Disentangling the Physical Origin of Emission Line Ratio Offsets at High Redshift with Spatially Resolved Spectroscopy

We present spatially resolved Hubble Space Telescope grism spectroscopy of 15 galaxies at z similar to 0.8 drawn from the DEEP2 survey. We analyze H alpha+[N ii], [S ii], and [S iii] emission on kiloparsec scales to explore which mechanisms are powering emission lines at high redshifts, testing which processes may be responsible for the well-known offset of high-redshift galaxies from the z similar to 0 locus in the [O iii]/H beta versus [N ii]/H alpha Baldwin-Phillips-Terlevich (BPT) excitation diagram. We study spatially resolved emission-line maps to examine evidence for active galactic nuclei (AGN), shocks, diffuse ionized gas (DIG), or escaping ionizing radiation, all of which may contribute to the BPT offsets observed in our sample. We do not find significant evidence of AGN in our sample and quantify that, on average, AGN would need to contribute similar to 25% of the H alpha flux in the central resolution element in order to cause the observed BPT offsets. We find weak (2 sigma) evidence of DIG emission at low surface brightnesses, yielding an implied total DIG emission fraction of similar to 20%, which is not significant enough to be the dominant emission line driver in our sample. In general we find that the observed emission is dominated by star-forming H ii regions. We discuss trends with demographic properties and the possible role of alpha-enhanced abundance patterns in the emission spectra of high-redshift galaxies. Our results indicate that photoionization modeling with stellar population synthesis inputs is a valid tool to explore the specific star formation properties which may cause BPT offsets, to be explored in future work.

Automated summary

Spatially resolved spectroscopy of 15 galaxies at z similar to 0.8 from the DEEP2 survey reveals that emission is mainly dominated by star-forming H ii regions, with weak evidence of DIG emission and no significant evidence of AGN. The study suggests that photoionization modeling with stellar population synthesis inputs is a valuable tool for exploring specific star formation properties that may cause observed BPT offsets.