VLT/MUSE spectroscopy suggests a central intermediate mass black hole in globular cluster NGC 6397

NGC6397-NRGBhiTwo new papers appearing today on the astro-ph preprint server have highlighted the capabilities of a new instrument at ESO/VLT, the Multi Unit Spectroscopic Explorer, MUSE. Studying the velocity dispersion of stars in the globular cluster NGC 6397, astronomers infer a central black hole of  some six hundred solar masses. They have also been able to construct the first complete spectroscopic HR diagram for a globular cluster, using nearly 19000 stellar spectra. Image: Antilhue/Chile; astrosurf.com, 14.5″ mirror, prime focus f/9. From the abstracts: (Paper I/Paper II)

We demonstrate the high multiplex advantage of crowded field 3D spectroscopy using the new integral field spectrograph MUSE by means of a spectroscopic analysis of more than 12,000 individual stars in the globular cluster NGC 6397. The stars are deblended with a PSF (point spread function) fitting technique, using a photometric reference catalogue from HST as prior, including relative positions and brightnesses. This catalogue is also used for a first analysis of the extracted spectra, followed by an automatic in-depth analysis using a full-spectrum fitting method based on a large grid of PHOENIX [theoretical model] spectra. With 18,932 spectra from 12,307 stars in NGC 6397 we have analysed the largest sample so far available for a single globular cluster. We derived a mean radial velocity of 17.84 ± 0.07 km/s and a mean metallicity of [Fe/H]= −2.120 ± 0.002, with the latter seemingly varying with temperature for stars on the RGB. We determine effective temperature and [Fe/H] from the spectra, and surface gravity from HST photometry. This is the first very comprehensive HRD for a globular cluster based on the analysis of several thousands of stellar spectra. Furthermore, two interesting objects were identified with one being a post-AGB star and the other a possible millisecond-pulsar companion.

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 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 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_sun 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 arcsec 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.

IDL TIFF fileHubble image of the central regions of NGC 6397 (Wikipedia). In 2006, a study using such data was published that showed a clear lower limit in the intrinsic brightness of the cluster population of faint stars at around visual magnitude 26. The authors therefore were able to deduce observationally the lower limit for the mass necessary for stars to develop a core capable of fusion: roughly 0.083 times the mass of the Sun.

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