Abstract
We take advantage of the Hubble Ultra Deep Field data to study the rest-frame optical and UV morphologies of the novel population of distant red galaxies (DRGs). Six galaxies with J-Ks > 2.3 are found to Ks = 21.5, five of which have photometric redshifts zphot ≳ 2, corresponding to a surface density of 0.9 arcmin-2. The surface brightness distributions of the zphot ≳ 2 galaxies are better represented by exponential disks than R1/4 laws. Two of the zphot ≳ 2 galaxies are extended, while three have compact morphologies. The rest-frame optical morphology of the zphot ≳ 2 galaxies is quite different from the rest-frame UV morphology: All the galaxies have red central components that dominate in the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) H160-band images, as well as distinct off-center blue features that show up in (and often dominate) the Advanced Camera for Surveys (ACS) images. The mean measured effective radius of the zphot ≳ 2 galaxies is ⟨re⟩ = 1.9 ± 1.4 kpc, similar (within the errors) to the mean size of Lyman break galaxies at similar redshifts. All the DRGs are resolved in the ACS images, while four are resolved in the NICMOS images. Two of the zphot ≳ 2 galaxies are bright X-ray sources and hence host active galactic nuclei (AGNs). One of these galaxies is resolved in the ACS and NICMOS images, which means that the AGN does not dominate its rest-frame UV/optical spectral energy distribution (SED), while the other is unresolved in the NICMOS images and hence could have an AGN-dominated SED. The diverse rest-frame optical and UV morphological properties of DRGs derived here suggest that they have complex stellar populations consisting of both evolved populations, which dominate the mass and the rest-frame optical light, and younger populations that show up as patches of star formation in the rest-frame UV light, in many ways resembling the properties of normal local galaxies. This interpretation is supported by fits to the broadband SEDs, which for all five zphot ≳ 2 galaxies are best represented by models with extended star formation histories and substantial amounts of dust.
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