Star clusters across the ages: internal kinematics from stellar nurseries to ancient globulars

Dalgleish, Hannah (2020) Star clusters across the ages: internal kinematics from stellar nurseries to ancient globulars. Liverpool John Moores University, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

This thesis comprises the dynamical study of star clusters in the Milky Way and Magellanic Clouds from early to ancient times. Much is unknown about the formation of high-mass stars and clusters — our understanding is deeply hindered by the obscuration of stars by thick columns of dust and gas. One can infer the motions of stars in star-forming regions, however, via radio observations of ionised gas. By way of example, I examine a young, bipolar H II region in the Galactic disc which lies at the centre of a massive (∼ 10^3 M⊙) infrared-dark cloud filament. Intriguingly, the region known as G316.81– 0.06, displays a large velocity gradient (47.81 ± 3.21 km s−1 pc−1) along the same direction as the filament — a phenomenon scarcely observed at this stage of evolution. Based on a qualitative comparison between G316.81–0.06 and simulations of young star-forming regions, the velocity gradient can be explained by rotation, inferred to be a direct result of the initial angular momentum of the natal molecular cloud. If true, this kinematic signature should be common in other young (bipolar) H II regions and may help to discern the scenario by which star clusters form and evolve. Star clusters at ancient times (i.e. globulars) appear in an entirely different form. Rid of their natal gaseous cocoons, globulars visibly contain 10^5 − 10^6 stars, held together by their mutual gravity. One particular conundrum appeared in recent decades: observed mass-to-light ratios (M/L) of metal-rich globular clusters (GCs) disagree with theoretical predictions. This discrepancy is of fundamental importance since stellar population models provide the stellar masses that underpin most of extragalactic astronomy, near and far. Using integral-field unit data from the WAGGS project, I have extracted radial velocities for 1,622 stars located in the centres of 59 Milky Way GCs — twelve of which have no previous kinematic information — in order to calculate dynamical masses and M/L_V ratios via N-body modelling. Most importantly, the sample includes NGC 6528 and NGC 6553, which extend the metallicity range of GCs with measured M/L up to [Fe/H] ∼ −0.1 dex. The results confirm that metal-rich clusters have M/L_V more than two times lower than what is predicted by simple stellar population models, and thus the discrepant M/L– [Fe/H] relation remains a serious concern. I have explored the potential origin of the divergence, and it appears that dynamical effects are the most likely explanation. With great technological advances in recent years, the internal kinematics of more distant star clusters can also be probed, such as massive star clusters in the Magellanic Clouds. These clusters are as young as ∼ 1 Myr and are thought to be the progenitors of ancient globulars. Thus, this provides a unique opportunity for the study of globular formation at a relatively unexplored snapshot in time. I have carried out a preliminary study of eleven (young, intermediate-age and old) massive clusters in the Clouds as an extension of the M/L–[Fe/H] study of GCs. With this, I can then test stellar population models and improve constraints on theories of dynamical evolution at early times. Newly discovered Gaia star clusters present another avenue for novel research. Home to a new area of parameter space, these clusters appear to be old and compact, yet they are faint (V-band magnitude < −2.5 mag). This is an exciting opportunity to advance our knowledge of (heavily dissolved) star clusters which seem to be approaching the end of their lifetime.

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