omega Centauri - a former nucleus of a dissolved dwarf galaxy? New evidence from Stroemgren photometry

TL;DR

This study uses Stroemgren photometry to analyze omega Centauri's complex star formation history, revealing multiple stellar populations with varying ages and metallicities, supporting the idea it was once a dwarf galaxy nucleus.

Contribution

It provides new evidence linking metallicity and age distributions in omega Centauri, suggesting a prolonged, clumpy mass inflow history consistent with a dwarf galaxy nucleus origin.

Findings

Multiple stellar populations with different ages and metallicities identified.

Metal-rich stars are more centrally concentrated and younger.

Evidence of early enrichment and complex star formation history.

Abstract

CCD vby Stroemgren photometry of a statistically complete sample of red giants and stars in the main sequence turn-off region in omega Centauri has been used to analyse the apparently complex star formation history of this cluster. From the location of stars in the (b-y),m_1 diagram metallicities have been determined. These have been used to estimate ages of different sub-populations in the color-magnitude diagram. The metal-poor population around -1.7 dex is the oldest population found. More metal-rich stars between [Fe/H]=-1.5 and -1.0 dex tend to be 1-3 Gyr younger. These stars are more concentrated towards the center than the metal-poor ones. The most-metal rich stars (-0.7 dex) might be up to 6 Gyr younger than the oldest population. They are asymmetrically distributed around the center with an excess of stars towards the South. We argue that the Stroemgren metallicity in terms of element abundances has another meaning than in other globular clusters. From a comparison with spectroscopic element abundances, we find the best correlation with the sum C+N. The high Stroemgren metallicities, if interpreted by strong CN-bands, result from progressively higher N and perhaps C abundances in comparison to iron. We see an enrichment already among the metal-poor population, which is difficult to explain by self-enrichment alone. An attractive speculation is that omega Cen was the nucleus of a dwarf galaxy. We propose a scenario in which omega Cen experienced mass inflow over a long period of time, until the gas content of its host galaxy was so low that star formation in omega Cen stopped, or alternatively the gas was stripped off during its infall in the Milky Way potential. A mass inflow occuringcn a clumpy and discontinuous manner could explain several of our findings.