The origins of supermassive black holes: a study of active galactic nuclei and dwarf galaxies

Abstract

The original seed conditions and growth processes of supermassive black holes (BHs) are currently poorly understood. Since we cannot directly observe BH seeds in the early Universe, astronomers have developed a novel method for studying them. This method studies the "fossil record" of BH seeds through BH-host galaxy scaling relations using observations of low-mass BHs residing in local dwarf galaxies. While this strategy avoids the biases (towards brighter objects) and limitations of high-redshift observations, it remains fraught with other issues; the signatures of BHs in dwarf galaxies can be confused with star-formation processes and true scaling relations are difficult to disentangle from empirically-derived scaling relations which can depend on galaxy type/sample. In this work, we 1. place samples of galaxies on two different scaling relations and 2. warn against multiple pitfalls involved with the "fossil record" strategy. We first use Sloan imaging with galaxy bulge-disk decompositions to place a sample of active galactic nuclei (AGNs) on the BH-bulge mass relation. We show that AGN host galaxies do not follow the same scaling relations defined for early-type galaxies, hinting that AGNs physically interact differently with their host galaxies compared to their inactive counterparts. Next, we investigate four galaxies with AGN-like, mid-infrared colors that do not show AGN signatures at other wavelengths. We use Hubble imaging and stellar population synthesis modeling to demonstrate that these colors could be coming instead from very young, massive nuclear star clusters. We therefore advise caution using mid- infrared color as an AGN selection technique specifically in dwarf galaxies. Lastly, using Magellan spectroscopy, we place a sample of six dwarf AGNs on the BH mass-stellar velocity dispersion and BH mass-stellar mass relations. These galaxies generally seem to follow the relations defined by larger, early-type galaxies, however, further samples are needed to draw conclusions about BH seeding. Overall, we make progress on populating scaling relations with active and low-mass galaxies, which is a crucial step in understanding BH seed formation/evolution. Additionally, we contribute to a body of work attempting to understand issues involved with extrapolating observational techniques derived for high-mass galaxies into the low-mass regime.