The Emission Nebula Sh 2-174: A Radio Investigation of the Surrounding Region

Sh 2-174 is believed to be either a planetary nebula (PN) or ionized, ambient interstellar medium (ISM). We present in this paper 1420 MHz polarization, 1420 MHz total intensity (Stokes-I), and neutral hydrogen (H I) images of the region around Sh 2-174. The radio images address not only the nature of the object, but also the history of the relationship between Sh 2-174 and its surrounding environment. The H I images show that Sh 2-174 sits presently at the center of a ~1.°2 × ~0.°4 cloud (with peak hydrogen density n _H = 4 ± 2 cm^-3). The Stokes-I image shows thermal-emission peaks (with electron densities n_e = 11 ± 3 cm^-3) coincident with the R-band optical nebula, as well as low-surface-brightness emission from an ionized "halo" around Sh 2-174 and from an ionized "plateau" extending southeast from the cloud. The polarization images reveal Faraday-rotation structures along the projected trajectory of Sh 2-174, including a high-contrast structure with "arms" that run precisely along the eastern edge of the H I cloud and a wide central region that merges with the downstream edge of Sh 2-174. The high-contrast structure is consistent with an ionized tail that has both early-epoch (before Sh 2-174 entered the cloud) and present-epoch (after Sh 2-174 entered the cloud) components. Furthermore, our rotation-measure analysis indicates that the ISM magnetic field is deflected at the leading edge of Sh 2-174. The downstream tail and upstream field deflection point to a PN-ISM interaction. Our estimated space velocity for the host white dwarf (GD 561) demonstrates that Sh 2-174 entered the cloud ~27,000 yr ago, and gives a PN-ISM interaction timescale <~ 2.0 × 10⁵ yr. We estimate an ambient magnetic field in the cloud of 11 ± 3 μG.