Gemini Focuses on a Mid-sized Galactic Black Hole

June 6, 2019

An international team of researchers led by astronomer Jong-Hak Woo obtained deep spectroscopy from Gemini, combined with light echo measurements from multiple observatories, to confirm a black hole “missing link.”

A team led by astronomer Jong-Hak Woo of Seoul National University have found strong evidence for an elusive intermediate mass black hole at the core of a small (dwarf) galaxy. The groundbreaking work is published on June 10 on Nature Astronomy. The preprint is available here.

Astronomers have long debated the existence of intermediate mass black holes with masses between those of individual giant stars and the supermassive black holes found at the cores of larger galaxies. Supermassive black holes can have masses with millions, or even billions, of solar masses.

The team used light echoes, or light that bounces off material surrounding the galaxy’s nucleus, to make the determination. “We have measured the shortest delay time for any echo ever observed in the light coming from the material falling into a black hole at the center of a galaxy,” said Woo. “When we combine that with the deep spectroscopic observations from Gemini, our team determined that this black hole has a mass of about 10,000 times the mass of our Sun.”

According to Woo, the Gemini observations were critical in determining the velocity of gases swirling around the black hole. “These velocities, which are over 400 kilometers per second, when combined with our light echo measurements, provide a solid basis for estimating the mass of the galaxy’s central black hole,” adds Woo.

To determine the black hole’s mass, Woo and his team measured the velocity of gas clouds orbiting around the black hole (using the Gemini spectroscopic observations) and the distance of the gas clouds from the black hole (using the echo delay observations). Based on these two measurements (velocity and distance), the mass of the black hole can be calculated using the basic physics of Newton’s Laws.

The galaxy targeted by the team is a dwarf galaxy and goes by the designation NGC 4395. Careful observations of the varying intensity of the light emitted from the center of the galaxy confirmed that the additional “travel time” for the echoes of the emissions from gasses swirling around the black hole is on the order of 80 minutes. This sets critical limits on the size of the black hole’s influence and thus its mass.

At a distance of 14 million light years, the center of the dwarf galaxy NGC 4395 has been the subject of extensive studies in the past. The brightness of its nucleus signals the presence of an actively accreting black hole at its center but nailing down its mass has been difficult. “We believe we have nailed it this time,” said Woo.

“Korea joined Gemini as an international partner less than a year ago. Clearly, Dr Woo and his colleagues are already making great use of our flagship optical-infrared observatory to contribute to Gemini science advances,” said Chris Davis of the National Science Foundation (NSF).

In addition to the Gemini observations, which used the Gemini Multi-Object Spectrograph (GMOS) on the Gemini North telescope on Hawaii’s Maunakea, multiple observatories provided the data used to measure the light echo delays. The light echo measurements utilized the MDM Hiltner 2.4-meter telescope, the 1-meter Lemmonsan Optical Astronomy Observatory (LOAO), and the 1-meter Mt. Laguna Observatory (MLO).

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