Cosmology Views

Galactic Collision in Cluster Abell 1185

This is the title for the Astronomy Picture of the Day on 2005 November 22.


Excerpt from its description:

What is a guitar doing in a cluster of galaxies? Colliding. Clusters of galaxies are sometimes packed so tight that the galaxies that compose them collide. A prominent example occurs on the left of the above image of the rich cluster of galaxies Abell 1185. There at least two galaxies, cataloged as Arp 105 and dubbed The Guitar for their familiar appearance, are pulling each other apart gravitationally. Most of Abell 1185's hundreds of galaxies are elliptical galaxies, although spiral, lenticular, and irregular galaxies are all clearly evident. Many of the spots on the above image are fully galaxies themselves containing billions of stars, but some spots are foreground stars in our own Milky Way Galaxy. Recent observations of Abell 1185 have found unusual globular clusters of stars that appear to belong only to the galaxy cluster and not to any individual galaxy. Abell 1185 spans about one million light years and lies 400 million light years distant.
(Excerpt end)


The "guitar" is a striking vertical alignment at the left.

Here is just the image from APOD but with the "guitar" at the bottom.:

I could find no better image of this "guitar."

Wikipedia identifies only  two NGC numbers for this Abell 1185 cluster. NGC 3558 and 3561.
Both offer very little information.

NGC 3558 : Its galaxy type is uncertain: E2, S0p

It is either an elliptical or lenticular.

There are 2 elliptical galaxies on the opposite side from the guitar.  NGC 3558 must be one while the other in not named.

NGC 3561

This  NGC 3561 is actually a pair of A and B galaxies:
galaxy: type
A: Sa - spiral
 B: E5 - elliptical

This pair is considered too close than allowed  so they "must be" colliding.

Without reliable data for distance and velocity, that conclusion cannot be justified, when based on only the line of sight.

This other  image is worth a view:

The 2 galaxies seem further apart in this image.

excerpt from Wikipedia:

NGC 3561, also known as Arp 105, is a pair of interacting galaxies NGC 3561A and NGC 3561B within the galaxy cluster Abell 1185 in Ursa Major. Its common name is "the Guitar" and contains a small tidal dwarf galaxy known as Ambartsumian's Knot that is believed to be the remnant of the extensive tidal tail pulled out of one of the galaxies.

(excerpt end)


Calling the "knot" a small dwarf galaxy does not seem justified.

There is another way to interpret this "collision":

NGC 3561B is a giant elliptical galaxy like M87, which is type E0.
M87 is known for its energetic jets in opposing directions from the plasmoid at the core.One jet is more than the other.

NGC 3561B has opposing non-symmetrical jets.

Ambartsumian's Knot  could be a very short jet in one direction.
The "guitar neck" could be a long jet in the opposite direction.

Collisions should never be proposed unless there is proper evidence. Proximity alone is not sufficient.

A more likely explanation than a collision is:

a) there are opposing jets from 3561B,
b) both the distance and amount of material are different for the jets.

c) A small spiral galaxy 3561A is in the line of sight, in front of the jet, on that side of 3561B.

d) the knot is not the remnants of a dwarf galaxy,

e) though the distant jet has substantial material,  it seems to have no NGC number and no name (unlike the knot).

The image quality does not justify observing a second jet behind 3561A, almost through a distant star on that side.

With no other data, especially the relative distances of both galaxies, that explanation matches the observation.

I wonder whether anyone has a convincing case for a collision...