The Luminosity Function and stellar Mass to Light ratio of the massive globular cluster NGC2419

TL;DR

This study uses Hubble data to analyze the luminosity function of globular cluster NGC2419, finding a constant mass-to-light ratio and providing constraints for dynamical models based on stellar mass distribution.

Contribution

It presents the first detailed luminosity function of NGC2419 over a wide radial range, confirming the absence of mass segregation and deriving a precise stellar mass-to-light ratio.

Findings

Luminosity function is consistent across different radial distances.

Mass-to-light ratio is estimated to be approximately 1.5 in the V band.

No significant mass segregation detected in the cluster.

Abstract

We used archival Hubble Space Telescope WFC3 images to obtain the Luminosity Function of the remote globular cluster NGC2419 from two magnitudes above the Horizontal Branch level down to \sim3.0 magnitudes below the Turn Off point (to M_I\sim6.4), approximately covering the range of initial stellar masses 0.5 M_sun<= m <= 0.9 M_sun. The completeness-corrected Luminosity Function does not display any change of shape over the radial range covered by the WFC3 data, out to ~6 core radii (r_c), or, equivalently, to ~2 half-light radii. The Luminosity Function in this radial range is also identical to that obtained from ground based data at much larger distances from the cluster centre (12r_c<= R<= 22r_c), in the magnitude range in which the two distributions overlap (M_I<= 4.0). These results support the conclusion by Dalessandro et al. that there is no significant mass segregation among cluster stars, hence the stellar mass-to-light ratio remains constant with distance from the cluster centre. We fitted the observed Luminosity Function with theoretical counterparts with the proper age and metallicity from different sets of stellar evolution models and we consistently derive a total V band mass-to-light ratio 1.2<= M/L_V<= 1.7, by extrapolating to the Hydrogen burning limit, with a best-fit value M/L_V=1.5 +/- 0.1. On the other hand, assuming that there are no cluster stars with m<= 0.3 M_sun, we establish a robust lower limit M/L_V> 0.8. These estimates provide useful constraints for dynamical models of the cluster that were forced to consider the stellar mass-to-light ratio as a (nearly) free parameter.