Measuring Interstellar Carbon Abundance via 158 um [CII] Absorption with SOFIA -- A Potential Detection, and Proof-of-Concept for Depletion Studies with Future Far-IR Facilities

TL;DR

This study introduces a novel far-infrared absorption method to measure interstellar carbon abundance, demonstrating a potential detection with SOFIA and assessing future telescope capabilities for depletion studies.

Contribution

It presents the first application of [CII] 158 μm absorption to measure carbon in the diffuse ISM and evaluates future FIR facilities for this purpose.

Findings

Potential detection of [CII] absorption towards IC342.

Future FIR telescopes can detect [CII] absorption in many sightlines.

The method offers a new way to study carbon depletion in the ISM.

Abstract

Carbon plays key roles in the InterStellar Medium (ISM) -- as a constituent of dust, as the carrier of the dominant far-infrared cooling line, and as a component of various important molecules. But despite this, there are very few measurements of the abundance and depletion of carbon in the diffuse ISM. As with other elements, these measurements are traditionally performed in the ultraviolet. But for carbon, such measurements are extremely difficult, and less than 20 have been reported in the literature to date. Here, we present a novel method of measuring the abundance and depletion of carbon in the diffuse ISM: by observing absorption of the 158 $\mu$m [CII] line in the far-infrared. We present a catalog of 432 candidate sightlines that use bright nearby galaxies as background sources, and predict the [CII] absorption expected towards each. We conducted a pilot study using SOFIA, targeting sightlines towards the galaxies IC342 and Circinus. We report a potential detection of Galactic [CII] absorption along the IC342 sightline, although it requires disentangling [CII] emission from IC342 itself. The Circinus sightline had an insufficiently stable instrumental baseline to allow a detection. This SOFIA study informs the prospects for [CII] absorption measurements with future facilities. To that end, we explore the potential for four proposed future FIR telescopes -- PRIMA, FIRSST, SALTUS, and Origins -- to detect [CII] absorption. We find that all four facilities would be able to detect [CII] absorption along a significant number of sightlines.