The 'Carina Flare' supershell : probing the atomic and molecular ISM in a Galactic chimney

The ‘Carina Flare’ supershell, GSH 287+04−17, is a molecular supershell originally discovered in ¹²CO (J = 1–0) with the NANTEN 4 m telescope. We present the first study of the shell's atomic ISM, using HI21-cm line data from the Parkes 64-m telescope Southern Galactic Plane Survey. The data reveal a gently expanding, ∼230 × 360 pc HI supershell that shows strong evidence of Galactic Plane blowout, with a break in its main body at z ∼ 280 pc and a capped high-latitude extension reaching z ∼ 450 pc. The molecular clouds form comoving parts of the atomic shell, and the morphology of the two phases reflects the supershell's influence on the structure of the ISM. We also report the first discovery of an ionized component of the supershell, in the form of delicate, streamer-like filaments aligned with the proposed direction of blowout. The distance estimate to the shell is re-examined, and we find strong evidence to support the original suggestion that it is located in the Carina Arm at a distance of 2.6 ± 0.4 kpc. Associated HI and H₂ masses are estimated as MHI ≈ 7 ± 3 × 10⁵M⊙ and MH₂ ≈ 2.0 ± 0.6 × 10⁵M⊙, and the kinetic energy of the expanding shell as EK ∼ 1 × 10⁵¹ erg. We examine the results of analytical and numerical models to estimate a required formation energy of several 10⁵¹ to ∼10⁵² erg, and an age of ∼10⁷ yr. This age is compatible with molecular cloud formation time-scales, and we briefly consider the viability of a supershell-triggered origin for the molecular component.14 page(s