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.

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Optical Observations of Ionized Gas in External Galaxies

Symposium - International Astronomical Union ◽

10.1017/s0074180900026528 ◽

1974 ◽

Vol 60 ◽

pp. 249-256

Author(s):

G. Monnet

Keyword(s):

Large Scale ◽

Spiral Galaxies ◽

Optical Observations ◽

Diffuse Emission ◽

Ionized Gas ◽

External Galaxies

This paper reviews recent optical results on the large scale distribution of ionized gas in spiral galaxies, including our own. There is a diffuse, inhomogeneous emission in the arm region in spirals, including our Galaxy, and in gas-rich galaxies a fainter diffuse emission between the arms.

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Diffuse Ionized Gas in Nearby Galaxies

Symposium - International Astronomical Union ◽

10.1017/s0074180900089117 ◽

1991 ◽

Vol 144 ◽

pp. 223-232

Author(s):

R.A.M. Walterbos

Keyword(s):

Interstellar Medium ◽

Spectral Characteristics ◽

High Ratio ◽

Radio Continuum ◽

Ionized Gas ◽

Star Forming ◽

Star Forming Regions ◽

Diffuse Ionized Gas ◽

External Galaxies ◽

Nearby Galaxies

We discuss the distribution and spectral characteristics of diffuse ionized gas in nearby galaxies. The existence of this elusive component of the interstellar medium (ISM), also referred to as the Warm Ionized Medium, is by now well established from deep imaging and spectroscopic surveys in several emission lines in external galaxies. Diffuse ionized gas is characterized by a relatively high ratio of [SII] over Hα intensities, typically twice as high as for discrete HII regions. The diffuse gas has been mapped in both edge-on and more face-on galaxies providing information on the radial and vertical distribitions. Emission from diffuse ionized gas is strongest around star forming regions. The vertical distribution appears related to the radio continuum thick-disk emission. We also briefly discuss ionization mechanisms, and the connection between star formation characteristics and morphology of the interstellar medium.

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A 158μm [CII] Map of NGC 6946: Detection in Extragalactic Atomic and Ionized Gas

Symposium - International Astronomical Union ◽

10.1017/s0074180900089816 ◽

1992 ◽

Vol 150 ◽

pp. 117-119

Author(s):

S. C. Madden ◽

N. Geis ◽

R. Genzel ◽

F. Herrmann ◽

A. Poglitsch ◽

...

Keyword(s):

Infrared Imaging ◽

Line Emission ◽

Far Infrared ◽

Max Planck ◽

Ionized Gas ◽

Ob Stars ◽

Ngc 6946 ◽

Total Luminosity ◽

Fabry Perot ◽

External Galaxies

The first observations of the [CII] line toward the nuclei of gas-rich external galaxies, showed that the far-infrared line emission contributes up to 1% of the total luminosity and most likely originates from dense photon-dominated regions (PDRs) associated with the surfaces of molecular clouds exposed to FUV from external or embedded OB stars (Crawford et al. 1985, Lugten et al. 1986, Stacey et al. 1991). We have mapped the [CII] emission toward NGC 6946 over an 8' × 6' (23 × 17 kpc) (Madden et al. 1991) using the Max-Planck Instutute/U.C.Berkeley Far-Infrared Imaging Fabry-Perot Interferometer (FIFI) on the Kuiper Airborne Observatory (KAO).

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The Large-Scale Activity in NGC 1068

Symposium - International Astronomical Union ◽

10.1017/s0074180900141270 ◽

1989 ◽

Vol 134 ◽

pp. 343-344

Author(s):

J. Bland ◽

G. N. Cecil

Keyword(s):

Large Scale ◽

Physical State ◽

Seyfert Galaxy ◽

Optical Emission ◽

Field Of View ◽

Ionized Gas ◽

Nuclear Activity ◽

Fabry Perot ◽

Ngc 1068 ◽

Resolution Study

While NGC 1068 has received much attention in recent years, little is known of the large-scale dynamics and physical state of the ionized gas in this nearby Seyfert galaxy and, in particular, its connection with the nuclear activity. We have used the Hawaii Imaging Fabry-Perot Interferometer (HIFI) at the CFHT to obtain detailed spectrophotometry at 65 kms−1 resolution (FWHM) over the Hα and neighbouring [N II] lines. The final maps are derived from 100 000 fits to spectra taken at 0.4″ increments over a 200″ field-of-view. (A higher resolution study which concentrates on the circumnuclear, optical emission and its relation to the radio jet is presented at this conference by Cecil & Bland.)

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Distribution of star-forming complexes and the structure of our Galaxy

Symposium - International Astronomical Union ◽

10.1017/s0074180900212618 ◽

2003 ◽

Vol 212 ◽

pp. 431-440 ◽

Cited By ~ 1

Author(s):

Delphine Russeil

Keyword(s):

Large Scale ◽

Stellar Population ◽

Line Of Sight ◽

H Ii Regions ◽

Spiral Arms ◽

Ob Stars ◽

Star Forming ◽

Optical Images ◽

External Galaxies

The determination of the external galaxies morphology is generally based on their appearance on optical images. At these wavelengths young stellar population and their associated H ii regions, which can be grouped into star-forming complexes, appear preferentially located along spiral arms. Hence, it is naturally to use the same tracers to delineate the arms of our own Galaxy. But, where for external galaxies the distribution of star-forming complexes along the spiral arms is generally evident from direct imaging, for our Galaxy the spiral arms are strung out along the line of sight, leading to the superposition and mixing of information from the different complexes in the spiral arms making it difficult to distinguish them. Thus to access to the spatial distribution of young objects, hence to the large scale structure of our Galaxy, it is required first to identify and collect star-forming complexes (molecular clouds – H ii regions – OB stars) and then to determine their distance. In this framework I review the observational results and difficulties concerning the distribution of star-forming complexes and the determination of the structure of our Galaxy.

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Diffuse Ionized Gas and the Disk-Halo Interaction in Spiral Galaxies

Astrophysics and Space Science Library - How does the Galaxy Work? ◽

10.1007/1-4020-2620-x_11 ◽

2005 ◽

pp. 61-70

Author(s):

R. -J. Dettmar

Keyword(s):

Spiral Galaxies ◽

Ionized Gas ◽

Diffuse Ionized Gas

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Diffuse Ionized Gas in a Sample of Edge‐on Galaxies and Comparisons with Hiand Radio Continuum Emission

The Astrophysical Journal ◽

10.1086/308975 ◽

2000 ◽

Vol 536 (2) ◽

pp. 645-662 ◽

Cited By ~ 48

Author(s):

Joseph A. Collins ◽

Richard J. Rand ◽

Nebojsa Duric ◽

Rene A. M. Walterbos

Keyword(s):

Radio Continuum ◽

Continuum Emission ◽

Ionized Gas ◽

Diffuse Ionized Gas

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The Large-Scale Radio Continuum Structure of Spiral Galaxies

Symposium - International Astronomical Union ◽

10.1017/s0074180900143645 ◽

1978 ◽

Vol 77 ◽

pp. 33-48 ◽

Cited By ~ 1

Author(s):

P.C. van der Kruit

Keyword(s):

Large Scale ◽

Spiral Galaxies ◽

Large Scale Structure ◽

Scale Structure ◽

Optical Image ◽

Radio Continuum ◽

Continuum Emission ◽

The Galaxy ◽

Continuum Structure ◽

Disk Component

This review concerns the large-scale structure of radio continuum emission in spiral galaxies (“the smooth background”), by which we mean the distribution of radio surface brightness at scales larger than, say, 1 kpc. Accordingly the nuclear emission and structure due to spiral arms and HII regions will not be a major topic of discussion here. Already the first mappings of the galactic background suggested that there is indeed a distribution of radio continuum emission extending throughout the Galaxy. This conclusion has been reinforced by the earliest observations of M31 by showing that the general emission from this object extended over at least the whole optical image. More recently, van der Kruit (1973a, b, c) separated the radio emission from a sample of spiral galaxies observed at 1415 MHz with the Westerbork Synthesis Radio Telescope (WSRT) into a nuclear, spiral arm and “base disk” component, showing that the latter component usually contains most of the flux density. This latter component is largely non-thermal and extends over the whole optical image (see also van der Kruit and Allen, 1976). Clearly it is astrophysically interesting to discuss the large-scale structure of the radio continuum emission.

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