I’m delighted to see the high level of interest in Dysonian SETI shown not only by reader comments here but in the scientific community at large. I wouldn’t normally return to the topic this quickly but for the need to add a quick addendum to our discussions of Project Hephaistos, the effort (based at Uppsala University, Sweden) to do a deep dive into data from different observatories looking for evidence of Dyson spheres in the form of quirks in the infrared data suggesting strong waste heat.

Swiftly after the latest Hephaistos paper comes a significant re-examination of the seven Dyson sphere candidates that made it through that project’s filters. You’ll recall that all seven were M-dwarfs, which struck me at the time as unusual. Only seven candidates emerged from over five million stars sampled, interesting especially because the possibility of a warm debris disk seemed to be ruled out. But Tongtian Ren (Jodrell Bank Centre for Astrophysics), working with Michael Garrett and Andrew Siemion, who share an affiliation with the same institution, has other ideas.

The researchers brought in new data from the Very Large Array Sky Survey, the NRAO VLA Sky Survey and two other sources that would allow a cross-matching of the seven Hephaistos candidates with radio sources. Hephaistos had been working with Gaia data release 3 along with the findings of the Two Micron All-Sky Survey (2MASS) and results from the Wide-field Infrared Survey Explorer, which now operates as NEOWISE. The search for radio counterparts to its Dyson candidates drew hits in three cases.

This looks strongly like data contamination, and the Jodrell Bank scientists think they’ve found the sources of the infrared signatures for these three:

Candidates A and G are associated with radio sources offset approximately ∼ 5 arcseconds from their respective Gaia stellar positions. We suggest that these radio sources are most likely to be DOGs (dust-obscured galaxies) that contaminate the IR (WISE) Spectral-Energy Distributions (SEDs) of the two DS candidates. The offsets for candidate B are smaller, approximately ∼ 0.35 arcsecond. Since M-dwarfs very rarely present persistent radio emission (≤ 0.5% of the sample observed by Callingham et al. (2021)), we suspect that this radio source is also associated with a background DOG lying very close to the line-of-sight. We note that the radio source associated with G has a steep spectral index with a best fit of α = −0.52 ± 0.02 – this value is typical of synchrotron emission from a radio-loud AGN with extended jets.

Let’s untangle this. A dust-obscured galaxy is generally studied at infrared wavelengths, being too difficult a target for visible light observations. There is likely strong star formation going on here, and perhaps an AGN, or active galactic nucleus, emitting energy across the electromagnetic spectrum. Usefully a DOG with an AGN can also be examined at radio wavelengths, which can tease out information about the gas content of the galaxy. So here we have background objects that can contaminate our infrared observations and can be identified by using surveys at different wavelengths.

All seven of the Hephaistos candidates are implicated in possible contamination if we bring in the objects known as hot dust-obscured galaxies, which have inevitably achieved the acronym Hot DOGs. The authors propose that the Spectral-Energy Distributions (SEDs) of each of the Hephaistos objects are “significantly contaminated” by background galaxies of this category. If this is the case, then the oddity of finding seven Dyson sphere candidates around M-dwarfs is resolved, but it will take deeper observations of all seven to confirm this, an effort the authors believe is warranted.

Image: Here is an artist’s impression of the Hot DOG W2246-0526, based on the results of a 2016 paper by Díaz-Santos et al. (2016). In that work (not connected with today’s paper), the authors used ALMA observations to show that the interstellar medium in the Hot DOG is dominated by turbulence, and may be unstable against the energy being injected by the AGN here, potentially producing an isotropic outflow. The WISE mission was essential to finding this galaxy because the galaxy is covered in dust, obscuring its light from visible-wavelength telescopes. But the radio signature of such objects, detected by other methods, raises questions about the recent Hephaistos findings. Image credit: NASA/JPL.

Bear in mind that only 1 out of every 3,000 galaxies that WISE observed fits into this category, so we are dealing with comparatively unusual objects. But given that the Hephaistos survey ran five million objects through its pipeline, the possibility of contamination in the data in the seven proposed candidates seems worth pursuing. The hunt continues, but more and more it appears that if Dyson spheres are achievable by advanced civilizations (and if such civilizations actually exist), they are seldom built.

The paper is Tongtian Ren et al., “Background Contamination of the Project Hephaistos Dyson Spheres Candidates,” available as a preprint.