Arp’s Peculiar Galaxies
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Arp observations 11-20 of 110 total to date.
| Thumbnail | Title/link | Arp Category | Date Observed | Observer Description |
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Arp 25 / NGC 2276 | Spiral galaxies with one heavy arm | 2024-04-15 21:30:00 | Arp 25 (upper left) was visible from the first light frame. Arp 114's center looked like a bright star at first and capturing the rest of its disc required most of the integration time of 40m. I presume Arp concluded the two galaxies were perturbing each other because the "one heavy arm" in Arp 25 is stretched in the direction of Arp 114. |
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Arp 26 / M101 / Pinwheel Galaxy | Spiral galaxies with one heavy arm | 2025-03-18 22:53:00 | This is a Méchain discovery dated March 27, 1781. No formal recording date is noted for M101 nor for any remaining entries in the Catalog: "Nebula without star, very obscure & pretty large, of 6 or 7 minutes [of arc] in diameter, between the left hand of Bootes & the tail of the great Bear [Ursa Major]. It is difficult to distinguish when one lights the [graticule] wires." Halton Arp selected M101 for his Atlas of Peculiar Galaxies based on "one heavy arm" <--> This is a favorite target to which I've returned many times. Featured image is a recent capture with my new ASI2600mc-pro camera, my first outing with that camera and the EdgeHD. This was the same night as my recent capture of M81, but transparency had worsened. Fourth image in the gallery was a July 2023 observation (compared to yet another recent one) to show supernova SN 2023ixf, which had been discovered on May 13, 2023. |
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Arp 27 / NGC 3631 | Spiral galaxies with one heavy arm | 2024-04-22 00:57:00 | This was a very challenging observation due to a 93% moon and high clouds; it took over an hour to capture 26m of data due to SharpCap's brightness filtering. In the event, I was very pleased with the quality of the capture despite the noise: the large arm is clearly visible. This is a charming spiral galaxy, largely symmetrical, with the one lower arm extended. With its compact diameter and apparently dense disc, it resembles a miniature Fireworks Galaxy. |
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Arp 28 / NGC 7678 | Spiral galaxies with one heavy arm | 2024-10-26 19:44:00 | This looks like a lovely, face-on spiral galaxy that is clipped a bit flat along the full length of the right side, and oddly expanded on the left. One can imagine another galaxy making a close approach to Arp 28 from the upper right, stripping off stars (though you can still see something close to Arp 28's original spiral shape in the faint dust cloud that remains). This hypothetical galaxy, as it continues past, then seems to have swung to the lower left (from our perspective) pulling open the main spiral arm of Arp 28, and drawing off more stars from the periphery. Arp categorizes this as having a single, heavy arm, though one could easily argue it has two, both on the lower left of this image. Either way, it's a lovely, intriguing target. |
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Arp 29 / Fireworks Galaxy / C12 | Spiral galaxies with one heavy arm | 2022-11-26 09:38:00 | As is often the case with Arp, there's some ambiguity about which is the "heavy" arm, though I believe it's the one towards the bottom of the frame. My Evolution telescope had a mirror diagonal in the image train; the image was flipped in the comparison to Arp's capture. This was literally the first, successful capture of a faint Galaxy in my EAA career. There had been a painful learning curve leading to this point: I was required to capture both darks and flat frames. It was also my second attempt; two days earlier I had captured the galaxy but the focus was horribly off. This capture was my personal, "I love this!" moment. I had no notion of Arp at the time, but in retrospect, this experience started me on the path to the Arp Challenge because this is when I learned that challenging targets are the most fun. I've added to the gallery a "bonus" image of Arp 29 and the open cluster NGC 6939, imaged with my 600mm refractor in August 2023. |
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Arp 37 / M77 | Spiral galaxies with low surface brightness companion on arms | 2023-11-01 22:36:00 | Recorded December 17, 1780: "Cluster of small stars, which contains some nebulosity, in Cetus & on the parallel of the star Delta, reported of the third magnitude, & which M. Messier estimated to be hardly of the fifth. M. Méchain saw this cluster on October 29, 1780 in the form of a nebula." Interesting... Messier didn't see the nebula. Just a cluster of stars on top? Was his eyesight starting to fail? <--> As a Messier galaxy, this is a pretty straight-forward spiral. Arp selected this as one of his peculiar galaxies, and I have a fundamental problem. I have basically NO IDEA what caused Arp to classify this as having a "low surface brightness companion". To try and figure it out, I studied Arp 38 to 48 which are all in the same classification. In almost all of them, the "companion" seems to be a galaxy. Usually smaller, often a dwarf by comparison. But a galaxy nonetheless. I also tried developing Arp's plate into a positive, and processed it as I do my own with sharpening and contrast enhancements. It's the second gallery image. I still don't see any companion galaxy. Arp's remarks, which can be found on the Caltech webiste comments: "Seyfert galaxy. Small knot in arm." Neither comment helps with this issue. I've looked at a half a dozen other images of this galaxy, including the Hubble image. I just don't see it. I searched the Internet. Most sources I found quote the Arp category and present it as if it's obvious. It's not. |
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Arp 38 / NGC 6412 | Spiral galaxies with low surface brightness companion on arms | 2024-06-12 22:59:00 | This was captured almost due north early in what is nearly the shortest evening of the year, with a 35% crescent moon in the west. This is a lovely, seemingly "normal" spiral galaxy with a slight bar. Arp categorized it as having a "low surface brightness companion on arms" which, frankly, I found challenging. Note Arp designates "companion", singular. Is Arp suggesting the glowing, slightly nebulous spot at the top (N) end of the disk is a dwarf "companion" galaxy? His remarks call out, "Small ring in arm on N side," so it seems likely. Like many "nebulae" in this part of the sky, this was discovered by William Herschel in 1797. I find the Herschel family's story inspiring and like to comment in these notes when I discover it's the case. What an amazing brother/sister/son combo they were! |
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Arp 41 / NGC 1232 and 1232A | Spiral galaxies with low surface brightness companion on arms | 2024-12-25 22:29:00 | Eridanus is pretty far south, and Arp 41 reached a maximum height of 28° elevation on the night I observed it. It barely cleared my southern horizon post-meridian, but the low elevation made it vulnerable to clouds, which obscured it after only 5m of integration. Amazingly, it was enough to show the key features which merited Arp 41's inclusion in Atlas, specifically "a low surface brightness companion on one of its arms". This is the grey smudge on the bottom right. While the bottom arm would require longer integration to render fully, there is a hint of it even in this brief capture. |
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Arp 46 / UGC 12265 | Spiral galaxies with low surface brightness companion on arms | 2024-10-26 21:33:00 | These are truly obscure galaxies. As far as I can tell, Arp 46 is alternatively listed only as UGC 12265, which is from the Upsala Catalog published in 1973. I believe there are TWO "low surface brightness companions" here, and possibly 3, best seen in the featured image which has been enlarged about 3x and rotated so N is up. The first is the small, slightly elongated, bright spot (with galaxy-like halo) to the NNW (top left) of the galactic disk (next to another bright circle, which appears to be a star within the Milky Way). The second is the small dot, with an even fainter halo, at the end of the long, looping tail that starts at the SE end (bottom right) of the central disk and loops all the way up the image so it's due north of the galactic center. Put another way, it's the faint dot at a roughly 45° angle up and to the right of the star in the Milky Way mentioned earlier. By itself, neither my image nor Arp's conclusively demonstrates it's a second companion. The third companion, imo, is at the base of the long looping tail where it connects to the main galaxy. It does look like a small galaxy attached at right angles to the center spiral, possibly which might explain how such a long trail was drawn-out to the far end of the bigger galaxy. |
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Arp 49 / NGC 5665 | Spiral galaxies with small high surface brightness companions on arms | 2024-06-07 22:22:00 | Arp classifies this as having "small high surface brightness companions on arms.". He notes, "Appearance of wake from stellar object in E arm." and the wake is pretty evident in the image extending towards the upper left corner (Note, east on Arp's images is counter-clockwise from north, and we can see from the comparison image that the white divot is right). Both considerations suggest the companion is the bright dot on the end of the arm that extends upwards in the image. That said, I don't feel this is the most compelling "high surface brightness companion" I've seen. This was the first observation during a brief, clear window on a mostly cloudy evening during a new moon. Significantly, I was able to complete this observation at 10:22 PM on an evening when Astronomical Dusk didn't hit until 10:43. Fortunately its location in Bootes to the SSW was sufficiently far from the residual sky-glow in the NW. |