Disk-halo Interface Research Articles

Revealing the Milky Way’s Hidden Circumgalactic Medium with the Cosmic Origins Spectrograph Quasar Database for Galactic Absorption Lines

Abstract Every quasar (quasi-stellar object; QSO) spectrum contains absorption-line signatures from the interstellar medium, disk-halo interface, and circumgalactic medium (CGM) of the Milky Way (MW). We analyze Hubble Space Telescope/Cosmic Origins Spectrograph (COS) spectra of 132 QSOs to study the significance and origin of Si iv absorption at km s−1 in the Galactic halo. The gas in the north predominantly falls in at km s−1, whereas in the south, no such pattern is observed. The Si iv column density has an average and a standard deviation of cm−2. At , does not significantly correlate with b, which cannot be explained by a commonly adopted flat-slab geometry. We propose a two-component model to reconstruct the –b distribution: a plane-parallel component to account for the MW’s disk-halo interface and a global component to reproduce the weak dependence on b. We find cm−2 and cm−2 on the basis of Bayesian analyses and block bootstrapping. The global component is most likely to have a Galactic origin, although its exact location is uncertain. If it were associated with the MW’s CGM, we would find for the cool gas at all velocities in the Galactic halo. Our analyses show that there is likely a considerable amount of gas at km s−1 hidden in the MW’s CGM. Along with this work, we make our QSO data set publicly available as the COS Quasar Database for Galactic Absorption Lines (COS-GAL).

Extraplanar H ii Regions in Spiral Galaxies. I. Low-metallicity Gas Accreting through the Disk-halo Interface of NGC 4013

Abstract The interstellar thick disks of galaxies serve as the interface between the thin star-forming disk, where feedback-driven outflows originate, and the distant halo, the repository for accreted gas. We present optical emission line spectroscopy of a luminous, thick disk H ii region located at z = 860 pc above the plane of the spiral galaxy NGC 4013 taken with the Multi-Object Double Spectrograph on the Large Binocular Telescope. This nebula, with an Hα luminosity ∼4–7 times that of the Orion nebula, surrounds a luminous cluster of young, hot stars that ionize the surrounding interstellar gas of the thick disk, providing a measure of the properties of that gas. We demonstrate that strong emission line methods can provide accurate measures of relative abundances between pairs of H ii regions. From our emission line spectroscopy, we show that the metal content of the thick disk H ii region is a factor of ≈2 lower than gas in H ii regions at the midplane of this galaxy (with the relative abundance of O in the thick disk lower by −0.32 ± 0.09 dex). This implies incomplete mixing of material in the thick disk on small scales (hundreds of parsecs) and that there is accretion of low-metallicity gas through the thick disks of spirals. The inclusion of low-metallicity gas this close to the plane of NGC 4013 is reminiscent of the recently proposed “fountain-driven” accretion models.

The Dragonfly Nearby Galaxies Survey. IV. A Giant Stellar Disk in NGC 2841

Abstract Neutral gas is commonly believed to dominate over stars in the outskirts of galaxies, and investigations of the disk-halo interface are generally considered to be in the domain of radio astronomy. This may simply be a consequence of the fact that deep H i observations typically probe to a lower-mass surface density than visible wavelength data. This paper presents low-surface-brightness, optimized visible wavelength observations of the extreme outskirts of the nearby spiral galaxy NGC 2841. We report the discovery of an enormous low-surface brightness stellar disk in this object. When azimuthally averaged, the stellar disk can be traced out to a radius of ∼70 kpc (5 R 25 or 23 inner disk scale lengths). The structure in the stellar disk traces the morphology of H i emission and extended UV emission. Contrary to expectations, the stellar mass surface density does not fall below that of the gas mass surface density at any radius. In fact, at all radii greater than ∼20 kpc, the ratio of the stellar mass to gas mass surface density is a constant 3:1. Beyond ∼30 kpc, the low-surface-brightness stellar disk begins to warp, which may be an indication of a physical connection between the outskirts of the galaxy and infall from the circumgalactic medium. A combination of stellar migration, accretion, and in situ star formation might be responsible for building up the outer stellar disk, but whatever mechanisms formed the outer disk must also explain the constant ratio between stellar and gas mass in the outskirts of this galaxy.

The Dispersion of Fast Radio Bursts from a Structured Intergalactic Medium at Redshifts z < 1.5

Abstract We analyze the sources of free electrons that produce the large dispersion measures, (in units of cm−3 pc), observed toward fast radio bursts (FRBs). Individual galaxies typically produce from ionized gas in their disk, disk-halo interface, and circumgalactic medium. Toward an FRB source at redshift z, a homogeneous intergalactic medium (IGM) containing a fraction of cosmological baryons will produce , where . A structured IGM of photoionized Lyα absorbers in the cosmic web produces similar dispersion, modeled from the observed distribution, , of H i (Lyα-forest) absorbers in column density and redshift with ionization corrections and scaling relations from cosmological simulations. An analytic formula for DM(z) applied to observed FRB dispersions suggests that for an IGM containing a significant baryon fraction, . Future surveys of the statistical distribution, DM( , of FRBs identified with specific galaxies and redshifts can be used to calibrate the IGM baryon fraction and distribution of Lyα absorbers. Fluctuations in DM at the level ±10 cm−3 pc will arise from filaments and voids in the cosmic web.

New Views From Galactoseismology: Rethinking the Galactic Disk-Halo Connection

AbstractWe present preliminary results from a study exploring the origin of Milky Way substructures, and show initial evidence of a common “kicked-out” formation mechanism for two low-latitude substructures. In this scenario, stars in these substructures formed in the disk and were subsequently “kicked-out” by an external perturbation, such as the merger of an accreted satellite, which created an oscillation in the Galactic disk. To test this origin scenario, we found the fraction of different stellar populations – M giants and RR Lyrae stars – in the Monoceros Ring (also known as GASS) and A13, supplementing a study of stellar populations in the Triangulum-Andromeda cloud. This work provides: (1) the first analysis of the GASS and A13 features based upon their stellar populations; and (2) preliminary evidence of disk stars in the Milky Way that have been relocated to the disk-halo interface due to vertical oscillations of the Milky Way’s disk.

Resolved magnetic structures in the disk-halo interface of NGC 628

Magnetic fields are essential to fully understand the interstellar medium (ISM) and its role in the disk-halo interface of galaxies is still poorly understood. Star formation is known to expel hot gas vertically into the halo and these outflows have important consequences for mean-field dynamo theory in that they can be efficient in removing magnetic helicity. We perform new observations of the nearby face-on spiral galaxy NGC 628 with the Karl G. Jansky Very Large Array (JVLA) at S-band and the Effelsberg 100-m telescope at frequencies of 2.6 GHz and 8.35 GHz. We obtain some of the most sensitive radio continuum images in both total and linearly polarised intensity of any external galaxy observed so far in addition to high-quality images of Faraday depth and polarisation angle from which we obtained evidence for drivers of magnetic turbulence in the disk-halo connection. Such drivers include a superbubble detected via a significant Faraday depth gradient coinciding with a HI hole. We observe an azimuthal periodic pattern in Faraday depth with a pattern wavelength of 3.7$\pm$ 0.1 kpc, indicating Parker instabilities. The lack of a significant anti-correlation between Faraday depth and magnetic pitch angle indicates that these loops are vertical in nature with little helical twisting, unlike in IC 342. We find that the magnetic pitch angle is systematically larger than the morphological pitch angle of the polarisation arms which gives evidence for the action of a large-scale dynamo where the regular magnetic field is not coupled to the gas flow and obtains a significant radial component. We additionally discover a lone region of ordered magnetic field to the north of the galaxy with a high degree of polarisation and a small pitch angle, a feature that has not been observed in any other galaxy so far and is possibly caused by an asymmetric HI hole.

HST/COS OBSERVATIONS OF IONIZED GAS ACCRETION AT THE DISK–HALO INTERFACE OF M33

ABSTRACT We report the detection of accreting ionized gas at the disk–halo interface of the nearby galaxy M33. We analyze Hubble Space Telescope/Cosmic Origins Spectrograph absorption-line spectra of seven ultraviolet-bright stars evenly distributed across the disk of M33. We find Si iv absorption components consistently redshifted relative to the bulk M33's ISM absorption along all the sightlines. The Si iv detection indicates an enriched, disk-wide, ionized gas inflow toward the disk. This inflow is most likely multi-phase as the redshifted components can also be observed in ions with lower ionization states (e.g., S ii, P ii, Fe ii, Si ii). Kinematic modeling of the inflow is consistent with an accreting layer at the disk–halo interface of M33, which has an accretion velocity of 110 − 20 + 15 km s − 1 at a distance of 1.5 − 1.0 + 1.0 kpc above the disk. The modeling indicates a total mass of ∼3.9 × 107 M ⊙ for the accreting material at the disk–halo interface on the near side of the M33 disk, with an accretion rate of ∼2.9 M ⊙ yr−1. The high accretion rate and the level of metal enrichment suggest the inflow is likely to be the fallback of M33 gas from a galactic fountain and/or the gas pulled loosed during a close interaction between M31 and M33. Our study of M33 is the first to unambiguously reveal the existence of a disk-wide, ionized gas inflow beyond the Milky Way, providing a better understanding of gas accretion in the vicinity of a galaxy disk.

High-resolution HI and CO observations of high-latitude intermediate-velocity clouds

Intermediate-velocity clouds (IVCs) are HI halo clouds that are likely related to a Galactic fountain process. In-falling IVCs are candidates for the re-accretion of matter onto the Milky Way. We study the evolution of IVCs at the disk-halo interface, focussing on the transition from atomic to molecular IVCs. We compare an atomic IVC to a molecular IVC and characterise their structural differences in order to investigate how molecular IVCs form high above the Galactic plane. With high-resolution HI observations of the Westerbork Synthesis Radio Telescope and 12CO(1-0) and 13CO(1-0) observations with the IRAM 30m telescope, we analyse the small-scale structures within the two clouds. By correlating HI and far-infrared (FIR) dust continuum emission from the Planck satellite, the distribution of molecular hydrogen (H2) is estimated. We conduct a detailed comparison of the HI, FIR, and CO data and study variations of the $X_\rm{CO}$ conversion factor. The atomic IVC does not disclose detectable CO emission. The atomic small-scale structure, as revealed by the high-resolution HI data, shows low peak HI column densities and low HI fluxes as compared to the molecular IVC. The molecular IVC exhibits a rich molecular structure and most of the CO emission is observed at the eastern edge of the cloud. There is observational evidence that the molecular IVC is in a transient and, thus, non-equilibrium phase. The average $X_\rm{CO}$ factor is close to the canonical value of the Milky Way disk. We propose that the two IVCs represent different states in a gradual transition from atomic to molecular clouds. The molecular IVC appears to be more condensed allowing the formation of H2 and CO in shielded regions all over the cloud. Ram pressure may accumulate gas and thus facilitate the formation of H2. We show evidence that the atomic IVC will evolve also into a molecular IVC in a few Myr.

Neutral hydrogen and magnetic fields in M83 observed with the SKA Pathfinder KAT-7

We present new KAT-7 observations of the neutral hydrogen (H i) spectral line, and polarized radio continuum emission, in the grand-design spiral M83. These observations provide a sensitive probe of the outer-disc structure and kinematics, revealing a vast and massive neutral gas distribution that appears to be tightly coupled to the interaction of the galaxy with the environment. We present a new rotation curve extending out to a radius of 50 kpc. Based on our new H i data set and comparison with multiwavelength data from the literature, we consider the impact of mergers on the outer disc and discuss the evolution of M83. We also study the periphery of the H i distribution and reveal a sharp edge to the gaseous disc that is consistent with photoionization or ram pressure from the intergalactic medium. The radio continuum emission is not nearly as extended as the H i and is restricted to the main optical disc. Despite the relatively low angular resolution, we are able to draw broad conclusions about the large-scale magnetic field topology. We show that the magnetic field of M83 is similar in form to other nearby star-forming galaxies, and suggest that the disc–halo interface may host a large-scale regular magnetic field.

GALAXIES PROBING GALAXIES AT HIGH RESOLUTION: CO-ROTATING GAS ASSOCIATED WITH A MILKY WAY ANALOG AT z = 0.4

ABSTRACT We present results on gas flows in the halo of a Milky-Way-like galaxy at z = 0.413 based on high-resolution spectroscopy of a background galaxy. This is the first study of circumgalactic gas at high spectral resolution toward an extended background source (i.e., a galaxy rather than a quasar). Using long-slit spectroscopy of the foreground galaxy, we observe spatially extended Hα emission with a circular rotation velocity km s−1. Using echelle spectroscopy of the background galaxy, we detect and absorption lines at an impact parameter kpc that are blueshifted from systemic in the sense of the foreground galaxy’s rotation. The strongest absorber ( Å) has an estimated column density ( cm−2) and line-of-sight velocity dispersion ( km s−1) that are consistent with the observed properties of extended disks in the local universe. Our analysis of the rotation curve also suggests that this gaseous disk is warped with respect to the stellar disk. In addition, we detect two weak absorbers in the halo with small velocity dispersions ( km s−1). While the exact geometry is unclear, one component is consistent with an extraplanar gas cloud near the disk–halo interface that is co-rotating with the disk, and the other is consistent with a tidal feature similar to the Magellanic Stream. We can place lower limits on the cloud sizes ( kpc) for these absorbers given the extended nature of the background source. We discuss the implications of these results for models of the geometry and kinematics of gas in the circumgalactic medium.

Star formation in a diffuse high-altitude cloud?

A recent discovery of two stellar clusters associated with the diffuse high-latitude cloud HRK 81.4-77.8 has important implications for star formation in the Galactic halo. We derive a plausible distance estimate to HRK 81.4-77.8 primarily from its gaseous properties. We spatially correlate state-of-the-art HI, far-infrared and soft X-ray data to analyze the diffuse gas in the cloud. The absorption of the soft X-ray emission from the Galactic halo by HRK 81.4-77.8 is used to constrain the distance to the cloud. HRK 81.4-77.8 is most likely located at an altitude of about 400 pc within the disk-halo interface of the Milky Way Galaxy. The HI data discloses a disbalance in density and pressure between the warm and cold gaseous phases. Apparently, the cold gas is compressed by the warm medium. This disbalance might trigger the formation of molecular gas high above the Galactic plane on pc to sub-pc scales.

Galaxy interactions in compact groups – II. Abundance and kinematic anomalies in HCG 91c

Galaxies in Hickson Compact Group 91 (HCG 91) were observed with the WiFeS integral field spectrograph as part of our ongoing campaign targeting the ionized gas physics and kinematics inside star forming members of compact groups. Here, we report the discovery of HII regions with abundance and kinematic offsets in the otherwise unremarkable star forming spiral HCG 91c. The optical emission line analysis of this galaxy reveals that at least three HII regions harbor an oxygen abundance ~0.15 dex lower than expected from their immediate surroundings and from the abundance gradient present in the inner regions of HCG 91c. The same star forming regions are also associated with a small kinematic offset in the form of a lag of 5-10 km/s with respect to the local circular rotation of the gas. HI observations of HCG 91 from the Very Large Array and broadband optical images from Pan-STARRS suggest that HCG 91c is caught early in its interaction with the other members of HCG 91. We discuss different scenarios to explain the origin of the peculiar star forming regions detected with WiFeS, and show that evidence point towards infalling and collapsing extra-planar gas clouds at the disk-halo interface, possibly as a consequence of long-range gravitational perturbations of HCG 91c from the other group members. As such, HCG 91c provides evidence that some of the perturbations possibly associated with the early phase of galaxy evolution in compact groups impact the star forming disk locally, and on sub-kpc scales.

Gaseous Galaxy Halos

Galactic halo gas traces inflowing star-formation fuel and feedback from a galaxy's disk and is therefore crucial to our understanding of galaxy evolution. In this review, we summarize the multiwavelength observational properties and origin models of Galactic and low-redshift spiral galaxy halo gas. Galactic halos contain multiphase gas flows that are dominated in mass by the ionized component and extend to large radii. The densest, coldest halo gas observed in neutral hydrogen (Hi) is generally closest to the disk (<20 kpc), and absorption line results indicate warm and warm-hot diffuse halo gas is present throughout a galaxy's halo. The hot halo gas detected is not a significant fraction of a galaxy's baryons. The disk-halo interface is where the multiphase flows are integrated into the star-forming disk, and there is evidence for both feedback and fueling at this interface from its temperature and kinematic gradients, and Hi structures. The origin and fate of halo gas are considered in the context of cosmological and idealized local simulations. Accretion along cosmic filaments occurs in both hot (>105.5 K) and cold modes in simulations, with the compressed material close to the disk being the coldest and densest, in agreement with observations. There is evidence in halo gas observations for radiative and mechanical feedback mechanisms, including escaping photons from the disk, supernova-driven winds, and a galactic fountain. Satellite accretion also leaves behind abundant halo gas. This satellite gas interacts with the existing halo medium, and much of this gas will become part of the diffuse halo before it reaches the disk. The accretion rate from cold and warm halo gas is generally below a galaxy disk's star-formation rate, but gas at the disk-halo interface and stellar feedback may be important additional fuel sources.

The Accretion of Fuel at the Disk-Halo Interface

We discuss the support for the cooling of gas directly at the disk-halo interface as a mechanism to continually fuel a galaxy. This may be an important reservoir as there is not enough cold gas observed in galaxy halos and simulations indicate the existing cold clouds will be rapidly destroyed as they move through the surrounding halo medium. We show possible evidence for a net infall of the WIM layer in the Milky Way, simulation results showing the recooling of warm clouds at the disk-halo interface, and GALFA HI data of small, cold HI clouds that could represent this recooling.

The Milky Way and its gas: Cold fountains and accretion

The Milky Way is acquiring gas from infalling high-velocity clouds. The material enters a disk-halo interface that in many places is populated with HI clouds that have been ejected from the disk through processes linked to star formation. The Smith Cloud is an extraordinary example of a high-velocity cloud that is bringing $>10^6$ M$_{\odot}$ of relatively low metallicity gas into the Milky Way. It may be part of a larger stream, components of which are now passing through the disk.

Properties of extra-planar H I clouds in the outer part of the Milky Way

<i>Context. <i/>There is mounting evidence for an extra-planar gas layer around the Milky Way disk, similar to the anomalous H I gas detected in a few other galaxies. As much as 10% of the gas may be in this phase.<i>Aims. <i/>We analyze H I clouds located in the disk-halo interface outside the solar circle to probe the properties of the extra-planar H I gas, which is following Galactic rotation.<i>Methods. <i/>We use the Leiden/Argentine/Bonn (LAB) 21-cm line survey to search for H I clouds which take part in the rotation of the Galactic plane, but are located above the disk layer. Selected regions are mapped with the Effelsberg 100-m telescope. Two of the H I halo clouds are studied in detail for their small scale structure using the Westerbork Synthesis Radio Telescope (WSRT)[The Westerbork Synthesis Radio Telescope is operated by ASTRON (Netherlands Foundation for Research in Astronomy) with support from the Netherlands Foundation for Scientific Research NWO.] and the NRAO Very Large Array (VLA)[The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.].<i>Results. <i/>Data from the 100 m telescope allow for the parameterization of 25 distinct H I halo clouds at Galactocentric radii 10 kpc <i><R<<i/>15 kpc and heights 1 kpc < <i>z<i/> < 5 kpc. The clouds have a median temperature of 620 K, column densities of ~ 10<sup>19<sup/> cm<sup>-2<sup/>, and most of them are surrounded by an extended envelope of warmer H I gas. Interferometer observations for two selected regions resolve the H I clouds into several arc-minute sized cores. These cores show narrow line widths (<i>FWHM<i/> ~ 3 km s<sup>-1<sup/>), they have volume densities of <i>n<i/> > 1.3 cm<sup>-3<sup/>, masses up to 24 , and are on average in pressure equilibrium with the surrounding envelopes. Pressures and densities fall within the expectations from theoretical phase diagrams (<i>P<i/> vs. ). The H I cores tend to be unstable if one assumes a thermally bistable medium, but are in better agreement with models that predict thermal fragmentation driven by a turbulent flow.

Detailed maps of interstellar clouds in front of ω Centauri: small-scale structures in the Galactic disc-halo interface

The multiphase interstellar medium (ISM) is highly structured, on scales from the size of the Solar System to that of a galaxy. In particular, small-scale structures are difficult to Study and hence are poorly understood. We used the multiplex capabilities of the AAOmega spectrograph at the Anglo-Australian Telescope to create a half-square-degree map of the neutral and low-ionized ;ISM in front of the nearby (similar to 5 kpc), most massive Galactic globular cluster, omega Centauri. Its redshifted, metal-poor and hot horizontal branch stars probe the medium-strong Ca II K and Na I D-2 line absorption, and weak absorption in the lambda 5780 and lambda 5797 diffuse interstellar bands (DIBs), on scales around a parsec. The kinematical and thermodynamical picture emerging from these data is that we predominantly probe the warm neutral medium and weakly ionized medium of the Galactic Disc-Halo interface, similar to 0.3-1 kpc above the mid-plane. A comparison with Spitzer Space Telescope 24 mu m and DIRBE/IRAS maps of the warm and cold dust emission confirms that both Na I and Ca II trace the overall column density of the warm neutral and weakly ionized medium. Clear signatures are seen of the depletion of calcium atoms from the gas phase into dust grains. Curiously, the coarse DIRBE/IRAS map is a more reliable representation of the relative reddening between sightlines than the Na I and Ca II absorption-line measurements, most likely because the latter are sensitive to fluctuations in the local ionization conditions. The behaviour of the DIBs is consistent with the;0780 band being stronger than the lambda 5797 band in regions where the ultraviolet radiation level is relatively high, as in the Disc-Halo interface. This region corresponds to a or-type cloud in which Ca I and small diatomic molecules Such as CH and CN are usually absent. In all, Our maps and simple analytical model calculations show in unprecedented detail that small-scale density and/or ionization structures exist in the extra-planar gas of a spiral galaxy.

Kinematics of diffuse ionized gas in the disk halo interface of NGC 891 from Fabry-Pérot observations

Context. The properties of the gas in halos of galaxies constrain global models of the interstellar medium. Kinematical information is of particular interest since it is a clue to the origin of the gas. Aims. Here we report observations of the kinematics of the thick layer of the diffuse ionized gas in NGC 891 in order to determine the rotation curve of the halo gas. Methods. We have obtained a Fabry-Perot data cube in H alpha to measure the kinematics of the halo gas with angular resolution much higher than obtained from HI 21 cm observations. The data cube was obtained with the TAURUS II spectrograph at the WHT on La Palma. The velocity information of the diffuse ionized gas extracted from the data cube is compared to model distributions to constrain the distribution of the gas and in particular the halo rotation curve. Results. The best fit model has a central attenuation tau H alpha = 6, a dust scale length of 8.1 kpc, an ionized gas scale length of 5.0 kpc. Above the plane the rotation curve lags with a vertical gradient of -18.8 kms(-1) kpc(-1). We find that the scale length of the H alpha must be between 2.5 and 6.5 kpc. Furthermore we find evidence that the rotation curve above the plane rises less steeply than in the plane. This is all in agreement with the velocities measured in the HI.

Further Evidence of a Merger Origin for the Thick Disk: Galactic Stars along Lines of Sight to Dwarf Spheroidal Galaxies

The history of the Milky Way is written in the properties of its stellar populations. Here we analyze stars observed as part of surveys of local dwarf spheroidal galaxies, but which from their kinematics are highly likely to be nonmembers. The selection function—designed to target metal-poor giants in the dwarf galaxies, at distances of ~100 kpc—includes F-M dwarfs in the Milky Way, at distances of up to several kiloparsecs. The stars whose motions are analyzed here lie in the cardinal directions of Galactic longitude l ~ 270° and l ~ 90°, where the radial velocity is sensitive to the orbital rotational velocity. We demonstrate that the faint F and G stars contain a significant population with Vphgr ~ 100 km s-1, similar to that found by a targeted, but limited in areal coverage, survey of thick disk and halo stars by Gilmore et al. This value of mean orbital rotation does not match either the canonical thick disk or the stellar halo. We argue that this population, detected at both l ~ 270° and l ~ 90°, has the expected properties of satellite debris in the thick disk-halo interface, which we interpret as remnants of the merger that heated a preexisting thin disk to form the thick disk.

Molecular Hydrogen in Infrared Cirrus

We combine data from our recent FUSE survey of interstellar molecular hydrogen absorption toward 50 high-latitude AGNs with COBE-corrected IRAS 100 ?m emission maps to study the correlation of infrared cirrus with H2. A plot of the H2 column density versus IR cirrus intensity shows the same transition in molecular fraction, f, as seen with total hydrogen column density, NH. This transition is usually attributed to H2 self-shielding, and it suggests that many diffuse cirrus clouds contain H2 in significant fractions, f ? 1%-30%. These clouds cover ~50% of the northern sky at b > 30?, at temperature-corrected 100 ?m intensities D ? 1.5 MJy sr-1. The sheetlike cirrus clouds, with hydrogen densities nH ? 30 cm-3, may be compressed by dynamical processes at the disk-halo interface, and they are conducive to H2 formation on grain surfaces. Exploiting the correlation between N and 100 ?m intensity, we estimate that cirrus clouds at b > 30? contain ~3000 M? in H2. Extrapolated over the inner Milky Way, the cirrus may contain 107 M? of H2 and 108 M? in total gas mass. If elevated to 100 pc, their gravitational potential energy is ~1053 ergs.