No rest for the wicked

A few months ago there was a very interesting paper showing a striking trend : the HI content of galaxies does not vary as a function of star formation rate. Even at very low levels of activity, galaxies apparently remain gas-rich. So apparently there's a substantial population of really gassy galaxies that just don't do anything. What's stopping the gas from forming stars ?

"Nothing", say the authors of this latest paper, "it's because the gormless twits used a really daft way to estimate star formation activity, and they should be ashamed of themselves."

... okay, they don't actually say that. But they have two very nice figures where they compare the trend using two different ways to estimate star formation rate : using optical SDSS data as the previous paper did, and using a combination of UV and mid IR. It's clear that the optical data gives crappy results, with there being almost no correlation at all between SFR and and HI content, whereas using the other data gives a very clear correlation indeed - i.e. low star formation activity always means a low HI content; gas-rich passive galaxies are not a thing. They show a visual sample of the so-called passive galaxies identified previously, and it's clear that they're absolutely normal star-forming discs.

It's pretty damning stuff. There are two things I didn't quite understand though :
1) The previous authors showed that there was a clear trend between SFR and molecular gas. How does the "correct" SFR alter this ?
2) The previous authors did at least try using alternative methods to estimate SFR and found that this didn't make much difference, except to reduce the number of passive galaxies. It's not clear to me what they did wrong here.

Oh well, science marches on...