The dominant origin of diffuse Ly$\alpha$ halos around LAEs explored by SED fitting and clustering analysis

Abstract

The physical origin of diffuse Ly$\alpha$ halos (LAHs) around star-forming galaxies is still a matter of debate. We present the dependence of LAH luminosity ($L(\mathrm{Ly}\alpha {)}_H$) on the stellar mass ($M_{\star }$), $SFR$, color excess ($E(B-V{)}_{\star }$), and dark matter halo mass ($M_{\mathrm{h}}$) of the parent galaxy for $\sim 900$ Ly$\alpha$ emitters (LAEs) at $z\sim 2$ divided into ten subsamples. We calculate $L(\mathrm{Ly}\alpha {)}_H$ using the stacked observational relation between $L(\mathrm{Ly}\alpha {)}_H$ and central Ly$\alpha$ luminosity by Momose et al. (2016), which we find agrees with the average trend of VLT/MUSE-detected individual LAEs. We find that our LAEs have relatively high $L(\mathrm{Ly}\alpha {)}_H$ despite low $M_{\star }$ and $M_{\mathrm{h}}$, and that $L(\mathrm{Ly}\alpha {)}_H$ remains almost unchanged with $M_{\star }$ and perhaps with $M_{\mathrm{h}}$. These results are incompatible with the cold stream (cooling radiation) scenario and the satellite-galaxy star-formation scenario, because the former predicts fainter $L(\mathrm{Ly}\alpha {)}_H$ and both predict steeper $L(\mathrm{Ly}\alpha {)}_H$ vs. $M_{\star }$ slopes. We argue that LAHs are mainly caused by Ly$\alpha$ photons escaping from the main body and then scattered in the circum-galactic medium. This argument is supported by LAH observations of H$\alpha$ emitters (HAEs). When LAHs are taken into account, the Ly$\alpha$ escape fractions of our LAEs are about ten times higher than those of HAEs with similar $M_{\star }$ or $E(B-V{)}_{\star }$, which may partly arise from lower HI gas masses implied from lower $M_{\mathrm{h}}$ at fixed $M_{\star }$, or from another Ly$\alpha$ source in the central part.

Published in PASJ; 35 pages, 13 figures, 6 tables

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