Gaseous Halos of Late-Type Spiral Galaxies

Abstract

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...

Warm dust as a tracer of galaxies with gaseous halos

“…Different components of the halo ISM in the target galaxies were detected in four cases, as previously done in more nearby systems (see e.g. Dahlem 1997). A list of the detection/non-detection of extraplanar radio and Hα emission is provided in Table 6.…”

“…Making sure that star formation dominates the energy input into the ISM As outlined by Dahlem (1997), several processes can contribute to the creation of gaseous halos around late-type spirals. In order to make sure that only star-formation related energy input is taken into account, galaxies with AGNs and closely interacting systems have been removed from the original sample above.…”

“…cosmic ray electrons) seems to be in general a rather rare phenomenon (Hummel et al 1991a found that only about 5.5% of the 181 galaxies in their initial optically selected sample show signs of extraplanar radio emission), their existence appears to be quite common among nearby galaxies with high star formation rates (SFRs; e.g. Dahlem 1997). We find that this apparent discrepancy is caused by two factors.…”

Large– and small–scale feedback in the Starburst centers of galaxies

“…Many theoretical and observational studies have investigated the mechanical feedback from star formation and its efficiency in transfering energy and material to regions far removed from the site of star formation (e.g., De Young & Heckman 1994;Suchkov et al 1994;Mac Low & Ferrara 1999;Strickland & Stevens 2000;Heckman, Armus & Miley 1990;Dahlem 1997;Ferrara & Tolstoy 2000;Cecil, Bland-Hawthorn & Veilleux 2002), as well as in triggering renewed star formation (e.g., Wada & Norman 2001). Yet many questions remain to be fully answered.…”

“…Among these, progressing from small to large scales: the impact of the spatial distribution, history, and duration of the star-forming event (Clarke & Oey 2002); the coupling of the feedback from star formation with the surrounding ISM; the dependence of the feedback efficiency on the conditions of the ISM (e.g. porosity; Clarke & Oey 2002); and the role of the global galaxy parameters (mass, metallicity, environment;Dahlem 1997;Strickland et al 2004). A review of some of these issues is given by D. Strickland (2004, these proceedings).…”