Last time, I discussed the appendix of our paper wherein we examined low- (and no-) surface brightness galaxies, and found that they’re not dim because the stars are all covered in Dyson spheres.

Top: Ellipticals are red, spirals are blue.
Bottom: Except when they’re not. The Galaxy Zoo project identified anomalous galaxies, the blue ellipticals (left) and red spirals (right). Credit: Galaxy Zoo blog.

We also looked at galaxies with anomalous populations of stars. The Galaxy Zoo project identified a new class of galaxy — the “red spiral”.  They have optically red colors, like elliptical galaxies, but clearly spiral morphology.  This is strange: spirals usually have lots of blue stars that dominate their light; why do some lack blue stars?

Blue stars are young stars (they don’t live long), so they trace star formation.  Since red spirals aren’t blue, they are sometimes called “passive” spirals, because they are apparently not “active”ly forming stars.

What does this have to do with SETI?  Well, it has been suggested (and if anyone knows a citation, please send it to me!) that galaxy-spanning civilizations would suppress high-mass star formation, because those stars tend to explode as supernovae, and supernovae are bad for the environment.  One might expect, therefore, that a signature of an arbitrarily advanced civilization would be an unnatural “initial mass function” (basically, the number of stars formed at various masses) that lacked any high mass stars.

So, red spirals seem to fit the bill!  Except that there is another explanation for them:  the color of these galaxies also traces star formation history.  If you produced lots and lots of stars in the past, many of them today will be red giants (like in ellipticals) and if you have enough of those, their light could overwhelm the young blue stars.  So red spirals could just be regular spirals that have an unusually large number of red giants, not an unusually small number of young stars.

And indeed, that’s what Luca Cortese found in his study of the near UV emission of red spirals (which traces star formation). The color formed by comparing the near UV to the r-band emission is consistent with other spirals, even if the galaxies look red in the optical.  Red spirals don’t seem to be deficient in star formation (so they’re not “passive”, they’re just red).

Left: near-UV emission (normalized by r-band stellar emission) on the y-axis (down is more UV emission), mid-infared color on the x-axis (tracing dust and, potentially, alien waste heat; right is more dust/heat). The horizontal line represents a conservative threshold for “passive” galaxies, which are usually ellipticals or very heavily extinguished (by dust) spirals.  Right: same x-axis, but the y-axis traces the W1-W2 color, which should be larger in cases of very warm or very luminous dust/heat. The green dots are the UV-dim galaxies above the line in the left figure.

BUT… not all of them.  We identified eight red spirals from the Galaxy Zoo sample that have very little near UV emission — so they could be red because they lack blue stars.  What’s more, five of these have very high amounts of MIR emission for spiral galaxies, especially in the W2 and W3 bands compared to W1.  Now, some spirals are edge-on, so their UV emission can blocked by dust in the disk, which could explain the low UV emission.  But we checked and all five of these galaxies are at modest inclination, as far as we can tell.

So… maybe aliens?  Here are “red spirals” that seem to lack star formation, and have high, but not off-the-charts, MIR emission.  That’s weird.  In a sample as big as ours (we checked 5,500 red spirals) there are bound to be a few natural outliers, so we didn’t highlight these in our press release, but we do think they’re worth following up, both as SETI targets and as interesting natural objects.

Next time: Next steps!