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Optical morphology of distant RATAN-600 radio galaxies
T. Pursimo1, K. Nilsson1, P. Teerikorpi1, A. Kopylov2, N. Soboleva3, Yu. Parijskij2, Yu. Baryshev4 , O. Verkhodanov2 , A. Temirova3, O. Zhelenkova2 , W. Goss5
2 3

5

Tuorla Observatory, University of Turku, FIN-21500 Piikkio, Finland Special Astrophysical Observatory, Radio Astronomy Sector, 35147 Niznij Arkhyz, Karachey-Cherkessia, Russia St.Petersburg Branch of the SAO, Radio Astronomy Laboratory, Pulkovskoe Shosse 65, 196140 St.Petersburg, Russia 4 Astronomical Institute of St.Petersburg University, St. Petersburg 198904, Russia National Radio Astronomical observatory,P.O.Box 0,Socorro,New Mexico 87801,USA
1

Introduction
We present direct imaging data of 22 ultra steep spectrum USS radio sources obtained at or near a subarcsecond seeing. The basic sample of 40 double radio sources was selected from the RATAN-600 catalogue Parijskij et al., 1991. The FRII-structure has been con rmed with VLA Kopylov et al., 1995a and preliminary optical identi cations which come from the 6 m-telescope Kopylov et al., 1995b. As the RATAN-600 ux limit at 3.9 GHz 10 mJy is fainter than that of ma jor surveys, the sample mayhave high-z contents. Based on the Hubble diagram Fig. 1 it is expected that most of the RC USS sources have z 0.7.

Figure 1: Hubble diagram in R-band for various galaxies from the literature. The histogram of RC USS sources is shown above.

Observations
Optical images were obtained using the 2.56 m Nordic Optical Telescope NOT at La Palma during three observing runs in March, May and December 1994. Typical exposure


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times were from 1200 to 2000 seconds depending on the brightness of the ob ject. On photometric nights, calibration stars from Landolt 1992 were observed several times each night at a range of air masses. The seeing varied from 0.500 to 2.000. For the analysis we have adopted cosmology with H0 =50 km s,1 Mpc,1 and q0 =0.0.

Figure 2: The central portion 6000 6000 or 252 252 kpc of RC0457+0452. North is up and East is to the left. Left, Gaussian smoothed R-band image 3600s, FWHM=0.800 . The position of the hotspots and centre of gravity of the radio source are marked with circles and a cross, respectively. On the right is V-band image 1800s, FWHM=0.700 . The contrasts are di erent in the images, the R-band image is for faint features and V-band for morphology of relatively bright emission. Note the double nucleus in the companions to the east and south.

Examples of individual ob jects
We could con rm 16 optical identi cations down to mR=24, which were obtained with the 6-m telescope. We show some examples of di erent galaxy morphologies and galaxy environments of RC USS ob jects. RC0457+0452: The redshift of this ob jects is 0.186 Dononov et al., 1999. The apparent magnitude is mR =19.7 which is consistent with the Hubble diagram Figure 1. The absolute magnitude is MR =,20.7, which suggests that the ob ject is slightly fainter than a typical FRI radio galaxy e.g. Colina & De Juan, 1995. The galaxy shows signs of interaction isophote twists, distorted morphology which is quite common for FRI host galaxies Colina & De Juan. This ob ject is possibly located in a cluster of galaxies. There are two apparently interacting companion galaxies, 2000 to the east and 2200 to the south. The pro jected distances between these ob jects are 84 and 92 kpc, respectively. Both of these ob jects appear to be interacting systems Fig 1.


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Figure 3: The central portion 6000 6000 or 656 656kpc of RC2144+0513. The left hand image R-band 1800s, FWHM=1.000 is slightly smoothed with a Gaussian function. East is to the left and North is up. Right, the same frame non smoothed after subtracting the scaled PSF. The circles radius of 200, pro jected distance of 2 kpc at z=1.01 show the position of the subtracted source. There are no or only weak signs of the host galaxy. RC2144+0513: Recently Dononov et al. 1999 measured the the redshift of the ob ject z=1.01. Astrometry suggests that the brighter, unresolved ob ject coincides with the radio source Fig. 3. This ob ject has mR =18.9, which corresponds with MR =,25.9 and is a typical value for a z=1 quasar e.g. Goldschmidt et al. 1999 There is a nearby companion 2.900 to the south west, which corresponds with the pro jected distance of 32 kpc. This ob ject has mR =20.9 and is unresolved, however the signal is rather weak and it could also be a compact companion galaxy. However this is unlikely, because in this case the absolute magnitude would be uncomfortable high MR =,23.9, unless this is the actual radio galaxy.

Summary
We obtained subarcsecond seeing images of 22 ob jects of which we could study the morphology of 16 ob jects. Five of these remained unresolved. Typically the resolved ob jects havea multicomponent structure with extended emission. The brightness of these ob jects varies from mR =18.8 to 24.0. Of the remaining ob jects three were very faint and optical identi cation remained uncertain for the other three ob jects. Ellipticities of the ob jects agree well with other studies of high-z radio galaxies Pursimo et al., 1999. They also found that many RC USS ob jects have close companions. The results of the imaging study and the rst redshifts make it imperative to measure spectroscopic redshifts for the RC USS galaxies.


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References
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