Beyond BPT: A New Multi-Dimensional Diagnostic Diagram for Classifying Power Sources Tested Using the SAMI Galaxy Survey

TL;DR

This paper introduces a multi-dimensional diagnostic diagram for classifying galaxy ionization sources, tested on the SAMI Galaxy Survey, revealing more diverse ionization mechanisms than traditional methods.

Contribution

A novel multi-dimensional diagnostic approach that improves differentiation of ionization sources in galaxy spectra using integral field data.

Findings

Identified 1433 galaxies with non-star-forming ionization sources, more than quadrupling previous counts.

Detected 886 galaxies with unique ionization signatures inconsistent with standard models.

Demonstrated that emission line diagnostics alone are insufficient for comprehensive ionization classification.

Abstract

Current methods of identifying the ionizing source of nebular emission in galaxies are well defined for the era of single fiber spectroscopy, but still struggle to differentiate the complex and overlapping ionization sources in some galaxies. With the advent of integral field spectroscopy, the limits of these previous classification schemes are more apparent. We propose a new method for distinguishing the ionizing source in resolved galaxy spectra by use of a multi-dimensional diagnostic diagram that compares emission line ratios with velocity dispersion on a spaxel by spaxel basis within a galaxy. This new method is tested using the SAMI Galaxy Survey Data Release 3, which contains 3068 galaxies at z $<$ 0.12. Our results are released as ionization maps available alongside the SAMI DR3 public data. Our method accounts for a more diverse range of ionization sources than the standard suite of emission line diagnostics; we find 1433 galaxies with significant contribution from non-star-forming ionization using our improved method as compared to 316 galaxies identified using only emission line ratio diagnostics. Within these galaxies, we further identify 886 galaxies hosting unique signatures inconsistent with standard ionization by H2 regions, AGN, or shocks. These galaxies span a wide range of masses and morphological types and comprise a sizable portion of the galaxies used in our sample. With our revised method, we show that emission line diagnostics alone do not adequately differentiate the multiple ways to ionize gas within a galaxy.