Starship Asterisk*

HCG 87: A Small Group of Galaxies

Explanation: Sometimes galaxies form groups. For example, our own Milky Way Galaxy is part of the Local Group of Galaxies. Small, compact groups, like Hickson Compact Group 87 (HCG 87) shown above, are interesting partly because they slowly self-destruct. Indeed, the galaxies of HCG 87 are gravitationally stretching each other during their 100-million year orbits around a common center. The pulling creates colliding gas that causes bright bursts of star formation and feeds matter into their active galaxy centers. HCG 87 is composed of a large edge-on spiral galaxy visible on the lower left, an elliptical galaxy visible on the lower right, and a spiral galaxy visible near the top. The small spiral near the center might be far in the distance. Several stars from our Galaxy are also visible in the foreground. The above picture was taken in 1999 July by the Hubble Space Telescope's Wide Field Planetary Camera 2. Studying groups like HCG 87 allows insight into how all galaxies form and evolve.

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There are some brave assertions in the APOD notes today. May I ask what is the scientific bases for these such comments?

* How do we know that the Hickson Compact Group 87 is actually compact? How do we know the distances between these galaxies?
* How do we know that the galaxies are self-destructing?
* How we even remotely guess that they have 100 million year orbits?

I think that scientists have ways of measuring these observations.
The picture is good and will become one of my wallpapers.

I believe that the central galaxy of the image can be part of the group or at least have had some contact as they arise from the deformation observed

verkle2 wrote:There are some brave assertions in the APOD notes today. May I ask what is the scientific bases for these such comments?

* How do we know that the Hickson Compact Group 87 is actually compact? How do we know the distances between these galaxies?
* How do we know that the galaxies are self-destructing?
* How we even remotely guess that they have 100 million year orbits?

The common redshift means that all are at the same distance. By considering the luminosities, mass and size can be estimated. By considering the mass of the galaxies and the much greater mass of the surrounding dark matter (a feature of these clusters), basic orbital parameters can be determined. Most of these galaxies have AGNs, so past collisions are probably. Many also show tidal distortion. Even simple n-body simulators show that systems like this are not stable, so galaxies pass close enough or through each other to slowly cannibalize each other. It isn't just the galaxies that are self-destructing, but also the cluster. Eventually, most of the material in the group will consolidated into a single galaxy.

WOW! Thanks Chris. I got it now except the 100-million year orbits around a common center. What's the common center?

moonstruck wrote:WOW! Thanks Chris. I got it now :? except the 100-million year orbits around a common center. What's the common center?

The "common center" is just the instantaneous center of mass of all the galaxies. Because the galaxies are in complex orbits around each other, that center is itself moving around.

The link for the "local group of galaxies" in the description above takes you to a mind-blowing list of galaxies that are considered the "local group" - I thought that I was starting to get a wee sense of ly distances in space until I read this - my little set of synapses is back to square one .....

Another thing cames to mind upon seeing today's APOD - the preface to the science fiction story "The Shadow and Night" by Chris Walley. Just to consider that someday our dear Earth might be called "Ancient Earth" and there could be "Made Worlds" out there on those galaxies .....

biddie67 wrote:The link for the "local group of galaxies" in the description above takes you to a mind-blowing list of galaxies that are considered the "local group" - I thought that I was starting to get a wee sense of ly distances in space until I read this - my little set of synapses is back to square one .....

The trick is to disconnect light-years from terrestrial distances. Trying to relate the two tends to cause the mind-blowing effect you refer to. The galaxies in the Local Group (our cluster) are all within 5 million ly of us. The cluster in today's APOD is something like 400 million ly away. So to scale, a compact galaxy cluster is like the buildings downtown in a big city, and clusters are separated by the typical distance between big cities.

I noticed that the large edge-on spiral in HCG 87 appears to have a cross going through its center, presumably our cross-sectional view of a double cone or hyperbolic structure. Is that likely a result of the galaxies' interactions? It seems too symmetrical to result from such a complicated set of orbits. Regardless, it's very interesting, especially since it's so large.

I too find it strange that your proposal of this/ these galaxies are 'slowly' destructing. in the brief time that we have been able to see these galaxies there certainly would be no evidence of destruction especially as you have surmised that its orbit is 100 million years. What basis/ evidence do you have that these galaxies are indeed destructing.

Is it just me, or does it look like those galaxies were just pasted there? The imagery just looks "off". It's still very pretty though!

Guest wrote:Is it just me, or does it look like those galaxies were just pasted there? The imagery just looks "off". It's still very pretty though!

Yeah, obviously it's fake http://www.gemini.edu/project/announcem ... 003-3.html

dan wrote:I too find it strange that your proposal of this/ these galaxies are 'slowly' destructing. in the brief time that we have been able to see these galaxies there certainly would be no evidence of destruction especially as you have surmised that its orbit is 100 million years. What basis/ evidence do you have that these galaxies are indeed destructing.

The value of a good theory is that it allows you to predict things you can't easily observe. These galaxies are subject to gravitational forces that have been well described since Newton's time. What would require a new explanation would be how such objects could orbit amongst each other and not evaporate (and such an explanation is lacking).

Chris Peterson wrote:The "common center" is just the instantaneous center of mass of all the galaxies. Because the galaxies are in complex orbits around each other, that center is itself moving around.

Wouldn't such moving-around violate conservation of momentum?

I'm afraid it is still unclear to me how we know both the relative positions of these galaxies and their mutual movement if all we have to go by is redshift measurements. The redshift gives a combination of distance and radial motion; is there any way to disentangle those (short of assuming the conclusion that these galaxies are close to each other in space)?

You write that th 100-megayear orbits are a deduction from, among other things, the amount of dark matter in the group. I thought we had concluded that there must be dark matter because that is needed to explain the already-known motions of galaxies. If the dark matter is there to explain the motion, and the motion is deduced from the dark matter, doesn't that lead to a really underspecified interpretation of the observations?

bystander wrote:

Guest wrote:Is it just me, or does it look like those galaxies were just pasted there? The imagery just looks "off". It's still very pretty though!

Yeah, obviously it's fake http://www.gemini.edu/project/announcem ... 003-3.html

Come on, I KNOW it's not fake!!! The photograph just looks funny....That's all.