14712521
A LARGE RESERVOIR OF IONIZED GAS IN THE GALACTIC HALO: IONIZED SILICON IN HIGH-VELOCITY AND INTERMEDIATE-VELOCITY CLOUDS
The low Galactic halo is enveloped by a sheath of ionized, low-metallicity gas, which can provide a substantial (1 M ☉ yr–1) cooling inflow to replenish star formation in the disk. Using absorption spectra from the Hubble Space Telescope and Far Ultraviolet Spectroscopic Explorer toward 37 active galactic nuclei at high latitude, we detect widespread interstellar Si III λ1206.50 absorption: 61 high-velocity clouds (HVCs) along 30 sight lines and 22 intermediate-velocity clouds (IVCs) along 20 sight lines. We find a segregation of redshifted and blueshifted absorbers across the Galactic rotation axis at l = 180°, consistent with a lag in the rotation velocity above the Galactic plane. The HVC sky coverage is large (81% ± 5% for 30 out of 37 directions) with Si III optical depth typically 4-5 times that of O VI λ1031.926. The mean HVC column density per sight line, log NSi III = 13.42 ± 0.21, corresponds to total column density N HII (6 × 1018 cm–2)(Z Si/0.2Z ☉)–1 of ionized low-metallicity gas, similar to that inferred in O VI. This reservoir could total 108 M ☉ and produce a mass infall rate ~1 M ☉ yr–1. By modeling Si II, Si III, Si IV, and H I in a subset of absorbers, we constrain the mean photoionization parameter in the low halo, log U = –3.0+0.3 –0.4, approximately 10 times lower than observed in the low-redshift intergalactic medium. The metallicities in some HVCs, derived from [Si II/H I], are 10%-30% solar, whereas values found from all three silicon ions are lower in the pure-photoionization models. These formally lower matallicities are highly uncertain, since some of the higher ions may be collisionally ionized. The HVC and IVC metallicities may be compared with the median photometric metallicity, [Fe/H] = –1.46 ± 0.30, for ~200,000 halo F/G stars in the Sloan Digital Sky Survey.
14287564
Kinematics of Diffuse Ionized Gas Halos: A Ballistic Model of Halo Rotation
To better understand diffuse ionized gas (DIG) kinematics and halo rotation in spiral galaxies, we have developed a model in which clouds are ejected from the disk and follow ballistic trajectories through the halo. The behavior of clouds in this model has been investigated thoroughly through a parameter space search and a study of individual cloud orbits. Synthetic velocity profiles have been generated in z (height above the plane) from the models for the purpose of comparing with velocity centroid data from previously obtained long-slit spectra of the edge-on spirals NGC 891 (one slit) and NGC 5775 (two slits). In each case, a purely ballistic model is insufficient to explain observed DIG kinematics. In the case of NGC 891, the observed vertical velocity gradient is not as steep as predicted by the model, possibly suggesting a source of coupling between disk and halo rotation or an outwardly directed pressure gradient. The ballistic model more successfully explains DIG kinematics observed in NGC 5775; however, it cannot explain the observed trend of high-z gas velocities nearly reaching the systemic velocity. Such behavior can be attributed to either an inwardly directed pressure gradient or a possible tidal interaction with its companion, NGC 5774. In addition, the ballistic model predicts that clouds move radially outward as they cycle through the halo. The mass and energy fluxes estimated from the model suggest that this radially outward gas migration leads to a redistribution of material that may significantly affect the evolution of the interstellar medium.
8084858
HIGHLY IONIZED HIGH-VELOCITY CLOUDS: HOT INTERGALACTIC MEDIUM OR GALACTIC HALO?
We use spectroscopic data from the Hubble Space Telescope (HST) and Far Ultraviolet Spectroscopic Explorer (FUSE) to study the wide range of ionization states of the "highly ionized high-velocity clouds" (HVCs). Studied extensively in O VI absorption, these clouds are usually assumed to be infalling gas in the Galactic halo at distances less than 50 kpc. An alternative model attributes the O VI (and O VII X-ray absorption) to cosmological structures of low-density, shock-heated intergalactic gas, distributed over 1-3 Mpc surrounding the Milky Way. The latter interpretation is unlikely, owing to the enormous required mass of gas (4 × 1012 M☉). Our detection, in 9 of 12 sight lines, of low-ionization stages (C II/III/IV; Si II/III/IV) at similar high velocities as O VI requires gas densities far above that (nH ≈ 5 × 10-6 cm-3) associated with the warm-hot intergalactic medium (WHIM). These HVCs are probably cooling, multiphase gas in the Galactic halo, bow shocks, and interfaces between clouds falling through a hot, rotating gaseous halo. The velocity segregation of these HVCs in Galactic coordinates is consistent with a pattern in which infalling clouds reflect the sense of Galactic rotation, with peculiar velocities superposed.
1954406
Hubble Space Telescope Survey of Interstellar High-Velocity Si III
We describe an ultraviolet spectroscopic survey of interstellar high-velocity cloud (HVC) absorption in the strong ?1206.500 line of Si III using the Space Telescope Imaging Spectrograph aboard the Hubble Space Telescope. Because the Si III line is 4-5 times stronger than O VI ?1031.926, it provides a sensitive probe of ionized gas down to column densities N Si III 5 ? 1011 cm?2 at Si III equivalent width 10 m?. We detect high-velocity Si III over 91% ? 4% of the sky (53 of 58 sight lines); 59% of the HVCs show negative local standard of rest velocities. The mean HVC column density per sight line is log N Si III = 13.19 ? 0.45, while the mean for all 90 velocity components is 12.92 ? 0.46. Lower limits due to Si III line saturation are included in this average, so the actual mean/median values are even higher. The Si III appears to trace an extensive ionized component of Galactic halo gas at temperatures 104.0-4.5 K indicative of a cooling flow. Photoionization models suggest that typical Si III absorbers with 12.5 < log N Si III < 13.5 have total hydrogen column densities N H 1018-1019 cm?2 for gas of hydrogen density n H 0.1 cm?3 and 10% solar metallicity. With typical neutral fractions N H I /N H 0.01, these HVCs may elude even long-duration 21 cm observations at Arecibo, the EVLA, and other radio facilities. However, if Si III is associated with higher density gas, n H ? 1 cm?3, the corresponding neutral hydrogen could be visible in deep observations. This reservoir of ionized gas may contain 108 M ? and produce a mass infall rate of 1 M ? yr?1 to the Galactic disk.
16974063
Metallicity and Ionization in High-Velocity Cloud Complex C
We analyze HST and FUSE ultraviolet spectroscopic data for 11 sight lines passing through the infalling high-velocity cloud (HVC) Complex C. These sight lines pass through regions with H I column densities in the range N = 1018.1-1020.1 cm-2. From [O I/H I] abundances, we find that Complex C metallicities range from 0.09 to 0.29 Z☉, with a column density weighted mean of 0.13 Z☉. Nitrogen (N I) is underabundant by factors of (0.01-0.07)(N/H)☉, significantly less than oxygen relative to solar abundances. This pattern suggests nucleosynthetic enrichment by Type II SNe, consistent with an origin in the Galactic fountain or infalling gas produced in winds from Local Group galaxies. The range of metallicity and its possible (2 σ) dependence on N could indicate some mixing of primordial material with enriched gas from the Milky Way, but the mixing mechanism is unclear. We also investigate the significant highly ionized component of Complex C, detected in C IV, Si IV, and O VI, but not in N V. High-ion column density ratios show little variance and are consistent with shock ionization or ionization at interfaces between Complex C and a hotter surrounding medium. Evidence for the former mechanism is seen in the Mrk 876 line profiles, where the offset in line centroids between low and high ions suggests a decelerating bow shock.
122083289
Calculating heavy quark distributions
Abstract A systematic calculation of the evolution of parton distribution functions including the effects of heavy-quark masses is presented. The method involves the use of a special renormalization scheme which ensures ordinary massless evolution with the correct number of active quark flavors at all stages, and specifies appropriate matching conditions at thresholds. This method is applicable to all orders of the perturbation expansion in principle, and it is simple to implement in practice. Results of this calculation using known distributions at low energies as input are examined and compared with published results. The heavy-quark distribution functions are found to be about a factor of two larger than the well-known EHLQ results.
122418663
The Theorems of Perturbative QCD
A review of perturbative QCD results, particularly factorization theorems, from the literature ''for which there is a statement of the result at all orders of perturbation theory and a reasonable approximation to a proof'' is presented. (AIP)
117957302
Ward identities, the renormalization group and the Callan-Symanzik equation
SummaryThe Ward identity for broken scale invariance inn dimensions is shown to be the equation obtained by dimensional analysis. Renormalizing and taking the limtn → 4 allows extraction of the anomaly atn=4. This is found to give either the renormalization group equation or the Callan-Symanzik equation, depending on the renormalization preseription used. Thus, the identity of the two equations is manifest.RiassuntoSi dimostra che l’identità di Ward per l’invarianza di scala infranta inn dimensioni coincide con l’equazione che si ottiene con l’analisi dimensionale. Rinormalizzando e passando al limite pern → 4 si può isolare l’anomalia an=4. Si trova che ciò produce o l’equazione del gruppo di rinormalizzazione o l’equazione di Callan-Symanzik, a seconda della regola di rinormalizzazione che si è usata. Pertanto è manifesta l’identità delle due equazioni.РеэюмеПокаэывается, что тождество Уорда для нарущенной масщтабной инвариантности в случае и иэмерений представляет уравнение, полученное иэ аналиэа раэмерности. Перенормировка и предельный переходn → 4 поэволяют исключить аномалию приn=4. Эта процедура приводит либо к уравнению группы перенормировки, либо к уравнению Челлена-Симанэика, которые эависят от испольэованной процедуры перенормировки. Таким обраэом, тождественность зтих двух уравнений является явно выраженной.
119225085
Gluon distribution function and factorization in Feynman gauge
A complication in proving factorization theorems in Feynman gauge is that individual graphs give a superleading power of the hard scale when all the gluons inducing the hard scattering are longitudinally polarized. With the aid of an example in gluon-mediated deep-inelastic scattering, we show that, although the superleading terms cancel after a sum over graphs, there is a residual nonzero leading term from longitudinally polarized gluons. This is due to the nonzero transverse momenta of the gluons in the target. The noncancellation, due to the non-Abelian property of the gauge group, is necessary to obtain the correct form of the gluon distribution function as a gauge-invariant matrix element.
46016111
Factorization of Hard Processes in QCD
We summarize the standard factorization theorems for hard processes in QCD, and describe their proofs.