oMEGACat II -- Photometry and proper motions for 1.4 million stars in Omega Centauri and its rotation in the plane of the sky

Abstract: Omega Centauri ($\omega$ Cen) is the most massive globular cluster of the Milky Way. It is thought to be the nucleus of an accreted dwarf galaxy because of its high mass and its complex stellar populations. To decipher its formation history and study its dynamics, we created the most comprehensive kinematic catalog for its inner region, by analyzing both archival and new Hubble Space Telescope (HST) data. Our catalog contains 1 395 781 proper-motion measurements out to the half-light radius of the cluster ($\sim$5.0') and down to $m_{F625W}\approx$25. The typical baseline for our proper-motion measurements is 20 years, leading to a median 1D proper motion precision of $\sim$11 $\mu$as yr$^{-1}$ for stars with $m_{F625W}\approx$18 mag, with even better precision ($\sim$6.6 $\mu$as yr$^{-1}$) achieved in the extensively observed centermost (r$<$1.5') region. In addition to our astrometric measurements, we also obtained precise HST photometry in seven filters spanning from the ultraviolet to the near-infrared. This allows detailed color-magnitude-diagram studies and to separate the multiple stellar populations of the cluster. In this work, we describe the data reduction used to obtain both the photometric and the proper-motion measurements. We also illustrate the creation and the content of our catalog, which is made publicly available. Finally, we present measurements of the plane-of-sky rotation of $\omega$ Cen in the previously unprobed inner few arcminutes and a precise measurement of the inclination $i = (43.9\pm1.3)^\circ$.

• The paper compiles an extensive catalog of 1.4 million stars with long baseline astrometry, achieving median precision as fine as 11 µas/yr.
• The paper employs HST photometry across seven filters to construct detailed color-magnitude diagrams that uncover multiple stellar populations.
• The paper measures Omega Centauri’s rotational dynamics in the plane of the sky, determining an inclination angle of 43.9° ±1.3°.

oMEGACat II: Photometric and Proper Motion Analysis of Omega Centauri Stars and Determination of Its Rotational Dynamics

The study of Omega Centauri, the most massive globular cluster in the Milky Way, has long intrigued astronomers due to its unique characteristics, suggesting it could be the nucleus of a once larger accreted dwarf galaxy. The paper discussed here provides a comprehensive analysis of 1.4 million stars in Omega Centauri, utilizing astrometric and photometric data gathered from both archival and new Hubble Space Telescope (HST) observations. This work is part of the ongoing oMEGACat project, which aims to unravel the formation history and dynamics of the cluster.

The catalog produced in this study is the most extensive to date, covering the core regions of Omega Centauri, which had remained uncharted territory for proper motion measurements in previous studies. The paper highlights several significant findings:

1. Astrometric Precision and Depth: The catalog includes 1,395,781 proper-motion measurements with a typical measurement baseline of 20 years. This long baseline allows the study to achieve a median 1D proper motion precision of approximately 11 µas yr$^{-1}$ for stars around 18th magnitude in the F625W filter. In the center where observational data is most dense, precision improves to 6.6 µas yr$^{-1}$.
2. Photometric Coverage: The study offers precise HST photometric measurements across seven filters ranging from ultraviolet to near-infrared. Importantly, this enables detailed color-magnitude diagram (CMD) analyses capable of distinguishing the multiple stellar populations of the cluster.
3. Catalog Availability: The full set of data has been made publicly available, providing a valuable resource for the astronomical community.
4. Rotational Dynamics: For the first time, this work presents measurements of the rotation of Omega Centauri in the plane of the sky, especially focusing on previously unprobed inner few arcminutes. Moreover, an inclination angle of $43.9^\circ$ with an uncertainty of $1.3^\circ$ was precisely determined, corroborating theoretical models predicting the internal dynamics of massive star clusters.

The implications of these findings are significant for both practical and theoretical astrophysics. Practically, the data sets a new standard for studying the dynamics of star clusters using proper motion data, which can potentially lead to understanding the interactions and accretion events in the Milky Way's history. Theoretically, this enhanced kinematic understanding of Omega Centauri may provide insights into the formation and evolution of globular clusters and potentially reveal the processes involved in the stripping of dwarf galaxies.

As the data here has been meticulously collected and is incredibly detailed, further studies could look into the individual stellar pathways and evolutionary status, offering insights into stellar dynamics within such a densely packed system. This foundational study will likely spur additional research, potentially utilizing similar methodologies to investigate other complex stellar systems.

In summary, this paper marks a substantial contribution to the field of astrophysical research through its comprehensive dataset and refined analysis of Omega Centauri, paving the way for further exploration and discovery in galactic astronomy.