Researchers report a lower stellar birth rate

For astrophysicists, learning that most star formation occurred when large galaxies were in their infancy would be akin to biologists discovering elephant remains in fossils next to the dinosaurs. But this is precisely what a team of about 25 astronomers, primarily at Yale, the Netherlands’ Leiden University and the University of Chile, discovered.

In this month’s issue of Astrophysical Journal, Yale researchers, in collaboration with other scientists, report that massive galaxies have a much lower stellar birth rate than expected. The researchers, collaborating as part of the Multiwavelength Survey by Yale-Chile, found that many of these galaxies had matured by forming most of their stars when the universe was only about 20 percent of its present age, 11 billion years ago. This effect appears to be unique to large galaxies.

The study was propelled by a desire to learn how galaxies in the nearby universe form, Mariska Kriek, the lead author of the study and a joint doctorate student of Leiden University and Yale, said.

“We’d like to see [galaxy formation] happening,” said Eric Gawiser, a postdoctoral fellow in the Astronomy Department who helped author the paper. “We’d love to see a ‘Jurassic Park’ of galaxy formation, but astronomy is not a laboratory science. We can’t make a galaxy in the basement of Gibbs Laboratory.”

In order to research galaxy formations, scientists must look into the past, said Pieter van Dokkum, a Yale professor of astronomy and physics and author of the paper. People look into the past every time they view the sun, for example, because what they actually see is an eight-minute-old picture of the sun, since the sun is distant and light travels at a finite speed.

Using a similar principle, by looking at distant galaxies, researchers can see what the galaxies looked like billions of years ago. Since the universe is fairly homogeneous, these distant galaxies should exhibit properties similar to other galaxies, he said.

Before this research, the prevailing opinion was that stars form gradually over time, Kriek said. The researchers expected, therefore, that the old galaxies they observed would still be actively forming many stars.

“However, our results show that most stars in large galaxies were born when the Universe was still very young, in the first few billion years after the Big Bang,” she said. “After this initial period of vigorous activity, the formation of new stars gets suppressed, and the stellar birth rate drops significantly.”

In other words, large galaxies have a violent growth spurt from youth to old age, van Dokkum said. These results seem counter-intuitive, because large galaxies with many stars exhibit a short period of star formation whereas smaller galaxies exhibit more gradual star formation. Researchers were surprised to learn that star formation ended at such an early epoch in large galaxies.

In order to learn about star formation, the researchers looked for a specific signature of light, as a marker for stellar birth, corresponding to ionized hydrogen, he said.

They did not find the spectrum coordinating to ionized hydrogen, indicating a low rate of new star formation, Gawiser said.

To search for these wavelengths of light, the researchers selected four regions of the sky that had very few bright objects, said Meg Urry, a professor of physics and astronomy. Telescope pictures were taken using the Gemini 8.1 m telescope in Chile, with cameras operating on a similar principle as digital cameras.

The pictures were filtered to isolate certain wavelengths of light and then combined to make spectrographs, Urry said. These images allowed the researchers to discover the galaxies and search for indications of ionized hydrogen.

The researchers remain uncertain as to why this low birth rate occurs considering that the fuel for star birth still exists in these galaxies. Black holes may be responsible for inhibiting new star growth by emitting enough energy to stop cosmic gases from condensing, leading to new stars, van Dokkum said.

The researchers also found evidence for massive black holes in the center of these galaxies.

Follow-up studies will further investigate their connection with decreased birth rates, Kriek said.

Researchers will try to look at even older galaxies, van Dokkum said. The Hubble Telescope will also be employed to capture pictures of some of the galaxies being studied.