High‐Resolution Ultraviolet Observations of the Highly Ionized Interstellar Gas toward Radio Loops I and IV

Abstract

We present new Goddard High-Resolution Spectrograph (GHRS) echelle observations of the high ionization lines of Si IV, C IV, and N V toward HD 119608, a halo star at d = 4.1 kpc behind the Loop I and IV supernova remnants. Absorption along the path to HD 119608 makes it possible to study energetic processes that may result in the flow of gas into the Galactic halo. The data have a resolution (FWHM) of ≈ 3.5 km s-1 and S/N ratios of 30:1-50:1. The integrated high ion column densities log N = 13.57 ± 0.02, 14.48 ± 0.06, and 13.45 ± 0.07 for Si IV, C IV, and N V, respectively, imply a factor of 2-4 enhancement in the amount of highly ionized gas compared to average sight lines through the Galactic disk and halo. The integrated high ion column density ratios, N(C IV)/N(Si IV) = 8.1 ± 1.1 and N(C IV)/N(N V) = 10.7 ± 2.1, are also several times larger than normal. These high ion results suggest the absorption is influenced by passage of the sight line through the center of Loop IV. The HD 119608 C IV absorption profile has a bimodal velocity structure indicative of an expanding shell; we tentatively derive an expansion velocity of 16 km s-1 for Radio Loop IV. A detailed analysis of the high ion profile structure indicates that multiple types of highly ionized gas with a range of properties exist along this sight line. We also reexamine the high ionization properties of the QSO 3C 273 sight line using new intermediate-resolution (FWHM ≈ 20 km s-1) GHRS data. We obtain log N = 14.49 ± 0.03 and 13.87 ± 0.06 for C IV and N V, respectively. The C IV column density, which is 0.12 dex smaller than earlier estimates, leads to somewhat smaller ionic ratios than previously determined. We find N(C IV)/N(Si IV) = 5.1 ± 0.6 and N(C IV)/N(N V) = 4.2 ± 0.6. However, as for HD 119608, the high ion column densities toward 3C 273 are larger than normal by factors of 2-4. The 3C 273 high ion absorption profiles are much broader than those seen toward HD 119608 and other sight lines near the center of Loop IV. The larger line widths may result because the sight line passes through the turbulent edge of Loop IV as well as the X-ray and radio continuum emission regions of the North Polar Spur. We have compiled a list of the highest quality IUE and GHRS high ion measurements available for interstellar sight lines through the disk and halo and find the following median averaged results: N(C IV)/N(Si IV) = 3.8 ± 1.9 and N(C IV)/N(N V) = 4.0 ± 2.4. These ratios are lower than those found for four Loop IV sight lines. We suggest a model for the production of highly ionized gas in Loop IV in which the contributions from turbulent mixing layers and conductive interfaces/SNR bubbles to the total high ion column densities are approximately equal. Much of the high ion absorption toward HD 119608 and 3C 273 may occur within a highly fragmented medium within the remnant or the outer cavity walls of the remnant.