Induced star formation and morphological evolution in very high redshift radio galaxies

Near-infrared, sub-arcsecond seeing images obtained with the W. M. Keck I Telescope of show strong evolution at rest–frame optical wavelengths in the morphologies of high redshift radio galaxies (HzRGs) with 1.9 < z < 4.4. The structures change from large–scale low surface brightness regions surrounding bright, multiple component and often radio– aligned features at z > 3, to much more compact and symmetrical shapes at z < 3. The linear sizes (∼ 10 kpc) and luminosities (MB ∼ −20 to −22) of the individual components in the z > 3 HzRGs are similar to the total sizes and luminosities of normal, radio–quiet, star forming galaxies seen at z = 3− 4. ‘R’-band, 0.1′′ resolution images with the Hubble Space Telescope of one of these HzRGs, 4C41.17 at z = 3.800, show that at rest–frame UV wavelengths the galaxy morphology breaks up in even smaller, ∼ 1 kpc– sized components embedded in a large halo of low surface brightness emission. The brightest UV emission is from a radio–aligned, edge-brightened feature (4C41.17-North) downstream from a bright radio knot. A narrow– band Ly-α image, also obtained with HST, shows an arc–shaped Ly-α feature at this same location, suggestive of a strong jet/cloud collision. Deep spectropolarimetric observations with the W. M. Keck II Tele-