On the Dynamical and Physical State of the “Diffuse Ionized Medium” in Nearby Spiral Galaxies

We have analyzed deep narrow-band H$\alpha$ images and high-resolutionlong-slit spectra for a sample of the nearest and brightest late-type galaxiesto study the morphology, physical state, and kinematics of the `Diffuse IonizedMedium' (`DIM'). We find that the DIM covers most of the star-forming disk, andis morphologically related to the presence of the giant HII regions. Inaddition, the DIM and the giant HII regions differ systematically in theirphysical and dynamical state. The DIM is characterized by enhanced emission inthe low-ionization forbidden lines ([OI], [NII], and [SII]), and even thehigh-ionization [OIII]$\lambda$5007 line is moderately strong in the DIM. Weverify the inference made by Lehnert & Heckman that the DIM contributessignificantly to the global emission-line ratios measured in late-typegalaxies. We also find that the DIM is more disturbed kinematically than thegas in the giant HII regions. The intrinsic FWHMs of the H$\alpha$ and[NII]$\lambda$6584 lines range from 30 to 100 km s$^{-1}$ in the DIM comparedto 20-50 km s$^{-1}$ in HII regions. The high-ionization gas in the DIM is evenmore kinematically disturbed than the low-ionization gas: the[OIII]$\lambda$5007 lines have intrinsic FWHMs of 70-150 km s$^{-1}$. Thediffering kinematics implies that `the DIM' is not a single monolithic phase ofthe ISM. Instead, it may consist of a `quiescent DIM' with a lowionization-state and small scale-height (few hundred pc) and a `disturbed DIM'with a high ionization state and moderate scale-height (0.5 to 1 kpc). We arguethat the quiescent DIM is most likely photoionized by radiation leaking out ofgiant HII regions, while the disturbed DIM is most likely heated by themechanical energy supplied by supernovae and stellar winds.Comment: 37 pages(including 7 tables) and 12 figures. To appear in the Dec 10, 1997 issue of The Astrophysical Journa