H$α$ emission in the outskirts of galaxies at z=0.4

Abstract

This paper reports detections of H$α$ emission and stellar continuum out to approximately 30 physical kpc, and H$α$ directionality in the outskirts of H$α$-emitting galaxies (H$α$ emitters) at $z=0.4$. This research adopts narrow-band selected H$α$ emitters at $z=0.4$ from the emission-line object catalog by Hayashi et al. (2020), which is based on data in the Deep and Ultradeep layers of the Hyper Suprime-Cam Subaru Strategic Program. Deep narrow- and broad-band images of 8625 H$α$ emitters across 16.8 deg$^2$ enable us to construct deep composite emission-line and continuum images. The stacked images show diffuse H$α$ emission (down to $\sim5\times10^{-20}$ erg~s$^{-1}$cm$^{-2}$arcsec$^{-2}$) and stellar continuum (down to $\sim5\times10^{-22}$ erg~s$^{-1}$cm$^{-2}$A$^{-1}$arcsec$^{-2}$), extending beyond 10 kpc at stellar masses $>10^9$ M$_\odot$, parts of which may originate from stellar halos. Those radial profiles are broadly consistent with each other. In addition, we obtain a dependence of the H$α$ emission on the position angle because relatively higher H$α$ equivalent width has been detected along the minor-axis towards galaxy disks. While the H$α$ directionality could be attributed to biconical outflows, further research with hydrodynamic simulations is highly demanded to pin down the exact cause.