A High-Resolution Hubble Space Telescope Study of Apparent Lyman Continuum Leakers at z~3
We present U_(336)V_(606)J_(125)H_(160) follow-up Hubble Space Telescope (HST) observations of 16 z ~ 3 candidate Lyman continuum (LyC) emitters in the HS1549+1919 field. With these data, we obtain high spatial-resolution photometric redshifts of all sub-arcsecond components of the LyC candidates in order to eliminate foreground contamination and identify robust candidates for leaking LyC emission. Of the 16 candidates, we find one object with a robust LyC detection that is not due to foreground contamination. This object (MD5) resolves into two components; we refer to the LyC-emitting component as MD5b. MD5b has an observed 1500 Å to 900 Å flux-density ratio of (F_(UV)/F_(LyC)_(obs) = 4.0 ± 2.0, compatible with predictions from stellar population synthesis models. Assuming minimal IGM absorption, this ratio corresponds to a relative (absolute) escape fraction of f_(esc,rel)^(MD5b) = 75%–100% f_(esc,abs)^(MD5b) = 14%–19%). The stellar population fit to MD5b indicates an age of ≾50 Myr, which is in the youngest 10% of the HST sample and the youngest third of typical z ~ 3 Lyman break galaxies, and may be a contributing factor to its LyC detection. We obtain a revised, contamination-free estimate for the comoving specific ionizing emissivity at z = 2.85, indicating (with large uncertainties) that star-forming galaxies provide roughly the same contribution as QSOs to the ionizing background at this redshift. Our results show that foreground contamination prevents ground-based LyC studies from obtaining a full understanding of LyC emission from z ~ 3 star-forming galaxies. Future progress in direct LyC searches is contingent upon the elimination of foreground contaminants through high spatial-resolution observations, and upon acquisition of sufficiently deep LyC imaging to probe ionizing radiation in high-redshift galaxies.
© 2015 The American Astronomical Society. Received 2015 June 24; accepted 2015 July 31; published 2015 September 3. We thank Anahita Alavi and Eros Vanzella for helpful discussions about the HST imaging reduction and photometry. Support for program GO-12959 was provided by NASA through a grant from the Space Telescope Science Institute (STScI), which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. A. E. S. acknowledges additional support from the David and Lucile Packard and Sloan Foundations, C.C.S. acknowledges support from the NSF grants AST-0908805 and AST-1313472, as well as STScI grants GO-11638.01 and GO-11694.02, and R.E.M. and A.E.S. acknowledge the generous support of Mr. Richard Kaplan. R.F.T. receives funding from the Miller Institute for Basic Research in Science at U.C. Berkeley. N.A.R. acknowledges support from the Sloan Foundation. We wish to extend special thanks to those of Hawaiian ancestry on whose sacred mountain we are privileged to be guests. Without their generous hospitality, most of the observations presented herein would not have been possible.
Published - Mostardi_2015.pdf
Accepted Version - 1506.08201v2.pdf