Origin of gas in the Magellanic Bridge: MeerKAT detection of HI 21-cm absorption

TL;DR

This work tackles the origin of neutral hydrogen in the Magellanic Bridge by using HI 21-cm absorption observed with MeerKAT in the MALS survey toward the background source J0332-7249, complemented by HI emission data from HI4PI and prior ATCA results. The authors map MB HI across 4–6 kpc, derive a spin temperature $T_ ext{Spin}$ (via $T_ ext{Spin} = \frac{N_ ext{HI}}{1.823\times 10^{18}\int \tau\, dv}$) and a gas-to-dust ratio, and compare these with SMC and LMC benchmarks. They find the absorption aligns with the lower-velocity, denser HI emission, yielding $T_ ext{Spin} \approx 69$ K and a gas-to-dust ratio around $1.29\times 10^{22}$ cm$^{-2}$ mag$^{-1}$, both of which favor an LMC origin for MB gas. The large-scale velocity structure supports a direct LMC–SMC collision scenario, in agreement with simulations and strengthening the view that MB gas largely originates from the LMC rather than the SMC. This study demonstrates the power of combining HI emission and absorption to constrain the origin and kinematics of gas in interacting dwarf galaxy systems.

Abstract

HI 21-cm absorption lines are investigated to determine the origin of the neutral atomic hydrogen (HI) of the Magellanic Bridge (MB). Using the MeerKat Absorption Line Survey (MALS) data we report the detection of an HI absorption line at a peak signal-to-noise ratio of 10 caused by MB gas against the radio source J033242.97-724904.5. In combination with earlier data obtained with the Australia Telescope Compact Array (ATCA) our new detected HI line permits the exploration of the MB atomic hydrogen gas across 4-6 kpc. The radial velocity profiles from the ATCA data and new data from MALS are analysed. Apart from the excitation conditions, the radial velocity structure of the HI gas seen in emission and absorption is investigated. Eventually the gas-to-dust ratio is quantified to identify the origin of the MB gas being either from the SMC (Small Magellanic Cloud) or the LMC (Large Magellanic Cloud). The HI absorption lines towards lines of sight separated by several kpc consistently coincide with the densest and perhaps coolest gas at the lower radial-velocity limit of the corresponding HI emission profiles. The gas-to-dust ratio is found to be consistent with an origin of the MB gas from the LMC. The large scale velocity distribution as seen from the HI absorption features favors the LMC-SMC direct collision scenario over the close fly-by scenario, as also currently found by numerical simulations.

Figures (3)

• Figure 1: HI4PI emission and MALS absorption spectrum towards J0332-7249. The top panel shows the HI4PI H i spectrum with $\Delta v_\mathrm{LSR}= 1.28\,\mathrm{km\,s^{-1}}$ velocity resolution. Single dish observations are sensitive to all H i gas phases. The bottom panel shows the corresponding MALS absorption line spectrum with a coarser velocity resolution of $\Delta v_\mathrm{LSR}= 6.1\, \mathrm{km\,s^{-1}}$. Absorption line detection among the more dominant emission requires high angular resolution and is a tracer of cold and diffuse H i gas. The Gaussian decomposition components of both H i profiles are compiled in Tab. \ref{['tab:emissionfits']} and Tab. \ref{['tab:absorptionfits']} respectively.
• Figure 2: Gas--to--dust ratio of the MCS in units of the Milky Way's median value of $\frac{N_{\mathrm{H}}}{E\mathrm{(B-V)}}=(0.63\pm0.05)\times 10^{22}\frac{cm^{-2}}{\mathrm{mag}}$. Marked by contour lines are the 2, 3, 4 and 5 times higher values, for comparision with Welty2012 and Koornneef1982. The circles mark the H i absorption lines detected by Kobulnicky1999 (black), the white one marks the MALS pointing position reported here.
• Figure 3: HI4PI emission spectra from the HI4PImapsite towards B0312-770 and B0202-765 at which Kobulnicky1999 detected absorption lines. The velocity component of the absorption line with the highest column density is marked with an orange triangle.