3. OBSERVATIONAL DIAGNOSTICS OF SUPERWINDS

3.1. X-Ray Observations of Superwinds

The overall X-ray properties of starburst galaxies have been recently reviewed by Petre (1993), so we will focus on the interplay between the X-ray data and the superwind phenomenon. X-ray observations of superwinds are crucial to our understanding of the phenomenon, because (potentially at least) they offer the most direct probe of the hot, tenuous superwind fluid itself (rather than simply probing the interaction of the wind with ambient gas, as is most likely the case for the optical line emission).

The characteristic temperature corresponding to the complete thermalization of the kinetic energy from an ensemble of supernovae and winds from massive stars is about kT = 10 keV (e.g., 10⁵¹ ergs per 10 M) - see Section 2 above. Thus we expect hard X-ray emission from a galaxy driving a superwind to come from the hot and tenuous supernova/wind-heated gas inside the starburst (i.e., inside the sonic radius of the wind). Since the corresponding terminal velocity of the outflowing superwind fluid is several thousand km s^-1, hard X-rays will also arise from internal shocks in the superwind (cf. Tomisaka & Ikeuchi 1988; Balsara, Suchkov, & Heckman 1993). In the idealized expanding bubble discussed in Section 2 above, these X-rays will be coming from zone 3.

Soft X-rays will be produced by dense ambient material that is shock-heated and/or evaporated by the superwind (cf. Watson, Stanger, & Griffiths 1984; White & Long 1991). In the idealized 'expanding bubble' model discussed in Section 2, this would correspond to the thin shell of shocked ambient gas at the interface with the undisturbed ambient gas (zone 4). In a more realistic situation, the soft X-rays will arise as the wind collides with, shock-heats, and evaporates density inhomogeneities ('clouds') in the galaxy halo. H I observations of starbursts show that such cool, dense material indeed exists in the environs of starbursts (see Section 3.4 below). Note that the typical outflow velocity of dense, wind-accelerated ambient material is observed in the optical to be several hundred km s^-1 (see Section 3.2 below), corresponding to post-shock temperatures of kT ~ 200 eV.

This theoretical/phenomenological picture is in reasonable agreement with the limited amount of X-ray data presently available for starburst galaxies. The X-ray luminosity of starburst galaxies correlates rather well with both the IR luminosity (e.g., Griffiths & Padovani 1990; David, Jones, & Forman 1992; Green, Anderson, & Ward 1992) and the Lyman continuum luminosity (Ward 1988) of the starburst. Moreover, the observed average ratio of X-ray and bolometric starburst luminosities agrees roughly with the predictions of simple theoretical models for starburst-driven superwinds (HAM - and see the discussion in Section 2 above). X-ray spectra of starbursts imply characteristic temperatures of kT = 6 to 9 keV for the global X-ray emission (e.g., Kim, Fabbiano, & Trinchieri 1992; Ohashi et al. 1990; Petre 1993), again in agreement with simple models (see Section 2 above).

Spatially-resolved X-ray images of well-studied starburst galaxies whose large-scale stellar disks are viewed nearly edge-on (M 82, NGC 253, NGC 2146, and NGC 3628) show striking X-ray 'halos' or 'plumes' that can extend out to a radius of 10 kpc or more along the galaxy minor axis. These have been interpreted as direct evidence for galactic superwinds (e.g., Watson, Stanger, & Griffiths 1984; Fabbiano 1988; Fabbiano, Heckman, & Keel 1990; Heckman & Fabbiano 1993; Armus, Heckman, & Weaver 1993). The transgalactic-scale X-ray nebulae associated with such 'ultraluminous' IR galaxies as Arp 220, NGC 3690, and Mrk 266 are likely to be more energetic versions of this phenomenon (Eales & Arnaud 1988; Armus, Heckman, & Weaver 1993).

In general, the estimated thermal energy content and mass of the X-ray gas are consistent with the time-integrated kinetic energy and mass output of the starburst (cf. HAM and references therein). However, these estimates are based on several simplifying assumptions. The most important of these are that all the observed X-rays arise from hot gas at a uniform - and high - temperature equal to that deduced on the basis of the integrated X-ray spectrum, and that this gas has a volume filling factor of unity. As we now discuss, the actual situation is likely to be considerably more complex.

The X-ray emission from the halo of M 82 is significantly softer than that from the central starburst (Petre 1993), as is also the case in NGC 3628 (Fabbiano, Heckman, & Keel 1990; Heckman & Fabbiano 1993) and NGC 3690 (Armus, Heckman, & Weaver 1993). Within the context of the superwind model, this could imply that the wind has broken free of the galaxy and has suffered severe adiabatic cooling on its way out (cf. CC and Equation 6 above). However, this is unlikely to be the case, since the X-ray nebulae are far too bright to simply be an adiabatically-cooled free-wind (e.g., Mathews & Doane 1993). Our new ROSAT images of several starbursts with superwinds also show a considerable amount of fine-scale structure: X-ray-bright 'lumps' and 'filaments' with sizes of a few hundred pc to a few kpc (Heckman & Fabbiano 1993; Armus, Heckman, & Weaver 1993). We believe it is most likely that the soft (kT Petre (1993) emphasizes that BBXRT data on starbursts show clear evidence for multiple X-ray spectral components, and Armus, Heckman, & Weaver (1993) have reached similar conclusions based on ROSAT PSPC data. X-ray images of the two nearest starburst galaxies (M 82 and NGC 253) also show that the X-ray emission is produced by both the spatially-extended component discussed above and a collection of discrete sources in or near the starburst (Watson, Stanger, & Griffiths 1984; Fabbiano & Trinchieri 1984). Thus, it may well be that much of the hard X-ray emission from starbursts is produced by the ensemble of X-ray binary systems in the starburst (e.g., Griffiths & Padovani 1990). Inverse Compton scattering of IR photons off the relativistic electrons responsible for the radio continuum emission may also contribute substantially to the hard X-ray emission (e.g., Schaaf et al. 1989; but Seaquist & Odegard 1991 and Tsuru 1992 argue otherwise). A composite thermal plus nonthermal origin for the hard X-rays could explain the relative weakness of the 6.7 keV Fe K line in M 82 and NGC 253 compared to expectations for optically thin thermal emission from gas with solar Fe abundances and kT = 5 to 10 keV (Tsuru 1992; Ohashi et al. 1990; Petre 1993).

One of the most interesting X-ray observations of a likely superwind is the Ginga data on the center of our own Milky Way (Yamauchi et al. 1990). Emission-line images using the 6.7 keV Fe K line imply a region of gas with a temperature of about 10⁸ K, dimensions of about 270 × 150 pc, gas pressures of P/k = 8 × 10⁶ K cm^-3, and total thermal energy content of 6times10⁵³ ergs. Since the sound speed in this gas (~ 1500 km s^-1) greatly exceeds the escape velocity from the Galactic Center, the gas should flow out as a high speed wind (unless it is confined by some other mechanism). If this outflow is a steady-state phenomenon, collisional heating with a supernova rate of about 0.6 per century within this volume is required to balance adiabatic cooling.

To summarize, while the current situation regarding X-ray emission from superwinds is unclear, we can expect dramatic progress over the next few years as the avalanche of new ROSAT, BBXRT, and ASTRO-D data on starbursts is analyzed and digested. Certainly the data already in hand are very tantalizing.

3.2. Optical Line Emission from Superwinds

Most of the existing data on superwinds concern the optical line emission associated with such outflows. Such data provide a whole array of diagnostics of the dynamical and physical state of the outflow.

In the simple schematic pictures described in Section 2 above, the optical line emission would not arise from the very hot and tenuous superwind fluid itself (the cooling times are excessively long, and most of the relatively feeble radiation that is produced there is in the form of hard X-rays). Instead, we would expect the optical line emission to come from relatively dense ambient material into which the wind's ram pressure drives slow (< few hundred km s^-1) radiative shocks (e.g., McKee & Hollenbach 1980 and see Section 2.2 above). In the wind-blown bubble 'onion-skin' model, this material would be the thin, dense shell of compressed and shock-heated ambient gas at the leading edge of the bubble. In a more realistic situation in which blow-out has occurred, and/or in which the wind is propagating through a inhomogeneous, multi-phase medium, the optical line emission will come from clouds (e.g., fragments of the ruptured shell, pre-existing density inhomogeneities in the halo, or material carried out of the galactic disk by the superwind).

We therefore expect that the optical data will provide a detailed - but indirect - view of the superwind phenomenon. The largest compilations of such data are found in the recent spectroscopic and narrow-band imaging surveys by HAM, Armus, Heckman, & Miley (1989, 1990 - hereafter AHM89, AHM9O), Lehnert (1992 - hereafter L92), and Lehnert & Heckman (1993 - hereafter LH).

3.2.1. Structure and Luminosity

Galaxies selected to be both luminous and warm in the far-IR have optical emission-line nebulae whose size and luminosity correlate reasonably well with the far-IR luminosity, and hence with the estimated formation rate of massive stars (AHM9O; L92; LH). The H luminosities of the extra-nuclear portion of the nebulae (e.g., well outside the starburst region) are just consistent with the predictions of the superwind model (see Equation 11 and the associated discussion in Section 2 and in HAM).

We (L92, LH) have recently completed an H imaging survey of a sample of edge-on disk galaxies selected on the basis of far-IR flux and color, and find many examples which show emission-line filaments, loops, or bubbles extending out one to ten kpc along the optical minor axis of the galaxy (four examples are shown in Figure 2). In general, we find a pronounced excess of optical line emission along the optical minor axis compared to what would be expected for ordinary emission-line gas confined to the galactic disk. At the highest levels of IR luminosity (L > few × 10¹¹ L), the H nebulae approach transgalactic dimensions (30-100 kpc), with the large-scale morphology often dominated by filaments, loops, or bubbles (e.g., Heckman, Armus, & Miley 1987; AHM90). Some examples of such nebulae are shown in Figure 3.

Figure 2. H+[NII] images of IR-bright, edge-on disk galaxies exhibiting large-scale optical emission-line filaments extending several kpc out of the galaxy disk and into the halo. (a) NGC 660; (b) NGC 3079; (c) NGC 3628; and (d) NGC 4666 - see AHM90; Fabbiano, Heckman, & Keel 1990; L92; LH).

Figure 3. H+[NII] images of two galaxies with IR luminosities greater than 1045 erg s-1: (a) Arp 220 (from Heckman, Armus, & Miley 1987) and (b) Mrk 266 (from AHM90). These represent the superwind phenomenon at the high-luminosity end, where the emission-line nebulae are tens of kpc in size

In the few starburst galaxies for which both H and X-ray images are available, there is a clear correspondence between the two images (e.g., Watson, Stanger, & Griffiths 1984; Armus, Heckman, & Weaver 1993). In some cases, the optical line emission is preferentially located along the outer boundary of the X-ray nebula (Fabbiano, Heckman, & Keel 1990; Heckman & Fabbiano 1993; McCarthy, Heckman, & van Breugel 1987 - and see Figure 4). This suggests that the optical line emission arises at the interface between the hot superwind and the ambient interstellar gas in the halo of the starburst galaxy (in good agreement with both the theoretical picture sketched above and the kinematics of the optical emission-line gas in several well-studied cases, as summarized below). New ROSAT images should allow us to conduct many more such comparisons, in order to better test this idea.

Figure 4. Overlaid H+[N II] image (greyscale) and Einstein HRI X-ray image (contour plot) of the central few kpc of the prototypical IR starburst galaxy NGC 253. The starburst nucleus is located in the upper right corner. Note how the filamentary optical line emission to the SE of the nucleus seems to enclose the X-ray plume (cf. McCarthy, Heckman, & van Breugel 1987; Fabbiano & Trinchieri 1984; LH; L92).

3.2.2. Kinematics and Dynamics

The kinematics of the optical emission-line gas associated with IR-luminous galaxies also suggest that outflows are common. Optical spectra of the nuclei of IR-luminous galaxies show that they often have blue-asymmetric emission-line profiles (e.g., AHM89; LH; Phillips 1992). Mirabel & Sanders (1988) find that the optical emission-line velocities in the nuclei of high-luminosity IR galaxies are blueshifted with respect to the galaxy systemic velocity by an average of about 100 km s^-1. Both the blue-asymmetric profiles and blueshifts suggest an outflow of ionized gas whose redshifted back side is obscured by dust.

For the edge-on starburst galaxies investigated by L92 and LH, in many cases the line widths actually increase with increasing radius along the minor axis. The emission-lines are also systematically broader along the minor than along the major axis (with typical FWHM's of 150 to 350 km s^-1 vs. × 100 to 200 km s^-1 respectively). The line widths along the minor axis correlate strongly with the starburst IR luminosity, but not with the galaxy rotation speed, thus favoring a starburst-driven outflow over simple orbital motions in the galaxy gravitational potential (see Figure 5). This interpretation is further supported by the strong trend found by L92 and LH for the largest measured velocity shears along the minor axis to occur in the galaxies viewed more nearly face-on (as expected for a starburst-driven radial outflow along the minor axis) and in the galaxies with the highest IR luminosities (see Figure 6).

Figure 5. Summary of the kinematics of the emission-line gas located outside the central starburst and along the optical minor axis for the IR-selected edge-on disk galaxies investigated by L92 and LH. The x-axis in both plots is the FWHM of the [N II]6584 line. This line width correlates strongly with the IR luminosity of the galaxy (left plot), but not with the rotation speed of the galaxy (right plot), implying that the gas dynamics is influenced much more strongly by the starburst-driven wind than by gravity.

Figure 6. Summary of the kinematics of the emission-line gas located along the optical minor axis for the IR-selected edge-on disk galaxies investigated by L92 and LH. These two plots show that the velocity shear along the minor axis is largest in the galaxies that are viewed more nearly face-on (left plot) and in the galaxies with the largest IR luminosities (right plot).

The detailed kinematic properties of the gas located along the minor axes of such nearby and well-studied edge-on starbursts as M 82 (e.g., Bland & Tully 1988; HAM), NGC 253 (Ulrich 1978; HAM), NGC 3079 (HAM; Filippenko & Sargent 1992), and NGC 4945 (HAM) provide more direct evidence for outflows. All four galaxies have bubble-like or filamentary emission-line nebulae that protrude out about a kpc along the optical minor axis. The kinematics of these structures (broad, double-peaked emission-line profiles in the center of the bubble, narrowing to single-peaked profiles along the bubble periphery) imply that they are either expanding bubbles or the walls of cone-like or cylindrical outflows, with inferred outflow/expansion speeds ranging from about 200 km s^-1 to nearly 1000 km s^-1 (see Figure 7). The sizes and expansion speeds of these structures are in good agreement with the quantitative predictions of the simple wind-blown-bubble model discussed in Section 2 (see equations 9 and 10 above, and HAM). Additional examples of similar outflow structures are found in the composite Seyfert/starburst galaxies NGC 1365 (Phillips et al. 1983a), NGC 5506 (Wilson, Baldwin, & Ulvestad 1985), NGC 7582 (Morris et al. 1985), and Mrk 509 (Phillips et al. 1983b).

Figure 7. Long-slit spectrum of (from left to right) the [N II]6548, H, [N II]6584 emission-lines in the central few kpc of NGC 253. The slit was oriented in a position angle of 135 degrees, along the minor axis of this edge-on galaxy. Note that each of the three lines has a double-peaked profile shape throughout a region about 600 pc in extent to the SE of the nucleus. This line-splitting has a magnitude of about 300 to 400 km s-1, and suggests either an expanding bubble or outflow along the surface of a hollow cone-like structure (cf. HAM). This region of expanding gas also corresponds to the bright X-ray plume seen in Fig. 4 (cf. Fabbiano & Trinchieri 1984; McCarthy, Heckman, & van Breugel 1987).

Qualitatively similar, but even larger and more energetic examples of such expanding structures, are found in several 'ultraluminous' IR galaxies (HAM). These are the 'double-bubble' emission-line nebula in Arp 220, the central 'hour-glass' structure in NGC 6240, and the extraordinary nebula associated with IRAS 00182-7112 in which line-widths are nearly 1000 km s^-1 over a region 30 kpc in extent. Colina, Lipari, & Macchetto (1991) have also published kinematic evidence for a superwind in the ultraluminous galaxy IRAS 19254-7245.

On the 'micro-starburst' level, Meurer et al. (1992) have found a kpc-scale bubble of ionized gas expanding at about 50 km s^-1 in the core of the post-starburst dwarf galaxy NGC 1705. They show that this expansion was probably powered by the kinetic energy supplied by a burst of star-formation that occurred some 10 Myr ago. Roy et al (1991) have found a kinematically similar 200 pc-scale expanding bubble of ionized gas in the starbursting dwarf irregular galaxy