An Overdensity of i-dropouts Among A Population of Excess Field Objects in the Virgo Cluster

Abstract

Using a set of deep imaging data obtained by the ACS on the HST shortly after its deployment, Yan, Windhorst & Cohen (2003) found a large number of F775W-band dropouts (i-dropouts), which are consistent with being galaxies at z~6. The surface density of i-dropouts thus derived, however, is an order of magnitude higher than those subsequent studies found in other deep ACS fields, including the HUDF. Here we revisit this problem, using both the existing and the new data. We confirm that the large overdensity of i-dropouts does exist in this field, and that their optical-to-IR colors are similar to those in the HUDF. However, we have discovered that the i-dropout overdensity is accompanied with an even larger excess of faint field objects in this region and its vicinity. This large excess of field objects is most likely caused by the tidal ``debris'' and/or halo stars related to an interacting galaxy pair in the Virgo Cluster, M60/NGC4647, which lies several arcminutes away from the region where the excess is found. This excess population is also red in color, and the red wing of its color distribution continuously extends to the regime where the i-dropouts reside. While we still cannot completely rule out the possibility that the overdensity of i-dropouts might be a genuine large-scale structure of galaxies at z~6, we prefer the interpretation that most of them are part of the excess stellar population related to M60/NGC4647. Future spectroscopic work will be needed to identify the nature of this i-dropout overdensity. (Abridged)