Arp’s Peculiar Galaxies
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Arp observations 81-90 of 171 total to date.
| Thumbnail | Title/link | Arp Category | Date Observed | Observer Description |
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Arp 159 / NGC 4747 | Galaxies disturbed by interior absorption | 2024-06-12 23:52:00 | This is a fascinating and challenging observation. NGC 4747 is a barred spiral galaxy categorized by Arp as exhibiting disturbances due to "interior absorption" which seems reasonably apt in this case. On the image, North is left. The overall galaxy appears elongated, though this may be exaggerated by the relatively low angle of view. The central disk, normally centered, is offset up and to the right. Huge knots are visible on both ends of core. The bright "core", usually circular, exhibits two, possibly three bright, circular centers, almost as if they were beaded. A huge debris trail flows from the lower left to underneath the core and is unconnected to any object near the far end. Merger relics? |
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Arp 160 / NGC 4194 / Medusa Merger Galaxy | Galaxies disturbed by interior absorption | 2026-02-27 21:41:00 | The modern consensus, backed up by a series of gorgeous Hubble images (see 1 of them in the gallery), and multi-band observing, is that this is a late stage merger whose disruption triggered starburst activity. Chandra X-Ray observing detected a black hole in the tidal tail at the top (N) of the image, which appears like the venomous snakes in the Greek goddess Medusa's hair. As usual, Arp almost never sees mergers in his Atlas, instead categorizing this as "disrupted with interior absorption". His remarks, cryptically to an amateur like myself, state "lambda3727 em" which is (according to Google's AI) a spectroscopic absorption line associated with new star formation. |
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Arp 165 / NGC 2418 | Galaxies with diffuse filaments | 2024-03-13 11:51:43 | This is one of my early Arp observations, and it benefited substantially when I reviewed it in preparation for submission. It was captured on an otherwise clear, moonless evening, when we experienced a thin layer of high clouds that limited integration time. The key problem then becomes how to process the image to show the "diverse filaments". My original processing of the color image wasn't terribly effective. I've since learned that working with a monochrome, inverse image -- even ones converted from an OSC color image -- can be very powerful. An advantage here is that other than converting to monochrome and tuning the white, black, and gamma levels, you really don't need to stretch the inverse image at all. The relatively low light gathering power of the EdgeHD also makes it easy to distinguish field stars (which are dark circles in inverse) versus small galaxies, which are light gray smears. Most of the darker filaments seem to point to such smears, particularly the main filament which you can see clearly in my capture at the root. It appears to point to three galaxies on a similar line, including the largest which is almost dead center on the north border of the comparison image. There's a second galaxy, just SE, and the smallest galaxy of the three, slightly SW, closer to NGC 2418. The 200" Hale telescope resolves the filament further up the image, but, hey, the Rockefeller's didn't pay for my telescope. It's worth looking at the color comparison image as these three galaxies are easily color-differentiated from the field stars and you can distinguish the core and disk on the two larger galaxies. Also in my capture, you can also see how some of the filaments are skewed NE towards several faint smears (N is up, East is counter clockwise from center), as well as NW towards another tiny galaxy.. The Hale telescope has so much light-gathering power that these tiny galaxies are nearly indistinguishable from field stars. |
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Arp 166 / NGC 750-51 | Galaxies with diffuse filaments | 2024-02-21 19:30:17 | Brief observation due to cloud conditions. This is another early observation that benefited from reprocessing prior to submission as an inverted image. The two, main elliptical galaxies, NGC 750 and 751 are evident enough, as are the filaments that connect them. In some ways, here, the limited light capture is helpful, as the two galactic centers are distinct, whereas they're blown out, and virtually merged in the original Arp image. The key "diverse filament" stretches to the NW (clockwise is West) to a dwarf galaxy. In the Arp image, you can resolve a spiral structure with an extended tail in the opposite direction. On my capture, this galaxy is barely a squiggle in the inverted image, and invisible in the color version. Nonetheless, while not nearly as distinct as in Arp's image, the NW trending filament is clearly visible in the inverted image. It is slightly visible as a brownish cast to the background in the color image, though admittedly I missed it in my original analysis. BTW, this is yet another pair of galaxies discovered by William Herschel in the late 18th century. |
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Arp 167 / NGC 2672 | Galaxies with diffuse counter-tails | 2024-03-24 23:43:00 | The two galaxy centers, NGC 2672 (the larger) and 2673 (the smaller) were evident in SharpCap from virtually the first frame. Time was required to capture the diffuse plumes which you can see, with sufficient time. Arp remarks, "Comp[anion]. galaxy very condensed, has curved plume". With North right, and East counter-clockwise, there's a hint of curvature towards the SE, comparing the density of the plume to the NE. This is slightly more clear in the inverted image, though the color image may be nearly as clear. |
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Arp 168 / M32 | Galaxies with diffuse counter-tails | 2024-02-20 20:00:00 | Messier recorded this object on August 3, 1764, the same date as M31, and attributed discovery to Guillaume-Joseph-Hyacinthe-Jean-Baptiste Le Gentil de la Galazière in 1749. <--> Note: this is the first of 11 Messier Galaxies that Halton Arp selected for his Atlas of Peculiar Galaxies. I developed this description as part of my qualifying for the AL Arp Observing award. This is a very famous galaxy, mostly imaged with its dominant partner, M31. It is extremely bright. I had fun with this, as I've never understood why Arp labelled it "peculiar", and after this "deep dive", I'm still skeptical. The recorded remarks on Arp 168 read as follows: "Faint diffuse plume curved away from M 31 disk." To be clear, M32 looks like a dwarf elliptical galaxy, which might qualify as peculiar by itself. But Arp's category here is defined by "diffuse counter-tails". As I understand it a counter-tail is an asymmetrical dust plume, today understood to be the result of a merger or impending merger. So, to test the "remarks" clarification, I needed to figure out what direction is Andromeda. So I converted my observation to an inverted image, and overlaid it on a personal capture of Andromeda (image 2). I then drew a line from the Center of M32 to M31 on top of my overlay, and cropped out the relevant bits (image 3). Note I left the overlay semi-transparent so you can judge the accuracy of the overlay. Then I took that overlay and placed it back on my original M32 inverted capture... placing an arrow on top of the line indicating towards M31's center (Image 4). -- At this point I removed the overlay to see my inverted capture, knowing the direction to Andromeda (Image 5). Do I think there is a longer plume due south, away from Andromeda? No. Arguably there is an extended plume SW (to lower left corner), but does that count? And is it enough to qualify M32 as peculiar? Just in case there is something about Arp's own capture, I overlaid it on this (image 6). Does it confirm a due south asymmetry? Not to my eye, and the posible SW asymmetry is not confirmed either. So, I continue to be skeptical of this one. |
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Arp 169 / NGC 7236-7237 | Galaxies with diffuse counter-tails | 2024-10-27 21:24:00 | Arp's remarks describe "Faint diffuse plumes coming away from two galaxies". Arp 169 consists of three relatively bright galaxies, lined up, all sharing a diffuse plume and presumably merging: lenticular galaxy NGC 7236, an eliptical galaxy NGC 7237, and a small elliptical: NGC 7237C. 7236's plume is extended to the Northeast, terminating at what appears to be a tiny dwarf galaxy. while 7237's even fainter plume appears extend to another tiny galaxy to the SW. |
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Arp 170 / NGC 7578 | Galaxies with diffuse counter-tails | 2024-11-03 22:17:00 | At the heart of Arp 170 is a merged, curved, diffuse envelope of what (from the shape) are at least two elliptical galaxies. However, a total of 5 or 6 possible galactic nucleii track the same curve as the envelope. Are they all galactic cores, or are some asterisms? I can't tell and even the Palomar image doesn't disambiguate it much. In the inverted image there is a triangular shaped counter-tail extending NNE which circumscribes a trapezoidal pattern of 4 likely, tiny galaxies or galaxy-fragments. There's just a hint in my capture that the plume continues to the medium sized spiral galaxy, seen edge-on, in the SE corner of the image. A smaller, roughly triangular counter-tail extends SSW from the last two bright object in the main curve. They extend towards another "spec" which could well be another tiny galaxy. |
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Arp 172 / IC 1178 & 1181 | Galaxies with diffuse counter-tails | 2025-07-15 23:50:00 | We're reaching the end of "galaxy season" and essentially all of the Arp galaxies are already past the meridian when it's dark enough to capture them. So I had no particular reason to capture Arp 172 except it was higher in the sky than most. This observation was cut short by clouds. It was also notable as the first time I'd used my Apollo-M camera without an IR/cut filter. I think it helped: I thought the capture was remarkably fast given the conditions. |
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Arp 175 / IC 3481 & 3483 | Galaxies with narrow counter-tails | 2025-07-15 22:40:00 | At the root of the controversy is whether a tidal connection exists between IC 3481 and 3483. In his observing notes, Arp says the connection goes only "two-thirds of the way". But later images with different telescopes and emulsions made it appear that the connection is complete, and my observation could certainly be interpreted that way (though I wouldn't suggest it's definitive). Most Arp fans thought the connection existed, but then spectroscopic analysis gave 3483 a much lower redshift suggesting it was much closer. Arp argued that redshift measurements were not definitive, and if what you could plainly see was in conflict with theory, believe your eyes. Arp ultimately ultimately developed his theory of "intrinsic redshift" which made him a pariah within the Palomar community... It was one thing to point out peculiar galaxies: it made some Palomar astronomers uncomfortable, but others -- in particular Zwicky -- were strong supporters. However, this disagreement placed Arp in direct opposition to a central principle of Hubble's cosmology -- that redshift was a reliable measure of relative distance due to the expansion of the universe. Arp ultimately lost his observing privileges as a result. |