Intermediate-velocity Clouds Research Articles

Multiwavelength observations of the M15 intermediate-velocity cloud

We present Westerbork Synthesis Radio Telescope HI images, Lovell Telescope multibeam HI wide-field mapping, Wisconsin H-alpha Mapper facility images, William Herschel Telescope longslit echelle CaII observations, and IRAS ISSA 60 and 100 micron coadded images towards the intermediate velocity cloud located in the general direction of the M15 globular cluster. When combined with previously-published Arecibo data, the HI gas in the IVC is found to be clumpy, with peak HI column density of 1.5x10^(20) cm^(-2), inferred volume density (assuming spherical symmetry) of 24 cm^(-3)/(D kpc), and maximum brightness temperature at a resolution of 81x14 arcsec of 14 K. The HI gas in the cloud is warm, with a minimum FWHM value of 5 km/s, corresponding to a kinetic temperature, in the absence of turbulence, of 540 K. There are indications in the HI data of 2-component velocity structure in the IVC, indicative of cloudlets. This velocity structure is also tentatively seen in the CaK spectra, although the SNR is low. The main IVC condensation is detected by WHAM in H-alpha with intensities uncorrected for Galactic absorption of upto 1.3 Rayleigh, indicating that the cloud is partially ionised. The FWHM of the ionised component, at a resolution of 1 degree, exceeds 30 km/s. The spatial and velocity coincidence of the H-alpha and HI peaks in emission is qualitatively good. Finally, the 100 and 60 micron IRAS images show spatial coincidence over a 0.7 degree field, with low and intermediate-velocity gas, respectively, indicating that the IVC may contain dust.

Optical spectroscopy of the M 15 intermediate velocity cloud

We present echelle spectrograph observations in the Na D lines, at resolutions of 6.2 8.5 km s 1 , for 11 stars located in the line-of-sight to the M 15 intermediate velocity cloud (IVC), which has a radial velocity of+70 km s 1 in the Local Standard of Rest. This cloud is a part of IVC Complex gp. The targets range in magnitude from mV =1 3:3 14.8. Seven of the observed stars are in the M 15 globular cluster, the remaining four being eld stars. Three of the observed cluster stars are located near a peak in intensity of the IVC H i column density as observed at a resolution of 1 arcmin. Intermediate velocity gas is detected in absorption towards 7 stars, with equivalent widths in Na D2 ranging from0.09 0.20 A, corresponding to log10(NNa cm 2 ) 11.8 12.5, and Na i/H i column density ratios (neglecting the H ii component) ranging from (1 3)10 8 . Over scales ranging from 30 arcsec to 1 arcmin, the Na i column density and the Na i/H i ratio varies by upto 70 per cent and a factor of2, respectively. Combining the current sightlines with previously obtained Na i data from Kennedy et al. (1998b), the Na i/H i column density ratio over cluster sightlines varies by upto a factor of 25, when using H i data of resolution 21 arcmin. One cluster star, M 15 ZNG-1, was also observed in the Ca i (air = 4226:728 A) and Ca ii (air = 3933:663 A) lines. A column density ratio N(Ca i)/N(Ca ii) < 0.03 was found, typical of values seen in the warm ionised interstellar medium. Towards this sightline, the IVC has aN ai/Ca ii column density ratio of 0.25, similar to that observed in the local interstellar medium. Finally, we detect tentative evidence for IV absorption in K i (air = 7698:974 A) towards 3 cluster stars, which have N(K i)/N(H i )r atios of0.5 310 9 .

Neutral Atomic Carbon in Intermediate‐Velocity Clouds

Using the Heinrich-Hertz-Telescope we have detected for the —rst time the [C I] line tran3P 1 ] 3P 0 sition at 492 GHz of neutral atomic carbon from two intermediate-velocity clouds (IVCs): IVC 135]54[46 and the Draco Nebula. Both clouds are located at high Galactic altitudes, more than 200 pc above the Galactic plane. The width of the [C I] line is quantitatively comparable to that of the 12CO rather than to that of the 13CO line, as commonly found for Galactic plane clouds. This indicates that neutral atomic carbon within the diUuse medium at high Galactic altitudes —lls a signi—cantly larger volume in comparison to the situation in dark or denser cirrus clouds close to the Galactic plane. Moreover, our observations suggest that for IVCs cooling by the [C I] line is as important as or even more important than that by the low level CO lines. PDR models used to approximate the observed line intensity ratios indicate that the molecular gas in high-altitude IVCs is exposed to a weak interstellar radiation —eld while its density must be high. . . . . ..

Distances and Metallicities of High‐ and Intermediate‐Velocity Clouds

A table is presented that summarizes published absorption line measurements for the high- and intermediate velocity clouds (HVCs and IVCs). New values are derived for N(HI) in the direction of observed probes, in order to arrive at reliable abundances and abundance limits (the HI data are described in Paper II). Distances to stellar probes are revisited and calculated consistently, in order to derive distance brackets or limits for many of the clouds, taking care to properly interpret non-detections. The main conclusions are the following. 1) Absolute abundances have been measured using lines of SII, NI and OI, with the following resulting values: ~0.1 solar for one HVC (complex C), ~0.3 solar for the Magellanic Stream, ~0.5 solar for a southern IVC, and ~ solar for two northern IVCs (the IV Arch and LLIV Arch). Finally, approximate values in the range 0.5-2 solar are found for three more IVCs. 2) Depletion patterns in IVCs are like those in warm disk or halo gas. 3) Most distance limits are based on strong UV lines of CII, SiII and MgII, a few on CaII. Distance limits for major HVCs are >5 kpc, while distance brackets for several IVCs are in the range 0.5-2 kpc. 4) Mass limits for major IVCs are 0.5-8x10^5 M_sun, but for major HVCs they are >10^6 M_sun. 5) The CaII/HI ratio varies by up to a factor 2-5 within a single cloud, somewhat more between clouds. 6) The NaIHI ratio varies by a factor >10 within a cloud, and even more between clouds. Thus, CaII can be useful for determining both lower and upper distance limits, but NaI only yields upper limits.

A Map of the Ionized Component of the Intermediate-Velocity Cloud Complex K

The Wisconsin H-Alpha Mapper (WHAM) Northern Sky Survey is revealing that many intermediate-velocity (|vLSR| ≤ 100 km s-1) neutral clouds and complexes have an associated ionized component. We present the first map of the Hα emission from an intermediate- or high-velocity structure: Complex K. This large, high-latitude feature stretches from l ≈ 10° to 70°, b ≈ +30° to +70° and peaks in velocity over vLSR ≈ -60 to -80 km s-1. The neutral and ionized gas generally trace each other quite well in the complex, but the detailed structure is not identical. In particular, the Hα emission peaks in brightness at slightly higher Galactic longitudes than corresponding 21 cm features. The ionized gas has a peak Hα intensity of 0.5 R, corresponding to an emission measure of 1.1 cm-6 pc. Structures in the complex are traced by WHAM down to about 0.1 R (0.2 cm-6 pc). Typical line widths of the Hα emission are ~30 km s-1, limiting temperatures in the ionized gas to less than 20,000 K. If radiation is the primary ionizing mechanism, the Lyman continuum flux required to sustain the most strongly emitting ionized regions is 1.2 × 106 photons cm-2 s-1. There appears to be no local, stellar source capable of maintaining the ionization of the gas; however, the required ionizing flux is consistent with current models of the escape of Lyman continuum radiation from OB stars in the disk and of ionizing radiation produced by cooling supernova remnants.

Variable Interstellar Absorption toward the Halo Star HD 219188: Implications for Small-Scale Interstellar Structure

Within the last 10 yr, strong, narrow Na I absorption has appeared at v☉ ~ -38 km s-1 toward the star HD 219188; that absorption has continued to strengthen, by a factor of 2-3, over the past 3 yr. The line of sight appears to be moving into/through a relatively cold, quiescent intermediate-velocity (IV) cloud, because of the 13 mas yr-1 proper motion of HD 219188; the variations in Na I probe length scales of 2-38 AU yr-1. UV spectra obtained with the Hubble Space Telescope GHRS in 1994-1995 suggest N(Htot) ~ 4.8 × 1017 cm-2, halo depletions, nH ~ 25 cm-3, and ne ~ 0.85-6.2 cm-3 (if T ~ 100 K) for the portion of the IV cloud sampled at that time. The relatively high fractional ionization, ne/nH 0.034, implies that hydrogen must be partially ionized. The N(Na I)/N(Htot) ratio is very high; in this case, the variations in Na I do not imply large local pressures or densities.

TheFUSESpectrum of PG 0804+761: A Study of Atomic and Molecular Gas in the Lower Galactic Halo and Beyond

We present an analysis of interstellar and intergalactic absorption lines in the Far-Ultraviolet Spectroscopic Explorer (FUSE) spectrum of the low-redshift quasar PG 0804]761 at intermediate (z em 0.100) resolution (FWHM D 25 km s~1) in the direction With a good signal-to-noise ratio l 138i.3, b 31i.0. (S/N) and the presence of several interesting Galactic and extragalactic absorption components along the sight line, this spectrum provides a good opportunity to demonstrate the ability of FUSE to do both interstellar and extragalactic science. Although the spectrum of PG 0804]761 is dominated by strong absorption from local Galactic gas at 0 km s~1, we concentrate our study on absorption by molecular hydrogen and neutral and ionized metals related to an intermediate-velocity cloud (IVC) in the lower Galactic halo at [55 km s~1, and on absorption from O VI extended to negative velocities. In the IVC, weak molecular hydrogen absorption is found in six lines for rotational levels 0 and 1, leading to a total column density of log N 14.71 ^ 0.30. We derive an O I gas-phase abundance for the IVC of H 2 solar. Lower abundances of other elements (Fe, Si) imply depletion onto dust grains or the 1.03 ~0.420.71 presence of higher, undetected ionization states. The presence of N II and Fe III absorption at [55 km s~1 indicates that a fraction of the hydrogen is ionized. From the relative abundances of O I and P II we estimate a degree of ionization H‘/(H0]H‘ )o fD19%. Absorption by O VI is found at velocities as negative as [110 km s~1, but no absorption from any species is found at velocities of D[180 km s~1, where absorption from the nearby high-velocity cloud complex A would be expected. We suggest that the extended O VI absorption traces hot gas situated above the Perseus spiral arm. Finally, we —nd intergalactic absorption by an intervening H I Lyb absorber at

H i and optical spectroscopy towards the M15 intermediate-velocity cloud

We present echelle spectrograph observations in NaD, at resolutions of 6.2-8.5 km/s, for 11 stars located in the line-of-sight to the M15 intermediate velocity cloud, which has a radial velocity of 70 km/s. This cloud is a part of IVC Complex gp. The targets range in magnitude from V=13.3-14.8. Seven of the observed stars are in the M15 globular cluster, the remaining four being field stars. Column density ratios of log(N cm^-2)=11.8-12.5 are derived. Combining the current sightlines with previously-existing data, we find the NaD/HI ratio in the IVC varies by upto a factor of 25. One cluster star, M15 ZNG-1, was also observed in Calcium. We find N(CaI)/N(CaII)<0.03 and NaI/CaII=0.25, similar to values seen in the local ISM. Finally, we detect tentative evidence for IV absorption in KI towards 3 cluster stars.

H i Spectra and Column Densities toward HVC and IVC Probes

We show 21 cm line profiles in the direction of stars and extragalactic objects, lying projected on high- and intermediate-velocity clouds (HVCs and IVCs). About half of these are from new data obtained with the Effelsberg 100 m telescope, about a quarter are extracted from the Leiden-Dwingeloo Survey (LDS), and the remaining quarter were observed with other single-dish telescopes. H I column densities were determined for each HVC/IVC. Paper I of this series uses these in combination with optical and ultraviolet high-resolution measurements to derive abundances. Here an analysis is given of the difference and ratio of N(H I) as observed with a 9' versus a 35' beam. For HVCs and IVCs the ratio N(H I-9')/N(H I -35') lies in the range 0.2-2.5. For low-velocity gas this ratio ranges from 0.75 to 1.3 (the observed ratio is 0.85-1.4, but it appears that the correction for stray radiation is slightly of). The smaller range for the low-velocity gas may be caused by confusion in the line of sight, so that a low ratio in one component can be compensated by a high ratio in another-for 11 low-velocity clouds fitted by one component the distribution of ratios has a larger dispersion. Comparison with higher angular resolution data is possible for 16 sight lines. Eight sight lines with H I data at 1'-2' resolution show a range of 0.75-1.25 for N(H I -2')/N(H I -9'), while in eight other sight lines N(H I-Ly alpha)/N(H I -9') ranges from 0.74 to 0.98.

ORFEUS echelle spectra: Molecular hydrogen in disk, IVC, and HVC gas in front of the LMC

In front of the LMC molecular hydrogen is found in absorption near 0 km s -1 , being local disk gas, near +60 km s -1 in an intermediate velocity cloud, and near +120 km s -1 , being a high velocity halo cloud. The nature of the gas is discussed based on four Orfeus far UV spectra of LMC stars and including data from the ground and from the IUE satellite. The local gas is cool and, given a span of sight lines of only $2.5^\circ$, rather fluffy. The fractional abundance of H 2 varies from $\log f=\log [N({\rm H}_2)/(2\cdot N({\rm H}_2)+N(\rm{\ion{H}{i}}))]=-5.4$ to -3.3. Metal depletions (up to -1.7 dex for Fe) are typical for galactic disk gas. In the intermediate and high velocity gas an apparent underabundance of neutral oxygen points to an ionization level of the gas of about 90% . H 2 is detected in intermediate and high velocity gas towards HD 269546. In the intermediate velocity gas we find an H 2 column density of $\log(N)\simeq15.6$. The H 2 excitation indicates that the line of sight samples a cloud at a temperature below 150 K. Column densities are too small to detect the higher UV pumped excitation levels. The high velocity H 2 ($\log(N)\simeq15.6$) is highly excited and probably exposed to a strong radiation field. Its excitation temperature exceeds 1000 K. Due to the radial velocity difference between the halo gas and the Milky Way disk, the unattenuated disk radiation is available for H 2 excitation in the halo. We do not find evidence for an intergalactic origin of this gas; a galactic as well as a Magellanic Cloud origin is possible.

Dependence of Gas-Phase Abundances in the Interstellar Medium on Column Density

Sight lines through high- and intermediate-velocity clouds allow measurements of ionic gas-phase abundances A at very low values of H I column density N(H I). Present observations cover over 4 orders of magnitude in N(H I). Remarkably, for several ions we find that the A versus N(H I) relation is the same at high and low column densities and that the abundances have a relatively low dispersion (factors of 2-3) at any particular N(H I). Halo gas tends to have slightly higher values of A than disk gas at the same N(H I), suggesting that part of the dispersion may be attributed to the environment. We note that the dispersion is largest for Na I; using Na I as a predictor of N(H I) can lead to large errors. Important implications of the low dispersions regarding the physical nature of the interstellar medium are (1) because of clumping, over sufficiently long path lengths N(H I) is a reasonable measure of the local density of most of the H atoms along the sight line; (2) the destruction of grains does not mainly take place in catastrophic events such as strong shocks but is a continuous function of the mean density; (3) the cycling of the ions becoming attached to grains and being detached must be rapid, and the two rates must be roughly equal under a wide variety of conditions; and (4) in gas that has a low average density the attachment should occur within denser concentrations.

A N[CLC]a[/CLC] [CSC]i[/CSC] Absorption Map of the Small-Scale Structure in the Interstellar Gas toward M15

Using the DensePak fiber-optic array on the KPNO WIYN telescope, we have obtained high signal-to-noise ratio echelle spectra of the Na I D wavelength region toward the central 27'' × 43'' of the globular cluster M15 at a spatial resolution of 4''. The spectra exhibit significant interstellar Na I absorption at LSR velocities of 3 km s-1 (local interstellar medium [LISM] component) and 68 km s-1 (intermediate-velocity cloud [IVC] component). Both components vary appreciably in strength on these scales. The derived Na I column densities differ by a factor of 4 across the LISM absorption map and by a factor of 16 across the IVC map. Assuming distances of 500 and 1500 pc for the LISM and IV clouds, respectively, these maps show evidence of significant ISM structure down to the minimum scales of 2000 and 6000 AU probed in these absorbers. The smallest scale N(Na I) variations observed in the M15 LISM and IVC maps are typically comparable to or higher than the values found at similar scales in previous studies of interstellar Na I structure toward binary stars. The physical implications of the small and larger scale Na I features observed in the M15 maps are discussed in terms of variations in the H I column density as well as in the Na ionization equilibrium.

High-resolution stellar and interstellar spectra of HD 100340

We present new, high-resolution, high signal-to-noise ratio, optical and 21-cm HI observations of the faint blue halo star HD 100340. A differential abundance analysis with respect to the Galactic disc star HR 2387 shows no significant peculiarities. We find that HD 100340 lies at a distance of 3 kpc, and a kinematical analysis strongly supports a `runaway star' origin. We also present improved cloud models for three intermediate velocity clouds seen in Ca II K or H I towards HD 100340.

The Distance to the Draco Intermediate-Velocity Cloud

The understanding of the nature of intermediate- and high-velocity gas in the Milky Way is hampered by a paucity of distance estimates to individual clouds. A project has been started at the David Dunlap Observatory to address this lack of distance measures by observing early-type stars along the line of sight toward these clouds and searching for sodium doublet absorption at the clouds' systemic velocities. Distances to foreground stars (no absorption) and background stars (with absorption) are estimated from spectroscopic parallax, and thus the distance to the bracketed cloud is estimated. In this Letter, we present the first result from this ongoing project: a measurement of the distance to the Draco Cloud, which is the most studied of the intermediate-velocity clouds. The result presented here is the first distance bracket that tightly constrains the position of the Draco Cloud. We briefly describe our target selection and observing methodology and then demonstrate absorption at the velocity of the Draco Cloud for one star (TYC 4194 2188) and a lack of absorption for several other stars. We derive a distance bracket to the Draco Cloud of 463+ 192−136 to 618+ 243−174 pc.

Radio H I and optical absorption-line spectra of an intermediate-velocity cloud in the general direction of the M15 globular cluster

Using H I spectra obtained with the Lovell telescope (FWHM ∼ 12 arcmin) we present maps showing the H I distribution and velocity structure of an intermediate-velocity cloud (IVC; vLSR ∼ 70 km s−1) which is observed in the general direction of the globular cluster M15. The gas is shown to be clumpy in nature and we examine its position and velocity structure. The IVC is detected in absorption in the Ca II K line towards five cluster stars in intermediate resolution spectra obtained with ISIS/WHT and in high resolution UES/WHT Na I D line spectra of two cluster stars (II-75; IV-38). The clumpy nature of the gas is indicated by the Na I and K I spectra obtained in the II-75 and IV-38 sightlines, which have angular separation ∼ 3.5 arcmin. The IVC is detected in K I in the higher column density II-75 sightline; this appears to be the first detection of IVC or HVC gas in K I. The IVC gas towards M15 has a similar velocity to that observed towards HD 203664, some 3.1° away from the cluster. Similarities in the IVC gas velocity suggest a gas structure that extends across both sightlines, although gas column densities are considerably higher towards M15. For a common feature, this would place the M15 IVC at a height above the Galactic plane (z-distance) of ≲ 1.5 kpc based on the Little et al. estimate of the HD 203664 distance. From the fine-scale structure and column density observations, estimates are made of the space density of the small-scale concentrations. However, these remain uncertain and the present observations emphasize the need for higher spatial and spectral resolution studies to provide firmer estimates of cloud properties. We report also on a radio H I and Ca II line survey towards a sample of 24 stars over a wider field. This was carried out in an attempt to detect any wider distribution of the IVC gas and to place better limits on its distance. Although these observations are of sufficient spectral quality, no new optical detections are reported.

Optical Studies of a Local Filament Toward the Magellanic Clouds and a Local IVC Region

AbstractWe present new spectroscopic results which constrain the distance of a large IR emitting filament near the LMC, as well as some new results for a local region with widely distributed detections of Intermediate Velocity Clouds (IVCs). We present spectroscopic observations of stars toward both regions which constrain the distances and kinematics of the interstellar features. New optical high resolution (λ/δλ ⋍ 106) spectra have been obtained for the star HD 22252, and are used to estimate approximate distances, temperatures, and pressures within the cloud. An unusual local region of the interstellar medium with several IVCs is also reported, with multiple components of absorption from -80 &lt; υLSR &lt; 70 km s−1 seen toward several stars in the region.

High-Velocity Clouds and their Soft X-ray Emission

AbstractDiffuse excess ¼ keV soft X-ray emission was found to be positionally correlated with the column density distribution of the high velocity cloud (HVC) complex C (Kerp et al. 1996). Here we point out that the detected diffuse X-ray emission is indeed associated with the HVC phenomenon. For this purpose we study the ¼ keV radiation transfer as well as the H I column density distribution of HVCs and intermediate velocity clouds (IVCs) towards HVC complex C in detail. We present evidence that on arcmin scales the ¾ keV soft X-ray emission is positionally anticorrelated with the HVC column density distribution of an individual HVC filament of complex C.

3D Hydrodynamical Simulations of a Galactic Fountain

AbstractGalactic fountains are thought to be responsible for the formation of the observed intermediate and high velocity clouds in the Galactic halo. Threedimensional simulations have been carried out to determine the evolution of the disk gas as it enters the fountain, cools and returns to the Galactic disk. The descending cold gas, headed by a shock, sweeps up the ascending flow, triggering the formation of an unstable layer where Rayleigh-Taylor instabilities grow, which may provoke the formation of the observed clouds.

The Distance to Two Neutral Hydrogen Clouds: The High‐Velocity Complex A and the Low‐Latitude Intermediate‐Velocity Cloud

A lower limit to the distance of the high-velocity cloud (HVC) complex A of 4 kpc (z > 3 kpc) is derived. The HVC is detected toward the Seyfert galaxy Mrk 106 in Mg n ΛΛ2796, 2803 absorption spectra taken with Hubble Space Telescope (HST) proving that Mg+ is present in the cloud. It is not detected in the Mg ii spectra of two stars, PG 0859 + 593 (distance 4 kpc) and PG 0906 + 597 (distance 0.7 kpc). The distances to the stars are derived by matching Stromgren photometry and intermediate-resolution spectroscopy with model stellar atmospheres; they are estimated to be accurate to within 1 kpc. From a combination of Effelsberg data and Westerbork maps with 2' or 3' resolution, we show that the H i column density and thus the Mg+ abundance in the direction of the two stars is sufficiently high for the nondetections to imply that the HVC is behind the stars. This distance limit can be used to eliminate several recent models for complex A that placed it nearby.We also derive a distance bracket of 1.7 < d < 4 kpc (1.1 < z < 3 kpc) for an intermediate-velocity cloud (IVC) at velocities of about –50 km s–1. This IVC was named the Low-Latitude Intermediate-Velocity Arch by Kuntz & Danly and is seen between l = 120°-160° and b = 30°-45°.

The Structure and Properties of an Intermediate‐Velocity Cloud in the Direction of the Globular Cluster M13

A study of an intermediate-velocity cloud (IVC; VLSR ≈- 70 km s–1) detected in the foreground of the globular cluster M13 has been made using H I spectra obtained with the Lovell Telescope, Jodrell Bank (FWHP beamwidth 12') and H I spectral maps derived from Dominion Radio Astrophysical Observatory Synthesis Telescope observations (FWHP synthesised beamwidth ≈2'.8 × 2'.0). The radio data complement the optical absorption line spectroscopy of M13 cluster stars reported by Bates et al. The H I data obtained with the Lovell Telescope show extended emission from a broad-velocity component (FWHM ≈ 30 km s–1) of Hi column density 7 × 1018 cm–2. Two cloudlets are revealed in the H I observations lying on either side of M13 and having a two-component structure; the broad component (FWHM ≈ 32 km s–1) has an Hi column density of ≈ 5 × 1019 cm–2, while the narrow component (FWHM ≈ 8 km s–1) has a column density of ≈ 6 × 1018 cm–2 as seen in the Synthesis Telescope observations. The cloudlets have an angular size ≈2' and are only partially resolved in the synthesis telescope data. Fine-scale gas structure is also evident from the cluster star spectroscopy with variations in Na I column density of a factor of 10 observed across the several arcminutes of the cluster face; there is also evidence for structure on an angular scale of several arcseconds. A column density ratio Na I/H I ≈ 4 × 10–8 is measured for a cloudlet in the foreground of the cluster near the strongest Na I detection. Estimates are made of cloudlet gas density that are comparable with those reported for high-velocity cloud (HVC) concentrations. However, the present observations emphasize further the need for higher spatial resolution H I studies to provide firmer estimates of cloud properties.