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

TL;DR

This study investigates the physical origin of diffuse Lyα halos around star-forming galaxies at z~2, finding they are mainly caused by Lyα photons escaping from the main galaxy body and scattering in the circum-galactic medium, challenging previous models.

Contribution

It provides observational evidence that Lyα halos are primarily due to photon scattering rather than cold streams or satellite star formation, using SED fitting and clustering analysis.

Findings

LAHs are mainly caused by Lyα photon scattering in the circum-galactic medium.

LAH luminosity remains almost unchanged with stellar mass and halo mass.

Lyα escape fractions are about ten times higher than in Hα emitters with similar properties.

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 ($L({\rm Ly}\alpha)_H$) on the stellar mass ($M_\star$), $SFR$, color excess ($E(B-V)_\star$), and dark matter halo mass ($M_{\rm h}$) of the parent galaxy for $\sim 900$ Ly$\alpha$ emitters (LAEs) at $z\sim2$ divided into ten subsamples. We calculate $L({\rm Ly}\alpha)_H$ using the stacked observational relation between $L({\rm Ly}\alpha)_H$ 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 $L({\rm Ly}\alpha)_H$ despite low $M_\star$ and $M_{\rm h}$, and that $L({\rm Ly}\alpha)_H$ remains almost unchanged with $M_\star$ and perhaps with $M_{\rm h}$. These results are incompatible with the cold stream (cooling radiation) scenario and the satellite-galaxy star-formation scenario, because the former predicts fainter $L({\rm Ly}\alpha)_H$ and both predict steeper $L({\rm Ly}\alpha)_H$ 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 $M_{\rm h}$ at fixed $M_\star$, or from another Ly$\alpha$ source in the central part.