Arp’s Peculiar Galaxies
Page 9 of 18
Arp observations 81-90 of 179 total to date.
| Thumbnail | Title/link | Arp Category | Date Observed | Observer Description |
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Arp 152 / Virgo A / M87 | Galaxies with jets | 2024-03-13 00:42:00 | The fourth of 9 entries added to the Messier catalog on March 8, 1781, also (#37) credited to Messier. "Nebula without star, in Virgo, below & very near a star of eighth magnitude.... This nebula appears at the same luminosity as the two nebulae Nos. 84 and 86." Halton Arp included M87 as Arp 152 in his Atlas of Peculiar Galaxies. He classified it with "Galaxies with Jets" and remarked "Short exposure to show jet. Virgo A radio source." <--> The toughest aspect for me was to set the black level appropriately to show the diffuse outer ring of this nebulous, elliptical galaxy while not obscuring the jet(s)... It's so large, I thought for a while my dark-subtraction in SharpCap was failing, and I was seeing amp glow! However, I was able to identify that the diffusion wasn't quite round, and not "quite" as large as uncontrolled amp-glow with my 294 sensor. This became obvious after about 15m of integration. The S/N ratio was so high, and since there is no structure to speak of in the outer disc, there seemed no advantage to continuing integration longer than the 25m once the two tiny galaxies, LEDA 139919 and UGC 7652, and the connecting jet were clear. Note that the "short exposure" Arp alludes to allows his image to show the jet right to the galactic center. My exposure wasn't so expert, and the inverted image is still blown out, but does appear to show the jet continuing a little deeper into the disk. Then a friend pointed out that it's actually second "jet" in this image that Arp did not detect: which appears as a tiny bump on the edge of the galactic core. You can see it the enlarged closeup is in the gallery. It is barely visible in the Arp photograph; note the Arp comparison is rotated 180° relative to the original capture, featured image, and jet enlargement. |
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Arp 155 / NGC 3656 | Galaxies disturbed by interior absorption | 2024-03-12 13:38:32 | The first challenge here is understanding what "disturbed by interior absorption" means. I searched carefully on the Internet for a definitive source and found found nothing very convincing. As is typical in the Arp world, many sources quote the category but explain nothing, as if Arp's perceptions were self-evident (they often aren't, I find). So I'm falling back onto the common meanings of these words, which would be a galaxy where the interior of the disk is seriously disturbed or disrupted. Examining the galaxies so categorized (153-160), I can confirm that this describes all of them to some degree. Certainly, in the case of Arp 153. the galaxy appears to be lenticular, but the core is bifurcated and divided by large dust lanes. This seems consistent with a galaxy consolidating its core after a merger. |
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Arp 157 / NGC 520 | Galaxies disturbed by interior absorption | 2026-02-24 20:30:00 | Discovered originally by William Herschel in 1784, this is a truly enigmatic galaxy of magnitude 11.8. Most modern commentator suggest its a merger in progress. Arp, himself, despised what he called "merger mania" and gave it a cryptic classification of "Galaxies, disturbed with interior absorption", whatever that means. Arp's remarks were "Note segment in NE direction" which presumably refers to the faint tidal trail continuing to the upper right from the galactic center. |
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Arp 158 / NGC 537 | Galaxies disturbed by interior absorption | 2024-02-29 19:39:57 | This is a highly disturbed galaxy, no doubt, but I'm a bit puzzled why "disturbance of interior absorption" is the most salient feature here. As a first approximation, young galaxies start with a coherent shape, often sprial and circular. As the galaxy matures and evolves through gravitational interactions with other galaxies, some sort of elliptical shape is often preserved, as well as a high degree of symmetry. This galaxy has been severely elongated in one direction and cut off in the other. The core appears to be separated into two planes: the top level, smaller, perhaps showing the remnants of a formerly "S" curved bar. The lower level, perhaps twice as long, and rotated counter-clockwise 10 or 15 degrees. |
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Arp 159 / NGC 4747 Revisit | Galaxies disturbed by interior absorption | 2026-07-10 22:56:00 | This is a revisit to 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. Arp's remarks note: "Very faint plume extending NE." The plumes are pretty clear; "internal abosorption" is a little trickier. The bright core, normally centered as a single circle of oval, now consists of 4 knots pushed off center. The faint plume isn't so faint in my capture (monochrome CMOS cameras being so sensitive). It turns out that the plume is a tidal tail caused by interactions with a nearby galaxy NGC 4725. I returned on July 12 to observe both galaxies together. There you'll find a full discussion of Tidal Tails, of which this is an example. |
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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. |