Hubble Space Telescope observations are enlightening astronomers about the evolution of galaxies in the distant universe.

CANDELS Ultra Deep Survey
3D-HST adds spectroscopy, and hence the third dimension, to this stunning two-dimensional image from the CANDELS Ultra Deep Survey. Our version doesn't do the data justice - click the image for a zoomable version.
NASA / ESA / A. van der Wel / H. Ferguson / A. Koekemoer / the CANDELS team

Astronomers are studying growing galaxies in the early years of the universe using a unique Hubble Space Telescope survey, and they presented a host of results at the winter meeting of the American Astronomical Society (AAS).

The survey, known as 3D-HST and led by Pieter van Dokkum (Yale University), fleshes out an existing set of Hubble images with spectroscopy, adding a third dimension to a previously two-dimensional view.

Hubble spent about a month collecting near-infrared spectra of galaxies living in a universe less than half its current age. The short observing time netted an incredible amount of data: “The unique ‘slitless’ nature of the 3D-HST observations provides simultaneous high-quality spectra of everything in the HST field-of-view,” says Gabriel Brammer (Space Telescope Science Institute).

The precise distance measurements add a third dimension to tens of thousands of galaxies detected in CANDELS, Hubble's single largest observing program, featured in Sky & Telescope's June 2014 issue.

3D-HST spectra
The spectra for 20,000 3D-HST galaxies are collapsed into a single figure. Arrows point out absorption lines that indicate star formation activity, active galactic nuclei activity, and stellar age. A galaxy at z=3 is observed when the universe was 11.6 billion years younger.
G. Brammer

In a younger universe, star formation and the galaxies’ growth spurts that accompany it are at their peak. By measuring galaxies’ spectra, the 3D-HST team homes in on galaxies’ hey-day to learn about their masses and star-formation histories. Combining those data with observations at other wavelengths, such as X-ray and radio, researchers have started exploring how galaxies formed their stars — and why they stopped.

Erica Nelson (Yale University) has been using 3D-HST to analyze the shapes and structures of star-forming galaxies similar to the Milky Way. Since stars ionize hydrogen gas in their vicinity, creating an H-alpha spectral line (which may be familiar to solar observers who view the Sun through H-alpha filters), tracking H-alpha emission traces the birth of stars.

Nelson mapped H-alpha in a thousand disk galaxies in a universe half its current age. She found that these galaxies grow by forming stars along their entire pancake-like structures. That’s unlike the precursors to modern-day elliptical galaxies, also studied with 3D-HST, which first form stars in their central bulges before star formation spreads outward.

Star formation in disk galaxies
Nelson and colleagues studied galaxies with pancake-like disk structures and spiral arms, comparing light from existing stars to light from gas ionized by forming stars. They find that stars are forming at all radii, rather than from the inside out.
Nelson & others

Astronomers are also using 3D-HST to investigate what causes a galaxy to leave its fiery youth behind and enter a leisurely retirement. Though “retired” galaxies were once thought to be rare in the distant universe, Katherine Whitaker (NASA Goddard Space Flight Center) and Gabriel Brammer (Space Telescope Science Institute) presented significant samples of quiescent massive galaxies with stars older than 1 billion years. These galaxies have stopped forming stars, and detailed analysis shows that some had already shut down star formation before the universe had turned 3 billion years old, earlier than previously thought.

But why did stars stop forming in these galaxies? One possibility is that these galaxies’ central supermassive black hole feeds energy back into the galaxy, stirring up gas and preventing it from condensing into stars. Whitaker, Brammer, and others will continue 3D-HST research to investigate this and other possibilities.

Finally, research with 3D-HST and CANDELS will guide observations planned for the James Webb Space Telescope (JWST), Hubble’s successor scheduled for launch in 2018, and Wide-Field Infrared Survey Telescope (WFIRST), which might launch in the early 2020s. Both of NASA’s largest upcoming missions will find and study many more galaxies in the universe’s distant past.




January 24, 2015 at 1:44 pm

There are suppose to be as many galaxies as stars in the Milky Way but are they counting the many small irregular galaxies of 10-12 billion years ago that can be seen in the Hubble Deep Sky Image or the mostly larger galaxies like the Milky Way and Andromeda that were created out of these smaller galaxies and exist today?

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