We Can Now Precisely Measure The Mass of Black Holes

by Monica Joshi

Long since black holes were discovered, we have been fascinated in trying to understand every property that they hold. When Interstellar came out, we marvelled at the graphics representing them and even appreciated Christopher Nolan's efforts (not enough for him to win an Oscar, though). We keep pushing. A research group led by Kyoko Onishi at the SOKENDAI (The Graduate University for Advanced Studies) observed the barred spiral galaxy NGC 1097 with ALMA and found that the central supermassive black hole (SMBH) has a mass 140 million times the mass of the Sun.

Galaxies are believed to have co-evolved with SMBHs. Therefore, SMBH mass is an important parameter in understanding their relation in the context of galaxy evolution. The results of this research is based on the ALMA observation data obtained within a two-hour observation. This demonstrates the outstanding capacity of ALMA in the SMBH mass measurement.

Majority of galaxies in the universe are thought to have a massive black hole in the galactic center. As these black holes have masses of several millions to tens of billions of solar masses, they are called "supermassive black holes (SMBHs)". Recent observation results suggest that there is a correlation between the SMBH mass and the central bulge mass/luminosity of the host galaxy. Such correlation indicates that SMBHs may have a key role in the growth and evolution of galaxies.

There are several ways to obtain the SMBH mass, one of which is using proper motion of stars and megamasers (astrophysical objects that emit strong radio waves) around the SMBH to estimate the gravity of the SMBH applied to the observed objects. This measurement method, however, is difficult and not suitable for the most galaxies because it requires observations of the regions around the SMBH with very high angular resolution. A second method is by using ionized gas dynamics distributed in the host galaxy bulge. Although since the ionized gas is easily affected by non-circular motion such as inflow or outflow of gas, in addition to the gravity of the SMBH, this makes it difficult to accurately measure SMBH mass for a large number of galaxies.

Finally, the method most commonly used to estimate SMBH mass is the one using stellar dynamics in host galaxies, although its application is rather limited to elliptical galaxies and thus it won't be a versatile SMBH mass measurement method applicable to wide ranging types of galaxies.

An alternative to these conventional methods is to derive the SMBH mass from molecular gas dynamics in the central region of a galaxy, which was formulated by Davis et al. at the European Southern Observatory (ESO). This method has the advantage that molecular gas is less susceptible to environmental conditions compared to stars and ionized gas. Thus, the motion affected by SMBH gravity can be measured more easily. The team made observations of a galaxy, NGC 4526, for countless hours with a radio telescope called CARMA and estimated the mass of the central SMBH.

The research team specifically measured the distribution and kinematics of molecular gas by observing emission lines from hydrogen cyanide and formyl cation with ALMA. They then examined the gravitational motion of the molecular gas by making some astrophysical models. As gravity applied to the molecular gas differs depending on the SMBH mass in conjunction with the density and distribution of stars in the bulge, the gas motion was calculated by making models assuming various cases. T

hey wanted to find a model which is best fitted for the observation data. It was found that the central SMBH of NGC1097 has a mass 140 million times the solar mass. This is the very first SMBH mass measurement using this method in late-type galaxies.

Onishi said, expressing her expectations for future ALMA observations:

"We could obtain the kinematics data of the central molecular gas in NGC 1097 within a two-hour ALMA observation. To reveal the relation between the SMBH and the host galaxy, we need to derive more SMBH masses in various types of galaxies. ALMA will enable us to observe a large number of galaxies in a practical length of time."

Measuring the mass of SMBHs is the first step to solving the long-standing mysteries about how galaxies and SMBHs have been formed and co-evolved. This research maintain that ALMA has the ability to do just that. Future research and measurements should be easier and more reliable.

However, while we are busy studying them a wandering black hole may catch up with us one day and eat us for breakfast and it wouldn’t even burp in the process. Michio Kaku weighs in on the matter.