Astron. Astrophys. 352, L51-L56 (1999)

4. Discussion

TN J1338-1942 shares several properties in common with other HzRGs but some of its characteristics deserve special comment. Here we shall briefly discuss these.

4.1. Ly emission

Assuming photoionization, case B recombination, and a temperature of  K we use the observed Ly emission to derive a total mass () of the HII gas (e.g., McCarthy et al. 1990) using  . Here is the filling factor in units of 10^-5, is the Ly luminosity in units of  ergs s^-1, and is the total volume in units of  cm³. Assuming a filling factor of 10^-5 (McCarthy et al. 1990), and a cubical volume with a side of 15 kpc, we find  . This value is on the high side, but well within the range that has been found for HzRGs (e.g., van Ojik et al. 1997)).

Previous authors have shown that gas clouds of such mass can cause radio jets to bend and decollimate (e.g., van Breugel Filippenko Heckman & Miley 1985 , Lonsdale & Barthel 1986 , Barthel & Miley 1988). Likewise, the extreme asymmetry in the TN J1338-1942 radio source could well be the result of strong interaction between the radio-emitting plasma and the Ly gas.

4.2. Ly absorption

Our spectrum also shows evidence for deep blue-ward absorption of the Ly emission line. We believe that this is probably due to resonant scattering by cold HI gas in a halo surrounding the radio galaxy, as seen in many other HzRGs (c.f., R"ottgering et al. 1995, van Ojik et al. 1997, Dey 1999). The spatial extent of the absorption edge as seen in the 2-dimensional spectrum (Fig. 3) implies that the extent of the absorbing gas is similar or even larger than the 4" (30 kpc) Ly emitting region.

To constrain the absorption parameters we constructed a simple model that describes the Ly profile with a Gaussian emission function and a single Voigt absorption function. As a first step, we fitted the red wing of the emission line with a Gaussian emission profile. Because the absorption is very broad, and extends to the red side of the peak, the parameters of this Gaussian emission profile are not well constrained. We adopted the Gaussian that best fits the lower red wing as well as the faint secondary peak, 1400 km s^-1 blue-wards from the main peak. The second step consisted of adjusting the parameters of the Voigt absorption profile to best match the sharp rise towards the main peak. The resulting model (shown along with the parameters of both components in Fig. 4) adequately matches the main features in the profile. We varied the parameters of both components, and all acceptable models yield column densities in the range cm^-2.

Fig. 4. Part of the spectrum around the Ly line. The solid line is the model consisting of a Gaussian emission profile (dashed line) and a Voigt absorption profile with the indicated parameters.

The main difference between our simple model and the observations is the relatively flat, but non-zero flux at the bottom of the broad depression. This flux is higher than the continuum surrounding the Ly line, indicating some photons can go through (i.e., a filling factor less than unity) or around the absorbing cloud. If the angular size of absorber and emitter are similar, the size of the absorber is 10 kpc. The total mass of neutral hydrogen then is , comparable to or somewhat less than the total mass of HII .

4.3. Continuum

Following Dey et al. 1997, and assuming that the rest frame UV continuum is due to young stars, one can estimate the star-formation rate (SFR) in TN J1338-1942 from the observed rest-frame UV continuum near 1400 Å. From our spectrum we estimate that Jy, resulting in a UV luminosity erg s^-1 Å^{- 1} and implying a SFR between 90 - 720 h yr^-1 in a  kpc² aperture. These values are similar to those found for 4C 41.17. In this case detailed HST images, when compared with high resolution radio maps, strongly suggested that this large SFR might have been induced at least in part by powerful jets interacting with massive, dense clouds (Dey et al. 1997; van Breugel et al. 1999b; Bicknell et al.1999). The co-spatial Ly emission-line and rest-frame optical continuum with the brightest radio hotspot in TN J1338-1942 suggests that a similar strong interaction might occur in this very asymmetric radio source.

The decrement of the continuum blue-wards of Ly (Fig. 2) due to the intervening HI absorption along the cosmological line of sight is described by the "flux deficit" parameter (Oke & Korycanski 1982). For TN J1338-1942 we measure , comparable to the that Spinrad et al. (1995) found for the radio galaxy 8C 1435+64 (uncorrected for Galactic reddening). This is only the second time the parameter has been measured in a radio galaxy.

The decrement described by is considered to be extrinsic to the object toward which it has been measured, and should therefore give similar values for different classes of objects at the same redshift. Because they have bright continua, quasars have historically been the most popular objects to measure . For , quasars have measured values of (e.g., Schneider, Schmidt & Gunn 1991, 1997). Similar measurements for color selected Lyman break galaxies do not yet exist.

Other non-color selected objects, in addition to radio galaxies, which do have reported measurements are serendiptiously discovered galaxies (, Dey et al. 1998) and narrow-band Ly-selected galaxies (, Hu, McMahon & Cowie 1999). Because of their larger redshifts these galaxy values can not directly be compared with those of quasars (, Songaila et al.1999). However, they seem to fall slightly () below the theoretical extrapolation of Madau (1995) at their respective redshifts, which quasars do follow rather closely. This is also true for the two radio galaxies () at their redshifts. Thus it appears that non-color selected galaxies, whether radio selected or otherwise, have values which fall below those of quasars.

Although, with only two measurements, the statistical significance of the low radio galaxy values is marginal, the result is suggestive. It is worthwhile contemplating the implications that would follow if further observations of radio galaxies and other objects selected without an optical color bias confirmed this trend. Given that optical color selection methods (often used to find quasars, and Lyman break galaxies) favour objects with large values, it is perhaps not surprising that non-color selected objects might have lower values of . Consequently, quasars and galaxies with low values might be missed in color-based surveys. This then could lead to an underestimate of their space densities, and an overestimate of the average HI columns density through the universe.

Radio galaxies have an extra advantage over radio selected quasars (e.g., Hook & McMahon 1998), because they very rarely contain BAL systems (there is only one such example, 6C 1908+722 at ; Dey 1999). Such BAL systems are known to lead to relatively large values of , indicating that part of the absorption is not due to cosmological HI gas, but due to absorption within the BAL system (Oke & Korycanski 1982). A statistically significant sample of radio galaxies would therefore determine the true space density of intervening HI absorbers.

© European Southern Observatory (ESO) 1999

Online publication: November 23, 1999
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