Some nuclear star clusters
Star clusters
Enviroments for the formation of central black holes
Star clusers are a large group where enormous stars are gravitationally bound. There are several kind of star clusters, but we focus on investigate two types: nuclear star cluster (NSC) and globular cluster (GC). Currently, it seem to have a scale relation of NSC mass and central black hole mass (Graham 2020, Greene 2020)
Nuclear star cluster
Nuclear star clusters (NSCs) are extremely dense and massive star clusters occupying the innermost region or ‘nucleus’ of most galaxies. Observationally, NSCs are identified as luminous and compact sources that clearly ‘stand out’ above their surroundings.
When central black holes formed, they would accreate the surround materials and grew up. In contrast, if they were massive enough, the nuclear star cluster could be destroyed. In low-mass galaxies, there was not enough gas and dust so that the black holes become dormant. There was a high incidence of black holes in nuclear star clusters (Greene 2020). Nuclear star clusters do not seem to replace black holes as the central compact object (Ferrarese 2006). Rather the two appear to coexist often at low galaxy mass (Seth 2008).
It is not obvious what (if any) causal relationship exists between black holes and nuclear star clusters.We have already discussed the possibility that black holes form via gravitational runaway processes in stellar clusters and the dearth of concrete observational evidence to date for black holes with mass more than 1,000 solar mass in globular clusters (Tremou 2018). Our research focus on studying the formation of nuclear star cluster and relation with massive black holes at the galaxy center.
Figure 1. The ratio of the masses of the central massive black hole and the NSC plotted against their host galaxy dynamical mass. The data from graph indicates that the fraction BH-mass/NSC-mass increases with host galaxy mass. However, the scatter in this ratio at a given galaxy mass is still up to three orders of magnitude. This likely reflects the fact that the growth history of both NSC and BH are very stochastic.
Methods
Spectrum & Kinematics
The gas and stars in galaxies have two type of motion: rotation and random motions. The rotation is caused by the galaxy disks while the random motion is the superposition of the individual stellar orbits. The mean of random motion velocity called vellocity dispersion, or sigma (σ). Suppose that a emitting gas atoms in an object have random motions along the line-of-sight draw a velocity distribution
Figure 2. (Left) An illustration of the broadening of a spectral line by the velocity dispersion of stars in a stellar system. A telescope collects light from all stars within a cylinder through the stellar system. Each star contributes a narrow spectral line with rest frequency f, which is Doppler shifted to a different frequency f '= f + Δf due to its motion along the line-of-sight. The superposition of many such line profiles produces a broadened line, with the profile given by the convolution of the original stellar spectral line and the velocity distribution of the stars in the cylinder. (Middle) An illustration of long-slit spectroscopy of a thin rotating disk along the major axis of the image. (Right) An illustration of Dopper effect and velocity dispersion on the observed spectrum.
Constraint black hole mass
We study the vicinity of central black hole kinematics to estimate its mass. We can determine the rotation speed by Doppler effect when investigate galaxy spectra. And the comparison between the spectrum of the galaxy and a fiducial spectral template to estimate velocity dispersion. The observed spectrum could originate from either a suitable star, whose spectral lines are not distinguishable at the current spectral resolution, or a blend of diverse stellar categories. Alternatively, it could be a high signal-to-noise ratio spectrum of a galaxy whose velocity dispersion is known.
Figure 3. Fitting the spectrum with template
Figure 4. Illutrate the difference of velocity dispersion in two circumstance. (upper) no black hole (M33) and (lower) with black hole (NGC205)
Nowadays, telescopes use a technique known as integral field spectroscopy. In this method, the signal from each cell or pixel of the field are fed to the spectrograph, which then generates a spectrum for individual pixel. All the spectra then are arranged into a datacube which contains entires 2D field of view while the third dimension is spectroscopy . We can use the wealth of information from integral field spectrograph to determine the velocity, kinematics, ... of the astronomical objects.
Figure 5. Investigating kinematics of FCC 202 galaxy (in Fonax galaxy cluster).
(Left) estimate kinematics of individual pixel of galaxy 2D field of view. We can determine velocity dispersion, rotational velocity, root-mean-square velocity from spectrum for each small parts of object by integral field spectroscopy. (Right) Extracting root-mean-square velocity of different black hole masses from kinematics on the left pictures.