Spatial Property of Multiple Metallic Populations in the Tidal Stream of ω Centauri

Abstract

ω Centauri, the remnant nucleus of an accreted dwarf galaxy, is a unique laboratory for studying complex stellar populations. The recently discovered Fimbulthul stream provides a fossil record of its ongoing tidal dissolution. In this work, we investigate the spatial distributions of metal-rich and metal-poor populations within ω Centauri and its stream to constrain the cluster's formation history. Using synthetic photometry from Gaia DR3 XP spectra, we classify stars via a Support Vector Classifier (SVC). The spatial distributions are then compared to a scaling N-body simulation performed with the PeTar code. Our analysis reveals no significant radial gradient in population ratios within the cluster, though the metal-rich stars may be slightly more extended. The population ratio in the tidal stream is consistent with that of the present-day cluster, albeit with large uncertainties. Our simulation indicates that any initial radial gradient must have been shallow, with a maximum fraction difference less than 0.15. Both observational and dynamical results suggest that the metal-rich population is not formed centrally concentrated. By combining our results and existing literature, we propose a new formation scenario for ω Centauri.

Figures (7)

• Figure 1: Color–magnitude diagram and pseudo–color–magnitude diagram of the training and predicted stars. The light–red and light–blue dots represent the metal–rich and metal–poor stars in the training sample, respectively. In the two upper panels, the cluster members are separated into metal–poor and metal–rich populations, indicated by blue and red stars, respectively. In the two lower panels, the stream members are likewise separated into metal–poor and metal–rich populations, marked by blue and red stars.
• Figure 2: Position of the simulated particles. In the left panel, all particles are shown as grey dots, the selected particles as blue dots, the observed stars as dark dots, and the cluster center as a red star. The right panel provides a zoom–in on the tidal stream, illustrating the cluster structure and its tidal extension; the yellow circle indicates the main body of the simulated cluster, with a radius of 20 arcmin.
• Figure 3: Radial distributions at four evolutionary stages: $t=0$ Myr (light blue), $300$ Myr (orange), $500$ Myr (green), and the present day at $800$ Myr (dark blue). No tidal stream is present at $t=0$, whereas the outer regions at later times consist of escaping stars. The initial $r_{\rm t}$ (at $t=0$) is indicated as a fixed boundary (black dashed line) to distinguish the stream, noting that the intrinsic $r_{\rm t}$ evolves with time.
• Figure 4: The three panels show the radial distributions at the same evolutionary stages but with different initial configurations: increasing, flat, and decreasing patterns. The initial radial distribution exhibited by fractions of metal-poor population in five bins, markered as light blue dots.The black dashed line denotes the tidal boundary, as in Fig.\ref{['fig:sim_radial']}
• Figure A1: Photometric error ($\sigma_{\text{m}}$) as a function of magnitude for the F435W, F625W, and F814W filters. The grey points represent the initial sample before quality cuts. The blue dashed curves indicate the upper boundaries used to define our high-accuracy sample. Following the strategy of 2005MNRAS.357..265S, the behavior of photometric errors has been modelled as a function of magnitude using analytical exponential functions, adopting the curves that provide $\sigma_{\text{F435W}} = 0.01$ at $m=18$, and $\sigma = 0.006$ at $m=17$ for both F625W and F814W. Stars lying above these boundaries were rejected from the final catalog. (See also 2007Sesar for the theoretical noise model).