One guess for cheap signaling would be to seed stellar atmospheres with stuff that should not belong. Stellar spectra are really good to measure, and very low concentration of are visible (create a spectral line). If you own the galaxy, you can do this at sufficiently many stars to create a spectral line that should not belong. If we observed a galaxy with “impossible” spectrum, we would not immediately know that it’s aliens; but we would sure point everything we have at it. And spectral data is routinely collected.
I am not an astronomer, though. So this is not meant as an answer, but rather as a starting point for others to do more literature research. I think I have seen this discussed somewhere, using technetium; but googling revealed that stars with technetium actually exist!
One guess for cheap signaling would be to seed stellar atmospheres with stuff that should not belong. Stellar spectra are really good to measure, and very low concentration of are visible (create a spectral line). If you own the galaxy, you can do this at sufficiently many stars to create a spectral line that should not belong. If we observed a galaxy with “impossible” spectrum, we would not immediately know that it’s aliens; but we would sure point everything we have at it. And spectral data is routinely collected.
I am not an astronomer, though. So this is not meant as an answer, but rather as a starting point for others to do more literature research. I think I have seen this discussed somewhere, using technetium; but googling revealed that stars with technetium actually exist!