There appears to be multiple methods for forming super-dense star clusters at the center of galaxies, according to a colloquium in Wilder Hall last Wednesday led by Anil Seth, a postdoctoral fellow at the Harvard-Smithsonian Center for Astrophysics. The lecture, entitled “The Formation of Galactic Nuclei,” was sponsored by the Department of Physics and Astronomy.
Nuclear star clusters exist in around 75 percent of galaxies, according to Seth. These objects, which are approximately 5 parsecs (pc) in diameter, have densities one million times that of their galaxies’ main disks, and are identifiable as distinct bulges visible in images of their respective galaxies. One parsec is equal to 3.08568025 × 1016 meters.
Seth’s research suggests two ways by which these clusters may be formed: “episodic disk accretion” and a merger mechanism. The former is a process recurring on the order of 50 to 100 million years in which gas is accreted inwards from elsewhere in the galactic disk, creating a super-dense region where many stars can form. In the latter, following the collision of two galaxies, gas from one galaxy is assimilated into the other, giving rise to a region of star creation.
Working with Nadine Neumayer of the European Southern Observatory and Michele Cappelari of the University of Oxford, Seth focused on nuclear star clusters between 2 and 5 Mpc away, which are close enough to be seen at high resolution through images from the Hubble Space Telescope. Spectra were also taken at optical and infrared wavelengths.
According to Seth, optical spectra are useful for providing constraints on the ages of stars. In the NGC 4244 spiral galaxy, the astronomers found two age populations of stars within its nuclear cluster: one with an approximate age of 70 million years and the other with an age of at least 1 billion years. This distinction indicates a “morphological separation” of the populations within the cluster.
Combining this information with kinematic data, including the knowledge that the star cluster is rotating in the same direction as the galaxy, the researchers posited episodic disk accretion as the method of formation in NGC 4244. The earliest stars will “get puffed up by subsequent accretion events,” Seth said, “creating a halo of older stars” within the nuclear cluster.
NGC 404, an S0-type galaxy whose shape is between elliptical and spiral, provides an example of the second posited merger method, according to Seth. The majority of stars in its disk and central bulge appear to be at least 10 billion years old, but there is cold gas on the galaxy’s outskirts providing evidence of a merger with another galaxy around 1 billion years ago.
When the astronomers examined the spectra of NCG 404’s nuclear cluster, they found a dominant population of stars that was about 1 billion years old, matching the estimated merger age. Seth suggested that a lot of gas from the merging galaxy was “funneled to the very center,” concentrating star formation at the center. A kinematic investigation found that the cluster has rotation counter to that of the galaxy itself, consistent with the merger theory.
Seth stated that knowledge about the formation of nuclear star clusters will also allow astronomers to investigate the formation of black holes, which, contrary to previous assumptions, can coexist with these clusters at the center of galaxies and to determine how these black holes correlate with the properties of their galaxies.
At least ten percent of nuclear star clusters have spectra consistent with accretion around a supermassive black hole, according to Seth. Models for the formation of such black holes can thus be constrained by studying galaxies that have both objects.
Recent results are suggesting the existence of central black holes with lower masses than previously found. “Nuclear star clusters are the best place to probe the low-mass end of the black hole mass function,” said Seth.
Citing continuing improvements in optics technology, Seth claimed that “we should be able to examine nuclear star clusters and low-mass black holes in an order of magnitude more systems within the next decade.”