Active Galaxies and Their Bizarre Centers Help to Measure Black Hole Masses

July 26, 2011

Ever since the revelation that the majority of galaxies in the universe host a supermassive black hole at their centers, astronomers have been working to measure the masses of these black holes in order to better understand galaxy formation and evolution.

Graduate student Catherine Grier is involved in the measurement of the masses of black holes at the centers of some of the brightest known galaxies in the universe. These galaxies, called active galactic nuclei (AGN), are defined in part by their luminous centers that undergo small fluctuations in brightness over a short period of time, similar to a firefly blinking at night.

“The fluctuations in the brightness of AGNs are what make it possible to indirectly measure the mass of the galaxy’s supermassive black hole,” said Grier. Grier works with faculty member and department chair Professor Bradley Peterson, who is widely known for his work on AGNs and his book An Introduction to Active Galactic Nuclei, which is a textbook for one of the astronomy graduate courses.

“The slight changes in brightness allow us to measure how far away the emitting gas is from the central black hole,” explained Grier. “After we have calculated the distance and velocity of the gas, we can use fairly basic gravity laws to infer the black hole’s mass,” she continued.

While this method has been in existence for more than two decades, it has been used to measure black hole masses in only around 40 AGNs. The process of determining the distance of the gas to the black hole is painstakingly long and difficult and requires many individual observations to collect the necessary amount of information.

Until recently, this method – known as reverberation mapping – was the only way to measure black hole masses in AGNs that are far away. However, the use of reverberation mapping has led to a very important discovery: the distance of the emitting gas from the black hole is related to the overall brightness of the AGN. This highly valuable relation is now being used to approximate the black hole mass of many AGNs simply by looking at how bright they are and using this single brightness measurement to obtain a gas distance estimate, explained Grier.

Picture of Catherine Grier.“The relationship between overall brightness and gas distance is very useful for obtaining black hole mass measurements for AGNs observed in large surveys that contain hundreds to thousands of candidates. This allows us to make black hole mass measurements across the universe rather than for just those objects that are near enough for us to apply reverberation mapping methods. It is important that we continue to make reverberation measurements to further develop this very important relation,” Grier remarked.

Grier is currently working with data taken of seven different AGNs from which she hopes to get accurate supermassive black hole mass measurements. The collection of these significant amounts of data was made possible by an impressive length of telescope time lasting 125 days that Grier was allotted in fall of 2010. This is far longer than the average time graduate students are given on publicly available telescopes, which tends to last from one to three weeks.

Grier used the 1.3-meter telescope at MDM observatory atop Kitt Peak in Arizona to observe these AGNs. “It was a little nerve racking at first because the first two months of observations were hindered by bad weather, but after the skies cleared up the data really started coming in,” Grier remarked.

Thanks to the many helpful astronomy graduate students at Ohio State and other MDM consortium institutions, Grier did not remain in Arizona for the full 4 months. She managed to have a different graduate student observe for her every week while she received and processed the data from Columbus.

Photo of the seed that lodged itself onto the telescope for a night.“One interesting and unexpected thing that happened during observations was the ‘flying spaghetti monster’ that appeared in one of the telescope’s images,” Grier said with a laugh. “Of course it wasn’t a real monster, but a seed that had lodged itself on the telescope, which impeded our ability to observe for the night. The seed eventually dislodged itself on its own – it’s a good example of how observations can be impacted by random factors,” she continued.

One of the AGNs observed during this campaign has been a part of Grier’s research for several years. A suggestion that this object should be named after her was picked up by one of the main astronomical databases, and as a result, the AGN in question is now fondly known (and officially listed) as “Kate’s Quasar”.

Grier will go on to write her PhD dissertation on her work with reverberation mapping of AGNs.

Written by Jessica Orwig