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Astron. Astrophys. 326, 505-527 (1997)

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Spectroscopy of ultra-steep spectrum radio sources:
a sample of [FORMULA] radio galaxies

H.J.A. Röttgering 1, 2, 3, R. van Ojik 1, G.K. Miley 1, K.C. Chambers 4, W.J.M. van Breugel 5 and S. de Koff 1, 6

1 Leiden Observatory, P.O. Box 9513, 2300 RA, Leiden, The Netherlands
2 Mullard Radio Astronomy Observatory, Cavendish Laboratory, Madingley Road, Cambridge, CB3 0HE, England
3 Institute of Astronomy, Madingley Road, Cambridge, CB3 0HA, England
4 Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, Hawaii 96822, USA
5 Institute for Geophysics and Planetary Physics, Lawrence Livermore National Laboratory, L-413, Livermore, CA 94550, USA
6 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore MD 21218, USA

Received 12 December 1995 / Accepted 22 July 1996

Abstract

We present spectroscopic observations for 64 radio galaxies having ultra steep radio spectra. Twenty-nine objects have redshifts [FORMULA], the largest redshifts being almost 4. Our ultra steep spectrum (USS) criterion ( [FORMULA] ) has proven to be the most efficient way of finding distant radio galaxies. We find that even among the USS sources, there is a strong statistical correlation between the spectral index and redshift. The most distant radio galaxies within the USS sample have the steepest radio spectra.

In our sample there are 3 radio galaxies at [FORMULA] compared with 26 at [FORMULA]. However, the present data do not allow us to decide whether there is a decrease in co-moving source density at the highest redshifts.

We have analyzed the spectra of the 30 objects with the highest redshifts ( [FORMULA] ). For these high redshift radio galaxies, Ly [FORMULA] is almost always the dominant emission line, with a rest frame equivalent width ranging from [FORMULA] Å to more than 1000 Å. The equivalent widths of the most important emission lines (Ly [FORMULA], C IV, He II, C III ]) are found to correlate strongly with each other. The large rest frame equivalent widths and the correlation between the equivalent widths of the emission lines, confirm that photoionization by a central continuum source is most likely the dominant ionization mechanism.

There are significant velocity differences between the various emission lines of our high redshift radio galaxies; in particular the Ly [FORMULA] line is shifted with respect to the higher ionization lines. Velocity shifts range from 100 to almost 1000 km s-1 in some cases. Simulations show that the effects of associated H I absorption on the Ly [FORMULA] emission line may be responsible for most of these velocity shifts. However, other mechanisms such as organized kinematics of the Ly [FORMULA] emission line gas (e.g. inflow or outflow) and obscuration of the line emission from the far side of the radio galaxy may also play a role.

Key words: galaxies: activegalaxies: redshiftsgalaxies: ISMradio continuum: galaxies

Send offprint requests to: H. Röttgering

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© European Southern Observatory (ESO) 1997

Online publication: October 15, 1997
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