Bart P. Wakker

HIGH-VELOCITY CLOUDS

▪ Abstract High-velocity clouds (HVCs) consist of neutral hydrogen (HI) at velocities incompatible with a simple model of differential galactic rotation; in practice one uses vLSR 90 km/s to define HVCs. This review describes the main features of the sky and velocity distributions, as well as the available information on cloud properties, small-scale structure, velocity structure, and observations other than in 21-cm emission. We show that HVCs contain heavy elements and that the more prominent ones are more than 2 kpc from the Galactic plane. We evaluate the hypotheses proposed for their origin and reject those that account for only one or a few HVCs. At least three different hypotheses are needed: one for the Magellanic Stream and possibly related clouds, one for the Outer Arm Extension, and one (or more) for the other HVCs. We discuss the evidence for the accretion and the fountain model but cannot rule out either one.

Accretion of low-metallicity gas by the Milky Way

Models of the chemical evolution of the Milky Way suggest that the observed abundances of elements heavier than helium (‘metals’) require a continuous infall of gas with metallicity (metal abundance) about 0.1 times the solar value. An infall rate integrated over the entire disk of the Milky Way of ∼1 solar mass per year can solve the ‘G-dwarf problem’—the observational fact that the metallicities of most long-lived stars near the Sun lie in a relatively narrow range. This infall dilutes the enrichment arising from the production of heavy elements in stars, and thereby prevents the metallicity of the interstellar medium from increasing steadily with time. However, in other spiral galaxies, the low-metallicity gas needed to provide this infall has been observed only in associated dwarf galaxies and in the extreme outer disk of the Milky Way. In the distant Universe, low-metallicity hydrogen clouds (known as ‘damped Lyα absorbers’) are sometimes seen near galaxies. Here we report a metallicity of 0.09 times solar for a massive cloud that is falling into the disk of the Milky Way. The mass flow associated with this cloud represents an infall per unit area of about the theoretically expected rate, and ∼0.1–0.2 times the amount required for the whole Galaxy.

Multiphase High-Velocity Clouds toward HE 0226–4110 and PG 0953+414*

We study the physical conditions, elemental abundances, and kinematics of the high-velocity clouds (HVCs) along the sight lines toward active galaxies HE 0226-4110 and PG 0953+414 using Hubble Space Telescope Space Telescope Imaging Spectrograph and Far Ultraviolet Spectroscopic Explorer data. No 21 cm H I emission is detected in these clouds, but our observations reveal multiple components of HVC absorption in lines of H I, C II, C III, C IV, O VI, Si II, Si III, and Si IV in both directions. We investigate whether photoionization by the extragalactic background radiation or by escaping Milky Way radiation can explain the observed ionization pattern. We find that photoionization is a good explanation for the C II, C III, Si II, and Si III features but not for the O VI or C IV associated with the HVCs, suggesting that two principal phases exist: a warm (T ≈ 104 K), photoionized phase and a hotter (T = 1-3 × 105 K), collisionally ionized phase; the broader line widths of the high ions are consistent with this multiphase hypothesis. The warm HVCs toward HE 0226-4110 have high levels of ionization (97%-99%) and metallicities ([Z/H] between -0.9 and -0.4) close to those in the Magellanic Stream, which lies 11° away on the sky at similar velocities. These HVCs may well be stripped fragments of the Stream that have been ionized by the pervading radiation field; they have thermal pressures that would place them close to equilibrium in a fully ionized 106 K Galactic corona with nH = 4-9 × 10-5 cm-3 at 50 kpc. The warm HVCs seen at -146 and 125 km s-1 toward PG 0953+414 have [Z/H] = -0.6 ± 0.2 and -0.8 ± 0.2, respectively, suggesting they are not formed from purely Galactic material. A minisurvey of the hot, collisionally ionized HVC components seen here and in five other sight lines finds that in 11/12 cases, the high ions have kinematics and ionic ratios that are consistent with an origin in conductive interfaces, where energy flows into the HVCs from a hot surrounding medium and produces O VI- and C IV-bearing boundary layers. However, the broad absorption wing on the O VI profile toward PG 0953+414 is not completely explained by the interface scenario. This feature may be tracing the outflow of hot gas into the Milky Way halo as part of a Galactic fountain or wind.

The Diversity of High- and Intermediate-Velocity Clouds: Complex C versus IV Arch

We present Far Ultraviolet Spectroscopic Explorer (FUSE) and Space Telescope Imaging Spectrograph (STIS) observations of interstellar ultraviolet absorption lines in the Galactic high-velocity cloud Complex C and the Intermediate-Velocity Arch (IV Arch) in the direction of the quasar PG 1259+593 (l = 1206, b = +581). Absorption lines from C II, N I, N II, O I, Al II, Si II, P II, S II, Ar I, Fe II, and Fe III are used to study the atomic abundances in these two halo clouds at VLSR ~ -130 km s-1 (Complex C) and -55 km s-1 (IV Arch). The O I/H I ratio provides the best measure of the overall metallicity in the diffuse interstellar medium because ionization effects do not alter the ratio, and oxygen is at most only lightly depleted from the gas into dust grains. For Complex C, we find an oxygen abundance of 0.093 times solar, consistent with the idea that Complex C represents the infall of low-metallicity gas onto the Milky Way. In contrast, the oxygen abundance in the IV Arch is 0.98 times solar, which indicates a Galactic origin. We report the detection of an intermediate-velocity absorption component at +60 km s-1 that is not seen in H I 21 cm emission. The clouds along the PG 1259+593 sight line have a variety of properties, proving that multiple processes are responsible for the creation and circulation of intermediate and high-velocity gas in the Milky Way halo.

Highly Ionized Gas Surrounding High-Velocity Cloud Complex C*

We present Far Ultraviolet Spectroscopic Explorer and Hubble Space Telescope observations of high-, intermediate-, and low-ion absorption in high-velocity cloud (HVC) Complex C along the lines of sight toward five active galaxies. Our purpose is to investigate the idea that Complex C is surrounded by an envelope of highly ionized material, arising from the interaction between the cloud and a hot surrounding medium. We measure column densities of high-velocity high-ion absorption and compare the kinematics of low-, intermediate-, and high-ionization gas along the five sight lines. We find that in all five cases, the H I and O VI high-velocity components are centered within 20 km s-1 of one another, with an average displacement of = 3 ± 12 km s-1. In those directions where the H I emission extends to more negative velocities (the so-called high-velocity ridge), so does the O VI absorption. The kinematics of Si II is also similar to that of O VI, with = 0 ± 15 km s-1. We compare our high-ion column density ratios to the predictions of various models, adjusted to account for both recent updates to the solar elemental abundances and relative elemental abundance ratios in Complex C. Along the PG 1259+593 sight line, we measure N(Si )/N(O ) = 0.10 ± 0.02, N(C )/N(O ) = 0.35, and N(N )/N(O ) < 0.07 (3 σ). These ratios are inconsistent with collisional ionization equilibrium at one kinetic temperature. Photoionization by the extragalactic background is ruled out as the source of the high ions since the path lengths required would make HVCs unreasonably large; photoionization by radiation from the disk of the Galaxy also appears unlikely since the emerging photons are not energetic enough to produce O VI. By themselves, ionic ratios are insufficient to discriminate between various ionization models, but by considering the absorption kinematics as well, we consider the most likely origin for the highly ionized high-velocity gas to be at the conductive or turbulent interfaces between the neutral/warm ionized components of Complex C and a surrounding hot medium. The presence of interfaces on the surface of HVCs provides indirect evidence for the existence of a hot medium in which the HVCs are immersed. This medium could be a hot (T 106 K) extended Galactic corona or hot gas in the Local Group.

Distances to galactic high-velocity clouds: Complex C

We report the first determination of a distance bracket for the high- velocity cloud (HVC) complex C. Combined with previous measurements showing that this cloud has a metallicity of 0.15 times solar, these results provide ample evidence that complex C traces the continuing accretion of intergalactic gas falling onto the Milky Way. Accounting for both neutral and ionized hydrogen as well as He, the distance bracket implies a mass of (3-14) x 10(6) M-circle dot, and the complex represents a mass inflow of 0.1-0.25 M-circle dot yr(-1). We base our distance bracket on the detection of Ca II absorption in the spectrum of the blue horizontal branch (BHB) star SDSS J120404.78 + 623345.6, in combination with a significant nondetection toward the BHB star BS 16034-0114. These results set a strong distance bracket of 3.7-11.2 kpc on the distance to complex C. A more weakly supported lower limit of 6.7 kpc may be derived from the spectrum of the BHB star BS 16079-0017.

Detection of Ne VIII in the low-redshift warm-hot intergalactic medium

High-resolution FUSE and STIS observations of the bright QSO HE 0226-4110 (zem = 0.495) reveal the presence of a multiphase absorption-line system at zabs(O ) = 0.20701 containing absorption from H I (Lyα to Lyθ), C III, O III, O IV, O VI, N III, Ne VIII, Si III, S VI, and possibly S V. Single-component fits to the Ne VIII and O VI absorption doublets yield log N(Ne ) = 13.89 ± 0.11, b = 23 ± 15 km s-1, and v = -7 ± 6 km s-1 and log N(O ) = 14.37 ± 0.03, b = 31 ± 2 km s-1, and v = 0 ± 2 km s-1. The Ne VIII and O VI doublets are detected at 3.9 and 16 σ significance levels, respectively. This represents the first detection of intergalactic Ne VIII, a diagnostic of gas with temperature in the range from ~5 × 105 to ~1 × 106 K. Through the entire absorber, N(Ne )/N(O ) = 0.33 ± 0.10. The O VI and Ne VIII are not likely to have been created in a low-density medium photoionized solely by the extragalactic background at z = 0.2, since the required path length ~11 Mpc (assuming [Z/H] = -0.5) implies that the Hubble flow absorption-line broadening would be ~10 times greater than the observed line widths. A collisional ionization origin is therefore more likely. Assuming [Ne/H] and [O/H] = -0.5, the value N(Ne )/N(O ) = 0.33 ± 0.10 is consistent with gas in collisional ionization equilibrium near T = 5.4 × 105 K with log N(H) = 19.9 and N(H)/N(H ) = 1.7 × 106. Various nonequilibrium ionization processes are also considered, because gas with T ~ (1-6) × 105 K cools efficiently. The observations of O VI and Ne VIII in the z = 0.20701 system support the basic idea that a substantial fraction of the baryonic matter at low redshift exists in hot, very highly ionized gaseous structures. Absorption by the moderately ionized gas (including O IV and S VI) is well modeled by gas in photoionization equilibrium with [Z/H] = -0.5 ± 0.2, log U ~ -1.85, T ~ 2.1 × 104 K, nH ~ 2.6 × 10-5 cm-3, P/k ~ 0.5 cm-3 K, and a path length of ~60 kpc. These values suggest that the moderately ionized absorber may be associated with the modestly enriched photoionized gas in a galaxy group or the outermost regions of a galaxy halo.

The Metallicity and Dust Content of HVC 287.5+22.5+240: Evidence for a Magellanic Clouds Origin*

?????We estimate the abundances of S and Fe in the high-velocity cloud HVC 287.5+22.5+240, which has a velocity of 240 km s-1 with respect to the local standard of rest and Galactic direction l ~ 287? and b ~ 23?. The measurements are based on UV absorption lines of these elements in the Hubble Space Telescope spectrum of NGC 3783, a background Seyfert galaxy, as well as new H I 21 cm interferometric data taken with the Australia Telescope Compact Array. We find S/H = 0.25 ? 0.07 times solar, Fe/H = 0.033 ? 0.006 times solar, and S/Fe = 7.6 ? 2.2 times solar. The S/H value provides an accurate measure of the chemical enrichment level in the HVC, while the supersolar S/Fe ratio clearly indicates the presence of dust, which depletes the gas-phase abundance of Fe. The metallicity and depletion information obtained here, coupled with the velocity and position of the HVC, strongly suggest that it originated from the Magellanic Clouds. It is likely, though not necessary, that the same processes that generated the Magellanic Stream are responsible for HVC 287.5+22.5+240.

A confirmed location in the Galactic halo for the high-velocity cloud 'chain A'

The high-velocity clouds of atomic hydrogen, discovered about 35 years ago,, have velocities inconsistent with simple Galactic rotation models that generally fit the stars and gas in the Milky Way disk. Their origins and role in Galactic evolution remain poorly understood, largely for lack of information on their distances. The high-velocity clouds might result from gas blown from the Milky Way disk into the halo by supernovae,, in which case they would enrich the Galaxy with heavy elements as they fall back onto the disk. Alternatively, they may consist of metal-poor gas—remnants of the era of galaxy formation,, accreted by the Galaxy and reducing its metal abundance. Or they might be truly extragalactic objects in the Local Group of galaxies. Here we report a firm distance bracket for a large high-velocity cloud, chain A, which places it in the Milky Way halo (2.5 to 7 kiloparsecs above the Galactic plane), rather than at an extragalactic distance, and constrains its gas mass to between 105 and 2×106 solar masses.

A Survey of O VI, C III, and H I in Highly Ionized High-Velocity Clouds

We present a Far Ultraviolet Spectroscopic Explorer survey of highly ionized high-velocity clouds (HVCs) in 66 extragalactic sight lines with (S/N)1030 > 8. We search the spectra for high-velocity (100 km s-1 180?. Eleven of these highly ionized HVCs are positive-velocity wings (broad O VI features extending asymmetrically to velocities of up to 300 km s-1). We find that 81% (30 of 37) of highly ionized HVCs have clear accompanying C III absorption, and 76% (29 of 38) have accompanying H I absorption in the Lyman series. We present the first (O VI selected) sample of C III and H I absorption line HVCs and find b(C ) = 30 ? 8 km s-1, log Na(C ) ranges from 14.4, b(H ) = 22 ? 5 km s-1, and log Na(H I) ranges from 16.9. The lower average width of the high-velocity H I absorbers implies the H I lines arise in a separate, lower temperature phase than the O VI. The ratio Na(C III)/Na(O VI) is generally constant with velocity in highly ionized HVCs, suggesting that at least some C III resides in the same gas as the O VI. Collisional ionization equilibrium models with solar abundances can explain the O VI/C III ratios for temperatures near 1.7 ? 105 K; nonequilibrium models with the O VI frozen in at lower temperatures are also possible. Photoionization models are not viable since they underpredict O VI by several orders of magnitude. The presence of associated C III and H I strongly suggests the highly ionized HVCs are not formed in the hotter plasma that gives rise to O VII and O VIII X-ray absorption. We find that the shape of the O VI positive-velocity wing profiles is well reproduced by a radiatively cooling, vertical outflow moving with ballistic dynamics, with T0 = 106 K, n0 ? 2 ? 10-3 cm-3, and v0 ? 250 km s-1. However, the outflow has to be patchy and out of ionization equilibrium to explain the sky distribution and the simultaneous presence of O VI, C III, and H I. We found that a spherical outflow can produce high-velocity O VI components (as opposed to the wings), showing that the possible range of outflow model results is too broad to conclusively identify whether or not an outflow has left its signature in the data. An alternative model, supported by the similar multiphase structure and similar O VI properties of highly ionized and 21 cm HVCs, is one where the highly ionized HVCs represent the low N(H I) tail of the HVC population, with the O VI formed at the interfaces around the embedded H I cores. Although we cannot rule out the possibility that some highly ionized HVCs exist in the Local Group or beyond, we favor a Galactic origin. This is based on the recent evidence that both H I HVCs and the million-degree gas detected in X-ray absorption are Galactic phenomena. Since the highly ionized HVCs appear to trace the interface between these two Galactic phases, it follows that highly ionized HVCs are Galactic themselves. However, the nondetection of high-velocity O VI in halo star spectra implies that any Galactic high-velocity O VI exists at z distances beyond a few kpc.