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The concept of an artist showing a view through a mysterious disc of young blue stars surrounding a supermassive black hole at the center of the Andromeda galaxy. Image: NASA / ESA.
Optical and infrared telescopes study radiation that crosses the universe with wavelengths in the visible and infrared parts of the electromagnetic spectrum. And using observations with these telescopes, astronomers today know that star-forming activity in the universe peaked about 8-10 billion years ago, and then dropped about 10 times its current rate.
However, we don’t know why this slowdown occurred, and not for lack of trying. The main problem is that we did not, and still do not have, enough information on a key ingredient in stars: neutral atomic hydrogen.
A neutral hydrogen atom consists of a proton, in the nucleus, and an electron orbiting around it. All subatomic particles have a property called spin, which denotes the angular momentum of the particle. If the proton and the electron in the hydrogen atom both have their spins oriented in the same direction, the atom as a whole has a little more energy than if the proton and electron were pointing in opposite directions (i.e. antiparallel ). The higher energy state is called the excited state.
Sometimes, in an extremely rare event, an excited hydrogen atom can switch to the lower energy state by emitting energy in the form of electromagnetic radiation – i.e. a photon – of a wavelength of 21.106 cm (rounded down to 21 cm for communication cheap). So observing the radiation of this wavelength is a sign that neutral hydrogen atoms are present.
This event is so rare that it is nearly impossible for physicists to observe it happening spontaneously in a laboratory on Earth. And even though astronomers know the precise wavelength of the radiation to look for, the emission itself is quite faint and difficult enough to spot when it comes from distant galaxies.
Axiomatically, observing and measuring 21 cm radiation from distant galaxies is bound to be a major undertaking. These are basically photons that carry an extra shadow one trillionth of a trillionth of one joule of energy each that has traveled billions of light years (a light year is about 9.4 trillion km) – and some of which have, by chance, encountered Earth.
A team of researchers from the National Center for Radio Astrophysics (NCRA), the Tata Institute of Fundamental Research, Pune and the Raman Research Institute (RRI), Bengaluru, recently reported that they only measure this 21cm radiation from distant galaxies, corresponding to a time when star-forming activity had just begun to decline. Their results, made with the Giant Meter-wave Radio Telescope (GMRT) in Pune, were released on October 14, 2020.
“This is the first epoch in the universe for which there is a measurement of the atomic gas content of galaxies and represents a significant leap in our understanding of gas in galaxies and its connection to star formation,” said Aditya Chowdhury, PhD student. at the NCRA and one of the study group members, he said The Wire Science.
Both space and time in the universe are united in some way by the speed of light. An important effect of this is that objects that are one light year away in space are actually one year away in time. If we also include the fact that the universe is expanding, the light emitted by a galaxy eight billion years ago will have to travel many more billions of light years to reach Earth – and when it does, it will carry information of a reality that existed. eight billion years ago. On the other hand, the information will have become much weaker due to the distance traveled.
Faced with this challenge, the NCRA-RRI team captured single 21 cm emission signals from 7,653 galaxies and aligned them in three dimensions: two corresponding to their position and one to redshift. In this way, they were able to adapt to different differences and get an average signal.
The researchers could figure out how far each of these galaxies was from Earth in a simple way. Suppose a galaxy has emitted a light that is traveling to Earth. The universe is constantly expanding, as a result of which space itself expands. So the wavelength of this light is stretched, making it appear redder. Astronomers know this distortion as redshift.
The higher the redshift of a galaxy, the further away it is.
Galaxies whose atomic hydrogen content the researchers had detected were at redshifts of 0.74 to 1.45. On this scale, 0 refers to the current time, 0.1 about a billion years ago, 0.75 about 5.78 billion years ago, and 1.45 about 9.2 billion years ago.
The older the galaxies, the further away they are in space. But since the universe is also expanding at an accelerating rate, distance and time are not linearly related. So galaxies that are about 8 billion years away in time are actually more than 20 billion light years in space.
According to Kanan Datta, assistant professor of physics at the Presidency University, Calcutta, it is difficult to overestimate the challenge of directly measuring the amount of neutral hydrogen from the early universe – that is, in distant parts of it – because the 21 cm signal from individual galaxies that they are so far apart it is very faint.
Chowdhury said detecting the same signal from a single galaxy at redshifts of 0.74-1.45 would require more than 10,000 hours of observation time and with only two currently operational radio telescopes: the GMRT and the MeerKAT array in South Africa.
Star formation is associated with regions that contain dust and cold, dense gas, including molecular hydrogen (H2), which can collapse to form stars. However, neutral atomic hydrogen (HI) eventually forms H2. “Hence, on larger galactic scales, HI is considered the fuel for star formation,” Khandai said.
In star-forming galaxies near us, astronomers have studied how the mass of galaxies and star-forming rates are related to their cold gas content, HI and H2. But estimates of HI content in distant galaxies were missing.
Chowdhury and his colleagues found that the rate at which new stars formed may have been mediated by the availability of hydrogen itself.
Specifically, they found that galaxies around 8-10 billion years ago contained nearly 2.5 times more atomic hydrogen than galaxies today. This historical availability of hydrogen may have contributed to the prolific creation of stars up to that point. The researchers also estimated that the abundance of the gas would decrease to the point where star formation would become too resource-intensive about a billion years or so later.
“Once the atomic hydrogen content is depleted, and if fresh atomic hydrogen does not accumulate from the environments of galaxies, star-forming activity will decline rapidly,” Chowdhury said. “This provides a plausible explanation for the decline in star-forming activity over the past eight billion years.”
Likewise, it expects based on the current rate – to accumulate 0.002-0.4 solar masses per year – in nearby star-forming galaxies, such as JKB18 and Andromeda, which will be able to continue creating new stars. for another 7-9 billion years.
Suman Majumdar, assistant professor of astronomy and astrophysics at IIT Indore, explained that the study provides observational evidence for theoretical studies that have predicted a decline in the rate of star formation in galaxies from the early universe to the present.
The work of the NCRA-RRI team is also remarkable because it used the GMRT, India’s most sensitive radio telescope, located near Pune. One of the fundamental objectives of this instrument – installed by the well-known radio astronomer Govind Swarup in 1995 – is to study the emission of 21 cm of neutral hydrogen into space. Swarup died on September 7 this year.
In 2016, the same group of researchers tried to detect 21cm emission signals with GMRT using the same approach, stacking signals from multiple galaxies. But the telescope’s specifications only allowed them to get an upper limit on the strength of the averaged signal.
The GMRT was updated the following year. The redshift of a galaxy determines the amount of elongation of the light from it. So the 21 cm signal from galaxies at different redshifts can only be observed at different frequencies. This in turn means that the telescope’s bandwidth determines the range of redshift that can be observed simultaneously. The pre-update GMRT had a 33 MHz bandwidth; after the updates, this became 400 MHz. The current studio was the product of this renewal.
“This work demonstrates that GMRT can play a very important role in direct estimation of the neutral hydrogen content of the early universe. We are about to see this field flourish in the near future and this work is definitely an important milestone, ”Datta said.
Indeed, radio astronomy is set to receive a boost with upcoming observers, perhaps most notably the Square Kilometer Array (SKA). The GMRT uses 30 parabolas or antennas for radiation collection, each 45 meters wide for a total collection area of 20,000-30,000 square meters, from low to high frequencies (120 MHz to 1.4 GHz). The SKA will use thousands of antennas, built in Australia and South Africa, covering a total collection area of 1,000,000 square meters – or 1 square km – to study radiation from 50 MHz to 14 GHz. Its first phase (which includes the MeerKAT telescope) is expected to be ready around 2023.
Majumdar agreed with Datta, saying that astronomers would be able to use SKA to conduct studies like those of the NCRA-RRI group, and faster and more accurately. He also noted that while the new findings marked the first step in solving a long-standing puzzle, they were also limited to a relatively small portion of the sky. “They only studied 7,653 galaxies for this – which may seem like a large number, but it’s not big enough on a cosmological scale,” Majumdar said.
“One of the main cosmological principles states that no single direction is special.” So astronomers will have to observe many more galaxies in different parts of the sky to validate and then build on the discovery.
“Astronomy is an observation-driven field and theories of galaxy formation depend on such important observations to make further progress,” said Khandai. “Future observations will further limit the HI content of galaxies to higher redshifts, which would answer many questions about how galaxies form and evolve.”
Joel P. Joseph is a science writer.
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