Starship Asterisk*

The Coma Cluster of Galaxies

Explanation: Almost every object in the featured photograph is a galaxy. The Coma Cluster of Galaxies pictured here is one of the densest clusters known - it contains thousands of galaxies. Each of these galaxies houses billions of stars - just as our own Milky Way Galaxy does. Although nearby when compared to most other clusters, light from the Coma Cluster still takes hundreds of millions of years to reach us. In fact, the Coma Cluster is so big it takes light millions of years just to go from one side to the other. Most galaxies in Coma and other clusters are ellipticals, while most galaxies outside of clusters are spirals. The nature of Coma's X-ray emission is still being investigated.

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Beautiful and Amazing. I wish they had mentioned the range of distances to galaxies (e.g. from 170 million to 10 billion light years away etc). Just to get and give a perspective, not everything that's "seen (looks) together and close" isn't so in reality.

shaileshs wrote: ↑Wed Mar 27, 2024 4:48 am Beautiful and Amazing. I wish they had mentioned the range of distances to galaxies (e.g. from 170 million to 10 billion light years away etc). Just to get and give a perspective, not everything that's "seen (looks) together and close" isn't so in reality.

Given that the diameter of the cluster is only a few percent of its distance from us, we can say that all of the galaxies in the image are approximately the same distance from us... around 300 million light years.

shaileshs wrote: ↑Wed Mar 27, 2024 4:48 am Beautiful and Amazing. I wish they had mentioned the range of distances to galaxies (e.g. from 170 million to 10 billion light years away etc). Just to get and give a perspective, not everything that's "seen (looks) together and close" isn't so in reality.

How can we know that most of the galaxies in the APOD are really at more or less the same distance from us?


We can know that most of these galaxies are at the same distance from us because most of them are blob-shaped and yellow in color. That means they are elliptical or lenticular galaxies with no spiral arms and practically no star formation. Also, the central galaxies are larger and brighter than the others. This is typical of dense galaxy clusters, where smaller elliptical galaxies crowd near larger elliptical galaxies.

Another giveway that we are dealing with a galaxy cluster where most of the galaxies in the picture are at the same distance from us is that we can find a few blue or bluish spiral or irregular galaxies some distance away from the center of the cluster. Let's have a look:


NGC 4889 and NGC 4874 are the two central galaxies of the Coma Cluster. Both are giant ellipticals. The blue-looking HD 113886 and HD 112887 are foreground stars of spectral class F, similar in color to Procyon, and they are much bluer than the elliptical galaxies of the Coma Cluster.

Near bottom of the image are two spiral galaxies, NGC 4921 and NGC 4911. They have spiral arms and new stars, but they are both affected by the hostile environment of the galaxy cluster, so they are transiting away from their spiral shape and gradually shutting down their star formation. That is especially true of NGC 4921:


NGC 4911 looks more normal, but it is a far cry from long-armed spectacular spiral galaxies like, say, M100. To me, NGC 4911 resembles Virgo Cluster spiral galaxy M58 (whose spiral nature is being slowly destroyed by its own galaxy cluster environment) in the character of its spiral arms and its degree of star formation:


Sometimes you see very blue and usually small galaxies at the outskirts of galaxy clusters. They are gas-rich galaxies that have been captured by the gravity of a nearby galaxy cluster, and now they are falling helplessly into it. As they are falling, their own gas meets the hot intracluster gas, triggering a torrent of star formation, while at the same time they are losing much of their gas due to what is called ram pressure (which can be translated roughly as a strong headwind as the galaxy is falling into the cluster).


I'm not aware of any hapless blue galaxies falling into the Coma Cluster. Maybe possibly maybe PGC 44779? It's the roundish blue galaxy near the lower right corner in the APOD. Though it looks more like a normal dwarf galaxy with a hint of spiral arms than a galaxy falling into a cosmic sinkhole. But it does have much the same radial velocity as Coma Cluster spiral NGC 4911, so who knows?


And that's it for me today! Gotta carpe diem! The sun is shining today.

Ann

Ann wrote: ↑Wed Mar 27, 2024 6:52 am
they are both affected by the hostile environment of the galaxy cluster, so they are transiting away from their spiral shape and gradually shutting down their star formation.

Ann

Why is the galaxy cluster such a hostile environment for star formation? I would have thought that all the chaotic movement would create shock waves to collapse the dust and gas and kick off star formation?

JimB wrote: ↑Wed Mar 27, 2024 9:21 am

Ann wrote: ↑Wed Mar 27, 2024 6:52 am
they are both affected by the hostile environment of the galaxy cluster, so they are transiting away from their spiral shape and gradually shutting down their star formation.

Ann

Why is the galaxy cluster such a hostile environment for star formation? I would have thought that all the chaotic movement would create shock waves to collapse the dust and gas and kick off star formation?

The way I understand it, large galaxy clusters and giant elliptical galaxies "grow up" together, and the giant elliptical galaxies contain monstrous black holes. When these black holes accrete matter, they eject tremendous jets that heat up and create turbulence in the intergalactic gas surrounding the galaxies.


Star formation, by contrast, needs cold molecular clouds to get started.

A. Ranjan et al. wrote:

Stars are known to form from the gravitational collapse of cold dense gas clumps in the interstellar medium (ISM; see Leroy et al. 2008). Observations in our own Galaxy and in the nearby Universe have further shown that this dense gas is in molecular form (H2). It is well known that gas giving birth to stars is molecular. Although the exact reason for this is not trivial since H2 is not a good coolant of the ISM.

Star formation requires cold molecular gas to get started. But in dense clusters, the intracluster gas, and probably also the gas inside galaxies, is heated up by outbursts of the supermassive black hole in the largest elliptical galaxies.

It is also highly likely that the wear and tear and constant interaction between galaxies in the crowded environment of galactic clusters quickly uses up the available gas inside galaxies. To see what I mean, consider nearby starburst galaxy M82:


Wait, you say. M82 isn't in a cluster, so why is it starforming so frantically and why is it losing so much gas? It's because it's interacting with M81 and NGC 3077:


When galaxies interact a lot, they tend to eventually exhaust their gas reservoirs. In galaxy clusters, elliptical galaxies keep interacting, but their interaction typically doesn't lead to more star formation:


So to summarize, too much jostling with other cluster members and too many X-rays from outside make the cluster galaxy members dull and yellow boys.

Ann

Hey y'all, am I the only one who is seeing gravitational lensing in this amazingly beautiful photo.

Ann wrote: ↑Wed Mar 27, 2024 10:28 am

JimB wrote: ↑Wed Mar 27, 2024 9:21 am

Ann wrote: ↑Wed Mar 27, 2024 6:52 am
they are both affected by the hostile environment of the galaxy cluster, so they are transiting away from their spiral shape and gradually shutting down their star formation.

Ann

Why is the galaxy cluster such a hostile environment for star formation? I would have thought that all the chaotic movement would create shock waves to collapse the dust and gas and kick off star formation?

The way I understand it, large galaxy clusters and giant elliptical galaxies "grow up" together, and the giant elliptical galaxies contain monstrous black holes. When these black holes accrete matter, they eject tremendous jets that heat up and create turbulence in the intergalactic gas surrounding the galaxies.

I think another factor involves the structure of spirals. The pattern of high density regions moving through low density ones and back often creates shock fronts, and that energy drives star formation. We often see that on the edges of spiral arms, no? But an elliptical is much more homogeneous, so we don't see those shock structures that stimulate new star formation so often in spirals.

Ed Siegel wrote: ↑Wed Mar 27, 2024 5:07 pm Hey y'all, am I the only one who is seeing gravitational lensing in this amazingly beautiful photo.

Where are you seeing it? Nothing jumps out to my eyes. I'm not sure that the scale here, 2.2 arcsec/pixel, would really allow us to see any lensed background objects.

Also, this cluster is probably too close to us to be an effective gravitational lens.

Chris Peterson wrote: ↑Wed Mar 27, 2024 5:08 pm

Ann wrote: ↑Wed Mar 27, 2024 10:28 am

JimB wrote: ↑Wed Mar 27, 2024 9:21 am

Why is the galaxy cluster such a hostile environment for star formation? I would have thought that all the chaotic movement would create shock waves to collapse the dust and gas and kick off star formation?

The way I understand it, large galaxy clusters and giant elliptical galaxies "grow up" together, and the giant elliptical galaxies contain monstrous black holes. When these black holes accrete matter, they eject tremendous jets that heat up and create turbulence in the intergalactic gas surrounding the galaxies.

I think another factor involves the structure of spirals. The pattern of high density regions moving through low density ones and back often creates shock fronts, and that energy drives star formation. We often see that on the edges of spiral arms, no? But an elliptical is much more homogeneous, so we don't see those shock structures that stimulate new star formation so often in spirals.

But that's it, Chris, isn't it? Elliptical galaxies lack spiral arms and dust lanes. And elliptical galaxies, both large and small, dominate dense galaxy clusters. Surely they must have changed and evolved over time to turn into ellipticals, by losing their gas?

Why would galaxy clusters be dominated by elliptical galaxies, both large and small, if there were not processes in these clusters that made virtually all their "central members" lose their arms? Because surely some of the galaxies in galaxy clusters must have started out as spirals?

We can see that there are spiral galaxies on the outskirts of of galaxy clusters. NGC 1365 in the Fornax Cluster is an almost painfully obvious example.


Indeed, Chris, elliptical galaxies don't form stars (broadly speaking). What I'm asking is why the central parts of (dense) galaxy clusters are so dominated by ellipticals. There must be a process that causes the central members of galaxy clusters to lose their gas and their arms.

Consider this map of the Virgo Cluster:


Note in the map of the Virgo Cluster that giant elliptical galaxy M87 (upper red dot) is located in the center of the dense clustering at top.


Further from the center of the Virgo Cluster, note starbursting spiral galaxies M99 and M100 right of center at top. Also note starbursting galaxy M61 far below the rest of the Virgo Cluster. The most "armful" of the Virgo Cluster galaxies are far away from the center of the cluster. Coincidence? I don't think so.

The central parts of dense galaxy clusters kill spirals, and therefore they also kill star formation.

Ann

Ann wrote: ↑Wed Mar 27, 2024 6:44 pm
Indeed, Chris, elliptical galaxies don't form stars (broadly speaking). What I'm asking is why the central parts of (dense) galaxy clusters are so dominated by ellipticals. There must be a process that causes the central members of galaxy clusters to lose their gas and their arms.

It may be as simple as the complex gravitational interactions not allowing spiral galaxies to last very long. After all, these are not static structures, but hundreds or thousands of galaxies dancing around each other like bees in a swarm. Which means collisions, near collisions, and complex tidal forces.

It just looks to me like everything is bent around the upper middle big yellow galaxy. I have been wrong before.

While the APOD itself left me with several questions, this Dicuss is one of the best ones ever.
I have a way to go before I finish all of it.
Thanks

Chris Peterson wrote: ↑Wed Mar 27, 2024 5:01 am

shaileshs wrote: ↑Wed Mar 27, 2024 4:48 am Beautiful and Amazing. I wish they had mentioned the range of distances to galaxies (e.g. from 170 million to 10 billion light years away etc). Just to get and give a perspective, not everything that's "seen (looks) together and close" isn't so in reality.

Given that the diameter of the cluster is only a few percent of its distance from us, we can say that all of the galaxies in the image are approximately the same distance from us... around 300 million light years.

And given (from the Wikipedia link) that there are, say, 5,000 galaxies in the cluster, and that "it takes millions of years for light to cross it", so, say it's 5,000,000 ly in diameter, I calculate that on average, each galaxy member is at the center of an 800,000 ly diameter sphere. Since that's probably about 8 times each galaxy's diameter, there's plenty of room in there! [unless my math is hopelessly off]

[ EDIT: Yup, my math was hopelessly off. With only 4000 galaxies in 5000 Kly cube, each galaxy would be only 250 Kly away from each of its neighbors. So I guess it's pretty crowded in this cluster after all. ]