The latest on the peas – do they lack metals?

It’s sometimes difficult to know which papers will excite other scientists and get them to follow-up what you’ve done.  Our peas paper already has seven references to it, so I wasn’t entirely surprised to find a whole paper discussing the peas on astro-ph today. Astro-ph is required reading for all astrophysicists and contains pre-prints of papers that are updated every day. Some papers are posted when they’re submitted to a journal, others only once they’ve been accepted.  A wonderful thing about the field of astronomy is the free access to data and the wide sharing of ideas through forums such as astro-ph.  This creates new and exciting scientific results at an amazing pace.

This paper, written by Ricardo O. Amorín, E. Pérez-Montero and J.M. Vílchez (all at the IAA-CISC), follows up on one of the aspects of the peas: the metallicity (amount of elements other than hydrogen and helium) that are polluting the gas in the peas. These elements (or metals, as astronomers confusingly say) are generated in supernovae, so the metallicity,and the ratios of specific elements, can give astronomers some idea of how “evolved” a galaxy is. The more metals, the more supernovae must have gone off and polluted the gas.

From: Amorin et al. (2010), arXiv:1004.4910. Horizontal axis: galaxy mass; Vertical axis: “metallicity”

What they find is different from our paper. Using a different method to measure the metallicity of the peas, they include the abundance of Nitrogen. This turns out to be anomalous in the peas, and suggests that the peas are less metal-enriched than we concluded. They then look at whether the peas have the amount of metals that other galaxies of similar mass have, and conclude that the peas are off the “mass-metallicity relation” (see plot above – green points are the peas,which are below the grey shaded area representing normal star forming galaxies). This is definitely different from what we concluded – we deduced that the peas are actually on the mass-metallicity relation.

They discuss what this means – if they are right, this makes the peas even more exceptional, since they don’t fit in with normal galaxies in our old, evolved Universe, and underscores their role as “living fossils” since the peas are more like primordial galaxies than evolved ones.  The differences in this nitrogen abundance tells us something about the way the peas convert gas into stars that is quite different from what occurs in galaxies like our own Milky Way.  Amorin et al. further suggest that the “pea” phase is likely short-lived as the intense star formation in the peas will quickly enrich the gas to make them appear more like their normal cousins.  The differences in this nitrogen abundance can imply

So who is right? We don’t know yet. The Amorin et al. paper is appearing in the Astrophysical Journal as a Letter and hopefully starts off a debate on the topic. Stay tuned!

Kevin & Carie