[Paper Review] Omega Centauri: Nucleus of a Milky Way Dwarf Spheroidal?

Steven R. Majewski, Richard J. Patterson+5 more|arXiv (Cornell University)|Oct 14, 1999

Stellar, planetary, and galactic studies5 citations

TL;DR

The paper proposes that Omega Centauri is the remnant nucleus of a disrupted Milky Way dwarf spheroidal galaxy, based on its anomalous metallicity distribution function (MDF) and strongly retrograde, disk-coplanar orbit—unlike typical globular clusters. This model explains its wide MDF (peaking at [Fe/H] = -1.7, extending to lower abundances) and orbital dynamics via tidal stripping by the Galactic disk, with implications for thick disk formation.

ABSTRACT

We focus on two aspects of Omega Cen that are rather uncharacteristic of globular clusters: its metallicity distribution and its orbit. We use (1) a Washington (M-T_2,M)_o color-magnitude diagram containing 130,000 stars, (2) DDO51 photometry to separate out giants from field dwarfs, and (3) a mapping of (M-T_2,M-DDO51)_o colors to [Fe/H] to derive a metallicity distribution function (MDF) for Omega Cen. The MDF is corrected for field giant contamination by a radial velocity membership study of a subsample of stars spanning all abundances. As with previous studies, the MDF is very wide with a peak near [Fe/H]=-1.7 and extending to both higher and lower abundance, but with few stars having [Fe/H] >~ -1.2. The orbit of Omega Cen is strongly retrograde, has small apogalacticon and is almost coplanar with the Milky Way disk -- i.e., an orbit unlike any known for almost all Galactic globular clusters, and unique among clusters with its horizontal branch type and [Fe/H]. We propose that the MDF and orbit of Omega Cen, when considered in light of other unusual Omega Cen properties are consistent with a picture wherein this object is the remains (nucleus) of a once larger satellite dwarf galaxy that has been substantially reduced by tidal stripping. A model wherein Omega Cen faced considerable stripping by the Galactic disk can account for the cluster's current state. A present-day counterpart of this process may be the Sagittarius dSph system, for which the globular cluster M54 (which has a mass similar to Omega Cen) has been purported to be the nucleus. A larger proto-Omega Cen on a retrograde orbit plowing through the Milky Way disk may have aided thick disk formation both through the contribution of tidally stripped stars and through substantial thin disk heating.

Motivation & Objective

• To investigate why Omega Centauri exhibits properties inconsistent with typical globular clusters, particularly its broad metallicity distribution and unusual orbit.
• To determine whether the observed metallicity distribution and orbital characteristics can be explained by a galactic nucleus origin rather than a standard cluster formation scenario.
• To assess the role of tidal stripping by the Milky Way disk in shaping Omega Cen's current structure and kinematics.
• To explore the potential connection between Omega Cen and the formation of the Milky Way's thick disk through stellar stripping.

Proposed method

• Constructed a high-cadence Washington (M-T₂, M)₀ color-magnitude diagram using 130,000 stars in Omega Cen.
• Applied DDO51 photometry to distinguish red giant stars from field dwarf contaminants.
• Mapped (M-T₂, M-DDO51)₀ colors to [Fe/H] to derive the metallicity distribution function (MDF).
• Corrected the MDF for field giant contamination using radial velocity membership data across the full abundance range.
• Analyzed the orbital parameters of Omega Cen, focusing on its retrograde motion, low apogalacticon, and coplanarity with the Milky Way disk.
• Modeled the effects of tidal stripping by the Galactic disk on a progenitor dwarf spheroidal, comparing it to current Omega Cen properties.

Experimental results

Research questions

• RQ1 Can the broad metallicity distribution of Omega Cen be explained by a galactic nucleus origin rather than standard cluster formation?
• RQ2 Why does Omega Cen have a retrograde orbit with low apogalacticon and disk coplanarity—characteristics unlike other globular clusters?
• RQ3 To what extent could tidal stripping by the Milky Way disk account for the current state of Omega Cen?
• RQ4 Is there a dynamical and chemical consistency between Omega Cen and a disrupted dwarf spheroidal galaxy?
• RQ5 Could the tidal debris from such a progenitor have contributed to the formation of the Milky Way's thick disk?

Key findings

• The metallicity distribution function (MDF) of Omega Cen is extremely broad, peaking at [Fe/H] = -1.7 and extending to both higher and lower metallicities, with few stars above [Fe/H] ≈ -1.2.
• The orbit of Omega Cen is strongly retrograde, has a small apogalacticon, and lies nearly coplanar with the Milky Way disk—unlike the orbits of most Galactic globular clusters.
• The combination of the wide MDF and unique orbit is inconsistent with standard globular cluster formation but consistent with the object being the remnant nucleus of a disrupted dwarf spheroidal galaxy.
• Tidal stripping by the Galactic disk likely reduced the original mass of the progenitor system, explaining Omega Cen's current compact and massive state.
• The model suggests that the progenitor system may have contributed to the formation of the Milky Way's thick disk via tidal stripping of stars and disk heating.
• A present-day analog may be the Sagittarius dSph, where the globular cluster M54 is proposed to be the nucleus of a similarly disrupted system.