With the aid of a new IRCAM image of 3C356, researchers question the common assumption that radiosource-stimulated starbursts are responsible for the extended optical emission aligned with radio structures in high-redshift radiogalaxies. They propose an alternative model in which the radiation from a hidden luminous quasar is beamed along the radio axis and illuminates dense clumps of cool gas to produce both extended narrow emission line regions and, by Thomson scattering, extended optical continua. Simple observational tests of this model are possible and necessary if we are to continue to accept that the color, magnitude and shape evolution of radiogalaxies are controlled by the active evolution of stellar populations

Eales, Stephen A.

Rawlings, Steve

English

Eales, Stephen Anthony

Online Research @ Cardiff

Non-steller light from high-redshift radio galaxies

NASA Technical Reports Server

Non-stellar light from high-redshift radiogalaxies? 3 P- 
Steve hwlings, MRAO, Cavendish Laboratory, Cambridge, UK. 
S.A. Eales, STScI, Baltimore, USA. 
Abstract 
With the aid of a new IRCAM image of 3C356, we question the common assumption that radiosource- 
stimulated starbursts are responsible for the extended optical emission aligned with radio structures in 
high-redshift radiogalaxies. We propose an alternative model in which the radiation from a hidden 
luminous quasar is beamed along the radio axis and illuminates dense clumps of cool gas to produce 
both extended narrow emission line regions and, by Thomson scattering, extended optical continua. 
Simple observational tests of this model are possible and, in QUI ykw, necessary if we are to continue 
to accept that the colour, magnitude and shape evolution of radiogalaxies is controlled by the active 
evolution of stellar populations. 
High-redshift (z > 0.5) radiogalaxies have different optical properties from their nearby counterparts: the 
rest-frame colours are far bluer ' q 2  and their optical structures are extended along the radio axes 314. These 
differences are commonly explained by models that invoke increased starburst activity at high z '; the 
alignment of optical and radio structures is then taken as evidence that the radiosource itself stimulates at 
least some of the star formation 3*5. This interpretation is not, however, without its problems and we here 
use recent observational results to suggest that the extended optical continua has a non-stellar origin. 
There is growing evidence that radiogalaxies, which by definition lack obvious central quasars, do possess 
such objects hidden from us by obscuration. It is now clear from the polarisation of optical-line and - 
continuum and from the detection of both scattered and highly attenuated broad emission lines 7*8 that 
Seyfert I1 galaxies contain obscured broad-line (Seyfert I) nuclei. A similar unification of quasars and 
radiogalaxies is suggested, but not proved, by the available data 'JO However, there is additional indirect 
support for this unification: quasars clearly radiate their optical emission anisotropically l1 and differences 
in the radio properties of quasars and radiogalaxies are most reasonably interpreted by differing average 
orientations 12913*14. Furthermore, there is a strong link between the bulk power developed in radio jets 
and the luminosity of the hidden quasar as inferred from the luminosities of the narrow emission lines 16. 
This link leads us to suspect that known high-z radiogalaxies possess extremely luminous central quasars 
as, being drawn from bright flux-limited samples such as 3CR, they are of high radio luminosity. 
Do we see any direct evidence for these hidden objects and can their presence explain any of the effects 
often attributed to starbursts? The answer is almost certainly yes: the extended emission line regions 
of high-z radiogalaxies have high effective ionisation parameters (U, - 10-2-3) and are most probably 
photoionized by the postulated hidden quasar ". How then might the associated extended optical con- 
tinua produced? Although the 'standard' model of radiosource-stimulated starbursts is generally accepted, 
various alternatives have been proposed (although some perhaps rather more for their 'erotic fascination' 
l7 than for their scientific validity): gravitational lensing la, the stimulation of radiosource activity by 
galaxy-galaxy interactions lo, inverse-Compton scattering of microwave background photons by the radio 
lobes ', non-thermal emission associated with the radio jets, and finally scattering of either blazar or quasar 
light by either dust or thermal electrons 2op21*22. 
For two high-z radiogalaxies, 3C356 and 3C368, images in the key infrared waveband have allowed us to 
compare the competing models 20. The off-nuclear infrared components (knots) identified in the 3C356 2o 
(Fig. 1) and 3C368 have a number of common properties: 
1. they are unresolved and on one side of the radio nucleus only (although aligned optical continua are 
seen on both sides of 3C368); there is no evidence for a smooth elongation in the infrared data. 
389 
https://ntrs.nasa.gov/search.jsp?R=19910004925 2020-03-19T20:42:23+00:00Z
2. 
3. 
4. 
they are red with spectral indices between B- and K-band of - 1.5 and - 2 respectively. 
they are undetected in the radio at the 1 mJy level. 
they are at similar distances from the radio core (M 50 and = 30 kpc respectively) despite having 
radio structure differing in linear size by a factor of - 10. 
These properties constrain the emission mechanism for the infrared knots as follows: (2) eliminates dust 
scattering as this would produce a spectrum biased strongly to the blue; (l), and other arguments ", 
eliminate inverse-Compton scattering by a lobe of relativistic electrons; (3) eliminates non-thermal jet 
emission, gravitational lensing and the electron-scattering of blazar light as each predicts coincident radio 
components. The galaxy interaction model is both physically implausible 2o and cannot explain aligned 
optical structures on both sides of the radio-core. Thus, if the infrared emission is starlight it must be 
from a young (< 10' yr) starburst stimulated by the radiosource. To be consistent with such a model (2) 
indicates that the starburst must either be reddened by associated dust, which would be hard to produce in 
the short time available, or involve highly codtrived stellar populations that seem, considering the differing 
ages of the proposed starbursts inferred from (4), unlikely to exist in both 3C356 and 3C368 ". 
We now focus on our preferred infrared emission mechanism, Thomson-scattering of hidden quasar light by 
thermal electrons associated with the extended narrow-line region. This mechanism will naturally produce 
a power-law spectra similar to that of the hidden quasar and furthermore highly ionized (U, - lo-') ,, 
emission line gas, coincident with the knot, provides evidence for both a highly efficient scattering medium 
and for a $idden luminous quasar. The small solid angle subtended by the knot (< 0.1 sr) and its total 
narrow line luminosity (- 5 x lo3' W 17) indicate that the hidden quasar must beam at least vL,  - 5 x lo3' 
52,/41r W along cones of solid angle 52,. Although this inferred luminosity is higher than that of the brightest 
quasar in 3CR, this is not a serious cause for concern: any distribution in the values of 52, will cause a 
classification bias, those termed radiogalaxies will, on average, have narrower cones than those termed 
quasars making it misleading to compare inferred luminosities for these classes of object. We can use the 
[011]3727 luminosity and size of the knot to put a lower limit to the electron number density n, of - 5 x 10' 
m -3 although the high U, suggests that ne is unlikely to orders of magnitude higher. At such densities the 
gas will scatter incident light with an efficiency approaching unity and, given the high luminosity inferred 
for the quasar, it is entirely feasible, on energetic grounds, that the infrared knot (with vL,  N lo3* W) 
results from Thomson scattering by thermal electrons associated with the narrow-line clouds. 
Finally we mention another possible problem for the standard starburst model, namely that there are 
reported correlations between radio properties, the blueness of the galaxy and emission line luminosity 
l l z 3 .  At first sight these correlations appear to support our model because: (a) the bulk of the emission 
lines are photoionised directly by the active nucleus; and (b) radio luminosity is far more likely to be linked 
to non-stellar rather than stellar radiation 15. However, using the available [OII] data at high z we find 
no more than marginal evidence for correlations between radio and optical properties (Fig. 2); moreover, 
using either the starburst or the scattering model, we have yet to find a convincing explanation for a link 
between optical properties and radio spectral index. 
In conclusion, the Thomson scattering model discussed here provides an alternative, and in some ways 
more attractive, solution to the intriguing problem of why optical and radio structures are aligned in high- 
z radiogalaxies. The model can also be tested relatively simply by looking for polarisation and/or scattered 
broad lines in the extended optical continua. 
390  
17h 23min 07s 17h 23min 06s 
Right axension 1 1950 1 
Fig. 1 K-band image of 3C356 'O; the contour levels are 2 + nu; n > 0. The position of the radio-core and 
the direction of the radio structures (Laing,R.A., priv. comm.) are marked. 
Fig. 2 The relations between [OH13727 line luminosity, radio luminosity and radio spectral index. The 
objects plotted are those from the Laing et al. sample 24 with z> 0.5 and with [OII]3727 data l6 (i.e. the 
sample is not necessarily unbiased). 
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Non-steller light from high-redshift radiogalaxies

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