Halo Clouds Research Articles

The moist halo region around shallow cumulus clouds in large eddy simulations

AbstractIn this study, the moist buffering halo region of shallow maritime cumulus clouds is systematically investigated using large eddy simulations with various grid resolutions and numerical choices. Autocorrelation analyses of cloud liquid water and relative humidity suggest a converged size of 200–300 m for moist patches outside clouds when the model resolution is below 50 m, but may overestimate this size due to noncloudy moist regions. Based on a composite analysis, the structure of the moist halo immediately outside individual clouds is examined. It is found that, regardless of model resolution, the distribution of relative humidity in the halo region does not depend on cloud size, but on the real distance away from the cloud boundary, indicating some size‐independent length scales are responsible for the halo formation. The relative humidity decays with distance more quickly with finer horizontal resolution, which is possibly related to the model resolution dependence of the cloud spectrum. The halo size near the cloud base is larger than that within the cloud layer and this feature is robust across all simulations. Further analyses of backward and forward Lagrangian trajectories originating from the moist halo region reveal the possible role for subcloud coherent structures in cloud‐base halo formation. Possible mechanisms explaining cloud halo sizes and associated length scales are discussed.

Supermassive black hole seeds from sub-keV dark matter

Quasars observed at redshifts z ∼ 6–7.5 are powered by supermassive black holes which are too large to have grown from early stellar remnants without efficient super-Eddington accretion. A proposal for alleviating this tension is for dust and metal-free gas clouds to have undergone a process of direct collapse, producing black hole seeds of mass M seed ∼ 105 M ⊙ around redshift z ∼ 17. For direct collapse to occur, a large flux of UV photons must exist to photodissociate molecular hydrogen, allowing the gas to cool slowly and avoid fragmentation. We investigate the possibility of sub-keV mass dark matter decaying or annihilating to produce the UV flux needed to cause direct collapse. To do so, we calculate the produced UV flux from dark matter annihilations and decays within the gas cloud's halo and compare these to the requirements of the UV spectrum found by previous hydrodynamical simulations. We find that annihilating dark matter with a mass in the range of 13.6 eV ≤ mdm ≤ 20 eV can produce the required flux while avoiding existing constraints. A non-thermally produced dark matter particle which comprises the entire dark matter abundance requires a thermally averaged cross section of 〈σv〉 ∼ 10-35 cm3/s. Alternatively, the flux could originate from a thermal relic which comprises only a fraction ∼ 10-9 of the total dark matter density. Decaying dark matter models which are unconstrained by independent astrophysical observations are unable to sufficiently suppress molecular hydrogen, except in gas clouds embedded in dark matter halos which are larger, cuspier, or more concentrated than current simulations predict. Lastly, we explore how our results could change with the inclusion of full three-dimensional effects. Notably, we demonstrate that if the H2 self-shielding is less than the conservative estimate used in this work, the range of both annihilating and decaying dark matter models which can cause direct collapse is significantly increased.

A Detection of H2 in a High-velocity Cloud toward the Large Magellanic Cloud

This work presents a new detection of H2 absorption arising in a high-velocity cloud associated with either the Milky Way or the Large Magellanic Cloud (LMC). The absorber was found in an archival Far Ultraviolet Spectroscopic Explorer spectrum of the LMC star Sk-70°32. This is the fifth well-characterized H2 absorber to be found in the Milky Way’s halo and the second such absorber outside the Magellanic Stream and Bridge. The absorber has a local standard of rest central velocity of +140 km s−1 and a H2 column density of 1017.5 cm−2. It is most likely part of a cool and relatively dense inclusion (T ≈ 75 K, n H ∼ 100 cm−3) in a warmer and more diffuse halo cloud. This halo cloud may be part of a still-rising Milky Way Galactic fountain flow or an outflow from the Large Magellanic Cloud.

Galactopause Formation and Gas Precipitation during Strong Galactic Outflows

Abstract Using X-ray constrained β-models for the radial distribution of gas in the outskirts of galaxies, we analyze the termination of galactic winds and the formation and evolution of halo clouds by thermal instability. At low mass-loss rates ( ) galactic winds are trapped within the halo, but they burst into the intergalactic medium during intermittent strong outflows with . We develop analytic models of halo clouds as they cool radiatively over condensation timescales for hydrogen number densities , gas temperatures T ≈ (106 K)T 6, and metallicities (Z/Z ⊙). Halo gas can form kiloparsec-scale clouds out to galactocentric distances r ≈ 30–65 kpc, where efficient radiative cooling from 106 K down to T ≈ 104–5 K occurs at Z ≥ 0.3 Z ⊙ on timescales less than 1 Gyr. After condensing to column densities N H ≥ 4 × 1016 cm−2, these clouds lose hydrostatic pressure support and fall inward on dynamical timescales of ∼200 Myr. Our baseline analysis will be followed by numerical calculations to understand the governing principles of halo cloud formation and transport of gas to the galactic disk.

Hydrogen recombination near-infrared line mapping of Centaurus A with IRSF/SIRIUS

Abstract Centaurus A (Cen A) is one of the most famous galaxies hosting an active galactic nucleus (AGN), where the interaction between AGN activities and surrounding interstellar and intergalactic media has been investigated. Recent studies reported detections of Hα emission from clouds in the galactic halo toward the northeast and southwest of the nucleus of Cen A, suggesting that AGN jets may have triggered star formation there. We performed near-infrared line mapping of Cen A with the IRSF 1.4 m telescope, using the narrow-band filter tuned for Paβ, from which we find that Paβ emission is not detected significantly from either of the northeast or southwest regions. The upper limit of the Paβ/Hα ratio in the northeast region is compatible with that expected for a typical H ii region, in line with the scenario that AGNs have triggered star formation there. On the other hand, the upper limit of Paβ/Hα in the southwest region is significantly lower than that expected for a typical H ii region. A possible explanation of the low Paβ/Hα ratio in the southwest region is the scattering of Hα and Paβ photons from the center of Cen A by dust grains in the halo clouds. From the upper limit of Paβ/Hα in the southwest region, we obtain constraints on the dust size distribution, which is found to be compatible with those seen in the interstellar medium of our Galaxy.

Hα Distances to the Leading Arm of the Magellanic Stream

Abstract The Leading Arm (LA) is a tidal feature that is in front of the Magellanic Clouds (MCs) on their orbit through the Galaxy’s halo. Many physical properties of the LA, such as its mass and size, are poorly constrained because it has few distance measurements. While Hα measurements have been used to estimate the distances to halo clouds, many studies have been unsuccessful in detecting Hα from the LA. In this study, we explore a group of H i clouds which lie 75°–90° from the MCs. Through ultraviolet and 21 cm radio spectroscopy, this region, dubbed the LA Extension, was found to have chemical and kinematic similarities to the LA. Using the Wisconsin Hα Mapper, we detect Hα emission in four out of seven of our targets. Assuming that this region is predominantly photoionized, we use a radiation model that incorporates the contributions of the Galaxy, MCs, and the extragalactic background at z = 0 to derive a heliocentric distance of d ⊙ ≥ 13.4 . We also use this model to rederive Hα distances of d ⊙ ≥ 5.0 kpc and d ⊙ ≥ 22.9 to two clouds in the literature that might also be associated with the LA. Using these new measurements, and others in the literature, we provide a general trend of the variation of LA heliocentric distance as a function of Magellanic Stream longitude, and explore its implications for the origin and closest point of approach of the LA.

Evolution of the Cool Gas in the Circumgalactic Medium of Massive Halos: A Keck Cosmic Web Imager Survey of Lyα Emission around QSOs at z ≈ 2

Abstract Motivated by the recent discovery of the near-ubiquity of Lyα emission around z ≳ 3 QSOs, we performed a systematic study of QSO circumgalactic Lyα emission at z ≈ 2 utilizing the unique capability of the Keck Cosmic Web Imager (KCWI)—a new wide-field, blue sensitive integral field spectrograph. In this paper, we present KCWI observations of a sample of 16 ultraluminous Type I QSOs at z = 2.1–2.3 with ionizing luminosities of erg s−1 Hz−1. We found that 14 out of 16 QSOs are associated with Lyα nebulae with projected linear sizes larger than 50 physical kpc. Among them, four nebulae have enormous Lyα emission with a Lyα surface brightness (SB) erg s−1 cm−2 arcsec−2 on the >100 kpc scale, extending beyond the field of view of KCWI. Our KCWI observations reveal that most z ≈ 2 QSO nebulae have a more irregular morphology compared to those at z ≳ 3. In turn, we measure that the circularly averaged SB at z ≈ 2 is 0.4 dex fainter than the redshift-corrected median SB at z ≳ 3. The Lyα SB profile (SBLyα ) of QSOs at z ≈ 2 can be described by a power law of erg s−1 cm−2 arcsec−2, with a slope similar to that at z ≳ 3. The observed lower redshift-corrected, circularly averaged SB may be mainly due to the lower covering factor of cool gas clouds in massive halos at z ≈ 2.

The Large-scale Ionization Cones in the Galaxy

Abstract There is compelling evidence for a highly energetic Seyfert explosion (1056–57 erg) that occurred in the Galactic center a few million years ago. The clearest indications are the X-ray/γ-ray “10 kpc bubbles” identified by the ROSAT and Fermi satellites. In an earlier paper, we suggested another manifestation of this nuclear activity, i.e., elevated Hα emission along a section of the Magellanic Stream due to a burst (or flare) of ionizing radiation from Sgr A*. We now provide further evidence for a powerful flare event: UV absorption line ratios (in particular / , Si iv/Si ii) observed by the Hubble Space Telescope reveal that some Magellanic Stream clouds toward both galactic poles are highly ionized by a source capable of producing ionization energies up to at least 50 eV. We show how these are clouds caught in a beam of bipolar, radiative “ionization cones” from a Seyfert nucleus associated with Sgr A*. In our model, the biconic axis is tilted by about 15° from the south Galactic pole with an opening angle of roughly 60°. For the Magellanic Stream at such large Galactic distances (D ≳ 75 kpc), nuclear activity is a plausible explanation for all of the observed signatures: elevated Hα emission and H ionization fraction (x e ≳ 0.5), enhanced / and Si iv/Si ii ratios, and high and Si iv column densities. Wind-driven “shock cones” are ruled out because the Fermi bubbles lose their momentum and energy to the Galactic corona long before reaching the Magellanic Stream. Our time-dependent Galactic ionization model (stellar populations, hot coronal gas, cloud–halo interaction) is too weak to explain the Magellanic Stream’s ionization. Instead, the nuclear flare event must have had a radiative UV luminosity close to the Eddington limit (f E ≈ 0.1–1). Our time-dependent Seyfert flare models adequately explain the observations and indicate that the Seyfert flare event took place T o = 3.5 ± 1 Myr ago. The timing estimates are consistent with the mechanical timescales needed to explain the X-ray/γ-ray bubbles in leptonic jet/wind models (≈2–8 Myr).

Recent results and perspectives on cosmic backgrounds from radio to far-infrared

Cosmological and astrophysical surveys from radio to far-infrared, in both temperature and polarization, offer a unique view of the universe properties and of the formation and evolution of its structures. The last release, close to be finalized, of the Planck mission results sets the scene for cosmological models and parameters, while the comparison with other types of data sets raises the issue of possible tensions about some parameters, first of all the Hubble constant. At the same time, on the extragalactic side, Planck carried out the deepest systematic all-sky survey of SZ galaxy clusters and detected thousands of dusty galaxies and many hundreds of extragalactic radio sources, also allowing us to investigate many specific topics, including molecular hydrogen clouds in galactic halos. The exploitation of future generation of CMB missions and the next radio facilities will allow us to deeply investigate several topics in cosmology and astrophysics, from the existence of primordial gravitational waves to the energy releases in the primeval plasma, from the dawn ages and the epoch of reionization to the formation and evolution of early galaxies and clusters, while a wide set of open astrophysical problems can be studied with future IR missions.

The Ultraviolet Detection of Diffuse Gas in Galaxy Groups

Abstract A small survey of the UV-absorbing gas in 12 low-z galaxy groups has been conducted using the Cosmic Origins Spectrograph on board the Hubble Space Telescope. Targets were selected from a large, homogeneously selected sample of groups found in the Sloan Digital Sky Survey. A critical selection criterion excluded sight lines that pass close (<1.5 virial radii) to a group galaxy, to ensure absorber association with the group as a whole. Deeper galaxy redshift observations are used both to search for closer galaxies and also to characterize these 1013.5–1014.5 M ⊙ groups, the most massive of which are highly virialized with numerous early-type galaxies (ETGs). This sample also includes two spiral-rich groups, not yet fully virialized. At group-centric impact parameters of 0.3–2 Mpc, these signal-to-noise ratios = 15–30 spectra detected H i absorption in 7 of 12 groups; high (O vi) and low (Si iii) ion metal lines are present in two-thirds of the absorption components. None of the three most highly virialized, ETG-dominated groups are detected in absorption. Covering fractions ≳50% are seen at all impact parameters probed, but do not require large filling factors despite an enormous extent. Unlike halo clouds in individual galaxies, group absorbers have radial velocities that are too low to escape the group potential well without doubt. This suggests that these groups are “closed boxes” for galactic evolution in the current epoch. Evidence is presented that the cool and warm group absorbers are not a pervasive intra-group medium (IGrM), requiring a hotter (T ∼ 106–107 K) IGrM to be present to close the baryon accounting.

Interaction of Cosmic Rays with Cold Clouds in Galactic Halos

We investigate the effects of cosmic ray (CR) dynamics on cold, dense clouds embedded in a hot, tenuous galactic halo. If the magnetic field does not increase too much inside the cloud, the local reduction in Alfv\'en speed imposes a bottleneck on CRs streaming out from the star-forming galactic disk. The bottleneck flattens the upstream CR gradient in the hot gas, implying that multi-phase structure could have global effects on CR driven winds. A large CR pressure gradient can also develop on the outward-facing edge of the cloud. This pressure gradient has two independent effects. The CRs push the cloud upward, imparting it with momentum. On smaller scales, the CRs pressurize cold gas in the fronts, reducing its density, consistent with the low densities of cold gas inferred in recent COS observations of local $L_{*}$ galaxies. They also heat the material at the cloud edge, broadening the cloud-halo interface and causing an observable change in interface ionic abundances. Due to the much weaker temperature dependence of cosmic ray heating relative to thermal conductive heating, CR mediated fronts have a higher ratio of low to high ions compared to conduction fronts, in better agreement with observations. We investigate these effects separately using 1D simulations and analytic techniques.

All-sky census of Galactic high-latitude molecular intermediate-velocity clouds

The HI halo clouds of the Milky Way, and in particular the intermediate-velocity clouds (IVCs), are thought to be connected to Galactic fountain processes. Observations of fountain clouds are important for understanding the role of matter recycling and accretion onto the Galactic disk and subsequent star formation. Here, we quantify the amount of molecular gas in the Galactic halo. We focus on the rare class of molecular IVCs (MIVCs) and search for new objects. The HI-FIR correlation was studied across the entire northern and southern Galactic hemispheres at Galactic latitudes $|b|>20^\circ$ to determine the amount and distribution of molecular gas in IVCs. We used the most recent large-scale HI and FIR data, the Effelsberg Bonn-HI Survey, the Parkes Galactic All-Sky Survey, and the Planck FIR surveys. We present a catalogue of 239 MIVC candidates on the northern and southern Galactic hemispheres. Among these candidates, all previously known MIVCs are recovered except for one single source. The frequency of candidates differs significantly between the northern and southern Galactic hemispheres and between negative and positive LSR velocities as well. In our approach we analyse the local Galactic environment. Extrapolating our results to the entire Galaxy, the global inflow of atomic and molecular IVC gas onto the Milky Way may account for the major fraction of the gaseous mass that is required to sustain the current Galactic star formation rate.

Identifying galaxy candidates in WSRT H i imaging of ultra-compact high velocity clouds

Ultra-compact high velocity clouds (UCHVCs) were identified in the ALFALFA HI survey as potential gas-bearing dark matter halos. Here we present higher resolution neutral hydrogen (HI) observations of twelve UCHVCS with the Westerbork Synthesis Radio Telescope (WSRT). The UCHVCs were selected based on a combination of size, isolation, large recessional velocity and high column density as the best candidate dark matter halos. The WSRT data were tapered to image the UCHVCs at 210" (comparable to Arecibo) and 105" angular resolution. In a comparison of the single-dish to interferometer data, we find that the line flux recovered in the WSRT observations is comparable to that from the single-dish ALFALFA data. In addition, any structure seen in the ALFALFA data is reproduced in the WSRT maps at the same angular resolution. At 210'" resolution all the sources are generally compact with a smooth HI morphology, as expected from their identification as UCHVCs. At the higher angular resolution, a majority of the sources break into small clumps contained in a diffuse envelope. These UCHVCs also have no ordered velocity motion and are most likely Galactic halo clouds. We identify two UCHVCs, AGC 198606 and AGC 249525, as excellent galaxy candidates based on maintaining a smooth HI morphology at higher angular resolution and showing ordered velocity motion consistent with rotation. A third source, AGC 249565, lies between these two populations in properties and is a possible galaxy candidate. If interpreted as gas-bearing dark matter halos, the three candidate galaxies have rotation velocities of 8-15 km/s, HI masses of 0.6-50 x 10^5 Msun, HI radii of 0.3-2 kpc, and dynamical masses of 2-20 x 10^7 Msun for a range of plausible distances. These are the UCHVCs with the highest column density values in the ALFALFA HI data and we suggest this is the best way to identify further candidates.

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.

High-velocity gas toward the LMC resides in the Milky Way halo

To explore the origin of high-velocity gas in the direction of the Large Magellanic Cloud (LMC) we analyze absorption lines in the ultraviolet spectrum of a Galactic halo star that is located in front of the LMC at d=9.2 kpc distance. We study the velocity-component structure of low and intermediate metal ions in the spectrum of RXJ0439.8-6809, as obtained with the Cosmic Origins Spectrograph (COS) onboard HST, and measure equivalent widths and column densities for these ions. We supplement our COS data with a Far-Ultraviolet Spectroscopic Explorer spectrum of the nearby LMC star Sk-69 59 and with HI 21cm data from the Leiden-Argentina-Bonn (LAB) survey. Metal absorption towards RXJ0439.8-6809 is unambiguously detected in three different velocity components near v_LSR=0,+60, and +150 km/s. The presence of absorption proves that all three gas components are situated in front of the star, thus being located in the disk and inner halo of the Milky Way. For the high-velocity cloud (HVC) at v_LSR=+150 km/s we derive an oxygen abundance of [O/H]=-0.63 (~0.2 solar) from the neighbouring Sk-69 59 sightline, in accordance with previous abundance measurements for this HVC. From the observed kinematics we infer that the HVC hardly participates in the Galactic rotation. Our study shows that the HVC towards the LMC represents a Milky Way halo cloud that traces low-column density gas with relatively low metallicity. It rules out scenarios in which the HVC represents material close to the LMC that stems from a LMC outflow.

Far-infrared excess emission as a tracer of disk-halo interaction

Given the current and past star-formation in the Milky Way in combination with the limited gas supply, the re-fuelling of the reservoir of cool gas is an important aspect of Galactic astrophysics. The infall of \ion{H}{i} halo clouds can, among other mechanisms, contribute to solving this problem. We study the intermediate-velocity cloud IVC135+54 and its spatially associated high-velocity counterpart to look for signs of a past or ongoing interaction. Using the Effelsberg-Bonn \ion{H}{i} Survey data, we investigated the interplay of gas at different velocities. In combination with far-infrared Planck and IRIS data, we extended this study to interstellar dust and used the correlation of the data sets to infer information on the dark gas. The velocity structure indicates a strong compression and deceleration of the infalling high-velocity cloud (HVC), associated with far-infrared excess emission in the intermediate-velocity cloud. This excess emission traces molecular hydrogen, confirming that IVC135+54 is one of the very few molecular halo clouds. The high dust emissivity of IVC135+54 with respect to the local gas implies that it consists of disk material and does not, unlike the HVC, have an extragalactic origin. Based on the velocity structure of the HVC and the dust content of the IVC, a physical connection between them appears to be the logical conclusion. Since this is not compatible with the distance difference between the two objects, we conclude that this particular HVC might be much closer to us than complex C. Alternatively, the indicators for an interaction are misleading and have another origin.

Dust-to-gas ratios of the GALFA-H I Compact Cloud Catalog

We have searched for infrared dust emission from subsets of compact, Galactic neutral hydrogen clouds, with the purpose of looking for dust in high-velocity clouds, identifying low-velocity halo clouds, and investigating the cloud populations defined in the GALFA-HI Compact Cloud Catalog. We do not detect dust emission from high-velocity clouds. The lack of dust emission from a group of low-velocity clouds supports the claim that they are low-velocity halo clouds. We detect dust in the remaining low-velocity clouds, indicating a Galactic origin, with a significantly greater dust-to-gas ratio for clouds with linewidths near 15 km/s. We propose that this is due to dust associated with ionized gas.

Kinetic temperatures toward X1/X2 orbit interceptions regions and giant molecular loops in the Galactic center region

Context: It is well known that the kinetic temperatures, Tkin, of the molecular clouds in the Galactic center region are higher than in typical disk clouds. However, the Tkin of the molecular complexes found at higher latitudes towards the giant molecular loops in the central region of the Galaxy is so far unknown. The gas of these high latitude molecular clouds (hereafter referred to as halo clouds) is located in a region where the gas in the disk may interact with the gas in the halo in the Galactic center region. Aims: To derive Tkin in the molecular clouds at high latitude and understand the physical process responsible for the heating of the molecular gas both in the Central Molecular Zone (the concentration of molecular gas in the inner 500 pc) and in the giant molecular loops. Methods: We measured the metastable inversion transitions of NH3 from (1,1) to (6,6) toward six positions selected throughout the Galactic central disk and halo. We used rotational diagrams and large velocity gradient modeling to estimate the kinetic temperatures toward all the sources. We also observed other molecules like SiO, HNCO, CS, C34S, C18O, and 13CO, to derive the densities and to trace different physical processes (shocks, photodissociation, dense gas) expected to dominate the heating of the molecular gas. Results: We derive for the first time Tkin of the high latitude clouds interacting with the disk in the Galactic center region. We find high rotational temperatures in all the observed positions. We derive two kinetic temperature components (150 K and 40 K) for the positions in the Central Molecular Zone, and only the warm kinetic temperature component for the clouds toward the giant molecular loops. The fractional abundances derived from the different molecules suggest that shocks provide the main heating mechanism throughout the Galactic center, also at high latitudes

GAS CONDENSATION IN THE GALACTIC HALO

Using adaptive mesh refinement (AMR) hydrodynamic simulations of vertically stratified hot halo gas, we examine the conditions under which clouds can form and condense out of the hot halo medium to potentially fuel star formation in the gaseous disk. We find that halo clouds do not develop from linear isobaric perturbations. This is a regime where the cooling time is longer than the Brunt-Vaisala time, confirming previous linear analysis. We extend the analysis into the nonlinear regime by considering mildly or strongly nonlinear perturbations with overdensities up to 100, also varying the initial height, the cloud size, and the metallicity of the gas. Here, the result depends on the ratio of cooling time to the time required to accelerate the cloud to the sound speed (similar to the dynamical time). If the ratio exceeds a critical value near unity, the cloud is accelerated without further cooling and gets disrupted by Kelvin-Helmholtz and/or Rayleigh-Taylor instabilities. If it is less than the critical value, the cloud cools and condenses before disruption. Accreting gas with overdensities of 10-20 is expected to be marginally unstable; the cooling fraction will depend on the metallicity, the size of the incoming cloud, and the distance to the galaxy. Locally enhanced overdensities within cold streams have a higher likelihood of cooling out. Our results have implications on the evolution of clouds seeded by cold accretion that are barely resolved in current cosmological hydrodynamic simulations and absorption line systems detected in galaxy halos.

A High-Resolution Study of Two Galactic H I Halo Clouds in the Ophiuchus Superbubble

Two compact HI clouds which seem to belong to the Ophiuchus superbubble were studied at ~30" resolution using the Very Large Array (VLA) in C and D configurations together with the Green Bank Telescope (GBT) providing the short-spacing flux. Here we present preliminary results of the data analysis.