This poster is the 42nd in my series of compilation posters.  All of the posters are contained in my Astrobin Poster Collection.

This poster is a collection of M51-type galaxies.  The M51 pair of objects (the main spiral NGC 5194 and its companion NGC 5195) has captured astronomers’ imaginations for over 100 years.  Because of its size and brightness, M51 was probably the first interacting galaxy pair to be observed.   Since then, hundreds of similar M51-type objects have been identified and discussed in numerous papers and collections, including the Arp catalog.

However, over the years, astronomers have concluded that the M51 pair is not quite as entangled as first assumed.  It is more accurate to say that NGC 5195 is passing behind NGC 5194.  Each galaxy is disturbing each other a bit but there is not believed to be a strong bridge between them.  Nevertheless, for the context of this work, I am using the term “M51-type” to indicate an interacting pair of a spiral galaxy and a companion.

In my review of the hundreds of M51-type galaxy pairs identified and documented in scientific papers as interacting galaxies, I have found that the majority of these pairs do not appear to be visually interacting.  The majority of the time, the pairs are simply the result of coincidental, misleading visual superposition.  We have the luxury today to have access to high quality imaging systems which allow us to study these objects in detail like never before.  For this poster, I have only included pairs which seem to be interacting visually.  

In case you are wondering, the calculated distances for these objects cannot be used to determine if they are interacting, due to the uncertainty of the measurements.   The data can certainty rule out interaction if there are many tens or hundreds of millions of light years difference in the pair’s distances.  But just because the pair distances are similar, it doesn’t mean they are interacting.  The consensus is that serious interaction takes place within a few galaxy diameters, or say 200,000 light years.  Such a distance is very small compared to the vast distances that these galaxies are away from us – say 200 million light years.  In that case, the interaction distance is only 0.1% of the total distance.  We simply don’t know the distances within that level of accuracy. 

For this M51-type poster, my criteria for object inclusion of the hundreds of potential M51-type objects are as follows:

1.    The main galaxy of the pair is a spiral galaxy, somewhat face-on.
2.    The companion galaxy is much smaller in size.
3.    Visual appearance of pair interaction, through tidal streams and deformation
4.    Reasonable object apparent size (>1 arc-minute in diameter)

In particular, I have not included pairs of objects of similar sizes, or where the largest galaxy is an elliptical.  Both would significantly expand the number of objects to be included.    

The poster shows each of these objects to the same apparent scale as M51, resulting in objects that are more distant being smaller.  The distances are shown on the poster in million of light years.  The objects range in distance from 25 million light years for M51 to 800 million light years for Arp 57. 

If you would like to read more about any of these objects, each of the objects in the poster has previously been uploaded and described individually on Astrobin. They all reside in my Astrobin M51-Type Galaxy Collection.