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ASU astronomers help locate obscure galaxies

October 10, 2007

If there is one thing parents know about looking for their children’s lost toys, it is to not rule out a possible spot just because the youngsters say they already looked there.

The same could be said for astronomers – they are just looking for much bigger Legos.
Recently, two ASU researchers helped to locate what they call the “Lego building blocks of galaxies.” They did so by looking in a place that other astronomers already had looked, but with fresh eyes.

Using NASA’s Hubble and Spitzer Space Telescopes, an international team of astronomers found nine of the smallest, faintest, most compact galaxies ever observed in the early universe – the building blocks of today’s larger, older galaxies.

The team, which includes associate professors Sangeeta Malhotra and James Rhoads of ASU’s School of Earth and Space Exploration, announced the discovery of these protogalaxies Sept. 6. Nor Pirzkal of the Space Telescope Science Institute in Baltimore led the study, which also included Chun Xu of the Shanghai Institute of Technical Physics in China.

Composed of millions of brilliant blue stars, each infantile galaxy is one-hundredth to one-thousandth as large as our Milky Way galaxy and formed about 12.5 billion years ago – just 1 billion years after the “Big Bang.”

Such galaxies are consistent with the conventional model of galactic formation, which holds that larger galaxies are formed when younger, smaller, less-massive galaxies merge. The sighting thus offers some much-needed support for the “hierarchical model,” which has become ever more contentious in recent years.

The controversy, like the discovery, is a question of observation.

One of the oddities of astronomy is that looking at distant objects also means looking back in time. Light, the fastest thing in the universe, still takes time to get from object to observer, meaning that scientists are always looking at snapshots from moments long ago.

If the conventional model is correct, then astronomers peering “deep” into the early days of the universe should detect “building block” galaxies akin to the ones announced Sept. 6. As yet, however, researchers have predominantly found older, more massive galaxies – causing some to question the hierarchical theory.

One explanation for observations skewing toward finding older galaxies is the Spitzer telescope itself, which specializes in seeing into the infrared spectrum.

“Spitzer is very good at detecting old stars,” Malhotra says. “When you see an old galaxy with Spitzer, it is exciting to publish articles saying, ‘Hey, old stars in a young universe.’ But when people don’t see galaxies with Spitzer, it’s a nondetection, and not much is made of it.”

Malhotra and Rhoads, along with their colleagues, wondered if these nondetections could be masking the very galaxies they sought. If these expanses of space did not contain old galaxies, might they not hold young ones?

To find out, the group turned to the Hubble Space Telescope.

“With the Hubble Space Telescope, we were able to find emissions in blue light, which meant that there were a lot of young stars in those galaxies,” Malhotra says. “The question was, underneath all of that, was there a substrate of old stars that came from a previous generation? For that, we had to look at the red light data, which came from Spitzer.”

By analyzing the spectral energy distribution data available from the two telescopes’ instrument arrays, the team determined that the nine blue galaxies indeed lack old red stars – and that they are also not very massive.

The spectral data also showed that the galaxies were very blue, implying that they are largely dust-free. Just as atmospheric dust makes sunsets red, dust between stars and observers on Earth makes starlight appear redder.

All of this was good news because, while Legos might be found in dusty corners of the house, building blocks of the galactic sort are more likely to be located in dust-free zones.

“Dust is a sign that a galaxy has been around for a while, because it is made of heavier elements that are themselves produced in stars,” Rhoads says. “So if there is a lot of dust, it means that the galaxy has been around long enough for dust to be formed in stars and dispersed back into gas between the stars at least once.”

Rhoads – who, along with Xu, first identified these galaxies – added that three of the galaxies appear to be stretched into tadpole-like shapes instead of the usual rounded blobs, which could suggest that they are in the process of merging with neighboring galaxies to form larger, cohesive structures.

In the effort to see the early universe, sometimes looking twice pays off.

“If you want to have a fair idea of what’s going on in the old universe, then you have to pay attention not only to what you see, but also to what you don’t see,” Malhotra says. “So, in this case, we don’t see old stars – and that’s big news.”