Implications of the SPEAR FUV Maps on Our Understanding of the ISM

TL;DR

This paper presents FUV emission maps of highly ionized species in the ISM, revealing the distribution and intensity of transition temperature gas, and compares observations with theoretical models to enhance understanding of diffuse galactic matter.

Contribution

It provides the first detailed spectral maps of CIV and OVI in the FUV, and compares their distribution with models, highlighting discrepancies and associations with supernova remnants.

Findings

Median emission consistent with evaporative interface models

Observed intensities are three times higher than model predictions

High emission regions linked to supernova remnants and cooling zones

Abstract

The distribution of a low-density transition temperature (10^4.5 - 10^5.5 K) gas in the interstellar medium conveys the character and evolution of diffuse matter in the Galaxy. This difficult to observe component of the ISM emits mainly in the far-ultraviolet (FUV) (912-1800 {\AA}) band. We describe spectral maps of FUV emission lines from the highly ionized species CIV and OVI likely to be the dominant cooling mechanisms of transition temperature gas in the ISM. The maps were obtained using an orbital spectrometer, SPEAR, that was launched in 2003 and has observed the FUV sky with a spectral resolution of \sim 550 and an angular resolution of 10'. We compare distribution of flux in these maps with three basic models of the distribution of transition temperature gas. We find that the median distribution of CIV and OVI emission is consistent with the spatial distribution and line ratios expected from a McKee-Ostriker (MO) type model of evaporative interfaces. However, the intensities are a factor of three higher than would be expected at the MO preferred parameters. Some high intensity regions are clearly associated with supernova remnants and superbubble structures. Others may indicate regions where gas is cooling through the transition temperature.