Galactic Fountain Model Research Articles

Detection of Dust in High-velocity Cloud Complex C–Enriched Gas Accreting onto the Milky Way * * Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the Data Archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. These observations are associated with program

We present the detection of dust depletion in Complex C, a massive, infalling, low-metallicity high-velocity cloud in the northern Galactic hemisphere that traces the ongoing accretion of gas onto the Milky Way. We analyze a very high signal-to-noise Hubble Space Telescope Cosmic Origins Spectrograph spectrum of active galactic nucleus (AGN) Mrk 817 formed by coadding 165 individual exposures taken under the AGN STORM 2 program, allowing us to determine dust-depletion patterns in Complex C at unprecedented precision. By fitting Voigt components to the O i, S ii, N i, Si ii, Fe ii, and Al ii absorption and applying ionization corrections from customized Cloudy photoionization models, we find subsolar elemental abundance ratios of [Fe/S] = −0.42 ± 0.08, [Si/S] = −0.29 ± 0.05, and [Al/S] = −0.53 ± 0.08. These ratios indicate the depletion of Fe, Si, and Al into dust grains, since S is mostly undepleted. The detection of dust provides an important constraint on the origin of Complex C, as dust grains indicate the gas has been processed through galaxies, rather than being purely extragalactic. We also derive a low metallicity of Complex C of [S/H] = −0.51 ± 0.16 (≈31% solar), confirming earlier results from this sight line. We discuss origin models that could explain the presence of dust in Complex C, including Galactic fountain models, tidal stripping from the Magellanic Clouds or other satellite galaxies, and precipitation of coronal gas onto dust-bearing “seed” clouds.

The Mass Inflow and Outflow Rates of the Milky Way

Abstract We present new calculations of the mass inflow and outflow rates around the Milky Way (MW), derived from a catalog of ultraviolet metal-line high-velocity clouds (HVCs). These calculations are conducted by transforming the HVC velocities into the Galactic standard of rest (GSR) reference frame, identifying inflowing (v GSR < 0 km s−1) and outflowing (v GSR > 0 km s−1) populations, and using observational constraints on the distance, metallicity, dust content, covering fractions, and total silicon column density of each population. After removing HVCs associated with the Magellanic Stream and the Fermi Bubbles, we find inflow and outflow rates in cool (T ∼ 104 K) ionized gas of dM in/dt ≳ (0.53 ± 0.23)(d/12 kpc)(Z/0.2Z ⊙)−1 M ⊙ yr−1 and dM out/dt ≳ (0.16 ± 0.07)(d/12 kpc)(Z/0.5Z ⊙)−1 M ⊙ yr−1. The apparent excess of inflowing over outflowing gas suggests that the MW is currently in an inflow-dominated phase, but the presence of substantial mass flux in both directions supports a Galactic fountain model, in which gas is constantly recycled between the disk and the halo. We also find that the metal flux in both directions (in and out) is indistinguishable. By comparing the outflow rate to the Galactic star formation rate, we present the first estimate of the mass loading factor (η HVC) of the disk-wide MW wind, finding η HVC ≳ (0.10 ± 0.06)(d/12 kpc)(Z/0.5Z ⊙)−1. Including the contributions from low- and intermediate-velocity clouds and from hot gas would increase these inflow and outflow estimates.

Galactic Gas Flows from Halo to Disk: Tomography and Kinematics at the Milky Way’s Disk–Halo Interface

Abstract We present a novel absorption-line survey using 54 blue horizontal branch stars in the Milky Way halo as background sources for detecting gas flows at the disk–halo interface. Distance measurements to high-latitude (b > 60°) background stars at 3.1–13.4 kpc, combined with unprecedented spatial sampling and spectral resolution, allow us to examine the 3D spatial distribution and kinematics of gas flows near the disk. We detect absorption signatures of extraplanar Ca ii and Na i in Keck HIRES spectra and find that their column densities exhibit no trend with distance to the background sources, indicating that these clouds lie within 3.1 kpc of the disk. We calculate covering fractions of f Ca ii = 63%, f Na i = 26%, and f H i = 52%, consistent with a picture of the circumgalactic medium (CGM) that includes multiphase clouds containing small clumps of cool gas within hotter, more diffuse gas. Our measurements constrain the scale of any substructure within these cool clouds to <0.5 kpc. Ca ii and Na i absorption features exhibit an intermediate-velocity (IV) component inflowing at velocities of −75 km s−1 <v < −25 km s−1 relative to the local standard of rest, consistent with previously studied H i structures in this region. We report the new detection of an inflow velocity gradient Δv z ∼ 6–9 km s−1 kpc−1 across the Galactic plane. These findings place constraints on the physical and kinematic properties of CGM gas flows through the disk–halo interface and support a galactic fountain model in which cold gas rains back onto the disk.

Interstellar magnetic cannon targeting the Galactic halo

We report the detection of a new Galactic bubble at the interface between the halo and the Galactic disc. We suggest that the nearby Lupus complex and parts of the Ophiuchus complex constitute the denser parts of the structure. This young bubble, ≲3 Myr old, could be the remnant of a supernova and it expands inside a larger HI loop that has been created by the outflows of the Upper Scorpius OB association. An HI cavity filled with hot X-ray gas is associated with the structure, which is consistent with the Galactic chimney scenario. The X-ray emission extends beyond the west and north-west edges of the bubble, suggesting that hot gas outflows are breaching the cavity, possibly through the fragmented Lupus complex. Analyses of the polarised radio synchrotron and of the polarised dust emission of the region suggest the connection of the Galactic centre spur with the young Galactic bubble. A distribution of HI clumps that spatially corresponds well to the cavity boundaries was found at VLSR ≃−100 km s−1. Some of these HI clumps are forming jets, which may arise from the fragmented part of the bubble. We suggest that these clumps might be “dripping” cold clouds from the shell walls inside the cavity that is filled with hot ionised gas. It is possible that some of these clumps are magnetised and were then accelerated by the compressed magnetic field at the edge of the cavity. Such a mechanism would challenge the Galactic accretion and fountain model, where high-velocity clouds are considered to be formed at high Galactic latitude from hot gas flows from the Galactic plane.

THE ORIGIN OF THE HOT GAS IN THE GALACTIC HALO: TESTING GALACTIC FOUNTAIN MODELS' X-RAY EMISSION

We test the X-ray emission predictions of galactic fountain models against XMM-Newton measurements of the emission from the Milky Way's hot halo. These measurements are from 110 sight lines, spanning the full range of Galactic longitudes. We find that a magnetohydrodynamical simulation of a supernova-driven interstellar medium, which features a flow of hot gas from the disk to the halo, reproduces the temperature but significantly underpredicts the 0.5-2.0 keV surface brightness of the halo (by two orders of magnitude, if we compare the median predicted and observed values). This is true for versions of the model with and without an interstellar magnetic field. We consider different reasons for the discrepancy between the model predictions and the observations. We find taking into account overionization in cooled halo plasma, which could in principle boost the predicted X-ray emission, is unlikely in practice to bring the predictions in line with the observations. We also find that including thermal conduction, which would tend to increase the surface brightnesses of interfaces between hot and cold gas, would not overcome the surface brightness shortfall. However, charge exchange emission from such interfaces, not included in the current model, may be significant. The faintness of the model may also be due to the lack of cosmic ray driving, meaning that the model may underestimate the amount of material transported from the disk to halo. In addition, an extended hot halo of accreted material may be important, by supplying hot electrons that could boost the emission of the material driven out from the disk. Additional model predictions are needed to test the relative importance of these processes in explaining the observed halo emission.

Galactic hail: the origin of the high-velocity cloud complex C

Abstract High-velocity clouds consist of cold gas that appears to be raining down from the halo to the disc of the Milky Way. Over the past 50 years, two competing scenarios have attributed their origin either to gas accretion from outside the Galaxy or to circulation of gas from the Galactic disc powered by supernova feedback (galactic fountain). Here, we show that both mechanisms are simultaneously at work. We use a new galactic fountain model combined with high-resolution hydrodynamical simulations. We focus on the prototypical cloud complex C and show that it was produced by an explosion that occurred in the Cygnus-Outer spiral arm about 150 Myr ago. The ejected material has triggered the condensation of a large portion of the circumgalactic medium and caused its subsequent accretion on to the disc. This fountain-driven cooling of the lower Galactic corona provides the low-metallicity gas required by chemical evolution models of the Milky Way's disc.

A DEEP X-RAY VIEW OF THE HOT HALO IN THE EDGE-ON SPIRAL GALAXY NGC 891

NGC 891 is a nearby edge-on galaxy that is similar to the Milky Way and has a hot X-ray emitting halo that could arise from accretion, a galactic fountain, or a combination of the two. The metallicity of the gas can help distinguish between these models, and here we report on results that use 138 ks of archival Chandra data and 92 ks of new XMM-Newton data to measure the temperature and metallicity of the hot halo of the galaxy. We find good fits for a thermal model with kT ~ 0.2 keV and Z ~ 0.1 solar, and rule out solar metallicity to more than 99% confidence. This result suggests accretion from the intergalactic medium as the origin for the hot halo. However, it is also possible to fit a two-temperature thermal model with solar metallicity where kT_1 = 0.1 keV and kT_2 = 0.25 keV. A consideration of the cooling rate and scale height prefers the single-temperature model. We also find that the cooling rate in the hot gas cannot explain the massive HI halo in the steady state. In addition, a galactic fountain model cannot eject enough mass to account for the HI halo, and we speculate that the neutral halo may be gas from a prior outflow that has since cooled.

Effects of galactic fountains and delayed mixing in the chemical evolution of the Milky Way

The majority of galactic chemical evolution models assumes the instantaneous mixing approximation (IMA). This assumption is probably not realistic as indicated by the existence of chemical inhomogeneities, although current chemical evolution models of the Milky Way can reproduce the majority of the observational constraints under the IMA. The aim of this paper is to test whether relaxing this approximation in a detailed chemical evolution model can improve or worsen the agreement with observations. To do that, we investigated two possible causes for relaxing of the instantaneous mixing: i) the ``galactic fountain time delay effect'' and ii) the ``metal cooling time delay effect''. We found that the effect of galactic fountains is negligible if an average time delay of 0.1 Gyr, as suggested in a previous paper, is assumed. Longer time delays produce differences in the results but they are not realistic. We also found that the O abundance gradient in the disk is not affected by galactic fountains. The metal cooling time delays produce strong effects on the evolution of the chemical abundances only if we adopt stellar yields depending on metallicity. If instead, the yields computed for to the solar chemical composition are adopted, negligible effects are produced, as in the case of the galactic fountain delay. The relaxation of the IMA by means of the galactic fountain model, where the delay is considered only for massive stars and only in the disk, does not affect the chemical evolution results. The combination of metal dependent yields and time delay in the chemical enrichment from all stars starting from the halo phase, instead, produces results at variance with observations.

ANOMALOUS H I IN NGC 2997

We present deep H I observations of the moderately inclined spiral galaxy, NGC 2997. The goal of these observations was to search for H I clouds in the vicinity of NGC 2997 analogous to the high-velocity clouds of the Milky Way and gain insight into their origins. We find evidence for the presence of a galactic fountain as well as the accretion of intragalactic material, however we do not identify any large clouds of H I far from the disk of the galaxy. NGC 2997 has a thick, lagging H I disk that is modeled with a vertical velocity gradient of 18-31 km s–1 kpc–1. Anomalous velocity H I clouds with masses of order 107 M ☉, which cannot be explained by galactic fountain models allow us to estimate a lower limit to the accretion of extragalactic gas of 1.2 M ☉ yr–1. The number and mass of these clouds have implications for cosmological simulations of large-scale structure and the presence of dark matter halos. We have used values from the literature to estimate a star formation rate of 5 ± 1 M ☉ yr–1 and to derive a new distance to NGC 2997 of 12.2 ± 0.9 Mpc using published Tully-Fisher relations.

Low and high velocity clouds produced by young stellar clusters

Intermediate and high velocity HI clouds rain onto the plane of our Galaxy. They are observed at heights of between 500 and 1500 pc, falling onto the Galactic plane at velocities from 50 to 140 km s$^{-1}$. To explain the origin of these clouds, we present a galactic fountain model, driven by the wind from a super stellar cluster (SSC). We solve the equations for a steady, radiative de Laval nozzle flow. We consider two effects not considered previously in astrophysical nozzle flow models: cooling functions for different metallicities, and the direct action of the galactic gravitational field on the gas flowing along the nozzle. For an adiabatic nozzle flow, the gravity acting directly on the gas within the nozzle "stalls" the nozzle flow for initial wind velocities lower than the escape velocity from the Galaxy. For the same wind velocity, a radiative nozzle flow stalls at lower altitudes above the galactic plane. We find that SSC winds with velocities of $v_w=500 - 800$ km s$^{-1}$ produce nozzles stall at heights of $x_m=1 - 15$ kpc. The stalled nozzle flow then rains back onto the galactic plane at velocities in the range observed in intermediate and high velocity HI clouds. We study a nozzle flow driven by a wind from a SSC close to the Galactic centre. We find that for velocities within the range expected for a SSC wind, we can produce nozzle flows that stall above the galactic plane. These stalled flows produce cool, infalling clouds with velocities similar to those of intermediate and high velocity HI clouds.

HIGH-VELOCITY CLOUDS IN THE NEARBY SPIRAL GALAXY M 83

We present deep H i 21 cm and optical observations of the face-on spiral galaxy M 83 obtained as part of a project to search for high-velocity clouds (HVCs) in nearby galaxies. Anomalous-velocity neutral gas is detected toward M 83, with 5.6 × 107 M☉ of H i contained in a disk rotating 40–50 km s−1 more slowly in projection than the bulk of the gas. We interpret this as a vertically extended thick disk of neutral material, containing 5.5% of the total H i within the central 8 kpc. Using an automated source detection algorithm to search for small-scale H i emission features, we find eight distinct, anomalous-velocity H i clouds with masses ranging from 7 × 105 to 1.5 × 107M☉ and velocities differing by up to 200 km s−1 compared to the H i disk. Large on-disk structures are coincident with the optical spiral arms, while unresolved off-disk clouds contain no diffuse optical emission down to a limit of 27 r' mag per square arcsec. The diversity of the thick H i disk and larger clouds suggests the influence of multiple formation mechanisms, with a galactic fountain responsible for the slowly rotating disk and on-disk discrete clouds, and tidal effects responsible for off-disk cloud production. The mass and kinetic energy of the H i clouds are consistent with the mass exchange rate predicted by the galactic fountain model. If the HVC population in M 83 is similar to that in our own Galaxy, then the Galactic HVCs must be distributed within a radius of less than 25 kpc.

Kinematics of the ionised gas in the spiral galaxy NGC 2403

We present a study of the kinematics of the ionised gas in the nearby spiral galaxy NGC 2403 using deep long-slit spectra obtained with the 4.2-m William Herschel Telescope. The data show the presence of a halo component of ionised gas that is rotating more slowly than the gas in the disk. The kinematics of this ionised halo gas is similar to that of the neutral halo gas. On small scales. broad line profiles (up to 300 km s(-1) wide) indicate regions of fast Outflows of ionised gas. We discuss these new results in the context of galactic fountain models.

A Galactic Example of a Massive Chimney

One mechanism for spreading hot, metal enriched gas away from galaxies is through gigantic chimneys formed in the disk of a galaxy. Chimneys form when shells or bubbles blown by many massive stellar winds and supernova explosions grow large enough to exceed the neutral hydrogen (H I) scale height of the disk. The shells then become unstable at their polar regions and expand rapidly, breaking out to the galaxy's halo. If galactic fountain models are correct the hot gas liberated by these chimneys should cool into H I cloudlets high above the galaxy's disk. The Milky Way provides the nearest laboratory to search for these objects in order to study how they form and the fate of the expelled gas. While we expect tens of chimneys in the Milky Way to account for the thermal support of the halo there are only a few known chimneys. Here we present an H I study of one Galactic chimney GSH 277+00+36. GSH 277+00+36 is the the only chimney known to have blown out of both sides of a galactic disk. We discuss the development of Rayleigh-Taylor instabilities in this object and the role those may have had in the formation of the chimney.

Extraplanar Emission‐Line Gas in Edge‐on Spiral Galaxies. II. Optical Spectroscopy

The results from deep long-slit spectroscopy of nine edge-on spiral galaxies with known extraplanar line emission are reported. Emission from Hα, [N II] λλ6548, 6583, and [S II] λλ6716, 6731 is detected out to heights of a few kiloparsecs in all of these galaxies. Several other fainter diagnostic lines such as [O I] λ6300, [O III] λλ4959, 5007, and He I λ5876 are also detected over a smaller scale. The relative strengths, centroids, and widths of the various emission lines provide constraints on the electron density, temperature, reddening, source(s) of ionization, and kinematics of the extraplanar gas. In all but one galaxy, photoionization by massive OB stars alone has difficulties explaining all of the line ratios in the extraplanar gas. Hybrid models that combine photoionization by OB stars and another source of ionization such as photoionization by turbulent mixing layers or shocks provide a better fit to the data. The (upper limits on the) velocity gradients measured in these galaxies are consistent with the predictions of the galactic fountain model to within the accuracy of the measurements.

Primordial Rain or Galactic Pollution?

High-Velocity Clouds--clouds of hydrogen gas that move faster than the Milky Way--were first identified in the 1960s, but as Zwaan discusses in this Perspective, their origin and location remain controversial. Today, there are two competing models, the Local Group model, which postulates that the clouds are extragalactic, primordial remnants, and the Galactic fountain model, in which supernova explosions are responsible for the formation of HVCs. Recent evidence points toward the Galactic fountain model, with implications for dark matter and galaxy formation theories.

The HiHalo of NGC 891

Neutral hydrogen observations of the nearby, edge-on spiral galaxy NGC 891 reveal the presence of an H I halo extending up to at least 5 kpc from the plane. This halo gas appears to rotate 25 to 100 km s(-1) more slowly than the gas in the plane. If this velocity difference is due to the gradient in the gravitational potential, then it may serve to discriminate between disk and spheroidal mass models. The classic picture of a large outer flare in the H I layer of NGC 891 may no longer be valid. A correlation is seen between the distributions of H I, H alpha and radio continuum emission; this, in accordance with galactic fountain models, supports the picture of a substantial disk-halo circulation related to star formation activity in the disk of NGC 891. There is now also clear evidence for the presence of a rapidly rotating (upsilon(rot) similar or equal to 230 km s(-1)) disk or ring of H I in the central part of NGC 891.

A Very Deep Spectrum of the Diffuse Ionized Gas in NGC 891

A deep long-slit spectrum of the diffuse ionized gas (DIG) in the edge-on spiral galaxy NGC 891 has been obtained. The slit crosses the plane at a galactocentric radius of 5 kpc and covers DIG on both the east and west sides of the galaxy. The primary motivation was to attempt to detect the recombination line He I λ5876, the strength of which relative to Hα provides a direct constraint on the hardness of the ionizing spectrum. In the DIG of the Milky Way, this line ratio has turned out to be surprisingly low (Reynolds & Tufte; Heiles et al.), implying an ionizing spectrum much softer than had been inferred from the more readily observable forbidden lines, such as [N II] λ6583 and [S II] λ6717. The He I line has been detected in NGC 891 to a height of about 1.5 kpc from the plane—well into the diffuse gas layer. In the DIG, He I/Hα ≈ 0.034, implying that helium is about 70% ionized and that the ionizing spectrum is significantly harder than in the Reynolds layer. The spectrum also allows very detailed mapping of the [N II] λ6583/Hα ratio as a function of height off the plane. This ratio reaches peak values of about 1.4 at heights of z = 2 kpc on the east side of the plane and z = 4 kpc on the west side. Beyond this height on the east side, the ratio clearly declines again, falling to about 1.0 at z = 4 kpc. Previous modeling indicates that such high values of [N II] λ6583/Hα require an ionizing spectrum significantly harder than that indicated by the He I/Hα ratio. Hence, despite the higher values of He I/Hα, the same dilemma exists as in the Reynolds layer case. These results suggest that we do not understand the heating and ionization of the diffuse gas well enough. A determination of the gas temperature would help, but the nondetection of the line [N II] λ5755 allows only upper limits to be set: 13,000 K on the east side and 10,000 K on the west side. Both the [N II] and Hα lines are detected beyond z = 5 kpc; thus, the DIG layer extends much further than indicated by previous narrowband images. A model of the electron density distribution consisting of two components with scale heights of about 1 kpc and 5-6 kpc provides a good fit to the Hα emission profile. The very extended component was also seen in the image of Rand, Kulkarni, & Hester, but the inferred scale height is now larger. Apart from the known effects of dust absorption on the velocity centroids at low z, there is a smooth gradient in velocity centroids with increasing z above the dust lane in the sense that they become closer to the systemic velocity, changing by about 30 km s-1 from z = 1 kpc to z = 4.5 kpc. We postulate that the effect is due in part to a decreasing rotation speed and an outward radial migration of gas with z. An illustrative model suggests that the rotation speed at z = 4.5 kpc may be about 20 km s-1 slower than in the disk, although other effects could change this estimate. Such a change in rotation velocity is expected in galactic fountain models, but some simple experiments with stellar orbits show that the flow in a fountain may be more complicated than previously thought.

Discovery of O VI Emission from the Galactic Corona with the Hopkins Ultraviolet Telescope

We have searched for far-UV emission from coronal gas in the Galactic halo along lines of sight with Galactic latitudes between 42^deg^ and 88^deg^ in spectra obtained by the Hopkins Ultraviolet Telescope during the Astro-2 space shuttle mission in 1995 March. Of four spectra taken through a 19" x 197" aperture, two show O VI λλ1032, 1038 emission at a significance of 4 σ. Two out of six spectra taken through a 10" x 56" aperture also show O VI emission, though at only 2 σ levels of significance. Three of the detections lie near regions of enhanced soft X-ray emission associated with Radio Loop I. The fourth, at l = 218^deg^, b = 56^deg^, may represent a more typical region of the halo. In its spectrum, we find I(O VI) = (3.59 +/- 0.96) x 10^-7^ ergs cm^-2^ s^-1^. This is the first detection of O VI emission from the Galactic halo. None of the spectra exhibit significant emission from C IV λλ1548, 1551, though our upper limits are greater than the intensities reported for other lines of sight. We set a limit on I(O VI)/I(C IV) >= 3.4, consistent with the predictions of self-photoionizing Galactic fountain models, but higher than those of models based on turbulent mixing layers. Combining our measured O VI intensity with estimates of N(O VI) through the halo, we find that, for 5.3 <= log T <= 5.8, the data are consistent with n_e_ ~ 0.06 cm^-3^ and 22,000 <= P/k <= 67,000 cm^-3^ K, values substantially greater than those derived from C IV observations, suggesting that the C IV and O VI emission arise from physically distinct clouds and/or that a substantial portion of the C IV absorption arises from cooler gas that does not contribute to the C IV emission. This result is consistent with Galactic halo models incorporating self-photoionization of the cooling gas.

HIGH-VELOCITY CLOUDS AND SUPERBUBBLES IN NEARBY DISK GALAXIES

ABSTRACT The galactic fountain model predicts that energetic stellar winds and supernovae in OB associations produce superbubbles containing hot gas that breaks out of the Galactic disk, cools radiatively as it rises upward, and recombines and returns to the disk ballistically. The hot (T ~ 106 K) gas can be observed with X-ray telescopes, while the cool returning neutral hydrogen (H I) is detectable as 21 cm emission from high-velocity clouds (HVCs). In the Milky Way Galaxy, a combination of infalling material tidally torn from the Magellanic Clouds and a galactic fountain can explain the high-velocity clouds that cover about 10% of the sky down to a column density of 2 to 3 X 1018 cm-2. Sensitive H I observations of nearby disk galaxies were performed with the Arecibo 305 m radio telescope to search for and measure the mass of HVCs in other galaxies. Ten of 14 galaxies have high-velocity wings that can be modeled as arising from a component of galactic gas with a velocity dispersion of 30 or 50 km s-1. The HVC mass for the 10 galaxies ranges from 6 X 107 solar mass to 4 X 109 solar mass, which corresponds to 4 to 14% of the total H I in the galaxies. This is the first survey to search for HVCs in more than a few galaxies, and the results imply that Galactic HVCs are a disk-wide phenomenon with a characteristic distance of 10 to 20 kpc, containing a substantial fraction (~10%) of the neutral hydrogen in the Galaxy and much of the random kinetic energy in neutral gas. 21 cm synthesis imaging of UGC 12732 and NGC 5668, performed with the Very Large Array, confirmed the Arecibo results that the former does not have high-velocity gas while the latter does. Two components of high-velocity gas are present in NGC~5668; one may be from an accretion event, while the other is visible due to the increased H I velocity dispersion throughout the optical disk and may be galactic fountain gas. Neither of these components are visible in the observations of UGC 12732, and this galaxy can be successfully modeled as a rotating disk of neutral hydrogen with a velocity dispersion of 7 km s-1 throughout. The star formation rate (SFR) in NGC 5668 is one to two orders of magnitude larger than the SFR in UGC 12732. Optical and far-infrared (FIR; measured with the IRAS satellite) observations of the 14 sample galaxies revealed that galaxies with high-velocity H I wings have an average SFR an order of magnitude larger than that of the galaxies without such wings. The FIR to blue light and FIR to H-alpha ratios imply that the galaxies with high-velocity H I wings have more dust-enshrouded FIR sources than the galaxies without high-velocity wings. Both of these results are expected if a substantial fraction of the HVCs are produced in galactic fountains. High-resolution X-ray observations of M33 were performed with the ROSAT satellite to search for superbubbles of hot gas within holes in the H I layer of this nearby spiral galaxy. These observations revealed a number of X-ray point sources (eight of which are coincident or nearly coincident with supernova remnants), diffuse X-ray emission around the nucleus of the galaxy, and two possible superbubbles. One has an X-ray luminosity of 2 X 1037 ergs s-1 within an H I hole of size 300 X 130 pc. The other has an X-ray luminosity of 8 X 1037 ergs s-1 and a gas temperature of 2 X 106 K. It is coincident with the giant H I region NGC 604 and is 13 ± 13 arcsec from a 150 X 130 pc H I hole. The luminosity limit of the ROSAT observations for the detection of soft (kT= 0.2 to 0.4 keV) X-ray emission is about 2 S 1037 ergs s-1, similar to the X-ray luminosities of the brightest superbubbles that have been found in the Large Magellanic Cloud. These observations cannot, therefore, be used to perform the meaningful survey of the superbubbles in M33 that would be required to constrain the galactic fountain model. Future observations with the Advanced X-ray Astrophysics Facility (AXAF) should be able to perform such a survey, which would enable a good estimate to be made of the covering factor of holes through which hot gas from OB associations can escape into the halo.

Global Models of the Interstellar Medium in Disk Galaxies

We have computed six two-dimensional simulations with two cospatial, interacting fluids that represent the stars and gas in a disk galaxy. The two fluids interact through star formation, mass loss, stellar winds, and supernovae, and the gas includes an optically thin cooling function. Our previous simulations have been able to produce the multiphase nature and the overall topology of the cold and warm interstellar medium (ISM) and here we extend the previous work by allowing for million-degree gas and an impulsive form of heating that represents supernovae. The six simulations differ in the form and rates of energy injection; for simulations with heating only from stellar winds and from both supernovae and stellar winds, we have run three simulations with low, moderate and high energy injection rates (with a factor of 4 difference between adjacent rates), where the moderate energy injection rate corresponds to that of the Galaxy. The three values of Edot in our simulations correspond to 2 (low), 8 (moderate), and 32 (high) × 10<SUP>41</SUP> ergs s<SUP>-1</SUP> for a disk of 16 kpc and are equivalent to supernova rates of 0.0075, 0.03, and 0.12 yr<SUP>-1</SUP>, respectively (assuming E<SUB>SN</SUB> = 6 × 10<SUP>50</SUP> ergs, and a ratio of energy released from supernovae to that from stellar winds of 3:1). We use a grid that is 2 kpc across and ±15 kpc in the vertical direction and impose a constant gravitational potential along this direction. <P />Our simulations create a three-phase medium with filaments of dense, cold and warm gas surrounding bubbles of hot gas, which are usually hundreds of parsecs across and can exceed 1 kpc in size. This filamentary topology is very similar to that inferred for our Galaxy and others, based largely on H I observations. The evolution of the cold, dense filaments is dominated by a loss of identity from filament-filament collisions, although in higher energy injection cases a Rayleigh-Taylor instability can cause a filament to fragment. <P />We calculate central densities, scale heights for the density, and filling factors of the three phases of gas, and demonstrate that the vertical density distribution of each phase is usually best fit with more than one component. The calculated central densities, scale heights, and filling factors for each gas phase reproduce the observational values made for the Galaxy in the moderate energy injection rate simulations. Also, the computed median pressure and pressure scale height best reproduce the Galactic values in the moderate energy injection rate simulations. <P />Velocity information for the cold (or neutral) gas is analyzed at one or two times in each simulation. The occasional multicomponent nature of H I emission profiles and the holes of H I in position-velocity plots occur in the simulations, although we fail to recreate quantitatively the amount and velocities of high-velocity gas that are observed in the Galaxy. Specifically, only the highest energy injection rate cases have significant amounts of neutral gas (more than 0.25% of the mass of cold gas) at |υ| ≥ 50 km s<SUP>-1</SUP>, and the maximum velocities are smaller than those observed in H I (≳100 km s<SUP>-1</SUP>) We attribute these shortcomings to numerical viscosity in the simulations. <P />Further analysis of the cold gas velocities reveals an anticorrelation between the column density of cold gas and its velocity dispersion for a galaxy viewed face-on. Also, one calculation of the net mass flux of each gaseous phase as a function of height demonstrates that most of the hot gas is rising, while cooler gas is falling, which is consistent with the galactic fountain model. <P />Varying the rate of energy injection has a large effect on the nature of the ISM, especially the extent of the cold gas, so we can constrain to a relatively narrow range the energy injection rate within the Galaxy. Our results suggest that neither the low-Edot simulations nor the high-Edot case can reproduce the distribution (i.e., central densities, scale heights, and filling factors) of the multiphase ISM in the Galaxy, although the high-E simulations are more effective at reproducing the high-velocity H I observed in the Galaxy.