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 Hubble image shows one such galaxy group, HCG 90, in startling detail. Three galaxies -- two visible here -- are revealed to be strongly interacting: a dusty spiral galaxy stretched and distorted in the image center, and two large elliptical galaxies. The close encounter will trigger furious star formation. On a cosmic timescale, the gravitational tug of war will eventually result in the merger of the trio into a large single galaxy. The merger process is now understood to be a normal part of the evolution of galaxies, including our own Milky Way. HCG 90 lies about 100 million light-years away toward the constellation of the Southern Fish (Piscis Austrinus). This Hubble view spans about 40,000 light-years at that estimated distance. Of course, Hickson Compact Groups also make for rewarding viewing for Earth-bound astronomers with more modest sized telescopes.
Hickson Compact Group 90.
Image Credit: NASA, ESA, and R. Sharples (University of Durham)
Thanks for posting that picture, alter-ego! You'll forgive me for posting it again.
APOD Robot wrote:
Three galaxies -- two visible here -- are revealed to be strongly interacting: a dusty spiral galaxy stretched and distorted in the image center, and two large elliptical galaxies. The close encounter will trigger furious star formation.
NGC 3314. Image Credit:
NASA, ESA, the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration,
and W. Keel (University of Alabama)
The claim that the close encounter of the galaxies of the Hickson 90 Group will trigger furious star formation in the dusty spiral member didn't seem right to me, because the dust didn't look "fertile" to me. As you can see in the picture at right, which shows one galaxy seen in silhouette in front of another galaxy, not all dust gives rise to star formation. An even more interesting example is this one, where the dusty tendrils of the foreground galaxy don't seem to form any stars at all.
There actually seems to be relatively small amounts of dust in Galaxy Group Hickson 90. Perhaps we should compare the dust in the compact Hickson group with the dust content in the large elliptical galaxy NGC 1316. The dust in NGC 1316 is, by all accounts, mostly "sterile", and it gives rise to very, very little star formation.
Also the galaxies in Hickson Group 90 seem to be very small.
APOD Robot wrote:
This Hubble view spans about 40,000 light-years at that estimated distance.
40,000 light-years? That's very little, and then we're talking about the whole field of view. What is the size of that elongated dust cloud - 25,000 light-years? 30,000 light-years? Bearing in mind that the size of the disk of the Milky Way is believed to be about 100,000 light-years, a disk as small as ~30,000 light-years suggests that the dusty galaxy started out as a dwarf galaxy and not a spiral galaxy. (Although it is still possible that the shredded dust originally belonged to a spiral galaxy, because there really are some tiny spiral galaxies out there. Such as M94, as seen in this picture by Leonardo Orazi.)
Post-collision galaxy NGC 2146.
Photo: Ken Crawford.
And by the way, I can't keep this from you. As I was searching for dusty colliding galaxies to compare with Hickson Compact Group 90, I considered post-collision galaxy NGC 2146. But I eventually rejected it, because it isn't sufficiently similar to the dusty spiral(?) of Hickson Group 90, also known as NGC 7174. NGC 2146 is dustier, bluer and more ultraviolet than NGC 7174. For NGC 2146, its B-V is 0.790, its U-B is 0.290, and its far infrared magnitude is almost 3 ½ magnitudes brighter than its B magnitude. That's dusty! For NGC 7174, its B-V is around 1.00, its U-B is around 0.5, and its far infrared magnitude is "only" 2 magnitudes brighter than its B magnitude.
NGC 2146. Credit: NASA/ESA/J. Schmidt (Geckzilla).
Yes, but guess what! As I was searching for pictures of NGC 2146, I came across this one - a picture of NGC 2146 processed by our own Geckzilla, Judy Schmidt!
Ann wrote:The claim that the close encounter of the galaxies of the Hickson 90 Group will trigger furious star formation in the dusty spiral member didn't seem right to me, because the dust didn't look "fertile" to me. As you can see in the picture at right, which shows one galaxy seen in silhouette in front of another galaxy, not all dust gives rise to star formation.
Star formation requires hydrogen. Dust plays some sort of only partly understood catalytic role, but the critical factor is hydrogen. Dust by itself (to the extent you find that) wouldn't do anything.
So the question here is how much hydrogen is present. Colliding galaxies often bring different H regions together at high speed, creating shocks and other local concentrations which result in star forming regions. We'd need to see a neutral hydrogen map (probably acquired with a radio telescope) of these galaxies to know more about the likelihood of star formation beginning in the next few million years.
Re: APOD: Galaxy Group Hickson 90 (2017 May 17)
Posted: Wed May 17, 2017 1:27 pm
by neufer
Click to play embedded YouTube video.
Ann wrote:
The claim that the close encounter of the galaxies of the Hickson 90 Group will trigger furious star formation in the dusty spiral member didn't seem right to me, because the dust didn't look "fertile" to me... There actually seems to be relatively small amounts of dust in Galaxy Group Hickson 90. Perhaps we should compare the dust in the compact Hickson group with the dust content in the large elliptical galaxy NGC 1316. The dust in NGC 1316 is, by all accounts, mostly "sterile", and it gives rise to very, very little star formation.
Re: APOD: Galaxy Group Hickson 90 (2017 May 17)
Posted: Wed May 17, 2017 2:28 pm
by Ann
Chris Peterson wrote:
Star formation requires hydrogen. Dust plays some sort of only partly understood catalytic role, but the critical factor is hydrogen. Dust by itself (to the extent you find that) wouldn't do anything.
So the question here is how much hydrogen is present. Colliding galaxies often bring different H regions together at high speed, creating shocks and other local concentrations which result in star forming regions. We'd need to see a neutral hydrogen map (probably acquired with a radio telescope) of these galaxies to know more about the likelihood of star formation beginning in the next few million years.
I agree, of course, that hydrogen is the critical factor.
My impression is that compact groups that have been interacting for a long time, and which are dominated ellipticals, don't seem to contain large quantities of "concentrated hydrogen". The hydrogen in such groups and such galaxies is typically "spread out", turbulent and rather "hot", and it is often unsuitable for star formation.
I believe that is the case for Galaxy Group Hickson 90.
Ann
Re: APOD: Galaxy Group Hickson 90 (2017 May 17)
Posted: Wed May 17, 2017 2:36 pm
by Chris Peterson
Ann wrote:
Chris Peterson wrote:
Star formation requires hydrogen. Dust plays some sort of only partly understood catalytic role, but the critical factor is hydrogen. Dust by itself (to the extent you find that) wouldn't do anything.
So the question here is how much hydrogen is present. Colliding galaxies often bring different H regions together at high speed, creating shocks and other local concentrations which result in star forming regions. We'd need to see a neutral hydrogen map (probably acquired with a radio telescope) of these galaxies to know more about the likelihood of star formation beginning in the next few million years.
I agree, of course, that hydrogen is the critical factor.
My impression is that compact groups that have been interacting for a long time, and which are dominated ellipticals, don't seem to contain large quantities of "concentrated hydrogen". The hydrogen in such groups and such galaxies is typically "spread out", turbulent and rather "hot", and it is often unsuitable for star formation.
Yes, but what collisions do is redistribute and concentrate such "spread out" regions which allows for new star formation.
Re: APOD: Galaxy Group Hickson 90 (2017 May 17)
Posted: Wed May 17, 2017 3:01 pm
by Ann
Chris Peterson wrote:
Yes, but what collisions do is redistribute and concentrate such "spread out" regions which allows for new star formation.
The Coma Cluster of galaxies.
Credit: NASA/ESA/JPL-Caltech/STScI
Yes, but in the long run, the constant interactions make the constituent hydrogen just too hot and turbulent for star formation.
Of course conditions are different in a huge cluster like the Coma Cluster, where jets from supermassive black holes may heat the hydrogen so much that it becomes unsuitable for star formation for many millions of light-years, than they are in a tiny compact group like Hickson 90. Even so, I doubt that Hickson 90 is in for any huge fireworks of upcoming star formation.
Ann
Re: APOD: Galaxy Group Hickson 90 (2017 May 17)
Posted: Wed May 17, 2017 5:15 pm
by douglas
I'd like to see a plausible explanation for why the dust lights up to the furthest extents of its presence in so many of these galaxies. A physical explanation. As if it were fluorescing, almost. Doesn't look like it's disturbed by gravitational acceleration, almost like it's been suspended from gravitational effects.
douglas wrote:I'd like to see a plausible explanation for why the dust lights up to the furthest extents of its presence in so many of these galaxies. A physical explanation. As if it were fluorescing, almost. Doesn't look like it's disturbed by gravitational acceleration, almost like it's been suspended from gravitational effects.
The dust isn't lighting up at all. It's attenuating the light of billions of galactic stars behind it.
The dust is orbiting due to gravity, but most of the disruption is probably electromagnetic in nature- disturbed by photons, winds of charged particles, and hydrodynamic interactions within itself.
Re: APOD: Galaxy Group Hickson 90 (2017 May 17)
Posted: Wed May 17, 2017 6:27 pm
by douglas
Chris Peterson wrote:
douglas wrote:I'd like to see a plausible explanation for why the dust lights up to the furthest extents of its presence in so many of these galaxies. A physical explanation. As if it were fluorescing, almost. Doesn't look like it's disturbed by gravitational acceleration, almost like it's been suspended from gravitational effects.
The dust isn't lighting up at all. It's attenuating the light of billions of galactic stars behind it.
The dust is orbiting due to gravity, but most of the disruption is probably electromagnetic in nature- disturbed by photons, winds of charged particles, and hydrodynamic interactions within itself.
Your proffering can only imply the galaxies have become incredibly dust-enriched, which I reject. And a uniformity of photon disturbance that makes it as an explanation truly unlikely?
No. It is an expression of something poorly understood at this time. The uniformity. Hydrodynamic interactions that leave no artifact of turbulence?
Re: APOD: Galaxy Group Hickson 90 (2017 May 17)
Posted: Wed May 17, 2017 6:35 pm
by Chris Peterson
douglas wrote:
Chris Peterson wrote:
douglas wrote:I'd like to see a plausible explanation for why the dust lights up to the furthest extents of its presence in so many of these galaxies. A physical explanation. As if it were fluorescing, almost. Doesn't look like it's disturbed by gravitational acceleration, almost like it's been suspended from gravitational effects.
The dust isn't lighting up at all. It's attenuating the light of billions of galactic stars behind it.
The dust is orbiting due to gravity, but most of the disruption is probably electromagnetic in nature- disturbed by photons, winds of charged particles, and hydrodynamic interactions within itself.
Your proffering can only imply the galaxies have become incredibly dust-enriched, which I reject. And a uniformity of photon disturbance that makes it as an explanation truly unlikely?
I don't understand what you're suggesting. Many galaxies have large amounts of dust. What we're seeing here isn't at all unusual. The only difference between the dust we see here and the dust we see in many individual galaxies is that this is stirred around more by the multiple interactions between the compact components of the cluster.
No. It is an expression of something poorly understood at this time. The uniformity. Hydrodynamic interactions that leave no artifact of turbulence?
There is really nothing in the structure of the dust that isn't well understood. It's the most trivial part of what's going on in these compact clusters. The real questions involve the distribution of neutral hydrogen and the way the the interstellar medium transfers to the intragalactic medium.
Re: APOD: Galaxy Group Hickson 90 (2017 May 17)
Posted: Wed May 17, 2017 10:13 pm
by Ann
douglas wrote:I'd like to see a plausible explanation for why the dust lights up to the furthest extents of its presence in so many of these galaxies. A physical explanation. As if it were fluorescing, almost. Doesn't look like it's disturbed by gravitational acceleration, almost like it's been suspended from gravitational effects.
It is possible that you are referring to Geck's (Geckzilla's) processed image of NGC 2146, which I posted earlier.
The reason why some of the dust in Geck's image is seen to light up is almost certainly that Geck had access to infrared data about NGC 2146, and there is ongoing star formation in NGC 2146.
When star formation is taking place inside dust clouds, so that it can't be seen in visual light, it is often visible in infrared. What happens is that the dust gets warmer as stars are forming inside it, and infrared filters react to heat.
However, dust is also more transparent to infrared light (i.e., heat) than to visual light. In NGC 2146, we are probably seeing the heat of billions of stars in the nucleus and the bulge of the galaxy shine through the dust lane in front of them.
So in infrared photography, dust may seem to flouresce because it emits heat, and because heat from various hot sources in it or behind it may well penetrate it.
douglas wrote:
Your proffering can only imply the galaxies have become incredibly dust-enriched, which I reject. And a uniformity of photon disturbance that makes it as an explanation truly unlikely?
The way I read Chris' answer to you, he wasn't suggesting that any of the galaxies in today's APOD are incredibly dust-enriched.
Dust is created in stars. Vast amounts of dust are formed in starbursts, and dust is also formed as stars die. The dust is then mixed with the hydrogen gas of galaxies. When the dust is sufficiently concentrated to be very visible, it is usually mixed with a lot of gas. Concentrated dust appears to help to cool gas to very low temperatures, which is what is needed in the nearby universe to form cool cores of gas and get star formation going. But after bursts of star formation, or after a lot of interaction with other galaxies, the dust may be quite hot and turbulent, and that way it will be unable to help hydrogen gas to cool down and form concentrated cores.
The dust that is visible in today's APOD is not very dark. It doesn't look very concentrated. We see some, but not many, point sources scattered in it. There are signs of outflows, but it is not clear that the outflows are related to star formation rather than to tidal forces or magnetic forces.
Ann
Re: APOD: Galaxy Group Hickson 90 (2017 May 17)
Posted: Thu May 18, 2017 1:04 am
by douglas
Ann wrote:
douglas wrote:I'd like to see a plausible explanation for why the dust lights up to the furthest extents of its presence in so many of these galaxies. A physical explanation. As if it were fluorescing, almost. Doesn't look like it's disturbed by gravitational acceleration, almost like it's been suspended from gravitational effects.
It is possible that you are referring to Geck's (Geckzilla's) processed image of NGC 2146, which I posted earlier.
The reason why some of the dust in Geck's image is seen to light up is almost certainly that Geck had access to infrared data about NGC 2146, and there is ongoing star formation in NGC 2146.
When star formation is taking place inside dust clouds, so that it can't be seen in visual light, it is often visible in infrared. What happens is that the dust gets warmer as stars are forming inside it, and infrared filters react to heat.
However, dust is also more transparent to infrared light (i.e., heat) than to visual light. In NGC 2146, we are probably seeing the heat of billions of stars in the nucleus and the bulge of the galaxy shine through the dust lane in front of them.
So in infrared photography, dust may seem to flouresce because it emits heat, and because heat from various hot sources in it or behind it may well penetrate it.
douglas wrote:
Your proffering can only imply the galaxies have become incredibly dust-enriched, which I reject. And a uniformity of photon disturbance that makes it as an explanation truly unlikely?
The way I read Chris' answer to you, he wasn't suggesting that any of the galaxies in today's APOD are incredibly dust-enriched.
Dust is created in stars. Vast amounts of dust are formed in starbursts, and dust is also formed as stars die. The dust is then mixed with the hydrogen gas of galaxies. When the dust is sufficiently concentrated to be very visible, it is usually mixed with a lot of gas. Concentrated dust appears to help to cool gas to very low temperatures, which is what is needed in the nearby universe to form cool cores of gas and get star formation going. But after bursts of star formation, or after a lot of interaction with other galaxies, the dust may be quite hot and turbulent, and that way it will be unable to help hydrogen gas to cool down and form concentrated cores.
The dust that is visible in today's APOD is not very dark. It doesn't look very concentrated. We see some, but not many, point sources scattered in it. There are signs of outflows, but it is not clear that the outflows are related to star formation rather than to tidal forces or magnetic forces.
Ann
I was referring to a more general observation that ellipticals appear so devoid of features. That's a signatory aspect of them.
Re: APOD: Galaxy Group Hickson 90 (2017 May 17)
Posted: Thu May 18, 2017 4:46 am
by Chris Peterson
douglas wrote: I was referring to a more general observation that ellipticals appear so devoid of features. That's a signatory aspect of them.
But the dust here is associated with an Sa spiral galaxy, not the two ellipticals or one irregular that make up the rest of HCG 90.
Re: APOD: Galaxy Group Hickson 90 (2017 May 17)
Posted: Thu May 18, 2017 8:32 am
by starsurfer
The primary member of Hickson 90, NGC 7172 isn't included in the Hubble image.
douglas wrote:
I was referring to a more general observation that ellipticals appear so devoid of features. That's a signatory aspect of them.
Indeed, ellipticals seem devoid of features, and they seem devoid of dust. But for one thing that doesn't necessarily mean that they have no dust at all, and for another, it doesn't mean that all spiral galaxies are the same, or that they have the same amount of dust.
Take a look at edge-on galaxies NGC 891 and NGC 5866. As you can see, NGC 891 has a pretty massive dust lane that appears to stretch along the full length of the visible disk. NGC 5866, by contrast, has a thin, small dust disk, much smaller in extent that the visible stellar disk. I believe that the dust disk of NGC 5866 has shrunk. Not only that, but it seems certain that galaxies in general used to contain very much more gas in the past than they do now.
All the hydrogen that the universe is ever going to get was created in the Big Bang. In the very, very early universe there were no stars at all. But then stars started to form, and very many of them were low-mass stars. Today, according to Ken Croswell and his book Planet Quest, 80% of all stars in the Milky Way are small, faint red dwarfs, and only 4% are G-type stars like the Sun. But although the red dwarfs are small and faint, they are more massive than you'd think. The typical mass of a red dwarf may be 30-50% of the mass of the Sun, but because the red dwarfs are so numerous, their combined mass is much greater than the combined mass of stars like the Sun.
The thing about red dwarfs is that they evolve so terrifically slowly that they, in effect, remove hydrogen gas from the universe and lock it up inside themselves as they form, and they don't give back appreciable amounts of this hydrogen for perhaps trillions of years. Stars like the Sun, by contrast, will give back probably at least half of their mass during their drawn-out death processes, and they are not likely to live for more than 10-12 billion years at most. This means that since most stars that in the universe are low-mass stars, star formation effectively removes large quantities of "free gas" from the universe. And as the amount of gas is continually being depleted in the universe, how can we expect the dust lanes of all galaxies to remain as thick and massive as they once were?
NGC 5866 is fast on its way to using up its gas and dust. At the same time its stellar disk is puffing up, becoming more similar to an elliptical galaxy. As spiral galaxies use up their "free gas" they seem bound to slowly evolve into more puffed-up disk galaxies with shrinking dust lanes and little star formation.
Ann
Re: APOD: Galaxy Group Hickson 90 (2017 May 17)
Posted: Thu May 18, 2017 8:43 pm
by douglas
Ellipticals appear to be rather a mystery if opaque at all wavelengths.
"Their appearance shows little structure and they typically have relatively little interstellar matter. Consequently, these galaxies also have a low portion of open clusters and a reduced rate of new star formation. Instead they are dominated by generally older, more evolved stars that are orbiting the common center of gravity in random directions. The stars contain low abundances of heavy elements because star formation ceases after the initial burst." https://en.wikipedia.org/wiki/Galaxy#Ellipticals
Yet they are illuminated to their furthest extents. I really "need" to understand how central black holes can cause such uniform extent of illumination. Totally uniform, at visible wavelengths? http://www.dailygalaxy.com/my_weblog/20 ... -dead.html
Re: APOD: Galaxy Group Hickson 90 (2017 May 17)
Posted: Thu May 18, 2017 9:03 pm
by Chris Peterson
douglas wrote:Yet they are illuminated to their furthest extents.
Well, not really. Their actual extents are typically defined by a much larger dark matter halo. However, in discussing the ordinary matter content of galaxies, they are all "illuminated to their furthest extents". They are, after all, made out of stars. Luminous matter. What would it mean for them not to be illuminated that way?
I really "need" to understand how central black holes can cause such uniform extent of illumination.
This has nothing to do with central black holes. As a rule, central black holes play a very small role in galaxy morphology. They simply don't have enough mass to have much effect.
Re: APOD: Galaxy Group Hickson 90 (2017 May 17)
Posted: Thu May 18, 2017 11:39 pm
by Ann
douglas wrote:
I really "need" to understand how central black holes can cause such uniform extent of illumination.
Elliptical galaxies can form in different ways, and it is true that black holes can be involved. Typically, however, when black holes inhibit star formation, they have jets that inject enormous energy into their interstellar medium (make that, their gas and dust), and this gas and dust becomes far too hot to be able to form any new stars. In order to form new stars, pockets of gas must cool, shrink in size and become ever more concentrated. Hot, turbulent gas generally can't form any new stars.
These nuclear jets form as matter is fed into a supermassive black hole in the center of the galaxy. Since the physical size of the black hole is small, even for supermassive black holes, it can't swallow huge amounts of matter in one go. Instead, inspiralling matter forms an accretion disk around the black hole, which generates enormous amounts of heat and light. Also magnetic lines get trapped and "wound up" in the accretion disk, eventually leading to a jet shooting out from the black hole at the same time as the black hole is ingesting matter. See this video for a simulation of what is happening.
A huge jet can affect not only the galaxy where the jet was formed, but it can actually affect nearby galaxies too, stirring up their gas and dust too and making those galaxies, too, unsuitable for star formation.
When no new stars are formed, the existing stars age. Hot, massive, blue stars die first, then intermediate stars like Sirius and Vega die, then the "high-end mass of low-mass stars" like the Sun die. It takes about 10-12 billion years. But the tremendous amounts of small red dwarf stars don't die for trillions of years. Not a single small red dwarf that has ever formed in the universe has died of old age.
As the bright blue stars die, and the intermediate stars die, and the Sun-like stars die, only red dwarf stars (and some evolved red giants) remain, orbiting the center of the galaxy like a swarm of bees. (Well, it seems certain that practically all elliptical galaxies still have some Sun-like stars in them, because Sun-like stars live for 10-12 billion years, and the universe is only about 14 billion years old. Not all Sun-like stars have had time to die in elliptical galaxies.)
As interactions with other galaxies have stirred up and destroyed the dust lane of the galaxy, no visible structure remains, only the bee-swarm of mostly old red stars buzzing around the center of the galaxy, with its supermassive black hole.
Watch this youtube video to see a simulation of what will happen when the Milky Way and the Andromeda galaxy collide and give rise to an elliptical galaxy.
Click to play embedded YouTube video.
Ann
Re: APOD: Galaxy Group Hickson 90 (2017 May 17)
Posted: Fri May 19, 2017 11:05 am
by douglas
Ann wrote:
douglas wrote:
I really "need" to understand how central black holes can cause such uniform extent of illumination.
(Well, it seems certain that practically all elliptical galaxies still have some Sun-like stars in them, because Sun-like stars live for 10-12 billion years, and the universe is only about 14 billion years old. Not all Sun-like stars have had time to die in elliptical galaxies.)
As interactions with other galaxies have stirred up and destroyed the dust lane of the galaxy, no visible structure remains, only the bee-swarm of mostly old red stars buzzing around the center of the galaxy, with its supermassive black hole.
Click to play embedded YouTube video.
Ann
I guess I was polling for info on the process of elliptical formation. I remember reading years ago of theories that ellipticals' lack of features represents some process that effectively 'pulses' constituent gas & dust to give their appearance in the visible. It is plausible that mergers account for it, but its uniformity still troubles the imagination.
Over time in this golden age of astronomy you'd think elliptical formation would be found along its formative continuum at every stage.
That was very good, sillyworm. I finally got around to reading it!
"Interaction-induced star formation may also account for the differing globular cluster specific frequencies of spirals and ellipticals."
Not so sure about that, but may be related to this:
"From this point of view, it may be wiser not to ask ``did mergers form ellipticals?" but rather `` what merged to form ellipticals?'' Observational studies have shown that cluster ellipticals must have formed very early, perhaps even before massive disk galaxies had formed. If so, the merging objects were probably very different from the types of galaxies we see involved in nearby mergers."
Surface brightness is the term to be referred to in their "uniformity" of appearance, as I was having it. Just trying to elucidate ellipticals' "learning curves" for such brightness ..
[/sarc]
Re: APOD: Galaxy Group Hickson 90 (2017 May 17)
Posted: Fri May 19, 2017 11:24 am
by neufer
douglas wrote:
I was polling for info on the process of elliptical formation. I remember reading years ago of theories that ellipticals' lack of features represents some process that effectively 'pulses' constituent gas & dust to give their appearance in the visible. It is plausible that mergers account for it, but its uniformity still troubles the imagination. Over time in this golden age of astronomy you'd think elliptical formation would be found along its formative continuum at every stage.
Elliptical formation probably has been found at every stage along its formative continuum
...but like with humanoid fossils it is hard to know exactly how to catalogue them all.
https://en.wikipedia.org/wiki/Elliptical_galaxy#Evolution wrote:
<<It is widely accepted that the evolution of elliptical galaxies is primarily composed of the merging of smaller galaxies. Many galaxies in the universe are gravitationally bound to other galaxies, which means that they will never escape the pull of the other galaxy. If the galaxies are of similar size, the resultant galaxy will appear similar to neither of the two galaxies merging, but will instead be an elliptical galaxy.
Such major galactic mergers are thought to have been common at early times, but may occur less frequently today. Minor galactic mergers involve two galaxies of very different masses, and are not limited to giant ellipticals. For example, our own Milky Way galaxy is merging with a couple of small galaxies right now. The Milky Way galaxy is also, depending upon an unknown tangential component, on a collision course in 4–5 billion years with the Andromeda Galaxy. It has been theorized that an elliptical galaxy will result from a merger of the two spirals.
It is believed that black holes may play an important role in limiting the growth of elliptical galaxies in the early universe by inhibiting star formation.>>
Galaxy Group Hickson 90
