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Register a new userHigh-Velocity Cloud Complex C: Galactic Fuel or galactic Waste?
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We present HST GHRS and STIS observations of five QSOs that probe the prominent high-velocity cloud (HVC) Complex C, covering 10% of the northern sky. Based upon a single sightline measurement (Mrk 290), a metallicity [S/H]=-1.05+/-0.12 has been associated with Complex C by Wakker et al. (1999a,b). When coupled with its inferred distance (5<d<30 kpc) and line-of-sight velocity (v=-100 to -200 km/s), Complex C appeared to represent the first direct evidence for infalling low-metallicity gas onto the Milky Way, which could provide the bulk of the fuel for star formation in the Galaxy. We have extended the abundance analysis of Complex C to encompass five sightlines. We detect SII absorption in three targets (Mrk 290, Mrk 817, and Mrk 279); the resulting [SII/HI] values range from -0.36 (Mrk 279) to -0.48 (Mrk 817) to -1.10 (Mrk 290). Our preliminary OI FUSE analysis of the Mrk 817 sightline also supports the conclusion that metallicities as high as 0.3 times solar are encountered within Complex C. These results complicate an interpretation of Complex C as infalling low-metallicity Galactic fuel. Ionization corrections for HII and SIII cannot easily reconcile the higher apparent metallicities along the Mrk 817 and Mrk 279 sightlines with that seen toward Mrk 290, since H-alpha emission measures preclude the existence of sufficient HII. If gas along the other lines of sight has a similar pressure and temperature to that sampled toward Mrk 290, the predicted H-alpha emission measures would be 900 mR. It may be necessary to reclassify Complex C as mildly enriched Galactic waste from the Milky Way or processed gas torn from a disrupted neighboring dwarf, as opposed to low-metallicity Galactic fuel.
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Tantalizing evidence has been presented supporting the suggestion that a large population of extragalactic gas clouds permeates the Local Group, a population which has been associated with the Galactic High-Velocity Clouds (HVCs). We comment on both the strengths and weaknesses of this suggestion, informally referred to as the Blitz/Spergel picture. Theoretical predictions for the spatial and kinematic distributions, metallicities, distances, and emission properties of Blitz/Spergel HVCs will be confronted with extant observational data.
We present Far Ultraviolet Spectroscopic Explorer (FUSE) observations of the sightline toward the Seyfert 1 galaxy Markarian 876, which passes through high velocity cloud (HVC) complex C. This sight line demonstrates the ability of FUSE to measure ionic absorption lines in Galactic HVCs. High velocity absorption is clearly seen in both members of the O VI doublet. This is the first detection of O VI in a neutral hydrogen HVC. One component of HVC complex C is resolved in multiple Fe II lines from which we derive N(Fe II)/N(H I)=0.48 (Fe/H)_solar. This value of N(Fe II)/N(H I) implies that the metallicity of complex C along this sightline may be higher than that along the Mrk 290 sightline (0.1 solar) found by Wakker et al. (1999). On the other hand, if the metallicity of complex C is also 0.1 solar along this line of sight, the observed value of N(Fe II)/(N(H I) suggests there may be a significan t amount of H+ along the line of sight. In any case, little, if any, iron can be depleted into dust grains if the intrinsic metallicity of complex C is subsolar. Absorption from complex C is also seen in C II, N I, and N II, and upper limits based on non-detections can be determined for Ar I, P II, and Fe III. Although molecular hydrogen in the Milky Way is obvious in the FUSE data, no H_2 absorption is seen in the high velocity cloud to a limit N(H_2)<2.0x10^14 cm^-2. Future FUSE observations of extragalactic objects behind Galactic high velocity clouds will allow us to better constrain models of HVC origins.
The high- and intermediate-velocity interstellar clouds (HVCs/IVCs) are tracers of energetic processes in and around the Milky Way. Clouds with near-solar metallicity about one kpc above the disk trace the circulation of material between disk and halo (the Galactic Fountain). The Magellanic Stream consists of gas tidally extracted from the SMC, tracing the dark matter potential of the Milky Way. Several other HVCs have low-metallicity and appear to trace the continuing accretion of infalling intergalactic gas. These assertions are supported by the metallicities (0.1 to 1 solar) measured for about ten clouds in the past decade. Direct measurements of distances to HVCs have remained elusive, however. In this paper we present four new distance brackets, using VLT observations of interstellar CaII H and K absorption toward distant Galactic halo stars. We derive distance brackets of 5.0 to 11.7 kpc for the Cohen Stream (likely to be an infalling low-metallicity cloud), 9.8 to 15.1 kpc for complex GCP (also known as the Smith Cloud or HVC40-15+100 and with still unknown origin), 1.0 to 2.7 kpc for an IVC that appears associated with the return flow of the Fountain in the Perseus Arm, and 1.8 to 3.8 kpc for cloud g1, which appears to be in the outflow phase of the Fountain. Our measurements further demonstrate that the Milky Way is accreting substantial amounts of gaseous material, which influences the Galaxys current and future dynamical and chemical evolution.
Using Milky Way data of the new Effelsberg-Bonn HI Survey (EBHIS) and the Galactic All-Sky Survey (GASS), we present a revised picture of the high-velocity cloud (HVC) complex Galactic Center Negative (GCN). Owing to the higher angular resolution of these surveys compared to previous studies (e.g., the Leiden Dwingeloo Survey), we resolve Complex GCN into lots of individual tiny clumps, that mostly have relatively broad line widths of more than 15 km/s. We do not detect a diffuse extended counterpart, which is unusual for an HVC complex. In total 243 clumps were identified and parameterized which allows us to statistically analyze the data. Cold-line components (i.e., w < 7.5 km/s) are found in about 5% only of the identified cloudlets. Our analysis reveals that Complex GCN is likely built up of several subpopulations that do not share a common origin. Furthermore, Complex GCN might be a prime example for warm-gas accretion onto the Milky Way, where neutral HI clouds are not stable against interaction with the Milky Way gas halo and become ionized prior to accretion.
Previous studies suggest that the estimated maximum accretion rate from approaching high velocity clouds (HVCs) on the Galactic disk can be up to ~ 0.4 solar mass per year. In this study, we point out that the hydrodynamic interaction between the HVCs and the Galactic disk is not considered in the traditional method of estimating the infall rate and therefore the true supply rate of fuel from HVCs can be different from the suggested value depending on the physical configurations of HVCs including density, velocity, and distance. We choose 11 HVC complexes and construct 4 different infall models in our simulations to give an idea of how the fuel supply rate could be different from the traditional infall rate. Our simulation results show that the fuel supply rate from HVC infall is overestimated in the traditional method and can be lowered by a factor of ~ 0.072 when the hydrodynamic interaction of the HVC complexes and the disk is considered.
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