Ĝ Paper III Part V: Why are “Dark” and Low Surface Brightness Galaxies Dim?

In the last few posts, I discussed some of the natural sources we found with Ĝ. In this post, I describe some examples of how one could apply the waste-heat approach to “suspicious” galaxies.

In the appendix of our paper, we took a look at two classes of optically anomalous galaxies.  That is, galaxies whose light in visible wavelengths is strange.

The first class are the so-called HI-dark galaxies.  Most galaxies have lots of stars, and so are very bright and obvious.  If you were to enshroud the stars in a galaxy with Dyson spheres, you would lower its surface brightness and it would get darker.  If you completely enshrouded every star in Dyson spheres, it would go totally dark.

In order to create a noticeable decrement in the surface brightness of a galaxy, you would have to cover a lot of the light — around 75% to make it really anomalously faint.  This is the threshold that James Annis used in his trailblazing paper on the topic.  He studied a group of about 100 galaxies all at the same distance, and used the Tully-Fischer relation to determine, from the rotation speeds of the galaxies, how bright their stars should be (their stellar mass content, strictly speaking, then he assumed a mass-to-light ratio).  He then looked for any galaxies that were significantly dimmer than this value.  He found none that would be consistent with 75% coverage by Dyson spheres, similar to our limits from Ĝ.

There is also a whole class of galaxy called a low surface-brightness galaxy (LSB).  These are often very hard to detect because they are so dim.  In principle, these could be examples of the galaxies Annis was looking for.

Of course, if the stars were completely covered with Dyson spheres, the galaxy wouldn’t be dim, it would be black.  But then, you wouldn’t know where to look for them in the first place.  Except, that galaxies don’t just contain stars, they also contain gas, like neutral hydrogen, and such gas can be detected with radio telescopes.

3-colour image of VIRGOHI 21 from the Isaac Newton Telescope

VirgoHI21: a dark galaxy.  “A three-colour (i, r, and B bands) image of VIRGOHI 21 from the Isaac Newton Telescope overlaid with contours of neutral hydrogen density (green) from observations with the Westerbork Synthesis Radio Telescope.” Original image here. Credit Robert Minchin for figure and caption.

Radio surveys have discovered many galaxies by searching for their neutral hydrogen emission, including many in the Virgo galaxy cluster.  But a few of these galaxies are very strange: they don’t seem to have any stars at all!  This is weird, but not impossible: we don’t know why these galaxies don’t seem to have formed any stars with their gas, but there are ideas.  These are called HI “dark” galaxies (HI = “aitch one” = hydrogen in its first ionization state).

But are they Type III Kardashev civilizations?

We went and studied 100,000 galaxies for emission, and if any of the LSB’s or dark galaxies were really dim because alien civilizations were blocking 99% of their starlight, they would have stood out as the very reddest objects in our survey.  Indeed, we didn’t search only known galaxies, but we searched every extended object that was bright in the W3 band, exactly so that we wouldn’t be biased against optically faint galaxies like these.

But just to be sure, in the appendix we specifically targeted a bunch of HI dark galaxies from the literature, and made sure to break out the LSB’s (as identified by SIMBAD) in our search.

We found… nothing among the HI dark galaxies.  No mid-infrared emission.  It turns out we weren’t the first to look, but we were probably the first to look in a SETI context.  So the HI dark galaxies are still strange, but they’re not K3’s.

Figure 11 from our paper. The Low surface brightness galaxies (LSBs) are the blue stars in the upper right panel. They seem to be typical of ordinary galaxies (black points, upper left), and perhaps star-forming galaxies (black points, upper right). Galaxy classifications from SIMBAD.

Figure 11 from our paper. The Low surface brightness galaxies (LSBs) are the blue stars in the upper right panel. They seem to be typical of ordinary galaxies (black points, upper left), and perhaps star-forming galaxies (black points, upper right). Galaxy classifications from SIMBAD.

The LSB’s , it turns out, aren’t especially bright in mid-infrared colors, and they are, indeed, dimmer than typical galaxies, on average. No surprises here: they’re just low on stars.

Next week: Red spirals!

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