Hot Ambient Gas Research Articles

Cooling flows within galactic haloes: the kinematics and properties of infalling multiphase gas

We study the formation of disks via the cooling flow of gas within galactic haloes using smoothed particle hydrodynamics simulations. These simulations resolve mass scales of a few thousand solar masses in the gas component for the first time. Thermal instabilities result in the formation of numerous warm clouds that are pressure confined by the hot ambient halo gas. The clouds fall slowly onto the disk through non-spherical accretion from material flowing preferentially down the angular momentum axis. The rotational velocity of the infalling cold gas decreases as a function of height above the disk, closely resembling that of the extra-planar gas recently observed around the spiral galaxy NGC 891.

Thermal X‐Ray Iron Line Emission from the Galactic Center Black Hole Sagittarius A*

We model thermal X-ray emission from the accreting supermassive black hole Sagittarius A* at the Galactic center. For the region inside 15 of the center, we use a generalized radiatively inefficient accretion flow (RIAF) model, and for the region out to 10'' we use published models of the ambient hot gas. We calculate the equivalent widths of hydrogen-like and helium-like emission lines of various elements, notably iron. We predict that a strong helium-like iron line with an equivalent width ~1 keV should be emitted by both the external medium and the RIAF. The equivalent width in the external medium is sensitive to the metallicity Z of the gas as well as the mean temperature. For reasonable choices of these parameters, the calculated results agree with Chandra's detection of an iron line with an equivalent width of 1.3 keV within 10''. The emission from within 15 is not sensitive to the external temperature, but is sensitive to the density and, especially, temperature profile inside the Bondi radius. For the range of profiles we consider, we calculate the equivalent width of the iron line to be ~0.6-1.5(Z/Z☉) keV, where Z☉ is the solar metallicity. We present a new Chandra spectrum of the quiescent emission within 15 of Sgr A*. The measured equivalent width of the iron line is 0.7 keV. Although this measurement has a large uncertainty, it is consistent with our predictions, provided the metallicity of the gas is approximately solar.

4226 放射状自由液膜流れの微粒化に及ぼす液体物性と周囲気体温度の影響(G05-13 気泡・液滴(2),G05 流体工学)

This paper describes flow patterns of a radial liquid sheet. A liquid film produced by a impingement of liquid to a disk center spreads out radially on the disk and forms a radial liquid sheet after the disk edge. The radial liquid sheet is unstable because of a internal velocity profile with a inflexional point. The instability cause strong tubulence through a laminar-turbulent transition immediately after the disk edge. Flow patterns of the radial liquid sheet depend completely on the frequency of perforations produced by the strong turbulence. The perforation frequency rely on flow conditions, liquid properties and temperture of ambient gas. Countless perforations lead to atomization of the liquid sheet and no perforation results in a smooth liquid sheet. This study reveals that the flow conditions each flow pattern emerges in and hot ambient gas promotes the atomization.

Accretion of cool stellar winds on to Sgr A*: another puzzle of the Galactic Centre?

Abstract Sgr A* is currently being fed by winds from a cluster of gravitationally bound young mass-losing stars. Using observational constraints on the orbits, mass-loss rates and wind velocities of these stars, we numerically model the distribution of gas in the ∼ 0.1–10 arcsec region around Sgr A*. We find that radiative cooling of recently discovered slow winds leads to the formation of many cool filaments and blobs, and to a thin and rather light accretion disc on a scale of about an arcsecond. The disc, however, does not extend all the way to our inner boundary. Instead, hot X-ray-emitting gas dominates the inner arcsecond. In our simulations, cool streams of gas frequently enter this region on low angular momentum orbits, and are then disrupted and heated up to the ambient hot gas temperature. The accreting gas around Sgr A* is thus two-phase, with a hot component, observable at X-ray wavelengths, and a cool component, which may be responsible for the majority of the time variability of Sgr A* emission on time-scales of 100–1000 yr. We obtain an accretion rate of a few ×10−6 M⊙ yr−1, consistent with Chandra estimates, but variable on time-scales even shorter than 100 yr. These results strongly depend on the chosen stellar orbits and wind parameters. Further observational input is thus key to a better modelling of the Sgr A* wind accretion.

Radiatively Inefficient Accretion Flow Models of Sgr A*

I review radiatively inefficient accretion flow models for the ≈2.6 × 106 M⊙ black hole (BH) in the Galactic Center. I argue for a ‘concordance model’ of Sgr A*: both theory and observations suggest that hot ambient gas around the BH is accreted at a rate ∼10–8 M⊙ yr–1 much less than the canonical Bondi rate. I interpret Chandra observations of Sgr A* in the context of such a model: (1) the extended ‘quiescent’ X-ray emission is due to thermal bremsstrahlung from gas in the vicinity of the Bondi accretion radius, and (2) the ∼104 second long X-ray flares are due to synchrotron or Inverse-Compton emission by non-thermal electrons accelerated in the inner ∼10 Schwarzschild radii of the accretion flow.

Momentum transfer across shear flows in smoothed particle hydrodynamic simulations of galaxy formation

We investigate the evolution of angular momentum in SPH simulations of galaxy formation, paying particular attention to artificial numerical effects. We find that a cold gas disc forming in an ambient hot gas halo receives a strong hydrodynamic torque from the hot gas. By splitting the hydrodynamic force into artificial viscosity and pressure gradients, we find that the angular momentum transport is caused not by the artificial viscosity but by the pressure gradients. Using simple test simulations of shear flows, we conclude that the pressure gradient-based viscosity can be divided into two components: one due to the noisiness of SPH and the other to ram pressure. The former is problematic even with very high resolution because increasing resolution does not reduce the noisiness. On the other hand, the ram pressure effect appears only when a cold gas disc or sheet does not contain enough particles. In such a case, holes form in the disc or sheet, and then ram pressure from intra-hole hot gas, causes significant deceleration. In simulations of galactic disc formation, star formation usually decreases the number of cold gas particles, and hole formation leads to the fragmentation of the disc. To circumvent these problem, we modify the SPH algorithm, decoupling the cold from the hot gas phases, i.e. inhibiting the hydrodynamic interaction between cold and hot particles. This, a crude modelling of a multi-phase fluid in SPH cosmological simulations, leads to the formation of smooth extended cold gas discs and to better numerical convergence. The decoupling is applicable in so far as the self-gravitating gas disc with negligible external pressure is a good approximation for a cold gas disc. (abridged)

A multi-phase model for simulations of galaxy formation

We discuss SPH simulations of galaxy formation which use a hybrid method to describe a two-phase structure of the star forming ISM on unresolved scales. Our modeling includes radiative cooling, heating due to a UV background, growth of cold clouds embedded in an ambient hot gas, star formation out of cloud material, feedback due to supernovae in the form of thermal heating and cloud evaporation, starbursts that can lead to galactic outflows, and metal enrichment. Our particular model for the treatment of the two-phase structure is based on a modified and extended version of the grid-based approach of Yepes et al. (1997). We discuss the properties of the feedback model and show how it stabilizes star forming disk galaxies and reduces the cosmic star formation rate to a level consistent with current observational constraints.

Swelling Behavior of Black Liquor from Soda Pulping of Wheat Straw

The primary aim of this research work was to characterize the swelling properties of black liquor and their dependence on the temperature and gas composition of the combustion or gasification environment. The measurements were made using an industrial black liquor from soda pulping of wheat straw. The swelling measurements were carried out in a single droplet reactor. This paper presents data that show the influence of the reactor temperature and the gas composition on the swelling during devolatilization, and compares the swelling characteristics of a straw black liquor with similar published data on black liquors from kraft pulping of wood. Experiments were carried out in inert, oxiding, and gasifying environments, at temperatures from 500 to 900 °C. One interesting result was that the straw liquor swelled much more than most kraft or soda liquors from pulping of wood. Another result was that droplets subjected to hot gas atmospheres containing O2, CO2 and/or H2O(v) swelled less than those subjected to hot N2 atmospheres. The data obtained suggest that the straw soda black liquor would burn more rapidly than most kraft liquors from pulping of wood because of its high external surface area when swollen. This highly swelling liquor caused problems with fluidization when straw liquors were injected in a laboratory-scale fluidized bed gasifier.

A Moving Cold Front in the Intergalactic Medium of A3667

We present results from a Chandra observation of the central region of the galaxy cluster A3667 with emphasis on the prominent sharp X-ray brightness edge spanning 0.5 Mpc near the cluster core. Our temperature map shows large-scale nonuniformities characteristic of the ongoing merger, in agreement with earlier ASCA results. The brightness edge turns out to be a boundary of a large cool gas cloud moving through the hot ambient gas, very similar to the fronts discovered by Chandra in A2142. The higher quality of the A3667 data allows the direct determination of the cloud velocity. At the leading edge of the cloud, the gas density abruptly increases by a factor of 3.9 ± 0.8, while the temperature decreases by a factor of 1.9 ± 0.2 (from 7.7 to 4.1 keV). The ratio of the gas pressures inside and outside the front shows that the cloud moves through the ambient gas at near-sonic velocity, M = 1 ± 0.2 or v = 1400 ± 300 km s-1. In front of the cloud, we observe the compression of the ambient gas with an amplitude expected for such a velocity. A smaller surface brightness discontinuity is observed further ahead, ~350 kpc in front of the cloud. We suggest that it corresponds to a weak bow shock, implying that the cloud velocity may be slightly supersonic. Given all the evidence, the cold front appears to delineate the remnant of a cool subcluster that recently has merged with A3667. The cold front is remarkably sharp. The upper limit on its width, 35 or 5 kpc, is several times smaller than the Coulomb mean free path. This is a direct observation of suppression of the transport processes in the intergalactic medium, most likely by magnetic fields.

CO (J = 1–0) Observation of the cD Galaxy of AWM 7: Constraints on the Evaporation of Molecular Gas

Abstract We have searched for molecular gas in the cD galaxy of a poor cluster of galaxies, AWM 7, using the Nobeyama 45 m telescope. We did not detect CO emission in the galaxy. Our limit of molecular gas in the inner 7.5 kpc is MH2< 4× 108 M⊙ . We estimate the total mass of molecular gas left in the cD galaxy when the gas deposited by a cooling flow once becomes molecular gas and the molecular gas is continuously evaporated by the ambient hot gas. The observational limit of molecular gas requires f≳ 10-3, where f is the ratio of the heat conduction rate to that of Spitzer. However, this contradicts recent X-ray observations showing f<10-5. Thus, the non-detection of CO cannot be explained by evaporation, and most of the cooled gas predicted by a cooling flow model may not change into molecular gas in the cD galaxy. Moreover, we estimate the evaporation time of molecular clouds brought to a cD galaxy through the capture of gas-rich galaxies and find that these clouds should not be evaporated if f≲ 10-3–10-4. Therefore, the non-detection of CO in a cD galaxy could constrain the total mass of the molecular clouds brought into it.

In-situ FTIR spectroscopic monitoring of a CVD controlled Si-N-O fibre growth process

High strength and high temperature composite materials such as CMCs represent a very promising family of future materials. A high temperature CVD-process has been developed to produce a new type of high-performance amorphous silicon-oxynitride (Si-N-O) fibres. The fibres were grown on a SiC substrate at 1450°C exposing a stoichiometric precursor powder mixture of SiO 2 + SiC, doped with 10 wt% Ti powder to flowing NH 3 . To improve CVD process control an in-situ FTIR monitoring system is in development. For application of a FTIR based monitoring to the fibre growth process a specific optical adaptation has been designed onto the growth reactor. The optical set-up allows an almost simultaneous in-situ measurement of the transmission and emission of the hot gas atmosphere just above the precursor powder mixture. In addition to the decomposition of NH 3 , different reaction products have been identified, such as CO, HCN and CH 4 . Gaseous Si-O species could be detected which are responsible for the silicon transport in the gas phase from the solid SiO 2 precursor powder to the fibre growth position. The assessment of the SiO bands has been supported by additional experiments which promote the formation of gaseous SiO.

Measurement of Transient Temperature Field Within a Falling Droplet

Fluorescence shift thermometry has been combined with droplet slicing imaging to capture images of the transient temperature field within a 500-μm-diam decane droplet falling into 370-420°C nitrogen. The temperature field becomes almost homogeneous within 5-10 ms of entry into the hot ambient gas and rises toward the boiling point of the liquid over 50-100 ms. The constraints faced in the design of such an experiment and the protocols required for successful measurement of such temperature images are addressed

Are Pressure-Confined Clouds in Galactic Halos Possible Models of Lyman Alpha Clouds?

Our understanding of the Lyα forest has changed considerably following observations by HST and Keck. Lyα clouds at low redshifts (z < 1.7) observed by HST showed two unexpected features: Lanzetta et al. (1995) found that most luminous galaxies at such redshifts produce Lyα absorptions at mean impact parameter ~160h−1kpc, and established the association between Lyα clouds and galaxies. Ulmer (1996) pointed out the strong clustering of Lyα clouds in this redshift range. Motivated by the above, we propose a two-component protogalaxy model for the Lyα clouds based upon our previous work (Miyahata & Ikeuchi 1995). In our model, the Lyα clouds are stable cold clouds confined by the pressure of ambient hot gas in a galactic halo. We determine the properties of these cold clouds and hot gas on the basis of theoretical and observational constraints. We take into account the stability of a cold cloud in the galactic halo in addition to the general stability conditions in a two-component medium (e.g. Ikeuchi & Ostriker 1986), and compare the derived quantities of Lyα clouds in the galactic halo and in the intergalactic medium at both high and low redshifts. We conclude that the ciondition that a cloud is stable against both evaporation and tidal disruption by a hot galactic halo is very restrictive. In the most noteworthy example at z ~ 0.5, a pressure-confined, stable spherical Lyα cloud with NHI = 1014cm−2 cannot survive in the galactic halo, although much higher column density clouds of NHI = 1017cm−2 can. Miyahata & Ikeuchi (1997) discuss how these results constrain an alternative model for Lyα clouds associated with the galaxies observed by Lanzetta et al. (1995).

On the Origin of Metallicity in Lyα Forest Systems

We investigate the hypothesis that $\lyal$ absorption lines arise in two populations of halos --- minihalos of small circular velocity ($V_c \la 55$ km s$^{-1}$) in which star formation and metal production are inhibited by photoionization of the UV background radiation, and large galactic halos ($55 \la V_c \la 250$ km s$^{-1}$) which possess stars and metals. Based on the model of $\lyal$-absorbing gas confined in both populations of halos, we attempt to explain the recent observations of (1) associations of visible galaxies with $\lyal$ lines at low redshifts $z \la 1$, and (2) metal lines associated with a non-negligible fraction of low H I column density $\lyal$ lines at $z \sim 3$. For galactic halos, we find that photoionized gas clouds confined in the pressure of ambient hot gas can produce $\lyal$ absorptions with H I column density as low as $10^{14}$ cm$^{-2}$, and that the impact parameter of a sightline for such absorptions matches well with the observed radius of gaseous envelope in a typical luminous galaxy. Using the Press- Schechter prescription for the mass function of halos, we also show that the fraction of $\lyal$ lines with associated metal lines can be understood in terms of the fraction of $\lyal$ absorbers that are associated with galactic halos. In particular, the reported fraction of $\sim 0.5--0.75$ at $z \sim 3$ is reproduced when the boundary value of $V_c$ to separate mini or galactic halos is $40 \sim 60$ km s$^{-1}$, which is consistent with the theoretical prediction of galaxy formation under photoionization. .....

Anomalous Heat Evolution from a Solid-State Electrolyte Under Alternating Current in High-Temperature D2Gas

AbstractA coin-shaped proton conductor made from metal oxides of strontium and cerium can be charged in a hot D2 gas atmosphere to produce excess heat. Anomalous heat evolution was observed from the proton conductors charged with alternating current at 5 to 45 V at temperatures ranging from 400 to 700°C. The anomalous heat produced temperature increases as much as 50°C. Excess heat was estimated as a few watts in most cases, totaling up to several kilojoules.

Dynamics of Collapsing Shells

In the last decade several scenarios for the formation of globular clusters have been suggested. One of these models starts with an OB-association exploding near the center of a molecular cloud (Brown et al., ApJ 376, 115 (1991)): The expanding shell sweeps up the cloud material and later the expansion will be decelerated or stopped by the external pressure of the ambient hot gas. The shell itself can break into fragments and form stars. If the total energy of these stars is negative, they will recollapse and eventually form a bound system. According to this idea the dynamics of a thin stellar shell has been studied.

Dynamics and X-ray emission of a galactic superwind interacting with disk and halo gas

There is a general agreement that the conspicuous extranuclear X-ray, optical-line, and radio-contiuum emission of starbursts is associated with powerful galactic superwinds blowing from their centers. However, despite the significant advances in observational studies of superwinds, there is no consensus on the nature of the emitting material and even on the emission mechanisms themselves. This is to a great extent a consequence of a poor understanding of dynamical processes in the starburst superwind regions. To address this issue, we have conducted two-dimensional hydrodynamical simulations of galactic superwinds. While previous similar studies have used a single (disk) component to represent the ISM of the starburst galaxy, we analyze the interaction of the wind with a two-component disk-halo ambient interstellar medium and argue that this two-component representation is crucial for adequate modeling of starbursts. The emphasis of this study is on the geometry and structure of the wind region and the X-ray emission arising in the wind material and the shocked gas in the disk and the halo of the galaxy. The simulation results have shown that a clear-cut bipolar wind can easily develop under a range of very different conditions. On the other hand, a complex 'filamentary' structure associated with the entrained dense disk material is found to arise within the hot bubble blown out by the wind. The flow pattern within the bubble is dominated equally by the central biconic outflow and a system of whirling motions r elated to the origin and development of the 'filaments'. The filament parameters make them a good candidate for optical-emission-line filamentary gas observed in starburst halos. We find that the history of mass and energy deposition in the starburst region of the galaxy is crucial for wind dynamics. A 'mild' early wind, which arises as a result of the cumulative effect of stellar winds from massive stars, produces a bipolar vertical cavity in the disk and halo gas without strongly affecting the gaseous disk, thus creating conditions for virtually free vertical escape of the hot gas at the later, much more violent supernova-dominated phases of the starburst. We calculate the luminosity, mass, and effective temperature of the X-ray emitting gas in the 'soft' (0.1 to 0.7 keV, 0.7 to 2.2 keV, and 0.1 to 2.2 keV) and 'hard' (1.6 to 8.3 keV) energy bands and estimate the contribution of different gaseous components to the X-ray flux in these bands. Analysis of these parameters enables us to make conclusions regarding the nature of the X-ray-emitting material. We have inferred that the bulk of the soft thermal X-ray emission from starbursts arises in the wind-shocked material of the disk and halo gas rather than in the wind material itself. This enables us to predict that the integrated soft X-ray spectra of starbursts need not show an overabundance of heavy elements which are believed to be produced copiously in the centers of starbursts. Unlike soft X-ray emission, the hard component of thermal X-ray emission is found to originate in the wind material ejected from the starburst region. However, the derived ratio of hard-to-soft X-ray luminosities is too small compared to that observed in starbursts. We conclude therefore that the observed hard X-ray emission of starbursts is probably not associated with the thermal emission of hot wind or ambient shocked gas. Typical temperatures of the bulk of the soft X-ray-emitting material in our very different models have been found to agree well with the ones estimated on the basis of the ROSAT data for the soft component of X-ray emission of nearby starbursts. We predict that temperatures of the extranuclear soft X-ray-emitting gas in starburst galaxies with heavy element abundances near solar should be close to T(sub Xs = 2 to 5 x 10(exp 6)K.

Lyman absorption at low redshifts and hot gas in galactic haloes

Motivated by recent observation of Lanzetta et al. that most luminous galaxies at low redshifts produce $\lya$ absorption at impact parameter $l\lsim 160 \kpch$, we propose that these absorbers are clouds confined by the pressure of ambient hot gas in galactic haloes. We determine the properties of this hot gas and of the absorption systems on the basis of observational and theoretical constraints. The absorbing clouds need to be replenished on about one orbital time ($\sim 10^9$ yrs) in the galactic halo. The pressure and temperature of the gas at radius $r\sim 100\kpc$ are $P=(10-100){\rm cm^{-3} K}$, $T=10^{(5.5-6.5)}{\rm K}$. The model requires that about 10 per cent of the gas in low-redshift galactic haloes is in the hot phase. Such gas in galactic haloes emits x-ray with bolometric luminosity of the order $10^{37-40}{\rm erg\,s^{-1}}$. The plausibility for such gas to exist in current models of galaxy formation is discussed.

Composition of interstellar clouds in the disk and halo. 2: Gamma[SUP]2[/SUP] Velorum

High-resolution observations of gamma(sup 2) Vel with the Goddard High-Resolution Spectrograph (GHRS) echelle on the Hubble Space Telescope reveal the presence of seven narrow absorption components, with LSR velocities between -23 and +9 km s(exp -1). Three of these show column density ratios N(S(++))/N(S(+)) and N(P(++))/N(P+)) of about 1 or more, and can be identified as H II regions, while the other four are H I regions, consistent with the O I profile and with the overall H(sup 0) column density of 5.9 x 10(exp 19) cm(exp -2), given the usual assumptions that S is undepleted while O has a depletion D(O) = -0.3 dex. The depletions of Fe, Si, and Mn, which could be measure accurately for two of the four H I regions (components 6 and 7), differ somewhat from the values of D(sub ws) found for slowly moving warm clouds in HD 93521; in particular, for the component at 4.0 km s(exp -1) (No. 6), abosolute of D exceeds absolute of D(sub ws) by 0.1-0.4 dex, while for that at 9.3 km s(exp -1) (No. 7), absolute of D equals absolute of D(sub ws) on the average. The observed ratio of Fe + Mg atoms to Si atoms in the grains of component 6 is 2.04 +/-0.10, consistent with an olivine grain composition; the Fe/Mg ratio is 1.5 +/- 0.2. The electron density in component 6, determined from the C II(sup *) feature, is 0.075 +/- 0.013 cm (exp -3), about two-thirds of that found for clouds of this velocity in HD 93521. In the two conspicuous H II regions, components 3 and 4, n(sub e), determined from the Si II(sup *) feature, is about 1 cm(exp -3). From the column density of S(+) + S(++) in these two components, the total H II path length is about 40 pc. With the radius of a wind-blown bubble around gamma(sup 2) Vel set equal to 60 pc, the effective Stromgren radius is about 100 pc, requiring that T approx. equal to 50,000 K for the Wolf-Rayet component of the gamma(sup 2) Vel binary. Since zeta Pup is a comparable source of ionizing radiation, this temperature is an upper limit. The profiles of the strongest H2 absorption features, from Copernicus archives, indicate that the absorbing molecules have a mean velocity identical with that of the strongest H II component (No. 4). We have no explanation for the possible presence of these H2 molecules in a region of ionized H. Alternatively, the H2 profiles can be explained by molecules in the two adjacent (in velocity) H I regions, components 2 and 5, provided their H I gas has densities and temperatures typical of normal cold clouds. The GHRS data show absorption by highly ionized atoms Si(3+) and C(3+), N(4+) in broad features, in addition to the narrow-line absorption by Si(3+) and C(3+) observed in the dominant H II components, Nos. 3 and 4. The broad C(3+) and N(4+) features have widths corresponding to T in the range (4-8) x 10(exp 5) K, consistent with the broad O(5+) line shown in Copernicus data. Despite some observational uncertainties, the ratios of column densities in the broad C(3+), N(4+), and O(5+) features agree to +/- 0.1 dex with theoretical values for warm gas, heating and evaporating by thermal conduction from an adjacent hot region. Outward evaporation from an isolated cloud in a hot ambient gas cannot be distinguished, on the basis of these data, from inward evaporation of a warm shell, compressed by an expanding, hot stellar-wind bubble. For several halo stars, the C IV/O VI ratio has a quite different average value, perhaps consistent with cooling of infalling hot gas instead of conductive heating and evaporation.

What determines the physical quantities of galaxies? - A two-component gas model for protogalaxies with energy input from supernovae

The equilibrium structure of a protogalaxy, which is assumed to be composed of cold clouds and hot ambient gas in pressure equilibrium, is examined. The underlying characteristic time scales are the free-fall time of the system and the cooling time of the hot ambient gas. The global collapse condition used here is the well-known condition that the cooling time must be less than the free-fall time. Feedback processes are incorporated into the model via an energy balance condition between the power injected from supernovae and related processes due to the evolution of newly formed massive stars and the luminosity radiated by cooling processes