[CII] Spectral Mapping of the Galactic Wind and Starbursting Disk of M82 with SOFIA

TL;DR

This study presents velocity-resolved [CII] spectral maps of M82's starburst disk and outflow, revealing the dominant neutral medium contribution and the low fraction of outflowing [CII], with implications for high-redshift observations.

Contribution

First velocity-resolved [CII] observations of M82's outflow, showing the neutral medium's dominance and low outflowing [CII] fraction, informing high-redshift galaxy studies.

Findings

Majority of [CII] in outflow is from neutral atomic medium.

Fraction of outflowing [CII] is small compared to halo or tidal streams.

FUV radiation in the outflow is significantly lower than in the disk.

Abstract

M82 is an archetypal starburst galaxy in the local Universe. The central burst of star formation, thought to be triggered by M82's interaction with other members in the M81 group, is driving a multiphase galaxy-scale wind away from the plane of the disk that has been studied across the electromagnetic spectrum. Here, we present new velocity-resolved observations of the [CII] 158$\mu$m line in the central disk and the southern outflow of M82 using the upGREAT instrument onboard SOFIA. We also report the first detections of velocity-resolved ($\Delta V = 10$ km s$^{-1}$) [CII] emission in the outflow of M82 at projected distances of $\approx1-2$ kpc south of the galaxy center. We compare the [CII] line profiles to observations of CO and HI and find that likely the majority ($>55$%) of the [CII] emission in the outflow is associated with the neutral atomic medium. We find that the fraction of [CII] actually outflowing from M82 is small compared to the bulk gas outside the midplane (which may be in a halo or tidal streamers), which has important implications for observations of [CII] outflows at higher redshift. Finally, by comparing the observed ratio of the [CII] and CO intensities to models of photodissociation regions, we estimate that the far-ultraviolet (FUV) radiation field in the disk is $\sim10^{3.5}~G_0$, in agreement with previous estimates. In the outflow, however, the FUV radiation field is 2-3 orders of magnitudes lower, which may explain the high fraction of [CII] arising from the neutral medium in the wind.