An Infrared and Radio Study of the Galactic Worm GW 46.4+5.5

In order to study the physical properties and origin of the Galactic worm GW 46.4+5.5, we have carried out high-resolution (~3') H I and CO (J=1-0) line observations and analyzed available infrared and radio emission survey data. GW 46.4+5.5 appears as a long (~8°), filamentary structure extending vertically from the Galactic plane in both median-filtered far-infrared and radio continuum maps. The I₆₀/I₁₀₀ ratio in GW 46.4+5.5 is estimated to be 0.29+/-0.05, which is significantly higher than the value determined for the solar neighborhood. The high ratio is consistent with a hypothesis that the dust grains in the worms have been processed by interstellar shocks. The radio continuum emission from GW 46.4+5.5 has spectral index α~=-0.47 and does not correlate with I₆₀ except for emission at low (|b|<=0.5d) latitudes. Thus, most of the radio continuum emission is likely to be nonthermal. Our one-dimensional H I observations show that the H I gas associated with GW 46.4+5.5 is mainly at v_LSR~=15-40 km s^-1. The H I gas is clumpy, and we detected two molecular clouds associated with the H I peaks. The molecular clouds have large internal velocity dispersions, 8.0 and 6.6 km s^-1, compared with their masses, 2.8×10³ and 1.7×10³ M_solar, which implies that they are not gravitationally bound.

Using the Leiden-Dwingeloo H I data, we identify an expanding H I supershell associated with GW 46.4+5.5, which is centered on (l,b)~=(42^deg,5^deg) with an angular size of 14^deg×22^deg (or 340×540 pc² at 1.4 kpc). The supershell appears between v_LSR~=18 and 40 km s^-1 and slowly decreases in size as the velocity increases. An averaged position-velocity diagram reveals that the supershell has a central velocity of ~18 km s^-1, giving a kinematic distance of 1.4 kpc and an expansion velocity of ~15 km s^-1. Assuming that it has been created by multiple stellar winds and supernova explosions, we estimate its kinematic age and the energy required to produce it to be about 5 Myr and 1.5×10⁵² ergs, respectively. The structure is also visible in median-filtered radio continuum maps, but not in the ROSAT maps. The observed molecular clouds might have condensed out of shock-compressed gas in GW 46.4+5.5 because they are closely associated with the H I gas in velocity as well as in position. Their altitudes are 80 and 100 pc, respectively, higher than the scale height of the thin molecular gas disk. The physical properties of the clouds are very similar to those of the high-altitude clouds observed recently in sensitive wide-latitude CO surveys. Our results suggest that at least some of the high-altitude clouds might have formed in Galactic worms (or swept-up H I shells and supershells).