From Sky & Telescope: “Astronomers Chart Star Formation History- Glimpse Fate of the Universe”

From Sky & Telescope

October 26, 2020
Monica Young

Astronomers have tallied how star-making material evolved over cosmic time — and predicted how long stars will keep forming before the universe goes dark.

ALMA has captured a gold mine of galaxies in the Hubble Ultra Deep Field: Those rich in carbon monoxide gas (which traces molecular hydrogen) have lots of star-forming potential (colored orange). Those galaxies imaged solely by Hubble appear in blue. This image from the ALMA Spectroscopic Survey (ASPECS) covers one-sixth of the full Hubble Ultra Deep Field.
Credit: B. Saxton (NRAO / AUI / NSF) / ALMA (ESO / NAOJ / NRAO) / NASA / ESA Hubble.

ESO/NRAO/NAOJ ALMA Array in Chile in the Atacama at Chajnantor plateau, at 5,000 metres.

Astronomers have always been historians, looking back through time to piece together the story of the universe.

Now, they have a new primary source in hand, a historical record of molecular hydrogen gas — the stuff that makes stars. The new observations enable not only a sweeping survey of the past, but a glimpse into our cosmic future.

A History of Star Stuff

Astronomers have spent decades charting the rise and fall of galaxies’ star formation over time. The stellar baby boom occurred about 10 billion years ago, at so-called “cosmic noon” [Annual Review of Astronomy & Astrophysics 2020]. During these early years of the universe, galaxies were bursting with newborn stars, sometimes birthing thousands per year. But rates have been falling ever since.

To explore this rise and fall, astronomers went a step earlier in the process, charting not just the stars born but the material used to make them. Molecular hydrogen gas is cool enough that hydrogen atoms pair up — and it’s also cool enough to collapse into stars. Fabian Walter (Max Planck Institute for Astronomy, Germany) and colleagues used the Atacama Large Millimeter/submillimeter Array (ALMA) to survey the Hubble Ultra Deep Field (HUDF), one of the best-studied regions of the sky. The results of that survey appear in The Astrophysical Journal.

ASPECS Project: ALMA and Hubble UDF

ALMA is a 66-dish array in Chile capable of spying cool gas and dust in galaxies whose light has been traveling for up to 12 billion years. “This is one of the largest programs executed at ALMA,” Walter says, adding that the program used almost 200 hours of ALMA observing time. Along with other studies of the HUDF, ALMA provided the data Walter and colleagues needed to trace the flow of gas into galaxies and into stars.

The gas that falls into galaxies is generally ionized, which means that the hydrogen atom is missing its electron. That gas has to cool, first recombining with electrons and then combining again into molecules, before it can form stars. Walter and colleagues are able to track both atomic and molecular gas to follow the flow of gas from the outermost reaches of a galaxy into its star-forming heart.

“I think we already knew that’s how it has to work, but this paper nicely quantifies, perhaps for the first time, the global rate at which that happened, averaged over all galaxies, and over most of cosmic history,” says Mark Dickinson (NOAO), who was not involved in the study.

This diagram shows the flow of gas from the outermost reaches of a galaxy into its star-forming core. Feedback also occurs, tossing some gas back out again.Credit: Tumlinson et al. / Annual Reviews of Astronomy & Astrophysics 2018.

The observations clearly show that galaxies never, at any one point in time, hold all the gas they need to make all their stars. The gas has to come from outside — the inflow of gas necessary for star formation has continued for all observed cosmic history.

“Those are very challenging millimeter and radio measurements that were impossible not long ago,” Dickinson notes. “I think the Walter et al. paper sets an important benchmark for future analyses as new data are collected.”

The Fate of the Universe

As ever, examining the past also hints at the future. Star formation rates have declined ever since cosmic noon 10 billion years ago. The inflow of gas will only continue to decline, the researchers write: “The cosmic star formation rate density will continue its steady descent to the infinitesimal.”

It’s a one-way street, Walter says: “I cannot think of a simple way to ‘reverse’ or ‘restart’ this trend.”

The good news is that we have billions of years before the universe goes dark. And even as the influx of star-making material continues to decrease over the next 5 billion years, galaxies will continue making new stars with what they still receive. We’re hardly at the end of times just yet.

See the full article here .


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Sky & Telescope magazine, founded in 1941 by Charles A. Federer Jr. and Helen Spence Federer, has the largest, most experienced staff of any astronomy magazine in the world. Its editors are virtually all amateur or professional astronomers, and every one has built a telescope, written a book, done original research, developed a new product, or otherwise distinguished him or herself.

Sky & Telescope magazine, now in its eighth decade, came about because of some happy accidents. Its earliest known ancestor was a four-page bulletin called The Amateur Astronomer, which was begun in 1929 by the Amateur Astronomers Association in New York City. Then, in 1935, the American Museum of Natural History opened its Hayden Planetarium and began to issue a monthly bulletin that became a full-size magazine called The Sky within a year. Under the editorship of Hans Christian Adamson, The Sky featured large illustrations and articles from astronomers all over the globe. It immediately absorbed The Amateur Astronomer.

Despite initial success, by 1939 the planetarium found itself unable to continue financial support of The Sky. Charles A. Federer, who would become the dominant force behind Sky & Telescope, was then working as a lecturer at the planetarium. He was asked to take over publishing The Sky. Federer agreed and started an independent publishing corporation in New York.

“Our first issue came out in January 1940,” he noted. “We dropped from 32 to 24 pages, used cheaper quality paper…but editorially we further defined the departments and tried to squeeze as much information as possible between the covers.” Federer was The Sky’s editor, and his wife, Helen, served as managing editor. In that January 1940 issue, they stated their goal: “We shall try to make the magazine meet the needs of amateur astronomy, so that amateur astronomers will come to regard it as essential to their pursuit, and professionals to consider it a worthwhile medium in which to bring their work before the public.”