Deep HST/PC imaging of a young elliptical radio galaxy at z=2.390

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Session 107 -- Galaxies: Morphology and Interactions
Display presentation, Thursday, 12, 1995, 9:20am - 6:30pm

[107.04] Deep HST/PC imaging of a young elliptical radio galaxy at z=2.390

R. A. Windhorst (Arizona State U.), W. C. Keel (U. Alabama)

We present deep 6-hr $HST/PC$ images in both the WFPC2 wide-V and I filters of the weak radio galaxy 53W002, a compact narrow-line object at z=2.390. These are among the first refurbished $PC$-images of a high redshift galaxy, and are nearly fully sampled at $\sim$0.07'' FWHM resolution. We subtract series of point sources to constrain its weak AGN ($\le$550 pc at z=2.390 for $H_o$=50, $q_o$=0.5) to 26$\pm$7\% of its total continuum in the I-band and 16$\pm$4\% in the V-band. This unresolved component is surrounded by an extended envelope that follows an $r^{1/4}$-law with effective radius $r_e\simeq$1.1'' (8.6 kpc), with a color gradient that indicates an aging stellar population of $\sim$0.3 Gyr in the center to $\sim$0.5 Gyr at $\sim$9 kpc.

53W002 has a very blue feature 0.45'' (3.5 kpc) to the West, which is $\sim$0.3 mag bluer than the galaxy's young stellar population, that shows a more streak-like structure in V (which contains redshifted C-IV) and is more uniform in I (which contains no strong emission lines). This ``cloud'' is aligned with the radio source and ground-based Ly-$\alpha$ image, but only on one side, and could be a combination of: (1) reflection of its AGN-light shining through a cone off an electron cloud and/or a dust screen, including Ly-$\alpha$ and C-IV emission lines from gas lit up by the cone. A cone is not seen to the East, so if AGN light is reflected on both sides, its far-side must be obscured by dust; or (2) a star-bursting region induced by the weak radio jet. Its bluer colors indicate that any such star-bursting region is $\le 10^{8}$ years old ($\simeq$ the typical radio source life-time), younger than the galaxy's dynamical time scale, or it would have disappeared into the galaxy's $r^{1/4}$ profile.

We conclude that the weak radio source 53W002 is likely a genuinely young elliptical galaxy that started forming stars $\le$0.3--0.5 Gyr before $z=2.390$, possibly induced by its weak and fading radio jet, and is just old enough to have developed a regular $r^{1/4}$ light-profile at z=2.390. This work was supported by HST grant GO-5308-01-94A from STScI, which is operated by AURA, Inc., under NASA contract NAS5-26555.

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