Massively parallel spectroscopy of sources in Galactic globular clusters

by Fabian Göttgens
Doctoral thesis
Date of Examination:2021-09-08
Date of issue:2021-10-04
Advisor:Prof. Dr. Stefan Dreizler
Referee:Prof. Dr. Stefan Dreizler
Referee:Prof. Nate Bastian
Persistent Address: http://dx.doi.org/10.53846/goediss-8829

 

 

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Abstract

English

Globular clusters (GCs) consist of hundreds of thousands of stars, densely packed into a spherical shape. Not only do GC contain ordinary main sequence and red giant stars, but also the products of frequent stellar encounters, stellar remnants, and potentially intermediate-mass black holes (IMBH). But how can we find these objects hidden between thousands of other stars? With the progress in observation techniques used in astronomy, it is possible to observe individual stars in the cores of GCs, the most crowded regions. In particular, the development of large integral-field spectrographs, such as Multi-Unit Spectroscopic Explorer (MUSE) at the Very Large Telescope, and adaptive optics to correct for atmospheric distortions, enable high-resolution observations from the ground. Using these techniques, we can efficiently observe GCs and measure individual spectra of thousands of stars simultaneously. During an automated search for emission-line objects in these spectra, we detected a previously unknown nebula in M22. The spectrum of this nebula shows emission lines of hydrogen, nitrogen, and sulphur, but it does not look like a typical spectrum of a planetary nebula (PN). Not only are its spectral lines unusual, but with its small size and a low luminosity, the nebula also does not resemble any of the four known PNe in Galactic GCs, including the one in M22. The literature contains many attempts to detect a central IMBH in a Galactic GC. So far, there is no unambiguous discovery. We use data taken with the MUSE narrow-field mode with a spatial resolution comparable to the Hubble Space Telescope to analyse the motion of stars in M80. To overcome the usual problems of previous attempts, we employ both a model based on the Jeans equations and an independent $N$-body model of M80. We find two equally probable solutions with different dynamical cluster centres: One solution has its centre close to the photometric centre from the literature, and it does not need an IMBH to explain the observed stellar motions. Another solution has a centre with a small offset from the first one. Here, a central IMBH with a mass of several thousand solar masses is needed. The $N$-body models exclude the existence of many stellar-mass black holes, which could mimic the effect of an IMBH on the stellar motions.
Keywords: Spectroscopy; Astronomy; Astrophysics; Black hole; Globular cluster; Messier 80; Messier 22; Emission nebula; Stellar dynamics
 
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