Arp 240 / NGC 5257 and 5258
| Object Type(s) | Galaxy |
| Relevant Catalog(s) | All (Chron), Arp, NGC |
| Arp Category | Galaxies with the appearance of fission |
| Obs. Lat/Long | 42° 17', 073° 57' |
| Constellation | Virgo |
| Date and Time Observed | 2025-07-21 22:16:00 |
| Instrument | EdgeHD 8" w/f7 reducer-1,422mm FL |
| Camera | Player One Apollo-M Mini |
| Image Details | Transparency: Good. Seeing: Good. Total integration time was 12m 30s. Exposures 15s@300g, No Filter. Dithered and recentered in SharpCap. No guiding. |
| Description | Arp categorizes this pair of interacting galaxies as providing "the appearance of fission". You can see why... two elongated galaxies roughly the same size, roughly mirroring each other. The best telescope in the world renders them with little internal structure visible. Could they have been, originally, one galaxy, somehow split apart? Continuing to share stars and other matter as they continued to separate? Possibly. At the time, galaxy mergers were thought nearly impossible due to the immense distances involved. So "fission" -- even absent any theoretical framework -- seemed a valid possibility. And welcome to the mid-1960s when some of the best observers in the world -- using the best telescopes in the world -- were seeing these phenomena for the first time and struggling to figure them out. Today, of course, we can see a lot more structure (see Hubble image). Far from being rare, we now know galaxy mergers are common, and that many galaxies experience several during their long evolutions. Mergers have a simple mechanism underlying them: gravity. When you look at the Hubble image, it's obvious that we're looking at two, independently formed spiral galaxies exchanging stars via the "bridges" and "tidal flows" we can see connecting them. What might have been round spirals at formation are now seriously distorted by the gravitational forces at play. They are "unwinding" not splitting. My image was captured on a night with a fair number of clouds, particularly low on the western horizon. We observed this image early in the evening, and integration was cut short by clouds. Remarkably, even with such short integration time we were able to capture the essence of what's going on here. |
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