AUDREY CLARENDON
Neutral Oxygen Features in Galaxies at z ∼ 2-3: Probing Galaxy Evolution through Faint Emission-Line Diagnostics
Abstract The faint emission line [OI]λ6302 is sensitive to the hardness of the ionizing spectrum, the presence of shocks, and diffuse ionized gas (DIG) in star-forming galaxies. It has been found frequently in early JWST surveys of z > 2 galaxies, indicating that it is a promising method for probing ionization mechanisms and tracing low-ionization gas in distant objects. We analyze a sample of galaxies from the Keck Baryonic Structure Survey (KBSS, z ∼ 2-3) to construct a line-ratio diagram analyzing [OI]. The combination of [OI] measurements and photoionization models help to uncover why distant galaxies differ in location on diagnostic emission-line diagrams from local ones. This redshift-dependent separation is captured on the O1-BPT as a pronounced offset between z ∼ 2-3 galaxies and local ones from the Sloan Digital Sky Survey (SDSS). The offset and the inability of the models (at the lower limit of Z∗ = 0.001) to capture the high redshift data suggest that DIG and shocks may be contributing to the observed [OI] emissions. In order to confirm these explanations, a larger, more representative study of [OI] is required.
Figure 1: The [OIII]λ5008/Hβ versus [OI]λ6302/Hα relationship (O1-BPT) for z ∼ 2-3. Note the clear offset between the high z (in blue) and low z (gray histogram) data. This offset is present in past examples of the N2-BPT and is indicated by the vertical separation between the median log([OIII]/Hβ) values (yellow stars) for KBSS and the cyan curve (encloses 50% of the SDSS data). The CLOUDY-BPASSv2 photoionization model is plotted for stellar metallicities Z ∗ = 0.002 and 0.001 in viridis (color map ranging from teal to green) and twilight (color map ranging from brown to tan), respectively. We plot the classification curves generated by Kewley et al. 2001, 2006 to discriminate ionization mechanisms.