Last week, my second paper on outliers from the mass–metallicity relation showed up on astro-ph. In the first one, which I described here, was on the low-mass high-metallicity outliers from the relation; I described in that first post more about what this so-called "mass–metallicity relation" thing actually is. We concluded in that paper that those galaxies must be running out of star-forming gas, and thus nearing the end of their star formation.
In Paper II, we are looking at the other corner of the mass–metallicity plane: massive low-metallicity galaxies. (Yes, it is easy to get tongue-tied in this game.) Most of the 42 galaxies in our sample look like this:They are very blue and what we astronomers called "disturbed." That's fancy-talk for "they've been playing rough with their neighbors and so their gas and stars have been all moved around so they look morphologically... disturbed." The key here is that simulations have shown that as galaxies interact, gas from really large scales will typically get drained into the centers of the galaxies. As it turns out, this large-scale gas will generically have a much lower metallicity than the gas originally at the galaxy center, so the large-scale gas inflow will effectively dilute the central gas. Relative to the amount of time we can expect for the star-formation to continue, it won't take very long for this new gas to get re-enriched by metals formed during the star formation itself, so we can expect for these luminous low-metallicity galaxies to be relatively rare.