3. Optical observations
The radio core position coincides perfectly with a galaxy of R=17.94 mag at , (J2000) according to the magnitude calibration and astrometric position in the Digitized Sky Survey (DSS) data base. The optical image of this galaxy is shown in the upper-right subset of Fig. 1 where the position of the very compact radio core is indicated.
In order to determine a redshift of the identified galaxy, we have made optical spectroscopic observations with the 2.1m telescope of the McDonald Observatory (Texas). These observations constituted a part of the larger project involving optical spectroscopy of the other giant candidates in our sample (cf. Introduction). The `Imaging Grism Instrument' (IGI) equipped with the TK4 CCD detector and cooled with liquid nitrogen was used. IGI allows direct imaging and spectroscopy with a spatial scale of per pixel within the field of view of 8 8 arc min. We used a grism sensitive to the wavelength range 3750 Å to 7600 Å and a 2" wide slit, that provided a dispersion of 3.7 Å per pixel and spectral resolution of about 12 Å. Three exposures of 30 min were taken on two nights of February 28 and 29, 2000, which allowed to improve S/N ratio without an increase of smearing of the spectrum during a too long single exposure and due to imperfect tracking of the telescope. The wavelength calibration was carried out using exposures to helium and mercury lamps. The flux calibration was provided by exposures of the standard star HZ 44. The data reduction and calibration were performed with the use of packages of the IRAF software. The reduced 1-D spectrum is shown in Fig. 4.
The spectrum reveals a weak continuum with prominent narrow emission lines [OIII] 4959 Å and 5007 Å, which give a redshift .
© European Southern Observatory (ESO) 2000
Online publication: December 5, 2000