Five Obscure Galaxies
So I’ve got a lot of classes today and I can’t be bothered to put together any too complex research. So y’all are getting the standard Low Quality Ranked List Filler Article™. So have five galaxies unloved by astronomers which I thought were pretty cool:
#5: Anon J201014-5638 / 'JO171'
JO171 and its glowing tentacles.
Moretti et al. (2018)
This horrid mouthful of numbers and letters refers to a galaxy buried deep in a vast cosmic city, the galaxy cluster Abell 36671. Like a college student crammed into an overfull studio apartment, the crowding has not been kind to JO171. The immense gravity of Abell 3667 slings it around at a thousand kilometers per second, fast enough that the thin, hot gas that permeates the cluster feels like a roaring wind. This wind rips away the star-forming gases of JO171 in a process called ram-pressure stripping, forming beautiful streamers of glowing nebulae and young stars that look a little like tentacles1. It is because of these streamers that we call JO171 and its rare kin jellyfish galaxies. But JO171 belongs not to just one rare galaxy type, but two—not only is it a jellyfish, it is also a Hoag-type ring galaxy. These rare and mysterious objects resemble scaled-up versions of the planet Saturn, with an inner sphere of old yellow stars and an outer ring of young blue ones1. All together, JO171 is a very unusual object that definitely deserves a second look.
#4: NGC 2915
At first glance, you might not see anything off about NGC 2915. Puny little galaxies like this are all over the universe, after all, and this one doesn't look at all out of the ordinary. But switch your optics over to the radio wave spectrum and everything changes. When we look at this galaxy in the 21-cm radio waves emitted by cold hydrogen gas, the piddly little midget transforms into a grand spiral2. Unlike the star-studded likes of our Milky Way and Andromeda Galaxy, NGC 2915's winding arms are utterly devoid of stars. Save for its core, this is a galaxy made of two things: hydrogen gas and 2. Why does NGC 2915 have so few stars to its name? We're not sure. NGC 2915 is a cosmic loner—perhaps without the gravitational tugs of close neighbours, the gas in its disk never had a chance to collapse and ignite2. All together, this is one galaxy that is certainly more than meets the (visible-light) eye.
#3: RCP 28
The (very bad) only image of RCP 28.
van Dokkum et al. (2023)
RCP 28 is another seemingly-unassuming dwarf. But that weird streak next to it isn't another galaxy. Rather, it's a trail of young stars 200,000 light-years long—way longer than RCP 28 itself! Looking at the galaxy in the green glow of shocked oxygen gas reveals another, fainter trail in the exact opposite direction. The movement of the gas in both trails indicates they were made by something very big moving very fast3. There are many galaxies with trails like these, formed when the at their hearts gorge on interstellar gas and spew out vast plumes of energetic particles. But RCP 28 has no sign of a feeding black hole3. There is one more key feature about the galaxy may explain this enigma: RCP 28 is not one galaxy, but three! It is an ongoing merger between three small galaxies3. Taking this into mind, we have a candidate for the very big thing and how it got moving so fast: a supermassive black hole, violently ejected from RCP 28 by another supermassive black hole! When galaxies collide, their supermassive black holes sink towards each other and collide too. But if a third galaxy and its supermassive black hole is added to the equation, things can get messy. The three black holes swing each other around until one, two, or even all three are tossed out3. With so much gravity being slung all over the place these unlucky black holes can get the boot at hundreds of kilometres per second—more than enough to exile them to deep space forever. RCP 28 shows us the titanic might of gravity on grand display.
#2: 2MASX J16273931+3002239
J16273: ugly, but fascinating.
Bogdán et al. (2022)
This somehow even worse scramble of letters and numbers, which we'll shorten to J16273, refers to a truly unique galaxy some 2.5 billion light-years away from Earth. In most galaxy clusters, the largest and most dominant galaxy will be an old, spherical elliptical galaxy. These have long exhausted their supplies of star-forming gases and collisions with other galaxies have battered them into globular shapes, so they lack the pretty disks and young blue stars of the more common spiral galaxies. With a few exceptions, they're pretty boring. But J16273, despite being dead-center in a galaxy cluster, is not an elliptical galaxy—it's a spiral4! Admittedly it's a pretty ugly and red spiral, but it's a spiral nonetheless. With the sheer number of galaxies buzzing around the cluster J16273 by all rights should have been bashed into the shape of a lumpy potato millions of years ago, so what gives? Well, the same forces that turn galaxies into ellipticals may sometimes make them go the other way! If a big elliptical galaxy collides with a small companion rich in gas, the gas can settle around it, form a new disk of stars4 and voilà! An out-of-place spiral galaxy! J16273 reminds us that there's an exception to every rule, and that's something I think we ought to keep in mind.
#1: 2MFGC 08638
2MFGC 08638: An enigmatic galactic giant.
NASA, ESA, P. Ogle (STScI), Joseph DePasquale (STScI), SDSS Consortium
Our final galaxy 2MFGC 08638 is a fellow super spiral and perhaps the most extreme one at that. With about ten times as many stars as our Milky Way it is impressive already, but what is not so visually apparent is its dark matter , which is forty times as massive as ours5! This is a tremendous amount of mass, more than almost all galaxies in the universe. A halo this large would usually contain dozens of galaxies, yet this one somehow has just one. It's possible for a galaxy as gargantuan as this to form if all the dozens of galaxies in the halo all merged into one, but 2MFGC 08638 actually has too few stars5 for its mass. Add that to its delicate morphology undisturbed by gravitational tugs from other galaxies and we have to assume 2MFGC 08638 has been alone for at least the past few billion years. How did a lone galaxy get so massive? We have no idea. 2MFGC 08638 shows us that there are always mysteries out there, no matter how confident we are of our worldview. And I think that's pretty neat!
References
1 Moretti, A., Poggianti, B. M., Gullieuszik, M., Mapelli, M., Jaffé, Y. L., Fritz, J., Biviano, A., Fasano, G., Bettoni, D., Vulcani, B., & D'Onofrio, M. (2018). GASP. V. Ram-pressure stripping of a ring Hoag's-like galaxy in a massive cluster. Monthly Notices of the Royal Astronomical Society, 475(3), 4055–4065. https://doi.org/10.1093/mnras/sty085
2 Meurer, G. R., Carignan, C., Beaulieu, S. F., & Freeman, K. C. (1996). NGC 2915. II. A dark spiral galaxy with a blue compact dwarf core. The Astronomical Journal, 111(4), 1551-1770. https://doi.org/10.1086/117895
3 van Dokkum, P., Pasha, I., Buzzo, M. L., LaMassa, S., Shen, Z., Keim, M. A., Abraham, R., Conroy, C., Danieli, S., Mitra, K., Nagai, D., Natarajan, P., Romanowsky, A. J., Tremblay, G., Urry, C. M., & van den Bosch, F. C. (2023). A candidate runaway supermassive black hole identiifed by shocks and star formation in its wake. The Astrophysical Journal Letters, 946(2), L50. https://doi.org/10.3847/2041-8213/acba86
4 Bogdán, Á., Lovisari, L., Ogle, P., Kovács, O. E., Jarrett, T., Jones, C., Forman, W. R., & Lanz, L. (2022). Detection of a superluminous spiral galaxy in the heart of a massive galaxy cluster. The Astrophysical Journal, 930(2), 138. https://doi.org/10.3847/1538-4357/ac62cd
5 Ogle. P. M., Lanz, L., Nader, C., & Helou, G. (2016). Superluminous spiral galaxies. The Astrophysical Journal, 817(2), 109. https://doi.org/10.3847/0004-637X/817/2/109
