Study of Galactic HI Shells/Supershells using the I-GALFA Survey Data

Abstract

Neutral atomic hydrogen (H i) gas in interstellar space is largely organized into filaments, loops, and shells. H i shells are the interstellar material swept up by su- personic shock waves produced by mechanical energy sources such as stellar winds and supernovae (SNe). Among H i shells, gigantic structures called as supershell require energy larger than 3 × 1052 erg to form and are thought to be produced by either the explosion of multiple SNe in OB associations or, alternatively, by the impact of high-velocity clouds (HVCs) falling into the Galactic disk. We carry out a systematic study of Hi shells/supershells in the first Galactic quadrant (l ≈ 32–77◦ and

b

≈10◦) using the Inner-Galaxy Arecibo L-band Feed Array (I-GALFA) H i 21-cm survey data. The high-resolution (4′) and high sensitiv- ity (0.2 K) of the survey allow us to exploit the true nature of the sources detected in previous low-resolution studies and also to identify weak or distant shells/supershells that were not detectable before. Our work is composed of three parts: (1) examine fast-expanding H i shells associated with Galactic supernova remnants (SNRs). (2) confirm the objects in the low-resolution (36′) catalog of Heiles (1979) and search for new H i shells/supershells. (3) examine detailed spatial and velocity structures of the supershell GS040.2+00.6−70 and the compact HVC HVC040+01−282 (here- after, GS040 and CHVC040, respectively). In the first part, among the 39 known Galactic SNRs in the I-GALFA survey area, we find that four SNRs have an associated H i shell: W44, G54.4−0.3, W51C, and CTB 80. All four were previously identified in low-resolution surveys, and three of those (excluding G54.4−0.3) were previously studied with the Arecibo telescope. A remarkable new result, however, is the detection of Hi emission at both very high positive and negative velocities in W44 from the receding and approaching parts of the H i expanding shell, respectively. This is the first detection of both sides of an expanding shell associated with an SNR in the inner Galaxy in Hi 21 cm emission. The high-resolution I-GALFA data also reveal a prominent expanding Hi shell with high circular symmetry associated with G54.4−0.3. We explore the physical characteristics of four SNRs and discuss what differentiates them from other SNRs in the survey area. We conclude that these four SNRs are likely the remnants of core-collapse SNe interacting with a relatively dense ( 1cm−3) ambient medium, and we discuss the visibility of SNRs in the H i line. In the second part, we report the detection of 38 Hi shells/supershells in the I-GALFA data: 13 are the previously-known Heiles shells while 25 are new shell candidates. Their geometric mean diameter and velocity extent have about 1–13◦ and 7–65 km s−1, respectively. The high-resolution I-GALFA images show that some of them, mostly in the inner Galaxy, are complicated with other chimney or more than one shell, and some might be located near active star-forming region(s). We estimate kinematic distances, radii, heights from the Galactic midplane (if available), and explosion energies of the found objects. Fifteen objects are likely to be supershells. We compare the Galactocentric distribution of H i shells/supershells with those of H ii regions and giant molecular clouds. The comparison implies that many inner Galactic H i supershells are missing in our shell list. One possibility would be that many H i supershells there had broken through the disk and evolved to chimneys or worms. We also present the result of investigations on the association of Hi shells with molecular clouds. In the third part, we report the discovery of a kiloparsec-size supershell GS040 in the Galactic outskirts with the CHVC040 at its geometrical center. Our Arecibo H i images reveal that GS040 appears as a complete circular ring with complicated structures inside while CHVC040 shows a bright elongated core surrounded by dif- fuse gas with a sharp boundary at the far side of the Galactic disk. The analysis of morphological and physical properties of the both objects suggests that CHVC040 collided with the disk ∼ 4.3 Myr ago to form a supershell and could be a fragment of a nearby disrupted galaxy or a cloud that originated from an intergalactic accreting flow. Our results imply that some compact HVCs can survive their journey through the Galactic halo and supply energy and momentum into the Milky Way disk.