Emission Line Metallicities from the Faint Infrared Grism Survey
We have derived direct measurement gas-phase metallicities of for 14 Emission Line Galaxies (ELGs) at from the Faint Infrared Grism Survey (FIGS). Using slitless G102 grism spectroscopy and VLT/MUSE optical spectra, we conducted a search for ELG candidates in the GOODS-South HUDF, and we were able to measure the weak auroral [Oiii]4363 Å emission line for this subsample, enabling the measurement of the metallicity from the electron temperature. Comparing the metallicity results of this sample with direct measurement metallicities from other samples at and indicates that we have identified a sample containing unusually low-metallicity galaxies, even among ELGs. With the metallicities and stellar masses () for these ELGs, we construct a mass-metallicty (MZ) relation and find that the relation is offset to lower metallicities compared to relations with metallicities derived from alternative methods. Using star formation rates derived from H emission lines, we also calculate our objects’ position on the Fundamental Metallicy Relation, where we also find a lower-metallicity offset. This demonstrates that these objects occupy a unique parameter space of key galaxy properties at redshifts .
The identification and study of nebular emission lines in galaxies can provide insight into star formation rates, ionization parameters, and gas-phase metallicities, among other physical parameters. The gas-phase metallicity can be related to star formation and mass growth in galaxies via the mass-metallicity (MZ) relation, an observed correlation between a galaxy’s stellar mass and its gas-phase metallicity, and by the Fundamental Metallicity Relation (man10; lar10), an empirical plane relating the metallicity and the stellar mass to the star formation rate.
These relations have been well-established for local star-forming galaxies (tre04), which show an increase in gas-phase metallicity as stellar mass increases from to , after which the metallicity flattens. Further surveys have pushed the study of the relation out to higher redshifts, typically finding lower levels of metallicity out to (lil03; mai05; erb06; man09). For these studies, the gas-phase metallicity is often measured through empirical and theoretical strong line ratio calibrations, such as (kk04), N2O2 (kd02), and O3N2 (pp04), using [O iii], [O ii], and Balmer-series hydrogen lines (see Table 1 for description of ratios). However, offsets between local and high-redshift galaxies on diagnostic plots such as the Baldwin-Phillips-Terlevich (BPT) diagram (bal81; ste14; san15), which compares the [O iii]5007/H line ratio to the [N ii]6568/H line ratio, indicate that conditions in the interstellar medium may differ at different redshifts (kew13). If so, there may be undetected biases in the line ratio calibrations. Some studies have also indicated, however, that the presence of very strong emission lines is itself an indicator of low gas-phase metallicity, regardless of the redshift (fin11; xia12; yan17). If this is the case, then selection effects need to be carefully considered when assessing properties of galaxy populations derived from ELGs.
|log(([O iii]4959+5007 + [O ii]3727+3729)/H)|
|N2O2||N2 - O2|
|O3N2||O3 - N2|