Plasma Diagnostics in the Era of Integral Field Spectroscopy

Toshiya Ueta

doi:10.3390/galaxies10010030

Abstract

To understand the physical conditions of various gaseous systems, plasma diagnostics must be performed properly. To that end, it is equally important to have extinction correction performed properly, even before performing plasma diagnostics. This means that the physical conditions of the target sources—the very quantities to be derived via plasma diagnostics—must be known even before performing extinction correction, because the degree of extinction is determined by comparing the observed spectra of the target sources with their theoretically predicted counterparts. One way to resolve this conundrum is to perform both extinction correction and plasma diagnostics together by iteratively seeking a converged solution. In fact, if these analyses are performed self-consistently, a converged solution can be found based solely on well-calibrated line intensities, given the adopted extinction law and the RV value. However, it is still rare to find these analyses performed numerically rigorously without unnecessary analytical approximations from start to finish. In this contribution for the APN 8e conference, we would like to review this convoluted problem and sort out critical issues based on the results of our recent experiments. It appears that the convoluted theoretical and observational progresses exacerbated by the highly numerical nature of these analyses necessitated a number of analytical simplifications to make the problem analytically tractable in the pre-computer era and that such analytical simplifications still remain rampant in the literature today, even after ample computational resources became readily available. Hence, the community is encouraged to do away with this old habit of sidestepping numerical calculations that was a necessary evil in the past. This is especially true in the context of spatially-resolved 2-D spectroscopy, which obviously conflicts with the uniformity assumption often blindly inherited from 1-D spectroscopy.

Highlights

To understand the physical conditions of various astrophysical gaseous systems, it is fundamental to perform plasma diagnostics [1,2]
The amount of extinction is usually determined by comparing observed diagnostic H I recombination line ratios (e.g., I(Hα)/I(Hβ) and I(Hβ)/I(Hγ)) with the corresponding theoretical counterparts [1,2]
To guarantee self-consistency between extinction correction and plasma diagnostics, the input ne and Te that define the unattenuated line ratios for comparison in extinction correction must be consistent with the resulting ne and Te to be derived via plasma diagnostics

Summary

Introduction

To understand the physical conditions of various astrophysical gaseous systems, it is fundamental to perform plasma diagnostics [1,2]. Ne and Te are determined from a set of equilibrium equations for the adopted n-level√system In these equilibrium equations, the collisional excitation coefficient has the ne Te exp(−∆E/kTe) dependence (where ∆E is the energy√difference between any two levels) and the collisional de-excitation coefficient has the ne Te dependence, while the radiation de-excitation coefficient has no dependence on ne and Te to the first order [9]. Uniform ne and Te (or c(Hβ)) are adopted across extended target sources, instead of performing calculations at each detector element (i.e., pixel or spaxel), defeating the purpose of spatially-resolved IFS observations. In such cases, self-consistency is regrettably nil.

Typical Procedure in the Literature

PPAP: Proper Plasma Analysis Practice

Historical Perspective

Findings

Plasma Diagnostics in the Era of IFS