The Highest Redshift Radio Galaxy Known in the Southern Hemisphere

We present the discovery of a z = 4.13 galaxy TN J1338-1942, the most distant radio galaxy in the southern hemisphere known to date. The source was selected from a sample of Ultra Steep Spectrum (USS; α < −1.3; S ∝ ν) radio sources using the Texas and NVSS catalogs. The discovery spectrum, obtained with the ESO 3.6m telescope, shows bright extended Ly-α emission. The radio source has a very asymmetric morphology, suggesting a strong interaction with an inhomogeneous surrounding medium. 1 Southern high redshift radio galaxy searches High Redshift Radio Galaxies (HzRGs) may be used to study the formation and evolution of massive elliptical galaxies (see, e.g. van Breugel, this volume). However, the sample of HzRGs at the highest redshifts (z > 3) is extremely small, despite vigorous searches by several groups. This is especially true in the southern hemisphere: of the 20 z > 3 radio galaxies known, only 3 are in the South; below declination −40, only one z > 2 radio galaxy is known! To provide samples of HzRGs to study with the soon to be operational 6−8m class telescopes in the southern hemisphere (VLT, Gemini−South, Magellan), we have constructed a sample of USS sources from the TEXAS 365 MHz (Douglas et al. 1996) and NVSS 1.4 GHz (Condon et al. 1998) surveys (δ > −35), and from the MRC 408 MHz (Large et al. 1981) and PMN 4.85 GHz (Griffith & Wright 1993) surveys (δ < −35). This USS selection makes our sample ∼ 65% efficient in selecting z > 2 radio galaxies (see van Breugel, this volume, and De Breuck et al. 1998). 2 The first southern z > 4 radio galaxy The highest redshift USS object we have found thus far is the radio galaxy TN J1338-1942. The source has an integrated spectral index α = −1.33, and a straight power-law spectrum between 365 MHz, 1.4 GHz and 4.8 GHz. High resolution VLA imaging at 4.8 GHz and 8.3 GHz shows that the source has a core with α 8.3 = −0.6, a northwestern lobe with α 4.8 8.3 = −1.6 at 1 ′′ from the 2 Carlos De Breuck et al. core, and a southeastern lobe with α 8.3 = −2.5 at 3. 6 from the core (Fig. 1). The coincidence of extended Ly-α emission with the NW lobe suggests that this asymmetry may be due to an inhomogeneous ambient medium. The R−band identification and spectroscopic observations were obtained with the EFOSC1 imaging spectrograph on the ESO 3.6m telescope. We obtained two spectra, one covering 3725Å to 6940Å (not shown here), and another covering 6000Å to 9200Å (Fig. 2). The 2 slit used to obtain the blue spectrum was offset 2 from the radio core, but nevertheless showed bright Ly-α , proving the large extent of the Ly-α emission. D E C L IN A T IO N ( J2 00 0) RIGHT ASCENSION (J2000) 13 38 26.4 26.3 26.2 26.1 26.0 25.9 -19 42 28