M. Rozas

The origin of the ionization of the diffuse ionized gas in spirals. II. Modelling the distribution of ionizing radiation in NGC 157

In this paper we make a quantitative study of the hypothesis that the diuse H emitted from the discs of spiral galaxies owes its origin to the ionizing photons escaping from H ii regions. The basis of the models is the assumption that a fraction of the Lyman continuum (Lyc) luminosity from the OB stars within each H ii region escapes from the region, leaking into the diuse gas. A basic input element of any such model is a position and luminosity catalogue in H of the H ii regions in the galaxy under examination, down to a low limiting luminosity, and we have previously produced a catalogue of this type for NGC 157. An initial family of models can then be generated in which the Lyc escaping from an H ii region is parametrized in terms of the observed H luminosity of the region and the escaping fluxes allowed through the diuse disc gas. These models can then be rened using a measured map of H i surface density to eect the down-conversion of the Lyc to H. For NGC 157 an H i map was available. Although its moderate angular resolution did limit the accuracy with which we could test our models, the predicted diuse H surface brightness distributions from our models were compared with the observed distributions showing that, in general terms, the hypothesis of density bounding for the H ii regions allows us to predict well the spatial distribution of the diuse ionized gas. In the model yielding the best t to the data, the regions of lower luminosity lose a constant fraction of their ionizing flux to their surroundings, while for H ii region luminosities above a specic transition value the ionizing escape fraction is a rising function of the H luminosity.

The internal dynamical equilibrium of H II regions: A statistical study

We present an analysis of the integrated Hα emission line profiles for the H uf769uf769 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 LHα) and non-thermal line width (log σnt )f or the Huf769uf769 regions of the three galaxies are studied and compared with the relation found taken all the H uf769uf769 regions of the three galaxies as a single distribution. In all of these distributions we find a lower envelope in log σnt. A clearer envelope in σnt is found when only those H uf769uf769 regions with σnt >σ s(13 km s −1 ) are considered, where σs is a canonical estimate of the sound speed in the interestellar medium. The linear fit for the envelope is log LHα = (36.8 ± 0.7) + (2.0 ± 0.5) log σnt where the Hα luminosity of the region is taken directly from a photometric H uf769uf769 region catalogue. When the Hα luminosity used instead is that fraction of the H uf769uf769 region luminosity, corresponding to the principal velocity component, i.e. to the turbulent non-expanding contribution, the linear fit is log LHα = (36.8 ± 0.6) + (2.0 ± 0.5) log σnt, i.e. unchanged but slightly tighter. The masses of the H uf769uf769 regions on the envelope using the virial theorem and the mass estimates from the Hα luminosity are comparable, which offers evidence that the H uf769uf769 regions on the envelope are virialized systems, while the remaining regions, the majority, are not in virial equilibrium.

Hα line profiles for a sample of supergiant HII regions I. The main spectral component

We present an analysis of the Hα emission line profiles of a sample of giant extragalactic HII regions, selected from among the brightest and most isolated in a group of spiral galaxies for which we have photometric and spectroscopic data: NGC 157, NGC 3631, NGC 6764, NGC 3344, NGC 4321, NGC 5364, NGC 5055, NGC 5985, NGC 7479. Our study confirms that the majority of the line profiles are composed of a bright, main component and two fainter, high velocity components that we denominate wings. Here, we analyze the kinematics of the principal components, finding a relation between the Hα luminosity, LHα, and the turbulent velocity dispersion, σnt. A linear fit to the relation between these quantities yields log LHα = (35.6 ± 0.4) + (2.87 ± 0.2) log σnt, in agreement with previous studies. We compute the mass of each HII region using both the virial theorem and the Hα luminosity, confirming that, though these estimates do not coincide exactly, they are comparable within the uncertainties and consequently that the HII regions in our sample are approximately virialized.

Fabry-Perot observations of the ionized gas in the spiral galaxy NGC 6951

We present two dimensional kinematic observations at high angular and velocity resolution measured via the H emission line in the active barred spiral NGC 6951, obtained using the TAURUS II Fabry-Perot system on the 4.2 m William Herschel Telescope. From the radial velocity map we produced the rotation curve, which has a rapidly rising inner portion, and then remains remarkably flat to large radii. Subtracting a two-dimensional projected model of this rotation from the observed map yields a residual velocity map which shows signicant non-circular motions, above all in the circumnuclear zone, where they reach projected values of 45 km s 1 .T he kinematic and morphological properties of this zone, a narrow well-dened annular region, point to inflow of gas resulting from perturbation in the stream-lined gas flow due to the bar, as predicted in dynamical models. The overall gas kinematics leads us to infer the presence of an inner disc within the main galactic disc, whose presence is marked by two inner Lindblad resonances at 180 pc and 900 pc from the centre respectively. Our map of the H velocity dispersion shows characteristic values of 20 km s 1 for the Hii regions, and an obvious correlation between higher values and the brightest Hii regions. The value rises to >100 km s 1 as the nucleus is approached.

H

We analyze the broad, low intensity, high velocity components that are seen in the H line profiles for a sample of HII regions. These HII regions are chosen from among the brightest and most isolated in a sample of spiral galaxies for which we have photometric and spectroscopic data: NGC 157, NGC 3631, NGC 6764, NGC 3344, NGC 4321, NGC 5364, NGC 5055, NGC 5985, and NGC 7479. We confirm that the line profiles of most of these bright, giant extragalactic HII regions contain broad kinematic components of low intensity, but high velocity, that we denote as wings. We analyze these components, deriving emission measures, central velocities, and velocity dispersions of the blue and red features, which are similar. We interpret these components as expanding shells within the HII regions and produced by the stellar winds from the ionizing stars. We compare the kinetic energies of these expanding shells with the kinetic energy available from the stellar winds. If we allow for the hypothesis that the brightest HII regions are density bounded, we show that, for these HII regions, the stellar wind mechanism can explain the observed shell kinetic energies.

\mathsf{\alpha}

We analyze the emission line profiles of the region populations in three disc spiral galaxies and find evidence of wing features at 40-90 from the central peak in a significant fraction of regions. We explain the wing features as due to a shell expanding inside the region, quantify the energy involved and present two possible mechanisms to drive the expanding shell.

line profiles for a sample of supergiant HII regions - II. Broad, low intensity components

We use Ha measurements of complete sets of H II regions in disc galaxies to compute the fraction of ionizing photons which escape from them using the hypothesis of density bounding. The escaping flux is more than enough to ionize the observed diffuse ionized gas (DIG) and the balance implies that an important fraction of the flux escapes into the intergalactic medium (IGM).