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Directly Measuring Masses of Supermassive Black Holes

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Quasar Schematic Representation
Quasar Schematic Representation

Schematic representation of a quasar. The hot accretion disk in the center surrounds the black hole, which is invisible here. A dense distribution of gas and dust surrounds it in which individual ionized gas clouds orbit the black hole at high speed. Stimulated by the intense and high-energy radiation of the accretion disk, these clouds emit radiation in the form of spectral lines, broadened due to the Doppler effect. The zone of these gas clouds is therefore called broad emission-line region (BLR). Credit: Graphics department/Bosco/MPIA

Testing a new, direct method for determining the masses of supermassive black holes.
Astronomers of the Max Planck Institute for Astronomy have, for the first time, successfully tested a new method for determining the masses of extreme black holes in quasars. This method is called spectroastrometry and is based on the measurement of radiation emitted by gas in the vicinity of supermassive black holes. This measurement simultaneously determines the rotational velocity of the radiating gas and its distance from the center of the accretion disk from which material flows into the Spectroastrometry Signal Origin

Schematic representation of the origin of the spectroastrometry signal. If the ionized gas were at rest, we would measure the same wavelength of the spectral line throughout the BLR. However, the gas clouds orbit the black hole. Seen from the side, they come towards us on one side while they move away again on the other. As a result, the spectral signal appears blue-shifted towards shorter wavelengths on one side. On the other side, it is red-shifted towards longer wavelengths. This difference in the measured wavelength depending on the position along the BLR results in the spectroastrometry signal indicated above. From this, researchers can determine the maximum distance of the observed BLR clouds from the center of the quasar and the prevailing velocity there. Credit: Graphics department/Bosco/MPIA

Even for today’s large telescopes, however, the extent of the BLR is far too small for this. “However, by separating spectral and spatial information in the collected light, as well as by statistically modeling the measured data, we can derive distances of much less than one image pixel from the center of the accretion disk,” Felix Bosco explains. The duration of the observations determines the precision of the measurement.

For J2123-0050, the astronomers calculated a black hole mass of at most 1.8 billion solar masses. “The exact mass determination was not yet the main goal of these first observations at all,” says Jörg-Uwe Pott, co-author and head of the “Black Holes and Accretion Mechanisms” working group at MPIA. “Instead, we wanted to show that the spectroastrometry method can in principle detect the kinematic signature of the central quasar masses using the 8-meter telescopes already available today.” Spectroastrometry could thus be a valuable addition to the tools that researchers use to determine black hole masses. Joe Hennawi adds, “With the significantly increased sensitivity of the Gemini North LGS by Moonlight

Photo of the dome of the Gemini North telescope in Hawaii, USA. This telescope has a primary mirror diameter of 8.1 meters and a laser guide star that, together with adaptive optics, helps minimize the influence of the atmosphere on observations. Gemini North was used for the spectroastrometry feasibility study. Credit: Gemini Observatory

However, the RM serves as a basis for calibrating other indirect methods first established for nearby quasars and then extended to more distant, luminous quasars with massive black holes. The quality of these indirect approaches stands and falls with the DOI: 10.3847/1538-4357/ac106a

“Spatially Resolving the Kinematics of the Quasar Broad-Line Region Using Spectroastrometry” by Jonathan Stern, Joseph F. Hennawi and Jörg-Uwe Pott, 30 April 2015, The Astrophysical Journal.
DOI: 10.1088/0004-637X/804/1/57