MUSE crowded field 3D spectroscopy of over 12 000 stars in the globular cluster NGC 6397

Abstract

We present a detailed analysis of the kinematics of the Galactic globular cluster NGC 6397 based on more than ∼18 000 spectra obtained with the novel integral field spectrograph MUSE. While NGC 6397 is often considered a core collapse cluster, our analysis suggests a flattening of the surface brightness profile at the smallest radii. Although it is among the nearest globular clusters, the low velocity dispersion of NGC 6397 of <5 km s −1 imposes heavy demands on the quality of the kinematical data. We show that despite its limited spectral resolution, MUSE reaches an accuracy of 1 km s −1 in the analysis of stellar spectra. We find slight evidence for a rotational component in the cluster and the velocity dispersion profile that we obtain shows a mild central cusp. To investigate the nature of this feature, we calculate spherical Jeans models and compare these models to our kinematical data. This comparison shows that if a constant mass-to-light ratio is assumed, the addition of an intermediate-mass black hole with a mass of 600 M brings the model predictions into agreement with our data, and therefore could be at the origin of the velocity dispersion profile. We further investigate cases with varying mass-to-light ratios and find that a compact dark stellar component can also explain our observations. However, such a component would closely resemble the black hole from the constant mass-to-light ratio models as this component must be confined to the central ∼5 of the cluster and must have a similar mass. Independent constraints on the distribution of stellar remnants in the cluster or kinematic measurements at the highest possible spatial resolution should be able to distinguish the two alternatives.

Far-ultraviolet observation of the globular cluster NGC 6397

“…Recently, Kamann et al (2016) used integral field spectroscopic data and suggested that the surface brightness profile and velocity dispersion profile could either be explained by the presence of an intermediate-mass black hole with a mass of 600 M ⊙ -which agrees with an earlier suggestion by de Rijcke et al (2006) of ≈ 1300 to 400M ⊙ , based on radio continuum emission -or with a dark stellar component.…”

How stellar rotation shapes the colour−magnitude diagram of the massive intermediate-age star cluster NGC 1846

“…To reliably separate physical radial velocity variations from those caused by the finite accuracy of our measurements, a proper knowledge of the measurement uncertainties is key. We calibrated the uncertainties from the observed epoch-to-epoch variations in a similar manner to Kamann et al (2016). It is based on the idea that (in the absence of physical variations due to binary or variable stars) the normalised velocity offsets,…”

“…Calibrated uncertainties are obtained by multiplying the original uncertainties with the actual standard deviation of the δv los distribution. In contrast to Kamann et al (2016), where the calibration was performed by grouping stars with extracted spectra of comparable S/N, we grouped the stars according to their T eff , log g, and M/H measurements. For each star, we inferred the correction factor for the formal uncertainties of its radial velocity measurements based on the distribution of the δv los values of the 100 closest stars in this threedimensional space.…”

Testing multimass dynamical models of star clusters with real data: mass segregation in three Galactic globular clusters

“…Indeed, radial velocities have been obtained essentially only for the brightest stars, giants and subgiants, which have very similar masses. To date, the measure of a significant number of radial velocities for stars in a range of masses has been performed through integral field spectroscopy only in the GC NGC 6397, leading however to uncertain results (Kamann et al 2016). The same kind of analysis made through proper motions requires superb astrometric accuracies that have been achieved only recently using Hubble Space Telescope (HST) multi-epoch observations providing evidence for mass-dependent kinetic temperature in M15 (Bellini et al 2014).…”