New column: Ask a Professor — Why is the recent discovery of an ancient trove of massive galaxies significant?

Vassar professor Debra Elmegreen is president-elect of the International Astronomical Union – the largest organization of professional astronomers in the world – and will begin her three-year term as president in 2021. Like Maria Mitchell more than 150 years ago, Elmegreen was recently elected into membership of the American Academy of Arts & Sciences. (Vassar College/ Tamar M. Thibodeau)

The programming and facilities provided by the mid-Hudson valley’s half dozen private and public colleges immeasurably enrich the cultural life of the community. In this, the inaugural installment of “Ask a Professor,” Almanac Weekly’s John Burdick attempts to leverage the area’s disproportionate resources of scholars, researchers, writers, scientists, artists and educators who call the region home, in the process stimulating intellectual commerce between the area’s academics and its communities.

In this installment, he talks with Debra Elmegreen, professor of Astronomy on the Maria Mitchell Chair at Vassar College. The historically significant astronomer Maria Mitchell (pronounced Mariah, like Carey) was Matthew Vassar’s very first professorial hire. Mitchell was already world-famous in the profession on the strength of her discovery of the telescopic comet (meaning a comet indivisible to the naked eye) that bears her name. For that discovery (which took place in her hometown of Nantucket), she was awarded a gold medal by the King of Denmark, who had offered a prize for the identification of telescopic comets.

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Mitchell and her amateur astronomer father moved into the Vassar Observatory – the first building completed in the Vassar campus – in 1865. From there, Mitchell led her students in original research, much of it eventually published in Silliman’s Journal, the American scientific journal founded at Yale in 1818 by Benjamin Silliman. Mitchell was the first woman elected into membership of the American Academy of Arts and Sciences, in 1848, and in 1850, of the American Association for the Advancement of Science. A proto-feminist per force as a woman competing for resources and attention at the highest levels of the old boys’ club of science, Mitchell used her observatory dome additionally as a venue for the discussion of politics and women’s issues. Famously, Julia Ward Howe, composer of the Civil War anthem “The Battle Hymn of the Republic,” lectured in the observatory on the subject: “Is Polite Society Polite?”

Professor Debra Elmegreen was the first woman to graduate from Princeton University with a degree in Astrophysics, as well as the first female postdoctoral researcher at the Carnegie Observatories. She has published an astronomy textbook for undergraduates titled Galaxies and Galactic Structure, as well as more than 200 academic papers. She was president of the American Astronomical Society from 2010 to 2012, and was appointed as a board member of the 2010 Astronomy and Astrophysics Decadal Survey through the National Academies of Sciences, Engineering and Medicine. In 2013, she and her husband authored a paper, “The Onset of Spiral Structure in the Universe,” published in the Astrophysical Journal. She is president-elect of the International Astronomical Union – the largest organization of professional astronomers in the world – and will begin her three-year term as president in 2021. Last year, she won the American Astronomical Society’s George van Biesbroeck Prize, awarded every two years to an American astronomer for outstanding service in the field. Like Maria Mitchell more than 150 years ago, she was recently elected into membership of the American Academy of Arts & Sciences.

Why is the recent discovery of an ancient trove of massive galaxies significant?

A galaxy is a collection of stars, gas, dust and dark matter, all held together by their own gravity. There are more than 100 billion, perhaps a trillion galaxies in the universe, and they span a wide range of masses. Galaxies like our own Milky Way contain more than 100 billion stars. Tiny galaxies, like the Large and Small Magellanic Clouds that orbit our galaxy, are 10, 100 or even 1,000 times less massive. The biggest galaxies are 10 to 100 times the mass of ours.

There are two consequences of observing galaxies very far away. First, the universe provides a time machine: We peer into the past as we look at distant galaxies, since their light takes a long time to reach us. The Hubble Space Telescope records what amount to baby pictures of those galaxies. Second, as the universe expands, light is stretched to longer wavelengths, so appears redder to us than what was emitted; this is called a redshift. A redshift of three, for example, means that the wavelength has been shifted to four times longer than emitted; visible light is shifted to the infrared, out of the range that Hubble can see. The Spitzer Space Telescope can detect such emission. The Atacama Large Millimeter Array (ALMA) is a large network of telescopes that can detect even-longer wavelengths, with much higher sensitivity and resolution. Recently, astronomers detected optically invisible distant galaxies with Spitzer, and then observed them in more detail with ALMA. That led to the discovery of dozens of very massive galaxies at redshifts greater than three, so the light we see was emitted when the universe was less than two billion years old (its current age is 13.8 billion years).

By conducting large surveys of galaxies at different distances, we find that galaxies grow as they age. The universe is filled with a web of hydrogen gas that drips onto dense pockets of matter, which become galaxies. Galaxies grow as they continue to accrete more gas (and sometimes merge with other galaxies); when the universe was half its current age, galaxies were about half their current size. We understand the basic process through observations and computer models, and we see distant small galaxies that are probably precursors to galaxies like the Milky Way. But we have never before detected so many galaxies that were very massive, even in the early universe. These already-large young galaxies evidently grow into today’s most massive galaxies. What we still don’t understand, based on our current models, is how they could have accumulated so much mass so quickly. We need to refine our theories and computer simulations as we stretch the limits of observation to find new cosmological surprises.

Professor of Astronomy on the Maria Mitchell Chair, Vassar College

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