Starship Asterisk*

Hickson 91 in Piscis Austrinus

Explanation: Scanning the skies for galaxies, Canadian astronomer Paul Hickson and colleagues identified some 100 compact groups of galaxies, now appropriately called Hickson Compact Groups (HCGs). This sharp telescopic image captures one such galaxy group, HCG 91, in beautiful detail. The group's three colorful spiral galaxies at the center of the field of view are locked in a gravitational tug of war, their interactions producing faint but visible tidal tails over 100,000 light-years long. Their close encounters trigger furious star formation. On a cosmic timescale the result will be a merger into a large single galaxy, a process now understood to be a normal part of the evolution of galaxies, including our own Milky Way. HCG 91 lies about 320 million light-years away in the constellation Piscis Austrinus. But the impressively deep image also catches evidence of fainter tidal tails and galaxy interactions close to 2 billion light-years distant.

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This is a fantastic group of galaxies indeed, and it is a splendid image!

It is very remarkable that so many galaxies of moderately the same size and therefore presumably located close together in space are spiral galaxies. There appear to be a couple of lenticulars too, but no ellipticals. But the spirals come in different shapes and colors. The bluest face-on arm-rich spiral is a typical Sc galaxy, a galaxy dominated by its arms. The dominant galaxy is a barred spiral of Hubble class SBb or SBbc. But the small galaxy tangled in the arms of the largest bully is harrassed and all yellow, lacking in star formation altogether. And a tidal arm from the big galaxy appears to stretch leftwards towards a small, rather neutral-colored "barred lenticular".

What a beautiful and fascinating group this is, and what a lovely APOD!

Ann

The "result will be a merger" link has some interesting info about mergers. Only about five pages long, but filled with explanations, graphs, and pictures.

From the top of the image along the diagonal, the galaxies of this image are : IC 5168, NGC 7214, HCG 91d, ESO 467-13 and ESO 467-15. Thank to Aladin, but i would like to know which are HCG 91a, HCG 91b and HCG 91c. Also, how many galaxies compose that group, 4 or more?

Donald Pelletier wrote:From the top of the image along the diagonal, the galaxies of this image are : IC 5168, NGC 7214, HCG 91d, ESO 467-13 and ESO 467-15. Thank to Aladin, but i would like to know which are HCG 91a, HCG 91b and HCG 91c. Also, how many galaxies compose that group, 4 or more?

You can use Aladin to search for HCG 91a and it will take you straight to NGC 7214. You can also search SIMBAD for HCG 91 and there is a little button which says "children" on it you can click to see a list of its members.
http://simbad.cfa.harvard.edu/simbad/si ... play=h_all

Donald Pelletier wrote:From the top of the image along the diagonal, the galaxies of this image are : IC 5168, NGC 7214, HCG 91d, ESO 467-13 and ESO 467-15. Thank to Aladin, but i would like to know which are HCG 91a, HCG 91b and HCG 91c. Also, how many galaxies compose that group, 4 or more?

The large central galaxy with the huge tidal streams is HICK 91A. also known as NGC 7214. The small yellow galaxy immediately below it is HICK 91D, also known as PGC 68155. The arm-rich blue galaxy which looks like a lovely flower is HICK 91C, also known as PGC 68160. At least this is what my software says!

I can't find HICK 91B, unfortunately. Unless, of course, HICK 91D really is HICK 91B!

Ann

HCG 91b is the edge-on spiral to the northeast (lower right in the image)

Ann wrote:

Donald Pelletier wrote:From the top of the image along the diagonal, the galaxies of this image are : IC 5168, NGC 7214, HCG 91d, ESO 467-13 and ESO 467-15. Thank to Aladin, but i would like to know which are HCG 91a, HCG 91b and HCG 91c. Also, how many galaxies compose that group, 4 or more?

The large central galaxy with the huge tidal streams is HICK 91A. also known as NGC 7214. The small yellow galaxy immediately below it is HICK 91D, also known as PGC 68155. The arm-rich blue galaxy which looks like a lovely flower is HICK 91C, also known as PGC 68160. At least this is what my software says!

I can't find HICK 91B, unfortunately. Unless, of course, HICK 91D really is HICK 91B!

Ann

Ann – Your lovely flower (PGC 68160) also caught my eye. In looking for a better image you run across newly published works such as" STAR FORMATION SUPPRESSION IN COMPACT GROUP GALAXIES: A NEW PATH TO QUENCHING?" and one is awestruck by the detail and complexity in a modern Astrophysical Journal article.

The specific use of color within the text of the article alone represents a new era in which the authors present their data in new and thought-provoking ways – and it's pretty too. The steps to publish such a thing must be mind-bending.

CHART32 keep producing some amazing images of some awesome galaxies! There's lots of other Hickson groups they can image and I look forward to seeing what they have in store for the future!

Also for identifying the members of Hickson 91, this guide might help.

Fred the Cat wrote: . . . STAR FORMATION SUPPRESSION IN COMPACT GROUP GALAXIES: A NEW PATH TO QUENCHING?" and one is awestruck by the detail and complexity in a modern Astrophysical Journal article.

The specific use of color within the text of the article alone represents a new era in which the authors present their data in new and thought-provoking ways – and it's pretty too. The steps to publish such a thing must be mind-bending.

That color is a standardized default color for any page anchor.

Fred the Cat wrote:

Ann wrote:

Donald Pelletier wrote:From the top of the image along the diagonal, the galaxies of this image are : IC 5168, NGC 7214, HCG 91d, ESO 467-13 and ESO 467-15. Thank to Aladin, but i would like to know which are HCG 91a, HCG 91b and HCG 91c. Also, how many galaxies compose that group, 4 or more?

The large central galaxy with the huge tidal streams is HICK 91A. also known as NGC 7214. The small yellow galaxy immediately below it is HICK 91D, also known as PGC 68155. The arm-rich blue galaxy which looks like a lovely flower is HICK 91C, also known as PGC 68160. At least this is what my software says!

I can't find HICK 91B, unfortunately. Unless, of course, HICK 91D really is HICK 91B!

Ann

Ann – Your lovely flower (PGC 68160) also caught my eye. In looking for a better image you run across newly published works such as" STAR FORMATION SUPPRESSION IN COMPACT GROUP GALAXIES: A NEW PATH TO QUENCHING?" and one is awestruck by the detail and complexity in a modern Astrophysical Journal article.

The specific use of color within the text of the article alone represents a new era in which the authors present their data in new and thought-provoking ways – and it's pretty too. The steps to publish such a thing must be mind-bending.

Thanks for the link to that fascinating article, Fred!

The focus of that article is the quenching of star formation in compact groups of galaxies. That's why I found it so interesting that the galaxy group of today's APOD is so totally dominated by spirals, the largest and most prominent of them relatively robustly starforming.

Which of the galaxies in the picture belong to Hickson 91? Well, the four galaxies labeled HCG 91 A, B, C and D belong to the group. (Geck was absolutely correct in her identification of HCG 91B.) I believe that the small rather neutral-colored seemingly barred galaxy located some distance to the left of one long curving tidal tail emanating from NGC 7214 may also be part of the group.

But the moderately large edge-on galaxy at 10 o'clock, IC 5168, is not part of the group. It is too fluffy, and its central region is too faint. Compare NGC 5168 with the fluffy blue foreground galaxy NGC 7320, seen in front of the compact group Stephan's Quintet.

In the upper left part of the picture you can find several small bright yellow-orange ellipticals, and at least one yellow-orange barred disk galaxy with distorted tidal tails. These galaxies are not part of Hickson 91, because they are clearly too far away.

There is also one small almost anomalously blue edge-on galaxy at 9 o'clock. It looks so different than the Hickson 91 galaxies that I doubt it is a member, but because it is less well resolved than the fluffy foreground galaxy to its upper right, it just might be at the same distance as the galaxies of Hickson 91. If it is, it would not be much farther away from the dominant galaxy NGC 7214 than the galaxy to the lower right, PGC 68164, and that would make it a member of the group.

Ann

Nice shot... really cool...

Dance of the Galaxies...

:---[===] *

Upon viewing the full size image I saw hundreds of faint, light, blurred dots in the distant background. Are these all galaxies?

Catalina wrote:Upon viewing the full size image I saw hundreds of faint, light, blurred dots in the distant background. Are these all galaxies?

It is always possible that a few of these blurry dots might be tiny blobs of nearby nebulosity. However, any blurry dot that is small but concentrated is almost always a galaxy, definitely so if it is yellowish and bright in the middle.

Ann

Faint, light, blurred dots can also be faint local stars. It is impossible to tell with just an image like this what they are unless it is obviously not circular or there are obvious extended features. I suppose even a faint, circular blob with extended features could also be a relatively local globular cluster, thought that seems unlikely to me for some reason, possibly because I've never considered that as an alternative before...

In regard to the names of galaxies in the APOD an inverted colours annotated image among the images that were brought up through the "deep image also catches" link should help.

Some of the faint dots in the background can also be distant quasars with distances of billions of light years.

Hi,

Totally new here and a complete amateur... I've registered mainly to ask a question regarding this fascinating image. Having read the posts on this image I'll assume the answer to my question is ...no, but I'll ask any way.

The face on galaxy near center at 4 O'clock to looks remarkably "similar" to the almost edge on galaxy more distant from center at 10 O' Clock. In fact the galaxy at 10 O' Clock looks a little distorted in comparison. Could these in fact be the same galaxy and the we are seeing a gravity lensing effect here?

Beautiful APOD!

Thanks,

Brian

Brian Mc wrote:The face on galaxy near center at 4 O'clock to looks remarkably "similar" to the almost edge on galaxy more distant from center at 10 O' Clock. In fact the galaxy at 10 O' Clock looks a little distorted in comparison. Could these in fact be the same galaxy and the we are seeing a gravity lensing effect here?

They do look similar, but what you are describing is strong gravitational lensing, which would be hard to miss if it was indeed going on in this cluster. In other words, more than one background galaxy would be very obviously affected by it.

Brian Mc wrote:The face on galaxy near center at 4 O'clock to looks remarkably "similar" to the almost edge on galaxy more distant from center at 10 O' Clock. In fact the galaxy at 10 O' Clock looks a little distorted in comparison. Could these in fact be the same galaxy and the we are seeing a gravity lensing effect here?

No. These galaxies are far too close to be lensed (which would require a massive galaxy or cluster much closer yet, blocking our view). Furthermore, gravitational lenses can't produce actual images the way an ordinary lens does, because unlike an ordinary lens, they are strongest at their center and weakest at their edges. So you end up with odd streaks and blobs, not true images.

Spiral galaxies share many features, so it's not uncommon to find many that look nearly alike.

Like Geck and Chris said, the face-on galaxy at 4 o'clock, PGC 68160, and the edge-on galaxy at 10 o'clock, IC 5168, are not lensed versions of the same galaxy. Chris and Geck have already stated their reasons, but I would like to add another one. Lensing always requires (or so I believe anyway) that the galaxies being lensed are far behind the massive cluster doing the lensing. That means that the lensing cluster can't "see" the lensed galaxies from two different directions, because even from different edges of the lensing cluster, the lensed galaxies can't be "turned over" so that they present different aspects of themselves.
Consider the famous Ferris wheel of London, the London Eye. At left you can see a face-on view of it. At right is a somewhat more edge-on view.












The distance between London and Paris, as the crow flies, is 343 kilometers. Let's assume there were two very tall buildings in Paris, one located at the western edge of the city limits and the other one at the eastern edge. Let's assume you could see the London Eye from both locations. Would they be sufficiently widely separated to see the London Eye show itself from two very different angles? I doubt it.

It is that way with background galaxies being lensed by foreground galaxy clusters. The lensed galaxies are being lensed by different amounts, and will show different amounts of distortion, but a the lensing will not present the same galaxy as a face-on galaxy in one lensed view and as an edge-on galaxy in another.



Here you can see an example of one and the same blue background galaxy being lensed by a foreground massive galaxy cluster. The multiple images of the blue galaxy show different amounts of distortion. We do see what might be a perfectly edge-on version of this galaxy, as well as one that is (apparently, but really) a rather face-on view of it. But what we see most of all is distortion.

Ann

I think he was suggesting that perhaps the edge-on galaxy was not an edge-on galaxy at all, but rather the same galaxy as the face-on spiral distorted in such a way that it appeared to be an edge-on galaxy.

Also forgot to mention, the tidal tails that are 2 billion light years away mentioned in the description are visible near the top left corner.

Another Hickson group in Piscis Austrinus is Hickson 90, which was previously on APOD although it doesn't include NGC 7172.

geckzilla wrote:I think he was suggesting that perhaps the edge-on galaxy was not an edge-on galaxy at all, but rather the same galaxy as the face-on spiral distorted in such a way that it appeared to be an edge-on galaxy.

Hello again.

Yes geckzilla that's exactly what I was suggesting. The comment that if lensing was occurring here there would be other galaxies similarly affected, was a great point. The fact that we don't is what lead me to thinking that gravity lensing was not occurring but like I first posted, I had to ask...

Thanks to all for the great detailed and quick responses. Now I know where to go for answers!

Cheers,

Brian