Re-evaluation of the central velocity-dispersion profile in NGC 6388

Abstract

Context. The globular cluster NGC 6388 is one of the most massive clusters in our Milky Way and has been the subject of many studies. Recently, two independent groups found very different results when measuring its central velocity-dispersion profile with different methods. While we found a rising profile and a high central velocity dispersion (23.3 km s −1 ), measurements obtained by Lanzoni et al. (2013, ApJ, 769, 107) showed a value lower by 40%. The value of the central velocity dispersion has a serious effect on the mass and possible presence of an intermediate-mass black hole at the center of NGC 6388. Aims. The goal of this paper is to quantify the biases arising from measuring velocity dispersions from individual extracted stellar velocities versus the line broadening measurements of the integrated light using new tools to simulate realistic observations made with integral field units (IFU). Methods. We used a photometric catalog of NGC 6388 to extract the positions and magnitudes from the brightest stars in the central three arcseconds of NGC 6388 and created a simulated SINFONI and ARGUS dataset. The IFU data cube was constructed with different observing conditions (i.e., Strehl ratios and seeing) reproducing the conditions reported for the original observations as closely as possible. In addition, we produced an N-body realization of a ∼10 6 M stellar cluster with the same photometric properties as NGC 6388 to account for unresolved stars. Results. We find that the individual radial velocities, that is, the measurements from the simulated SINFONI data, are systematically biased towards lower velocity dispersions. The reason is that the velocities become biased toward the mean cluster velocity as a result of the wings in the point spread function of adaptive optics (AO) corrected data sets. This study shows that even with AO supported observations, individual radial velocities in crowded fields do not reproduce the true velocity distribution. The ARGUS observations do not show this kind of bias, but they were found to have larger uncertainties than previously obtained. We find a bias toward higher velocity dispersions in the ARGUS pointing when fixing the extreme velocities of the three brightest stars, but these variations are within the determined uncertainties. We reran Jeans models and fit them to the kinematic profile with the new uncertainties. This yielded a black-hole mass of M • = (2.8 ± 0.4) × 10 4 M and M/L ratio M/L = (1.6 ± 0.1) M /L , consistent with our previous results.

N-body modelling of globular clusters: masses, mass-to-light ratios and intermediate-mass black holes

“…If the central velocity dispersion is as low as 13 km/sec as found by Lanzoni et al (2013) the cluster definitely does not contain an IMBH, while if the velocity dispersion profile is rising as found by Lützgendorf et al (2015) an IMBH could be present. Interestingly, we have difficulties reproducing both the low velocity dispersion from Lanzoni et al (2013) with a no IMBH model as well as the high velocity dispersion found by Lützgendorf et al (2015) with our best-fitting IMBH model, which could be an indication that both values are biased to too low/high values. A final decision on whether an IMBH is present in NGC 6388 or not can only be made once the velocity dispersion profile in the center of the cluster is known.…”

“…Lanzoni et al (2013) and Lapenna et al (2015) on the other hand obtained VLT FLAMES and KMOS spectra of 52 and 82 giant stars near the cluster center and found a low central velocity dispersion of about 13 km/sec, which limited the mass of any central black hole to less than 2000 M⊙. In a re-analysis of all existing data, Lützgendorf et al (2015) found that individual radial velocities in the core of NGC 6388 are systematically biased towards the mean cluster velocity due to the blending of stars as a result of the high central density. By simulating this effect using artificially created IFU data cubes, they confirmed their initial high value for the velocity dispersion and derived an IMBH mass of 2.8 ± 0.4 · 10 4 M⊙.…”

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

“…For example, Lanzoni et al (2013) used AO-assisted integral field spectroscopy in the near-infrared and measured velocities in the centre of NGC 6388 via aperture spectroscopy around the brighter stars. Lützgendorf et al (2015) claim that this approach is biased because blends of unresolved or fainter neighbouring stars pull the measured velocities to the cluster mean, which explains the discrepancy to their measurements that suggest the presence of a black hole.…”

Observational evidence for intermediate-mass black holes

“…Baumgardt (2017) also found that the velocity dispersion profile of ω Centauri is best fitted by an IMBH of 10 4 M . Using integral-field spectroscopy and HST photometry, Lützgendorf et al (2012Lützgendorf et al ( , 2013Lützgendorf et al ( , 2015 reported upper limits on the mass of a putative BH in the globular clusters NGC 1851, NGC 2808, NGC 5694, NGC 5824, and NGC 6093 (see Table 1) and predicted the presence of an IMBH of (3 ± 1) × 10 3 M in NGC 1904, of (2 ± 1) × 10 3 M in NGC 6266, and of (2.8 ± 0.4) × 10 4 M in NGC 6388. An IMBH of (1.5 ± 1.0) × 10 3 M is also suspected in the globular cluster NGC 5286 (Feldmeier et al 2013) and Ibata et al (2009) reported the possible presence of an IMBH of ∼9400 M in NGC 6715 (M54), a globular cluster located at the center of the Sagittarius dwarf galaxy.…”