Identification of Type Ia Supernovae at Redshift 1.3 and Beyond with the Advanced Camera for Surveys on HST

Adam G. Riess, Louis-Gregory Strolger, John Tonry, Zlatan Tsvetanov, Stefano Casertano, Henry C. Ferguson, Bahram Mobasher, Peter Challis, Nino Panagia, Alexei V. Filippenko, Weidong Li, Ryan Chornock, Robert P. Kirshner, Bruno Leibundgut, Mark Dickinson, Anton Koekemoer, Norman A. Grogin, Mauro Giavalisco

Published 2003-08-11Version 1

We present discoveries of SNe Ia at z > 1 and the photometric diagnostic used to discriminate them from other types of SNe detected during the GOODS Hubble Space Telescope Treasury program with the Advanced Camera for Surveys (ACS). Photometric redshift measurements of the hosts combined with deep f606w, f775w, and f850lp imaging discriminates hydrogen-rich SNe II from SNe I at z > 1 by exploiting the ultraviolet (UV) deficit in the energy distributions of SNe Ia. This sorting allows rapid follow-up of space-based discoveries. Subsequent spectroscopy of 11 GOODS SNe Ia obtained from the ground and with the grism on ACS confirmed the reliability of our photometric screening. We present the highest-redshift spectrum of any supernova published to date, SN Ia 2002fw at z = 1.3 observed near maximum brightness with the ACS grism. The lack of UV flux for true SNe Ia provides an effective tool for our ongoing efforts to build a sample of SNe Ia at 1.2 < z < 1.8 which will extend the useful range of the magnitude-redshift relation of SNe Ia.