I just finished taking a tour through the latest papers posted on the astronomy preprint server. In general, few things do more to attract a my attention than a good title. One title in particular stopped my casual scrolling dead on its pixels: Redshifted formaldehyde from the gravitational lens B0218+357. Now, I knew beforehand that formaldehyde – the chemical people who store dead things used to use to preserve their specimens – existed in clouds of interstellar material. Other than knowing it is out there, I have to admit that I really don’t know why anyone cares that it exists beyond the “Oh neat” factor. So… I stopped to read the paper. (Image: CASTLES Survey)
B0218+357 is a galaxy at a z=0.944. (This means it took the light 6.2 billion years to get here.) It is a special type of active galaxy called a BL Lac object, which has an actively feeding supermassive black hole that we are viewing essentially face on (down the jet). This view makes the object stand out brightly in radio light. Between us and B0218+357 is a face on spiral galaxy that gravitationally lens light from B0218+357, causing us to see B0218+357 as two objects that are slightly separated on the sky. (See this site for explanation of gravitational lenses.) The light from each of these images took a slightly different path through the universe to reach us. Here is where things get interesting: the light that forms the image of the galaxy on the left passes through a rouge molecular cloud while the light from the right image does not. This means that astronomers can use differences in the spectra of the two galaxy images to study a distant molecular cloud.
Here’s how it works: The spectral differences in the light of B0218+357’s two images was used to identify the location on the sky of a molecular cloud. This part was simple. Take a spectra of each image, look for differences that correspond to atoms at a different redshift than the object come from the light passing through a cloud/galaxy/etc. Once the molecular cloud was found (and a lot of complicated work was done to sort out the differences in the galaxies light in each image) it was possible to study the gas cloud.
And here is where the formaldehyde comes into play. This molecule (H2CO) comes in two varieties: molecules with the hydrogen nuclei spins in parallel (ortho-H2CO) and anti-parallel (para-H2CO). The ratio of the number of molecules in each of these states is related to the temperature and densities of the gas when the formaldehyde forms. By studying the ratios of these two molecules, one can figure out the temperature of a blob of gas a few billion light years away. In this case, it looks like the gas is a meager 55K. This is 22 degrees (K or C) colder than liquid Nitrogen.
And, strangely, this brings us to the Cosmic Microwave Background. It is the light from the CMB that triggers some of the observed transitions in this distant molecular cloud, and it is possible to use the cloud to study what the observed temperature of the CMB was in the past, back when the light from B0218+357 passed through this nameless blob of gas. From what the scientists learned, the identify lines in CS, HCN, HCO+, and N2H+ as all possible candidates for future study of this cloud, and the historic temperature of the CMB.
All in all, I can’t say this is the most exciting paper I’ve ever read, and it reinforced my “I really don’t ever want to do work in the area of interstellar medium” feeling, but… It is still neat to know that intergalactic formaldehyde can be used to learn something and that gravitational lenses can be used to find something.
So, I guess if you want to get someone (like me) with almost no interest in molecular clouds (which make me think of intergalactic dust bunnies waiting to get cleaned up by star forming vacuum cleaners) to read a paper, give it a weird title that includes “Redshifted formaldehyde.”