Most large galaxies in the universe have low rates of star formation at this time. Our Milky Way, for example, is estimated to form less than 10 new stars per year, while the star formation rate is other galaxies such as nearby M31 is even lower. With an estimated hundreds of billions of stars in these galaxies, past star formation rates were obviously much higher.
Some galaxies, though, are currently forming many more stars. Instead of the typical yellow disk color, these galaxies often have arms with bright blue star formation regions. A few of these are shown in the top row of the poster – M61, M66, M83, and M99. Higher star formation rates in a galaxy are believed to be due to the presence of dust, gas, and a triggering mechanism such as a close encounter with another galaxy.
In contrast to the Messier galaxies above, I have noticed that a few galaxies that I have imaged in the past few years have isolated areas of regional star formation. In these cases, a portion of a single arm appears bright and blue, indicating a local area of high star formation. I have included 15 of these galaxies in the last 3 rows of the poster. These galaxies are all oriented so that the bright arm portion is in the upper right quadrant.
Higher star formation rates in a galaxy are easy for me to understand. But how do the 15 galaxies in the poster have star formation is such a local region? I don’t know. In each of the 15 galaxies, the star formation occurs along a long and thin length of the mid-region of the arm, without extending all the way to the core or to the end of the arm. In a few cases, the opposite arm also shows a slight amount of star formation as well.
The most logical cause would be that a dwarf companion has interacted with the arm portion and initiated star formation. But I can’t find good examples of such a local encounter in progress.
Each of the galaxies in the poster is posted individually on Astrobin at the following links:
Typical Star-Forming Galaxies:M61M66M83M99Single Bright Star-Forming Arms:UGC 5644 (Hickson 47)UGC 1775 (Arp 10)NGC 7678 (Arp 28)NGC 2543NGC 6926NGC 4615 (Arp 34)NGC 1421NGC 2713NGC 2715NGC 1532NGC 5371NGC 7479NGC 986NGC 2196NGC 5693The 15 above galaxies are shown in my Astrobin
Star-Forming Single Arm Collection.
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