On the Fundamental Line of Galactic and Extragalactic Globular Clusters

Abstract

In a previous paper we found that the Globular Clusters of our Galaxy lie around a line in the log(Re), SBe, log(sigma) parameter space, with a moderate degree of scatter and remarkable axi-symmetry. This implies the existence of a purely photometric scaling law obtained by projecting such a line onto the log(Re), SBe plane. Such photometric quantities are readily available for large samples of clusters, as opposed to stellar velocity dispersion data. We study a sample of 129 Galactic and extragalactic clusters on such photometric plane in the V-band. We look for a linear relation between SBe and log(Re) and study how the scatter around it is influenced by age and dynamical environment. We interpret our results as a test on the evolutionary versus primordial origin of the Fundamental Line. We perform a detailed analysis of surface brightness profiles, which allows us to present a catalogue of structural properties, without relying on a given dynamical model. We find a linear relation between SBe and log(Re), in the form SBe = (5.25 +- 0.44) log(Re) + (15.58 +- 0.28), where SBe is measured in mag/arcsec^2 and Re in parsec. Both young and old clusters lie on the scaling law, with a scatter of approximately 1 mag in SBe. However, young clusters display more scatter and a clear trend of such scatter with age, which old clusters do not. Such trend becomes tighter if cluster age is measured in units of the cluster half-light relaxation time. Two-body relaxation therefore plays a major role, together with passive stellar population evolution, in shaping the relation between SBe, log(Re), and cluster age. We argue that the log(Re)-SBe relation and hence the Fundamental Line scaling law is not primordially set at cluster formation, but rather is the result of combined stellar evolution and collisional dynamical evolution.

Accepted for publication on Astronomy and Astrophysics, official acceptance date November 2, 2009

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Cited by in corpus (3)

• Na-O anticorrelation and HB. IX. Kinematics of the program clusters. A link between systemic rotation and HB morphology?
• Some Systematics of Galactic Globular Clusters
• Stellar systems following the $R^{1/m}$ luminosity law. IV. The total energy and the central concentration of galaxies