A Frankenstein Galaxy of Spare Parts

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Photo: At left, in optical light, UGC 1382 appears to be a simple elliptical galaxy. But spiral arms emerged when astronomers incorporated ultraviolet and deep optical data (middle). Combining that with a view of low-density hydrogen gas (shown in green at right), scientists discovered that UGC 1382 is huge, about 718,000 light-years wide.
Credit: NASA/JPL/Caltech/SDSS/NRAO

In a sleepy section of the visible universe there lurks a huge galaxy with a bizarre patchwork of features. The galaxy was known to astronomers, but its large size and strange attributes went unnoticed for decades. Research spearheaded by Lea Hagan, a graduate student at Pennsylvania State University, University Park, has found there is more to galaxy UGC 1382 than meets the eye. Hagen and her colleagues published their findings last month in The Astrophysical Journal.

A galaxy is a vast system of stars, gas, dust, and other matter held together in space by their mutual gravitational pull. Astronomers classify galaxies into three main types based on shape: (1) spiral galaxies, (2) elliptical galaxies, and (3) irregular galaxies. Spiral galaxies feature a thin, disklike structure with sweeping arms of stars wrapped about the galaxy’s center. Elliptical galaxies have forms like centrally concentrated spheres or flattened globes. Irregular galaxies are those that don’t have spiral or elliptical forms.

In visible-light images, UGC 1382 appears to be a modestly sized elliptical galaxy, uninteresting to all but the most serious astronomer.Ultraviolet images of the galaxy gathered by the orbiting Galaxy Evolution Explorer (GALEX) telescope, however, hinted that UGC 1382 had a more complicated structure. Hagen and her team noticed previously unseen huge spiral arms billowing out from the elliptical center. The team then used observations from other telescopes to learn more about this mysterious galaxy.

Hagen’s team found that UGC 1382 is over 700,000 light-years across with its spiral arms. A light-year is the distance light travels in a year in the vacuum of space, or about 5.88 trillion miles (9.46 trillion kilometers). The Milky Way, the spiral galaxy that contains our solar system, is only about 100,000 light-years across. The scientists also found UGC 1382 to possess a puzzling structure. They determined that the stars in the spiral arms are older than the stars in the galaxy’s bright core. This arrangement is opposite of what is seen in most spiral galaxies.

The team thinks that this odd formation arose from the combination of several galaxies long ago. Billions of years ago, a group of small galaxies formed, containing much gas and dark matter. Dark matter is a substance thought to make up most of the matter in the universe but does not give off, reflect, or absorb light rays. Much later, a kind of elliptical galaxy called a lenticular galaxy formed near the group. A lenticular galaxy spins, but has no spiral arms. This galaxy floated through the group of older galaxies. The lenticular galaxy’s gravity warped and pulled at the older galaxies, eventually twisting them around itself to form spiral arms. Because these galaxies were composed mostly of gas and dark matter, the spiral arms are dim and do not appear in visible-light images of UGC 1382.

Hagen and her colleagues think this kind of galaxy could only form in emptier sections of the universe, without gravitational interference from other galaxies. They hope to do more research to better understand UGC 1382, a sort “Frankenstein” galaxy cobbled together from cosmic spare parts, and to find more oddball galaxies that are stranger than they first appeared

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