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03. April 2025

A Map of the Cold Gas in the Milky Way International Team of Astronomers Maps Locations Where Stars Can Form

An international team of astronomers from the Inter-University Centre for Astronomy and Astrophysics (IUCAA) in India, the University of Bonn, and the Ioffe Institute in St. Petersburg, Russia, has surveyed the cold gas in the Milky Way using the most sensitive radio telescope on Earth—the MeerKAT telescope in South Africa. The "MeerKAT Absorption Line Survey" (MALS) is the most comprehensive published catalog of its kind to date. Astronomers are using the MALS survey to understand why the Milky Way continues to form new stars, even though it is billions of years old. The study has been accepted for publication in the journal Astronomy & Astrophysics, and a preprint is available at http://arxiv.org/abs/2504.00097.

The distribution of the 21-cm brightness across the entire sky is shown in grayscale.
The distribution of the 21-cm brightness across the entire sky is shown in grayscale. - Dark areas are regions with a high density of hydrogen, while bright areas have a low density. The Galactic Center is located at the center of the image. The plane of the Milky Way is rich in both stars and hydrogen gas. This map was created by Bonn astronomers in 2016, using observations from the largest fully movable radio telescopes on both continents: the Effelsberg 100-meter and the Parkes 64-meter telescope in Australia. The circles mark the positions of the 391 MALS observations, which led to the discovery of approximately 20,000 absorption lines from cold gas towards bright radio sources. Some of these absorption lines are shown as examples. The combination of emission and absorption signals allows the exploration of the gas physics of the Milky Way in unprecedented detail. © Grafik: Jürgen Kerp, AIfA
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“This publicly available catalog will help researchers answer a wide range of questions about the distribution of cold gas in the Milky Way's halo and the processes by which galaxies convert gas into stars over cosmic time,” says Neeraj Gupta, astronomer at IUCAA in India and lead of the MALS project.

Atomic hydrogen makes up the majority of matter in the universe and is the basic building block of all stars. Understanding the spatial distribution of atomic hydrogen in space therefore provides key insights into star formation, galaxy dynamics, the activity of black holes, and the past and future evolution of our Milky Way. As part of the "MeerKAT Absorption Line Survey" (MALS), an international team of astronomers has investigated the distribution of HI in the Milky Way.

 
A Complete Picture of the Gas Distribution in the Milky Way
The 21-centimeter emission line of neutral atomic hydrogen (HI) is used by astronomers to determine the distribution and velocity of atomic gas in galaxies. It enables the detection of both cold and warm gas. Cold gas can also be observed in absorption against the bright centers of galaxies that host supermassive black holes.

Radio astronomers at the University of Bonn have significantly contributed to surveying the sky using the 21-cm emission line with the world’s largest fully steerable radio telescopes, through projects like the Effelsberg-Bonn HI Survey (EBHIS) and HI4PI. However, measuring absorption lines requires the use of radio interferometers, such as MeerKAT. These allow astronomers to produce highly detailed images of the sky—focusing exclusively on cold gas.

Now, for the first time, combining the Bonn sky surveys (EBHIS/HI4PI) with the MeerKAT data provides a complete picture of the gas distribution in the Milky Way. “Simultaneous detection of the 21-cm line of atomic hydrogen in both emission and absorption has the potential to unlock the mystery of ongoing star formation in the Milky Way,” explains Dr. Jürgen Kerp, a senior researcher at the Argelander Institute for Astronomy, University of Bonn.

 
A Shipping Container Full of Data Analyzed
To create the MALS catalog, the international research team processed the enormous volume of data collected by MeerKAT—equivalent to a shipping container full of magnetic tapes—using a sophisticated processing pipeline and data storage system set up at IUCAA.

The MALS team combined this catalog with existing Bonn sky surveys, optical data, and far-infrared thermal radiation observations to better understand the interwoven atomic gas phases. “Multi-wavelength data are crucial to uncover the relationships between atomic gas and local excitation conditions caused by stellar radiation,” says Sergei Balashev, researcher at the Ioffe Institute in St. Petersburg, Russia.

 
Collaborating Institutions and Funding
The MALS team is an international collaboration of researchers from around the world, including the University of Bonn. The project is led by N. Gupta from IUCAA in India. The MeerKAT telescope is a facility of the National Research Foundation (NRF) in South Africa and is operated by the South African Radio Astronomy Observatory (SARAO). The enormous amount of raw data (1.6 petabytes) collected by SARAO is hosted at IUCAA and processed using an automated pipeline developed in collaboration with Thoughtworks Technologies India Pvt. Ltd. The pipeline extensively uses tools from the Common Astronomy Software Applications (CASA), developed by the National Radio Astronomy Observatory (NRAO) in the USA.

The catalog of Galactic HI absorption lines and the accompanying scientific results of this study are detailed in Gupta, Kerp, Balashev et al. (2025), which has been accepted for publication in Astronomy & Astrophysics:
Gupta, Kerp, Balashev et al. (2025): The MeerKAT Absorption Line Survey (MALS) Data Release 3: Cold Atomic Gas Associated with the Milky Way.
The publication is available as a preprint at https://arxiv.org/abs/2504.00097.

This is the third in a series of publications presenting radio continuum and spectral line data from MALS.

The MALS catalogs and images are publicly accessible at: https://mals.iucaa.in/

P.D. Dr Jürgen Kerp

Argelander-Institut für Astronomie

Universität Bonn

Tel.: +49 228 73 3667

E-mail: jkerp@astro.uni-bonn.de