I get your point, Art, but it seems to me that NGC 7318 A and B are not necessarily "less merged" than the two central ellipticals of SDSS J1531+3414. And while the shock front of Stephan's Quintet may well be something totally different than the blue bridge that is the the topic of this thread, please note that there is indeed a sort of bridge of star formation between the central bulges of NGC 7318 A and B. It is not obvious that this bridge is part of the shock front of Stephan's Quintet. If it isn't part of the shock front, then it might be slightly similar to the blue bridge between the ellipticals of SDSS J1531+3414.neufer wrote:Except for the fact that the Stephan's Quintet case is supposed to represent a shock between merging galaxiesAnn wrote:
When all is said and done, maybe the closest analog to the blue bridge between the central elliptical galaxies of SDSS J1531+3414 might be the well-known star forming shock front caused by the collision between galaxies NGC 7318 A and B in Stephan's Quintet. According to my software Guide, NGC 7318A is an elliptical while NGC 7318B is a spiral galaxy. The blue shock front in the picture you can see in my post is glowing from invisible X-rays, mapped as blue light. However, there is real star formation producing optical and ultraviolet light there too, as can be seen in this picture.
whereas today's APOD is supposed to represent a gravitational condensation in the wake of two already merged galaxies.
But if NGC 7318 A and B are a bad analog to the elliptical galaxies with their blue bridge in today's APOD, then we might compare the ellipticals and their bridge in SDSS J1531+3414 to Perseus A, NGC 1275. There is obviously a chain of star formation asymmetrically placed across the "body" of NGC 1275. Clearly NGC 1275 itself is a product of a galactic merger (if not numerous galactic mergers). Also, like SDSS J1531+3414, the Perseus galaxy cluster (where Perseus A is the central and by far most active member) is a massive cluster dominated by elliptical galaxies.
Admittedly, though, there is one huge difference between Perseus A (and Centaurus A, NGC 5128) and the two ellipticals and their bridge in today's APOD. Perseus A (and Centaurus A) is one elliptical galaxy colliding and merging with a much smaller spiral galaxy, which leads to the complete disruption of the spiral galaxy as well as a lot of tidal messiness. There are no signs of a chaotic collision in today's APOD. Is it even remotely possible that we could be seeing the merger between three galaxies in SDSS J1531+3414, two ellipticals and a small and gas rich spiral or irregular galaxy? And if we are seeing such a merging threesome here, is it even remotely possible that none of the filters that were used to produce today's APOD would have detected the tidal tails and general disorderliness that we must expect from such a complex merger?
Ann