where is Arp 220?

while peculiar galaxies might not seem as symmetrically pleasing as their spiral counterparts, the events creating their odd structures are quite exciting! the galaxy, arp 220 (the 220th object in halton arp's atlas of peculiar galaxies), represents a well-studied, relatively nearby (250 million light-years away) galaxy, that doesnt appear typical at all!


the hubble space telescope image above shows the optical view of a mashed-up, non-symmetric mess! arp 220 appears as one object, but as you can infer from the wisps of faint starlight curling around the outskirts of the galaxy, something unusual is occurring!

arp 220 is also the brightest object in the local universe, as it shines extremely brightly at both x-ray and infrared wavelengths. here is an image of arp 220 in the infrared, where it radiates 90 percent of its energy, making it a classic example of an ultra-luminous infrared galaxy (ULIRG = "you-lerg"):


so what explains all these crazy characteristics of arp 220? this peculiar galaxy is the result of two separate spiral galaxies colliding together to become one! the wispy tidal tails seen in the optical image are the result of some stars and gas getting flung out of each galaxy during their initial close encounters. other chunks of gas and dust get concentrated into dense clumps during the entire merger process, seen shining brightly in the infrared image. these dense regions of gas, cool enough to form stars - lots of stars! the newly-formed stars shine very brightly, but enough dust surrounds the star-forming gas clouds that the dust absorbs huge amounts of the stellar light and heats up! the dust naturally wants to cool off and does so by releasing light energy in the infrared... thus, we see arp 220 shining very brightly in infrared observations!

at its current rate of star formation, arp 220 will shine brightly in the infrared for about the next 40 million years! this sounds like a long time to our human scales, but its not that long, astronomically speaking. after 40 million years, nearly all the gas in arp 220 will have condensed into shining stars and the resulting galaxy should eventually look like a regular elliptical galaxy.

although arp 220 lives pretty close to our milky way galaxy, it represents potential to study what may have happened to the earliest galaxies that formed in our universe, since early galaxies experienced interactions and mergers with surrounding galaxies quite often!

my fellow graduate students and i used to joke about arp 220 because it seemed to be a favorite galaxy of a particular professor of ours. every time any of us presented results on any aspect of galaxies evolving, this professor would ask "and how would arp 220 fit in with your scenario?" or "where does arp 220 lie in that plot?" he was so predictable with these questions, that we all learned to expect them and prepared ourselves, and each other, for his inevitable comments.

even though we joked about his insistence with this particular galaxy, he taught me a good lesson about the universe - when trying to understand how things occur very far away and long, long ago, its good to take examples of local, well-studied galaxies to see if they can be explained by your new theories. if arp 220, or the milky way, or andromeda, or the magellanic clouds cannot be explained to some degree by your theory, then some aspect of it is not sufficiently comprehensive in its current form.

ironically, i now find that i ask myself quite regularly - "where would arp 220 be in that plot?" and the thought experiment often proves to be useful!