I’m enough of a perfectionist that when I get something wrong, I can’t rest easy until I figure out how and why I missed the story. Such a case occurred in an article I wrote for Aeon Magazine called Distant Ruins. The article covered the rise of so-called ‘Dysonian SETI,’ which is adding an entirely new dimension to current radio and optical methods by looking into observational evidence for advanced civilizations in our abundant astronomical data.
In the story, I homed in at one point on the work that Jason Wright and his colleagues Matthew Povich and Steinn Sigurðsson are doing with the Glimpsing Heat from Alien Technologies (G-HAT) project at Penn State. Keith Cooper went over the basics of this effort on Friday, putting his own spin on the group’s recent search of 100,000 galaxies. For more background, see Jason Wright’s Glimpsing Heat from Alien Technologies essay.
I noted in the Aeon article that the G-HAT team was examining infrared data from the Wide-field Infrared Survey Explorer (WISE) and the Spitzer Space Telescope in search of the signs of an advanced civilization. What I had wrong in my description was the statement that “Wright’s group is also looking for ‘Fermi bubbles’, patches of a galaxy that show higher infrared emissions than the rest, which could be a sign that a civilisation is gradually transforming a galaxy as it works its way across it.” I know I drew the idea of Fermi bubbles from Richard Carrigan’s work, and generalized from there, but generalizing was a mistake, because it turns out that the G-HAT team doesn’t believe Fermi bubbles are something we could detect.
Below is the ‘Whirlpool’ galaxy, M51, a beautiful image and a useful object for study because we are looking at a spiral galaxy in many ways like the Milky Way from an angle that lets us see it face-on. Could we see Fermi bubbles here?
Image: The Whirlpool Galaxy is a classic spiral galaxy. At only 30 million light years distant and fully 60 thousand light years across, M51, also known as NGC 5194, is one of the brightest and most picturesque galaxies in the sky. The above image is a digital combination of a ground-based image from the 0.9-meter telescope at Kitt Peak National Observatory and a space-based image from the Hubble Space Telescope. Credit: N. Scoville (Caltech), T. Rector (U. Alaska, NOAO) et al., Hubble Heritage Team, NASA.
Richard Carrigan has studied this galaxy closely, looking for such Fermi bubbles, which he described in a 2010 paper. Here’s my description in Toward an Interstellar Archaeology, written for these pages in the same year:
Suppose a civilization somewhere in the cosmos is approaching Kardashev type III status. In other words, it is already capable of using all the power resources of its star (4*1026 W for a star like the Sun) and is on the way to exploiting the power of its galaxy (4*1037 W). Imagine it expanding out of its galactic niche, turning stars in its stellar neighborhood into a series of Dyson spheres. If we were to observe such activity in a distant galaxy, we would presumably detect a growing void in visible light from the area of the galaxy where this activity was happening, and an upturn in the infrared. Call it a ‘Fermi bubble.’
Carrigan (Fermi National Accelerator Laboratory) studied M51 and concluded that there were no unexplained ‘bubbles’ at the level of 5 percent of the galactic area. The Whirlpool galaxy seems like an ideal place to mount such a search given its orientation towards us. A Fermi bubble, if such things exist, might manifest itself as a void in the visible light we see in the image.
Carrigan talked about an expanding front of colonization as an advanced civilization moved through its galaxy, engulfing the galaxy on a time scale comparable to the galaxy’s rotation period or even less. But M51 produced no ‘bubbles,’ and James Annis would suggest that elliptical, rather than spiral, galaxies might be a better place to look for Fermi bubbles because ellipticals exhibit little structure, so that a potential void would stand out.
Here’s Carrigan in the 2010 paper (citation below) on how a civilization on its way to Kardashev Type III status might proceed:
If it was busily turning stars into Dyson spheres the civilization could create a “Fermi bubble” or void in the visible light from a patch of the galaxy with a corresponding upturn in the emission of infrared light. This bubble would grow following the lines of a suggestion attributed to Fermi that patient space travelers moving at 1/1000 to 1/100 of the speed of light could span a galaxy in one to ten million years. Here “Fermi bubble” is used rather than “Fermi void”, in part because the latter is also a term in solid state physics and also because such a region would only be a visible light void, not a matter void.
Wright and the G-HAT team are not persuaded by Carrigan’s Fermi bubbles. For one thing, as Carrigan has noted himself, bubble-like structures are not unusual in extragalactic astronomy, and spiral galaxies include areas that might mimic a void that would be hard to regard as anything but natural. In one of their recent papers, the G-HAT researchers add that with galactic arm widths on the order of ~ kpc, it is difficult to identify structures below this size scale.
The Annis idea, therefore, seems more useful, but for now let’s home in on that word ‘void.’ In the Aeon story, I referred to the galaxy VIRGOHI21 as a galaxy that contains a ‘void.’ But that’s a mistake, for as Jason Wright explained in a recent email, Virgo HI21 has no emissions at any wavelength except 21cm. It may, in fact, be a starless or ‘dark’ galaxy, a galaxy composed of dark matter, although the nature of the object is still controversial. The G-HAT team, according to Wright, has studied Virgo HI21 and found no infrared emission.
In any case, as Wright explained, the word ‘void’ isn’t appropriate, for galaxies do not actually contain them. Areas where there has been no star formation for the past 10 million years or so may manifest themselves as darker lanes between the spiral arms, and dust lanes may also appear dark, but Wright does not believe the shape of these darker lanes is consistent with the spread of a civilization. In any case, these are not voids. They contain just as many stars as other regions in the galaxy. So detecting Fermi bubbles gets to be more and more problematic.
Fermi bubbles would be hard to detect for other reasons as well, as explained by the G-HAT team and presented in their recent work. This is intriguing stuff, having to do with the time scales involved in the spread of a civilization and the motions of stars in that period — these ‘bubbles’ would not be static! I want to look at this issue next but probably won’t be able to get the piece written and published before Wednesday due to an intersection of competing duties elsewhere.
The Carrigan paper is “Starry Messages: Searching for Signatures of Interstellar Archaeology,” JBIS Vol. 63 (2010), p. 90 (preprint). The G-HAT paper I am discussing today and on Wednesday is Wright et al., “The Ĝ Infrared Search for Extraterrestrial Civilizations with Large Energy Supplies. I. Background and Justification,” The Astrophysical Journal Vol. 792, No. 1 (2014), p. 26 (abstract / preprint).