Sergey E. Koposov

Gaia 1 and 2. A pair of new Galactic star clusters

We present the results of the very first search for faint Milky Way satellites in the Gaia data. Using stellar positions only, we are able to re-discover objects detected in much deeper data as recently as the last couple of years. While we do not identify new prominent ultra-faint dwarf galaxies, we report the discovery of two new star clusters, Gaia 1 and Gaia 2. Gaia 1 is particularly curious, as it is a massive (2.2

The slight spin of the old stellar halo

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The first all-sky view of the Milky Way stellar halo with Gaia+2MASS RR Lyrae

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Discovery of two neighbouring satellites in the Carina constellation with MagLiteS

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Snake in the Clouds: a new nearby dwarf galaxy in the Magellanic bridge*

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The VISTA ZYJHKs photometric system: Calibration from 2MASS

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The Discovery of Tidal Tails around the Globular Cluster NGC 7492 with Pan-STARRS1

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Unmixing the Galactic Halo with RR Lyrae tagging

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Co-formation of the disc and the stellar halo

9 pc) and nearby (4.6 kpc) cluster, situated 10 away from the brightest star on the sky, Sirius! Even though this satellite is detected at significance in excess of 10, it was missed by previous sky surveys. We conclude that Gaia possesses powerful and unique capabilities for satellite detection thanks to its unrivalled angular resolution and highly efficient object classification.

The Sausage Globular Clusters

We combine Gaia data release 1 astrometry with Sloan Digital Sky Survey (SDSS) images taken some ∼10–15 years earlier, to measure proper motions of stars in the halo of our Galaxy. The SDSS–Gaia proper motions have typical statistical errors of 2u2009mas yr−1 down to r ∼ 20 mag, and are robust to variations with magnitude and colour. Armed with this exquisite set of halo proper motions, we identify RR Lyrae, blue horizontal branch (BHB), and K giant stars in the halo, and measure their net rotation with respect to the Galactic disc. We find evidence for a gently rotating prograde signal (〈Vϕ〉 ∼ 5–25u2009kmu2009s−1) in the halo stars, which shows little variation with Galactocentric radius out to 50 kpc. The average rotation signal for the three populations is 〈Vϕ〉 = 14 ± 2 ± 10 (syst.) km s−1. There is also tentative evidence for a kinematic correlation with metallicity, whereby the metal richer BHB and K giant stars have slightly stronger prograde rotation than the metal poorer stars. Using the Auriga simulation suite, we find that the old (T >10 Gyr) stars in the simulated haloes exhibit mild prograde rotation, with little dependence on radius or metallicity, in general agreement with the observations. The weak halo rotation suggests that the Milky Way has a minor in situ halo component, and has undergone a relatively quiet accretion history.