This article reviews observations and models of the diffuse ionized gas that permeates the disk and halo of our Galaxy and others. It was inspired by a series of invited talks presented during an afternoon scientific session of the 65th birthday celebration for Professor Carl Heiles held at Arecibo Observatory in August 2004. This review is in recognition of Carl's long standing interest in and advocacy for studies of the ionized as well as the neutral components of the interstellar medium.Comment: 29 pages, 19 figures; accepted by Reviews of Modern Physic

IPHASX J194359.5+170901 is a new high-excitation planetary nebula with remarkable characteristics. It consists of a knotty ring expanding at a speed of 28 km s −1 , and a fast collimated outflow in the form of faint lobes and caps along the direction perpendicular to the ring. The expansion speed of the polar caps is ∼100 km s −1 , and their kinematical age is twice as large as the age of the ring.Time-resolved photometry of the central star of IPHASX J194359.5+170901 reveals a sinusoidal modulation with a period of 1.16 d. This is interpreted as evidence for binarity of the central star, the brightness variations being related to the orbital motion of an irradiated companion. This is supported by the spectrum of the central star in the visible range, which appears to be dominated by emission from the irradiated zone, consisting of a warm (6000-7000 K) continuum, narrow C III, C IV and N III emission lines, and broader lines from a flat H I Balmer sequence in emission. IPHASX J194359.5+170901 helps to clarify the role of (close) binaries in the formation and shaping of planetary nebulae (PNe). The output of the common-envelope (CE) evolution of the system is a strongly flattened circumstellar mass deposition, a feature that seems to be distinctive of this kind of binary system. Also, IPHASX J194359.5+170901 is among the first post-CE PNe for which the existence of a high-velocity polar outflow has been demonstrated. Its kinematical age might indicate that the polar outflow is formed before the CE phase. This points to mass transfer on to the secondary as the origin, but alternative explanations are also considered.

Abstract.We present an analysis of the integrated Hα emission line profiles for the H  region population of the spiral galaxies NGC 1530, NGC 6951 and NGC 3359. We show that ∼70% of the line profiles show two or three Gaussian components. The relations between the luminosity (log L Hα ) and non-thermal line width (log σ nt ) for the H  regions of the three galaxies are studied and compared with the relation found taken all the H  regions of the three galaxies as a single distribution. In all of these distributions we find a lower envelope in log σ nt . A clearer envelope in σ nt is found when only those H  regions with σ nt > σ s (13 km s −1 ) are considered, where σ s is a canonical estimate of the sound speed in the interestellar medium. The linear fit for the envelope is log L Hα = (36.8 ± 0.7) + (2.0 ± 0.5) log σ nt where the Hα luminosity of the region is taken directly from a photometric H  region catalogue. When the Hα luminosity used instead is that fraction of the H  region luminosity, corresponding to the principal velocity component, i.e. to the turbulent non-expanding contribution, the linear fit is log L Hα = (36.8 ± 0.6) + (2.0 ± 0.5) log σ nt , i.e. unchanged but slightly tighter. The masses of the H  regions on the envelope using the virial theorem and the mass estimates from the Hα luminosity are comparable, which offers evidence that the H  regions on the envelope are virialized systems, while the remaining regions, the majority, are not in virial equilibrium.