Astronomers use observations of a gravitationally lensed galaxy to measure the properties of the early universe October 26, 2016 by Sean Nealon Please Register or Log in to view the hidden image! The Cosmic Horseshoe, as photographed by the Hubble Space Telescope. Credit: ESA/Hubble & NASA. Although the universe started out with a bang it quickly evolved to a relatively cool, dark place. After a few hundred thousand years the lights came back on and scientists are still trying to figure out why. Astronomers know that reionization made the universe transparent by allowing light from distant galaxies to travel almost freely through the cosmos to reach us. However, astronomers don't fully understand the escape rate of ionizing photons from early galaxies. That escape rate is a crucial, but still a poorly constrained value, meaning there are a wide range of upper and lower limits in the models developed by astronomers. Read more at: http://phys.org/news/2016-10-astronomers-gravitationally-lensed-galaxy-properties.html#jCp
http://iopscience.iop.org/article/1...DAB7052CFB96C0C8800.c3.iopscience.cld.iop.org THE LYMAN CONTINUUM ESCAPE FRACTION OF THE COSMIC HORSESHOE: A TEST OF INDIRECT ESTIMATES Abstract High-redshift star-forming galaxies are likely responsible for the reionization of the universe, yet direct detection of their escaping ionizing (Lyman continuum [LyC]) photons has proven to be extremely challenging. In this study, we search for escaping LyC of the Cosmic Horseshoe, a gravitationally lensed, star-forming galaxy at z = 2.38 with a large magnification of ~24. Transmission at wavelengths of low-ionization interstellar absorption lines in the rest-frame ultraviolet suggests a patchy, partially transparent interstellar medium. This makes it an ideal candidate for direct detection of the LyC. We obtained a 10-orbit Hubble near-UV image using the Wide Field Camera 3 (WFC3)/UVIS F275W filter that probes wavelengths just below the Lyman limit at the redshift of the Horseshoe in an attempt to detect escaping LyC radiation. After fully accounting for the uncertainties in the opacity of the intergalactic medium (IGM) and accounting for the charge transfer inefficiency in the WFC3 CCDs, we find a Please Register or Log in to view the hidden image! upper limit for the relative escape fraction of Please Register or Log in to view the hidden image!. This value is a factor of five lower than the value (0.4) predicted by the 40% transmission in the low-ion absorption lines. Though possible, it is unlikely that the nondetection is due to a high-opacity line of sight through the IGM (Please Register or Log in to view the hidden image!% chance). We discuss several possible causes for the discrepancy between the escape fraction and the covering fraction and consider the implications for future attempts at both direct LyC detection and indirect estimates of the escape fraction.
