Directly measuring the masses of supermassive black holes – the Hamden Journal

Testing of a new direct method to determine the masses of supermassive black holes.
Astronomers at 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 the radiation emitted by gases in the vicinity of supermassive black holes. This measurement simultaneously determines the speed of rotation of the radiant gas and its distance from the center of the accretion disk from which the material flows into the black hole. Compared to other methods, spectroastrometry is relatively simple and efficient if performed with large modern telescopes. The high sensitivity of this method makes it possible to study the environment of luminous quasars and supermassive black holes in the early Universe.

In cosmology, determining the mass of supermassive black holes in the young Universe is an important measure for tracking the temporal evolution of the cosmos. Now Felix Bosco, in close collaboration with Jörg-Uwe Pott, both of the Max Planck Institute for Astronomy (MPIA) in Heidelberg, and former MPIA researchers Jonathan Stern (now Tel Aviv University, Israel) and Joseph Hennawi (now UC Santa Barbara; USA and Leiden University, Netherlands), succeeded for the first time in demonstrating the feasibility of the direct determination of the mass of a quasar by spectroastrometry.

This method makes it possible to determine the mass of distant black holes in light quasars directly from optical spectra, without the need for in-depth hypotheses on the spatial distribution of the gas. The spectacular applications of spectroastrometric measurements of quasar masses were systematically studied at the MPIA several years ago.

Quasars: beacons of the Universe

Quasars contain supermassive black holes in the center of galaxies and are among the brightest cosmic objects. Consequently, they are detectable over great distances and thus allow the exploration of the early Universe.

If there is gas near a black hole, it cannot fall into it directly. Instead, an accretion disk forms, a vortex that helps matter flow into the black hole. High frictional forces in this gas flow, which ultimately feeds into the black hole, typically heat the accretion disk to fifty thousand degrees. The intensity of the radiation emitted during the process makes the quasars so bright that they eclipse all the stars in the galaxy.

Other components of quasars have been known for several decades, such as the "wide emission line region" (BLR), an area in which clouds of ionized gas orbit around the central black hole at speeds of several thousand. kilometers per second. The intense and energetic radiation of the accretion disk stimulates the emission of the gas in the BLR, which is visible in the spectra in the form of spectral lines. However, due to the Doppler effect, they are greatly enlarged by the high orbital speeds, thus giving the BLR its name.

A new method of measuring the masses of black holes

Now Felix Bosco and his colleagues have measured the optically brightest spectral line of hydrogen (Ha) in the BLR of quasar J2123-0050 in the constellation Aquarius. Its light dates from a time when the Universe was only 2.9 billion years old. Using the spectroastrometry method, they determined the putative distance from the radiation source in the BLR to the center of the accretion disk, the location of the potential supermassive black hole. At the same time, the Ha line provides the radial velocity of hydrogen gas, that is, the velocity component that points towards the Earth. Just as the mass of the Sun determines the orbital speeds of the planets in the solar system, the mass of the black hole at the center of the quasar can be accurately deduced from this data if the distribution of gases can be spatially resolved.

However, even for today's large telescopes, the scope of the BLR is far too small for this. "However, by separating the spectral and spatial information in the collected light, as well as statistically modeling the measured data, we can derive distances well below an image pixel from the center of the accretion disk," says Felix Bosco. . The duration of the observations determines the accuracy of the measurement.

For J2123-0050, astronomers calculated a black hole mass of at most 1.8 billion solar masses. "The exact determination of mass was not yet at all the main objective of these early observations," explains Jörg-Uwe Pott, co-author and leader of the "Black holes and accretion mechanisms" working group at the MPIA. "Instead, we wanted to show that the spectroastrometry method can in principle detect the kinematic signature of the quasar's central masses using the 8-meter telescopes already available today." Spectroastrometry could thus be a valuable addition to the tools that researchers use to determine the masses of black holes. Joe Hennawi adds, "With the dramatically heightened sensitivity of James Webb Space Telescope (JWST) and the Extremely Large Telescope (ELT with a primary mirror diameter of 39 meters) currently under construction, we will soon be able to determine the masses of the quasars at the highest redshifts. Jörg-Uwe Pott, who also leads Heidelberg's contributions to ELT's first near infrared camera, MICADO, adds: "The now published feasibility study is helping us define and prepare our planned ELT research programs.

Spectroastrometry valuable complement to classical methods

Among the alternatives for probing the BLR in nearby quasars, there is a widely used method: "Reverberation Mapping" (RM). It uses the transit time of light that any fluctuation in brightness in the accretion disk needs to excite the surrounding gas to increased radiation. From this, astronomers estimate the average extent of the BLR. In addition to the sometimes considerable uncertainties in the hypotheses, this method presents decisive drawbacks compared to spectroastrometry for the investigation of the most massive black holes and the most distant. The diameter of the BLR correlates with the mass of the central black hole. Therefore, the signal delay between the accretion disk and the BLR becomes very large for massive black holes in the first Universe. The series of measurements required for several years are becoming too long.

Additionally, fluctuations in brightness and measurability tend to decrease with increasing black hole mass and quasar brightness. The RM method is therefore rarely applicable to light quasars. Consequently, it is not suitable for measuring quasars at large cosmological distances.

However, the RM serves as the basis for calibrating other indirect methods first established for nearby quasars, then extended to more distant luminous quasars with massive black holes. The quality of these indirect approaches comes and goes with the precision of the RM method. Here, too, spectroastrometry can help broaden the determination of the mass of massive black holes. For example, the evaluation of the data from J2123-0050 indicates that the correlation between the size of the BLR and the luminosity of the quasars, initially established with the method RM for rather close and weak quasars, seems in fact also to be valid for the luminous quasars. . However, other measures are needed here.

BLR can also be measured interferometrically in nearby active galaxies, such as with the GRAVITY instrument of the Very large telescope Interferometer (VLTI). The great advantage of spectroastrometry, however, is that only one very sensitive observation is required. In addition, it does not require the technically very complex coupling of several telescopes as required by interferometry or long series of measurements over months and years as is the case with MR. For example, a single series of observations with an exposure time of four hours with the 8-meter-class Gemini North telescope in Hawaii, supported by a correction system consisting of a laser guide star and adaptive optics, was sufficient for the research group led by Félix Bosco.

Open a new door to the exploration of the early Universe

Researchers have high hopes for the next generation of large optical telescopes such as ESOis ELT. The combination of an enlarged light collecting surface with five times the image sharpness would make the observation presented here possible in a matter of minutes at the ELT. Felix Bosco explains, "We will use the ELT to astrometrically measure many quasars at different distances in a single night, allowing us to directly observe the cosmological evolution of black hole masses. With the success of the astrometric feasibility study, the authors have opened a whole new door to the exploration of the early Universe.

The references:

"Spatial resolution of the kinematics of the mainline region of the quasar?" 100 µas using spectroastrometry. II. The First Tentative Detection in a Luminous Quasar at z = 2.3 "by Felix Bosco, Joseph F. Hennawi, Jonathan Stern and Jörg-Uwe Pott, September 22, 2021, The Journal of Astrophysics.
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, April 30, 2015, The Journal of Astrophysics.
DOI: 10.1088 / 0004-637X / 804/1/57