next previous
Up: The revised GB/GB2

4. The optical and X-ray data

4.1. Optical identifications

The radio sources were identified with their optical counterpart on the basis of its positional coincidence with the radio centre. If a compact radio core was detected, then it undoubtedly coincided with the identification. If a source was an extended one and no core could be detected, then a much larger area near the "centre" of the source had to be considered. Usually the source was identified with the brightest galaxy or stellar object in the "centre". Majority of the sample sources were identified with galaxies and quasars, as well as galaxy or quasar candidates seen on the Palomar Observatory Sky Survey (POSS) prints. The tentative samples of GB/GB2 radio galaxies and quasar candidates were analyzed by Machalski & Condon (1985b, 1986). Later on, deeper identifications were collected from the literature and our own research (e.g. Machalski & Magdziarz 1993b). In Notes to Table A3 (click here) some ambiguous identifications are discussed. Unfortunately, still about 34 per cent of the sources in Part 1, about 45 per cent in Part 2, and about 52 per cent in Part 3 remain unidentified.

4.2. Optical magnitudes and redshift

Red (R) and blue (B) magnitudes of the identified objects are either photometric ones in the R Kron-Cousins VRI system (Cousins 1976) and in the UBV system of Johnson (1966), or estimated from the POSS(E) and POSS(O) prints, respectively. In particular, UBVRI photometry for the brightest elliptical galaxies and quasars in the sample was provided by Machalski & Wiśniewski (1988), and absolute MR and MB magnitudes as well as four intrinsic colours of the elliptical galaxies, corrected for galactic extinction, aperture, and reddening (K-correction), was given by Machalski (1988). Statistics of the optical type is given in Sect. 6.

Spectroscopic redshift is available for majority of the sample quasars, and for many brighter galaxies. References to the available photometry and redshifts are provided in Table A3 (click here). For elliptical galaxies without spectroscopic redshift but with photoelectrical photometry, a redshift estimate, based on corrected R-magnitude and B-R colour, is provided by Machalski (1988). A statistical analysis, described in Sect. 6 suggests that at least 11-12 sources in the sample may have redshift greater than 3.16.

4.3. X-ray identifications

The sample sources are identified with X-ray sources observed with the Imaging Proportional Counter (IPC) on the EINSTEIN observatory (Wilkes et al. 1994), and the Position Sensitive Proportional Counter (PSPC) on the ROSAT observatory. The latter data were taken from the "WGA" catalogue, and an updated version of the "First ROSAT source Catalogue of Pointed Observations". An X-ray source is considered as a counterpart for a radio source if its normalized distance to the radio position is
where: tex2html_wrap_inline2800 and tex2html_wrap_inline2802 are the differences between the X-ray and radio position, and tex2html_wrap_inline2804 and tex2html_wrap_inline2806 are the combined X-ray-radio position errors tex2html_wrap_inline2808 in right ascension and declination, respectively. Majority of the X-ray sources are identified with compact radio sources (mostly quasars and BL Lac objects) for which accuracy of the VLA position is tex2html_wrap_inline2810 arcsec.

Because the X-ray data available are not homogeneous, and the GB/GB2 sky region is only partly covered by the above X-ray observations, their details (epoch of observation, integration time, resultant counts, etc.) are not cited in this paper. Only a reference to the original data is given.

next previous
Up: The revised GB/GB2

Copyright by the European Southern Observatory (ESO)