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| When Galaxies Collide |
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It is now the general doctrine that many (estimated at more than 50%) galaxies have been involved in one or more "collisions" with other galaxies, or even merged within galaxy clusters. Although it is claimed this was more common in the "early Universe" when galaxies were supposedly much closer, there is no evidence to support this view. The word "collision" is put in quotes to call attention to the actual nature of the event. In effect, two galaxies that meet 'interact' in the sense that they join into one with only a few stars actually bumping together in a destructive manner, since the distance between stars is much greater than star sizes themselves. The term "merge" is probably more descriptive of the process involved. It is not necessarily evident from a telescope image that galaxies are colliding. For instance, look at this Hubble view of ARP 274: This pair was won out in an Internet "contest" run by NASA as the first choice of amateur viewers for the target of 100 hours of observation by the HST to advance our knowledge of the Universe. No rationale for this choice was given: it is extremely unlikely that anything by way of noticeable change will occur in the lifetimes of the contestants. But the viewers seemed to want more detail. One thing that a collision leads to is the production of new stars as the gases involved are stirred up by the interaction. A release on October 21, 1997 from one of the HST research teams describes
important information on star formation within an evolving galaxy system that is undergoing collision. The image below shows
a ground-based telescope view of two colliding
galaxies which together make up what is called the Antennae galaxy, so-named
from the long wisps of luminous gas extending like an insect's antennae. The central panel shows the pair together. Because
of its proximity (63 million light years away), it has been a prime candidate for a closer look. The left and right images are much higher resolution HST views (note boxes) of the central galactic mass of merged stars from the two once separated galaxies. More information about the central interior of this developing supergalaxy, and about regions of active star formation appears in this image:
The surprise is the numerous clusters of blue stars. Each appears to be groups of up to a million young (hence bright and hot) individual stars. The clusters likely are still developing, as cold Hydrogen gas in giant molecular clouds (typically 100s of light years across) distributed in pockets through each galaxy are being squeezed during the collision process. They contract and heat up into individual stars as this goes on, often collapsing rapidly enough for many of the stars to explode almost like "firecrackers". Other pre-existent stars are likely to be destroyed as the collision continues. The two orange centers are the older surviving parts of each galaxy. Mayall's Object, ARP 148, is an example of a collision which has drawn out one of the galaxies:  Still another spectacular collision seen by HST is that just beginning between NGC2007 and IGC2163. As the two merge, most individual stars will not collide with another star, since in any such galaxy the distance between stars is actually huge, lessening the changes of direct collisions (although as the pair of galaxies pass through, any given star must always face the possibility of encounter with a star somewhere along their mutual paths):  As galaxies approach, materials (gases; dust) are exchanged between them, new stars are born, and Black Holes grow. This is clearly going on between two (unidentified) galaxies that were imaged by Chandra:  Another example is galaxy 3C321. A jet of gas/dust is being expelled to a smaller nearby galaxy (about 20000 l.y. away), but some of that is also deflected into outer space. The actual Chandra image is in the inset at lower right; an artist's rendering is the main illustration (which portrays the intergalactic jet that is poorly displayed in the Chandra view):  The galaxy pair at ARP 194 shows large blobs of hot material exchanging between the two:  These observations conflict with the view that these collisions
were a more common process in the early Universe as they continue to occur. To claim they were more common in the early universe is pure speculation without foundation. The very fact that they are so common tell us that the big bang model is wrong. Galaxies are not speeding away from one another. They are crashing into each other and this is opposite to what might be expected if there was a big bang.
Sometimes more than two galaxies are involved in the colliding process. One solid indication of collisions is the notable irregularity of the galaxy composite, with irregular center(s) and distorted spiral arms. These three HST examples of multiple collisions, imaged in the infrared, illustrate that:
In this recent image, made by combining images obtained by the NICMOS and ACS sensors, 4 galaxies can be resolved individually. As a pair merge, the increased gravitational attraction of the composite can draw in nearby galaxies to foster further enlargement of a galactic grouping. Another well known grouping of close-spaced galaxies that appear to be headed for some kind of amalgamation is Stephan's Quintet, with three of the five seen in this HST view: One type of galaxy with a distinct Active Galactic Nucleus is the Seyfert class, mentioned on page 20-3. Seyfert galaxies have strongly ionized Hydrogen, Helium, Oxygen, and Nitrogen in their central region. Interaction between infalling gases and a Black Hole is the probable cause. The Seyfert Sextet is a group of six galaxies, 190 million light years away in the constellation Serpens. They consist of 3 ellipticals and 3 spirals (only five visible in this orientation; the small spiral galaxy seen face on is not in this group, being much more distant). None of the galaxies is more than 35000 l.y. across. This configuration has been interpreted as a congregation of galaxies in the process of colliding and being ripped apart by gravitational interaction. The elongate bright central areas in two regions of the cluster may be the cores of merging pairs of galaxies. Unlike some colliding galaxies, there is no visible evidence of bright new stars being formed in these phases of collision. This next pair of spiral galaxies are also starting their collision interactions. Note the stream of gas and dust between them. Stars are forming in this bridge. The usual end product of the merging of two spiral galaxies is an elliptical
galaxy; many elliptical galaxies formed this way. Collisions can also give
rise to spiral structure. Some globular clusters also presumably originated
from interactive collision. One of the more visually intriguing results of a collision is the Cartwheel Galaxy (below), another Ringed Galaxy, in which the passage of one galaxy through another generated shock waves traveling at high velocities. As these waves moved outward, they condensed Hydrogen into a huge collection of new stars that lie along the front of the advancing waves. This image released appears to locate one or two galaxies that may be the ones that passed through the Cartwheel galaxy, producing shock waves responsible for several rings in which the new stars are forming. The prominent outer ring contains the largest number of resulting stars: Here are three more galactic collision images:    Galaxy collisions can release copious amounts of energy. The Chandra X-Ray telescope has detected a huge release of X-rays stemming from the elliptical galaxy NGC 1700, located 160 million light years from Earth:  At 90000 light years in diameter, this is the largest X-ray source near Earth yet discovered in the Universe. The emissions come from a vast spinning cloud of Hydrogen gas excited to temperatures in excess of 8 million degrees. Astronomers studying this cloud surmise that the collision was between a spiral and an elliptical galaxy.
But the biggest collision by far is Abell754, found within the Hydra constellation (but more distant), in which two small galaxy clusters have been colliding over the last half billion years. This yields a tremendous source of X-rays as detected by ESA's XMMM-Newton Observatory. Below is a view produced from data received, but with the image touched up by an artist for emphasis. And beneath that is a plot of energy variations within the colliding clusters that will eventually produce one huge galaxy (supporting the prevailing concept that most galaxies in the Universe have built up by the collision process). Galaxy collisions are taking place throughout
the Universe, as illustrated by recent HST observations of the Centaurus A (NGC5128),
the closest active galaxy to our own Milky Way galaxy. Centaurus A, itself much larger than the Milky Way, is a
known radiation "hot spot", being the source of intense X-rays and Radio waves. This galaxy is only 10 million light years away; what we see now represents its condition at 10,000,000 years prior to today.
The circular inset shows Centaurus A as seen optically through
a ground-based telescope. The detailed view to the right was acquired by
HST's Wide Field Camera. An elongate disc, marked by dark dust, is spread
across a large white glow that is identified as an elliptical galaxy. This
pairing is interpreted to be an intermingling of a spiral galaxy in collision
with this elliptical galaxy. The Infrared Camera on HST can penetrate the
dust to reveal a hot, turbulent mass of stars, dust and gas from the spiral
galaxy falling into the core of the elliptical one, as seen in the larger
view.
The Chandra X-ray telescope captured an unsuspected feature of Centaurus A - namely, a jet
of material ejected to a distant of 25000 l.y. from the core. This single jet of intense X-ray energy is roughly
at right angles to the plane of the disc.
A Black Hole is postulated to occur towards the center
of the two interacting Centaurus systems. This B.H. may be as massive as 10 billion
solar masses, occupying a volume similar to our Solar System. The Black
Hole is "sucking" matter from both galaxies into its growing body. This
set of observations is the most detailed yet of the consequences of galactic
collisions.
One model of future Universe expansion paths indicates that
the nearby Andromeda spiral galaxy could come close to our Milky Way galaxy and might even collide with us, several billion years in the future. There is considerable recent evidence that
a small galaxy is presently passing through the Milky Way. Known as the Sagittarius
dwarf spheroidal galaxy (or Sgr), its presence has been deduced from motions
of certain stars that do not fit the motions of M.W. stars in the spiral arms;
also star "tails" stretch out in the galactic halo, suggesting that
Sgr is in a broad orbit that has caused it to intersect the Milky Way before.
The vast distances between stars keeps interactions to a
minimum. Sagittarius is the closest small galaxy to the M.W. Here is an artist's idea of Sagittarius as it is presently encroaching on the M.W.
The role of galaxy destruction by merging, leading to a new supergalaxy, has been underappreciated until recently. In the early Universe, proximity of galaxies before expansion had separated them must have led to frequent collisions as the norm. Some astronomers think that most galaxies as seen today were products of earlier collisions. |
