The giant radio galaxy 8C0821+695 and its environment

We present new VLA and Effelsberg observations of the radio galaxy 8C0821+695. We have obtained detailed images in total intensity and polarization of this 2 Mpc sized giant. The magnetic field has a configuration predominantly parallel to the source main

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A&A manuscript no.

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ASTRONOMY AND ASTROPHY SICS

1.Introduction

Giant Radio Galaxies (GRGs)constitute an unusual class of radio sources with projected linear sizes larger than 11We assume H 0=75km s ?1Mpc ?1and q 0=0.5throughout this paper.

We present new VLA and Effelsberg observations of the radio galaxy 8C0821+695. We have obtained detailed images in total intensity and polarization of this 2 Mpc sized giant. The magnetic field has a configuration predominantly parallel to the source main

2Lara et al.:GRG8C0821+695and its environment

ters(Subrahmanyan et al.1996)and the X-ray emission around their host galaxies is usually weak(Mack et al.

1997a).However,information about the ambient medium

at very large distances from the host galaxies can still be gained through the study of their radio properties.Most

GRGs probe the intergalactic medium(IGM)providing

information about the density of matter outside galactic halos,adding important observational constraints to cur-

rent cosmological models(Begelman&Cio?1989;Nath 1995).

We discuss in this paper new observations of the GRG 8C0821+695,a Fanaro?-Riley type II(Fanaro?&Riley

1974).It is optically identi?ed with a faint M R～22.2 galaxy at a redshift of0.538(Lacy et al.1993).Its high

redshift,compared to other GRGs,renders8C0821+695

a very interesting object since it provides information on the external environment at large cosmological distances. No X-ray source coincident with the radio source is found in the Bright Point Source catalogue from the ROSAT All Sky Survey.At the distance of8C0821+695,one arcsec-ond corresponds to4.9kpc.

2.Observations and data analysis

We observed8C0821+695with the VLA in the framework of a complete sample of large angular size radio sources selected from the NVSS(see Lara et al.1999for a sample description),and with the100-m E?elsberg telescope.We also incorporate for the discussion maps from the NVSS and WENSS,and maps presented by Lacy et al.(1993). The data have been calibrated according to the scale of Baars et al.(1977).

2.1.WENSS and NVSS maps

8C0821+695appears as a straight～7′long FR II radio source in the NVSS and the WENSS maps,with its main axis at a position angle(P.A.)of11?,measured north through east.

The WENSS map(Fig.1a),at a frequency of327 MHz and an angular resolution of57.′′7×54.′′0,presents two prominent lobes(N and S)connected by a continuous bridge of emission,although the position of the core is not evident at all.

The NVSS map(Fig.1b),made at a frequency of1400 MHz and an angular resolution of45′′,shows a prominent central core straddling the two radio lobes.The N-lobe has higher surface brightness than the S-lobe.The mean frac-tional polarization(p m)at1400MHz is p m=23%in the N-lobe and p m=26%in the S-lobe,while the core is unpo-larized.The E-vectors have a similar and rather uniform orientation in the N-and S-lobes,oblique to the source main axis.2.2.E?elsberg observations

We observed8C0821+695with the100-m E?elsberg tele-scope at10.6GHz(Tab.1)in order to obtain informa-tion about the morphology and polarization properties at high frequencies.The observational and data reduction procedures were those detailed by Gregorini et al.(1992). The maps were CLEANed as described by Klein&Mack (1995).We made40coverages resulting–after combining–in a?nal noise level of0.5mJy/beam in total power and 0.1mJy/beam in the polarized channels.The polarization maps were corrected for the non-Gaussian noise distribu-tion of the polarized intensity,as described by Wardle& Kronberg(1974).This is of particular importance in case of polarized low-brightness regions,e.g.in extended radio lobes.

The10.6GHz map(Fig.1c),with an angular resolu-tion of69′′,shows three components corresponding to the core and the two lobes.The superimposed vectors rep-resent the electrical?elds.Since at this high frequency Faraday e?ects are most probably negligible,a rotation by90?immediately yields the direction of the projected magnetic?eld.It is oriented predominantly parallel to the source main axis.The degree of polarization at this fre-quency is26%in both lobes.

We also observed8C0821+695with the100-m tele-scope at4850MHz.After combination of10coverages we reached the confusion limit of0.6mJy/beam in to-tal intensity,while the noise level was0.1mJy/beam in the polarization channels.Because of the large beam size (143′′),this map does not reveal any additional morpho-logical details,so we do not need to display it here.The degree of polarization derived from the E?elsberg map at 4850MHz is19%in the N-and S-lobes.

2.3.VLA observations

We made continuum observations of8C0821+695with the VLA in the B-and C-con?gurations at1425and4860 MHz in dual polarization(see Tab.1).The interferometric phases were calibrated with the nearby source J0903+679, except during4860MHz observations with the B-array,for which J0841+708was used as phase calibrator.The radio

Table1.Observations of8C0821+695

NVSS1400100–23Nov93 WENSS3275––VLA-C1425501019Feb96

48601001019Feb96 VLA-B1425501019Nov95

48601001025May97

E?elsberg10550300–28Aug94

4850500–27Feb98

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