Numerical simulations of hot halo gas in galaxy mergers

Galaxy merger simulations have explored the behaviour of gas within the galactic disc, yet the dynamics of hot gas within the galaxy halo have been neglected. We report on the results of high-resolution hydrodynamic simulations of colliding galaxies with metal-free hot halo gas. To isolate the effect of the halo gas, we simulate only the dark matter halo and the hot halo gas over a range of mass ratios, gas fractions and orbital configurations to constrain the shocks and gas dynamics within the progenitor haloes. We find that (i) a strong shock is produced in the galaxy haloes before the first passage, increasing the temperature of the gas by almost an order of magnitude to   T ∼ 10^6.3 K  . (ii) The X-ray luminosity of the shock is strongly dependent on the gas fraction; it is  ≳10³⁹ erg s⁻¹  for halo gas fractions larger than 10 per cent. (iii) The hot diffuse gas in the simulation produces X-ray luminosities as large as  10⁴² erg s⁻¹  . This contributes to the total X-ray background in the Universe. (iv) We find an analytic fit to the maximum X-ray luminosity of the shock as a function of merger parameters. This fit can be used in semi-analytic recipes of galaxy formation to estimate the total X-ray emission from shocks in merging galaxies. (v) ∼10–20 per cent of the initial gas mass is unbound from the galaxies for equal-mass mergers, while 3–5 per cent of the gas mass is released for the 3:1 and 10:1 mergers. This unbound gas ends up far from the galaxy and can be a feasible mechanism to enrich the intergalactic medium with metals.     

Spatial pattern formation of interstellar medium

Population dynamics of multi-phased interstellar medium (ISM) is investigated by using the lattice model in position-fixed reaction. Interactions between three distinct phases of gas, cold clouds, warm gas, and hot gas give rise to cyclic phase changes in ISM. Such local phase changes are propagated in space, and stochastic steady-state spatial pattern is finally achieved. We obtain the following two characteristic patterns:

1. When the sweeping rate of a warm gas into a cold component is relatively high, cold clouds associated with warm gas form small-scale clumps and are dispersively distributed, whereas hot gas covers large fraction of space.
2. When the sweeping rate is relatively low, in contrast, warm gas and cold clouds are diffusively and equally distributed, while hot gas component is substantially localized.

     

Noble gases in enstatite chondrites released by stepped crushing and heating

Abstract– Enstatite chondrites (ECs) were subjected to noble gas analyses using stepped crushing and pyrolysis extraction methods. ECs can be classified into subsolar gas‐carrying and subsolar gas‐free ECs based on the ³⁶Ar/⁸⁴Kr/¹³²Xe ratios. For subsolar gas‐free ECs, elemental ratios, and Xe isotopic compositions indicate that Q gas is the dominant trapped component, the Q gas concentration can be correlated with the petrologic type, reasonably explained by gas release from a common EC parental material during subsequent heating. Atmospheric Xe with sub‐Q elemental ratios is found in Antarctic E3s at 600–800 °C and through crushing. The ¹³²Xe released in these fractions accounts for 30–60% of the bulk concentrations. Hence, the sub‐Q signature is generally due to contamination of elementally fractionated atmosphere. Subsolar gas is mainly released (up to 78% of the bulk ³⁶Ar) at 1300–1600 °C and through crushing, suggesting that enstatite and friable phases are the host phases. Subsolar gas is isotopically identical to solar gas, but elementally fractionated. These observations are consistent with a previous study, which suggested that subsolar gas could be fractionated solar wind having been implanted into chondrule precursors ( Okazaki et al. 2001 ). Unlike subsolar gas‐free ECs, the primordial gas concentrations of subsolar gas‐carrying ECs are not simply correlated with the petrologic type. It is inferred that subsolar gas‐rich chondrules were heterogeneously distributed in the solar nebula and accreted to form subsolar gas‐carrying ECs. Subsequent metamorphic and impact‐shock heating events have affected noble gas compositions to various degrees.     

Interaction between the X-ray gas and radio gas in the galaxy cluster PKS 0745-191

By analyzing the Chandra data of the central region of the galaxy cluster PKS 0745-191, the properties of a patch of bright X-ray gas distributed along the radio structure in the west of the central galaxy are investigated. This gas is found to be cooler and denser than the ambient gas. According to the calculation based on radio observations, the pressure gradient of the radio gas in the west is greater than that in the east. It means that there is interaction between that patch of cool X-ray gas and the radio gas. The cool gas is either formed by outer cool gas supported and disturbed by the radio gas, or is brought out from the central galaxy by radio buoyant bubbles. Assuming that the gas is in pressure-gravity balance, the volume filling factor of the X-ray gas in the central region is calculated to be b = 0.69 ± 0.28, and the properties of the relativistic particles in the radio gas, as well as the expansion effect of the radio gas on the cooling flow, are discussed.     

Molecular hydrogen emission from discs in the η Chamaeleontis cluster

Discs in the 6 Myr old cluster η Chamaeleontis were searched for emission from hot H₂. Around the M3 star ECHA J0843.3−7905, we detect circumstellar gas orbiting at ∼2 au. If the gas is ultraviolet excited, the ro-vibrational line traces a hot gas layer supported by a disc of mass  ∼0.03 M_⊙  , similar to the minimum mass solar nebula. Such a gas reservoir at 6 Myr would promote the formation and the inwards migration of gas giant planets.     

Axisymmetric dusty gas jet in the inner coma of a comet

The behavior of expanding pure and dusty gas jets is investigated in the inner coma of an H₂O-dominated comet by numerically solving the axisymmetric, time-dependent, coupled hydrodynamic equations for H₂O gas and single-sized dust (0.65 μ) in polar coordinates (r, θ, φ). The jet profile is assumed to be Gaussian on the surface of a nucleus. The viscosity of the gas is taken into account. Two-dimensional distributions of the densities, velocities in the r and θ directions, and temperatures for the gas and dust have been obtained. For the dusty jet, the axisymmetric transonic solution for the gas has been calculated time-dependently. For a narrow dusty gas jet (i.e., of breadth 10°), the gas density peaks shift from the central axis of the jet (θ = 0°) to its wings (θ ∼ 30°) with the gas flowing away from the cometary nucleus, owing to a steep density gradient in the θ direction. Dragged by this laterally expanding gas outflow, the dust particles are swept away from the central axis and are also concentrated more sharply at θ ∼ 35° than the gas particles. This lateral expansion of the jet is overwhelming only within the innermost region (r ≦ 10 km). The jet feature for the gas becomes indiscernible by the time the flow reaches the outer boundary (r = 100 km), while the corresponding dust feature remains even at the outer boundary. The radial velocities of the gas and dust are enhanced inside the jet, compared with those in the background. For a broad pure gas jet (i.e., of breadth 30°), on the other hand, the gas density peaks do not shift to the wings and the jet feature can still be seen at the outer boundary, in contrast to the narrow case.     

Molecular gas associated with the IRAS-vela shell

We present a survey of molecular gas in theJ = 1 → 0 transition of¹²CO towards the IRAS Vela Shell. The shell, previously identified from IRAS maps, is a ring-like structure seen in the region of the Gum Nebula. We confirm the presence of molecular gas associated with some of the infrared point sources seen along the shell. We have studied the morphology and kinematics of the gas and conclude that the shell is expanding at the rate of ~ 13 km s^-1 from a common center. We go on to include in this study the Southern Dark Clouds seen in the region. The distribution and motion of these objects firmly identify them as being part of the shell of molecular gas. Estimates of the mass of gas involved in this expansion reveal that the shell is a massive object comparable to a GMC. From the expansion and various other signatures like the presence of bright-rimmed clouds with head-tail morphology, clumpy distribution of the gas etc., we conjecture that the molecular gas we have detected is the remnant of a GMC in the process of being disrupted and swept outwards through the influence of a central OB association, itself born of the parent cloud.     

Fields and filaments in the core of the Centaurus cluster

We present high-resolution images of the Faraday rotation measure (RM) structure of the radio galaxy PKS 1246−410 at the centre of the Centaurus cluster. Comparison with Hα-line and soft X-ray emission reveals a correspondence between the line-emitting gas, the soft X-ray emitting gas, regions with an excess in the RM images and signs of depolarization. Magnetic field strengths of 25 μG, organized on scales of ∼1 kpc and intermixed with gas at a temperature of 5 × 10⁶ K with a density of ∼0.1 cm⁻³, can reproduce the observed RM excess, the depolarization and the observed X-ray surface brightness. This hot gas may be in pressure equilibrium with the optical line-emitting gas, but the magnetic field strength of 25 μG associated with the hot gas provides only 10 per cent of the thermal pressure and is therefore insufficient to account for the stability of the line-emitting filaments.     

A model of supernova feedback in galaxy formation

A model of supernova feedback during disc galaxy formation is developed. The model incorporates infall of cooling gas from a halo, and outflow of hot gas from a multiphase interstellar medium (ISM). The star formation rate is determined by balancing the energy dissipated in collisions between cold gas clouds with that supplied by supernovae in a disc marginally unstable to axisymmetric instabilities. Hot gas is created by thermal evaporation of cold gas clouds in supernova remnants, and criteria are derived to estimate the characteristic temperature and density of the hot component and hence the net mass outflow rate. A number of refinements of the model are investigated, including a simple model of a galactic fountain, the response of the cold component to the pressure of the hot gas, pressure-induced star formation and chemical evolution. The main conclusion of this paper is that low rates of star formation can expel a large fraction of the gas from a dwarf galaxy. For example, a galaxy with circular speed 50 km s¹ can expel 6080 per cent of its gas over a time-scale of 1 Gyr, with a star formation rate that never exceeds 0.1 M yr¹. Effective feedback can therefore take place in a quiescent mode and does not require strong bursts of star formation. Even a large galaxy, such as the Milky Way, might have lost as much as 20 per cent of its mass in a supernova-driven wind. The models developed here suggest that dwarf galaxies at high redshifts will have low average star formation rates and may contain extended gaseous discs of largely unprocessed gas. Such extended gaseous discs might explain the numbers, metallicities and metallicity dispersions of damped Lyman systems.     

Virial shocks in galactic haloes?

We investigate the conditions for the existence of an expanding virial shock in the gas falling within a spherical dark matter halo. The shock relies on pressure support by the shock-heated gas behind it. When the radiative cooling is efficient compared with the infall rate, the post-shock gas becomes unstable; it collapses inwards and cannot support the shock. We find for a monatomic gas that the shock is stable when the post-shock pressure and density obey     . When expressed in terms of the pre-shock gas properties at radius r it reads as  ρ r Λ( T )/ u ³ < 0.0126  , where ρ is the gas density, u is the infall velocity and Λ( T ) is the cooling function, with the post-shock temperature   T ∝ u ²  . This result is confirmed by hydrodynamical simulations, using an accurate spheri-symmetric Lagrangian code. When the stability analysis is applied in cosmology, we find that a virial shock does not develop in most haloes that form before   z ∼ 2  , and it never forms in haloes less massive than a few  10¹¹ M_⊙  . In such haloes, the infalling gas is not heated to the virial temperature until it hits the disc, thus avoiding the cooling-dominated quasi-static contraction phase. The direct collapse of the cold gas into the disc should have non-trivial effects on the star formation rate and on outflows. The soft X-ray produced by the shock-heated gas in the disc is expected to ionize the dense disc environment, and the subsequent recombination would result in a high flux of Lα emission. This may explain both the puzzling low flux of soft X-ray background and the Lα emitters observed at high redshift.     

Magnetic fields and the dynamics of spiral galaxies

We investigate the dynamics of magnetic fields in spiral galaxies by performing 3D magnetohydrodynamics simulations of galactic discs subject to a spiral potential using cold gas, warm gas and a two-phase mixture of both. Recent hydrodynamic simulations have demonstrated the formation of interarm spurs as well as spiral arm molecular clouds, provided the interstellar medium model includes a cold H  i phase. We find that the main effect of adding a magnetic field to these calculations is to inhibit the formation of structure in the disc. However, provided a cold phase is included, spurs and spiral arm clumps are still present if β≳ 0.1 in the cold gas. A caveat to the two-phase calculations though is that by assuming a uniform initial distribution, β≳ 10 in the warm gas, emphasizing that models with more consistent initial conditions and thermodynamics are required. Our simulations with only warm gas do not show such structure, irrespective of the magnetic field strength.
Furthermore, we find that the introduction of a cold H  i phase naturally produces the observed degree of disorder in the magnetic field, which is again absent from simulations using only warm gas. Whilst the global magnetic field follows the large-scale gas flow, the magnetic field also contains a substantial random component that is produced by the velocity dispersion induced in the cold gas during the passage through a spiral shock. Without any cold gas, the magnetic field in the warm phase remains relatively well ordered apart from becoming compressed in the spiral shocks. Our results provide a natural explanation for the observed high proportions of disordered magnetic field in spiral galaxies and we thus predict that the relative strengths of the random and ordered components of the magnetic field observed in spiral galaxies will depend on the dynamics of spiral shocks.     

Are HVCs Produced in Galactic Fountains?

Three-dimensional simulations of the disk-halo interaction show the formation of a thick HI and HII gas disk with different scale heights. The thick HI disk prevents the disk gas from expanding freely upwards, unless some highly energetic event such as chimneys occurs, whereas the thick HII disk acts as a disk-halo interaction region from where the hot ionized gas flows freely into the halo. The upflowing gas reaches the maximum height at z ∼ 9.3 ± 1 kpc becoming thermally unstable due to radiative losses, and condenses into HI clouds. Because the major fraction of the gas is gravitationally bound to the Galaxy, the cold gas returns to the disk. The descending clouds will have at some height high velocities. In a period of 200 Myr of fountain evolution, some 10 percent of the total number of clouds are HVCs. This revised version was published online in July 2006 with corrections to the Cover Date.     

Gas Flows and Star Formation in the Ringed Galaxy NGC 4736

We present high-resolution (∼5″) BIMA CO observations of the ringed galaxy NGC 4736, along with previously published VLA HI data (Braun, 1995). Strong CO emission is detected from the star-forming ring at r=45″ and in the central region, where a molecular bar is apparent. The azimuthally averaged gas surface density is still much less than the Toomre critical density within r=60″, despite the starburst conditions in the ring (gas depletion time ≲1Gyr). Both CO and HI velocity fields show strong departures from a circular rotating disc model. The velocity residuals are consistent with inflowing gas near the ends of the central bar, outflowing gas between the bar and the ring, and inflowing gas outside the ring. We propose that the high star formation efficiency in the ring results from gas being driven out towards the OLR of the bar and in towards the ILR of the larger oval distortion. However, the strong signature of inflow outside the ring is probably due in part to gas motion in elliptical orbits. This revised version was published online in July 2006 with corrections to the Cover Date.     

12CO, 13CO and HCN in the Barred Galaxy NGC 1530

Bars probably have a great importance in galactic evolution. The barred potential is able to concentrate large quantities of interstellar gas in the vicinity of the nucleus, feeding any nuclear activity, be it central starbursts or black hole accretion discs. The IRAM millimeter interferometer and 30 m telescope have allowed a precise analysis of the molecular gas in the bar and the nucleus of a typical barred spiral galaxy, NGC 1530. In this galaxy, I have detected CO(1→0) along two lanes that trace shocks in the molecular gas. In these lanes, the gas moves toward the centre of the galaxy, with typical in fall velocities of 100 km s^-1. I have shown in these shocks an anticorrelation between shear in the gas and star formation efficiency by comparing H^α and CO maps. I have also studied the centre of this galaxy at higher resolution in¹²CO(1→0), ¹²CO(2→1),¹³CO(1→0) and HCN(1→0). In the central region, the gas distribution is a ring or an unresolved spiral, surrounded by two curved arcs. The nuclear ring contains large amounts of dense gas traced by HCN and ¹³CO, and shows intense star formation, as indicated by the non-thermal centimetre continuum. The arcs, in contrast, are poor in dense gas and form few stars. This revised version was published online in July 2006 with corrections to the Cover Date.     

The 6.4-keV fluorescent iron line from cluster cooling flows

The fate of the cooling gas in the central regions of rich clusters of galaxies is not well understood. In one plausible scenario clouds of atomic or molecular gas are formed. However the mass of the cold gas, inferred from measurements of low-energy X-ray absorption, is hardly consistent with the absence of powerful CO or 21-cm emission lines from the cooling flow region. Among the factors which may affect the detectability of the cold clouds are their optical depth, shape and covering fraction. Thus, alternative methods to determine the mass in cold clouds, which are less sensitive to these parameters, are important.   For the inner region of the cooling flow (e.g. within a radius of ∼50–100 kpc) the Thomson optical depth of the hot gas in a massive cooling flow can be as large as ∼ 0.01. Assuming that the cooling time in the inner region is few times shorter than the lifetime of the cluster, the Thomson depth of the accumulated cold gas can be accordingly higher (if most of the gas remains in the form of clouds). The illumination of the cold clouds by the X-ray emission of the hot gas should lead to the appearance of a 6.4-keV iron fluorescent line, with an equivalent width proportional to τ_T. The equivalent width only weakly depends on the detailed properties of the clouds, e.g. on the column density of individual clouds, as long as the column density is less than a few 10²³ cm⁻². Another effect also associated exclusively with the cold gas is a flux in the Compton shoulder of bright X-ray emission lines. It also scales linearly with the Thomson optical depth of the cold gas. With the new generation of X-ray telescopes, combining large effective area and high spectral resolution, the mass of the cold gas in cooling flows (and its distribution) can be measured.     

Stellar winds and globules in Hii regions

The interaction of a plane-parallel hypersonic stellar wind with a globule in an Hii region is considered in two approximations. In both approximations, the ionization front on the globule remains strong-D type, and a flow pattern containing two oppositely facing shock waves results. In the first approximation, the structure of the shocked region is calculated assuming that globule gas and stellar wind gas mix well and move at the same velocity. However, this assumption results in a very thick shocked layer and the assumption of good mixing is consequently not well justified. This approximation provides an upper limit on the gas velocities expected in the shocked gas which originated at the globule. In the second approximation, the stellar wind merely applies pressure to balance the momentum flux in the globule gas. The structure of the shocked region is calculated on the assumption that a tangential discontinuity exists between shocked stellar wind and shocked glubule gas. Structures may be produced having velocities ～10 km s^?1 and emission measures ～10³ cm^?6 pc with reasonable stellar luminosities and mass loss rates.     

Simulation of large-scale gas structures formed in the interaction between galaxies. I. Method and preliminary results

Computer programs developed to study large-scale, transient gas structures in galaxies are described and test results are given. Gas-dynamic quantities are determined on the basis of a three-dimensional algorithm using so-called "smoothed particle hydrodynamics" (SPH). Preliminary calculations were made to simulate the formation of a gas ring around a spheroidal galaxy when it absorbs a low-mass, gas-rich companion, as well due to gas accretion during the flyby of a spiral galaxy of comparable mass. The evolution of tidal gas tails of disk galaxies is investigated.Translated from Astrofizika, Vol. 39, No. 2, pp. 265–284, April–June, 1996.     

The fragmentation of pre-enriched primordial objects

Recent theoretical investigations have suggested that the formation of the very first stars, forming out of metal-free gas, was fundamentally different from the present-day case. The question then arises which effect was responsible for this transition in the star formation properties. In this paper, we study the effect of metallicity on the evolution of the gas in a collapsing dark matter mini-halo. We model such a system as an isolated 3 σ peak of mass     that collapses at     , using smoothed particle hydrodynamics. The gas has a supposed level of pre-enrichment of either     or 10⁻³ Z_⊙. We assume that H₂ has been radiatively destroyed by the presence of a soft UV background. Metals therefore provide the only viable cooling at temperatures below 10⁴ K. We find that the evolution proceeds very differently for the two cases. The gas in the lower metallicity simulation fails to undergo continued collapse and fragmentation, whereas the gas in the higher metallicity case dissipatively settles into the centre of the dark matter halo. The central gas, characterized by densities     , and a temperature,     , that closely follows that of the cosmic microwave background, is gravitationally unstable and undergoes vigorous fragmentation. We discuss the physical reason for the existence of a critical metallicity,     , and its possible dependence on redshift. Compared with the pure H/He case, the fragmentation of the     gas leads to a larger relative number of low-mass clumps.     

On the soft X-ray spectrum of cooling flows

Strong evidence for cooling flows has been found in low-resolution X-ray imaging and spectra of many clusters of galaxies. However, high-resolution X-ray spectra of several clusters from the Reflection Grating Spectrometer on XMM-Newton now show a soft X-ray spectrum inconsistent with a simple cooling flow. The main problem is a lack of the emission lines expected from gas cooling below 1–2 keV. Lines from gas at about 2–3 keV are observed, even in a high-temperature cluster such as A1835, indicating that gas is cooling down to about 2–3 keV, but is not found at lower temperatures. Here we discuss several solutions to the problem: heating, mixing, differential absorption and inhomogeneous metallicity. Continuous or sporadic heating creates further problems, including the targeting of the heat at the cooler gas and also the high total energy required. So far there is no clear observational evidence for widespread heating, or shocks, in cluster cores, except in radio lobes which occupy only part of the volume. Alternatively, if the metals in the intracluster medium are not uniformly spread but are clumped, then little line emission is expected from the gas cooling below 1 keV. The low-metallicity part cools without line emission, whereas the strengths of the soft X-ray lines from the metal-rich gas depend on the mass fraction of that gas and not on the abundance, since soft X-ray line emission dominates the cooling function below 2 keV.