Galactic Rims: News and a Reminiscence

by Paul Gilster on July 22, 2008

The image below is striking enough that I would have run it even without the interesting story it tells about the presence of organic materials in Messier 101. Viewed at infrared wavelengths and color-coded, the Pinwheel galaxy’s spiral arms are visible, as is an outer zone, marked by a coral color, in which the organic molecules called polycyclic aromatic hydrocarbons disappear. These hydrocarbons are typically found in areas of star formation, with interesting implications for the appearance of life. So what does an organic-free zone tell us about the Pinwheel galaxy?

“If you were going look for life in Messier 101, you would not want to look at its edges,” said Karl Gordon of the Space Telescope Science Institute in Baltimore, Md. “The organics can’t survive in these regions, most likely because of high amounts of harsh radiation.”

Image: The Pinwheel galaxy, otherwise known as Messier 101, sports bright reddish edges in this new infrared image from NASA’s Spitzer Space Telescope. Research from Spitzer has revealed that this outer red zone lacks organic molecules present in the rest of the galaxy. The red and blue spots outside of the spiral galaxy are either foreground stars or more distant galaxies. Credit: NASA/JPL-Caltech/STScI.

The Pinwheel is also interesting because of its high metal gradient (in astronomical terms, metals are elements heavier than helium), the galaxy showing a wide range between the concentration of metals at its center and those in the outer disk, the result of metal-producing stars being found primarily in the central regions. The gradient of polycyclic aromatic hydrocarbons here acts much the same, decreasing in concentration with distance from the center, but in the case of the organics, these molecules simply disappear in the area of the galactic rim, the victim of radiation.

All of which makes Messier 101 useful indeed. The lack of polycyclic aromatic hydrocarbons sets up a laboratory for the study of star formation in such environments. Their normal contribution is to help cool star-forming clouds, but stars in the rim regions of the Pinwheel must form, as did stars in the early universe, without the help of organic dust. The paper is Gordon et al., “The Behavior of the Aromatic Features in M101 H ii Regions: Evidence for Dust Processing,” Astrophysical Journal 682 (July 20, 2008), pp. 336–354.

Centauri Dreams note: A news item about a galactic rim invariably brings to mind the remarkable A. Bertram Chandler, sailor, merchant marine captain and author of forty novels, as well as over 200 short stories. Born in England, he wound up an Australian, and despite his demanding profession, the chronicler of the distant worlds of the Rim. Chandler’s nautical background fed his settings and plot in both the Rim World series and his novels about space sailor John Grimes, of the Federation Survey Service. I always imagined Chandler in his quarters typing away at a battered keyboard, his log for the day complete, sketching the farthest reaches of space with the calm, practiced skill of a man used to command. I wonder what this ship’s master would have made of the ‘no-organics’ zone around the Pinwheel galaxy…

Adam July 22, 2008 at 16:17

Hi Paul

Another Bertram Chandler fan! Joy! I adored the Rim series and even have a few quite fun short stories from “Astounding” of his. He also wrote a very Australian alt-history story and novel, “Kelly Country”, about what Australia might have become if Ned Kelly had over-turned the charges against him. Moebius strip antenna, cat’s brain computers and all the Rim weirdness… had a certain appeal, like being on the edge of an infinite sea.

Thomas July 22, 2008 at 19:12

Please forgive the naive question, but where does the radiation come from? Why is it only in the outer regions of the galaxy? Why isn’t the inner regions of the galaxy affected (as much?) by the radiation?

Administrator July 22, 2008 at 19:54

Thomas, re your question:

Please forgive the naive question, but where does the radiation come from? Why is it only in the outer regions of the galaxy? Why isn’t the inner regions of the galaxy affected (as much?) by the radiation?

Because I’m unclear on this myself, I’m going to quote from a Spitzer news release:

“Spitzer found that the polycyclic aromatic hydrocarbons decrease in concentration toward the outer portion of the Pinwheel galaxy, then quickly drop off and are no longer detected at its very outer rim. According to astronomers, there’s a threshold at the rim where the organic material is being destroyed by harsh radiation from stars. Radiation is more damaging at the far reaches of a galaxy because the stars there have less heavy metals, and metals dampen the radiation.”

I find this answer unsatisfying, and hope some of the readers may have other insights.

Administrator July 23, 2008 at 10:11

Adam, we must return to the Chandler tradition by seeing if we can get Centauri Dreams reader Paul Titze to start writing novels. Paul works out of Sydney harbor, has extensive maritime experience and a deep background in physics, and practices the art of astronomy. Paul, if you see this message, please tell us you have a novel in the works to get us back to the days of nautically-based science fiction! Paul’s excellent Web site (check out the photography) is here:

http://wizlab.com/marine/

Minoan July 23, 2008 at 11:51

Does some colour in that picture explicitly indicate the polycyclic aromatic hydrocarbons? I get the part about the outer edges being unlikely for the evolution of life; however i am confused about the specific identification of the polycyclic aromatic hydrocarbon gradients througout Messier 101.

Administrator July 23, 2008 at 15:15

Minoan, I have no information as to color-coding for PAHs in the image other than the lack of them being indicated by the coral color in the rim. I’ll drop a note to the researchers and see what we can learn.

Administrator July 23, 2008 at 18:04

And here’s the answer to Minoan’s question, from Karl Gordon (Space Telescope Science Institute):

“The Green color (IRAC 8um) is the dominant color of the PAHs. The
gradient in the aromatic features (PAH features) is seen in the image as
the center changes from green (strong aromatic features) to red
(weak/non-existent aromatic features, but still strong hot dust grain
emission) near the rim. The clear indication that this color change is
due to the aromatic features dramatically decreasing in strength at a
particular radius/harsh radiation field level is seen in complimentary
IRS spectra taken of select HII regions at a variety of radii in the
galaxy. Lots of detail in the paper, but this is the quick summary.”

Paul Titze July 25, 2008 at 3:17

Re: Scifi novels

Unfortunetly Paul, no nautical based sci-fi novels in the pipeworks at this stage! :-)) Too busy studying physics in my spare time… maybe later ;-)

Cheers, Paul.

Minoan July 25, 2008 at 6:24

Hi Admin,

Thanks very much for the follow up. Very interesting results!

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