Older galaxies composed of neutral hydrogen gas, different from younger ones: Study

Putting a lid on all existing beliefs, a new study by a group of Pune-based astronomers has concluded that galaxies that formed during the early Universe were largely composed of neutral hydrogen gas, and are in stark difference from the composition of younger galaxies.

The conclusions are based on an average result calculated after observing about 11,000 galaxies formed about 9 billion years ago.

Final-year PhD student Aditya Chowdhury along with scientists Nissim Kanekar and Jayaram Chengalur of the Pune-based National Centre for Radio Astrophysics (NCRA) under the Tata Institute of Fundamental Research (TIFR) studied 510 hours of data obtained from the Giant Metrewave Radio Telescope (GMRT) – CATz1 survey to arrive at these findings.

Generally, galaxies are composed of either atomic or molecular hydrogen and stars. During the galaxy’s lifetime, the atomic hydrogen cools and gets converted to molecular hydrogen. It further collapses to form stars as we know them today.

Over a period of 9 billion years, scientists said, gas reserves of these formative galaxies condensed and converted to form several galaxies, like the Milky Way. However, it was the mass of stars, and not gases, that dominated, scientists noted. For instance, a highly evolved galaxy, which has continued giving birth to stars for a long time, would have consumed most of the galaxy’s gas. Such galaxies, the scientists said, would have significant amounts of the mass stored in the stars than the actual gaseous matter.

“In the galaxies formed 9 billion years ago, we found they were composed of 70 per cent atomic hydrogen and 16 per cent of the mass concentrated in stars. Whereas, in the composition of galaxies today, 2/3 of mass is within the stars, while 33 per cent mass (1/3) of the total ordinary mass (inclusive of dark matter) is atomic hydrogen gas. Only 6 per cent of the total ordinary mass (inclusive of dark matter) is molecular hydrogen gas,” said Aditya Chaudhury, lead author of the study published in the Astrophysical Journal Letters.

The new findings have now solved the critical missing information about the mass of atomic gas in galaxies, said Nissim Kanekar, senior scientist at NCRA and co-author of the study.

“The recent observations of molecular gas in the early galaxies have shown that the molecular gas is comparable to the stars in mass, providing the first hints that these galaxies are very different from galaxies today,” he said.

“The relative amounts of atomic and molecular gas and the stellar material in a galaxy indicate its evolutionary stage,” the researchers said.

To measure the mass of atomic hydrogen gas, researchers normally study the spectral line in hydrogen atom at 21cm wavelength. But the 21 cm spectral line emerging from distant galaxies is extremely weak when captured using ground-based telescopes. So, the scientists combined this information with deep observations done over focused regions of the sky during the GMRT-CATz1 survey done between 2018 -2020.

The current study settles a long-standing debate about the early galaxies, providing a complete picture of what these galaxies were made up of, said Jayaram Chengalur, director, TIFR, and co-author of the study.

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