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Astron. Astrophys. 353, 507-527 (2000)

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The optical properties of low redshift radio galaxies *

F. Govoni 1,3, R. Falomo 1, G. Fasano 1 and R. Scarpa 2

1 Osservatorio Astronomico di Padova, Vicolo dell'Osservatorio 5, 35122 Padova, Italy
2 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
3 Dipartimento di Astronomia dell'Universitá di Bologna, Via Ranzani 1, 40127 Bologna, Italy

Received 29 July 1999 / Accepted 22 October 1999


We present morphological and photometric properties of 79 low redshift ([FORMULA]) radio galaxies extracted from two radio flux limited samples of radio sources.

All objects are imaged in the R band and for a subsample we have also obtained B band images. The sample includes sources of both FRI and FRII radio morphological type. Through the decomposition of the luminosity profiles and the analysis of the structural profiles (ellipticity, PA, c4) of the galaxies we are able to characterize in detail the optical properties of the radio galaxies.

It is found that most of host galaxies are luminous bulge dominated systems similar to normal giant ellipticals. Some cases of additional disk components are found whose spheroid-to-disk luminosity ratio is similar to that found in S0 galaxies.

The average absolute magnitude is [FORMULA] with a clear trend for FRI sources to be [FORMULA] mag brighter than FRII galaxies.

In about 40% of the objects observed we find an excess of light in the nucleus that is attributed to the presence of a nuclear point source whose luminosity is on average [FORMULA] 1-2% of the total flux of the host galaxy. The luminosity of these nuclear point sources appears correlated with the core radio power of the galaxies.

Radio galaxies follow the same [FORMULA] - Re relationship as normal elliptical galaxies.

The distribution of ellipticity, the amount of twisting and shape of isophotes ([FORMULA]) do not differ significantly from other ellipticals. The evidence for recent interactions is therefore rather modest.

Finally on average radio galaxies are bluer and have a color dispersion larger than normal elliptical galaxies, and the average color gradient in radio galaxies appears slightly steeper than in normal ellipticals.

These results support a scenario where radio emission is weakly related with the overall properties and/or the activity have negligible effects on the global characteristics of the host galaxy.

Key words: galaxies: active – galaxies: fundamental parameters – galaxies: nuclei – galaxies: photometry

* Based on observations collected at the European Southern Observatory, La Silla, Chile. Based on observations collected at the Nordic Optical Telescope, La Palma.

Send offprint requests to: R. Falomo (falomo@pd.astro.it)

© European Southern Observatory (ESO) 2000

Online publication: December 17, 1999