How did Rubin's Galaxy, UGC 2885, grow so large? Professor A. Mateur will explain! Let's begin by considering the famous Whirlpool galaxy, M51.
Inner (original?) arms of M51 (marked as white) versus
the full size of the spiral arms. NASA/ESA.
I read someplace (and just because you read it doesn't make it true) that the original spiral arms of M51 were much shorter than the ones we see today, and that the arms were "drawn out" by the interactions with neighboring galaxy NGC 5195. But the impressive size of the optical spiral arms is nothing compared with the enormous tidal "radio arm" that has been drawn out of M51 as a consequence of its interaction with galaxy NGC 5195.
It seems obvious that galactic interactions can enhance the spiral structure of a galaxy. Another example of this is M100
, whose outer diffuse arm seems to be connected with a small satellite galaxy, NGC 4323.
But interaction can affect spiral and elliptical galaxies quite differently:
Interacting galaxies IC 1178/1181 and NGC 6050 A and B in the Hercules Cluster.
Look at the interacting pairs IC 1178/1181 at left and NGC 6050 A and B at right in the Hercules cluster. IC 1178/1181 is a pair of elliptical (or possibly lenticular) galaxies with no spiral arms or dust structures. Their interaction has spread old yellow stars in two huge clouds around them. But NGC 6050 A and B is a tight, small, well-formed pair of starforming galaxies with spiral arms.
To get a truly, truly large and well-formed spiral galaxy, we need just the right conditions.
The galaxy has to grow large perhaps through collisions.
The collisions must all enhance the spiral structure, not destroy it.
The interlopers should probably be completely absorbed by the large galaxy. We may in fact be talking about very large reservoirs of hydrogen surrounding the galaxy and raining down on it as discrete clouds.
I think, although I'm not sure, that the central black hole in the major galaxy should not grow too massive. Because if it is very massive, I believe it tends to prevent the galaxy's major feature from growing too extensive. So the collisions should feed the disk more than the core of the galaxy.
Once the spiral galaxy has grown extremely large, all major collisions must cease, so as not to disrupt the spiral structure of the galaxy. This is probably best achieved if the galaxy is located in a void.
Galactic voids tend to grow over time. If UGC 2885 originally had several neighbors and managed to absorb very many of them like a vacuum cleaner, and the space around the galaxy actually grew pretty empty because it was located in a growing void, then that might just possibly explain the galaxy's humongous size.
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