High S/N Observations of Low-Ionization Gas Through the Galactic Halo
Session 21 -- Diffuse Galactic Emission
Display presentation, Wednesday, January 12, 9:30-6:45, Salons I/II Room (Crystal Gateway)

## [21.03] High S/N Observations of Low-Ionization Gas Through the Galactic Halo

D. M. Meyer (Northwestern Univ.), K. C. Roth (STScI), B. D. Savage, L. Lu (Univ. of Wisconsin)

Optical absorption-line studies of extragalactic objects can now provide a sensitive probe of low-ionization gas over the full extent of the Galactic halo. Such work is particularly pertinent to a better understanding of the distances, metallicities, and sky coverage of the high-velocity clouds (HVCs) primarily observed in H I 21 cm emission. We have recently obtained high S/N, high resolution (20 km s$^{-1}$) KPNO 4-m echelle observations of the Ca II absorption toward the QSO 1821+643, the Seyfert galaxy Mkn 231, and SN 1993J in the galaxy M81. We detect a weak ($W_\lambda\approx30$ m\AA) Ca II component at an LSR velocity of -140 km s$^{-1}$ toward 1821+643 that corresponds to the Outer Arm H I HVC Complex. In the case of Mkn 231, we find no evidence of high-velocity Ca II absorption despite the location of this sightline near H I HVC Complex C. Our Ca II observations toward SN 1993J are especially exceptional in quality ($S/N\approx500$) and reveal absorption due to the Galactic halo, the disk of M81, and intergalactic material in the M81 group. Although the M81 disk gas dominates the absorption in the velocity region encompassing HVC Complex C, we do find a high-velocity component at +228 km s$^{-1}$ in Ca II that has also been seen in Mg II absorption with HST (Bowen et al. 1994, Ap. J. (Letters) , in press). Our echelle spectra of SN 1993J also reveal detections of other interstellar atoms and molecules such as Ti II, Ca I, and CH$^+$\@. The Ti II measurement is particularly interesting in that it represents the first detection of Ti II toward an extragalactic object and indicates a Ti II scale height of about 1200 pc which is somewhat less than expected from observations of Ti II toward halo stars.