Nuclear Star Clusters in the Vicinity of Black hole

Adaptive optics spectroscopy has allowed for spectral identifications of bright stars within ~1 pc of the supermassive black hole and has revealed two very unexpected results. The first surprise was the detection of a wealth of young (<10 Myr) stars in a region where none were expected due to the strong tidal field of the black hole and the low present-day gas densities. The second surprise was the unanticipated dearth of late-type stars near the black hole.

A young (4-8 Myr) nuclear cluster surrounds the Galactic Center black hole and extends out to a radius of 0.5 pc. What is their formation history? This is an essential question in the study of the co-evolution and growth of Super Massive Black Hole and the nuclear star clusters that surround them.

The origin of young stars, where and when the clusters formed, is difficult to explain since the gas densities observed today are orders of magnitude too low for a gas clump to overcome the extreme tidal forces and collapse to form stars. The star clusters' proximity should cause them to be torn apart, and thus their presence within the strong tidal field of our Galaxy's central SMBH still remains a puzzle. If the star clusters formed in situ, then the events leading up to its formation were likely unique and extreme compared to those found in the local solar neighborhood.

For further reading:

• Do et al 2013 Three-dimensional Stellar Kinematics at the Galactic Center: Measuring the Nuclear Star Cluster Spatial Density Profile, Black Hole Mass, and Distance

• Do et al 2011 Testing Stellar Cusp Formation Theories with Observations of the Milky Way Nuclear Star Cluster

Arches and Quintuplet Clusters

The Arches and Quintuplet clusters are some of the most massive, young clusters in the Milky Way. They are located ~30 pc in projection from the black hole, and formed in one of the most extreme environments in the Galaxy. Again, these clusters experience strong tidal shear forces from the supermassive black hole. Along with the young star clusters in the immediate vicinity of the Galactic Center, these clusters provide an ideal opportunity to study how stars and clusters form under extreme initial conditions.

For further reading:

• Clarkson et al. 2012 Proper Motions of the Arches Cluster with Keck Laser Guide Star Adaptive Optics: The First Kinematic Mass Measurement of the Arches

• Stolte et al. 2006 The Arches cluster - evidence for a truncated MF in the GC?

• Hosek et al. 2015 The Arches Cluster: Extended Structure and Tidal Radius

• Hosek et al. 2018 The Unusual Initial Mass Function of the Arches Cluster

Arches Cluster. Credit: Hubble Space Telescope/NICMOS

Dearth of Old Stars

Dynamically relaxed star clusters with a massive black hole are predicted to have a steep increase in stellar density toward the black hole. The red giant stars in the nuclear cluster are likely old enough (1-10 Gigayears) to have formed such a cusp. However, star counts using AO spectroscopy and medium-band imaging have shown that the red giants have a very flat projected surface dnsity profile close to Sgr A*, the central black hole, strongly suggesting that the observed number is far below the predicted. Converting the projected profile to a 3D profile is difficult but it is essential for understanding how the nuclear cluster's formation and evolution differ from current theories. The density profile is also a key input for models of black hole growth. Thus

For further reading:

• Do et al 2009 High Angular Resolution Integral-Field Spectroscopy of the Galaxy's Nuclear Cluster: A Missing Stellar Cusp?

Plot of the surface number density as a function of projected distance from SgrA*. Old and young stars are represented by red and blue solid circles respectively. The image is taken from Do et al 2009.

Objects Interacting with the Black Hole

The detection of a very red infrared object approaching the supermassive black hole was reported in 2012. This object, G2, was predicted to be in an orbit, with its closest approach only 3000 times the radius of the event horizon. The Brackett-gamma emission was also detected from G2, indicating that this object is a dusty, three-solar-mass object. The Galactic Center Group members closely monitored G2 during its closest approach to the black hole during summer of 2014 and found that G2 survived. If G2 were a gas cloud, it would have been torn apart as it approached the black hole. Instead, our team demonstrated that G2 is a pair of binary stars that had been orbiting the black hole in tandem and merged together into an extremely large star, cloaked in gas and dust.

For further reading:

• Witzel et al 2014 Detection of Galactic Center Source G2 at 3.8 μm during Periapse Passage

Paradox of Youth: Remnant Clockwise Disk of Young Stars in the Galactic Center

Spectroscopic observations of the Galactic Center region have revealed large population of ~200 hot, eary-type stars, including massive Wolf-Rayet stars (more than 20 solar massses) and O and B type main sequence stars. Observed local gas densities in the Galactic Center region are insufficient for self-gravity to overcome tidal forces and to collapse to form stars. Using high precision kinematic measurements and modeling of 116 young stars in the central 1 pc of the Galaxy, a particularly prominent feature has been observed: a stellar disk containing a large fraction of the young stars orbiting the black hole in a clockwise sense. The estimated eccentricities can be achieved through dynamical relaxation in an initially circular disk with a moderately top-heavy mass function. We also find that the B stars have similar kinematic properties as the more massive O and WR stars, suggesting a common star formation event.

For further reading:

• Lu et al 2013 Stellar Populations in the Central 0.5 pc of the Galaxy. II. The Initial Mass Function

• Yelda et al. 2014 Properties of the Remnant Clockwise Disk of Young Stars in the Galactic Center

Images shows an increase in density of stars (red/blue region). From Yelda et al. 2014.