Explanation: The colorful, spiky stars are in the foreground of this image taken with a small telescope on planet Earth. They lie well within our own Milky Way Galaxy. But the two eye-catching galaxies in the frame lie far beyond the Milky Way, at a distance of over 300 million light-years. The galaxies' twisted and distorted appearance is due to mutual gravitational tides as the pair engage in close encounters. Cataloged as Arp 273 (also as UGC 1810), these galaxies do look peculiar, but interacting galaxies are now understood to be common in the universe. Closer to home, the large spiral Andromeda Galaxy is known to be some 2 million light-years away and inexorably approaching the Milky Way. In fact the far away peculiar galaxies of Arp 273 may offer an analog of the far future encounter of Andromeda and Milky Way. Repeated galaxy encounters on a cosmic timescale ultimately result in a merger into a single galaxy of stars. From our perspective, the bright cores of the Arp 273 galaxies are separated by only a little over 100,000 light-years.
I wish this APOD hadn't been so large in size! As it is, I have to post it as an attachment.
Image171a[3].jpg
Peculiar Galaxies of Arp 273
Image Credit & Copyright: Dave Doctor
In view of the fact that this picture was taken with a small telescope, and that the galaxies are 300 million light-years away from us, I think the picture is impressive. I also think it is right and good that the APOD editors should pick such fine efforts from people with moderate equipment and post their images as APODs.
For all of that, there is no way - of course! - that today's image can compete with Hubble's portrait of Arp 273:
The Hubble image helps us understand what we are seeing in the APOD. For example, there is a bright yellow object to the lower right of the center of the larger component of Arp 273. The Hubble image proves to us that this object is a star.
But in my opinion, the APOD helps us understand the colors of the Hubble image. The Hubble images are often "close to RGB" or "true color", but not always. An example of a Hubble image that is "not true color" is the Hubble portrait of NGC 3310. Compare the colors of the Hubble pictutre with the colors of the galaxy in a picture by Adam Block:
Why do the two images look so different? Adam Block wanted to create a "true" RGB color image, but he probably also used an Hα filter to bring out the fantastic pink "jet-like" features of emission nebulas from furious star formation. The Hubble picture, by contrast, aimed at separating different populations of ultraviolet stars in this very blue galaxy, which is why the entire palette of the Hubble image has been shifted to the red!
Let's leave NGC 3310 behind and concentrate on today's APOD. Note the two bright stars "above" Arp 273 in the APOD. The blue star is
TYC 2835-2364-1, and if you ask me, the B-V color index, 0.28, coupled with the luminosity of this star, 15 times the luminosity of the Sun, makes it an almost perfect equivalent of the bright star Altair in the Summer Triangle!
According to Wikipedia, the B-V index of Altair is 0.22 and its luminosity is 11 times that of the Sun. Altair is an A7V type star. I find it very likely that TYC 2835-2364-1 is a late A-type star, too. In short, the color of the blue star in the APOD looks "just right" and is "RGB-OK"!
I spent far less time finding info on the yellow star next to TYC 2835-2364-1, but according to my software this is a star of spectral class G5 with a color index of 0.889. The color of that star in the APOD looks good, too. So we can use the colors of the stars in the APOD to assess the color of the galaxies, and when we compare the APOD with the Hubble image we can see, therefore, that the Hubble image did a good job of bringing out the RGB color of this galactic pair.
I like this feature that can only be seen in the Hubble image:
Mini galaxy nestled in arm of UGC 1810 NASA ESA Hubble Heritage.png
Note the yellow bar and small bright core of this tiny galaxy-like feature! Also note that the apparent galactic bar seems to be surrounded by an incipient spiral-like or ring-like feature. And please note that this little thing appears to be sitting square in the middle of a spiral arm from UGC 1810, which is the larger component of Arp 273. Can we think of this structure as a true mini-galaxy? There are probably many reasons why we should not, but in my opinion, the little thing sure looks like a galaxy and has all the (main) features of a barred galaxy.
And more importantly, there is another interacting galaxy pair, NGC 6050, that also features what looks like a mini galaxy sitting right where two spiral arms meet!
The apparent mini-galaxy in NGC 6050 is sitting right where two major spiral arms meet, and it features a bright bar and what looks like a spiral or a galactic disk around it. There is an even smaller galaxy to the right of it, in an opening between two spiral arms of NGC 6050, but my guess is that this other feature is a background object.
All right, so how small can a galaxy get? I'm just saying, the Mandelbrot set...
Click to play embedded YouTube video.
The Mandelbrot set is saying, the way I understand it, that similar-looking structures can be seen at smaller and smaller scales in nature (and in mathematics). What about galaxies? I don't know of any miniature free-floating galaxies far away from larger galaxies. But what about tiny galaxy-like structures inside major arms of normal-sized spiral galaxies?
Let's return to the main features of Arp 273. (Oh, and I just have to add, did you know that NGC 6050 that I just talked about is also known as Arp 272??)
All right. Arp 273 has been compared with a rose, and the smaller component, UGC 1813, can be thought of as the stem of a rose, whereas the larger component, UGC 1810, is the flower itself. Surely you can see the similarity? UGC 1810 has fantastic spiral arms!
UGC 1810 and UGC 1813, the two components of Arp 272, are interacting and almost colliding. According to today's caption, the separation between the two components is as small as 100,000 light-years.
Well, the big bully of Andromeda is approaching us and may turn us into a galactic rose in the future, for all I know. But we are safe for now, since the distance to Andromeda is more than two million light-years.
"From our perspective, the bright cores of the Arp 273 galaxies are separated by only a little over 100,000 light-years."
What's that mean? I doubt we know the true distance to each of these galaxies separately to within 5 million lys. So their cores could very well be several million lys apart, right?
-- "To B̬̻̋̚o̞̮̚̚l̘̲̀᷾d̫͓᷅ͩḷ̯᷁ͮȳ͙᷊͠ Go......Beyond The F͇̤i̙̖e̤̟l̡͓d͈̹s̙͚ We Know."{ʲₒʰₙNYᵈₑᵉₚ}
johnnydeep wrote: ↑Thu Jan 09, 2025 12:53 pm
"From our perspective, the bright cores of the Arp 273 galaxies are separated by only a little over 100,000 light-years."
What's that mean? I doubt we know the true distance to each of these galaxies separately to within 5 million lys. So their cores could very well be several million lys apart, right?
Maybe it means that the angular separation at their distance corresponds to 100,000 ly separation... a distance only meaningful to the extent that both bodies are at substantially the same distance from us. Which we probably can't determine.
Chris
*****************************************
Chris L Peterson
Cloudbait Observatory https://www.cloudbait.com
johnnydeep wrote: ↑Thu Jan 09, 2025 12:53 pm
"From our perspective, the bright cores of the Arp 273 galaxies are separated by only a little over 100,000 light-years."
What's that mean? I doubt we know the true distance to each of these galaxies separately to within 5 million lys. So their cores could very well be several million lys apart, right?
Maybe it means that the angular separation at their distance corresponds to 100,000 ly separation... a distance only meaningful to the extent that both bodies are at substantially the same distance from us. Which we probably can't determine.
Ok. So, a pretty meaningless statement.
-- "To B̬̻̋̚o̞̮̚̚l̘̲̀᷾d̫͓᷅ͩḷ̯᷁ͮȳ͙᷊͠ Go......Beyond The F͇̤i̙̖e̤̟l̡͓d͈̹s̙͚ We Know."{ʲₒʰₙNYᵈₑᵉₚ}
This is from Halton Arp’s Atlas of Peculiar Galaxies published in 1966. The formation of galaxies was not well understood. Ann’s question about small galaxies within spiral arms may fit with Arp’s conjectures about ejection from galactic cores. IMHO, every astronomer should have a copy of the atlas. I, being a retired computer jock and school teacher, do not.
Roy wrote: ↑Thu Jan 09, 2025 5:37 pm
This is from Halton Arp’s Atlas of Peculiar Galaxies published in 1966. The formation of galaxies was not well understood. Ann’s question about small galaxies within spiral arms may fit with Arp’s conjectures about ejection from galactic cores. IMHO, every astronomer should have a copy of the atlas. I, being a retired computer jock and school teacher, do not.
Arp's atlas is interesting. His conjectures, however, are absurd and almost entirely discredited. No astronomer takes them seriously.
Chris
*****************************************
Chris L Peterson
Cloudbait Observatory https://www.cloudbait.com
What makes this pair so interesting is that either NGC 1232A is a background object, and then we may wonder why it appears to be absolutely unaffected by redshift-reddening, or else it is at more or less the same distance from us as NGC 1232. And if so, we may wonder why such a small galaxy has such a very well-established bar structure.
Could NGC 1232A be a background object? Yes, because there are some wildly discrepant radial velocities measured for this object. While the radial velocities measured for NGC 1232 vary between 1517 kilometers per second and 1705 km/s, the radial velocities of measured for NGC 1232A vary between 1439 km/s and 4062 km/s!! That's wild!
But the physical appearance of NGC 1232A suggests that it may be at the same distance as NGC 1232. Note that the blue star clusters in the outer arms of NGC 1232 and in the broken ring structure of NGC 1232A appear to be the same size and resolved to the same degree:
Also note that while the inner spiral structure of NGC 1232 is really quite yellow, only the bar of NGC 1232A shows some yellow stars. And even the bar is not very yellow, but shows clear signs of blue star formation, too.
Star formation in bars is not unheard of, but my impression is that it is mostly (though not exclusively) found in smallish galaxies where the bar structure is still in the process of either establishing or reaffirming itself. An example of this is NGC 7741:
NGC 7741 is a smallish galaxy dominated by young stars, even though its brightest feature, the bar, contains a good number of old stars. Note the similarity in overall structure, shape and stellar populations between NGC 7741 and NGC 1232A. Unlike NGC 1232A, NGC 7741 is not located either actually or just apparently close to another, major galaxy.
As for NGC 1232A, another interesting feature about it is that it is located exactly where the outermost bright spiral arm of NGC 1232 ends. I'm not suggesting that the arm of NGC 1232 has somehow "thrown out" NGC 1232A - I don't see how that could happen - but I do find the juxtaposition interesting. It might mean absolutely nothing, of course.
If NGC 1232A is located at more or less the same distance from us as NGC 1232, which seems likely, then the properties of this small galaxy suggest that, if nothing else, even very small galaxies can form bright and stable bars surrounded by rings or disks.
Ann wrote: ↑Fri Jan 10, 2025 6:13 am
...
I would like to draw your attention to the odd pair of NGC 1232 and NGC 1232A.
...
I'm not sure what NGC 1232 has to do with ARP 273
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk. — Garrison Keillor
Ann wrote: ↑Fri Jan 10, 2025 6:13 am
...
I would like to draw your attention to the odd pair of NGC 1232 and NGC 1232A.
...
I'm not sure what NGC 1232 has to do with ARP 273
I went off on a tangent, noticing what looks like a tiny galaxy inside an arm of UGC 1810. That reminded me of NGC 6050, which also seems to have a tiny galaxy in one of its arms. And then I was reminded of NGC 1232 and NGC 1232A, because NGC 1232A appears to be a very small galaxy with a well-developed bar, thus making it slightly similar to the tiny "galaxy" in the arm of UGC 1810.
I hope it is allowed to compare galaxies with one another. I love doing it.
Thank you for allowing me to log in again, Geck and Owlice!
Roy wrote: ↑Thu Jan 09, 2025 5:37 pm
This is from Halton Arp’s Atlas of Peculiar Galaxies published in 1966. The formation of galaxies was not well understood. Ann’s question about small galaxies within spiral arms may fit with Arp’s conjectures about ejection from galactic cores. IMHO, every astronomer should have a copy of the atlas. I, being a retired computer jock and school teacher, do not.
Well. I admire Halton Arp as an old-school astronomer like Zwicky was, who froze hundreds or thousands of hours in front of or next to their telescopes. Luckily, I am also one of the proud owners of a copy of his atlas and I love it even though it is now available online ( https://ned.ipac.caltech.edu/level5/Arp/Arp11.html ), just as I like all other old astro images, before our aesthetic perception was ruined by Photoshop filters and AI.
Regarding your comment. I don't think the examples are comparable. In his hypothesis, Arp is primarily interested in quasars as the origin of new "galaxy children". As Chris also says, this hypothesis could never be confirmed. Personally, I find this hypothesis quite interesting, but I have problems imagining how a galaxy core could give birth to and eject quasars. Since Arp was unable to provide any more precise information on this, it ultimately remained a hypothesis that is still pretty much the only one in classical science. I myself have analyzed a large number of examples of "seeing red" but have not been able to find any clear evidence, even using more recent images and at other wavelengths.
But well, Ann's galaxy in the galaxy arm is not a quasar and will not become one. However, I find Ann's assumption that we are dealing with a galaxy, wherever it comes from or was formed, interesting. If you study the original Hubble images in maximum resolution, you can see that there is an almost circular shell around the twisted core, which is much larger than the core area. We should therefore always be careful with galaxy dimensions, as their boundaries and areas of influence are notoriously underestimated. which in my opinion is a mistake. But what is also clear is the fact that this is not a galaxy lying behind it, but rather an effective interaction taking place.
Perhaps the JWST will shed more light on the matter in the future, as there are currently no JWST images of the galaxy.
Peculiar Galaxies of Arp 273
