6. GASEOUS HALOS OF EXTERNAL SPIRAL GALAXIES - MULTI-FREQUENCY OBSERVATIONS

Investigations of halos in external galaxies face several observational problems:

(1)	Only a few nearby galaxies have detectable gaseous halos. Hummel, Beck & Dettmar (1991) conducted a VLA snapshot survey to search for radio halos in 181 nearby highly inclined galaxies. Of these 181, only 6 candidates show high-z radio emission. This scarcity of (detectable) galactic halos makes it difficult to establish general rules. Due to the lack of statistically relevant object samples, most research is based on in-depth investigations of the few available targets.
(2)	Halos are very extended, intrinsically faint (i.e., low surface brightness) emitters, which makes them hard to find; this warrants high-sensitivity measurements. Over the last 20 years, in some fields even more recently, telescope and detector technology has improved so much that we now possess the necessary sensitivity to extend the range of our studies to galaxies at the distance of the Virgo cluster (D = 16.8 Mpc; Tully 1988) and beyond.
(3)	It is difficult to morphologically disentangle emission from galaxy disks and halos. Thus, an edge-on geometry is favorable. For this reason I will concentrate here on observations of very highly inclined systems (3). Note that in edge-on galaxies disk-halo interactions occur in the tangential plane of the sky, making it extremely difficult to obtain kinematic information for the halo gas.

In this chapter I will give an overview of observations showing that the gas in the halos of external galaxies has a very complex composition, similar to that of the ISM in our Galaxy.

6.1 Edge-on Starburst Galaxies

Due to their high level of SF and the correspondingly high brightness in various wavebands, starburst galaxies are the best laboratories for studies of SF and related processes. Of these, M82 is a prime example.

6.1.1 M82

Diffuse ionized gas: The first detection of extraplanar emission in an external spiral galaxy was made in M82 by Lynds & Sandage (1963). They used the 5-m Palomar telescope, the most powerful instrument available at that time, to obtain - among other data sets - a red optical plate of M82 (6100-6700 Å) and an H plate. Subtracting the broad-band image from the H frame revealed the presence of a plethora of emission line filaments, tracing the WIM associated with an approximately conical outflow coming from the circumnuclear starburst (their Fig. 9). Spectroscopy of these lines proved that the direction of the flow is outward. This result was confirmed by Bland & Tully (1988), Götz et al. (1990), and McKeith et al. (1995), again based on optical emission line spectroscopy. A new wide-field H image of M82 obtained by M. Lehnert (unpublished) is displayed in Fig. 1. The gas filaments reach out to z-distances of ~ 6 kpc from the galaxy disk (light gray areas in Fig. 1), see also the image by Shopbell (on the cover page of the August 1996 issue of PASP).

Figure 1. H+[N II] image of M82 by M. Lehnert (unpublished). North is to the top, east is to the left.

New observing techniques have been developed, providing access to diffuse gaseous halos via previously unexploited emission processes. Achtermann & Lacy (1995), for example, used the forbidden [Ne II] line in the infra-red ( 12.8 µm) to study the emission line gas. Close to the central starburst Golla et al. (1996) find a spatial correlation of [Ne II] and radio continuum wisps directed out of the disk of M82, connecting to the halo.

Double-peaked emission line profiles and line ratios measured in the halos of several edge-on starburst galaxies, including M82, are the prime tracers of outflowing, shock-heated gas in so-called galactic ``superwinds'', as defined by Heckman, Armus, & Miley (1990; hereafter HAM90).

CRs and B-fields: A radio halo perpendicular to the inner disk of M82, tracing CRs, was detected by Seaquist et al. (1985). Subsequently, more sensitive measurements were conducted by Seaquist & Odegard (1991) and Reuter et al. (1992, 1994). The observed steepening of the radio spectral index indicates synchrotron and inverse Compton losses of the particles leaving the disk. CRs are escaping the starburst together with the thermal gas, reaching z heights of up to 5 kpc above the plane (Reuter et al. 1992; their Fig. 2).

The radio synchrotron emission of M82 is also highly polarized, revealing the presence of a highly ordered magnetic field associated with the outflow. Assuming equipartition between CRs and the magnetic field, Klein et al. (1988) calculated a B-field strength of ~ 50 µG in the disk of M82, which is extraordinarily high compared to other galaxies. Reviews on magnetic fields in galaxies - including B-fields in their halos - have been given by various authors, e.g., Beck et al. (1996), Kronberg (1994), and Wielebinski & Krause (1993).

Dust: In the optical domain dust was detected in absorption ~ 1.5 kpc above the disk of M82 (see e.g., Ichikawa et al. 1994). The detection of (thermal continuum) emission from dust in the halos of galaxies is not easy. Only the IRAS Chopped Photometric Channel (CPC) receiver had an angular resolution (~ 1') high enough to spatially resolve halos of nearby galaxies (van Driel et al. 1993). M82 is the only galaxy in which FIR emission from dust in its halo could be detected. The distribution of dust is similar to that of the optical emission line gas as well as the synchrotron radio continuum (Fig. 10 by van Driel et al. 1993).

The continuum emission of cold dust (~ 10 K) extends into the millimeter radio band. However, to date only the brightest spur of thermal emission perpendicular to the disk of M82, previously seen in H and X-ray emission, was detected at 450 µm by Hughes et al. (1994). No millimeter continuum has yet been detected from the halo of any other galaxy.

Polarized optical emission arises from dust grains aligning with the magnetic field permeating the ISM. M82 is again one of the first objects in which optical polarization studies led to a detection of these grains (Scarrott et al. 1991). By observing polarized H line emission, these authors find that the outflow structure in M82 shows characteristics of a galactic superwind. Reflection of light from the galaxy center was also found to be important, consistent with the detection of scattered red optical light from dust grains in the halo (Perrin et al. 1995).

Molecular gas: CO above the thin disk of M82 was detected by Stark & Carlson (1984). Loiseau et al. (1988, 1990) mapped the distribution of CO in more detail and showed that the molecular gas in M82 is particularly warm.

Hot ionized gas: Soft X-ray emission from the HIM in the halo of M82 was detected by Watson et al. (1984) and Kronberg et al. (1985). More detailed studies (Fabbiano 1988; Schaaf et al. 1989; Bregman et al. 1995; Moran & Lehnert 1997) show that the soft X-ray emission distribution is correlated with the shape of the CR radio halo and that the HIM fills the interior of the outflow cone outlined by the optical emission line filaments in Fig. 1. The HIM can also be traced out to 6 kpc above the disk plane, as far out as the DIG. X-ray spectroscopy reveals that at least two thermal plasma models, with temperatures of 0.2-0.3 keV and 0.6-0.8 keV (Moran & Lehnert 1997; Weaver et al., in prep.), are needed to fit the data. This - although partly a technical artefact - reflects the inhomogeneity of the ISM on galaxy scales. Norman & Ferrara (1996) therefore suggest to describe the ISM as a phase continuum (see Section 8.3), rather than dividing it up into individual phases with different temperatures.

Warm neutral gas: It is difficult to associate specific H I gas features with the outflow cone of M82. The reason is that the outermost parts of M82 - and thereby its H I gas - are heavily disturbed by its tidal interaction with M81 (Yun et al. 1994).

Summary of M82 observations: In summary, basically all phases of the ISM were detected in the halo of M82 and have been studied in great detail, yielding plenty of evidence for a gaseous outflow powered by the central starburst.

6.1.2 Other edge-on starburst galaxies

Extraplanar optical emission-line gas has been detected in several galaxies; for a compilation of optical emission line observations of starburst galaxies see e.g., Lehnert & Heckman (1995, 1996a, 1996b). Examples of edge-on starburst galaxies with very prominent extraplanar H line emission are NGC 253, NGC 3079, NGC 3628, and NGC 4945. HAM90 used double-peaked optical emission lines to kinematically trace outflows in a sample of starburst galaxies, including NGC 253, M82, NGC 3079, and NGC 4945. In some cases, the optical emission line filaments can be traced back into SF regions in the circumnuclear starbursts. The spatial extent of the starbursts in the disks determines the structure of the outflows. In NGC 253, for example, the diameter of the circumnuclear starburst ring is ~ 600 pc and the base of the optical outflow cone is correspondingly small (e.g., Schulz & Wegner 1992), while in NGC 4666 (mixed type of SF activity; see below) the starburst has a diameter of ~ 6.5 kpc (Dahlem et al. 1997) and the H filaments in the halo are accordingly far apart from each other.

NGC 253: NGC 253, member of the Sculptor group of galaxies, is very nearby (D = 3.2 Mpc). Above the starburst, with its compact stellar clusters (Watson et al. 1996) and radio knots (Antonucci & Ulvestad 1988), optical emission lines trace an outflow cone (Schulz & Wegner 1992). An associated extended CR halo was found by Carilli et al. (1992), and investigated in more detail by Beck et al. (1994). NGC 253 also has an extended halo of hot X-ray emitting ionized gas (Pietsch 1996; Fig. 2). Outside the central cone optical filaments were found as well (Sofue 1987).

Figure 2. Overlay of 0.1 to 2.4 keV X-ray emission contours on top of an optical image of NGC 253 by Pietsch (1996). Reproduced with kind permission by Birkhäuser Verlag, Basel.

NGC 3628: NGC 3628 in the Leo triplet is the starburst galaxy with the most extended hot gaseous halo (Dahlem et al. 1996; their Figs. 5 and 6). The HIM extends to up to 25 kpc above the disk and has a total mass of about 4 x 10⁸ M. Optical emission line gas, which appears to be associated with the outflowing hot gas near the center, was detected by Fabbiano et al. (1990). Reuter et al. (1991) reported the detection of (partly polarized) radio emission outside the thin disk with, however, low signal-to-noise and much smaller extent than the hot ionized gas. More radio emission from the halo is visible in the data by Condon et al. (1982) at 4.9 GHz. CO gas outside the thin disk was detected by Irwin & Sofue (1996).

H I gas in the halo of NGC 3628 had already been found by Haynes et al. (1979); however, it is unlikely that these H I plumes and filaments in the halo have been driven out of the disk by the starburst. The more probable cause for these large-scale disturbances is the ongoing interaction with NGC 3627 and NGC 3623.

NGC 1808: NGC 1808 is not viewed edge-on, but at an inclination of about 60°. Prominent dust filaments, seen in absorption against the stellar continuum of the underlying disk, make it clear that disk-halo interactions are going on (e.g., Laustsen et al. 1987). Baggett & Dahlem (in prep.) are using high-resolution HST images to study dust filaments in NGC 1808 (Fig. 3 and Fig. 4) and other galaxies. The filaments are very narrow and elongated, with typical axial ratios of order 10 to 100; Scarrott et al. (1993) could not detect poloidal magnetic fields associated with these filaments. Reflected light from the nuclear area dominates the polarized optical emission. In the case of NGC 1808 there is observational proof from H I measurements by Koribalski et al. (1993) and optical absorption line studies by Forbes et al. (1992) and Phillips (1993) that the direction of the flow is outward. Although Dahlem et al. (1990) could also not detect polarized radio emission along the outflow, the alignment of the dust filaments in NGC 1808 suggests the existence of a poloidal B-field, along which matter (like the H I gas mentioned above) is streaming. Dahlem et al. (1994b) also reported the detection of soft X-ray emission from hot gas in the outflow cone.

Figure 3. HST WFPC2 F606W image of NGC 1808, showing conspicuous dust filaments pointing from the nuclear region into the halo. The little compass points out the direction of north and east (from Baggett & Dahlem; in preparation).

Figure 4. The same image as in Fig. 3, now after subtraction of a median-filtered background image, mimicking the extended stellar continuum background light. This technique lets the dust filaments stick out clearly (from Baggett & Dahlem; in preparation).

NGC 4945: A member of the Centaurus group of galaxies. H emission tracing a central outflow cone was detected by Lehnert (1992); the cone has been imaged in both optical and near-infrared emission lines by Moorwood et al. (1996), see Fig. 5. Currently, there is no other detection of emission from its halo. Searches for high-z X-ray emitting gas are hampered by the low Galactic latitude and thus high foreground H I column density of NGC 4945 of N_H 1.5 x 10²¹ cm^-2.

Figure 5. The central outflow cone of NGC 4945 in H + [N II] emission, with H2 line contours overlaid (from Moorwood et al. 1996; reproduced with kind permission by Astronomy & Astrophysics). The displayed field of view is 32" x 34".

NGC 3079: In some cases, the exact cause of the disk-halo interactions is uncertain. In NGC 3079, for example, Ford et al. (1986) detected an H cone with an opening angle of ~ 90°. Filippenko & Sargent (1992) found kinematic evidence for an outflow of shock-heated gas from the nuclear region with velocities of up to 2000 km s^-1. Veilleux et al. (1995) argue that simultaneous infall of gas is also possible. The most likely sources driving the general outflow, according to these authors, are SF processes in the central disk. However, based on near-infrared spectroscopy of the nuclear area, Hawarden et al. (1995) argue that the active nucleus in NGC 3079 is the dominant energy source for the outflow. CRs and hot gas associated with the outflow were found by Duric & Seaquist (1988) and Dahlem et al. (1998). Womble et al. (1992) find from optical absorption line studies that the gas in the halo of NGC 3079 has relatively low metallicities, comparable to those of IVCs in our Galaxy and the Magellanic stream.

NGC 2146: In the halo of the very disturbed starburst galaxy NGC 2146, Armus et al. (1995) detected soft X-ray emission. They interpret their X-ray and optical emission line observations as evidence for a galactic superwind. An extended radio halo was mapped at several frequencies by Lisenfeld et al. (1996a); the slope of the radio spectral index away from the disk plane is consistent with diffusion of CR electrons away from the disk.

Summary: There is plenty of evidence that halos of other starburst galaxies also have a complex ISM, like that of M82. The short list of starburst galaxies presented above is complete in one sense: all nearby edge-on starburst galaxies have gaseous halos and thus are included here. The omnipresence of extraplanar hot gas is also one of the results of a soft X-ray mini-survey of all nearby edge-on starburst galaxies by Dahlem et al. (1998).

6.2 Edge-on normal spiral galaxies

6.2.1 NGC 891

NGC 891 is the best example of a ``normal'' galaxy with an extended gaseous halo. It does not have a kpc-scale circumnuclear starburst like M82, but still shows clear signs of ongoing disk-halo interactions. The prime observational advantage of NGC 891 compared to other normal galaxies is that there is no visible disturbance due to interactions with companion galaxies. External influences therefore appear to be negligible and disk-halo interactions in NGC 891 are most likely caused only by internal processes. The following gas components were detected in its halo:

Dust: NGC 891 is morphologically similar to the Milky Way, but has an SFR that is about 2-3 times higher (Dahlem et al. 1995; hereafter DLG95). It has long been known from optical images that dust filaments exist far outside the disk plane of NGC 891 (Keppel et al. (1991) published an extraordinary photographic plate obtained in 1975). Dust in the halo has not yet been detected in emission, e.g., in the FIR (van Driel et al. 1993) or in the millimeter regime (Guélin et al. 1993), although other components of the ISM have been discovered above the thin disk of NGC 891.

eDIG: Optical line emission from eDIG was found out to 4.5 kpc (Dettmar 1990; Rand et al. 1990). Part of the H emission is filamentary, while some of it might be genuinely diffuse; the heating source of this gas is not yet completely understood. Photo-ionization of massive stars in the disk is most likely a significant contributor, but cannot explain the observed high [N II]/H line ratios. Most recently, Rand (1997) detected for the first time the important diagnostic He I optical emission line at 587.6 nm. Dettmar & Schulz (1992) argue that the observed line ratios in the halo of NGC 891 are not compatible with heating by decaying dark matter (DDM), as proposed by Sciama (1990a, b), thus ruling out one potential energy source of the eDIG.

Cosmic rays: Extended synchrotron radio emission was detected by Allen et al. (1978). This synchrotron halo was investigated by others at more frequencies (Klein et al. 1984; Howarth 1990) and/or with higher sensitivity (e.g., Dahlem et al. 1994b). Figure 6 shows that most of the synchrotron emission (~ 90%) comes from the halo, while the thin disk appears only as a small hump in the minor axis profiles. The exponential scale height of the radio halo is of order 4.5 to 5.5 kpc. Investigations of the radio spectral index as a function of z distance above the plane were carried out by Allen et al. (1978), Klein et al. (1984), and Hummel et al. (1991c): a steepening of the radio spectral index above the thin disk indicates energy losses of CRs escaping from the disk. Out in the halo, at z 1 kpc, the slope of the spectral index might flatten again, which would indicate particle reacceleration in a galactic wind (e.g., Hummel et al. 1991c).

Figure 6. Minor axis cuts through the synchrotron emission distribution of NGC 891 at 1.49 GHz, showing a clear distinction between the thin disk and the exponentially declining halo emission.

Magnetic fields: Polarized radio synchrotron emission, tracing large-scale ordered magnetic fields in the halo of NGC 891, was measured by Hummel et al. (1991a), Sukumar & Allen (1991), and Dahlem et al. (1994a). The magnetic field strength in the halo of NGC 891 is about 8 µG, somewhat weaker than the disk field. In NGC 891, polarized optical emission has also been found, implying that in parts of the galaxy, the dust is aligned along vertical magnetic field lines (Scarrott & Draper 1996; Fendt et al. 1996).

Molecular gas: García-Burillo et al. (1992) detected CO in the halo of NGC 891. Handa et al. (1992) reported the detection of one particular CO gas filament perpendicular to the plane above one of the most actively star-forming regions in the disk. This filament originates in an H II region detected via its radio continuum emission (Dahlem et al.; in prep.).

Hot ionized gas: Bregman & Pildis (1994) and Bregman & Houck (1997) found soft X-ray radiation, tracing hot gas above parts of the disk of NGC 891. The gas temperature is about 3.6 x 10⁶ K, with a total gas mass of ~ 10⁸ M.

Summary: Thus, although NGC 891 has no circumnuclear starburst, it nevertheless has a prominent gaseous halo in which many different ISM phases coexist. The halo emission is very extended, as is the distribution of SF regions in the disk and - although there is little doubt about the origin of the halo gas (the high-level SF in the galaxy disk) - many details of the disk-halo interactions, in particular the heating of the optical emission line gas, are not yet understood (Rand 1997). For a review, with emphasis on the DIG in NGC 891, see Dettmar (1992).

6.2.2 Other normal edge-on galaxies

In DLG95 we showed that the properties of gaseous halos depend on the level of SF activity in the underlying disks. The most actively star-forming galaxies have the most prominent halos. There appear to be three categories of normal galaxies with respect to their SF and halo properties.

Galaxies with high SFRs: In these systems a multitude of individual disk-halo interactions creates pervasive gaseous halos.

NGC 5775 is in some respects similar to NGC 891. Its SFR is much higher than in the Milky Way and it has no central starburst, just high-level SF activity spread over a large part of the galaxy disk. Duric & Bloemen (1989) detected an extended radio halo around NGC 5775, which is confirmed by data by Golla (1996; priv. comm.). Associated H from the halo was detected by Lehnert & Heckman (1996a) and several large H I arcs were found in NGC 5775 by Irwin (1994). Although interacting with its companion, NGC 5774, the optical appearance of NGC 5775 seems quite undisturbed and therefore the assumption appears to be justified that the halo emission (except possibly the H I) is not predominantly caused by tidal disturbances. Note that the SFR of NGC 5775 is very high; it is a borderline case which might also be considered a starburst galaxy.

NGC 2820 is one out of six galaxies in the sample of 181 spirals by Hummel et al. (1991b) showing indications of a radio continuum halo. This galaxy has not yet been investigated in depth and, although possibly disturbed by an interaction partner, might offer a chance to gain some new insights.

Galaxies with intermediate SF levels: In these galaxies disk-halo interactions occur only in a few locations where (locally) the SF rate is high.

NGC 3044, NGC 4565, NGC 5907, and other edge-on galaxies have SFRs and SN rates similar to those of our Galaxy (DLG95). Only marginal - if any - signs of emission from gaseous halos have been detected in these systems, all of which are at distances of about 10 to 20 Mpc.

In NGC 3044 only one huge radio continuum spur, but no other radio emission, was found in the halo (Sorathia 1995). In NGC 4565 and NGC 5907 Hummel et al. (1984) could find only marginal indications of the existence of radio halo emission. While NGC 891 does show some halo emission at 10.7 GHz, Dumke et al. (1995) could not detect any from galaxies such as NGC 4565, NGC 5907, and NGC 7331, all of which have intermediate levels of SF. In NGC 4565 there are only a few tiny dust filaments oriented perpendicular to the disk and a tentative detection of soft X-ray emission close to the disk was reported by Vogler et al. (1996). In NGC 5907 and NGC 7331, the polarized optical emission observed by Fendt et al. (1996) is consistent with reflection of stellar emission from the disk. No clear signs of vertical dust filaments were detected. Neininger et al. (1996) found dust emission from the warped disk of NGC 4565, but not from its halo.

Despite many searches in M31, the closest spiral galaxy, no conclusive evidence has yet been found for the existence of a gaseous halo, indicating that non-detections in more distant galaxies with similar SF levels, like e.g., NGC 4565 or NGC 5907, might not be entirely due to sensitivity limitations.

Galaxies with low SF rates: Such systems show no detectable signs of disk-halo interactions.

Galaxies like NGC 4244 represent the lowest level of SFR and SN rates (DLG95). This galaxy (currently) appears to be almost entirely quiescent; its radio continuum total flux is so extremely low that - although located at a distance of only 3.1 Mpc (like M82) - even the disk emission is hardly detectable (Hummel et al. 1984). Not surprisingly, no indication has been found of any emission from its halo. For an image of the H/[S II] emission line ratio in NGC 4244 see Marcus (1997).

6.3 Galaxies with both starbursts and wide-spread SF

Some of the galaxies mentioned above, e.g., NGC 4666 and NGC 4631, exhibit both circumnuclear starburst activity and high-level SF in their outer disks. This leads to halo emission which can be subdivided into two components: the typical hour-glass shaped nuclear outflow cone of starburst galaxies, plus more extended emission above the outer disk (Fig. 7; from Dahlem et al. 1997). The disk of NGC 4666, for example, beyond the radius of its starburst ( 3 kpc) is that of a normal spiral galaxy. The SFR is high and individual outflows can be detected far from the center of NGC 4666 that have no direct connection with its activity. An H filament was detected above one of the most conspicuous complexes of H II regions not associated with the central starburst, thus providing evidence for an outflow driven by the local SF activity.

Figure 7. H + [N II] image of NGC 4666, with contours of 4.89 GHz radio continuum emission and magnetic field vectors superimposed.

6.3.1 NGC 4666

Sukumar et al. (1988) presented initial evidence for the existence of a radio halo around NGC 4666. Dahlem et al. (1997) detected halo emission in several wavebands - optical emission lines, radio continuum (incl. polarized radio emission; see Fig. 7), and soft X-ray emission from hot gas. This means that several different phases of the ISM are participating in the outflow from both the central starburst and SF regions in the outer disk. Dahlem et al. (1997) presented the first spatially resolved optical line diagnostics for several lines, including [N II]/H vs. [O III]/H along the minor axis of a galaxy. The line ratios exhibit three emission components along the minor axis of NGC 4666: a low-ionization narrow emission-line region (LINER; Heckman et al. 1983) nucleus in the center, emission typical of photo-ionized gas in H II regions in the disk, and high [N II]/H and [O III]/H line ratios in the halo up to 5 kpc from the plane, which are not consistent with pure photo-ionization, but imply shock-heating of the gas (see Section 8.4 for more details). At a distance of D = 26.3 Mpc, NGC 4666 is currently the most distant galaxy in which halo gas has been detected in emission. Systems at higher redshifts are currently only detectable in absorption.

6.3.2 NGC 4631

NGC 4631 is the spiral galaxy with the most extended radio halo (Ekers & Sancisi 1977; Hummel et al. 1988; Hummel et al. 1991c; Golla & Hummel 1994). The latter identified a radio continuum spur reaching from the brightest SF region in the central disk plane into the northern halo of NGC 4631, up to a z-distance of ~ 10 kpc. In addition, both diffuse and filamentary H was detected in the halo (Rand et al. 1992; Golla et al. 1996). NGC 4631 also has an extended hot gaseous halo, which was imaged by Wang et al. (1995) and Vogler & Pietsch (1996).

The magnetic field in the halo of NGC 4631 is extraordinary. It is well ordered, highly symmetric, and everywhere directed away from the central disk (Golla & Hummel 1994). Such behavior has not been observed in any other galaxy - and the reason is not yet understood.

Extended H I spurs and arcs in NGC 4631 have long been known (Weliachew et al. 1978). However, as in the case of NGC 3628 (above), these gas streams are most likely not caused by SF in the disk of NGC 4631, but by tidal interactions with its companions NGC 4656 and NGC 4627 (Combes 1978). Donahue et al. (1995) found evidence that the entire NGC 4631 group of galaxies is immersed in an extended ionized gas cloud that is visible in H emission. Due to the disturbance of NGC 4631, the interpretation of the processes dominating the outflow and the excitation of gas in its halo is even more difficult than in other systems.

6.4 Starbursts and active galactic nuclei

Some starburst galaxies, like NGC 1808 and NGC 4945, have mixed types of (circum-)nuclear activity, showing signs of both low-luminosity active galactic nuclei (AGNs) and at the same time kpc-scale circumnuclear starbursts, often in rings (Awaki & Koyama 1993; Colbert et al. 1996). The question as to which of the two sources dominates the disk-halo interactions must be addressed individually for each galaxy. In NGC 1808, for example, the widespread distribution of the observed dust filaments suggests that the starburst dominates the disk-halo interactions. The primary energy source for the outflow in NGC 3079, however, has not yet been identified (Veilleux et al. 1995; Hawarden et al. 1995). A distinction between the two processes can be attempted morphologically (by measuring the width of the base of the outflow) and spectroscopically (by using line ratios to distinguish between different continua heating the gas).

6.5 Irregular galaxies

The title of this paper implies that the emphasis here will be on L_* spiral galaxies. Nevertheless, a few words on Sm and Irr type galaxies are appropriate. These galaxies do not have well-defined disks as those of Sa - Sd type spirals; therefore, it is difficult to distinguish between disks and halos, even in an edge-on viewing geometry. Two examples of such nearby edge-on galaxies are NGC 55 and NGC 2188.

An extremely sensitive H image of NGC 55 by Ferguson et al. (1996) exhibits a wealth of filaments, loops and arcs in optical line emission. Dahlem et al. (1998) find low surface brightness soft X-ray emission associated with some of the H structures; however, there is no evidence for any one of them being clearly ``extraplanar'', i.e., forming part of a halo.

Domgörgen et al. (1996) and Domgörgen & Dettmar (1997) observed the edge-on irregular galaxy NGC 2188. Here, clear signs of disk-halo interactions were found, with H filaments reaching as far as > 500 pc above the most actively star-forming region in the disk. Expanding gaseous shells were kinematically identified through H I line observations.

More information on other dwarf and irregular galaxies, including e.g., NGC 1569, NGC 1705, NGC 4449, and NGC 5253, can be found for example in Heckman et al. (1995), Bomans et al. (1997), Meurer et al. (1992), Marlowe et al. (1995), Hunter & Gallagher (1997), and references therein.

6.6 Quasar absorption line studies

The lines of sight towards powerful background continuum sources provide an independent source of information on the absorber. This motivated searches for background light sources close to nearby (extended) galaxies (e.g., Bowen et al. 1995, 1996, and references therein). These authors report that the outer parts of galaxies (including their halos) have sufficient column densities to produce Mg II absorption lines and that galaxy halos are Ly absorbers with radii of up to 400 (H₀ = 75 km s^-1 Mpc^-1) kpc, and covering factors of ~ 40%.

Due to the small number of sightlines per intervening object only limited spatial information can be obtained on the foreground objects. On the other hand, the high sensitivity in absorption lines (10^13-14 cm^-2, compared to 10^18-19 cm^-2 in emission lines) renders possible very detailed studies of element abundances along these few sightlines.

Many absorbers found in front of QSOs show metal absorption lines (e.g., Vogel & Reimers 1995), indicating the presence of matter that has been metal-enriched through star formation processes. The absorption properties of the gas are so heterogeneous that simple object geometries are ruled out. It is much more likely that multiple absorptions occur in individual galaxies (e.g., Churchill et al. 1996). The most likely identification of the intervening systems is that they are the disks and halos of galaxies, which have been metal-enriched by SF-driven outflows (Heckman et al. 1993). NGC 1569, a nearby dwarf galaxy, is one object in which an outflow of matter at or close to escape velocity has been observed (Heckman et al. 1995).

6.7 Disk-halo interactions in face-on galaxies

Currently, only few methods exist to trace disk-halo interactions in face-on galaxies:

``Blue'' wings of (optical) emission lines, emanating from outflowing gas moving toward us, have been detected in spectra of several starburst galaxies (e.g., Lehnert & Heckman 1996a). There is also evidence for a blueshift of optical emission lines with respect to the H I lines in FIR galaxies (Mirabel & Sanders 1988), implying outflows of optical emission line gas.

Schulman et al. (1994) traced extragalactic H I HVCs as evidenced by faint wings in the integral H I spectra of 10 out of 14 galaxies in their sample, indicating the presence of extraplanar high-velocity H I gas. More details on one particular galaxy, NGC 5668, were presented by Schulman et al. (1996). In NGC 6946 Kamphuis & Sancisi (1993) also found H I HVCs.

Berkhuijsen et al. (1997) deduced the existence of a magneto-ionic halo from a Faraday rotation analysis based on multi-band radio data of M51. This is the first time that a radio halo has been (analytically) separated from the disk emission of a face-on galaxy. Similar results have been obtained by Ehle et al. (in prep.) for M83.

These three new observing techniques will help to expand our knowledge on disk-halo interactions in the future, because there are many more face-on (or mildly inclined) galaxies than edge-on. In particular, optical and high-resolution H I spectroscopy of individual bright H II regions, not only in starbursts but also in the outer disks of actively star-forming galaxies, will provide new results in the next decade.