Explanation: This spectacular intergalactic skyscape features Arp 227, a curious system of galaxies from the 1966 Atlas of Peculiar Galaxies. Some 100 million light-years distant within the boundaries of the constellation Pisces, Arp 227 consists of the two galaxies prominent above and left of center, the shell galaxy NGC 474 and its blue, spiral-armed neighbor NGC 470. The readily apparent shells and star streams of NGC 474 are likely tidal features originating from the accretion of another smaller galaxy during close gravitational encounters that began over a billion years ago. The large galaxy on the bottom righthand side of the deep image, NGC 467, appears to be surrounded by faint shells and streams too, evidence of another merging galaxy system. Intriguing background galaxies are scattered around the field that also includes spiky foreground stars. Of course, those stars lie well within our own Milky Way Galaxy. The telescopic field of view spans 25 arc minutes or just under 1/2 degree on the sky.
Yes, the NGC 474/NGC 470/NGC 467 group of galaxies form an amazing skyscape indeed!
And, for the understandable lack of a Hubble portrait of this extended group, George Williams, today's photographer, has done a fine job indeed portraying these galaxies!
I searched for other pictures of the same galaxies, because it is always interesting to compare several images of the same objects. The best one I could find was this one:
What I particularly liked about Warren Keller's image is that it provided the highest resolution I could find of the peculiar blue dwarf(?) galaxy, PGC 4755:
PGC 4755 near NGC 467. Credit: Warren A. Keller.
PGC 4755 is the fourth remarkable galaxy in this galactic field. (Or the third, if you don't think that the bright ring galaxy NGC 470 is remarkable.) PGC 4755 looks like an isolated bright blue bar that flares out at the ends. The bar is the galaxy! And there is a faint tail extending down and to the right in Warren Keller's image. In the APOD, whose resolution is not quite so high, the tail (which is more noticeable), is extending up and to the left.
PGC 4755 with a noticeable tail. Credit: George Williams.
A final note on PGC 4755: It is seen at the top of Warren A. Keller's image, but at the bottom of the APOD.
Of course, the real star of this APOD is NGC 474. Just look at that jumble of chaotic shells!!!
What causes such shells? Let's look at well-groomed shell galaxy NGC 3923 and ask Wikipedia about their prevalence and cause:
NGC 3923 is an example of a shell galaxy where the stars in its halo are arranged in layers. NGC 3923 has up to 42 shells, the highest number among all shell galaxies, and its shells are much more subtle than those of other shell galaxies. The shells of this galaxy are also symmetrical, while other shell galaxies are more skewed and asymmetrical.
Concentric shells of stars enclosing a galaxy are quite common and are observed in many elliptical galaxies. In fact, every tenth elliptical galaxy exhibits this onion-like structure, which has never been observed in spiral galaxies.
The shell-like structures are thought to develop as a consequence of galactic cannibalism, when a larger galaxy ingests a smaller companion. As the two centers approach, they initially oscillate about a common center, and this oscillation ripples outwards forming the shells of stars just as ripples on a pond spread when the surface is disturbed.
Click to play embedded YouTube video.
One more note on NGC 3923. The way I understand it, NGC 3923 underwent a minor (and gentle) merger, which is why its shells are so regular.
Did you know that fairly well-known elliptical galaxy NGC 1316 is a shell galaxy too?
It is possible that small spiral galaxy NGC 1317 (to the upper right of NGC 1316 in the picture above) is involved in creating the huge shell of NGC 1316. However, NGC 1317 looks fairly intact, and does not appear to be in the process of being eaten.
Of course, it's possible that NGC 1317 used to have a nice pair of flowing arms that NGC 1316 has eaten, which has given the big bully of a galaxy indigestion!
Soo... What happens to spiral galaxies when they eat a smaller companion? Well, they get twisted and distorted or cartwheel-like or grow enormous tails, but they don't grow shells. Don't ask me why.
P.S. Of course, speaking of shells, then there is Wolf-Rayet star WR 140 and its hot massive O-type companion, dancing together to create a stunning series of shells. But let's leave that for another time, shall we?
Ann wrote:And, for the understandable lack of a Hubble portrait of this extended group...
And why is the lack of a Hubble image understandable? Is it just because Hubble's FOV is too small to take it all in?
Most or all of these objects have been imaged by the HST. It's just the wide field that encompasses all of them that Hubble can't do (as is the case with the majority of APOD images, of course).
Chris
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Chris L Peterson
Cloudbait Observatory https://www.cloudbait.com
Ann wrote:And, for the understandable lack of a Hubble portrait of this extended group...
And why is the lack of a Hubble image understandable? Is it just because Hubble's FOV is too small to take it all in?
Most or all of these objects have been imaged by the HST. It's just the wide field that encompasses all of them that Hubble can't do (as is the case with the majority of APOD images, of course).
That's what I meant.
But if Hubble has imaged all of the remarkable objects in the APOD separately, can you, Chris, direct me to Hubble's portrait of PGC 4755?
And why is the lack of a Hubble image understandable? Is it just because Hubble's FOV is too small to take it all in?
Most or all of these objects have been imaged by the HST. It's just the wide field that encompasses all of them that Hubble can't do (as is the case with the majority of APOD images, of course).
That's what I meant.
But if Hubble has imaged all of the remarkable objects in the APOD separately, can you, Chris, direct me to Hubble's portrait of PGC 4755?
Ann
That object seems to have received very little attention from anybody. The only papers are catalog lists, nothing specific to the galaxy at all.
Chris
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Chris L Peterson
Cloudbait Observatory https://www.cloudbait.com
Shell Galaxies in Pisces
