The structure of Andromeda II dwarf spheroidal galaxy

TL;DR

This study reveals that Andromeda II's two stellar populations differ in spatial distribution and kinematics, suggesting a complex formation history possibly involving a merger around redshift 1.75.

Contribution

It provides a detailed analysis of the spatial and kinematic properties of two stellar populations in Andromeda II, highlighting their differences and implications for galaxy formation.

Findings

Old stars are round and rotate around the major axis.

Intermediate-age stars are centrally concentrated and rotate around the minor axis.

The populations' different centers of rotation suggest a merger history.

Abstract

We analyze in detail the spatial distribution and kinematic properties of two different stellar populations in Andromeda II (And II) dwarf spheroidal galaxy. We obtained their detailed surface density maps, together with their radial density profiles. The two populations differ not only in age and metallicity, but also in their spatial distribution and kinematics. Old stars ($\gtrsim 11$ Gyr) follow a round distribution well fitted by truncated density profiles. These stars rotate around the projected optical major axis of the galaxy with line-of-sight velocities $v_{los}(r_h) = 16 \pm 3$ km s$^{-1}$ and a velocity gradient of $2.06 \pm 0.21$ km s$^{-1}$ arcmin$^{-1}$. Intermediate-age stars ($\lesssim 9$ Gyr) concentrate in the centre of the galaxy and form an elongated structure extending along the projected optical major axis. This structure appears to rotate with a velocity gradient of $2.24 \pm 0.22$ km s$^{-1}$ arcmin$^{-1}$, and around the optical minor axis. The centres of rotation and kinetic position angles (PA$_{\rm kin}$) of both populations differ. For intermediate-age stars we obtained PA$_{\rm kin} = 18^\circ \pm 2^\circ$ and for the old ones PA$_{\rm kin} = 63^\circ \pm 3^\circ$ in good agreement with photometric PA measured from isopleths fitted to the photometry. We conclude that the two stellar populations may not be fully relaxed and thus be the result of a merger occurred at redshift $z\sim 1.75$.