Evaluating the Morphology of the Local Interstellar Medium: Using New Data to Distinguish Between Multiple Discrete Clouds and a Continuous Medium

TL;DR

This study compares two models of the local interstellar medium, finding that a multiple cloud model better fits observational data than a single continuous cloud model, supported by spectral and structural evidence.

Contribution

The paper provides empirical evidence favoring a multiple cloud morphology over a single cloud model for the local interstellar medium using spectroscopic data.

Findings

Multiple cloud model fits the data better than the single cloud model.

Spectroscopic data reveal multiple velocity components along lines of sight.

Structural features support the existence of discrete interstellar clouds.

Abstract

Ultraviolet and optical spectra of interstellar gas along the lines of sight to nearby stars have been interpreted by Redfield & Linsky (2008) and previous studies as a set of discrete warm, partially ionized clouds each with a different flow vector, temperature, and metal depletion. Recently, Gry & Jenkins (2014) have proposed a fundamentally different model consisting of a single cloud with nonrigid flows filling space out to 9 parsecs from the Sun that they propose better describes the local ISM. Here we test these fundamentally different morphological models against the spatially unbiased Malamut et al. (2014) spectroscopic data set, and find that the multiple cloud morphology model provides a better fit to both the new and old data sets. The detection of three or more velocity components along the lines of sight to many nearby stars, the presence of nearby scattering screens, the observed thin elongated structures of warm interstellar gas, and the likely presence of strong interstellar magnetic fields also support the multiple cloud model. The detection and identification of intercloud gas and the measurement of neutral hydrogen density in clouds beyond the Local Interstellar Cloud could provide future morphological tests.