The not so simple globular cluster $\omega$ Cen. I. Spatial distribution of the multiple stellar populations

Abstract

We present a multi-band photometric catalog of $\approx$ 1.7 million cluster members for a field of view of $\approx$ 2x2 degree across $\omega$ Cen. Photometry is based on images collected with the Dark Energy Camera on the 4m Blanco telescope and the Advanced Camera for Surveys on the Hubble Space Telescope. The unprecedented photometric accuracy and field coverage allowed us for the first time to investigate the spatial distribution of $\omega$ Cen multiple populations from the core to the tidal radius, confirming its very complex structure. We found that the frequency of blue main-sequence stars is increasing compared to red main-sequence stars starting from a distance of $\approx$ 25' from the cluster center. Blue main-sequence stars also show a clumpy spatial distribution, with an excess in the North-East quadrant of the cluster pointing towards the direction of the Galactic center. Stars belonging to the reddest and faintest red-giant branch also show a more extended spatial distribution in the outskirts of $\omega$ Cen, a region never explored before. Both these stellar sub-populations, according to spectroscopic measurements, are more metal-rich compared to the cluster main stellar population. These findings, once confirmed, make $\omega$ Cen the only stellar system currently known where metal-rich stars have a more extended spatial distribution compared to metal-poor stars. Kinematic and chemical abundance measurements are now needed for stars in the external regions of $\omega$ Cen to better characterize the properties of these sub-populations.

20 pages, 17 figures, accepted for publication in The Astronomical Journal

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• On a new method to estimate distance, reddening and metallicity of RR Lyrae stars using optical/near-infrared ($B$,$V$,$I$,$J$,$H$,$K$) mean magnitudes: $\omega$ Centauri as a first test case
• Evolutionary and pulsation properties of Type II Cepheids
• SHOTGLAS I: The ultimate spectroscopic census of extreme horizontal branch stars in $\omega$ Centauri
• Hot UV-bright stars of galactic globular clusters
• The Most Metal-poor Stars in Omega Centauri (NGC 5139)
• oMEGACat III. Multi-band photometry and metallicities reveal spatially well-mixed populations within $\omega$ Centauri's half-light radius
• Horizontal branch morphology: A new photometric parametrization
• The not so simple stellar system Omega Cen. II. Evidence in support of a merging scenario
• The STREGA survey. II. Globular Cluster Palomar 12
• A stellar census in globular clusters with MUSE. A new perspective on the multiple main sequences of $\omega$ Centauri
• A DECam View of the Diffuse Dwarf Galaxy Crater II: The Colour-Magnitude Diagram
• oMEGACat. VI. Analysis of the overall kinematics of Omega Centauri in 3D: velocity dispersion, kinematic distance, anisotropy, and energy equipartition
• Habitability in the Omega Centauri Cluster
• Impact of distance determinations on Galactic structure. II. Old tracers
• Omega Centauri: weak MgH-band in red giants directly trace the helium content
• A Wide View of the Galactic Globular Cluster NGC 2808: Red Giant and Horizontal Branch Star Spatial Distributions