Nebular Excitation in z~2 Star-forming Galaxies from the SINS and LUCI Surveys: The Influence of Shocks and AGN

TL;DR

This study uses spatially-resolved data of z~2 star-forming galaxies to investigate excitation mechanisms, revealing the influence of shocks and AGN, and evaluating diagnostic tools' effectiveness at high redshift.

Contribution

It provides new insights into the excitation sources in high-z galaxies and assesses the applicability of local diagnostic methods after accounting for metallicity evolution.

Findings

Some galaxies host AGN or shocks affecting emission lines.

The MEx diagnostic needs adjustment for high-z galaxies.

Metallicity calibrations based on [NII]/Ha are biased by shocks and AGN.

Abstract

Based on high-resolution, spatially resolved data of 10 z~2 star-forming galaxies from the SINS/zC-SINF survey and LUCI data for 12 additional galaxies, we probe the excitation properties of high-z galaxies and the impact of active galactic nuclei (AGN), shocks and photoionization. We explore how these spatially-resolved line ratios can inform our interpretation of integrated emission line ratios obtained at high redshift. Many of our galaxies fall in the `composite' region of the z~0 [NII]/Ha versus [OIII]/Hb diagnostic (BPT) diagram, between star-forming galaxies and those with AGN. Based on our resolved measurements, we find that some of these galaxies likely host an AGN, while others appear to be affected by the presence of shocks possibly caused by an outflow or from enhanced ionization parameter as compared with HII regions in normal local star-forming galaxies. We find that the Mass-Excitation (MEx) diagnostic, which separates purely star-forming and AGN hosting local galaxies in the [OIII]/Hb versus stellar mass plane, does not properly separate z~2 galaxies classified according to the BPT diagram. However, if we shift the galaxies based on the offset between the local and z~2 mass-metallicity relation (i.e. to the mass they would have at z~0 with the same metallicity), we find better agreement between the MEx and BPT diagnostics. Finally, we find that metallicity calibrations based on [NII]/Ha are more biased by shocks and AGN at high-z than the [OIII]/Hb/[NII]/Ha calibration.