Study of a large arched structure of interstellar H i in the southern Milky Way

Abstract

We have revisited the sky region delimited by 290° < l < 320° and 3° < b < 17°, which includes a large arc of neutral hydrogen (H i), and extended the study region using the Leiden–Argentine–Bonn H i survey. The H i arc is almost an open downward semi-circumference of ≈10° of radius, with the symmetry axis perpendicular to the Galactic plane and the vertex at (l, b) ≈ (308°, 11°). The velocity component in the spectra of the 21 cm H i line that reveals the H i arc has a mean radial velocity (local standard of rest) of − 19 ± 2 km s − 1. Analysing H i maps covering the region lying below the arc (b < 0), we have investigated whether the arc could be interpreted as the cap of an expanding shell originated by a strong explosion occurred below the Galactic plane. The lower cap of the shell would have been blown out into the Galactic halo. In these conditions, the evolution of the shock wave and the associated shell was calculated by means of the Kompaneets approximation. By comparing the model’s predictions with the observations, we obtained that the distance, age and H i mass of the shell would be 640 pc, 9.3 × 106 yr and 3.4 × 105 M⊙, respectively. The shell’s sizes along and perpendicular to the Galactic plane would be 295 pc and ≈ 540 pc, respectively. By means of the model, we obtained that the energy of the explosion that gave origin to the shell would be around 5 × 1050 erg. Data from future surveys and from Gaia are expected to help confirming the proposed model. Other interstellar complexes of the region are also considered in relation to the H i shell.