By Molly Peeples, Postdoctoral Fellow at STScI
As all heavy elements are produced in stars and stellar deaths, the eventual fates of metals are unique tracers of the large scale gas flows driving galaxy evolution. Some metals will remain in the ISM, some will get trapped in stars during subsequent episodes of star formation, and some will be blown out of the galaxy via large-scale galactic winds. I have recently conducted an inventory of metals in and around star forming galaxies at z~0 in order to place constraints on the histories of gas flows into, within, and out of galaxies (Peeples et al. 2014).
Figure 1 shows our main result, the relative distribution of metals in galactic and circumgalactic components relative to the total amount of metals galaxies have produced, as a function of galaxy stellar mass. “100%” on this diagram denotes the total mass of metals a galaxy has produced by Type II supernovae, Type Ia supernovae, and AGB stars throughout its lifetime.
Figure 1: Cumulative fraction of metals in interstellar gas (blue), stars (red), interstellar dust (orange), the highly ionized circumgalactic medium (CGM; green), the low-ionization CGM (purple), circumgalactic dust (brown), and the hot X-ray traced CGM (yellow) of star forming galaxies. The points correspond to the median stellar mass of the COS-Halos galaxies. 100% corresponds to the total mass of metals a typical star forming galaxy of a given stellar mass has produced in its lifetime. Adapted from Peeples et al. (2014).
With regards to the metals that remain in galaxies, there are a few surprising results this analysis has uncovered. The relative distribution of metals within galaxies depends on the galaxy stellar mass: massive galaxies have most of their retained metals in stars, while low mass galaxies have the bulk of their retained metals in the interstellar medium. Remarkably, star forming dwarf galaxies have more metals in interstellar dust than in stars; models of the chemical evolution of these galaxies cannot ignore this relatively massive component. Most strikingly, by combining the metals in stars and the ISM gas and dust, we find that star forming galaxies have retained a nearly constant 20-25% of the metals they have produced. That this fraction is so constant is extremely counter-intuitive because our current understanding of galaxy evolution has as a central facet the idea that low mass galaxies are more efficient at driving galactic winds, owing to their relatively shallow potential wells.
An important implication of the fact that galaxies have retained only ~20% of their metals is that, because this fraction is so low, most of the metals galaxies have produced must no longer be in the galaxies, but instead have been expelled into their surroundings. With the installation of the Cosmic Origins Spectrograph (COS) on HST, we can finally systematically characterize galaxies’ gaseous halos’ baryons and metals. With 129 orbits, the COS-Halos program (Tumlinson et al., 2013) has measured the mass of metals and baryons in the circumgalactic medium (CGM) of galaxies with stellar mass ~1010 Msun out to impact parameters of 150 kpc (Peeples et al., 2014; Werk et al., 2014). We find that the CGM is multi-phase: there is a warm, diffuse, highly-ionized phase (traced by the highly ionized oxygen ion, OVI) and a colder, denser low-ionization phase (traced by lower ionization species such as Si II, Mg II, and C II). The relative masses we measure for these phases are shown as the purple and green wedges in the Figure; the points denote the median stellar mass of the COS-Halos galaxies. (The wedge-like shape owes to the fact that we do not detect a difference in the CGM mass within 150kpc that depends on the galaxy stellar mass.) We find that the CGM out to 150kpc has a mass of metals comparable to the mass remaining in the interstellar medium.
The COS-Halos team is now working on observationally extending this metal inventory by characterizing the CGM beyond 150kpc, and, with the COS-Dwarfs survey, of lower mass galaxies within 150kpc. From a theoretical standpoint, I am using analytic models and hydrodynamic simulations to try to reconcile how galaxies can have outflow efficiencies that strongly depend on the depth of their potential wells while simultaneously retaining a constant fraction of the metals they produce and maintaining a CGM structure that is consistent with our observations.