The proximity of Veterans Day made me think of the American flag and then, about stars. Astronomers like to think about the properties of stars, including their nuclear burning histories, in order to appreciate how they gave us the life-friendly conditions we enjoy today. Part of that stellar history involves detection of neutral hydrogen in galaxies more than 8 billion light-years away. Now, a team of astronomers has employed an innovative telescope technique to detect this hydrogen, a find that helps us understand star formation history and the conditions conducive to life today.
A Just-Right Time for Life
Using anthropic principles (there’s a weak and a strong type), which is the knowledge that humans exist in the universe today, astronomers have determined what the history of star formation in the universe looked like. Stars produce the elements needed for humans to efficiently function over several decades and for a planetary home on which to thrive. However, aggressive star formation in the universe today would shower those humans with deadly radiation and gravitationally disturb their home planet. For human beings to possibly exist, star formation must begin early in the universe, roughly 200 million years after the cosmic creation event. Then it must ramp up to a peak star formation rate per unit volume of the universe that lasts from about 2.5–4.5 billion years ago, and then exponentially decline to where it is today. Star formation is now about 10 times less than what it was at its peak.
Astronomical observations have affirmed much of what astronomers have derived from anthropic principles. Measurements of the stellar content in galaxies over a distance range of 13 billion years (look-back times spanning the past 13 billion years) establish that about half of all stars in the universe formed when the universe was between 2.5 and 4.5 billion years old.1 During the last 10 billion years, the star formation rate per unit volume has decreased to about 10 percent of what it was when the universe was 4 billion years old.2
Based on observations of ongoing star formation in our galaxy, astronomers have discerned that stars form as a result of diffuse clouds of neutral hydrogen that cool and collapse into dense clouds of molecular hydrogen (H2). What astronomers have lacked, however, are observations of neutral hydrogen in galaxies sufficiently distant to correspond to the epoch of peak star formation to determine exactly how star formation rates are linked to the properties of neutral hydrogen clouds.
Observations of Distant Neutral Hydrogen Gas
This lack is understandable given how feeble neutral hydrogen emission is in galaxies located 8–13 billion light-years away. Presently, there is no radio telescope powerful enough to detect neutral hydrogen (even at concentrations tens of times greater than any source in our galaxy) in galaxies more distant than 8 billion light-years. However, researchers have overcome this deficiency thanks to a novel technique developed by a team of five astronomers led by Aditya Chowdhury.
Chowdhury’s team used what’s called a stacking analysis of the individual 21-centimeter radio observations of neutral hydrogen in 7,653 star-forming galaxies located between 9.4 and 6.7 billion light-years away. The observations were recently obtained with the upgraded Giant Metrewave Radio Telescope (near Pune, India), an array of 30 45-meter-diameter (150-foot-diameter) radio telescopes (see figure 1). They stacked the radio spectra of the 7,653 galaxies on top of one another. This stacking enhanced the sensitivity of their observations by the square root of 7,653. That is, they gained a factor of 87.5 times greater detectability of neutral hydrogen.
Figure 1: One of the 30 Radio Telescopes in the Giant Metrewave Radio Telescope Image credit: Megan Argo, Creative Commons Attribution
Though it was impossible for Chowdhury’s team to detect neutral hydrogen in any individual galaxy, they did achieve helpful results thanks to the enhanced detectability from their stacking. They were able to determine that, on average, the 7,653 galaxies contained about 2.5 times more neutral hydrogen gas relative to total stellar mass than galaxies do today. The team’s detection established that, indeed, excess neutral hydrogen gas explains the high star formation in galaxies 9.4–6.7 billion years ago. They showed that the amount of neutral hydrogen gas they measured in these distant galaxies implies that the hydrogen gas would have been consumed by star formation in only 1–2 billion years. This rapid rate of transformation of neutral hydrogen gas into stars explains the exponential drop-off in the star formation rate that occurred thereafter.
The stacking technique only reveals how much neutral hydrogen gas, on average, resides in the 7,653 galaxies. It gives no information as to where in the galaxies the hydrogen gas resides or where star formation is most aggressively occurring within the galaxies.
Such information awaits the completion of the Square Kilometer Array (see figure 2). When completed, the Square Kilometer Array will consist of several thousand radio telescopes located in Australia and South Africa with a total light–collecting area of 1 square kilometer. This collecting area will exceed by 21.5 times the collecting area of the Giant Metrewave Radio Telescope. The Square Kilometer Array will possess the capability of detecting neutral hydrogen gas emission in individual galaxies 8–12 billion light–years away, including the emissions’ locations within the galaxies.
Figure 2: Artist’s Impression of One of the Cores of the Square Kilometer Array Image credit: SKA Project Development Office and Swinburne Astronomy Productions
As astronomers eagerly await the new technique, they celebrate the detection by Chowdhury’s team. It is a breakthrough in understanding how and when hydrogen gas is taken up by galaxies to form stars. The discovery provides concrete evidence that the star formation rates throughout the history of the universe have been fine-tuned to make possible the existence of human beings.
Endnotes
Piero Madau and Mark Dickinson, “Cosmic Star-Formation History,” Annual Review of Astronomy and Astrophysics 52 (August 2014): 415–86, doi:10.1146/annurev-astro-081811-125615.
Madau and Dickinson, “Cosmic Star-Formation.”
Aditya Chowdhury et al., “HI 21-Centimetre Emission from an Ensemble of Galaxies at an Average Redshift of One,” Nature 586 (October 15, 2020): 369–72, doi:10.1038/s41586-020-2794-7.
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