The power of teaming up HST and Gaia: the first proper motion measurement of the distant cluster NGC 2419

TL;DR

This paper combines HST and Gaia data to measure the proper motion of the distant globular cluster NGC 2419, revealing its orbit and suggesting it originated from the Sagittarius dwarf galaxy.

Contribution

It provides the first proper motion measurement of NGC 2419 using combined HST and Gaia data, and models its orbit in the Milky Way.

Findings

NGC 2419's proper motion is measured as (-0.17±0.26, -0.49±0.17) mas/yr.

The cluster's orbit oscillates between 53 and 98 kpc on a nearly polar path.

NGC 2419 likely originated from the Sagittarius dwarf galaxy.

Abstract

Aims: We present the first measurement of the proper motion and orbit of the very distant and intriguing globular cluster NCG 2419. Methods: We have combined data from HST and Gaia DR1 to derive the relative proper motions of stars in the direction to the cluster. To tie to an absolute reference frame we have used a background galaxy located in the field. Results: We find the absolute proper motion of NGC 2419 to be $(\mu_{\alpha}\cos(\delta)$, $\mu_{\delta}$)=($-0.17\pm0.26,-0.49\pm0.17$) mas/yr. We have integrated the orbit of the cluster in a Galactic potential and found it to oscillate between $\sim$53 kpc and $\sim$98 kpc on a nearly polar orbit. This makes it very likely that NGC 2419 is a former cluster of the Sagittarius dwarf spheroidal galaxy, also because it shares the same sense of rotation around the Milky Way.