Despite everything astronomers have learned about the nature and structure of galaxies, there are still mysteries about the Milky Way. The reason for this is simple: since we are embedded in the Milky Way’s disk, we have difficulty mapping it and observing it as a whole. It’s also very challenging to observe the center of the galaxy, what lies beyond it, and features in the disk itself because of all the gas and dust between stars- the Interstellar Medium (ISM). However, by observing the Milky Way in the non-visible spectrum (radio, x-ray, gamma-ray, etc.), astronomers can see more of what’s out there.
There’s also the spectral line that corresponds to the emission frequency (1420 MHz) of cold neutral hydrogen gas (HI), which makes up the majority of the ISM. Using the Five-hundred-meter Aperture Spherical Telescope (FAST) – the most powerful radio telescope in the world near Guizhou, China – a team of scientists located more than 500 new faint pulsars. During the survey, the team simultaneously recorded the spectral line data with high spectral and spatial resolution, making it an extremely valuable resource for studying the structure of the Milky Way Galaxy and the life cycle of its stars.
The research was conducted by members of the Galactic Plane Pulsar Snapshot HI (GPPS HI) survey, which consists of astrophysicists with the Chinese Academy of Sciences (CAS) National Astronomical Observatories, the CAS Key Laboratory of FAST, the University of Chinese Academy of Sciences in Beijing, Peking University, Guangzhou University, and Southwest University in Chongqing. Their results were published in a series of papers (“Peering into the Milky Way by FAST“), which recently appeared in the Science China Physics Mechanics and Astronomy journal.
Observing neutral hydrogen in the ISM is vital to astronomers since it is from dense hydrogen clouds that new stars are born. When young and bright, these stars are detectable from how their radiation ionizes surrounding gas. In time, stars will undergo gravitational collapse and go supernova, shedding their outer layers and becoming white dwarf remnants (like a pulsar). The shock waves produced by these explosions compress interstellar gas and accelerate electrons (and nuclei) to close to the speed of light that interact with the interstellar magnetic field and radiate faint radio waves.
The study of the ionized gas in the interstellar medium is therefore fundamental to the study of the birth and death of stars. It is also the last major component of the Milky Way that hasn’t been explored in detail. The FAST array is especially suited for this purpose because it is the most sensitive single-dish radio telescope in the world and is equipped with a highly-sensitive L-band 19-beam cryogenic receiver. This corresponds to radio frequencies of 1 to 2 gigahertz (GHz) in the Ultra High Frequency (UHF) domain, making it an excellent instrument for hunting pulsars and exploring the ISM.
In its first data release, FAST detected the distribution of neutral hydrogen (HI) gas across 88 square degrees of the night sky, between 33° and 55° Galactic longitude and ±2° Galactic latitude. Though the fine calibration is still in progress, the results of this survey are already the most sensitive to date, showing unprecedented detail about the distribution of HI gas in our galaxy. As emeritus professor John M. Dickey of the University of Tasmania in Australia and the University of Minnesota wrote of the GPPS HI survey:
“[T]he improvement in angular resolution and sensitivity over all previous surveys is impressive. FAST can go deeper and detect fainter emission over wider areas than other telescopes both because it has a larger collecting area (aperture diameter 300 m within the reflector diameter of 500 m), and because it uses a sensitive, 19-beam receiver with Tsys?22 K to collect the radiation… Meanwhile, [the] publication of this first paper on the GPPS HI survey is a landmark accomplishment, worthy of celebration and international attention.”
For the first study in the series, the CAS team processed the hydrogen radio recombination lines (RRL) in the spectral line data for the same sky area covered by the GPPS HI survey. This revealed luminous regions resulting from bright star ionization and patches of diffuse ionized gas (DIG) of unknown origin. The team also tested the FAST observations for radio continuum radiation within our galaxy in an area of 5°× 7° in the sky. The results confirmed the presence of two large faint radio-emissions structures (G203.1+6.6 and G206.7+5.9) that correspond to supernova remnants.
They further confirmed that one of these supernovae took place roughly 1,400 light-years from the Sun. This data is likely to have significant value for studies of the cycle of gas and star formation in the Milky Way. Dr. Dana S. Balser, a scientist at the National Radio Astronomy Observatory (NRAO), was quoted by China Daily Mail, saying:
“This GPPS RRL survey is the most sensitive survey to date and has sufficient angular resolution to separate DIG emission from HII regions… Large single-dish telescopes such as the FAST are the best to probe the DIG, the last major component of the Milky Way Galaxy to be well characterized.”
In addition, the team relied on FAST data to measure the polarization and Faraday rotation of 134 faint pulsars in the Galactic halo – the part of our galaxy that extends beyond the main visible component. This required that they measure the magnetic fields that permeate the ISM in this region of the galaxy, which is no easy feat. Nevertheless, using FAST’s extreme sensitivity, the team not only located these pulsars but obtained a magnetic field strength reading of about two microgauss in the Galactic halo. These new measurements provide evidence for magnetic field reversals in the spiral arms of the outer Milky Way.
The interstellar magnetic field in such a wide region was impossible before FAST became operational. Jing Yipeng, a CAS researcher from Jiao Tong University in Shanghai, looks forward to the research that the GPPS HI and future surveys using FAST will allow. “The sensitive FAST observations can reveal unprecedented details of the Milky Way,” he said. “The databases of neutral hydrogen and ionized hydrogen published by these papers are valuable resources for astronomers [all] over the world.”
Further Reading: EurekAlert!, Science China Physics, Mechanics & Astronomy