ON THE DYNAMICAL AND PHYSICAL STATE OF THE DIFFUSE IONIZED MEDIUM IN NEARBY SPIRAL GALAXIES

We report the initial results from a program to study the morphology, physical state, and kinematics of the ''diffuse ionized medium'' (DIM) in a sample of the nearest and brightest late-type galaxies. For each of five galaxies (NGC 2403, M81, NGC 4395, M51, and M101), we have an alyzed deep narrowband H alpha images of the entire star-forming disk and long-slit spectra of the inner (similar to 10 kpc) disk with a res olution of 40-75 km s(-1). We find that the DIM covers most of the sta r-forming disk and is morphologically related to the presence of high surface brightness gas (the giant H II regions). The DIM and the giant H II regions differ systematically in their physical and dynamical st ate. The DIM is characterized by enhanced emission in the low-ionizati on forbidden lines ([O I], [N II], and [S II]), and even the high-ioni zation [O III] lambda 5007 line is moderately strong in the DIM in at least three cases. This last result contrasts with upper limits on the [O III] surface brightness in the local DIM of our own Galaxy (the '' Reynolds Layer''). We directly verify the inference made by Lehnert an d Heckman that the DIM contributes significantly to the spatially inte grated (global) emission-line ratios measured in late-type galaxies. W e also find that the DIM is more disturbed kinematically than the gas in the giant H II regions. The deconvolved (intrinsic) widths of the H alpha and [N II] lambda 6584 lines range from 30 to 100 km s(-1) (FWH M) in the DIM compared to 20-50 km s(-1) in the giant H II regions. Th e high-ionization gas in the DIM is more kinematically disturbed than the low-ionization gas: the [O III] lambda 5007 lines have intrinsic w idths of 70-150 km s(-1). The differing kinematics implies that the DI M is not a single monolithic phase of the ISM. Instead, it may consist of a ''quiescent'' DIM with a low ionization state and small scale he ight (few hundred parsec) and a ''disturbed'' DIM with a high ionizati on state and moderate scale height (0.5-1 kpc). We argue that the quie scent DIM is most likely photoionized by radiation from O stars leakin g out of giant H II regions (although this requires fine-tuning the op acity of galactic disks to ionizing radiation). The disturbed DIM is m ost likely heated by the mechanical energy supplied by supernovae and stellar winds. Since the disturbed DIM accounts for only a minority (< 20%) of the H alpha emission in the regions we have studied, there is no fundamental energetics problem with this model, but it does require mechanically heated gas to have a high areal covering factor in the i nner disk (which needs to be confirmed observationally). We find no cl ear discontinuity between the physical and dynamical properties of the giant H II regions and the quiescent DIM. The quiescent DIM is morpho logically related to the giant H II regions and there is a smooth depe ndence of the emission-line ratios and emission-line widths on the sur face brightness of the emission. Thus, we suggest that a unified appro ach to the study of the DIM and giant H II regions in star-forming gal axies will prove fruitful.