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Astron. Astrophys. 326, 45-50 (1997)

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3. Evidence for dust-lanes/gaseous disks around massive ellipticals

3.1. The optical evidence

As mentioned above, since galaxy interactions/mergers seem to be frequently involved in the genesis of the quasar phenomenon, gaseous disks are expected to be a common feature of massive ellipticals hosting a quasar nucleus. CCD imaging surveys of nearby elliptical galaxies have frequently revealed absorption features identified as dust-lanes extended across them (e.g., Goudfrooij & de Jong 1995; Zeilinger et al. 1990). Inspite of the difficulty in detecting dust-lanes, unless viewed almost edge-on, they have been observed in [FORMULA] of bright ellipticals (e.g., Sadler & Gerhard 1985; Jura et al. 1987; Ebneter et al. 1988; Goudfrooij & de Jong 1995; Zeilinger et al. 1990). At large redshifts ([FORMULA]), evidence for such disks of dusty gaseous material surrounding radio-loud galaxies and quasars comes from the apparent suppression of [FORMULA] emission from the region of the radio lobe located on the far side of the parent galaxy, both in cases of radio galaxies (Gopal-Krishna et al. 1995) and quasars (Heckman et al. 1991). Many authors have linked the onset of nuclear activity in galaxies to the formation of dust lanes around them, due to accretion of a gas-rich galaxy (e.g., Colina & de Juan 1995; Ellingson et al. 1991; Sparks et al. 1989; Sect. 2).

3.2. The radio evidence

The evidence for large gaseous disks is further bolstered by the detection of millimetric continuum radiation from high-z radio sources, which is interpreted to arise mainly from massive dusty disks (e.g., Dunlop et al., 1994; Chini & Krügel 1994; Ivison 1995; Andreani et al. 1993). For the [FORMULA] radio galaxy 4C41.17 (Chambers et al. 1990), the gas mass associated with the dusty disk is estimated to be enormous ([FORMULA]) (see, Chini & Krügel 1994; Dunlop et al. 1994), albeit, consistent with the inference made from the far-infrared emission of powerful radio sources (Sect. 4).

A key evidence for fat and highly extended gaseous disks around the elliptical galaxy hosts of powerful double radio sources comes from the detection of sharp and often quasi-linear inner boundaries of the radio lobes, as highlighted recently by Gopal-Krishna & Wiita (1996). These sharp inner edges of the lobes (on the side facing the parent galaxy), give rise to strip-like central gaps in the radio bridges. Their detection requires not only maps with high sensitivity and dynamic range, but also a favourable orientation of the lobe axis, i.e., close to the plane of the sky. Despite these difficult demands, clear examples of the sharp inner boundaries of radio lobes have been observed in a number of powerful double radio galaxies. A few examples are: 3C34 (Johnson et al. 1995); 3C192 (Leahy et al. 1997); 3C227 (Black et al. 1992); 0828+32 (Capetti et al. 1993) and 3C16, 3C33, 3C61.1, 3C184.1, 3C341, 3C381, 4C14.11 and 4C14.27 (Leahy & Perley 1991). In the nearby radio galaxy M87, a recent interferometric map at 90 GHz has revealed a linear feature which is likely to be an enormously massive ([FORMULA]) disk-like structure of very cold atomic gas, oriented perpendicular to the jet (Despringre et al. 1996).

The sharply bounded emission gaps seen near the middle portions of radio bridges are commonly attributed to a blocking of the `back-flowing' radio plasma of the lobes by a denser thermal plasma (ISM) associated with the parent elliptical galaxy (Leahy & Williams 1984; Black et al., 1992; Gopal-Krishna & Wiita 1996). The detection of quasi-linear inner boundaries of the radio lobes is strongly indicative of a disk-like geometry for the thermal ISM blocking the radio plasma of the lobes. Very large minimum values for the disk diameter are thus inferred from the fact that the strip-like radio emission gaps are seen to extend across the radio bridges to great distances. From the radio maps of the 12 relatively nearby ([FORMULA] radio galaxies cited above, we infer a median value of [FORMULA] for the disk diameter (clearly a lower limit). Likewise, the median value for disk thickness is found to be [FORMULA]. In the following discussion about high-z radio sources, these estimates will be taken as representative values.

Further, it may be noted that the appreciable offset of the active nucleus from the mid-point of the radio emission gap, as witnessed in several of these sources, probably arises from the motion of the parent galaxy. Recently, some interesting implications of this possibility have been discussed by Gopal-Krishna & Wiita (1996) who have argued that the correlated radio-optical asymmetry of powerful double radio sources (McCarthy et al. 1991) can be more readily understood in terms of an asymmetric disposition of the obscuring dusty ISM of the parent elliptical galaxy, resulting from the motion of the galaxy (instead of the widely discussed explanation invoking a large-scale density asymmetry of the circum-galactic medium). Here we explore the nature and some likely consequences of the disky thermal ISM, in particular the possibility that it could account for the L-G effect, thus obviating the need to postulate clusters with a dense ICM at high redshifts, with dimensions fairly closely matched to the radio source size (Sect. 1).

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© European Southern Observatory (ESO) 1997

Online publication: April 20, 1998
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