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T Pyxidis is the only recurrent nova surrounded by knots of material ejectedin previous outbursts. Following the eruption that began on 2011 April 14.29,we obtained seven epochs (from 4 to 383 days after eruption) of Hubble SpaceTelescope narrowband Ha images of T Pyx . The flash of radiation from the novaevent had no effect on the ejecta until at least 55 days after the eruptionbegan. Photoionization of hydrogen located north and south of the central starwas seen 132 days after the beginning of the eruption. That hydrogen recombinedin the following 51 days, allowing us to determine a hydrogen atom density ofat least 7e5 cm^-3 - at least an order of magnitude denser than the previouslydetected, unresolved [NII] knots surrounding T Pyx. Material to the northwestand southeast was photoionized between 132 and 183 days after the eruptionbegan. 99 days later that hydrogen had recombined. Both then (282 days afteroutburst) and 101 days later, we detected almost no trace of hydrogen emissionaround T Pyx. There is a large reservoir of previously unseen, cold diffusehydrogen overlapping the previously detected, [NII] - emitting knots of T Pyxejecta. The mass of this newly detected hydrogen is probably an order ofmagnitude larger than that of the [NII] knots. We also determine that there isno significant reservoir of undetected ejecta from the outer boundaries of thepreviously detected ejecta out to about twice that distance, near the plane ofthe sky. The lack of distant ejecta is consistent with the Schaefer et al(2010) scenario for T Pyx, in which the star underwent its first eruptionwithin five years of 1866 after many millennia of quiescence, followed by thesix observed recurrent nova eruptions since 1890. This lack of distant ejectais not consistent with scenarios in which T Pyx has been erupting continuouslyas a recurrent nova for many centuries or millennia.

HST Images Flash Ionization of Old Ejecta by the 2011 Eruption of Recurrent Nova T Pyxidis