A 300 PC thermal spur associated with the HII region S 54.

The authors report multifrequency radio continuum observations of a long narrow thermal spur emerging from the large H II region S54 (W35, NGC 6604). The spur runs perpendicular to the galactic plane at l ≅ 18°.5 from b ≅ 2° up to b ≅ 8°. H110α recombination line emission was detected from both the spur and the H II region at the same radial velocity (v_lsr ≅ 30 km s^-1), indicating their physical association. At a kinematic distance of about 2.9 kpc the linear extent of the spur is 25 pc by 300 pc. The authors derive a mean electron temperature of T_e ≅ 4000K and a continuous decrease of the mean electron density from N_e ≅ 15 cm^-3 near S54 to N_e ≅ 1 - 2 cm^-3 at its largest observed z-distance.