, Santiago, Chile
4  Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
5  Astrophysics Group, Blackett Laboratory, Imperial College, Prince Consort Road, London SW7 2AZ, UK

(Received 10 May 2004 / Accepted 9 August 2004)

Abstract
We present a sample of mid-infrared detected sources from the European Large Area ISO Survey (ELAIS) regions characterised by strong mid-IR radiation with faint near-IR and optical counterparts. These extreme mid-to-near-IR objects (EMNOs) are defined here by a flux ratio of $f_{\nu}
(15~\mu{\rm m}) / f_{\nu} (2.2~\mu{\rm m}) > 25$ . This population is not obvious in deeper small area ISO surveys, though it produces more than 20% of the observed cosmic IR background radiation (CIRB) at 15 $~\mu$m above 1 mJy. Near-future large area deep mid-IR surveys with the Spitzer Space Telescope, however, are bound to uncover large amounts of these objects, which we argue to most likely be obscured AGNs, based on SED shapes and X-ray data. Very strong dusty starbursts at z>1 may also have high mid-to-near-IR flux ratios, but using the MIR/NIR and FIR/MIR ratios these may be separated. Most of our EMNOs appear to be ULIRGs, half are also extremely red objects (ERO). A curious case of a low redshift, less luminous object with a very young stellar population is also found. We predict that the simple broad band selection method makes EMNOs a useful window into high-redshift obscured nuclear activity and its sought after relation to star-formation, in a similar way that EROs have been used to define samples of high-redshift early type galaxies.


Key words: galaxies: evolution -- galaxies: starburst -- galaxies: active -- infrared: galaxies -- cosmology: observations

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© ESO 2004