AAS 201st Meeting, January, 2003
Session 42. Galaxies and Clusters
Poster, Tuesday, January 7, 2003, 9:20am-6:30pm, Exhibit Hall AB

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[42.12] ACS imaging observations of a protocluster around the z=4.1 radio galaxy TN1338-1942

R.A. Overzier (JHU/Leiden Obs.), G.K. Miley (Leiden Obs.), Z.I. Tsvetanov, D.R. Ardila (JHU), F. Bartko (Bartko Sci. & Tech.), N. Benitez, J.P. Blakeslee (JHU), R. Bouwens (UCO/Lick Observatory), T.J. Broadhurst (Racah Inst. Physics, Hebrew U.), R.A. Brown, C. Burrows (STScI), E. Cheng (GSFC), M. Clampin (STScI), N. Cross, P.D. Feldman, H.C. Ford (JHU), M. Franx (Leiden Obs.), D.A. Golimowski (JHU), C. Gronwall (Pennsylvania State Univ.), G. Hartig (STScI), G.D. Illingworth (UCO/Lick Obs.), L. Infante (Pontificia Univ. Catolica de Chile), R.A. Kimble (GSFC), J.E. Krist (STScI), M. Lesser (Steward Obs.), A.R. Martel, F. Menanteau, G.R. Meurer (JHU), M. Postman (STScI), P. Rosati (ESO), M. Sirianni (JHU), W.B. Sparks (STScI), H.D. Tran (JHU), R.L. White (STScI), W. Zheng (JHU), ACS Science Team

We will present a preliminary analysis of Advanced Camera for Surveys imaging observations of the (proto-)cluster environment around radio galaxy TN1338-1942 at z=4.1. This structure was discovered by searching for Ly\alpha emitting galaxies around luminous, high-redshift radio galaxies in a large VLT program, and is presently the most distant structure of galaxies known.

We obtained ACS imaging of this cluster field at g-band (4 orbits), r-band (4 orbits) and i-band (5 orbits) using the Wide Field Camera, as part of the ACS Guaranteed Time Observations program. These observations allow us to 1) study the morphology and colours of the 10 spectroscopically confirmed Ly\alpha emitting cluster galaxies, and 2) probe an entirely different population of possible cluster galaxies selected using the Lyman-break technique. We will present and discuss our findings of an anomalously high number of g-band dropouts (~50 objects having IAB<26 and g-r>1.5), which additionally seem to be strongly clustered towards the radio galaxy. The field around radio galaxy TN1338--1942 provides us with a unique laboratory for the study of structure formation in an overdense region of the early Universe.

ACS was developed under NASA contract NAS 5-32865, and this research is supported by NASA grant NAG5-7697. We are grateful for an equipment grant from the Sun Microsystems, Inc.

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Bulletin of the American Astronomical Society, 34, #4
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