5. Composition and heating of the gaseous disks
As mentioned above, the gaseous disks seen as dust-lanes across elliptical galaxies are believed to be the remnants of gas-rich galaxies captured by the massive ellipticals. In order to understand the physical composition of the disks, it is therefore important to recapitulate the current knowledge about the warm and cool ISM of well studied spiral galaxies. Most of the information on the warm ionized medium (WIM) at comes from the studies of our Galaxy within the solar neighbourhood. It is estimated that the WIM fills roughly 20% of the total disk volume, and has a typical electron density of with a vertical height of (e.g., Walterbos & Braun 1996). Most of the mass of the ISM is, however, contained in the neutral HI phase whose properties are best determined from the observations of face-on spirals, such as the Sc galaxy M 101. The clumpy and the diffuse cool components of the ISM in this galaxy are estimated to contribute comparable amounts of HI line emission, though the clumped HI gas is essentially confined to within the optically luminous part of the galactic disk (Walterbos & Braun 1996).
When, following its capture, such a gaseous disk would begin to settle around the massive elliptical, the various components of its ISM would respond differently to the surrounding hot corona associated with the elliptical. The thermal pressure within the corona typically corresponds to . Such a high ambient pressure would compress the larger gas clumps further to densities of which are typical of molecular clouds capable of shielding the embedded dust particles against sputtering by the hot coronal gas (e.g., Sparks et al., 1989; de Jong et al. 1990). On the other hand, the diffuse component and the smaller clumps of the cool ISM inside the disk would get rapidly depleted via a heat transfer from the hot ambient gas (and join the existing warm medium: WIM), following a turbulent mixing of the two phases. The thus augmented warm ISM of the disk will be the subject of focus in the present work. Since the details of the heating/evaporation process are highly model dependent (e.g., Sparks 1992; de Jong et al. 1990; Sparks et al. 1989), our attempt here is only to check broad consistency of our proposed scenario with the available observations.
5.1. Heating of the accreted disk material by the ambient hot corona
In order to sketch the thermal and dynamical evolution of the volume-filling warm ISM (WIM) of the captured disk, we shall make a few simplifying, plausible assumptions. As a first clue, recall that the estimated mass of the diffuse ionized gas in the disk of our Galaxy is M , taking a scale height of kpc, a radius of kpc, and a filling factor , corresponding to a volume-averaged density of cm-3, as estimated from the dispersion measures of pulsars and faint optical emission from the WIM (e.g., Kulkarni & Heiles 1988; Reynolds 1989). Based on Sect. 4, we shall assume that the gaseous disks around the hosts of high quasars and radio galaxies contained an order-of-magnitude more ionized gas (i.e., M ) than our Galaxy. This is also consistent with the evidence for intense star-formation activity occuring in the large disk galaxies at those early epochs (Sect. 4), and their additional heating/ionization after the capture, as discussed below.
The tidal shearing of the captured disk by a massive elliptical is expected to stretch the disk, as the accretion progresses. To make a rough allowance for this and also paying attention to the observational evidence (Sect. 3.2), we adopt for the accreted disk a radius of kpc and an initial thickness kpc. The implied volume, together with the above mentioned gas content M yields an initial mean electron density cm-3. At , this warm disk medium would be in pressure equilibrium with the ambient hot gas ( K) at a typical density in the outskirts of the corona, as inferred for nearby massive ellipticals (e.g., Fabbiano 1987; Sarazin 1990). Note that our adopted value of is on the higher side of the range established for the coronae of nearby massive ellipticals, consistent with the prevailing notion that a deep gravitational potential is condusive to the formation of powerful radio sources.
We next consider the thermal evolution of the warm ISM of the captured disk at as it gravitates towards the inner regions of the hot corona of the elliptical. Assuming a uniform gas density within the disk (see below), the net heating rate, Q, of the disk ISM due to thermal conduction through both surfaces of the disk, is given by (Spitzer 1962):
Expecting a low metallicity (Z ) to characterize disk ISM at high redshifts, the peak cooling rate of the gas would be erg s-1 cm-3 at K (Fall & Rees 1985). This peak value being comparable to the heating rate (cf. Eq. 1), even for the above estimated initial density, of the disk ISM (Eq. 1), the ISM would continue to tap heat from the massive ambient corona and its temperature would rise along the range . The heating would simultaneously cause the disk to expand, thereby maintaining pressure equilibrium with the ambient corona. In view of the cylindrical geometry of the disk we assume that, to a first order, its expansion occurs mainly along the axial direction, thus conserving the ISM column density (, while broadening the disk from to in yr at the local sound speed of km s-1. Such a width of the disk would be consistent with the extent of the emission gaps observed in the radio bridges (Sect. 3.2). Note that moving towards a temperature of , the disk ISM would primarily cool via hydrogen line emission in the blue/UV region, which is likely to be absorbed by the dust within the disk and re-emitted in the far-infrared. It may be recalled that far-infrared is the energetically dominant spectral region for powerful radio galaxies, with (Heckman et al. 1992).
Realistically, the ISM of the captured disk is expected have a substantial density structure, due to which the external heating would give rise to a range of temperature, even extending beyond K. Conceivably, the hotter, more rapidly expanding phases of the ISM would determine the width of the disk. Another potential contributor to the widening of the disk is the stripping of its ISM due to external ram-pressure generated by the motion of the elliptical galaxy itself, as inferred from the frequently observed positional offsets of the active nucleus from the mid-plane of the radio emission gap (Sect. 3.2).
While the present treatment of the disk heating is very approximate, we note that observational evidence is already available to support the basic picture of depletion of the cool diffuse ISM of the disk embedded within a massive elliptical bulge component having a hot corona. This is seen on a recent HI map of the Sa type galaxy NGC1291 in which ROSAT PSPC observations have revealed a corona of X-ray emitting gas centred on the bulge component (Bregman et al. 1995). This hot corona is similar to the coronae of other nearby massive early-type galaxies, with central densities in the range (e.g., Sarazin 1990). Interestingly, the large HI mass of in NGC1291 is found to be concentrated within an annular portion of the disk; at 10 kpc radius inside which HI emission becomes undetectable, the hot gas becomes detectable in X-rays and attains a pressure given by , before rising by two orders of magnitude near the centre (Bregman et al. 1995). This spatial anti-correlation between the cool and hot gaseous components shows that if any significant neutral gas is at all present within the portion of the disk coinciding with bulge, it must be in the form of molecular clumps. The lack of HI in the bulge region can be understood as a consequence of thermal interaction between the cool and hot gas phases through turbulent mixing and conduction, leading to a heating/depletion of the HI within the inner disk. This finding lends strong support to the scenario we have sketched above for the heating of the captured gaseous disk by the hot coronal gas of the captor elliptical galaxy. Note that the possibility of dust-lanes receiving heat input from the hot ambient corona, via grain-gas collisions, and then re-radiating it in the far-infrared has also been considered by Sparks & Collier-Cameron (1988) in a different context.
To summarize our basic picture, the capture of a gas-rich galactic/proto-galactic disk by a massive elliptical with a hot corona would, in addition to an occasional triggering of radio jets, also lead to the formation of an extended disk filled with diffuse ionized gas and dense, dusty clumps of molecular gas embedded in it. We have estimated that over the formation time-scale of the dust lane (typically yrs) the heated portion of the accreted disk would have steadily expanded in width to , thus accounting for the large, sharply bounded central emission gaps which have been detected in the middle portions of the radio bridges. The other possible signatures of such fat disks, e.g., soft X-rays, have not yet been picked up in imaging observations, for which several plausible reasons exist. Firstly, the radiation from the putative disk component could easily be outshined by the coronal X-rays of the powerful radio galaxies and quasars. Moreover, conceivably, the captured disk may only attain a temperature which is substantially lower than that of the ambient corona (), in which case the PSPC images would not be sensitive to the disk emission. In any event, X-ray images are not available presently even for moderately distant ellipticals, let alone the distant ones being discussed here in the context of the L-G effect.
5.2. The gaseous superdisk as a Faraday screen
Quasars at typically have apparent radio sizes of to (taking and ) (e.g., Kapahi 1990; Singal 1993), and their radio axes are inclined, on average, at an angle of from the line-of-sight (Barthel 1989). Hence, the radio lobes on the far side could well be hidden behind the large magneto-ionic disks discussed above, which are oriented roughly perpendicular to the radio axis (Sect. 3.2). Conceivably, such disks could provide a significant coverage even to the lobes of radio galaxies (despite their axes being oriented closer to the sky plane), because the disks are expected to be often appreciably warped in their outer parts (see, Sanders et al. 1989; Phinney 1989).
Depolarization due to a magneto-ionic screen can arise from multiple patches of varying rotation measure present within the beam. A quantitative estimation of this would require the knowledge of several parameters, such as the detailed geometry of the screen and the distribution of the electron density, as well as the magnetic field within the disk. Lacking this information, we shall only attempt to make a gross estimate of the Faraday effects, with the objective of checking the basic viability of the proposed new model for the L-G effect. Taking a typical value of for the ordered component of the disk magnetic field along the line-of-sight and an electron column density (Sect. 5.1), the average rotation measure across the disk = would be . This translates to an average Faraday rotation angle of at an emission wavelength where the L-G effect has been observed (see, Garrington et al. 1991). Plausibly, such a magnitude of Faraday rotation can fulfil the basic requirement for depolarization of a background source, assuming that significant irregularities exist in the Faraday screen on a scale , creating several patches of varying rotation measure within the beam (as found by Garrington & Conway 1991, in their analysis of the L-G effect). To a first order, the Faraday dispersion parameter would amount to , for the disk parameters estimated above. Such values are consistent with those inferred from the analysis of the radio depolarization maps of quasars (cf. Table 1 of Garrington & Conway 1991). It may also be noted that the uniform magnetic field component of assumed here for the fat disk considered here is well within the corresponding value of for our Galaxy (Wielebinski 1988) and also modest compared to the typical field strength of estimated by Krause (1990) for a set of 11 nearby disk galaxies.
© European Southern Observatory (ESO) 1997
Online publication: April 20, 1998