Gaia 1 and 2. A pair of new Galactic star clusters (1702.01122v2)

Abstract: 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$\times$10$^4$ M$_\odot$), large ($\sim$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.

Overview of Gaia 1 and 2: Newly Identified Galactic Star Clusters

In this paper, Koposov et al. present the discovery of two new star clusters in the Milky Way, named Gaia 1 and Gaia 2, using data from the European Space Agency's Gaia mission. The authors conducted an exhaustive search for faint satellites in Gaia's Data Release 1 (DR1), leveraging its high angular resolution and astrometric capabilities. They showed that Gaia can effectively detect stellar overdensities, even in regions influenced by systematic survey biases or dominated by bright foreground stars.

Detection and Analysis

The search utilized the GaiaSource catalog, focusing solely on stellar positions and employing kernel density estimation techniques to identify potential satellite candidates across the sky. The absence of photometric color data in DR1 necessitated a modified approach to confirming star-like objects, relying heavily on parameters such as astrometric excess noise for star-galaxy separation.

Of notable success in their endeavor was the re-detection of several known ultra-faint dwarf galaxies, including Canes Venatici I and Bootes I, affirming the robustness of Gaia's detection capabilities. However, the most striking outcomes of their survey were the identifications of Gaia 1 and Gaia 2, which had eluded previous surveys despite their apparent brightness and mass.

Properties of Gaia 1

Gaia 1 emerged as particularly intriguing due to its mass (approximately $2.2 \times 10^4$ M$_\odot$), size (~9 pc), and proximity (~4.6 kpc), located just 10 arcminutes away from Sirius, the brightest star in the sky. Gaia's ability to detect this cluster so close to Sirius is attributed to its unique multi-epoch astrometric methodology, which efficiently filters out artifacts caused by saturated CCD images of bright stars.

The analysis of Gaia 1 relied on both Gaia and ancillary photometry from 2MASS and Pan-STARRS1 (PS1) datasets. The object shows signs typical of a globular or old open cluster, with an age estimated at 6.3 Gyr and metallicity around $\text{[Fe/H]} = -0.7$. The available PS1 data allowed for a clear observation of its red clump and turn-off stars, vital for constraining the cluster's distance and age.

Characteristics of Gaia 2

Gaia 2, positioned close to the Galactic plane, was observed with a significance of 9, demonstrating its standout presence even in a densely populated stellar field. The PS1 data were critical in confirming its nature, revealing a main sequence that corroborated its cluster status. Preliminary parameters suggest an age of 8 Gyr and $\text{[Fe/H]} = -0.6$, indicating it too may be an old globular cluster or enrichment-enhanced open cluster.

The structural analysis using Gaia data shows Gaia 2 possesses a half-light radius of 1.9 arcminutes (~3 pc); this compact nature, alongside its stellar population traits, suggests it as an interesting target for further detailed observation and paper in subsequent Gaia releases.

Implications and Future Perspectives

These discoveries highlight Gaia's unmatched potency in detecting Galactic satellites, even in the face of challenging observing conditions. The authors convincingly argue that Gaia's ability to eliminate false detections caused by variable sky brightness and poor weather conditions is instrumental in unveiling new celestial objects hidden in plain sight. Gaia's astrometric dexterity, especially in regions affected by bright stars like Sirius, opens up prospects for discovering more such satellites in future Gaia data releases. As Gaia continues to improve its catalog completeness and introduces additional photometric and motion-based identification criteria, the potential for identifying novel and diverse Galactic structures is exciting.

In conclusion, the work by Koposov et al. lays a foundation for future investigations into similar hidden star clusters, reaffirming Gaia's position as a pivotal instrument for Galactic astronomy. These discoveries prompt a reassessment of galactic structure models and our understanding of star cluster distributions within the Milky Way.