Astron. Astrophys. 326, 580-596 (1997)

The radio galaxy 1138262 at z 2.2:
a giant elliptical galaxy at the center of a proto-cluster?

L. Pentericci ¹, H.J.A. Röttgering ¹, G.K. Miley ¹, C.L. Carilli ² and P. McCarthy <sup>3

1</sup> Leiden Observatory, P.O. Box 9513, 2300 RA Leiden, The Netherlands
² NRAO, PO Box 0, Socorro, NM, 87801
³ The Observatories of the Carnegie Institution of Washington,813 Santa Barbara Street, Pasadena, California 91101

Received 7 March 1997 / Accepted 5 May 1997

Abstract

We present a detailed observational study of a remarkable radio galaxy at a redshift of : 1138-262.

This object was selected from our compendium of ultra steep spectrum radio sources on the basis of its distorted radio morphology. High resolution VLA radio observations show that the radio source consists of a series of knots and jet-like protrusions, with a sharp bend, strongly suggesting that the jet propagation is constrained by a dense external medium. Further evidence that 1138-262 resides in a dense environment comes from the extremely large rotation measure (6200 rad m^-1 ), the highest yet measured in a high redshift radio galaxy. This suggest that the source is at the center of an hot X-ray emitting halo, possibly a massive cooling flow ( ). Finally, a small emission line galaxy is observed to the north of the main galaxy: on the basis of both broad band and narrow band images we conclude that it is at the same redshift as 1138262; therefore it is a further evidence of the overdense environment of the radio galaxy.

The bright K-band magnitude of 1138262 suggest that the host of the radio source is a very massive galaxy, of the order 10 . The K-band morphology can be fitted well by a classical De Vaucouleurs profile, indicating that 1138262 is a well formed elliptical galaxy. However the UV/optical broad band morphology is very clumpy, consisting of a central region and a number of knots with sizes 10 to 15 kpc, which are systematically bluer that the central emission. All these components are embedded in a giant ( kpc) Ly emission gas halo, which is also very clumpy and shows a high velocity dispersion. We argue that in the outer regions star formation is still occurring, either triggered by the passage of the radio jets or by the tidal interactions between the clumps that are falling on the parent galaxy.

Our favored interpretation of the observations is that 1138262 is a massive elliptical galaxy, at the center of a dense, cluster-like region. The galaxy is still accreting mass from its environment, and will therefore become even more massive. This strongly supports the idea that 1138262 will eventually become a cD galaxy.

Key words: galaxies: activegalaxies: radiogalaxies: clustersgalaxies: G 1138-262

Send offprint requests to: L. Pentericci

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Contents

• 1. Introduction
• 2. Selection and observations
  • 2.1. High resolution spectroscopy
  • 2.2. Narrow band Ly imaging
  • 2.3. Broad band imaging
• 3. Results
  • 3.1. Radio polarimetric imaging
    • 3.1.1. Identification of the nucleus
    • 3.1.2. Comparison with typical powerful radio sources
    • 3.1.3. Physical properties
  • 3.2. Optical and IR continuum
    • 3.2.1. Structure and orientation
    • 3.2.2. Optical color index
  • 3.3. The Ly emission gas
    • 3.3.1. Narrow band imaging
    • 3.3.2. Kinematics of the gas
    • 3.3.3. Ionization, density and confinement of the gas
• 4. Discussion
  • 4.1. The peculiar properties of 1138262
  • 4.2. Companion galaxy
  • 4.3. The nature of the infrared emission
  • 4.4. The Ly -radio source connection
    • 4.4.1. Deflection of the radio jet
    • 4.4.2. Correlation between radio parameters and Ly emission
  • 4.5. High rotation measure: further evidence that 1138262 is at the center of a protocluster
  • 4.6. Scenarios of galaxy formation: is 1138262 a proto-cD galaxy?
• 5. Conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 1997

Online publication: October 15, 1997
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