The discovery of tight scaling relations between properties of the spheroidal component of a galaxy (i.e., the nuclear bulge of a spiral galaxy, or the region within the effective radius of an elliptical galaxy) has led to the realization that most, if not all, galaxies harbor a super-massive black hole (105 < MBH < 1011 MSun) at their cores. Early work correlated the host galaxy mass and luminosity with an inferred nuclear black-hole mass, while later work showed that stellar velocity dispersion presents a tighter correlation. These relationship point to an intimate connection between the growth of the central black hole and the formation of the spheroidal component of a galaxy.
Accurate measurements of black hole masses in galaxies of diverse types and masses are required to test possible origins of these correlations. Black-hole masses are inferred by measuring the integrated motions of stars orbitting the black hole inside the small volume within which the gravitational field of the black hole dominates that of the host galaxy - the sphere of influence of the black hole. Generating significant samples requires access to a large volume of the local Universe, which in turn requires high angular resolution to resolve the black-hole sphere of influence in distant objects.
For lower-mass galaxies, available evidence suggests that a compact nuclear star cluster may form in preference to a central black hole. These clusters follow and extend the same correlations found for more massive black holes, as if they form two possible end-products of a similar formation process. GMTIFS will contribute to studies of massive nuclear black holes in two broad areas:
- Its high angular resolution will be used to measure the masses ofthe most massive black holes in the Universe out to large distance, and
- Its high angular resolution and high spectral resolution will be used to probe low-mass nuclear black holes and compact nuclear star clusters in nearby low-mass galaxies.
The most massive nuclear black holes
There is considerable interest in determining how massive nuclear black holes can become. To date, M87 contains the highest mass black hole that has been measured directly at 6.6x109 MSun. However, even higher black-hole masses are required if our understanding of quasars is correct. Determining whether the most massive black holes obey the same host-galaxy scaling relations as lower-mass black holes will constrain theories of galaxy evolution.
Low-mass nuclear black holes and nuclear star clusters
Identifying the smallest black holes at the centers of low-mass galaxies will constrain models of black-hole formation. Seed black holes are a salient feature of every theoretical model of black-hole formation and growth. However, no well-founded mechanism exists for their initial genesis. Detection of low-mass black holes is challenging because their sphere of influence is small, so high angular resolution is required to resolve this region even in nearby galaxies, and the orbital motions of stars in the vicinity of the black hole are slower, so higher spectral resolution is required to measure these motions. GMTIFS will have both the high angular resolution and the high spectral resolution to perform these measurements.
Low-mass galaxies predominantly form nucleated star clusters in preference to super-massive black holes, but both structures follow indistinguishable scaling relations with host galaxy properties. This suggests that a threshold exists above which a massive black hole forms and below which nuclear material forms into stars, rather than a nuclear black hole. A few transition objects are known that appear to contain both a nuclear star cluster and a nuclear black hole at its center. Investigating this transition region will provide crucial data on the conditions under which massive black holes form and how the nuclear regions of galaxies evolve with time.