Images from the edge: new views of star birth and baby galaxies.
Recorded by the orbiting observatory known as the Galaxy Evolution Explorer (GALEX), the images show that about 20 percent of spiral galaxies appear to harbor clusters of newborn stars in the galactic equivalent of Siberia. In this outermost fringe, the density of gas is so low that star formation had been deemed too infrequent to be of interest. Most astronomers expected the outlying parts of the galaxies to appear uniformly dim, but in the GALEX images, they're riddled with bright spots.
Intrigued but cautious, astronomer Schiminovich, who's now at Columbia University, and his colleagues initially attributed the seeming anomaly to instrumental noise or image-processing errors. But when they laid the UV images on top of radio wave maps of the galaxies' cold hydrogen gas--the raw material for making stars--the researchers became convinced they were on to something new.
The match between peaks in the gas distribution and the UV radiation left "no doubt that the [bright] features were real," recalls Schiminovich. Somehow, new stars were forming thousands of light-years beyond the visible outlines of these galaxies.
The images are among the earliest products of GALEX, which is conducting the first comprehensive survey of the universe at UV wavelengths. This portrait holds special interest because UV light is typically associated with the flame of recent star birth.
In addition to capturing star birth at the fringes of nearby spiral galaxies, GALEX has found evidence of the birth of galaxies in the modern-day universe. The satellite is also beginning to gather images that will enable researchers to reconstruct the past 10 billion years of star formation in the universe.
GALEX researchers presented their findings last month at a meeting of the American Astronomical Society (AAS) in San Diego. An upcoming issue of the Astrophysical Journal is devoted to the GALEX observations.
STAR HUNT Launched in April 2003, GALEX is designed to hunt for young, relatively nearby, massive stars, about 10 times as heavy as the sun. Such stars emit most of their light at UV wavelengths to which GALEX is most sensitive.
These stars last at most only a few hundred million years. Because the stars live and die so quickly, their UV emissions provide a snapshot of some of the universe's most recent star formation, notes Susan Neff of NASA's Goddard Space Flight Center in Greenbelt, Md. "In the ultraviolet ... you can tell what's doing at one particular recent slice of time in [a galaxy's] life," she says.
GALEX's telescope, featuring a primary mirror only 0.5 m in diameter, can't render galaxies with the exquisite sharpness that the Hubble Space Telescope offers. But it can survey an area of sky about 1,000 times as large as Hubble can, says Neff.
This wide field of view enables researchers to trace star-forming regions at the outskirts of nearby galaxies, rather than just at their star-filled cores. In one spiral galaxy known as M83, GALEX researcher David A. Thilker of Johns Hopkins University in Baltimore and his colleagues found clusters of UV-emitting stars as far out as 80,000 light-years from the core. That's about four times as far from the center of M83 than is the outermost region previously seen in visible light.
Astronomers already knew that some stars could form in the outer regions of a galaxy, comments Robert Kennicutt of the University of Arizona in Tucson. "What is surprising is the number of such objects seen in the GALEX images," he says. "This is new and is truly exciting."
No one is sure why these remote outposts of star birth show up so faintly, if at all, in visible light. But one possibility, Thilker says, is that the tenuous outer parts of galaxies generate a mix of stars that are, on average, less massive than those formed in the central parts. He and other astronomers speculate that hydrogen gas in the outer regions is typically warmer than the gas in the central parts of the galaxies. However, star making requires cool hydrogen.
If that's the ease, then star formation ought to be relatively sparse in the galactic fringes. The pockets of cool, dense clouds of gas that do manage to form would give birth to a smaller number of massive stars than do clouds in the inner regions of galaxies.
Because the stars that form on the periphery would, on average, have lower birthweights, they would convert less of the surrounding gas into visible-light--emitting ions compared with stars in the inner regions.
"What I think might be happening is that we're seeing a change in the average properties of hydrogen gas beyond the [visible] star-forming edge of spiral galaxies," says Thilker. "Inside the edge, you're dominated by the cold gas; outside, you're dominated by warm gas."
Newborn stars seen by GALEX in the outlying regions are likely to forge an array of heavy elements, which astronomers define as all elements heavier than helium. Studies using the brilliant beacons from distant quasars to probe the chemistry of foreground galaxies often show a higher abundance of heavy elements than astronomers had calculated considering only the visible portions of those galaxies.
Quasars are more likely to pierce the vast, normally invisible, outermost parts of galaxies rather than their cores, notes Neff, and the outer regions simply may have a higher concentration of heavy elements.
The new findings might force theorists to revise some of their notions about star formation, such as the density of gas required to trigger star birth. However, there are less-dramatic possibilities to consider, notes Kennicutt.
For example, although UV-bright stars last for only a few hundred million years, that's still about 30 times as long as a group of more-massive stars would last. These massive stars ionize surrounding gas, causing it to glow brightly at a specific wavelength of visible light.
Therefore, the absence of visible features in galactic fringes could indicate that some disturbance in these outlying regions squelched star formation in just the past few million years, after the UV-bright stars had finished forming. This could explain why the outlying regions of some galaxies look bright at UV wavelengths but not in visible wavelengths. Thilker notes, however, that in some galaxies, such as M83, it seems unlikely that star formation would have stopped at the same time across such large regions of the outer disk.
Another possibility, says Kennicutt, is that a substantial number of very massive, short-lived stars do reside in these low-density outposts, but that their strong winds have cleared out all the surrounding gas. Without ionized gas, the massive stars wouldn't radiate visible light and so would be invisible, he says.
Other astronomers, says Kennicutt, have an even more mundane explanation. They suggest that the ultraviolet-bright concentrations of light that have so captivated Schiminovich and other astronomers studying GALEX images aren't star-forming regions at all but merely clouds of gas and dust that reflect ultraviolet light.
"The nice thing is that no matter what the explanation is, we will learn something new and important about star formation and the interstellar medium in galaxies," Kennicutt says.
BABY ALBUM GALEX has also uncovered some entirely expected stellar activity. When two massive galaxies collide, they often throw out streamers or tails of gas that become sites for new-star formation. Examining several of these tails, the observatory has found neighborhoods of young stars less than 400 million years old. These newcomers appear to have formed in a single burst of activity, rather than over an extended time.
The very youngest concentrations of stars, which reside at the distant ends of the tails, weigh between 10 million and 1 billion times as much as the sun, Neff and her colleagues reported at the January AAS meeting. That's roughly the weight of much older dwarf galaxies, such as the ones that now orbit the Milky Way. Astronomers say that the dwarfs were generated by galactic collisions billions of years ago.
The nascent clusters of stars now seen with GALEX could be the youngest known examples of newly formed galaxies undergoing their first wave of star formation, Neff says. She notes that it's unclear whether the stars in these tails will separate from their parent galaxies and become independent galaxies or will eventually fall back into their parents.
On a larger scale, GALEX has begun to examine the UV emissions from thousands of galaxies, providing new data on the rate of star formation throughout most of cosmic history. The telescope is sensitive enough to detect star formation at a rate comparable to that of the Milky Way, about one sun a year, back to a time when the universe was half its current age.
In contrast, large surveys with infrared telescopes, which can look at star formation much further back in time, are limited to examining galaxies that churn out stars at 10 to 100 times the Milky Way's rate. Such telescopes simply can't detect star formation in the fainter, less active, but more-typical galaxies that GALEX can image, says Schiminovich.
Studying star formation with GALEX in nearby reaches of the universe can address questions raised by astronomers studying much more distant galaxies. Compared with nearby galaxies, distant ones recede faster, and the light they emit is shifted to longer, or redder, wavelengths. For instance, light from an extremely distant galaxy, recorded by an Earth-orbiting telescope as infrared radiation, was originally emitted by the galaxy as ultraviolet light.
To make sure they're making the proper comparisons when they're observing infrared light that began billions of years ago as UV light from distant galaxies, astronomers want to study the same sort of UV light in the nearby reaches of the universe. There, redshift is minimal and UV light remains UV.
"If we are observing the ultraviolet light from distant galaxies ... we certainly need to have a good knowledge of what the local galaxies look like in the ultraviolet," says Schiminovich. Preliminary data from GALEX confirm earlier findings that there's been a downturn in star formation over the past few billion years of cosmic history. The decline, however, appears to be steadier and more gradual than previous studies had indicated.
Schiminovich suggests that the decline may be linked to activity of supermassive black holes and quasars, which peaked in activity several billion years ago. Winds and jets from these powerhouses may have slowly but steadily blown away large amounts of gas in galaxies, hobbling star formation (SN: 1/22/05, p. 56).
Nevertheless, says Neff, GALEX has revealed that now, 14 billion years after the Big Bang, "there's still life in the universe. Things are still being lit up, and star-forming galaxies are still being born."
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|Date:||Feb 19, 2005|
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