A High-resolution Study of the Cold Neutral Medium in and around 30 Doradus

TL;DR

This work presents a high-resolution study of the cold neutral medium (CNM) in and around 30 Doradus using GASKAP-H I, resolving CNM structures on ~7 pc scales and decomposing HI absorption into 862 Gaussian components across four velocity bands. It uncovers a main dense CNM structure at B2 and identifies B1 as an outflow while B3 and B4 trace inflows, with a combined inflow rate comparable to the region's star formation rate, and a comparatively small HI outflow component. The CNM here exhibits lower spin temperatures and relatively uniform HI column densities, implying efficient HI shielding layers that facilitate H2 formation even in a highly energized, feedback-dominated environment. Comparisons with the Milky Way and LMC indicate 30 Dor hosts a colder, denser CNM under higher thermal pressures, highlighting the CNM’s critical role in sustaining star formation and regulating the cloud-scale baryon cycle in extreme extragalactic environments.

Abstract

With the aim of evaluating the roles of the cold neutral medium (CNM) in the cloud-scale baryon cycle, we perform a high-resolution study of the CNM in and around the extreme star-forming region 30 Doradus (30 Dor). For our study, we use Galactic Australian Square Kilometre Array Pathfinder H I Survey data and produce H I emission and absorption cubes on 7 pc scales. To examine the CNM structures toward 30 Dor, we decompose the H I absorption cube into 862 Gaussian components and find that these components are distributed at four velocity ranges (B1, B2, B3, and B4, respectively): 200$-$230 km s$^{-1}$, 230$-$260 km s$^{-1}$, 260$-$277 km s$^{-1}$, and 277$-$300 km s$^{-1}$. We derive line-of-sight average spin temperatures and opacity-corrected total H I column densities and show that the B1$-$B4 structures have systematically different properties, indicating that they are physically distinct. As for the nature of the observed CNM structures, we find that B2 is associated with the main dense structure where ionized, atomic, and molecular gases are concentrated. B3 and B4 trace inflows whose combined mass flux rate of 0.14 $M_{\odot}$ yr$^{-1}$ is comparable to the current star formation rate, while B1 probes outflows with a much lower mass flux rate of 0.007 $M_{\odot}$ yr$^{-1}$. Interestingly, the H I column densities in B1$-$B4 are nearly uniform with a factor of two spatial variations, implying the presence of H I shielding layers for H$_{2}$ formation.

Figures (20)

• Figure 1: Three-color composite image of 30 Dor (Spitzer 8, 4.5, and 3.6 $\micron$ emission in red, green, and blue, respectively; meixner2006). The CO integrated intensity image from wong2022 is overlaid as the black contours with levels ranging from 10% to 90% of the maximum value of 44.1 K km s$^{-1}$ with increments of 20%. The blue box outlines the coverage of the GASKAP-H i data, while the red cross marks the location of the central star cluster R136.
• Figure 2: 1.4 GHz continuum image at a resolution of 30$"$ with a pixel size of 12$"$. The location of R136 is indicated as the red cross.
• Figure 3: Derivation of the leakage spectrum. (Left) 184 off-source points that were used to derive the leakage spectrum in the high-resolution cube are shown as the white dots. The colorscale continuum image is the same as the one in Figure \ref{['f:cont_pair']}, but with a slightly larger coverage. (Right) Median of the spectra at the off-source points is shown in black, while the 1$\sigma$ error computed from the distribution of the brightness temperatures in each velocity channel is indicated as the gray envelope. This error dominates the precision of our absorption and emission-only spectra.
• Figure 4: (Left) 1/$\sigma_{\textrm{emt}}^{2}$-weighted mean $1 - e^{-\tau(\varv)}$ spectrum. (Right) 1/$\sigma_{\textrm{em-only}}^{2}$-weighted mean emission-only spectrum. To produce these two spectra, the same unmasked 225 pixels in the absorption and emission-only cubes were used.
• Figure 5: Spatial distributions of the integrated $1 - e^{-\tau(\varv)}$ (left) and $T_{\rm{em-only}}$ (right). The entire velocity range of 200--300 km s$^{-1}$ over which H i absorption and emission are clearly detected was used to produce the maps.