4. Mass estimates for the gaseous disks of distant radio galaxies
As mentioned in Sect. 3.2, the z = 3.8 radio galaxy 4C41.17 exhibits a dusty disk also detected in the millimetric continuum, whose total gas content is estimated to be , assuming the galactic gas-to-dust ratio of 500. Even down to moderate redshifts, , it is now evident from sensitive spectroscopy of optically selected field galaxies that star formation continued to remain very active in large disk galaxies and, hence, much of their mass had yet not condensed into stars (Cowie et al. 1996). Independent evidence for occurence of such massive gaseous disks obtains from studies of radio-loud quasars; they are found to emit a few times more of the far-infrared radiation compared to the disks of radio galaxies of matched radio lobe power (Heckman et al. 1992). Attributing this excess to the difference in the disk orientation for the two classes of sources and to the implied high far-infrared opacity of the dust in the disk, these authors have estimated a minimum gas content of in a kiloparsec-scale disk component, assuming the far-infrared radiation to be thermal, as argued earlier by Sanders et al. (1989), Phinney (1989) and Antonucci et al. (1990). Allowing for the possibility that even in lobe-dominated quasars, a fraction of the far-infrared emission can be the beamed nonthermal continuum of the nucleus (Hes et al. 1995), we shall adopt as a representative value for the total gas content of the disks/dust-lanes associated with intermediate to high redshift radio sources (see, also, Eales & Edmonds 1996).
© European Southern Observatory (ESO) 1997
Online publication: April 20, 1998