Galaxy evolution and neutral hydrogen

By Neeraj Gupta, Professor, Inter-University Centre for Astronomy and Astrophysics (IUCAA), India, and Bärbel S. Koribalski, Senior Principal Research Scientist, Commonwealth Scientific and Industrial Research Organisation (CSIRO), Australia

Hydrogen is the most abundant element in the Universe. Every galaxy contains vast reservoirs of neutral hydrogen gas, which is the primary fuel for star formation.

The faint 21-cm radio signal that neutral hydrogen gas emits at 1420 megahertz (MHz) has, since its discovery in 1951, been a workhorse of modern astronomy. It allows us to map galaxies in their entirety, revealing structures invisible even to the largest telescopes at other wavebands on the ground and in space.

Yet in recent years, this signal – arriving at frequencies shared with navigation satellites, mobile communications, and other services that billions of people rely on every day – is becoming ever harder to detect.

Finding the right balance will enable society to continue the quest to understand our own origins by answering a question that has driven astronomy for decades: How did galaxies, including our own Milky Way, emerge from the hot gas created in the Big Bang 13.8 billion years ago?

Neutral surveys of galactic transformation

Galaxies are often two to three times larger in neutral hydrogen gas than in their bright stellar disks. Neutral hydrogen gas streams and bridges between galaxies are tell-tale signs of tidal interactions, and such interactions are now recognized as a common and important part of galaxy evolution.

Closer to home, the Magellanic Stream is a ribbon of neutral hydrogen gas spanning over 100 degrees on the sky, connecting the Large and Small Magellanic Clouds with the Milky Way. The 21-cm line reveals all this that would otherwise remain invisible.

The signal from distant galaxies arrives at lower frequencies due to the Doppler shift caused by the expansion of the Universe.

Modern radio telescopes with wide frequency coverage and large instantaneous bandwidths, such as the Jansky Very Large Array (JVLA) in the United States, the Giant Metrewave Radio Telescope (uGMRT) in India, the Australia Telescope Compact Array (ATCA), MeerKAT in South Africa, the Australian Square Kilometre Array Pathfinder (ASKAP), and the Five-hundred-meter Aperture Spherical Telescope (FAST) in China, are capable of surveying neutral hydrogen gas in galaxies across billions of years of cosmic history.

Low-frequency facilities (~50–350 MHz) such as the Low-Frequency Array (LoFAR) in Europe and the Murchison Widefield Array (MWA) in Australia are specifically designed to probe neutral hydrogen gas from the earliest stages of the Universe.

The challenge of crowded spectrum

The upcoming World Radiocommunication Conference (WRC-27) being organized by the International Telecommunication Union (ITU) will shape future spectrum use, particularly as the emphasis on space and satellite services grows.

The ITU Radio Regulations protect the 1400–1427 MHz frequency band for radio astronomy to observe the 21-cm spectral line from neutral hydrogen gas. While this is sufficient to study the Milky Way and relatively nearby galaxies, the faint cosmic signal from distant galaxies must compete with radiation from global navigation satellite systems (GNSS), mobile communications, and aviation radar.

The further back in time we look, the fainter the signal from galaxies and the harder it becomes to separate it from the terrestrial signals. The WALLABY (Widefield ASKAP L-band Legacy All-sky Blind survey) project, one of the largest neutral hydrogen gas surveys underway, illustrates the challenge well.

Originally designed to cover frequencies from 1130 to 1430 MHz, interference from GNSS signals below 1300 MHz forced a reduction of this range to 1300–1440 MHz. The MeerKAT Absorption Line Survey (MALS) in South Africa has detected neutral hydrogen gas in absorption in this frequency range, but only because the signal happens to be exceptionally rare and strong.

Discerning faint signals

The challenges and limitations of signal strength significantly reduce the reach of such surveys into the distant Universe.

Today, only a handful of galaxies have been detected in 21-cm line emission beyond a distance where the signal arrives below 1000 MHz. The MIGHTEE-HI survey with MeerKAT in South Africa and the CHILES survey with the JVLA in the United States represent the current frontier, with detections reaching back to when the Universe was roughly two-thirds of its current age.

Stacking experiments with the uGMRT have confirmed that neutral hydrogen gas does exist in galaxies at these distances. But stacking, by definition, measures the average signal from many galaxies, and therefore cannot reveal the properties of individual galaxies.

Neutral hydrogen gas can, as noted, also be detected in absorption against bright background radio sources, offering a complementary probe of individual distant galaxies.

The current record absorption detection is at a redshift of 3.5, corresponding to when the Universe was just 15 per cent of its current age. But detections at these low frequencies are rare: Radio frequency interference (RFI) increases the difficulty of pushing the sensitivity limits of telescopes.

The collaborative way forward

The upcoming Square Kilometre Array (SKA) telescopes in South Africa and Australia, together with the FAST array in China, the Deep Synoptic Array (DSA-2000) and the Next Generation Very Large Array (ngVLA) in the United States, are specifically designed to overcome the sensitivity limitations of current telescopes, promising to detect neutral hydrogen gas in millions of galaxies across cosmic time.

However, if the interference environment in this range continues to worsen, the damage to our understanding of galaxy evolution will be severe. Recent measurements from observatories show that entire redshift ranges are already inaccessible due to human-made interference, and without improved mitigation, large swaths of cosmic history will remain permanently out of reach.

To minimize the effect of terrestrial RFI, radio observatories are increasingly being built in radio quiet zones, far from dense populations (e.g. ASKAP, MeerKAT and SKA).

Advanced receiver designs with high dynamic range, sophisticated signal processing, and artificial intelligence and machine learning-based RFI mitigation techniques are being developed to distinguish cosmic signals from artificial ones.

Collaboration between astronomers and industry offers another avenue. The National Radio Astronomy Observatory (NRAO) in the United States has developed the Operational Data Sharing (ODS) system, through which telescope scheduling information is shared directly with satellite operators.

SpaceX’s Starlink constellation is using this information to redirect or briefly turn off its beam-forming electronics when passing over the JVLA.

Coordinated regulation needed

Current developments in advanced engineering, computing, big data handling, machine learning and the physical sciences can bring wide-ranging benefits to society, our learning and curiosity, engaging everyone from primary school onwards.

Admittedly, we are witnessing important contributions, but they are not sufficient to bridge the gap between current interference levels and the protection that radio astronomy requires.

Coordinated international regulatory action is essential to ensure the quest to unravel the full story of how galaxies form and evolve can continue.

Header image credit: Created by B. Koribalski; Reference: Koribalski, Gordon & Jones 2003, MNRAS 339, 1203