Lyα photons scattered by neutral hydrogen atoms in the circumgalactic media or produced in the halos of star-forming galaxies are expected to lead to extended Lyα emission around galaxies. Such low surface brightness Lyα halos (LAHs) have been detected by stacking Lyα images of high-redshift star-forming galaxies. We study the origin of LAHs by performing radiative transfer modeling of nine z=3.1 Lyman-Alpha Emitters (LAEs) in a high resolution hydrodynamic cosmological galaxy formation simulation. We develop a method of computing the mean Lyα surface brightness profile of each LAE by effectively integrating over many different observing directions. Without adjusting any parameters, our model yields an average Lyα surface brightness profile in remarkable agreement with observations. We find that observed LAHs cannot be accounted for solely by photons originating from the central LAE and scattered to large radii by hydrogen atoms in the circumgalactic gas. Instead, Lyα emission from regions in the outer halo is primarily responsible for producing the extended LAHs seen in observations, which potentially includes both star-forming and cooling radiation. With the limit on the star formation contribution set by the ultra-violet (UV) halo measurement, we find that cooling radiation can play an important role in forming the extended LAHs. We discuss the implications and caveats of such a picture.
@article{arxiv.1502.01349,
title = {On the Diffuse Lyman-alpha Halo Around Lyman-alpha Emitting Galaxies},
author = {Ethan Lake and Zheng Zheng and Renyue Cen and Raphael Sadoun and Rieko Momose and Masami Ouchi},
journal= {arXiv preprint arXiv:1502.01349},
year = {2015}
}
Comments
11 pages, 8 figures, accepted for publication in ApJ. Our conclusion is slightly updated as a result of an improved evaluation of the sky-subtraction systematics in the observed UV profile. The diffuse Lya halo can have contributions from both star formation and cooling radiation