Study of structural parameters and systemic proper motion of Sextans dwarf spheroidal galaxy with Subaru Hyper Suprime-Cam data

We use the Subaru Hyper Suprime-Cam (HSC) data to study structural parameters and systemic proper motion of the Sextans dwarf spheroidal galaxy at the heliocentric distance of 86 kpc, which is one of the most important targets for studies of dark matter nature and galaxy formation physics. Thanks to the superb image quality and wide area coverage of the Sextans field, the HSC data enables a secure selection of member star candidates based on the colour-magnitude cut, yielding about 10,000 member candidates at magnitudes down to $i\sim 24$. We use a likelihood analysis of the two-dimensional distribution of stars to estimate the structural parameters of Sextans taking into account the contamination of foreground halo stars in the Milky Way, and find that the member star distribution is well-fitted by an elliptical King profile with ellipticity $\epsilon \simeq 0.25$ and the core and tidal radii of $R_c=(368.4\pm 8.5)$ pc and $R_t=(2.54\pm 0.046)$ kpc, respectively. Then using the two HSC datasets of 2.66 years time baseline on average, we find the systemic proper motions of Sextans to be $(\mu_\alpha, \mu_\delta)=(-0.448\pm{0.075},0.083\pm{0.078})$ $\mathrm{mas}\ \mathrm{yr}^{-1}$, which is consistent with some of the previous works using the $Gaia$ data of relatively bright member stars in Sextans. Thus, our results give a demonstration that a ground-based, large-aperture telescope data which covers a wide solid angle of the sky and has a long time baseline, such as the upcoming LSST data, can be used to study systemic proper motions of dwarf galaxies.