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The Structure and Composition of High Redshift Radio Galaxies

R. A. E. Fosbury1 ST-ECF, Garching bei München, Germany, (

J. Vernet ESO, Garching bei München, Germany

M. Villar-Martín Department of Physics & Astronomy, University of Sheffield, UK

M. H. Cohen Astronomy Department, California Institute of Technology, Pasadena, USA

A. Cimatti, S. di Serego Alighieri Osservatorio Astrofisico di Arcetri, Firenze, Italy

P. J. McCarthy The Observatories of the Carnegie Institution, Pasadena, USA

(1)Affiliated to the Astrophysics Division, Space Science Department, European Space Agency




Keck spectropolarimetry, giving spectral coverage from Ly-$\alpha $ to beyond CIII], and HST imaging of a sample of powerful radio galaxies with $z\sim2.5$ has been obtained. These data are giving us a clear picture of the nature of the `alignment effect' and are revealing new correlations between polarization and emission line ratios which may be interpreted in the context of the stellar evolutionary histories of these massive galaxies. In particular, we see the 2200Å dust absorption feature in the radio galaxy continua and a large variation in the NV/CIV line ratio amongst objects having a similar ionization level. VLT infrared spectroscopy of this and similar samples will give us a view of a period of galaxy history during which rapid chemical evolution was taking place.

radio galaxies, spectropolarimetry, dust, nitrogen


This is a short progress report on a rather extensive programme we are carrying out to study the structure and composition of high redshift radio galaxies (HzRG) -- and, by implication, the host galaxies of radio quasars -- using observations in the optical, IR and mm bands. A separation of the stellar and the AGN-related components is made using a combination of Keck spectropolarimetry and high resolution WFPC 2 imaging in the rest-frame UV (below the 4000Å break), NICMOS imaging in the rest-frame optical, and photometric measurements of cool dust thermal emission at longer wavelengths. The relevance to this meeting is the use of NICMOS to image the evolved stellar population in these galaxies (see the following talk by McCarthy) during the epoch when powerful AGN were most common. In addition, we plan to use VLT (ISAAC) IR-spectroscopy to measure the rest-frame optical emission line spectrum, allowing us to perform the kind of detailed ionization/composition analysis which has already been carried out on local objects.

The Sample

We have selected RG with $z\sim2.5$ which allow us to study the strong UV emission lines from Ly-$\alpha $ to CIII], the UV continuum, resonance absorption lines and the 2200Å dust feature in the optical band and to straddle the 4000Å break in the 1-2$m\mu$ region.

Our principal sample consists of eight objects (six of which have already been analysed) with 2.3 < z < 2.9 and this is supplemented by three sources from the literature having similar quality data but extending the redshift range to 1.8 < z < 3.8. More data are currently being obtained on sources with z>3.


The Keck spectropolarimetric observations for the first two sources in the programme are described in Cimatti et al. (1998). Four more sources have been observed and reduced and three further objects were observed during the period of this Workshop. An example of the spectropolarimetry is shown in Figure 1.

Figure: A Keck II, LRISp spectropolarimetric observation of the radio galaxy TXS 0211-122. The three panels show respectively the total flux (in 10-17 erg cm-2 s-1 Å-1), the fractional polarization in continuum (wide horizontal bars) and line bands and the position angle of the E-vector. The strong emission lines are, from short wavelengths, Ly-$\alpha $, NV, CIV, HeII and CIII].

The HST WFPC 2 and NICMOS images, where available, were taken from the public archive at the ST-ECF and from the McCarthy et al. program (ID 7498). An example of the NICMOS and WFPC 2 imaging is shown in Figure 2.
Figure: Images from NICMOS/F160W and WFPC 2/F702W of the z=2.93 radio galaxy MRC 0943-242 represented at the same scale. This object has an aligned component which still dominates in the H-band. Although bluer than the underlying galaxy, the aligned light is somewhat redder than usual in these objects although the range in UV colours is small (see text). There is a prominent Ly-$\alpha $ absorption component.

Some deep groundbased imaging data have been taken from the literature to complement the higher resolution but shallower HST images.

Principal Results

Here we summarise the principal results to date. These will be described more fully in papers in preparation.

Figure: Plots of the fractional continuum polarization against (a) the Ly-$\alpha $/CIV emission line ratio and, (b) the NV/CIV ratio. The sources in the plots are: 4C 23.56 (components a and b, z=2.482), 4C -00.54 (z=2.366), TXS 0211-122 (z=2.338), B3 0731+438 (z=2.429), USS 0828+193 (z=2.572) and MRC 0943-242 (z=2.93) from our own observations and 4C 41.17 (z=3.798), MRC 2025-218 (z=2.63) and 3C 256 (z=1.824) from the literature.


The conclusions we are drawing from these studies can be summarised as follows:

These objects are telling us the story of the formation of massive galaxies and their quasar nuclei during the epoch when AGN had their maximum space density. The UV emission lines can give us some clues to the chemical composition of the extended nebulosity but to make inferences with more confidence, we need to measure the optical forbidden line spectrum in the infrared with ISAAC at the VLT.

We thank Laura Pentericci for making available to us the reductions of the NICMOS images. We are grateful to Bob Goodrich for frequent help with the polarimetric observations and many discussions. Our Keck programme is supported by NATO Collaborative Research Grant number 971115. This paper is based partially on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.

\begin{references}% latex2html id marker 6952
\par\reference Cimatti, A., di Ser...
...A., Miley, G.\ K. \&
Hunstead, R.\ W.\ 1997, A\&A, 317, 358

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