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A Grand Spiral

The ultraviolet GALEX satellite just revealed the largest known spiral galaxy! NGC 6872 (a barred spiral) is 522,000 light-years across from the tip of one outstretched arm to the tip of the other, which makes it about 5 times the size of our home galaxy, the Milky Way! We recently covered this story on our Facebook but we wanted to go a little more in-depth. So, here is an interview with one of the people involved with this discovery, grad student Rafael Eufrasio.

NASA's GALEX Reveals the Largest-Known Spiral Galaxy
This composite of the giant barred spiral galaxy NGC 6872 combines visible light images from the European Southern Observatory’s Very Large Telescope with far-ultraviolet (1,528 angstroms) data from NASA’s GALEX and 3.6-micron infrared data acquired by NASA’s Spitzer Space Telescope. A previously unsuspected tidal dwarf galaxy candidate (circled) appears only in the ultraviolet, indicating the presence of many hot young stars. IC 4970, the small disk galaxy interacting with NGC 6872, is located above the spiral’s central region. Images of lower resolution from the Digital Sky Survey were used to fill in marginal areas not covered by the other data. Credit: NASA’s Goddard Space Flight Center/ESO/JPL-Caltech/DSS

Blueshift: What is your current role at Goddard? What research are you working on?

Rafel: I am a research assistant at Goddard, but I am also a PhD candidate at the Catholic University in DC. I am a 5th year PhD student and [have been] working at Goddard since I started it in 2008. If everything goes as planned, I am defending my thesis next year.

I am working on the Spectral Energy Distribution (SED) of nearby face-on spiral galaxies. For my research I use as much data as I can possibly gather for these galaxies, from the ultraviolet (UV) to the radio (i.e. UV, optical, IR, sub-millimeter, and radio), from space and ground-based telescopes. My goal is to describe the content of stars, gas, and dust of these galaxies based on the information from the SEDs, as well as their Star Formation History (SFH).

Blueshift: We read the news feature on the GALEX discovery of NGC 6872, which is currently the largest known spiral galaxy, more than five times the size of the Milky Way. Can you tell us more about the object and how this discovery came about? Did you stumble on this discovery or did you specifically use GALEX to study this object in more detail?

Rafael: I was introduced to this galaxy almost a year and a half ago by Dr. Duilia de Mello, a professor at the Catholic University who also works at Goddard. She was studying young stellar systems outside of galaxies, specially the ones with strong UV emission, that she likes to call “blue blobs”. She knew I was working on the SED of spiral galaxies and we started this project. Initially, [we] chose 16 large regions (10 kiloparsecs or kpc in diameter) all over the galaxy, [which] produced UV-to-IR [light] and we presented preliminary results at the American Astronomical Society (AAS) meeting in January of 2012.

However something seemed off when I checked the sizes of the regions, stellar masses, and star formation rates. These 10 kpc regions would encompass a large chunk of most spiral galaxies I am studying, but they looked small when superposed in NGC 6872.

Then we spent a year asking astronomers in different places and searching catalogs to finally claim that it is the largest-known spiral galaxy at the AAS meeting this January. With an extended disk of at least 160 kpc in diameter.

Blueshift: Why is this galaxy so much bigger than the Milky Way – and do you think this is an uncommon size for a galaxy, or have we just not discovered others like this yet? Once upon a time, we didn’t think exoplanets were common and now we know differently, for example.

Rafael: The interaction/collision between IC 4970 [the small galaxy near the disk of the larger one] and NGC 6872 contributes to this galaxy being so big (in physical extent and gas masses). IC 4970 lost a lot of gas to NGC 6872 in the collision. However, NGC 6872 had already a huge stellar mass before the interaction and formed a lot more after that. First, NGC 6872 is in a group (Pavo group) with at least 12 other companions and that might have contributed to its size and mass in the past.

Since galaxies grow by “mergers and acquisitions,” the largest spiral galaxies are expected to be in the local universe (100 megaparsecs or ~326,000,000 light years from us). In that sense, we don’t expect to see larger spirals in the early universe, billions of light years from us. However, it is certain that something bigger is out there waiting for us to find it.

Despite the dimensions of NGC 6872 not being common, the process that shaped it seems to be very common, specially when the universe was younger. As a matter of fact, they might be the connection (or part of it) between the clumpy galaxies in the high-redshift [early] universe and the spiral disks we observe in the local universe.

Even the largest and best-studied interacting systems with large tails, like the Mice, Antennae, and the Tadpole (from tip to tip of their tails) are smaller than the disk of NGC 6872!

The Mice (NGC 4676): Colliding Galaxies With Tails of Stars and Gas
The Mice (NGC 4676): Colliding Galaxies With Tails of Stars and Gas Credit: NASA, H. Ford (JHU), G. Illingworth (UCSC/LO), M.Clampin (STScI), G. Hartig (STScI), the ACS Science Team, and ESA

Blueshift: Are you collaborating with others to learn more about this galaxy? Is this galaxy being studied at different wavelengths?

Rafael: Yes, we are. So far we have collaborators in Chile and in Brazil. Since this galaxy is in the southern hemisphere (declination -70 degrees), it has been observed by the Very Large Telescope in Chile and is a good target for any instrument in the Southern hemisphere. Previous to this study, NGC 6872 has been observed in X-rays, optical, near-infrared and
radio. Our study added the archival GALEX ultraviolet [data], taken in 2005. The full extent of the galaxy can be more easily seen in the UV, by unveiling the young, hotter stars formed after the collision.

Blueshift: Where does this research go next? What else do you hope to learn about this galaxy, or about galaxies in general?

Rafael: We are still working on this galaxy and there is still a lot to learn about it. Right now I’m spending most of my time on its stellar populations and star formation rate, by decomposing its UV-to-IR spectral energy distribution.

Blueshift: What’s your next research topic, and where do you hope to be after your time at Goddard?

Rafael: Right know I am trying to focus on my thesis and not thinking too much about the future. I am planning on graduating next year and I definitely plan to continue my career as an Astrophysicist.

Thanks, Rafael, and best of luck in the future!

If you want to learn more about this galaxy, be sure to read the NASA feature!


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