This is the title for the Astronomy Picture of the Day on 2004 November 21.
Excerpt from its description:
Billions of years from now, only one of these two galaxies will remain. Until then, spiral galaxies NGC 2207 and IC 2163 will slowly pull each other apart, creating tides of matter, sheets of shocked gas, lanes of dark dust, bursts of star formation, and streams of cast-away stars. Astronomers predict that NGC 2207, the larger galaxy on the left, will eventually incorporate IC 2163, the smaller galaxy on the right. In the most recent encounter that peaked 40 million years ago, the smaller galaxy is swinging around counter-clockwise, and is now slightly behind the larger galaxy. The space between stars is so vast that when galaxies collide, the stars in them usually do not collide.
First of all, neither of the two galaxies has a measured 3-dimensional velocity.
Without that detail, it is impossible to determine convergence or divergence. Assigning a time for an earlier "encounter peak" or a time for any fuure result cannot be justified.
"sheets of shocked gas" must be a reference to plasma.
APOD provies only " a brief explanation written by a professional astronomer."
For comparison, here is the Chandra story on this "collision" of 2 spiral galaxies.
Excerpt from Chandra:
NGC 2207 and IC 2163 are two spiral galaxies in the process of merging.
This pair contains a large collection of super bright X-ray objects called "ultraluminous X-ray sources" (ULXs).
Astronomers have found evidence for three supernova explosions within this pair in the past 15 years.
A new composite image of the system contains X-rays from Chandra (pink) along with optical and infrared data.
ULXs have far brighter X-rays than most "normal" X-ray binaries. The true nature of ULXs is still debated, but they are likely a peculiar type of X-ray binary. The black holes in some ULXs may be heavier than stellar mass black holes and could represent a hypothesized, but as yet unconfirmed, intermediate-mass category of black holes.
This composite image of NGC 2207 and IC 2163 contains Chandra data in pink, optical light data from the Hubble Space Telescope in red, green, and blue (appearing as blue, white, orange, and brown), and infrared data from the Spitzer Space Telescope in red.
The new Chandra image contains about five times more observing time than previous efforts to study ULXs in this galaxy pair. Scientists now tally a total of 28 ULXs between NGC 2207 and IC 2163. Twelve of these vary over a span of several years, including seven that were not detected before because they were in a "quiet" phase during earlier observations.
The scientists involved in studying this system note that there is a strong correlation between the number of X-ray sources in different regions of the galaxies and the rate at which stars are forming in these regions. The composite image shows this correlation through X-ray sources concentrated in the spiral arms of the galaxies, where large amounts of stars are known to be forming. This correlation also suggests that the companion star in the binary systems is young and massive.
Chandra's page includes tabs to select optical, like APOD, and also X-ray and Infrared.
The X-ray image reveals those "ULXs [whose true nature] is still debated."
Those 28 ULXs in the image are seen in optical, and X-ray.
Infrared can be obscured, unlike X-ray, so its image is not always consistent.
That broad frequency spectrum from each object matches synchrotron radiation.
The 28 objects are all plasmoids.
In the big bang cosmology, every X-ray point source is a black hole. M87 demonstrated they are often plasmoids.
On May 21, I posted "Plasmoid Ejection" which observed the Birkelund current pair with a pinch or bend appears the combination required for ejecting plasmoids. That combination is not restricted to only a galaxy core.
Both galaxies in this claimed "collision" are spiral galaxies. I did not count the 28 but whatever the number they seem mostly in the spiral arms.
The pinch at the core of IC 2163 at the right is more intense in X-ray than the core of NGC 2207 at the left.
Perhaps, these are coincidences but around the IC 2163 core:
At 4 o'clock and 10 o'clock, there is a dimmer pair of X-ray points nearby, and a brighter pair of point pairs further away.
Arp famously observed ejected pairs of quasars which are just plasmoids and surrounded by clouds of metal atoms. Seyfert galaxies (Arp's parent of quasars) provide the metals. Neither of these galaxies are Seyferts so no quasars.
Chandra images are always entertaining. X-rays are possible only with synchrotron radiation. Thermal radiation never exceeds ultraviolet.
The infrared in the image from Spitzer is at the lower end of synchrotron radiation.
Apparently Chandra has been observing this collision awhile to note 12 ULXs have varied "over a span of several years."
In a laboratory, plasmoids don't last long. On the galactic scale they last much longer.
The collision story with many ULXs is much more exciting than 2 electrically active spiral galaxies.