GSH 23.0--0.7+117: A Neutral Hydrogen Shell in the Inner Galaxy

GSH 23.0-0.7+117 is a well-defined neutral hydrogen shell discovered in the Very Large Array (VLA) Galactic Plane Survey (VGPS). Only the blueshifted side of the shell was detected. The expansion velocity and systemic velocity were determined through the systematic behavior of the H I emission with velocity. The center of the shell is at (l, b, v) = (23°.05, -0°.77, +117 km s-1). The angular radius of the shell is 6′.8, or 15 pc at a distance of 7.8 kpc. The H I mass divided by the volume of the half-shell implies an average density nH = 11 ± 4 cm-3 for the medium in which the shell expanded. The estimated age of GSH 23.0-0.7+117 is I Myr, with an upper limit of 2 Myr. The modest expansion energy of 2 × 1048 ergs could have been provided by the stellar wind of a single O4-O8 star over the age of the shell. The 3 σ upper limit to the 1.4 GHz continuum flux density (S1.4 < 248 mJy) is used to derive an upper limit to the Lyman continuum luminosity generated inside the shell. This upper limit implies a maximum of one O9 star (i.e., 08-09.5, taking into account the error in the distance) inside the H I shell, unless most of the incident ionizing flux leaks through the H I shell. To allow this, the shell should be fragmented on scales smaller than the beam (2.3 pc). If the stellar wind bubble is not adiabatic or the bubble has burst (as suggested by the H I channel maps), agreement between the energy and ionization requirements is even less likely. The limit set by the nondetection in the continuum provides a significant challenge for the interpretation of GSH 23.0-0.7+117 as a stellar wind bubble. A similar analysis may be applicable to other Galactic H I shells that have not been detected in the continuum.