EDP Sciences
Free Access
Volume 427, Number 3, December I 2004
Page(s) 795 - 801
Section Extragalactic astronomy
DOI https://doi.org/10.1051/0004-6361:20041616
Published online 16 November 2004

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)

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

SIMBAD Objects

© ESO 2004

Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.

Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.

Initial download of the metrics may take a while.