Detection of the Galactic Warm Neutral Medium in HI 21cm absorption

Abstract

We report a deep Giant Metrewave Radio Telescope (GMRT) search for Galactic H{\sc i} 21cm absorption towards the quasar B0438$-$436, yielding the detection of wide, weak H{\sc i} 21cm absorption, with a velocity-integrated H{\sc i} 21cm optical depth of $0.0188\pm 0.0036$~km~s${}^{-1}$. Comparing this with the H{\sc i} column density measured in the Parkes Galactic All-Sky Survey gives a column density-weighted harmonic mean spin temperature of $3760\pm 365$~K, one of the highest measured in the Galaxy. This is consistent with most of the H{\sc i} along the sightline arising in the stable warm neutral medium (WNM). The low peak H{\sc i} 21cm optical depth towards B0438$-$436 implies negligible self-absorption, allowing a multi-Gaussian joint decomposition of the H{\sc i} 21cm absorption and emission spectra. This yields a gas kinetic temperature of ${\mathrm{T}}_{\mathrm{k}}\le (4910\pm 1900)$~K, and a spin temperature of ${\mathrm{T}}_{\mathrm{s}}=(1000\pm 345)$~K for the gas that gives rise to the H{\sc i} 21cm absorption. Our data are consistent with the H{\sc i} 21cm absorption arising from either the stable WNM, with ${\mathrm{T}}_{\mathrm{s}}\ll {\mathrm{T}}_{\mathrm{k}}$, ${\mathrm{T}}_{\mathrm{k}}\approx 5000$~K, and little penetration of the background Lyman-$\alpha$ radiation field into the neutral hydrogen, or from the unstable neutral medium, with ${\mathrm{T}}_{\mathrm{s}}\approx {\mathrm{T}}_{\mathrm{k}}\approx 1000\mathrm{K}$.

5 pages, 3 figures; accepted for publication in MNRAS-Letters

References in corpus (2)

• GASS: The Parkes Galactic All-Sky Survey. Update: improved correction for instrumental effects and new data release
• The 21-SPONGE HI Absorption Survey I: Techniques and Initial Results

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