In last night’s patio #LockDownAstronomy, I was back in the Virgo Cluster. One nice thing about imaging a galaxy cluster is that you can get multiple galaxies in a single shot: working clockwise from the top right, we have NGC4492, M86, NGC4438 and NGC4435.

These galaxies lie in part of a feature called “Markarian’s Chain,” which comprises a sweeping curve of 8 bright Virgo Cluster galaxies, named after a 1961 paper presented by Benjamin Markarian ( https://ui.adsabs.harvard.edu/abs/1961AJ.....66..555M/abstract).">https://ui.adsabs.harvard.edu/abs/1961A...

Markarian suggested that they formed a physical entity on the basis that the probability of such a feature occurring by chance was very small, although noted that it wouldn’t stay this way for long as it isn’t gravitationally bound together.

Interestingly, he got some serious push-back on the dubious nature of his statistics in the discussion of his paper (it was apparently a conference presentation), pointing out the dangers of such a posteriori analysis:

It is, however, apparent that there is a level of physical interaction between at least some of these galaxies: my image is just deep enough to show how disturbed the outer parts of NGC4438 are, due to the gravitational tug of its neighbours.

But even this is not as simple as it seems: while the obvious candidate for tugging at NGC4438 is its near neighbour in this image, NGC4435, there is a stream of gas connecting it all the way back to M86, making it pretty clear where the damage was done ( https://iopscience.iop.org/article/10.1086/593300).">https://iopscience.iop.org/article/1...