The document summarizes two astronomy stories from the American Astronomical Society meeting. 1) Hubble images of very early galaxies from 600 million years after the Big Bang show they were tiny, only 5% the size of the Milky Way, but already had populations of older stars. This implies that the transition from the earliest stars to later populations occurred very quickly in the early universe. 2) A study of over 100 galaxies found their ratio of detectable "baryonic" matter to dark matter decreases with galaxy size, with the largest galaxies having up to 14% baryonic matter but the smallest dwarfs having less than 1%. This suggests larger galaxies were better able to retain their baryonic matter over time.

1. NEWS OF THE WEEK
ASTRONOMY
Oldest Galaxies Show Stars Came Together in a Hurry
Sifting through images taken by the Theorists think the very first
newly refurbished Hubble Space stars, known as Population III
Telescope, astronomers have spot- stars, were massive stellar objects
ted five galaxies that date back to a made only of hydrogen and
mere 600 million years after the big helium and didn’t live for very
bang—the earliest galaxies found long. The first normal low-mass
so far by 200 million years. The dis- stars—called Population II and I
coveries take researchers close to stars—could form only after Pop-
the primordial stage of cosmic evo- ulation III stars had exploded as
lution, when the first galaxies were supernova and enriched the uni-
taking shape. verse with heavier elements
Astronomers have already forged in the process.
learned a few things about the newly The imaging of the new
Downloaded from www.sciencemag.org on January 15, 2010
discovered galaxies. For one, they galaxies, with the discovery
are tiny compared with contempo- of Population I and II
rary galaxies—barely 5% the size of stars within them,
the Milky Way and less than 1% its implies that “the
mass. “These are the seeds of the transition in cos-
great galaxies of today,” says Garth mic star formation
Illingworth of the University of Cal- Looking back. Hubble’s new camera has found several galaxies from mode, from Pop III
ifornia, Santa Cruz, who presented 600 million to 800 million years after the big bang. to Pop I and II, took
the findings at the American Astro- place quickly, in dark-
nomical Society meeting in Washington, D.C., That pushes back the birth of the earliest stars matter systems that were even smaller” than
last week.* Illingworth led the survey team that of the universe to within a few hundred mil- the refurbished Hubble can see, Bromm says.
took the new images using Wide Field Camera lion years of the big bang—a blink of an eye He adds that the “true moment of first light
CREDITS: NASA, ESA, G. ILLINGWORTH, R. BOUWENS (UNIVERSITY OF CALIFORNIA, SANTA CRUZ), AND THE HUDF09 TEAM; (INSET) COURTESY NASA
3, one of two new instruments mounted on in astronomical time. remains elusive, and its discovery has to await
Hubble in a servicing mission last year. Volker Bromm, an astrophysicist at the the James Webb Space Telescope”—planned
Another striking fact about the galaxies is University of Texas, Austin, says the new for launch in 2014.
that they are populated by stars that had galaxies “clearly demonstrate the hierarchical In addition to Illingworth’s team, four
already been burning for 300 million years. nature of structure formation—small objects research groups have reported similar
formed first—and provide interesting con- f indings from their analyses of the new
*215th AAS Meeting, Washington, D.C., 3–7 January. straints for early star formation.” Hubble data. –YUDHIJIT BHATTACHARJEE
ASTRONOMY
Inventory Asks: Where Is All the Non-Dark Matter Hiding?
Astrophysicists know that 83% of the matter from the motion of its stars. They estimate its expect,” which is about 5%. And in the small-
in the universe is dark matter—invisible stuff baryonic mass from the amount of light the est dwarfs, baryonic matter is a hundredth of
as yet undetected. The other 17% is galaxy emits, which can be converted to the what’s expected—as minuscule as 0.2%.
detectable “baryonic matter,” the atoms and total mass of its stars, and a measure of atomic “These are very interesting results” that
ions that make up stars, planets, dust, and hydrogen in the galaxy, which provides an quantify the “missing baryonic matter prob-
gas. To astronomers’ surprise, the ratio of estimate of the interstellar gas. lem,” says Joel Bregman, an astronomer at
baryonic matter to dark matter seems to vary “What we find is that there is a very sys- the University of Michigan, Ann Arbor.
from galaxy to galaxy like the ratio of choco- tematic variation in the ratio with scale,” says Where is all the missing baryonic matter
late chips to dough in different batches of McGaugh, who presented the findings at the lurking? One hypothesis is that its particles are
home-baked cookies. Now, a team led by American Astronomical Society meeting in interspersed within the galaxy’s dark matter
Stacy McGaugh at the University of Mary- Washington, D.C., last week.* “When you go halo in the form of undetectable hot gas.
land, College Park, has determined that the to the very large galaxies, the baryonic mat- Another is that supernova explosions have
proportion varies by scale: The largest galax- ter can be as much as 14%. As you go down blown it into intergalactic space. This second
ies have the highest percentage of baryonic in size, you see that galaxies fall short of the idea would square with McGaugh’s findings:
matter, although not quite 17%; whereas the cosmic fraction [17:83] by an ever-increasing Large galaxies, with stronger gravitational
smallest galaxies have less than 1%. amount.” In galaxies the size of the Milky pulls, would be able to retain more of their
McGaugh and colleagues compiled the Way, “all the stars and gas add up to only a baryonic matter, whereas smaller galaxies
ratios for more than 100 galaxies ranging third of the baryonic matter you would would let more escape. But so far, McGaugh
from supermassive ones to dwarfs. Researchers says, that explanation is just one of several lines
infer the amount of dark matter in a galaxy *215th AAS Meeting, Washington, D.C., 3–7 January. of speculation. –YUDHIJIT BHATTACHARJEE
258 15 JANUARY 2010 VOL 327 SCIENCE www.sciencemag.org
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