SIMBAD references

We discovered a large-scale shell G53.9+0.2 around the Crab-like pulsar wind nebula (PWN) G54.1+0.3 with 1420 MHz continuum Very Large Array observations. This is confirmed by a new infrared (IR) image at 8 µm from the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire project, which reveals an intriguing IR shell just surrounding the large radio shell. We analyze the 21 cm H I absorption spectra and ¹³CO emission spectra toward PWN G54.1+0.3 and bright sources on both radio and IR shells. Continuous H I absorption up to the tangent point and absence of negative H I absorption features imply that PWN G54.1+0.3 has a distance beyond the tangent point but within the solar circle, i.e., 4.5-9 kpc. G54.1+0.3 is likely at a distance of ≃6.2 kpc due to the morphological association of the PWN with a CO molecular cloud at a velocity of ≃53 km/s, as revealed by high-resolution ¹³CO images. Based on the H I absorption spectrum and recombination line velocity (≃40 km/s) of the bright H II region G54.09-0.06, which is on the IR shell, the IR shell is likely located at a distance of ≃7.3 kpc, which is also the distance of the associated large-scale radio shell. At this distance, the radio shell has a radius of ∼30 pc. The radio shell may be thermal and lack IR emission due to dust destruction, or it may be nonthermal and part of a newly found old supernova remnant. In either case, it is located at a distance different from PWN G54.1+0.3.