The Making of a “Frankenstein” Galaxy

Astronomers take a second look at UGC 1382 — previously considered a typical elliptical galaxy — and reassess everything they know about its size, age, and formation.

We all know the story: a crazy scientist named Frankenstein creates a monster by assembling different human parts. Now, astronomers have found a similar “Frankenstein” galaxy about 250 million light-years away in a quiet and unremarkable neighborhood — but this Frankenstein is beautiful!

The monster galaxy, UGC 1382, was originally thought to be old, small, and typical of other elliptical galaxies. Several surveys done in the 2000s looked for structural features like star rings and bars but didn’t find anything other than a simple elliptical galaxy. Then Lea M. Hagen (Pennsylvania State University) noticed something interesting about UGC 1382 when her team was investigating star formation in early-type galaxies.

Using NASA’s Galaxy Evolution Explorer (GALEX), which images the universe in ultraviolet, they noticed UGC 1382 had very extended spiral arms. Further investigation using optical and infrared light observations showed it was actually ten times bigger than previously thought and, unlike most galaxies, its innermost stars are younger than the stars on the outskirts. It’s almost as if the galaxy had been built using spare parts — like Frankenstein.

"This rare 'Frankenstein' galaxy formed and is able to survive because it lies in a quiet little suburban neighborhood of the universe, where none of the hubbub of the more crowded parts can bother it," said Mark Seibert (Observatories of the Carnegie Institution for Science) in a press release. The galaxy is so delicate that even a small nudge from a neighboring galaxy would make it disintegrate.

Frankenstein’s Formation

In most galaxies, the innermost region forms first and has the oldest stars. As time goes on, newer stars form in the outer regions of the galaxy. But this wasn’t the case with UGC 1382. Instead of a crazy scientist putting pieces together, this unique galaxy may have been the result of separate entities merging together — each with its own history.

First, a group of dwarf galaxies composed mostly of gas and dark matter formed. A rotating galaxy without spiral arms, called a lenticular galaxy, came together nearby. Then, at least 3 billion years ago, the smaller galaxies fell into orbit around the lenticular, eventually becoming its spiral arms. This process would make the center of UGC 1382 younger than the spiral disk surrounding it.

But Lynn Matthews (Haystack Observatory) says one has to be cautious with this scenario. It’s difficult to find the precise age of a galaxy because stars form at different times and that process doesn’t always go smoothly. The amount of heavy elements, or metallicity, present in the galaxy can also impact the ages of the stars. “What the authors have shown so far is intriguing, and new observations to measure metallicity of the stars and gas in the different parts of this galaxy would be a very interesting next step,” she says.

Finding Similar Galaxies

At about 520,000 light-years across, UGC 1382’s spiral disk is roughly five times wider than the Milky Way’s* and, according to the study, is one of the three largest isolated disk galaxies known. The increasing availability of sensitive optical, ultraviolet, and hydrogen line observations of early-type galaxies may reveal more giant spirals like UGC 1382.

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* Corrected, September 13, 2016: The original version of this sentence reported that UGC 1382 is 718,000 light-years across and therefore more than seven times wider than the Milky Way Galaxy; however, 718,000 light-years is the size of the neutral hydrogen (HI) gas disk, which extends much farther than a spiral galaxy's arms (the Milky Way's HI disk is about 230,000 light-years across, whereas its spiral arms span roughly 100,000 light-years).