Diffuse Ionized Gas Research Articles

Turbulent Heating in the Galactic Diffuse Ionized Gas

AbstractThe observed properties of the diffuse ionized gas (DIG) in our Galaxy are not easily reconcilable with simple photoionization models. This suggests that there are different or additional physical processes at work in the DIG. We have developed a model of the DIG whereby it is ionized by a relatively soft ionizing spectrum (Teff ≤ 32,000 K) and is also heated by an additional thermal mechanism: the dissipation of turbulence. This model predicts the same electron temperature, [N II] λ 6583/Hα ratio, [S II] λ 6716/Hα ratio and He I λ 5876/Hα ratio as observed in the DIG. Without the turbulent thermal heating term, this model will not reproduce the observed properties of the DIG. The dissipation of turbulence may also be important in other phases of the ISM.

Diffuse Ionized Gas in Halos of Spiral Galaxies

AbstractOver the last couple of years Diffuse Ionized Gas (DIG) has been identified as an important constituent of the interstellar medium (ISM) in the halos of spiral galaxies. Imaging in and spectroscopy of optical emission lines allow us to study the distribution and excitation of this gas with a spatial resolution not achievable for other phases of the ISM in external galaxies. Its origin and ionization is under debate and give important constraints for models of the ISM in general and on the large scale exchange of matter between disk and halo in particular. This review summarizes more recent observational results and compares them with model predictions. The data available now demonstrate that the presence of DIG in the disk-halo interface of spiral galaxies is related to star formation processes in the underlying disk. While photoionization by OB stars in the disk seems a viable source for the power required to ionize the DIG, additional processes are needed to explain some of the spectral features. The observed correlation with properties of the non-thermal radio continuum indicate that magnetic fields and cosmic rays could play a role for the physics of this medium.

Diffuse Ionized Gas in Three Sculptor Group Galaxies

We present a study of the diffuse ionized gas (DIG) in three Sculptor group galaxies: NGC 55, NGC 253, and NGC 300. The study is based on narrow band imagery in H-alpha+[NII](6548+6583A) and [SII] (6717+6731A). We find that DIG contributes 33 to 58% of the total H-alpha luminosity in these galaxies, or 30 to 54% after correcting for scattered light. We find that NGC 300 has a higher fractional DIG luminosity than the other galaxies in our sample, but it is not clear whether this is a significant difference or an effect of the high inclination of the other galaxies. The diffuse emission, averaged over the optical extent of the disk, has a face-on emission measure of 5 to 10 pc cm^{-6}. The DIG is concentrated near HII regions, although significant emission is seen at large distances from HII regions, up to 0.5 to 1 kpc. The [SII]/(H-alpha+[NII]) ratio is enhanced in the DIG, typically around 0.3 to 0.5, compared to 0.2 for the HII regions in these galaxies. These properties are similar to those measured for the DIG in the Milky Way and in other nearby spirals. The line ratios, large-scale distribution, and energy requirement suggest that photoionization is the dominant ionization mechanism.

The Effect of Light Scattering by Dust in Galactic Halos on Emission-Line Ratios

We present results from Monte Carlo simulations describing the radiation transfer of $H\alpha$ line emission, produced both by HII regions in the disk and in the diffuse ionized gas (DIG), through the dust layer of the galaxy NGC891. This allows us to calculate the amount of light originating in the HII regions of the disk and scattered by dust at high $z$, and compare it with the emission produced by recombinations in the DIG. The cuts of photometric and polarimetric maps along the $z$-axis show that scattered light from HII regions is still 10\% of that of the DIG at $z\sim 600$~pc, whereas the the degree of linear polarization is small ($<1$\%). The importance of these results for the determination of intrinsic emission line ratios is emphasized, and the significance and possible implications of dust at high $z$ are discussed.

Diffuse Ionized Gas in Spiral Galaxies: Probing Lyman Continuum Photon Leakage From H II Regions?

view Abstract Citations (176) References (51) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Diffuse Ionized Gas in Spiral Galaxies: Probing Lyman Continuum Photon Leakage From H II Regions? Ferguson, Annette M. N. ; Wyse, Rosemary F. G. ; Gallagher, J. S., III ; Hunter, Deidre A. Abstract As part of a large study to map the distribution of star formation across galactic disks, we have obtained deep Hα images of the nearby Sculptor Group spirals NGC 247 and NGC 7793. These images are of sufficiently high quality that they allow identification and analysis of diffuse Hα emission at surface brightness levels ranging from those of extremely low density H II regions down to those of the local Galactic disk diffuse emission. This paper presents a study of the large scale distribution and global energetics of diffuse ionized gas (DIG) in these galaxies and investigates the association between DIG and discrete H II regions. The radial and azimuthal intensity distributions of the DIG reveal that it is highly correlated with bright H II regions over both small and large scales. The observed Hα luminosities of the DIG are considerable and account for 30%-50% of the total Hα emission of each galaxy. The integrated minimum Lyman continuum (Lyc) power required to sustain the DIG in each galaxy is enormous and can only easily be met by the ionizing output from massive star formation; the mechanical energy from supernovae and stellar winds falls short of that required by at least a factor of two. Our results support the hypothesis that the DIG is photoionized by Lyc photons which leak out of traditional H II regions, and suggest that the local H I column density plays a role in regulating the amount of leakage which can occur. This interpretation has profound implications for the derivation of global and radial star-formation rates based on Hα emission-line fluxes, leading to qualitatively and quantitatively different behaviors depending on whether the Hα emission from the DIG is included or not. The contribution of the diffuse Hα component to the total Hα emission; i.e., the diffuse fraction, in these galaxies is similar to values found in other galaxies with differing Hubble types and star-formation rates. The constancy of the diffuse fraction is rather unexpected and implies that the overall fraction of photons which can leak out of H II regions and ionize the ISM over large scales is relatively invariant from one galaxy to another. Publication: The Astronomical Journal Pub Date: June 1996 DOI: 10.1086/117961 Bibcode: 1996AJ....111.2265F Keywords: GALAXIES: SPIRAL; GALAXIES: ISM; HII REGIONS full text sources ADS | data products SIMBAD (9) NED (2)

The ionization of the diffuse ionized gas

The diffuse ionized gas (DIG), responsible for producing the faint but pervasive H(sub alpha), requires 15% of the power of all Galactic O stars for its ionization. No other source of ionization seems practical. The spectrum is strong in low stages of ionization (N II, SII), weak in (O III), and very weak in (O I) lambda 6300, at least in one well-observed direction, all significantly different from denser H II regions. We give low-density, low-excitation photoionization models that explain the observed spectrum. The lambda 6300 observation eliminates the simplest models for the DIG (Mathis 1986), in which neutral H extends beyond the edge of the very dilute stellar radiation field. Our present models include two components. One, representing the edges of interstellar H I clouds, extends to the point where H becomes neutral. In the second, the fraction of H(sup o) is not allowed to exceed 0.05 to 0.1. Both have very low values of the ionization parameter, or ratio of the number densities of ionizing photons to electrons. The ionization parameter required by our models is shown to be compatible with observed values of photon and electron densities in the diffuse interstellar medium. Interstellar dust is not important in either model. Predictions of the model are that the Galaxy is leaking about 4% of the ionizing radiation of at least the local O stars into the intergalactic medium, that (O II) lambda 3727 approximately equals 1.1 H(sub alpha), and that He(sup +)/H (sup +)) approximately equals 0.6 He/H. A major difficul ty of this picture is having ionizing radiation propagate from the ionizing stars in the plane of the Galaxy to the DIG. We suggest, as do Miller & Cox (1993), that the ionizing radiation from O stars produces holes or tunnels in the distribution of neutral H as seen from their positions, while we see a sky covered with H I sheets or filaments because we are not near a source of ionizing radiation. With our models, several interfaces between ionized and neutral gas are allowed before the observational limit on lambda 6300 is exceeded. Another difficulty is the gas-phase N(S(sup +))/N(H(sup o)) ratio toward the high-latitude star HD 93521 (Spitzer & Fitzpatrick 1993). We require (S/H) to be somewhat lower than solar, either because of depletion onto grains or a Galactic abundance gradient, so that the S(sup +) can be produced in each of separate H(sup o) and H(sup +) regions.

The Diffuse Ionized Gas Perpendicular to the Plane of NGC 891

Hα images and long-slit spectra of NGC 891 show the presence of a thick layer of diffuse ionized gas (DIG) in this edge-on galaxy. Hα emission originating in the DIG of NGC 891 is detected out to several kpc above the midplane with a typical scale height for the electron density ne(z) of he~ 600 pc. The distribution of the gas is very inhomogeneous with a large scale asymmetry and filamentary structure. Individual Hα emitting structures can be traced out to 4-5 kpc above the midplane. The emission line ratios of [NII] and [SII] to Hα vary with height above the disk. We compare these properties with the analogous gas phase in our Galaxy.The distribution of the DIG is related to star formation in the underlying galactic disk and we conclude that in NGC 891 the interstellar medium is heavily effected by processes related to star formation.

The photoionization of the diffuse galactic gas

view Abstract Citations (116) References (36) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Photoionization of the Diffuse Galactic Gas Mathis, J. S. Abstract Observations recently published by Reynolds show that there is diffuse ionized gas (DIG) both in the Galactic plane and at high Galactic latitudes. Relative to normal H II regions, this gas shows strong [S II]/Hα, normal [N II]/Hα, and very weak [O III]/Hα. Models are presented for this gas, assuming that it is photoionized by O stars in the Galactic plane, that the ionized zone is in a steady state as regards both ionization and thermal balance, and that the DIG is also responsible for the pulsar dispersion measures. It is possible to obtain an excellent fit to the observations with a plausible mixture of spectral types for the ionizing stars. It is also shown that shocks are unlikely to be producing the diffuse emission lines. Publication: The Astrophysical Journal Pub Date: February 1986 DOI: 10.1086/163910 Bibcode: 1986ApJ...301..423M Keywords: Galactic Structure; Hydrogen Clouds; Interstellar Gas; Ionized Gases; O Stars; Photoionization; H Alpha Line; Ion Production Rates; Milky Way Galaxy; Shock Waves; Star Distribution; Supernova Remnants; Astrophysics; INTERSTELLAR: MATTER; STARS: EARLY-TYPE full text sources ADS |