A Redshiftz≈ 5.4 Lyα Emitting Galaxy with Linear Morphology in the GRAPES/Hubble Ultra Deep Field

Abstract

We have discovered an extended Lyα plume (UDF 5225) associated with a compact source at redshift z ≈ 5.4 in slitless spectroscopic data from the Grism ACS Program for Extragalactic Science (GRAPES) project. The spatial extent of the emission is about 6 × 1.5 kpc (1'' × 025). Combining our grism data and the broadband images from the Hubble Ultra Deep Field (UDF) images, we find a Lyα line flux of ~2.2 × 10^-17 ergs cm^-2 s^-1 and surface brightness ~7 × 10^-17ergs cm^-2 s^-1 arcsec^-2. The UDF images show diffuse continuum emission associated with UDF 5225, including three embedded knots. The morphology of UDF 5225 is highly suggestive of a galaxy in assembly. It is possible that the prominent Lyα emission from this object is due to an active nucleus, and that we are seeing the simultaneous growth through accretion of a galaxy and its central black hole. Follow-up observations at higher spectral resolution could test this hypothesis.