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Gaseous halos of spiral galaxies

Part of the science presented here, based on the level of knowledge in 1997, is summarized in a review article available online via the NASA Extragalactic Database (NED; "Level 5").

MOTIVATION

The primary goal of my research is to improve our observational knowledge on the feedback processes of massive star formation (on kpc scales) into the ambient Interstellar Medium (ISM). One interesting question in this research field that I am particularly interested in is: Under which circumstances can high-level star formation (SF) lead to outflows of gas from the disks of spiral galaxies into their halos? Answering this question is not only important in the context of understanding the nature of the ISM in galaxies, but also because metal-enriched gas leaving spiral galaxies might be a significant contributor to the metal enrichment (and thus chemical evolution) of intergalactic gas, as observed along the lines of sight to distant quasars. The newly detected damped Lyman-alpha systems at near-0 redshifts might actually be galaxy halos or tidally disrupted gas, like plumes or tails.

DISK-HALO INTERACTIONS CAUSED BY MASSIVE STAR FORMATION

My work in this area started with my Ph.D. Thesis (1990, Bonn University) and the papers in which the thesis was published, investigating gaseous emission from the spiral galaxies NGC 891 and NGC 1808. The immediate scientific goals were to investigate the properties of the well-known radio halo of NGC 891 and to find out whether there is a starburst-related outflow from the center of NGC 1808, as suggested by the appearance of dust filaments visible in optical images.

This work was continued, based primarily on radio continuum and X-ray observations of actively star-forming galaxies, such as NGC 1569, NGC 1792, NGC 1808 and a few others:

One important general conclusion reached by us is that gaseous halos, especially radio halos, are present only in those galaxies with a sufficiently high energy input into their ISM, and within these, only in regions with an amount of energy input above a threshold value, as described by us in

A similar behaviour had previously been observed in our Galaxy by Beuermann et al. (1985; A&A, 153, 17). Galaxies with high levels of energy input are bright far-infrared sources and at the same time have relatively high mean dust temperatures, as traced by the ratio of the IRAS 60 mu to 100 mu flux ratio. We found that all nearby edge-on far-infrared-warm starburst galaxies do have gaseous halos. It turns out that all far-infrared warm galaxies studied by us have gaseous halos:

and that therefore warm dust in the galaxy disks is a good tracer of galaxies with gaseous halos:

These papers are direct follow-ups of an earlier one on the dependence of halo properties on the level of activity in the underlying disk (M. Dahlem et al. 1995). As an example, have a look at the newly detected radio halo of the southern edge-on spiral NGC 7090.

It also became evident that gaseous halos comprise all components of the ISM known previously from studies of the Milky Way and the disks of external galaxies:

The study of NGC 4666 also proved for the first time the fact that pure photo-ionization from massive stars within the disk of the galaxy cannot account for the excitation conditions of the diffuse ionized gas in its halo, as deduced from optical line ratios in an Osterbrock diagnostic diagram. A second source of heat, most likely shock-heating, must play an important role, at least in the kpc-scale starburst-driven superwind of NGC 4666.

We have also gathered evidence proving that magnetic fields are important in either allowing or disallowing outflows from galaxy disks. In areas beyond the disks of some edge-on galaxies where extraplanar emission is found also the magnetic field has open field lines along which matter can escape into the halo. Such evidence was found by us in NGC 891, NGC 4631 and NGC 4666:

The effect is also known to be prominently visible in M 82 .

An analysis of the X-ray spectra of the hot gas in the halos of the two nearby edge-on starburst galaxies M 82 and NGC 253 showed that they can be fitted with near-Solar metallicities, indicating that the material in the halos is chemically evolved gas processed by stars in the galaxy disks and that shocks are the most likely source of heating:

Also included in our studies are observations of the gas from which stars form, to investigate which processes initiate starbursts which then can lead to disk-halo interactions. Related papers are, e.g.

There is a separate page on related studies regarding the environment of spirals and groups of galaxies.