Early galaxy bulges in the middle

Concentration of newborn stars provides more evidence that galaxies grow from the inside out

1:01pm, February 4, 2009
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Peering 12.8 billion years back in time to examine the flame of star birth in one of the earliest known galaxies, astronomers say they have captured the first snapshot of the formation of a galaxy’s bulge — a central concentration of stars that’s one of the oldest components of galaxies.

Fabian Walter of the Max Planck Institute for Astronomy in Heidelberg, Germany, and his colleagues had already established in 2004 that a remote galaxy called J114816.64+525150.3, seen as it appeared when the universe was less than a billion years old, was already going gangbusters, forging the equivalent of 1,000 suns every year. (By contrast, the Milky Way galaxy today makes about one sun per year.) But by studying the galaxy’s infrared glow, the team couldn’t tell whether the rash of star formation was distributed evenly across the entire galaxy or confined to a small region.

Walter and his collaborators recently re-examined the galaxy, recording emissions from singly ionized carbon atoms, which closely trace the location of star birth. These carbon ions emit light at infrared wavelengths, but the expansion of the universe shifts the emissions to longer, radio wavelengths. Using the IRAM interferometer, a network of radio telescopes in the French Alps, to record the radiation, the researchers found that star birth in the young galaxy is limited to a central region only 4,000 light-years in diameter. For comparison, the Milky Way has a diameter of 100,000 light-years. The findings are reported in the Feb. 5 Nature.

The compact, central star-forming region found by the team provides additional evidence that galaxies grow from the inside out. Other episodes of star formation can occur throughout galaxies. The team proposes that the central starburst is the first flicker of what will become a spherical nuclear bulge like the one in the center of the Milky Way, but much larger. Bulges are one of the oldest components of galaxies, older than a galaxy’s spiral arms, for example.

Most galaxies in the universe have bulges, which generally range from 5 billion to 10 billon years of age, Fabian says. But researchers don’t know whether J114816.64+525150.3 is typical or an extreme example of bulge formation.

“The crucial element is that for the first time someone has measured the size of the region in which the dust-obscured starburst [of one of the cosmos’s first galaxies] is occurring,” comments Ian Smail of Durham University in England. “The scale of the starburst is indeed well-matched to the formation of a bulge.”

The study also reveals that the core of J114816.64+525150.3 produces about as many stars in a given volume of space as models would allow, Walter says. Winds from newborn stars, as well as the rapid, explosive death of massive stars as supernovas erode the clouds of gas and dust from which stars arise, limit the rate of star birth. 

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