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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 Msun) - 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 Halpha 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 Halpha 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 Lsun), the Halpha 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

Figure 2. Halpha+[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

Figure 3. Halpha+[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

HAM, L92, and LH have measured radial electron density profiles in the emission-line nebulae associated with about 20 IR-selected starburst galaxies. Densities range from 103 to < 102 cm-3. The observed Halpha luminosities and the measured densities allow us to estimate the mass and the volume-filling factor of optical emission-line gas. The derived masses range from 105 Msun to 107 Msun, and the volume-filling factors are typically 10-3 to 10-4. Thus, the emission-line gas represents a modest amount of relatively dense material distributed in the form of clumps, sheets, or filaments that occupy only a very small fraction of the volume of the halo of the starburst galaxy.

In the few starburst galaxies for which both Halpha 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

Figure 4. Overlaid Halpha+[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

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

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

Figure 7. Long-slit spectrum of (from left to right) the [N II]Lambda6548, Halpha, [N II]Lambda6584 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 NGC 2366, while de Vaucouleurs, de Vaucouleurs, & Pence (1974) found evidence for large-scale outflow in the similar system NGC 1569. JHU graduate student A. Marlowe with T. Heckman, R. Wyse, and R. Schommer have now found an additional half-dozen similar examples in their echelle and Fabry-Perot Halpha survey of UV-bright dwarf galaxies (Marlowe et al. 1993). Because the potential wells of dwarfs are so shallow, there is a broad theoretical consensus that starburst-driven mass loss plays a crucial role in the evolution of such systems (e.g., Silk, Wyse, & Shields 1987). We will discuss related H I observations of several dwarf galaxies in Section 3.4 below.

As a closing comment on the kinematics of the optical emission-line gas, we note that the outflow velocities as deduced from all the evidence summarized above are quite modest (typically < several hundred km s-1). These velocities are much lower than the predicted outflow speed for the hot, tenuous superwind fluid (a few thousand km s-1 - see Section 2), but are quite consistent with our expectations for ambient gas that is being shocked and accelerated outward by the wind (see the discussion in Sections 2.2 and 2.4 above). This may be seen by considering Equations 10, 16, and 18 above, or by noting that strong optical line emission will only be produced by material into which radiative shocks are being driven (e.g., shocks in which the radiative cooling time behind the shock is short compared to the shock dynamical timescale). As reviewed by McKee & Hollenbach (1980), radiative shocks in the ISM generally have shock speeds less than a few hundred km s-1. Since we expect the velocity to which a shocked cloud is accelerated by the superwind to be similar to the speed of the shock driven into it, the observed outflow velocities therefore represent additional indirect evidence for the schematic picture we have developed above.

3.2.3. Emission-Line Ratios and the Ionization Source

The relative emission-line intensities in the nebulae associated with superwinds provide indirect evidence for a high-speed outflow. Key emission-line ratios like [S II]LambdaLambda6716,6731 / Halpha, [N II]Lambda6584 / Halpha, and [O I]Lambda6300 / Halpha have values that are more similar to those seen in gas that has been shock-heated (e.g., old supernova remnants and Herbig-Haro objects - cf. Hartigan, Raymond, & Hartmann 1987; Fesen, Blair, & Kirshner 1985) than to what is observed in H II regions in spiral and irregular galaxies (e.g., McCall, Rybski, & Shields 1985). We (L92; LH) find that all these ratios tend to increase with increasing radius along the minor axis in edge-on starburst galaxies (see also AHM89 and HAM) and are also systematically larger along the minor axis than along the major axis.

The simplest picture is that the heating and ionization of the gas in the central starburst and in the large-scale disk of the galaxy is dominated by photoionization by hot stars. In contrast, shocking of ambient gas by the superwind probably makes a significant contribution to heating the gas located out in the galaxy halo and seen most clearly along the minor axis. The transfer of energy from the hot superwind gas to imbedded cool, dense material via the 'mixing layer' phenomenon (Begelman & Fabian 1990; Slavin, Shull, & Begelman 1993) may also contribute to the heating and ionization of the optical emission-line gas. It is also possible that the halo gas is photoionized by an extremely dilute radiation field 'leaking' out of the starburst (e.g., Sokolowski & Bland-Hawthorn 1993; see also Dettmar 1993 and references therein).

Perhaps the strongest indirect evidence for mechanical heating of the gas by the superwind is the strong correlation found by HAM, L92, and LH between the dynamical state of the gas (as measured by the emission-line widths) and the thermal/ionization state of the gas (as probed by the strengths of the [O I], [S II], and [N II] lines relative to Halpha). This strongly suggests that the same mechanism is responsible for both heating and accelerating the gas. The most decisive way of testing whether the gas is collisionally ionized or photoionized would be to determine whether the electron temperature in the O III zone is higher than can be explained by photoionization (e.g., T > 20,000 K - cf. Osterbrock 1989). HAM attempted such measurements using the ratio of the [O III]Lambda4363 and Lambda5007 lines, but the results were inconclusive (in some cases shock-heating was favored and in other cases photoionization).

3.2.4. Densities and Pressures as Diagnostics

One of the most useful diagnostics of superwinds is to use the ratio of the [S II]Lambda6716 and Lambda6731 lines to determine the radial run of density within the superwind nebulae. In many cases, the average temperature of the nebula can also be deduced from the ratio of the [N II]Lambda5755 and Lambda6584 lines, so that the radial run of pressure can be determined. According to superwind models, the central pressure in the starburst is set by the starburst radius and the rate at which momentum is injected by supernovae and stellar winds (cf. CC; HAM; see Equation 4 above). HAM have used the measured central pressures and the predicted momentum injection rates to estimate starburst radii for 12 IR-selected galaxies. These radii agree surprisingly well (typically within 0.1 dex) with radii measured directly from radio, IR, and optical imaging of the starbursts (see also L92; LH).

The superwind model also predicts that once the wind 'blows out' of the galactic disk and is in free expansion, the pressure within the wind is dominated by ram pressure. The ram pressure will drop like the inverse square of the radius and will be proportional to the rate at which the starburst injects momentum into the wind (see Equation 5 above). HAM have used the measured pressure profiles in 12 IR-selected galaxies to estimate the momentum flux of the wind, and find excellent agreement between these estimates and the predicted rate of momentum input from the starburst. This analysis has been extended to an additional set of galaxies by L92 and LH, and the good agreement between theory and observation verified over a range of nearly three orders-of-magnitude in starburst luminosity.

It is worth emphasizing that an ultraluminous starburst/superwind can produce pressures over galactic-scale volumes that are three to four orders-of-magnitude higher than the general pressure in the ISM in the Milky Way: e.g., P/k > few × 106 K cm-3 (see HAM, L92, and LH for details). The radius of the over-pressured region (as measured out to some fixed isobar) should scale as the square-root of the star-formation rate, and this dependence has been confirmed by HAM, L92, and LH. HAM and LRD have further shown that the P DeltaV work represented by the measured pressure profiles in the halos of starbursts with superwinds is in satisfactory agreement with the time-integrated kinetic energy input predicted from the starburst (cf. Equation 1 above).

The high measured pressures and the systematic dependence of the radial pressure profile upon the starburst luminosity clearly establish that starbursts are able to regulate the gas pressure over very large volumes (at least one hundred times the volume of the starburst itself). But must this regulation be due to a wind? Might it not be possible for the measured pressure profiles to reflect the static thermal pressure in a hot volume-filling phase that is gravitationally bound to the starburst galaxy?

In fact, it is rather easy to rule out the above idea for those starbursts in HAM that have X-ray data available. Since the X-ray luminosity depends on density squared, and since the density (at fixed pressure) is inversely proportional to the temperature, it follows that the hotter the volume-filling phase (at fixed pressure), the lower its X-ray luminosity. Specifically, we calculate that a volume-filling hot phase in pressure-balance with the optical emission-line clouds would have an X-ray luminosity that exceeds the measured X-ray luminosity unless it has a temperature in excess of 4 × 107 K. The sound-speed in gas at T > 4 × 107 K is cS > 1000 km s-1, which is in turn much greater than the escape velocity from the starburst potential well. Thus, any such gas could not be bound to the starburst and will flow out as a wind (QED!).

The shapes of the measured pressure profiles can also be used to argue against an AGN as the source of the wind. If, as would be the case with an AGN, the region of mass/energy injection is small compared to the seeing-disk (which has a typical radius of 100 pc for the starbursts in HAM and LH), then the radial pressure profile should follow an inverse square law all the way in to the center. Such a r-2 radial density/pressure profile is directly measured in the classical AGN (type 1 Seyfert nucleus) NGC 4151 by Mediavilla et al. (1992) over a range in radii between about 70 and 400 pc (e.g., into the approximate radius of the seeing disk). Pronik (1989) has used the observed correlations between critical density and line width for many different forbidden lines in the nucleus to conclude that the density in NGC 4151 continues to follow a r-2 law all the way in to sub-pc scales. In contrast, the observed radial pressure profiles in most IR-bright galaxies show flat central zones (within the starburst, where mass and energy are injected) and then steepen to approximately r-2 at large radii (in reasonable agreement with theoretical models - cf. CC; HAM; LH). This behavior can be clearly seen in Figure 8 for the two galaxies in HAM for which the measured radial pressure profiles are of the highest quality (M 82 and NGC 3256). The shapes of the radial pressure profiles are remarkably similar in the two galaxies, and the larger radial scale associated with the 8-times-more-luminous starburst in NGC 3256 is exactly as predicted by the superwind model. Note that both profiles are relatively flat in the central region (within the starburst radius denoted r* in the plot), and then steepen to an r-2 dependence at large radii.

Figure 8

Figure 8. A log-log plot of the electron density (left axis) and gas pressure (right axis) vs. radius for two well-studied starburst galaxies (M 82 - solid dots; NGC 3256 - hollow dots). Note that the radial scale for NGC 3256 (upper axis) has been shifted by 0.45 dex (factor 2.8) with respect to that for M 82 (lower axis). The radial pressure profile predicted for both galaxies by the simple spherically-symmetric wind model of CC is indicated by the solid curve. We have also indicated estimates of the pressures in the X-ray and radio plasma in M 82. See HAM for further details.

3.2.5. The Role of Scattering

Finally, in closing our discussion of optical line emission, it must be emphasized that imaging polarimetry of the famous M 82 nebula shows that Halpha emission from a portion of the starburst nucleus is being scattered into our line-of-sight by dust grains in the halo of M 82, and that this scattered light makes a significant contribution to the observed Halpha emission in the halo (Scarrott, Eaton, & Axon 1991 and references therein). On the other hand, the systematic spatial variations in the optical emission-line ratios measured in the inner portion of the minor-axis nebula (that is, the gas located within about 30 degrees of the minor axis and within about a kpc from the nucleus) show that the majority of the Halpha emission from this region must be intrinsic to the filament system rather than scattered light (e.g., McCarthy, Heckman, & van Breugel 1987; Bland & Tully 1988; HAM). Indeed, the polarization of the Halpha within this region is typically only 4 to 10%, compared to values of 10 to 30% at greater angles off the minor axis (Scarrott, Eaton, & Axon 1991). It is also this inner region with lower polarization in which the clear kinematic signature of an outflow is seen (HAM; Bland & Tully 1988). We suggest that scattered light contributes no more than about 30% of the Halpha flux in this inner region. A spectropolarimetric, multi-emission-line mapping of the M 82 nebula with a spectral resolution of an Ångstrom or better would clearly be a magnificent Ph.D. thesis project!

3.3. The Relativistic Plasma Associated with Superwinds

Several starburst galaxies have been discovered to have large-scale nonthermal radio halos. These presumably arise as relativistic magnetized plasma is convected by the superwind out of the central starburst, where it was created by supernovae (e.g., Lerche & Schlickeiser 1980).

The galaxy M 82, studied most recently by Seaquist & Odegard (1991), has a synchrotron-emitting halo extending out to a radius of at least 8 kpc. The spectrum of the radio emission steepens markedly with increasing radius, which Seaquist & Odegard attribute to the more rapid cooling of the most energetic electrons by inverse Compton scattering of the starburst IR photons. They then derive an outflow speed for the relativistic plasma of about 2000 km s-1 (consistent with the expected outflow speed of the tenuous superwind fluid - cf. CC and the discussion in Section 2 above). They also use the radio data to argue that inverse Compton radiation contributes a negligible fraction of the X-ray emission from M 82 (see Section 3.1 above), and they estimate that relativistic particles comprise only a few percent of the energy content of the wind. They note that this ratio is similar to that observed in supernova remnants, as expected in the superwind model. Reuter et al. (1992) have obtained higher resolution maps of the M 82 halo, and find a considerable amount of structure (radially-oriented filaments and gaps). They interpret this as clear evidence that magnetized plasma is being convected out of the M 82 starburst, creating a bipolar (poloidal) magnetic field geometry.

Hummel & Dettmar (1990) have studied the nonthermal radio halo of the edge-on actively-star-forming galaxy NGC 4631, and have argued that the radial spectral index gradient they measure for the radio emission is consistent with models in which relativistic plasma is convected out of the galaxy by a wind, and is then cooled by synchrotron, inverse Compton, and/or adiabatic-expansion losses. As in M 82, the magnetic field geometry in the halo of NGC 4631 is inferred to be poloidal (field lines directed radially outward). Hummel & Dettmar suggest this is due to the operation of a strong galactic wind in this galaxy (though a dynamo model for the field can not be ruled out).

Most recently, Carilli et al. (1992) have discovered a radio halo around the edge-on starburst galaxy NGC 253 which extends out to at least 9 kpc. As in the case of M 82 and NGC 4631, the radio spectrum steepens with increasing distance from the disk/starburst, and Carilli et al. interpret this as evidence for an outflow from the galactic disk into the halo. The magnetic field geometry in this case appears more nearly toroidal (field lines parallel to the galactic disk), except along a bright radio spur that attaches to the inner disk of the galaxy. Dettmar (1993) has also discussed the radio halo of the IR-bright edge-on disk galaxy NGC 5775 (Hummel 1991), and emphasized the close connection between the radio halo and the system of faint Halpha filaments extending many kpc out of the galactic disk.

There are also less complete data indicating the presence of thick radio disks and/or radio halos in the edge-on starburst systems NGC 1808 (Dahlem et al. 1990), NGC 3628 (Reuter et al. 1991), and NGC 4945 (Harnett et al. 1989). The smaller (few-kpc-scale) radio sources extending out along the minor axes of some edge-on radio-bright disk galaxies (e.g., Duric et al. 1983; Hummel, van Gorkom, & Kotanyl 1983) may also be related phenomena.

3.4. The Cool Gas Associated with Superwinds

Studies of the interaction of superwinds with cool atomic and molecular gas are potentially very important, since such gas is expected to comprise the major fraction of the mass of the ISM inside starburst galaxies or in their surrounding halos. One of the most important insights provided by such studies is that starburst galaxies are often immersed in large (transgalactic-scale) HI envelopes or halos, presumably of tidal origin (e.g., Yun & Ho 1993; Hibbard et al. 1993; Fisher & Tully 1977; Haynes et al. 1979; Stanford 1990; Stanford & Wood 1989; Gottesman & Mahon 1990). This is of crucial significance, because the interaction between the wind and this comparatively dense halo gas probably accounts for the relative brightness of superwinds in the optical and X-ray domain. Were such gas not present - i.e., if superwinds instead flowed into a very low density intergalactic medium - they would quickly adiabatically cool, their density would rapidly drop, and they would then become undetectably faint sources of either X-rays or optical line emission (cf. Mathews & Doane 1993). Thus, it is likely that the interaction of the superwind with the H I envelope/halo is fundamental in determining what we observe at optical and X-ray wavelengths.

Studies of the cool gas in the environs of starbursts have also provided direct evidence for outflows. Indeed, possibly the single most convincing piece of evidence to date for an outflow from a starburst galaxy is the recent analysis of the kinematics of the Na I D doublet seen in the well-known starburst galaxy NGC 1808 (Phillips 1992). He has studied the kinematics of the system of radial dust-filaments that extend several kpc out along the minor axis of this galaxy. The Na D doublet is seen in absorption on the near side of the outflow, and has a strongly blueshifted component (by up to 400 km s-1 with respect to the systemic velocity). Since the kinematics are being probed with absorption-lines in this case, there is no possible ambiguity in the sign of the observed (outward) radial motions. Assuming a standard gas-to-dust ratio in the outbound dust filaments, the implied mass of the ejected material is about 6 × 107 Msun. The kinetic energy of this material is then about 1056 ergs, or about 20% of the time-integrated kinetic energy of the starburst in NGC 1808 over a period of 107 yr (the dynamical lifetime of the dust filament system). This is in good agreement with the prediction that the kinetic energy in the shell of an energy-conserving wind-blown bubble should be about 20% of the total time-integrated amount of injected kinetic energy (cf. Dyson 1989).

There are also several pieces of evidence for outflowing molecular gas in starbursts. High-resolution 115GHz CO observations of M 82 by Nakai et al. (1987) suggest that molecular gas is flowing out of the central starburst, and Irwin & Sofue (1992) have reached similar conclusions for the CO in the center of NGC 3079. Turner (1985) has found a molecular outflow traced by the OH emission lines extending out from the nucleus to about 1.5 kpc along the minor axis of NGC 253, and van der Werf et al. (1994) have found plumes of near-IR-emitting molecular hydrogen at the base of this apparent outflow. In the most detailed near-IR study to-date of the famous IR-luminous galaxy NGC 6240, van der Werf et al. (1993) have uncovered convincing kinematic and morphological evidence that the relatively warm molecular gas (as probed with the near-IR vibration-rotation lines of H2) in this system is being ejected from the starburst at velocities of several hundred to 1000 km s-1 (in agreement with the kinematics of the ionized gas discussed by HAM).

Radio observations of H I in several starburst galaxies also reveal the impact a superwind can have on the cool ISM. Yun, Ho, & Lo (1992) have suggested that the wind from the starburst in M 82 has carved a channel through the inner part of that galaxy's H I envelope. Koribalski et al. (1992) have found clear kinematic evidence for H I flowing out along the minor axis of the starburst galaxy NGC 1808 at a velocity of at least 150 km s-1. This is the same outflow studied optically at much higher spatial resolution via the Na I D line by Phillips (1992 - see above).

Recent VLA H I maps of several star-forming dwarf galaxies have revealed the presence of kpc-scale H I shells expanding at about 10 km s-1 (e.g., Puche et al. 1992; Westpfahl & Puche 1992). These authors argue that such expanding, hollow structures are the result of the collective effect of multiple stellar winds and supernovae. Moreover, since the sizes and expansion speeds of the bubbles are comparable to the total sizes and internal velocity dispersions in the smallest dwarf galaxies, these structures probably play a major role in the evolution of the ISM and of star-formation in such galaxies (e.g., Westpfahl & Puche 1992).

Perhaps the most spectacular phenomenon ascribed to a superwind is the ram pressure stripping of the H I out of the outer parts of the galaxy NGC 3073 by its starbursting, superwind-driving neighbor NGC 3079 (Irwin et al. 1987). The ram pressure that Irwin et al. estimate as being required to strip the H I from NGC 3073 implies a superwind momentum flux that agrees well with that estimated for NGC 3079 by HAM via three independent techniques (from the kinematics of the expanding bubble, from the radial pressure profile, and from the starburst luminosity using Equation 3 above).

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