Milky Way’s cold gas clouds mapped in unprecedented detail by MeerKAT Absorption Line Survey
An international team of astronomers led by professor Neeraj Gupta, astronomer at the Inter University Centre for Astronomy and Astrophysics (IUCAA), India, on Wednesday released the most comprehensive catalogue of cold hydrogen gas in the Milky Way galaxy as part of the MeerKAT Absorption Line Survey (MALS) project using data from the most sensitive radio telescope on Earth, the MeerKAT telescope in South Africa.
Such hydrogen atoms may eventually transform into molecular gas, the basic fuel for star formation. Therefore, these cold gas structures are key to the conditions that enable star formation and are of considerable interest to astronomers.
Professor Gupta who is the lead investigator of the MALS project, said, “This publicly released catalogue will help us address a wide range of questions about the distribution of cold gas in the halo of the Milky Way and the processes through which galaxies convert gas into stars over cosmic time.”
“A hydrogen atom consists of an electron and a proton. Through their interaction, it can produce a spectral line in emission or absorption at a wavelength of about 21 centimetres. The strength of this 21 cm spectral line of hydrogen in emission is insensitive to temperature and hence traces both cold and warm components of the gas. In turn, the absorption signal is more sensitive to colder phases. Over the last several decades, many astronomical surveys have tried to map the complex structures of the gas in the Milky Way in emission at 21 centimetres wavelength. Still, the absorption line measurements require more sensitive measurements and hence remained sparse. MALS changes this fundamentally with about 3500 detections,” said Gupta who is currently on a sabbatical at NRAO in USA.
Juergen Kerp, researcher at the University of Bonn in Germany for multi-wavelength studies, said, “The simultaneous detection of this line in emission and absorption has the power to unravel the distribution of various thermal components and complex microphysics through which cold gas structures are formed.”
Vast volumes of MeerKAT data were processed using a sophisticated processing pipeline and data storage facility set up at IUCAA to generate the MALS catalogue. The MALS team combined this catalogue with the existing maps of 21 centimetres line emission, optical and far-infrared radiation to disentangle the distribution of the interwoven atomic gas phases.
Sergei Balashev, researcher at Loffe Institute, St. Petersburg, Russia, said, “These multi-wavelength data allow us to establish relationships between the properties of the thermal components of atomic gas and the local conditions such as radiation and turbulence generated by the release of energy from stars.”
Figure 1: In this full-sky map presented in Galactic coordinates, the background shows the brightness of 21 centimetres line emission tracing both warm and cold components of neutral hydrogen gas from the Milky Way galaxy as mapped by the HI4PI collaboration. The horizontal stripe at the centre is the Milky Way disc. Also seen are filamentary structures of gas flowing in or out of the disc. The circles mark the positions of 391 MALS pointings containing a total of about 20,000 bright radio sources, typically supermassive black holes at the heart of galaxies located at cosmological distances. Also shown are examples of small absorption signals caused by cold hydrogen gas structures in the Milky Way. The combination of emission and absorption signals enables the exploration of gas physics in the Milky Way in unprecedented detail.
