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[ Instrument Network R & D ] Since the discovery of quasars for more than half a century, measuring their cosmological distance has been a major problem for astronomers. Recently, a team led by Wang Jianmin, a researcher at the Institute of High Energy Physics of the Chinese Academy of Sciences, developed a new method of geometric ranging, which successfully measured the cosmological distance of the quasar 3C 273. A related article "A parallax distance to 3C 273 through spectroastrometry and reverberation mapping" was published in Nature Astronomy on January 13, 2020.
Quasar geometric distance measurement requires observations with extremely high spatial resolution, and can only be achieved by interfering with Rayleigh limits. In the past ten years, the European Southern Observatory has made great efforts to successfully achieve the observation of the first quasar 3C 273 with a spatial resolution of 10 micro-arc seconds for the first time in the World's Large Telescope Optical Interference Array (VLTI). Wang Jianmin's team used interference data to cleverly combine the 10-year response mapping data of the Lijiang 2.4-meter telescope of the Yunnan Observatory of the Chinese Academy of Sciences and the Bok 2.3-meter telescope of the Steward Observatory of the United States to achieve high-precision ranging. This method does not rely on any existing distance steps, nor does it rely on corrections such as extinction, reddening, and standardization necessary for traditional tools, and systematic errors can be observed and inspected to accurately measure the geometry of the universe, study the expansion speed and history of the universe Opened up a new path.
Cosmology is based on measuring distances with high accuracy. In the 1920s, the American astronomer E. Hubble discovered that the universe was expanding: most galaxies are moving away from us, and the speed of regression (redshift) is directly proportional to the distance of the galaxy. This scaling factor is now called the Hubble constant, and it characterizes the current rate of expansion of the universe. One of the cores of observing cosmology is the measuring distance-redshift relationship. It describes the history of the expansion of the universe, and can directly answer basic questions about the age, geometry, and composition of the universe. It can even test many new physical predictions. In astronomical observation, the redshift of distant celestial bodies can be accurately obtained, but the accurate measurement of distance has always been the biggest problem for astronomers.
Traditional distance measurement tools are mainly Cepheid variable stars and type Ia supernovae. In the early days of Hubble's law discovery, distance measurement was mainly based on the weekly light relationship between Cepheid variable stars discovered by American astronomer HS Leavitt in 1912, that is, the period of light change is directly proportional to the luminosity. Therefore, by measuring the period of Cepheid variable stars, the absolute luminosity can be calculated and the distance can be estimated. This method has strong vitality and is still one of the main tools for distance measurement from more than 100 years ago. At present, the furthest Cepheid variable stars observed by astronomers are 29 Mpc (approximately 100 million light years) from the earth. Farther Cepheid variables are too dark to observe, and this tool is affected by extinction and reddening. Fortunately, based on the theoretical mass limit of the famous Chandrasekhar white dwarf, it was discovered that the Ia supernova can be used as a standard candlelight, opening a new door for measuring farther distances. The supernova's luminosity at the time of the eruption was high, comparable to that of the entire galaxy, allowing astronomers to measure farther distances than Cepheid variable stars. With this method, S. Perlmutter, B. Schmidt, and A. Riess measured high redshifted supernova samples, obtained distance-redshift relationships, and discovered the accelerated expansion and dark energy of the universe. In 2011 they won the Nobel Prize in Physics. Similar to Cepheid variable star ranging, this method also relies on extinction and reddening corrections because it involves luminosity, and is also limited by the standardization process of the Phillips relationship.
Another major breakthrough of the 20th century was the cosmic microwave background radiation, and its measurement brought astronomy into the era of "precise cosmology". Given a parametric cosmological model, cosmological parameters, including the Hubble constant, can be obtained from the anisotropy of microwave background radiation. However, with the improvement of observation accuracy, a deviation of up to 4.4σ occurs between the traditional method and the Hubble constant given by microwave background radiation. This is called the "Hubble constant crisis." This crisis means that either the effects of unknown factors are observed, or the standard model of cosmology needs to be modified, and new physics is likely to be embedded in it. At such a crossroads, astronomers are increasingly pressing for new precision tools. The new tool should not rely on existing distance ladders or standard cosmological models, but also have accuracy comparable to existing measurements (around 2%).
High spatial resolution is the eternal pursuit of astronomers, and also provides a rare opportunity for geometric methods to measure cosmological distances with high accuracy. GRAVITY is a terminal instrument costing nearly 100 million Euros and completed in ten years, assembled on VLTI. It achieves a spatial resolution of up to 10 micro-arcseconds in the near-infrared band, which is equivalent to a telescope with a diameter of 130 meters. Since it was put into use in 2017, it has obtained a lot of brand-new results in the fields of exoplanets, silver hearts, black holes, and micro-gravity lenses, and constantly refreshed human understanding of the universe. From 2017 to 2018, the GRAVITY team successfully measured the wide-line area of ​​the quasar 3C 273 at an angle of 46 microseconds. This is the current spatial resolution observation of the wide-line area of ​​the active galaxy's core. At the same time, Wang Jianmin's team has been using Lijiang's 2.4-meter telescope since 2012 to perform long-term spectral monitoring of the wide-line region of the active galaxy nucleus. By measuring the delay between the emission line relative to the continuous spectral light change, the physical scale of the wide line region can be directly obtained. A more detailed analysis can also obtain the geometry and dynamics of the gas in the wide line area, and measure the mass of the central black hole. This observation technique is called response mapping. The team observed that the super-Eddington accretion has special properties of active galaxy nuclei, and found that the delay is shortened and the black hole is saturated with luminosity. These phenomena have been confirmed by the observation of the US Sloan Sky Survey. In the past ten years, they have systematically developed various necessary analysis methods and software, which have laid a solid foundation for high-precision measurement of black hole mass and cosmological distance.
In addition, this study will also combine GRAVITY / VLTI interference and response mapping observations to achieve direct measurement of quasar distances and provide a new way to resolve the Hubble constant crisis. After the GRAVITY team released the interference observation results of the quasar 3C 273, Wang Jianmin's team was keenly aware of the complementarity between the two sets of independent observation data: GRAVITY observes the opening angle of the wide line area, and the response map observes the physics size. Through the comprehensive analysis of modeling, they obtained the angular distance and Hubble constant of 3C 273. Using only observations from a single quasar, the statistical error of the Hubble constant measurement is only 16%. 3C 273 is about 2 billion light-years from Earth, far beyond the limits of the distance measurement method using Cepheid variable stars. The reviewers believe that this work is a necessary solution to improve the quality of black holes and the accuracy of cosmological distance measurement. It is very timely and exciting and will be welcomed by the academic community.
At present, GRAVITY team and Wang Jianmin team are actively collaborating to observe and expand the sample. According to GRAVITY's existing observation capabilities, about 50 active galaxy nuclei can be used as GRAVITY-response mapping collaborative observation targets. It is expected to increase the Hubble constant measurement accuracy to more than 2% in the next few years. "Provides independent and accurate measurements. In the next 5 years, the observation capability of the next-generation GRAVITY will be greatly improved. At that time, distance measurement of quasars with redshifts up to z = 3 will be realized, and the distance-redshift relationship of wide redshift ranges will be established. Parameters, studying the history of the expansion of the universe, and examining cosmological models. This will open up a deep understanding of cosmology, dark matter and dark energy, and new physics.
This research was supported by major projects of the National Natural Science Foundation of China and key special projects of the Ministry of Science and Technology.