I report the discovery of a stellar stream (Sutlej) using Gaia DR3 (third data release) proper motions and XP metallicities located $\sim$15° north of the Small Magellanic Cloud (SMC). The stream is composed of two parallel linear components (‘branches’) approximately $\sim$8° × 0.6° in size and separated by 2.5°. The stars have a mean proper motion of ($\mu _{\rm RA},\mu _{\rm Dec.}$) = (+0.08 mas yr−1, −1.41 mas yr−1), which is quite similar to the proper motion of stars on the western side of the SMC. The colour–magnitude diagram of the stream stars has a clear red giant branch, horizontal branch, and main-sequence turn-off that are well matched by a parsec isochrone of 10 Gyr, [Fe/H] = −1.8 at 32 kpc, and a total stellar mass of $\sim$33 000 M$_{\odot }$. The stream is spread out over an area of 9.6 deg2 and has a surface brightness of 32.5 mag arcsec−2. The metallicity of the stream stars from Gaia XP spectra extends over $-2.5$$\le$ [M/H] $\le$$-1.0$ with a median of [M/H] = −1.8. The tangential velocity of the stream stars is 214 km s−1 compared to the values of 448 km s−1 for the Large Magellanic Cloud and 428 km s−1 for the SMC. While the radial velocity of the stream is not yet known, a comparison of the space velocities using a range of assumed radial velocities shows that the stream is unlikely to be associated with the Magellanic Clouds. The tangential velocity vector is misaligned with the stream by nearly 90°, which might indicate an important gravitational influence from the nearby Magellanic Clouds.
The Milky Way halo is one of the few galactic haloes that provides a unique insight into galaxy formation by resolved stellar populations. Here, we present a catalogue of ∼47 million halo stars selected independent of parallax and line-of-sight velocities, using a combination of Gaia DR3 proper motion and photometry by means of their reduced proper motion. We select high tangential velocity (halo) main sequence stars and fit distances to them using their simple colour-absolute-magnitude relation. This sample reaches out to ∼21 kpc with a median distance of 6.6 kpc thereby probing much further out than would be possible using reliable Gaia parallaxes. The typical uncertainty in their distances is $0.57_{-0.26}^{+0.56}$ kpc. Using the colour range 0.45 < (G0 − GRP, 0) < 0.715, where the main sequence is narrower, gives an even better accuracy down to $0.39_{-0.12}^{+0.18}$ kpc in distance. The median velocity uncertainty for stars within this colour range is 15.5 km s−1. The distribution of these sources in the sky, together with their tangential component velocities, are very well-suited to study retrograde substructures. We explore the selection of two complex retrograde streams: GD-1 and Jhelum. For these streams, we resolve the gaps, wiggles and density breaks reported in the literature more clearly. We also illustrate the effect of the kinematic selection bias towards high proper motion stars and incompleteness at larger distances due to Gaia’s scanning law. These examples showcase how the full RPM catalogue made available here can help us paint a more detailed picture of the build-up of the Milky Way halo.
more » « less- PAR ID:
- 10401844
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- Volume:
- 521
- Issue:
- 2
- ISSN:
- 0035-8711
- Page Range / eLocation ID:
- p. 2087-2102
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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