Abstract
We report the detection of a new Galactic bubble at the interface between the halo and the Galactic disc. We suggest that the nearby Lupus complex and parts of the Ophiuchus complex constitute the denser parts of the structure. This young bubble, ≲3 Myr old, could be the remnant of a supernova and it expands inside a larger HI loop that has been created by the outflows of the Upper Scorpius OB association. An HI cavity filled with hot X-ray gas is associated with the structure, which is consistent with the Galactic chimney scenario. The X-ray emission extends beyond the west and north-west edges of the bubble, suggesting that hot gas outflows are breaching the cavity, possibly through the fragmented Lupus complex. Analyses of the polarised radio synchrotron and of the polarised dust emission of the region suggest the connection of the Galactic centre spur with the young Galactic bubble. A distribution of HI clumps that spatially corresponds well to the cavity boundaries was found at VLSR ≃−100 km s−1. Some of these HI clumps are forming jets, which may arise from the fragmented part of the bubble. We suggest that these clumps might be “dripping” cold clouds from the shell walls inside the cavity that is filled with hot ionised gas. It is possible that some of these clumps are magnetised and were then accelerated by the compressed magnetic field at the edge of the cavity. Such a mechanism would challenge the Galactic accretion and fountain model, where high-velocity clouds are considered to be formed at high Galactic latitude from hot gas flows from the Galactic plane.
Highlights
The Galactic centre is one of the most complex regions of the Galaxy
We suggest that the western void, rather than being produced by the UpperCentaurus Lupus (UCL) subgroups feedback, is probably filled with hot gas escaping from the bubble through the fragmented Lupus molecular cloud
If the Ophiuchus streamers have the same origin as the HI stream seen at VLSR = −57.71 km s−1, they might have left the Galactic plane for the lighter hot gas in the large Upper Scorpius (USco) HI shell before the supernova explosion that created the smaller bubble
Summary
The Galactic centre is one of the most complex regions of the Galaxy. Its complicated star formation history and the presence of multiple massive star clusters have shaped the large-scale structures, which appear as a superposition of loops observed through the atomic hydrogen (HI) line emission, gamma-rays (the “Fermi bubbles”), and through synchrotron radio emission (Heiles 1984; Egger & Aschenbach 1995; Wolleben 2007; Su et al 2010; Vidal et al 2015; Crocker et al 2015). Vidal et al (2015) identified four possible explanations for the origin of these loops: old and nearby supernova remnants, outflows from the Galactic centre, bubbles powered by OB associations, and magnetic field loops illuminated by synchrotron emission. The authors explain these two voids by the cumulative feedback from massive stars in the USco and UCL subgroups, which would have triggered the star formation in the region. The multiple compression scenario, where the bubble expansion colliding with the larger USco HI shell would be at the origin of the Lupus molecular complex, will be explored in the discussion section This model places the Lupus complex at the edge of a shelllike structure that is probably filled with hot X-ray gas (see section). We propose that its fragmented morphology might be at the origin of the outflow of hot gas seen in the western void
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