The globular cluster NGC7492 and the Sagittarius tidal stream: together but unmixed

TL;DR

This study uses spectroscopy to analyze stars around NGC7492, revealing its separation from the Sagittarius stream and identifying potential cluster members in tidal tails, enhancing understanding of their kinematic relationship.

Contribution

First spectroscopic analysis distinguishing NGC7492 from Sagittarius stream components, clarifying the cluster's independent kinematics and tidal tail membership.

Findings

NGC7492's systemic velocity differs from Sagittarius stream components.

Stars at ~125 km/s and ~-110 km/s are linked to Sagittarius stream.

Potential cluster members identified in NGC7492's tidal tails.

Abstract

We have derived from VIMOS spectroscopy the radial velocities for a sample of 71 stars selected from CFHT/Megacam photometry around the Galactic globular cluster NGC7492. In the resulting velocity distribution, it is possible to distinguish two relevant non-Galactic kinematic components along the same line of sight: a group of stars at $\langle{v_{\rm r}}\rangle \sim 125$km s$^{-1}$ which is compatible with the velocity of the old leading arm of the Sagittarius tidal stream, and a larger number of objects at $\langle{v_{\rm r}}\rangle \sim -110$km s$^{-1}$ that might be identified as members of the trailing wrap of the same stream. The systemic velocity of NGC7492 set at $v_{\rm r} \sim -177$km s$^{-1}$ differs significantly from that of both components, thus our results confirm that this cluster is not one of the globular clusters deposited by the Sagittarius dwarf spheroidal in the Galactic halo, even if it is immersed in the stream. A group of stars with $<v_{\rm r}> \sim -180$km s$^{-1}$ might be comprised of cluster members along one of the tidal tails of NGC7492.