Let's make a galaxy: astronomers have identified a cosmic infant "nearby"—70 million light-years from Earth

Galaxies in the universe are rather like the cells in an animal. Just as cells combine to make an animal's organs and systems, so, too, do galaxies come together to make the superclusters and filaments that define the large-scale structure of the cosmos. Not surprisingly, in much the same way that biologists examine cell development to understand the aging process in animals, astronomers study galaxy formation to decipher the evolution of the cosmos. Hence, the study of the origin of galaxies is one of the most important topics in modern astronomical research.

In this case, though, the, biologists have it much easier. Animals reproduce, and so biologists have a steady supply of newborn cellular agglomerations for comparison and scrutiny. But astronomers have only one universe to observe, and it's mighty long in the tooth--13 billion years old, according to the best current estimates (about three times the age of Earth). Worse, according to current thinking, the vast majority of galaxies 'formed long, long ago. So when we astronomers want to study the earliest moments in galactic "life" we have to approach our work more like paleontologists than like biologists, seeking to understand an ancient world with only fossilized remains as a guide. To glimpse galaxies in their embryonic stages, astronomers have to look far back in time, across distances amounting to billions of light-years. Of course, such great distances pose severe challenges even for the most powerful telescopes. Unfortunately, there is no other choice.

Then again, maybe there is. In the past several years, new evidence has suggested that some galaxies may still be forming. Now, according to astronomers Michael R. Corbin of the Space Telescope Science Institute in Baltimore and William D. Vacca, now at the University of California in Berkeley, a smoking gun may be in view--a nearby galaxy, caught in the act of birth.

What do we astronomers mean by "galaxy"? We usually recognize one when we see one, but ask us for a definition and we have a much tougher time. Here's a reasonable working definition: a galaxy is a vast, contiguous collection of stars, gas, dust, and other matter, totaling at least a few million times the mass of the Sun, all held together by mutual gravitational attraction. At least our own Milky Way fits the definition pretty well--though it's on the hefty side as galaxies go, made up of about 100 billion stars that stretch across 100,000 light-years.

According to current models, between two and four billion years after the universe began with a (big) bang, clumps of matter had formed in vast numbers. Each clump was larger than a typical cluster of stars, but smaller than a modern-day galaxy. As gravity acted on these subgalactic clumps, pulling them ever closer together, regions of space that were already relatively dense with clumps became even denser, and did so more quickly, than regions where the clumps were initially relatively sparse. Large collections of clumps created deep gravity wells that sucked in smaller groups of clumps, even as those collections coalesced to make single, larger bodies. Today, billions of years later, we observe the resultant hierarchy of cosmic structure: subgalactic clumps that combined to form galaxies, which in turn gathered into groups and clusters, which then collected into filaments and superclusters.

Since all that clumping and clustering started so long ago, cosmologists don't expect, by and large, to find such primordial subgalactic objects in the universe today. Much of the current observational research on galaxy formation therefore focuses on dwarf galaxies, with less than one-hundredth the mass of the Milky Way. Some kinds of dwarfs have many more young stars than do their larger siblings, and offer the possibility of studying present-day galaxy evolution on a manageable scale.

Corbin and Vacca examined a sample of dwarf galaxies chosen for their compact size and the youth of their star populations. One of the dwarfs, called POX 186, caught their eye. (The nomenclature has nothing to do with skin disease--"POX" is a kind of shorthand for the informal name of the survey that discovered the object in 1981.) With the Hubble Space Telescope, Corbin and Vacca made a high-resolution image of the galaxy--and found a small, apparently new-born minigalaxy just 70 million light-years away and a mere 100 million years old. The shape, size, and age of the dwarf galaxy all seem consistent with the idea that POX 186 is actually made up of two partly coalesced subgalactic clumps, in the act of coming together to make a new galaxy.

How could such a young galaxy be forming before our eyes? After all, according to conventional wisdom, subgalactic clumps were all swept up long ago into galaxies like our own. The location of POX 186 may provide a critical clue. Nestled between matter-rich filaments and superclusters are "voids" of intergalactic space. Only a sparse smattering of galaxies occurs in these vast, empty volumes of space. POX 186 resides near the edge of such a void, in the direction of the constellation Bootes; Corbin and Vacca found no other galaxies within 15 million light-years of the dwarf. Maybe that's why the two subgalactic clumps survived so long: exiled in the void, they remained undisturbed for more than 10 billion years, never encountering any other clumps--until now.

The discovery that a nearby dwarf galaxy is actually in its infancy is, though fascinating, hardly heretical. In fact, understanding the ancient, distant subgalactic clumps is still essential for unraveling the mysteries of galaxy birth and formation. But POX 186 does open the door to a new line of inquiry, because astronomers now know that it's also worthwhile to look closer to home. More little blobs of matter may be lurking in the voids.

Charles Liu is an astrophysicist at the Hayden Planetarium and a research scientist at Barnard College in New York City.

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