On the fundamental line of galactic and extragalactic globular clusters
Dipartimento di Fisica, Università di Pisa, Largo Bruno Pontecorvo 3, 56127 Pisa, Italy e-mail: email@example.com
2 Dipartimento di Fisica, Università degli Studi di Milano, via Celoria 16, 20133 Milano, Italy
Accepted: 2 November 2009
Context. In a previous paper, we found that globular clusters in our Galaxy lie close to a line in the (log Re, SBe, log σ) parameter space, with a moderate degree of scatter and remarkable axi-symmetry. This implies that a purely photometric scaling law exists, that can be obtained by projecting this line onto the (log Re, SBe) plane. These photometric quantities are readily available for large samples of clusters, as opposed to stellar velocity dispersion data.
Aims. We study a sample of 129 Galactic and extragalactic clusters on this photometric plane in the V-band. We search for a linear relation between SBe and log Re and study how the scatter around the best-fit relation is influenced by both age and dynamical environment. We interpret our results in terms of testing the evolutionary versus primordial origin of the fundamental line.
Methods. 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.
Results. 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/arcsec2 and Re in parsec. Both young and old clusters follow the scaling law, which has a scatter of approximately 1 mag in SBe. However, young clusters display more of a scatter and a clear trend in this with age, which old clusters do not. This 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 does not have a primordial origin at cluster formation, but is rather the result of a combination of stellar evolution and collisional dynamical evolution.
Key words: globular clusters: general / Galaxy: structure
© ESO, 2010