Galaxy formation and evolution

Abstract:

The first UV F300W and F218W WFPC2 observations of the rich galaxy cluster CL 0939 + 4713 at z = 0.41 are presented and discussed. UV/optical two-color and color-magnitude diagrams of the sources detected in the F300W waveband are constructed. Thanks to preexisting Hubble Space Telescope (HST) optical images of the same field, a morphological classification for the majority of these objects is also provided. Moreover, taking advantage of recent redshift surveys along the CL 0939 + 4713 line of sight, separate diagrams comparing the properties of galaxies belonging to the cluster and to its close projected field are presented. Possible evolutionary effects in the UV from z ∼ 0.4 to the present time are investigated by comparing the rest-frame (mid-UV/optical) colors of galaxies in CL 0939 + 4713 with balloon-borne data of the Coma Cluster, as well as by resorting to suitable galaxy evolution models. Finally, current attempts to constrain the epoch of the UV-upturn onset in evolved populations by means of HST UV observations are discussed.

Abstract:

We present deep near-infrared Keck Near-Infrared Camera (NIRC) imaging of a recently discovered z = 4.04 galaxy. This is lensed by the rich foreground cluster Abell 2390 (z ≈ 0.23) into highly magnified arcs ″-5″ in length. Our H and K′ band NIRC imaging allows us to map the Balmer + 4000 Å break amplitude. In combination with high-quality archival Hubble Space Telescope (HST) WFPC2 data, we can spatially resolve stellar populations along the arcs. The WFPC2 images clearly reveal several bright knots, which correspond to sites of active star formation. However, there are considerable portions of the arcs that are significantly redder, consistent with being observed ≳ 100 Myr after star formation has ceased. Keck Low-Resolution Imaging Spectrograph (LRIS) long-slit spectroscopy along the arcs reveals that the Lyα emission is spatially offset by ≈1″ from the rest-UV continuum regions. We show that this line emission is most probably powered by star formation in neighboring H II regions and that the z = 4 system is unlikely to be an active galactic nucleus.