AAS Meeting #194 - Chicago, Illinois, May/June 1999
Session 71. Between the Stars II: The ISM, Galactic and Extragalactic
Display, Wednesday, June 2, 1999, 10:00am-6:30pm, Southwest Exhibit Hall

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[71.14] The Multiphase Halo of NGC 891: Deep WIYN H\alpha and BVI Images

J.C. Howk (Wisconsin/Johns Hopkins), B.D. Savage (Wisconsin)

We present new deep sub-arcsecond BVI and H\alpha observations of the edge-on Sb galaxy NGC 891 taken with the WIYN 3.5-m telescope. These observations are on more than 10 times deeper than our earlier imaging work of this galaxy (Howk & Savage, 1997, AJ, 114, 2463) and show an extensive web of inhomogeneously distributed (or clumped) dust structures far from the midplane. These dust structures are traceable to heights in excess of 2.0 kpc, although the number and apparent extinction of the features decreases markedly at heights above z=1.8 kpc. The hydrogen column densities of the dust structures are estimated to be of order 1021 cm-2, while the masses are likely in excess of 105 solar masses.

Our deep, high-resolution (0.9 arcsec) H\alpha images recover the high-z diffuse ionized gas (DIG) studied in early works. Though the DIG contains many filaments, the distribution of DIG is much smoother than that of the material traced by the dust absorption. In fact, there is little direct physical association of the DIG emission and the dusty filaments seen in absorption, though some of the structure seen in the DIG seems to be caused by absorption of the H\alpha emission by the high-z dusty filaments visible in our broadband images. We suggest that the high-z ISM in this galaxy is a multiphase medium, with the dusty absorbing structures tracing the cold neutral medium in the interstellar thick disk or halo of NGC 891. This conclusion is supported by our identification of several discrete H II regions, tracing recent star formation, at heights between 600 and 2000 pc from the midplane. Gas in the halo of NGC 891 evidently has a large enough pressure (P/k > 600 K cm-3) to permit the existence of a multiphase medium, including a dense phase (see, e.g., Wolfire et al. 1995, ApJ, 443, 152).

J.C.H. was supported in this work by a NASA Graduate Student Researcher Fellowship under the grant NGT-5-50121.

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