Book Reviews

Sokolowski, Joseph W.

doi:10.1093/milmed/159.11.a6c

Cited by 6 publications

(12 citation statements)

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“…The same trend in [S II]/Hα was also seen in narrowband images (Rand, Kulkarni & Hester 1990). The spatial behaviour is as expected in photoionisation models (the ratios rise with distance from the disk ionising sources as the radiation field is diluted) but values as high as 1 • 4 are difficult to explain (Sokolowski 1994). Note that in M31 (Walterbos 1998, this issue p. 99) and the Milky Way, [N II]/Hα is roughly constant at only 0 • 3-0 • 5, and well modelled by Domgörgen & Mathis (1994).…”

Section: Emission Line Ratios As Diagnostics Of Ionisation and Heatingsupporting

confidence: 76%

“…The He I emission is much weaker than expected from models, given the high [N II]/Hα and [S II]/Hα values (see Domgörgen & Mathis 1994). The forbidden lines are highly temperature-sensitive and their interpretation is complicated by issues of abundances, gas heating, and depletion of important coolants such as Fe, Si, and Ca (Sokolowski 1994). But regardless of the forbidden lines, the low He I/Hα is problematic in itself and may indicate that we do not understand stellar atmospheres in the extreme UV well enough (see e.g.…”

Section: Emission Line Ratios As Diagnostics Of Ionisation and Heatingmentioning

confidence: 95%

“…The available energy easily satisfies that required to maintain the layer, and photoionisation models [e.g. Mathis 1986;Sokolowski 1994(partially reproduced in Bland-Hawthorn, Freeman & Quinn 1997] have been the most successful in explaining the behaviour of line ratios (enhanced [S II]λλ6716, 6731/Hα and [N II]λ6583/Hα, and very weak [O I]λ6300/Hα, and [O III]λ5007/Hα relative to HII regions are due to dilution of the stellar radiation as it propagates away from the Galactic plane). However, there are observations that these models cannot explain, one of which is the weakness of He Iλ5876/Hα (Reynolds & Tufte 1995;Reynolds 1998), a ratio with a fairly straightforward interpretation in terms of the hardness of the ionising radiation [see also Heiles et al (1996) for an even more extreme result from radio recombination lines].…”

Section: Emission Line Ratios As Diagnostics Of Ionisation and Heatingmentioning

confidence: 99%

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Diffuse Ionised Gas in Edge-on Galaxies

Rand¹

1998

Publ. – Astron. Soc. Aust

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We review observations of diffuse ionised gas (DIG) in edge-on spiral galaxies. Deep imaging has revealed a variety of morphologies for ‘extraplanar’ (above the HII region layer) or ‘halo’ DIG: some show widespread prominent layers of truly diffuse gas and filamentary structures, others show just one patch or a few patches of extraplanar emission, while still others show no detectable extraplanar DIG at all. The key galactic property which appears to govern the prominence of the DIG layer is the star-formation activity. Smaller-scale connections between DIG and local star formation activity can be seen in some of the galaxies.DIG halos are also related to radio continuum, X-ray and HI halos. Those galaxies with the most prominent DIG layers also have the most prominent radio halos. In NGC 891, there are spatial correlations between extraplanar DIG, radio emission, X-ray emission and HI, and these in turn are related to the underlying disk star-formation activity.Progress on identifying the sources of ionisation and heating of the gas has come from spectroscopy, imaging in different emission lines, and Fabry–Perot observations. By comparing with theoretical predictions, much evidence is found for photoionisation by disk stars as the primary ionisation and heating mechanism, but recent observations are beginning to point towards the need for additional sources of ionisation and/or heating.

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Section: Emission Line Ratios As Diagnostics Of Ionisation and Heatingsupporting

confidence: 76%

Section: Emission Line Ratios As Diagnostics Of Ionisation and Heatingmentioning

confidence: 95%

Section: Emission Line Ratios As Diagnostics Of Ionisation and Heatingmentioning

confidence: 99%

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Diffuse Ionised Gas in Edge-on Galaxies

Rand¹

1998

Publ. – Astron. Soc. Aust

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“…Only in the particular case of the irregular galaxy NGC 2188 has photoionization been shown to explain all data, including optical emission line ratios (Domgörgen and Dettmar 1997). Several attempts have been made to alleviate the problem of line ratios being inconsistent with photoionization, e.g., by invoking a hardening of the optical continuum impinging on the halo gas via differential absorption and/or scattering of radiation due to dust (Sokolowski 1994;Ferrara et al 1996). In some cases this still leaves open the possibility that photoionization is the only heating source of halo gas.…”

Section: Photoionization By Uv and Optical Continuum Of Massive Youngmentioning

confidence: 99%

Gaseous Halos of Late-Type Spiral Galaxies

Dahlem¹

1997

PASP

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ABSTRACT. This article reviews the most recent observational results on disk-halo interactions in nearby galaxies. The implications of these results on our understanding of the structure of the interstellar medium (ISM) in our Galaxy and external spiral galaxies, with particular emphasis on the halo ISM, are discussed. Disk-halo interactions occur only above the brightest HII regions, which are found almost exclusively in late-type spirals and dwarf and irregular galaxies. Circumnuclear starbursts are the most energetic flavor of this phenomenon. The existence, the shapes, and the properties of gaseous halos depend on the level of energy input into the disk ISM per unit surface area, i.e., on the feedback of mass, momentum, and energy produced by vigorous star formation. Thus, current theoretical models of the ISM take into account that gaseous halos are natural extensions of disk ISMs and are heated by the winds and ionizing radiation of massive stars, and by the shock waves of Supernovae and their remnants. Observationally, this is reflected by the fact that all phases of the ISM known to exist in galaxy disks have also been detected in halos. Disk-halo interactions are a very effective way of redistributing energy, metals, and magnetic fields in galaxies and of expelling them into intergalactic space when outflows reach escape velocity. MOTIVATION AND OUTLINESpiral galaxies consist of three spatially and dynamically dinstinct components, namely bulge, disk, and halo. Of these, the central bulge consists primarily of old stars and has a low gas fraction. The disk and halo consist of stars plus gas, dust, and in many cases "dark matter." Here I want to concentrate on the interstellar medium (ISM) in the disks and halos of galaxies, with emphasis on late-type spirals. Massive dark halos will not be described (see, e.g., Ashman 1992; Holt and Bennett 1995). In the current context "latetype" stands for disk-dominated spirals with rather inconspicuous bulges (Hubble types Sb -Sdm). The ISM of elliptical galaxies and early-type spirals (S0-Sab) will also not be presented here (cf. Walsh et al. 1989;and Fabbiano 1989).Studies of the ISM contribute significantly to our understanding of the structure, kinematics, and evolution of galaxies. The level of star formation (SF), its spatial distribution, and its temporal changes depend strongly on the properties of the interstellar gas from which stars can form. Stars feed energy and metal-enriched matter back into the ambient medium via stellar winds and supernovae (SNe), thereby distributing processed material throughout the ISM. Hence, through this gas-star-gas cycle, the ISM becomes a mediator of SF.In Sec. 2, the different phases of the ISM in galaxies will be introduced, and Sec. 3 presents observations of the ISM in the Milky Way. Section 4 will show that the Galactic ISM is l A\\ correspondence should be directed to this address. not confined to the thin disk, but extends to high distances (z) above the plane, forming a gaseous halo. However, being located within the...

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“…The majority of the ionized gas in the interstellar medium of the Milky Way resides in a vertically extended layer known as the Reynolds Layer or Warm Ionized Medium (WIM). Photoionization by massive stars in the disk is likely the dominant source of energy input into the layer given its energetic requirements (Reynolds 1993) as well as the first order agreement between observed line ratios and photoionization models (Domgörgen & Mathis 1994;Sokolowski 1994;Bland-Hawthorn et al 1997;Sembach et al 2000).…”

Section: Introductionmentioning

confidence: 98%

Kinematics of Diffuse Ionized Gas Halos: A Ballistic Model of Halo Rotation

Collins

Benjamin

Rand

2002

ApJ

112

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To better understand diffuse ionized gas kinematics and halo rotation in spiral galaxies, we have developed a model in which clouds are ejected from the disk and follow ballistic trajectories through the halo. The behavior of clouds in this model has been investigated thoroughly through a parameter space search and a study of individual cloud orbits. Synthetic velocity profiles have been generated in z (height above the plane) from the models for the purpose of comparing with velocity centroid data from previously obtained long-slit spectra of the edge-on spirals NGC 891 (one slit) and NGC 5775 (two slits). In each case, a purely ballistic model is insufficient in explaining observed DIG kinematics. In the case of NGC 891, the observed vertical velocity gradient is not as steep as predicted by the model, possibly suggesting a source of coupling between disk and halo rotation or an outwardly directed pressure gradient. The ballistic model more successfully explains DIG kinematics observed in NGC 5775; however, it cannot explain the observed trend of high-z gas velocities nearly reaching the systemic velocity. Such behavior can be attributed to either an inwardly directed pressure gradient or 1 Visiting Astronomer, National Optical Astronomy Observatories, Tucson, AZ -2a possible tidal interaction with its companion, NGC 5774. In addition, the ballistic model predicts that clouds move radially outward as they cycle through the halo. The mass and energy fluxes estimated from the model suggest this radially outward gas migration leads to a redistribution of material that may significantly affect the evolution of the ISM.

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Book Reviews

Cited by 6 publications

References 0 publications

Diffuse Ionised Gas in Edge-on Galaxies

Diffuse Ionised Gas in Edge-on Galaxies

Gaseous Halos of Late-Type Spiral Galaxies

Kinematics of Diffuse Ionized Gas Halos: A Ballistic Model of Halo Rotation

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