Arp’s Peculiar Galaxies

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Arp observations 1-10 of 107 total to date.
Thumbnail | Title/link | Arp Category | Date Observed | Observer Description |
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Arp 1 / NGC 2857 | Spiral galaxies of low surface brightness | 2024-03-01 18:20:45 | Captured in an observation with Arp 285 in the same frame. Arp 285 was substantially brighter, but Arp 1 emerged eventually. No moon. A faint spiral galaxy with extended arms. Lovely once you could see it clearly! Significantly, the slightly extended upper arm of Arp 1 points at the two Arp 285 galaxies (see full image in the gallery), suggesting it may have been caused by a gravitational interaction with them. |
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Arp 6 | Bear Paw Galaxy | Spiral galaxies of low surface brightness | 2024-03-12 11:35:58 | What a "peculiar" target: tiny, moderate brightness! It appears to be 3 tiny dwarf galaxies merging, or else one or two tightly folded on themselves. I don't quite understand why Arp considered the peculiar aspect here to be the low surface brightness. For example, I've observed Coddington's Nebula, IC 2574, in the M81 Group, and it has much lower surface brightness besides being much larger and presumably much closer. Arp 6 seems much brighter. It seems to me that the three segments represent the more peculiar aspect.... |
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Arp 8 / NGC 497 | Spiral galaxies with split arms | 2025-01-02 19:58:00 | A wild, windy night in the Catskills where I needed both SharpCap's brightness filters (to eliminate cloud-obscured images) and FWHM filtering (to eliminate images blurred by wind or periodic error). I managed 38m with the Apollo-M mono-sensor which was enough. The split arms that Arp refers to are presumably the short stubby arms pulled away from the spiral disk in the top left and bottom right of the galaxy disk. I infer these were caused by "fly byes" of smaller galaxies that pulled the arms away from the original spiral, but were not absorbed. In the third gallery image, I've marked several tiny galaxies in the visual vicinity that represent the most obvious fly-by candidates. Note that most of the gray splotches in the area could also be galaxies. However, all of these galaxies -- including the 3 marked -- could as easily be distant with no gravitational relationship at all. There are no visible connections such as dust lanes connecting them to NGC 497. |
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Arp 9 / NGC 2523 | Spiral galaxies with split arms | 2024-02-02 19:30:00 | This was a difficult night because of clouds. I set the brightness filters on SharpCap, but I was only able to capture 24 minutes in an over an hour, after which observing became impossible. While I would have preferred to spend more time on this target, the bar through the galactic center was very obvious, and the elongated, split arms on both ends were faintly visible. The original color images, which suffered terribly from color noise, were converted to grayscale and processed in Affinity Photo as a monochrome image. This brought out substantially more detail than was possible in RGB. NGC 2523 is a barred spiral which, along with its split arms, implies a mature galaxy with past interactions with other galaxies. |
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Arp 10 / UGC 1775 | Spiral galaxies with split arms | 2025-01-03 20:42:40 | Another challenging observation because 1) there is a big disparity in surface brightness, compared to 2) the outer disk showing very faintly. The conventional Arp negative plate is difficult to interpret, so we've "developed" it in Affinity Photo in the comparison image. I'm frankly puzzled why Arp called out the "split arms" (presumably the arms to the left and right of the image) when the ring feature is so dominant and unusual in its own right. A number of papers have been written in the 2000's attributing the ring to the results of a galaxy collision or "accretion". One paper by Russian Astronomers (Bizyaev et al, The Astrophysical Journal, 662:304-321, 2007 June 10) attributes the ring structure to a galaxy collision and, if I interpret the paper correctly, their candidate for galaxy that did the deed is the bright knot or bulge in the galactic center. This is evident in my capture and slightly more distinct in Arp's. |
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Arp 12 / NGC 2608 | Spiral galaxies with split arms | 2024-04-22 23:32:00 | This is a barred spiral galaxy with arms split and bending towards the north and south. Magnitude 13 capture was challenging on a full moon night. No obvious object that might have split the arms via gravitational attraction. A local star and the galaxy center appear as an "optical double"... I believe the galaxy center is above the star. I converted the color image to grayscale and processed in monochrome, which allowed me to reduce noise and sharpen more aggressively to make the arms more obvious. |
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Arp 13 / NGC 7448 | Spiral galaxies with detached segments | 2024-10-24 20:57:00 | Arp categorized this as "Spiral Galaxies with detached segments". Presumably this refers to the bright lobes outside of the core. Four, arrayed in an arc from 0° to 100° in the image (280° to 20° in the sky) are fully "detached" from the core, and quite evident in my capture. Another five inner lobes, less distinct, abut the central core. I presume the outer lobes are dwarf galaxies being absorbed, while the inner lobes are segments of the original spiral structure getting pulled apart by the gravitational influence of the merging dwarfs. I thought the original negative plate from the Atlas was not terribly informative, so I "developed" it in Affinity Photo using the "invert" and "levels" tool. No sharpening or smoothing was applied to the Arp capture. |
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Arp 16 / M66 / NGC 3627 | Spiral galaxies with detached segments | 2024-03-25 23:03:40 | This was a lovely, challenging barred spiral galaxy on a Full Moon evening. Obviously, the broad outline of the target was instantly visible in SharpCap, but the surface brightness was lower than expected. I presume Arp's category refers to the fact that the upper arm of the galaxy is pulled away from the disk which gives M66 its distinctive, teardrop shape. I imagine that the arm was pulled away by an encounter with another galaxy at some point. Also indicating some sort of disturbance, the plane of the central bar is angled-in slightly towards the back of the disk compared to the plane of the disk and outer arms. |
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Arp 18 / NGC 4088 | Spiral galaxies with detached segments | 2024-04-16 22:33:00 | This appears to be a heavily distorted, formerly barred spiral galaxy. There are two "detached segments" that could account for Arp's categorization. The first is the stretched arm to the eastern (lower) end of the disc. This appears to be a former spiral arm, that was shifted by a gravitational encounter of some kind. The second is the arm sticking out from the western (top) end of the galaxy. This appears to be a partially digested dwarf galaxy merger. It could also account for the lower arm's detachment if it (the dwarf galaxy) approached from the East (bottom), passed westward to detach the spiral arm, then swung back to collide with the main galaxy after gravity attracted it sufficiently to begin a merger. |
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Arp 19 / NGC 145 | Three-armed spiral galaxies | 2025-01-03 20:50:00 | At magnitude 10.3 and 1.8x1.3 arc minutes this is a challenging galaxy to observe with a small telescope. Overall the galaxy is circular, but extremely dim and the disc appears as background dust in both my capture and Arp's. Presumably, if you exclude the center bar, two of the three arms include the long, circular arm across the top and the nearly vertical descending bar at the right. The third arm is a bit more ambiguous, but presumably is the short descending bar to the left. |