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Astron. Astrophys. 335, 443-448 (1998)

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1. Introduction

26W20 was discovered in the Westerbork radio survey by Harris et al. (1980). The galaxy lies in a sub-cluster of Abell 754 . The structure of the cluster and the reality of the sub-cluster containing 26W20 are detailed by Kriesslar and Beers (1997).

The radio morphology and classification has been described as a Fanaroff and Riley class I object, either a 'tailed radio galaxy' (TRG) or a single-sided lobe (Harris et al. 1980; Harris, Costain and Dewdney, 1984 - 'HCD' hereafter). A single jet/tail is seen extending 170 kpc from the core ([FORMULA] = 50 km s-1 Mpc-1) and ending in a low surface brightness lobe (Fig. 1). Because no jet or lobe was detected on the other side of the galaxy, and since the host galaxy is a member of a small group of galaxies, a TRG model was favored over a single-sided jet model (HCD).

[FIGURE] Fig. 1. A naturally weighted [FORMULA] 21 cm map of 26W20 from 1980 VLA observations. The image has been restored with a [FORMULA] circular beam in order to delineate the low surface brightness features.

Galaxies with X-ray bright nuclei are usually characterized by strong emission lines. 26W20 is one of four known galaxies with a high apparent X-ray luminosity from the nucleus but with only weak or undetectable emission lines. The other three are 3C264 (Baum et al. 1997), J2310-43 (Tananbaum et al. 1997) and PKS 2316-423 (Crawford and Fabian 1994). Nuclear parameters are given in Table 1.


Table 1. Comparative parameters for the nucleus of similar sources.
References: (1) as reported in Tananbaum et al. 1997 (2) as reported in Tananbaum et al. 1997, Elvis et al. 1981; (3) Crawford and Fabian 1994

Notes: (a) Monochromatic luminosity at 5 GHz in units of ergs s-1 Hz-1 ; (b) Energy range of (0.5-3.0 keV); (c) Energy range of (0.1-2.4 keV); (d) The core magnitude is assumed to be less than 1/10 of the magnitude of the whole galaxy; (e) This lower limit is due to the inclusion of optical emission not associated with the nucleus; (f) An extended region of 400 km s-1 wide H [FORMULA] +[NII] emission is seen (Baum et al. 1997)

26W20 was first observed in X-rays with the EINSTEIN Observatory's imaging proportional counter (IPC) and high resolution imager (HRI). The X-ray source was unresolved with the HRI (FWHM= [FORMULA]) and the IPC spectrum could be equally well fit by a power law with an energy index of 0.8 [FORMULA] 0.4 or a thermal bremsstrahlung spectrum with kT=3 keV (constrained to be greater than 1 keV).

Motivated to understand the head-tail morphology of 26W20, we have obtained new radio and X-ray measurements to study the inner jet region. While there is possible evidence for a TRG model, these observations reveal core emission properties similar to those of a BL Lac object. However, there is no evidence for excess optical emission from the nucleus. This distinguishes 26w20 from a typical BL Lac object.

We use ROSAT HRI data to provide upper limits on hot ambient gas which would be required to provide the 'ram pressure' if nuclear gas is to be 'stripped' from the galaxy. MERLIN observations were obtained to see if twin jets are present in the core and to see if they show severe bending necessary for a beaming model in order to change the jet from being close to the line of sight to that observed for the large scale tail which lies more in the plane of the sky.

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© European Southern Observatory (ESO) 1998

Online publication: June 18, 1998