A&A 436, 457-464 (2005)
DOI: 10.1051/0004-6361:20052682

Optical and radio observations of a sample of 52 powerful ultra-steep spectrum radio sources

Gopal-Krishna¹, C. Ledoux², J. Melnick², E. Giraud³, V. Kulkarni¹ and B. Altieri<sup>4

1</sup>  National Centre for Radio Astrophysics, TIFR, Post Bag 3, Ganesh Khind, Pune 411 007, India
    e-mail: krishna@ncra.tifr.res.in; kulkarni@ncra.tifr.res.in
²  European Southern Observatory, Alonso de Córdova 3107, Casilla 19001, Vitacura, Santiago, Chile
    e-mail: cledoux@eso.org; jmelnick@eso.org
³  GAM, Univ. Montpellier II, Place E. Bataillon, 34095 Montpellier Cedex, France
    e-mail: edmond.giraud@gamum2.in2p3.fr
⁴  European Space Agency, Villafranca del Castillo, Apartado 50727, 28080 Madrid, Spain
    e-mail: baltieri@xmm.vilspa.esa.es

(Received 12 January 2005 / Accepted 22 February 2005 )

Abstract
We present the results of radio (VLA) and optical (ESO/La Silla) imaging of a sample of 52 radio sources having an ultra-steep radio spectrum with  mostly steeper than -1.1 at decimetre wavelengths (median ). Radio-optical overlays are presented to an astrometric accuracy of ~. For 41 of the sources, radio spectral indices are newly determined using unpublished observations made with the 100-m Effelsberg radio telescope. For 14 of the sources identified with relatively brighter optical counterparts, spectroscopic observations were also carried out at La Silla and their redshifts are found to lie in the range 0.4 to 2.6. These observations have revealed three distant clusters of galaxies with redshifts of 0.55, 0.75 and 0.79, and we suggest that, together with an ultra-steep radio spectrum and relaxed radio morphology, the presence of a LINER spectrum in the optical can be used as a powerful indicator of rich clusters of galaxies. Additional candidates of this type in our sample are pointed out. Also, sources exhibiting particularly interesting radio-optical morphological relationships are highlighted. We further note the presence of six sources in our sample for which the optical counterpart (either detected or undetected) is fainter than and the radio extent is small (<). These ultra-steep spectrum radio sources are good signposts for discovering massive galaxies out to very large redshifts.

Key words: cosmology: observations -- galaxies: active -- cooling flows -- galaxies: clusters: general -- quasars: general -- radio continuum: galaxies

SIMBAD Objects

© ESO 2005

The OpenURL standard is a protocol for transmission of metadata describing the resource that you wish to access. An OpenURL link contains article metadata and directs it to the OpenURL server of your choice. The OpenURL server can provide access to the resource and also offer complementary services (specific search engine, export of references...). The OpenURL link can be generated by different means.

• If your librarian has set up your subscription with an OpenURL resolver, OpenURL links appear automatically on the abstract pages.
• You can define your own OpenURL resolver with your EDPS Account. In this case your choice will be given priority over that of your library.
• You can use an add-on for your browser (Firefox or I.E.) to display OpenURL links on a page (see http://www.openly.com/openurlref/). You should disable this module if you wish to use the OpenURL server that you or your library have defined.