Figure 1: The first image of the black hole at the center of the Milky Way
(Image credit: Courtesy of the EHT Partnership)
Previously, the Nobel Prize in Physics was awarded “for the discovery of the black hole at the center of the Milky Way”. Photos released by the EHT today provide direct visual evidence of the existence of this supermassive black hole.
1. Decades ago, the black hole at the center of the Milky Way was “discovered”
In the late 1950s, with the development of the all-sky radio source census, it was discovered that there is a class of strong radio sources whose optical counterparts appear to be stars, but have incomprehensible optical spectra, which are called by astronomers. Quasar. In 1963, Schmidt (Schmidt 1963) solved the problem in one fell swoop by identifying the strongly redshifted hydrogen Barr line in the spectrum of the quasar 3C 273, and he concluded that 3C 273 is not a star, but a distant galaxy of extremely bright nuclei.
Since most quasars have very large redshifts, are very far from humans, and because the brightness of these quasars is not much different from that of ordinary stars in the Milky Way, they have enormous energies that can radiate more power than an ordinary galaxy Thousands of times the total radiated power. However, their luminosity can change dramatically within days to weeks, suggesting that quasars are only a few light days to a few light weeks in size. So the question is, where does the enormous energy of quasars come from?
After the discovery of quasars, people have successively proposed various models to explain the energy production mechanism of quasars. In these models, radiation from supermassive black holes accreting matter has gradually become the accepted explanation.
In the late 1960s, Lynden-Bell proposed that many galaxies have a supermassive black hole at their center with a mass from one million to several billion solar masses. He asserted that such a supermassive black hole is a remnant of an active “quasar phase” in the past (Lynden-Bell 1969). Likewise, the Milky Way should not be an exception. Two years later, Lynden-Bell and Rees (1971) demonstrated the existence of a supermassive black hole at the center of the Milky Way, and proposed that the Very Long Baseline Interferometry (VLBI) technique could soon determine the size of the black hole at the center of the Milky Way.
The nature of quasars has been further understood, but detecting the dense radio source associated with the black hole at the center of the Milky Way has been a difficult and intriguing process. (Interested readers can refer to Goss, Brown & Lo 2003)
In February 1974, Balick & Brown formally detected the dense radio source corresponding to the black hole at the center of the Milky Way with the Green Bank radio interferometer in the United States. Since then, different names have been proposed for the compact radio source, but in the end only the name Sgr A* has withstood the test of time and was accepted (Brown 1982). Brown’s explanation is that the naming is analogous to how excited atoms are named in atomic physics.
It is no exaggeration to say that human beings realize that “Sgr A* is the radio source corresponding to the black hole of more than 4 million times the mass of the sun in the center of the Milky Way” represents a fundamental progress in our understanding of galactic nuclei. In the following decades, people’s desire to directly detect the black hole has continuously promoted the development of technology, enabling humans to “close” to the edge of the black hole step by step.
2. From centimeter-wave to millimeter-wave, approaching Sgr A* with VLBI
The first detection of Sgr A* took many attempts, mainly because the galactic center was affected by strong interstellar scattering (Davies, Walsh & Booth 1976). Due to the dominance of scattering effects, the observed shape of Sgr A* in the centimeter and longer wavelength bands is an east-west ellipse Gaussian whose size is proportional to the square of the observed wavelength. In the early days of the development of VLBI technology, due to the very limited number of radio telescopes at that time, it was necessary to detect Sgr A* at the “correct” observation wavelength and between radio telescopes at the “right” distance.
Since the scattering effect will decrease rapidly with the increase of the observation frequency, only in the (sub) millimeter waveband can we get rid of the effect of scattering and see the true face of Sgr A*. Indeed, at wavelengths longer than a few centimeters, the observed structure of Sgr A* is entirely dominated by scattering. The intrinsic structure of Sgr A* only gradually emerges when observed at wavelengths of about 1 cm and less. As the observation wavelength continues to decrease to the (sub) millimeter waveband, on the one hand, the resolving power of the interferometer will continue to increase, and on the other hand, it will be easier to overcome the opacity effect caused by the self-absorption of synchrotron radiation. These are all conducive to gradually seeing the ring-shaped (sub)millimeter-wave radiation structure (the “black hole shadow”) that is getting closer and closer to the black hole and determined by its gravitational bending.
In VLBI observations, two methods are often used to analyze and interpret the observed “visibility” data:
Model fitting is performed directly on the visibility data, usually using some geometric models, such as two-dimensional circle or ellipse Gaussian shape, ring, disk or crescent model, etc. The complexity of the model here is determined by the characteristics of the data.
The visibility data is imaged, and then the image is modeled and analyzed to obtain the relevant model parameters, so as to quantitatively describe the observed radiation structure.
The two methods have their own advantages and disadvantages, and the model fitting is relatively straightforward, especially when the number of telescopes is small and the baseline coverage is not enough for imaging, some more reliable conclusions can be drawn. A typical case is Whitney et al. (1971). In the case of the observation data of two telescopes (one telescope baseline), the apparent superluminal phenomenon in 3C 279 was found by the method of model fitting. This is why many early observations use this method. But this tends to lose detail due to the simplicity of the model. On the contrary, the results of imaging will be more intuitive, but the imaging process will introduce some additional uncertainties. In many jobs, both methods are used simultaneously to obtain the most reliable results, and these processes are often combined with the calibration of the data.
With the development of VLBI technology and observation equipment, a series of high-resolution observations of Sgr A* have been carried out, especially in the millimeter waveband in the past two decades.
At the 7 mm wavelength, the first imaging results were obtained by Krichbaum et al. in 1993 (Krichbaum et al. 1993), but due to the small number of telescopes involved in the observation, these results still have large uncertainties. Although there are many subsequent observations in this band, due to the large uncertainty in the data calibration, people have not been able to accurately determine and deduct the influence of the scattering effect, and thus cannot know the intrinsic structure of Sgr A*. One of the main reasons is that the vast majority of telescopes participating in the observation are not specially built for millimeter-wave observations, and are mostly located in the northern hemisphere, which suffers from severe atmospheric influences when observing Sgr A* in the southern sky. In 2004, Bower et al. measured the intrinsic magnitude of Sgr A* after determining and deducting scattering effects by using a closed-amplitude method to remove uncertainty in data calibration (Bower et al. 2004).
At 3 mm wavelength, Rogers et al. first detected Sgr A* in 1994. In 2002, an international team led by Shen Zhiqiang, a researcher at the Shanghai Astronomical Observatory of the Chinese Academy of Sciences, carried out the first high-resolution imaging observation of Sgr A* using the Very Long Baseline Interferometry Array VLBA in the United States (as shown in Figure 2), and measured Sgr A* at 3 With the intrinsic size of millimeters, we found convincing evidence for the existence of supermassive black holes in the center of the Milky Way (Shen Zhiqiang et al. 2005).
Figure 2: CLEAN image of Sgr A* at 3 mm, the left and right images correspond to the reconstructed images using elliptical and circular beam cleaning respectively (Image source: Shen Zhiqiang et al. 2005)
With the addition of millimeter-wave telescopes located in the southern hemisphere (eg, the Large Millimeter-Wave Telescope LMT, the Akatama Large Millimeter-Submillimeter Array ALMA), observations in recent years have been able to better confine the 2D intrinsic structure of Sgr A* and the nature of interstellar scattering (eg Issaoun et al. 2019, 2021).
In the 1 mm wavelength band, true VLBI imaging has not been achieved due to the limitation of the number of millimeter-wave telescopes. In 1998, Krichbaum et al. (1998) achieved the first 1 mm fringe detection for SgrA* between two IRAM telescopes in France and Spain, and obtained its angular size at 1 mm. Doeleman et al. (2008) carried out 1 mm observations using a three-station array and found that Sgr A* has a dense structure at the scale of the event horizon. By fitting a circular Gaussian geometry, the size of the structure was found to be 37 microarcsec. Due to data limitations, these observations cannot yet be used to determine models more complex than a circular Gaussian. Fish et al. (2011) used similar later observations to find that although the flux density of Sgr A* changed significantly over several days, its magnitude over time was not. Johnson et al. (2015) found that the dense structure of Sgr A* has obvious linear polarization characteristics, which means that there is an ordered magnetic field structure around the black hole at the center of the Milky Way. By analyzing the closed-phase information in the VLBI data, Fish et al. (2016) found that Sgr A* has an asymmetry in the radiation structure at 1 mm. After the Atacama Pathfinder Experiment (APEX) in Chile was added to the 1mm VLBI array, Lu Rusen et al. (2018) found in 2018 that the observation data of Sgr A* could no longer be explained by a single Gaussian model . By considering a slightly more complex model than this, it was found that there are denser substructures within the overall 50 microarcsec structure. In particular, the crescent-shaped model (Fig. 3) that best fits the observational data, with a diameter of 52 microarcseconds, is surprisingly consistent with the results of black hole shadows predicted by general relativity. This is also the latest result of the 1-millimeter VLBI observation before the image of the black hole at the center of the Milky Way.
Figure 3: Model schematic diagram of the dense structure of Sgr A* (Image source: Lu Rusen et al. 2018 [Note: Because the collaborators reserved space for the first black hole imaging results, the optimal image of the model fitting could not be published in the original paper, Only gray schematics are available.]).
3. Its first photo, why was it “taken” for five years?
Since the EHT collaboration announced the results of the first M87 black hole imaging as early as 2019 (Lu Rusen & Zuo Wenwen 2019), the first imaging of the black hole at the center of the Milky Way can be said to be a long-awaited one. However, one can’t help but ask, since the EHT observed M87* and Sgr A* almost simultaneously in April 2017, why was the “photograph” of the latter so time-consuming?
Because “processing” this photo is more technically difficult.
On the one hand, in addition to the angular broadening caused by the diffraction effect in the aforementioned interstellar scattering, there is also the effect of refraction scattering. As a result, the introduction of the so-called “refraction noise” will be superimposed on the visibility corresponding to Sgr A* itself. amplitude information.
On the other hand, the more important reason is that the pattern and brightness of the radio emission of Sgr A* close to the black hole will show rapid changes (typically on the time scale of several minutes), much shorter than the observations usually required for VLBI imaging time (hours). Therefore, VLBI image reconstruction for such a variable source violates the basic assumption of the comprehensive imaging of the earth’s rotation aperture (Lulusen et al. 2016).
Combined with the still relatively sparse coverage of the current telescope baseline, these factors together make reconstructing the image of Sgr A* at the event horizon scale challenging. impact.
Since VLBI-reconstructed images are usually not unique, the EHT collaboration team used simulated data that was consistent with the characteristics of the observational data to “train” various imaging methods to select the optimal set of parameters required for imaging. Using these optimal parameter sets, we found that the vast majority of the resulting images showed ring-like structures whose diameter, width, and central darkness were consistent across different imaging methods and parameter choices. However, the reconstructed images showed diversity in their specific morphology, especially the intensity distribution along the azimuthal angles of the rings. This diversity is due to the still limited telescope baseline coverage of the EHT coupled with structural changes in Sgr A*.
All reconstructed images can be divided into four subsets according to their morphology. The images in three subsets show a ring-shaped structure, but the distribution of the brightness of the rings along the azimuth angle is different, and the fourth subset contains a relatively small number of The images, although they also fit the data, don’t look like rings. Finally, a representative image of Sgr A* was generated by averaging thousands of images obtained using different imaging methods (shown in Figure 4). Based on our understanding of the coverage of the telescope’s baseline, the time-varying characteristics, and the nature of interstellar scattering, combined with simulation data, we can say that the EHT observations strongly demonstrate that the image of Sgr A* is indeed composed of a ring with a diameter of 50 microarcseconds. This is very consistent with the size of the “shadow” expected for a black hole with a mass of 4 million solar masses and a distance of 8 kpc from Earth.
The imaging results provide direct evidence for the existence of a supermassive black hole at the center of the Milky Way, and for the first time link predictions from measurements of stellar orbital dynamics on the 103-105 gravitational radius scale to images and time-variation on the event horizon scale. Further, the comparison with the EHT imaging results of the supermassive black hole M87∗ shows the consistency of the predictions of general relativity in systems spanning three mass levels, fully proving that “the world is black as black holes”!
Figure 4: Above is a representative image of Sgr A* obtained by the EHT from the April 7, 2017 observations. The lower four figures show, from left to right, the average image of three subsets of images exhibiting ring structures and the average image of a subset of images that are not ring structures. The histogram in the figure shows the relative number of images belonging to each subset, and its height represents the relative contribution of each subset to the final photo. (Image credit: EHT Partnership)
4. The next step of “Black Hole Photo Studio” is “Black Hole Video”
As the closest supermassive black hole to humans, Sgr A* provides us with a unique laboratory for testing general relativity and exploring black hole astrophysics. With the release of the first photo of the black hole at the center of the Milky Way, follow-up work will use polarization observation data to study the magnetic field around the black hole and further study the structural changes associated with the observed X-ray flare activity.
After 2017, with the addition of new telescopes and the continuous increase of data recording bandwidth, the sensitivity of the EHT array has also been continuously improved, and the imaging capability of the variable source Sgr A* has been continuously enhanced. In the future, with the addition of more submillimeter wave telescopes, it is expected to achieve 24-hour uninterrupted relay imaging observations, and we will eventually be able to achieve dynamic imaging of the physical environment around the black hole. In this regard, building a submillimeter-wave VLBI telescope in China and participating in related observations will play a key role.
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Lu Rusen, Jiang Wu, Shen Zhiqiang/WeChat account of Shanghai Observatory of Chinese Academy of Sciences