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Silhouettes of Early Galaxies Reveal Few Seeds for New Stars

Artist’s impression of the gas surrounding a young galaxy in the distant Universe. The gas, shown as red streams on the left, is actually invisible, and the starlight from the galaxy is too faint for astronomers to see directly. Instead, the gas is seen in silhouette against a bright, background quasar. Molecules in the gas imprint a shadow, or absorption line, onto the quasar light at a very specific color, as seen on the right, and astronomers can detect this shadow. Image credits: ESO/L. Calçada/ESA/AOES Medialab, Swinburne Astronomy Productions.

Regina Jorgenson, a National Science Foundation Postdoctoral Fellow working at the IfA, co-led an international team that has discovered that gas around young galaxies is almost barren, devoid of the seeds from which new stars are thought to form—molecules of hydrogen.

Without starlight to see them directly, the team observed the young galaxies’ outskirts in silhouette. They searched for telltale signs of hydrogen molecules absorbing the light from background objects called quasars—supermassive black holes sucking in surrounding material—that glow very brightly.

“Previous experiments led us to expect molecules in about 10 of the 90 young galaxies we observed, but we found just one case,” said Michael Murphy of the Swinburne University of Technology in Australia, who co-led the study with Jorgenson.

Astronomers believe that stars begin to form in cold gas that is rich in molecules. The team observed galaxies at a time when the Universe was most actively forming stars, about 12 billion years ago.

“This is a little mystery. This is when most stars are born, and we think this gas forms stars eventually, but it lacks the key ingredient—molecules—to do so,” Murphy said.

The team believes that location and time are the key. “The gas we observe in silhouette probably lies too far from the galaxies to form stars,” Jorgenson said. “It’s got lots of potential, but it hasn’t had time to fall into the richer, denser parts of the galaxies which might be better stellar nurseries.”

The researchers made new observations of more than 50 quasars for this study using the 6.5-meter Magellan telescopes in Chile. The study will be published in the September 21 issue of the Monthly Notices of the Royal Astronomical Society.

Other researchers on the team work at the University of Cambridge in England and the University of Arizona.