In galaxies other than our own, the wide-spread H+ is most often referred to as the diffuse ionized gas, or DIG. While its physical properties can be measured in much more detail for the interstellar medium of our Galaxy, the detection of the DIG in other galaxies provides the much needed "outside" perspective. Rand (1997, 1998), for example, was able to detect the DIG far into the halo of the edge-on spiral galaxy NGC 891 and found that not only was [S II] / H anomalously high, as observed in the solar neighborhood, but that [S II] / H, [N II] / H, and even [O III] / H increased significantly with increasing distance from the midplane. This opened up a broader discussion about the ionization and heating processes within the gas and their variation with location within a galaxy.
Furthermore, observations of other galaxies provide important new information about the links between the DIG and global properties of galaxies, such as their star formation activity. In particular, the structure of this gas perpendicular to the galactic plane, that is, in the main direction of the gravitational potential, is an excellent tracer of the dynamics of the interstellar medium driven by energetic galactic processes. The observations corroborate the picture of a dynamic interstellar medium driven by multiple and clustered supernova, producing the so-called disk-halo interaction between the star formation regions near the midplane and energized interstellar matter that extends a kiloparsec or more above the disk. Early discussions of these ideas can be found in Supernovae, the Interstellar Medium, and the Gaseous Halo: The Swashbuckler's Approach by Heiles(1986), The disk-halo interaction - Superbubbles and the structure of the interstellar medium by Norman and Ikeuchi (1989), and Galactic worms by Koo et al. (1992).
3.1. Diffuse ionized gas in the halo and star formation in the disk
From an observational point of view, the study of the warm H+ in galactic halos-the extraplanar diffuse ionized gas-is a good start toward understanding the nature of gaseous halos and the disk-halo connection. Of all tracers of halo gas, the warm ionized gas is the easiest to observe with regard to sensitivity and resolution (Dettmar 1992, 1998, Rand 1997, 1998), allowing us to study the global influence of the energy released by young and massive stars into the interstellar medium.
A halo component of the DIG for external galaxies was first discovered in NGC 891 (Dettmar 1990, Rand et al. 1990), followed by studies of a number of other galaxies (e.g., Pildis et al. 1994, Rand 1996, Rossa and Dettmar 2000). In Rossa and Dettmar (2003a, 2003b) a large sample of edge-on spiral galaxies clearly demonstrates a correlation of DIG in the halo with the star formation rate (SFR) in the disk (see also Miller and Veilleux 2003 for comparison). This survey covered a broad range in SFR extending the observations to less active galaxies. Until then, emphasis had been given mainly to galaxies with high SFR or even starburst galaxies (Lehnert and Heckman 1995). This relationship between the presence of DIG and far infrared (FIR) emission (a measure of the SFR) is shown in Fig. 5. If the star formation rate per unit area is low, as determined by the FIR luminosity normalized to the disk surface area, the presence of halo DIG indeed is observed to diminish.
Figure 5. The dependence of DIG detections in halos of galaxies on the normalized star formation rate (per unit area). The ratio of the FIR fluxes at 60 µm and 100 µm (S60 / S100) is given versus the FIR luminosity normalized to the optical galaxy diameter at 25th mag in units of 1040 erg s-1 kpc-2. Filled symbols indicate detections of H+ gas in the halo while open symbols represent non-detections. Galaxies from the () starburst sample of Lehnert and Heckman (1995) are included with more normal spirals. Integrated values for the Galaxy are taken from Bloemen et al. (1990) and Cox and Mezger (1989) for the FIR. A Galactic disk radius of 22.5 kpc is assumed. Adopted from Rossa and Dettmar (2003a).
The FIR luminosity per unit area therefore seems to be a promising indicator for the presence of halo H+ and suggests that there is a minimum SFR per area required to drive the disk-halo interaction (Rossa and Dettmar 2003a). Given the known large uncertainties in the normalization of supernova rates from FIR-fluxes, a reasonable estimate for the local break-out condition is on the order of ~ 15 SNe over the lifetime of the O star association. An obvious shortcoming of this analysis is the use of global measurements for the FIR luminosities. Future observations with higher angular resolution, for example, with the Spitzer-satellite, could allow a much better comparison of the local SFR with associated halo gas properties.
3.2. The disk-halo connection and hot gas
The scenario of the disk-halo connection predicts outflows of hot (106 K), supernova-created gas away from the disk and into the halo via superbubbles (or "chimneys"). In this way, transparent (i.e., H I-free) pathways are then also provided for the transport of ionizing radiation into the halo and beyond. For galaxies with strong starburst activity this is well supported by observations (see, e.g., Cecil et al. 2002, Strickland et al. 2004a, Strickland et al. 2004b, Veilleux et al. 2005) and described well by theory and numerical modeling (e.g., Mac Low and Ferrara 1999). However, for more normal (i.e., non-starburst) galaxies, we have little direct evidence for such outflows. High resolution ground-based (e.g., Howk and Savage 2000) and HST imaging (e.g., Rossa et al. 2004) studies of the DIG distribution in edge-on galaxies does not reveal the number and specific morphology of superbubble- or chimney-like structures predicted by some models (Norman and Ikeuchi 1989). Such structures have been found, but their number is small. The difficulty in finding them could be an observational problem, e.g., confusion due to superpositions along the line of sight, or it could mean the the current models of the disk-halo connection need further work. Thus, the idea of actual mass exchange between disk and halo is still a working hypothesis only.
On the other hand, there is circumstantial evidence for hot gas outflows in more normal disk galaxies that is indicated by the detection of diffuse interstellar X-ray emission, both in our Galaxy (e.g., Smith et al. 2007) and others (e.g., Wang et al. 2001). In a sample by Ranalli et al. (2003) that included disk galaxies with more normal star formation rates, a correlation was found between the galaxy's global X-ray emission, presumably associated with the very hot (106 K) gas located inside supernova remnants and superbubbles, and the amount of star formation activity in the disk. With the current generation of X-ray satellites it is now possible to extend the X-ray studies from the starbursting and luminous galaxies to less active galaxies. One result of a small survey with XMM-Newton is shown in Fig. 6. The EPIC X-ray image of the galaxy NGC 5775 is integrated over all energies and summed over all detectors. It demonstrates the presence of smaller scale X-ray emission features associated with H filaments far above the disk.
Figure 6. The X-ray halo NGC 5775 from integrated EPIC XMM-Newton observations exhibits substructure correlating on galactic scales with filaments of diffuse ionized gas and spurs in the radiocontinuum. Adopted from Tüllmann et al. (2006).
The diffuse H+ in galactic halos is, in most of the observed cases, also associated with synchrotron radiocontinuum halos, suggesting that there is also a relationship between DIG and the presence of cosmic rays and magnetic fields in the halo. For example, NGC 5775 shows a prominent radio continuum halo, and a study of its polarization characteristics with the Very Large Array (VLA) radio synthesis telescope reveals a highly structured large-scale magnetic field with an ordered component to the field lines which opens up into the halo (Fig. 7). This particular structure of the magnetic field could be of interest for the interpretation of global dynamo theory in galaxies and is discussed in more detail by Beck et al. (1996). The condition that allows cosmic rays to break out of the disk as a function of star formation rate per unit area is discussed by Dahlem et al. (1995). A detailed study of the kinematics of the DIG in NGC 5775 is presented by Heald et al. (2006). A compilation of observations of edge-on galaxies in H, radio continuum, and X-rays is given in Dettmar (1998) and Rossa and Dettmar (2003a, 2003b).
Figure 7. VLA radiocontinuum map of NGC 5775 at 6cm. The polarized intensity is given in contours on a grey scale representation of the H emission. The bars represent the magnetic field direction and strength. Adopted from Tüllmann et al. (2000), courtesy M. Soida.
In summary, studies of edge-on galaxies have shown that warm ionized gas halos are found in galaxies where the SFR per unit area is sufficiently high. Typically these layers of extraplanar DIG can be traced out to distances of z 1000-2000 pc, sometimes even up to 5000 pc or more from the mid-plane of the disk. The H+ halos seem to be associated with halos of cosmic rays and X-ray emitting plasma, which is expected if superbubble or chimneys provide absorption free pathways for hydrogen ionizing photons as well as being conduits for the vertical transport of hot gas and cosmic rays away from the disk and into the halo.