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

Abstract

This paper reports detections of H$\alpha$ emission and stellar continuum out to approximately 30 physical kpc, and H$\alpha$ directionality in the outskirts of H$\alpha$-emitting galaxies (H$\alpha$ emitters) at $z=0.4$. This research adopts narrow-band selected H$\alpha$ emitters at $z=0.4$ from the emission-line object catalog by Hayashi2020, 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$\alpha$ emitters across 16.8 deg² enable us to construct deep composite emission-line and continuum images. The stacked images show diffuse H$\alpha$ emission (down to $\sim 5\times 10^{-20}$ erg s^-1cm^-2arcsec^-2) and stellar continuum (down to $\sim 5\times 10^{-22}$ erg s^-1cm^-2Å^-1arcsec^-2), extending beyond 10 kpc at stellar masses $>10^{9}$ M_⊙, 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$\alpha$ emission on the position angle because relatively higher H$\alpha$ equivalent width has been detected along the minor-axis towards galaxy disks. While the H$\alpha$ directionality could be attributed to biconical outflows, further research with hydrodynamic simulations is highly demanded to pin down the exact cause.