Near-Infrared Observations of Powerful High-Redshift Radio Galaxies: 4C 40.36 and 4C 39.37

Abstract

We present near-infrared imaging and spectroscopic observations of two Fanaroff-Riley type II high-redshift radio galaxies (HzRGs), 4C 40.36 (z = 2.3) and 4C 39.37 (z = 3.2), obtained with the Hubble Space Telescope and the Keck and Hale Telescopes. High-resolution images were taken with filters both in and out of strong emission lines, and together with the spectroscopic data, the properties of the line and continuum emissions were carefully analyzed. Our analysis of 4C 40.36 and 4C 39.37 shows that strong emission lines (e.g., [O III] λ5007 and Hα+[N II]) contribute to the broadband fluxes much more significantly than previously estimated (80% vs. 20%–40%) and that when the continuum sources are imaged through line-free filters, they show an extremely compact morphology with a high surface brightness. If we use the R^(1/4)-law parameterization, their effective radii (r_e) are only 2h_(50)^(-1) to 3h_(50)^(-1) kpc, while their rest-frame B-band surface brightnesses at r_e are I_e(B) ~ 18 mag arcsec^(-2). Compared with z ~ 1 3CR radio galaxies, the former is 3–5 times smaller, while the latter is 1–1.5 mag brighter than what is predicted from the I_e(B)-r_e correlation. Although exponential profiles produce equally good fits for 4C 40.36 and 4C 39.37, this clearly indicates that with respect to the z ~ 1 3CR radio galaxies, the light distribution of these two HzRGs is much more centrally concentrated. Spectroscopically, 4C 40.36 shows a flat (f_ν = const.) continuum, while 4C 39.37 shows a spectrum as red as that of a local giant elliptical galaxy. Although this difference may be explained in terms of a varying degree of star formation, the similarities of their surface brightness profiles and the submillimeter detection of 4C 39.37 might suggest that the intrinsic spectra is equally blue (young stars or an active galactic nuclei) and that the difference is the amount of reddening.