How did galaxies form?While observational tests on the details of cosmology pro-

ceed apace, astronomers are focusing on the mechanics

of how matter came together in the early universe. Thekey question is: Did galaxies, stars, or blackholes come first?

The infant universe was a relatively uni-form sea of several-thousand-degree gasand dark matter — the unseen, mysterious,and much more predominant form of mat-ter that is indirectly known to exist becauseof its huge gravitational influence on galax-ies. But how galaxies, stars, and black holescame together is the key to understandingthe puzzle of the early universe.

Based on the microwave backgrounddata, astronomers think matter coalescedwhen the universe cooled and became"transparent" 380,000 years after the BigBang. Structures like stars and galaxiesformed about 1 billion years after the BigBang. But exactly how matter clumped isopen to future research.

Deciphering galaxy formation goes backto Walter Baade, who studied stars in galax-

ies and tried to interpret how the galaxiesformed. One of the premier researchers atCalifornia's Mt. Wilson Observatory in the1950s, Baade discovered a group of starsaround the Milky Way with few metals (ele-ments heavier than hydrogen and helium).These stars are ancient, probably 11 billionyears old. Metals thrown out into interstellarspace by supernovae and other processeswere eventually incorporated into youngerstars in our galaxy.

Baade's discovery led to a model ofgalaxy formation in the 1960s nicknamedELS, after Olin Eggen, Donald Lynden-Bell,and Allan Sandage.The ELS model saysgalaxies collapsed as single objects outof gas clouds. As the gas fell in by gravity,it first formed a spherical halo. As moregas coalesced, it began spinning andwas enriched with metals, creating disksinside galaxies.

A different idea proposed recently is themerger theory. It could have been hatchedon Wall Street when the merger buzz wasabout AOL with Time-Warner and Exxonwith Mobil. But those mergers are minus-cule compared with the unions of proto-galaxies — blobs of gas without stars thatgravitated together and merged to form

galaxies in the early uni-verse — and galaxies ofvarious sizes merging laterwith other galaxies,

Indeed, over the pastfew years it has becomeincreasingly clear thatmany galaxies, perhapsthe vast majority, formedwhen small gas cloudscame together, merginginto larger and largerstructures as time wenton. This is called thebottom-up path.

"We don't really knowwhich is the dominantpath yet," says JohnS.

A STAR-LADEN SOMBRERO.Beautifully formed spiralgalaxies like the SombreroGalaxy, seen from ourline of sight as edge-on,coalesced as clumps ofmatter aggregated in theearly universe.