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

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3. VLT observations

Because of the importance of TN J1338-1942 as a southern laboratory for studying HzRGs, we obtained a spectrum of this object with high signal-to-noise and intermediate spectral resolution with FORS1 on the ANTU unit of the VLT on UT 1999 April 20. The purpose of these observations was to study the Ly[FORMULA] emission and UV-continuum in detail.

The radio galaxy was detected in the acquisition images ([FORMULA] s; [FORMULA] in a 2" aperture). We used the 600R grism with a [FORMULA] wide slit, resulting in a spectral resolution of 5.5 Å (FWHM). The slit was centered on the peak of the [FORMULA]band emission at a position angle of 210o North through East. To minimize the effects of fringing in the red part of the CCD, we split the observation into two 1400 s exposures, while offsetting the object by 10" along the slit between the individual exposures. The seeing during the TN J1338-1942 observations was [FORMULA] and conditions were photometric.

Data reduction followed the standard procedures using the NOAO IRAF package. We extracted the one-dimensional spectrum using a 4" wide aperture, chosen to include all of the Ly[FORMULA] emission. For the initial wavelength calibration, we used exposures of a HeArNe lamp. We then adjusted the final zero point of the wavelength scale using telluric emission lines. The flux calibration was based on observations of the spectrophotometric standard star LTT2415, and is believed to be accurate to [FORMULA]. We corrected the spectrum for foreground Galactic extinction using a reddening of [FORMULA] determined from the dust maps of Schlegel, Finkbeiner & Davis (1998).

In Fig. 2 we show the observed one dimensional spectrum and in Fig. 3 the region of the two-dimensional spectrum surrounding the Ly[FORMULA] emission line. Most notable is the large asymmetry in the profile, consistent with a very wide ([FORMULA] km s-1) blue-ward depression. Following previous detection of Ly [FORMULA] absorption systems in HzRGs (R"ottgering et al. 1995; van Ojik et al. 1997; Dey 1999) we shall interpret the blue-ward asymmetry in the Ly[FORMULA] profile of TN J1338-1942 as being due to foreground absorption by neutral hydrogen.

[FIGURE] Fig. 2. VLT spectrum of TN J1338-1942. The lower panel has been boxcar smoothed by a factor of 15 to better show the shape of the Ly[FORMULA] forest and the Lyman limit. The horizontal dotted line is the extrapolation of the continuum at 1300 Å [FORMULA] Å, and the vertical dotted line indicates the position of the [FORMULA] Å Lyman limit.

[FIGURE] Fig. 3. Two dimensional FORS1 spectrum of the Ly[FORMULA] region. Note the strong, 1400 km s-1 wide depression in the blue half.

The rest-frame equivalent width of Ly[FORMULA] in TN J1338-1942 [FORMULA] Å, is twice as high as in the well-studied radio galaxy 4C 41.17 ([FORMULA]; Dey et al. 1997). The large Ly[FORMULA] luminosity ([FORMULA] erg s-1 after correction for absorption) makes TN J1338-1942 the most luminous Ly[FORMULA] emitting radio galaxy known.

Following Spinrad et al. (1995), we measure the continuum discontinuity across the Ly[FORMULA] line, defined as [[FORMULA]] = [FORMULA]. Similarly, for the Lyman limit at [FORMULA] = 912 Å, we find [[FORMULA]] = [FORMULA], though this value is uncertain because the flux calibration at the edge of the spectrum is poorly determined.

The presence of these continuum discontinuities further confirm our measured redshift. However, the redshift of the system is difficult to determine accurately because our VLT spectrum does not cover C [FORMULA] [FORMULA] 1549 or He [FORMULA] [FORMULA] 1640. Furthermore, since the Ly[FORMULA] emission is heavily absorbed, it is likely that the redshift of the peak of the Ly[FORMULA] emission (at [FORMULA], [FORMULA]) does not exactly coincide with the redshift of the galaxy. We shall assume [FORMULA].

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© European Southern Observatory (ESO) 1999

Online publication: November 23, 1999