Detection of Highly Ionized Nitrogen at Large Galactocentric Distances

Previous abstract Next abstract

Session 29 -- General Interstellar Medium
Display presentation, Tuesday, 31, 1994, 9:20-6:30

[29.06] Detection of Highly Ionized Nitrogen at Large Galactocentric Distances

Blair D. Savage, Limin Lu (Univ of Wisconsin ), Kenneth R. Sembach (MIT)

We present intermediate resolution GHRS observations of interstellar S II, Si II, Mg II, and N V absorption toward the bright quasar H 1821+643. Galactic rotation in the direction of H 1821+643 ($l=94.0^0$, $b=27.4^0$) causes distant matter in the warped outer Galaxy to emit and absorb at LSR velocities approaching $-150$ km/s. We detected strongly saturated absorption by Si II and Mg II extending from +40 km/s to $-150$ km/s indicating the presence of gas containing these metals in the warped outer Galaxy. Strong N V absorption is found at low to intermediate velocity (+30 to $-80$ km/s ) and at very high velocity ($-120$ to $-170$ km/s). The low and intermediate velocity N V is associated with Galactic disk and halo gas similar to that previously studied with the IUE and the HST. The high velocity N V absorption appears to be centered on the velocity where the Si II and Mg II absorption end. We believe the high velocity N V is most likely associated with gas at $R_{G}>30$ kpc, but can not rule out a more local explanation involving high velocity phenomena associated with energetic events occurring above a nearby spiral arm. The high level of ionization may be produced by collisional ionization within a hot/warm gas interface at the outer gaseous boundary of the Milky Way or in a region of low density warm gas photoionized by the extragalactic EUV background. The presence of N V at large Galactocentric distances in the direction of H 1821+643 has important implications for the origin of highly ionized gas seen in QSO absorption line systems.

Support for this work was provided by NASA through grant numbers GO-3463.01-91A and HF-1038.01-92A.

Tuesday program listing