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R. A. E. Fosbury1
ST-ECF, Garching bei München, Germany, (firstname.lastname@example.org)
ESO, Garching bei München, Germany
Department of Physics & Astronomy, University of Sheffield, UK
M. H. Cohen
Astronomy Department, California Institute of Technology,
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-
beyond CIII], and HST imaging of a sample of powerful radio galaxies
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.
We have selected RG with
which allow us to study the strong UV
emission lines from Ly-
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
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.
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,
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.
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-
Some deep groundbased
imaging data have been taken from the literature to complement the higher
resolution but shallower HST images.
Here we summarise the principal results to date. These will be described
more fully in papers in preparation.
- All sources show a strong `alignment effect' between their UV and
radio morphologies although the structures are complex. One case,
4C 23.56 (Knopp & Chambers 1997), shows a beautiful `ionization cone' in
The brightest UV emission is extended and does not
necessarily coincide with the nucleus (radio core).
- The continuum colours are remarkably similar to one another and can be
fitted by a power law absorbed by a standard Galactic (in the RG rest-frame)
extinction law with
which corresponds to
at 1500Å. Example fits to three of the sources are shown in Figure 3.
Total flux spectra of three of the radio galaxies scaled to show the
continuum. The crosses mark continuum bins chosen to be free of emission lines
and atmospheric absorption features with the vertical bars representing one
sigma statistical errors. The fitted curves are two parameter fits of a power law
absorbed by a Galactic extinction curve
in the rest-frame of the RG. The derived values from
fits are shown in the labels.
- Interstellar absorption lines are seen and, in some objects, there
is evidence for wind and photospheric absorption lines from hot stars.
Several sources show complex, spatially extended absorption structures at
(see also van Ojik et al., 1997).
- The emission lines are spatially extended (up to arcsec
or kpc for Ly-)
and show complex kinematic structures
extending over km s-1.
- The continuum linear polarization, measured just longward of
Ly-/NV, ranges from
The E-vector is
perpendicular to the UV extension as seen at HST resolution (but not
necessarily precisely to the radio axis).
- The emission line spectra indicate a rather constant level of
ionization with a small range in the observed CIII]/CIV and HeII/CIV
- Amongst the spatially integrated properties, the strongest
correlations, shown in Figure 4, are observed to be:
- between continuum polarization, P, and the Ly-/CIV
emission line ratio (anticorrelation)
- between P and the NV/CIV ratio
Plots of the fractional continuum polarization against (a) the
Ly-/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
The conclusions we are drawing from these studies can be summarised as follows:
- Powerful radio galaxies contain (hidden) QSO nuclei whose EUV emission
ionizes the extended gas along the radio axis and whose FUV emission we see
scattered by extended dust structures.
- The scattered component can -- but does not always -- dominate the
observed UV continuum and there is evidence for an unpolarized hot stellar
component in addition to a nebular continuum contribution from the emission line
gas. The presence of an old, red stellar population becomes apparent above
the 4000Å break, although this does not always dominate the observed flux
in the infrared bands.
- There is a direct connection between the scattering mechanism (which
produces the polarization) and the destruction of Ly-
could simply result from the abundance and spatial distribution of dust,
although orientation effects may be important as well.
- The continuum exhibits dust extinction signatures in the form of the
2200Å dip and a peak at the position of the extinction minimum
around 1400Å. Some of the extinction may arise in an extended halo outside
the regions which see the QSO radiation field directly. This would be
consistent with the absence of the 2000Å dip in radio quasars.
- There appears to be strong connection between the dominance of scattered
light (dust abundance and/or intrinsic quasar luminosity) and nitrogen/carbon
ratio. The behaviour of the NV/CIV, NV/HeII diagram indicates that the effect is
due to nitrogen abundance variations and not to carbon depletion. This may be
related to the suggestion of a relative overabundance of nitrogen in high
redshift QSOs (Hamann & Ferland 1993).
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
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.
Next: Investigating the Evolution of
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