Measuring the Cosmic Equation of State with Counts of Galaxies

We show that the gas in growing density perturbations is vulnerable to the influence of winds outflowing from nearby collapsed galaxies that have already formed stars. This suggests that the formation of nearby galaxies with masses less, similar10(9) M( middle dot in circle) is likely to be suppressed, irrespective of the details of galaxy formation. An impinging wind may shock-heat the gas of a nearby perturbation to above the virial temperature, thereby mechanically evaporating the gas, or the baryons may be stripped from the perturbation entirely if they are accelerated to above the escape velocity. We show that baryonic stripping is the most effective of these two processes, because shock-heated clouds that are too large to be stripped are able to radiatively cool within a sound crossing time, limiting evaporation. The intergalactic medium temperatures and star formation rates required for outflows to have a significant influence on the formation of low-mass galaxies are consistent with current observations, but may soon be examined directly via associated distortions in the cosmic microwave background and with near-infrared observations from the Next Generation Space Telescope, which may detect the supernovae from early-forming stars.     

Collapse of primordial clouds — II. The role of dark matter

In this article we extend the study performed in our previous article of the collapse of primordial objects. We here analyse the behaviour of the physical parameters for clouds ranging from 10⁷ to 10¹⁵ M_⊙. We study the dynamical evolution of these clouds in two ways: as purely baryonic clouds and as clouds with non-baryonic dark matter included. We start the calculations at the beginning of the recombination era, following the evolution of the structure until the collapse (which we defined as the time when the density contrast of the baryonic matter is greater than 10⁴). We analyse the behaviour of several physical parameters of the clouds (e.g. the density contrast and the velocities of the baryonic matter and the dark matter) as a function of time and radial position in the cloud. In this study all physical processes that are relevant to the dynamical evolution of the primordial clouds, such as for example photon drag (due to the cosmic background radiation) and hydrogen molecular production, besides the expansion of the Universe, are included in the calculations. In particular we find that the clouds with dark matter collapse at higher redshift when we compare the results with the purely baryonic models. As a general result we find that the distribution of the non-baryonic dark matter is more concentrated than the baryonic one. It is important to stress that we do not take into account the putative virialization of the non-baryonic dark matter; we just follow the time and spatial evolution of the cloud, solving its hydrodynamical equations. We also studied the role of cooling–heating processes in the purely baryonic clouds.     

Giant HVC’s and the Rotation Curves of the Milky Way and M31

We have modelled, for the cases of Milky Way and M31, the effects on the galactic discs, of the arrival at high velocity (≥150 km s⁻¹) of giant HI clouds, with masses of up to 10⁸M⊙. Predictions are compared with the detailed structure of the observed rotation curves for these two galaxies. The model explains the rises and falls observed at large distances from the centre of each galaxy, distributed with a degree of regularity in radius, in terms of a specific type of perturbations driven by the infall of the high velocity clouds (HVC's) arriving from the intracluster medium of the Local Group. The underlying rotation curve is explained conventionally via the distribution of the baryonic and dark matter components of the galaxy in question. This scenario, though tested here on the two major Local Group objects, is in principle applicable to galaxies undergoing minor mergers with subgalactic mass gas clouds.     

The transparency of the Universe limited by Lyα clouds

The brightnesses of supernovae are commonly understood to indicate that cosmological expansion is accelerating due to dark energy. However the entire discussion presumes a perfectly transparent universe because no effects of reddening associated with the interstellar extinction law are seen. We note that with two kinds of dark matter (baryonic and nonbaryonic) strongly dominating the known mass of the universe, it is seriously premature to assume that these dark matter components have not reduced the transmission of the universe for cosmological sources. We show that the long‐known Lyα clouds, if nucleated by the population of baryonic dark matter primordial planetoids indicated by quasar microlensing, would act as spherical lenses and achromatically fade cosmologically distant sources. We attempt to estimate the amount of this cosmological fading, but ultimately the calculation is limited by lack of a satisfactory model for the tenuous outer parts of a primordial planetoid. We also consider the effects of such cosmological fading on the light of quasars. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The martian mesosphere as revealed by CO2 cloud observations and General Circulation Modeling

Different missions have observed mesospheric clouds on Mars in the last years. The presence of these clouds implies, among other conditions, mesospheric temperatures below CO₂ condensation temperature. We use a General Circulation Model to study the mesospheric temperatures and compare the observed distribution of the mesospheric clouds and the predicted climatology of mesospheric temperatures. Although the model does not usually predict temperatures below condensation for daytime conditions, in some regions the predicted temperatures are close enough to condensation that perturbations caused by small scale processes could produce local excursions below condensation. The location and time of the lowest temperatures predicted by the GCM correspond to a first order with the two observed populations of mesospheric clouds: equatorial clouds observed before and after the Northern summer solstice, and mid-latitude clouds observed around the Northern winter solstice. For the equatorial clouds season, the model predicts temperatures close to condensation at the longitude, latitude, altitude and local time where they have been observed. We find that the diurnal migrating thermal tide and non-migrating tides are at the root of the spatial confinement of the equatorial clouds. For the mid-latitude clouds season, the temperatures predicted by the model at the location of the observed clouds is too high. Stereo observations by two different instruments allow for the determination of the zonal speed of these clouds producing a rare dataset of mesospheric winds. We compare the mesospheric zonal winds predicted by the model with these observations, finding a good agreement, although in some cases the observed variability exceeds that predicted by the model.     

Analytic solutions for coupled linear perturbations

Analytic solutions for the evolution of cosmological linear density perturbations in the baryonic gas and collisionless dark matter are derived. The solutions are expressed in a closed form in terms of elementary functions, for arbitrary baryonic mass fraction. They are obtained assuming =1 and a time-independent comoving Jeans wavenumber, k _J. By working with a time variable ln( t ^2/3), the evolution of the perturbations is described by linear differential equations with constant coefficients. The new equations are then solved by means of Laplace transformation, assuming that the gas and dark matter trace the same density field before a sudden heating epoch. In a dark matter-dominated Universe, the ratio of baryonic to dark matter density perturbation decays with time roughly as exp(5 /4) t ^5/6 to the limiting value 1/[1+( k k _J)²]. For wavenumbers the decay is accompanied by oscillations with a period in . In comparison, as increases in a baryonic matter-dominated Universe, the ratio approaches 1( k k _J)² for k k _J, and zero otherwise.     

Jet Propagation Through Irregular Media and the Impact of Lobes on Galaxy Formation

We review results of two- and three-dimensional simulations of jets striking clouds with a view toward determining the conditions under which extragalactic jets might stably survive such collisions, and thereby produce “dog-leg” or wide-angle-tail morphologies. Under most circumstances, the jet either destroys the cloud and has its stability little affected or it stalls and is rapidly destabilized by the impact. But there does appear to be a limited range in parameter space where jets can be deflected by clouds but still survive for an extended period. Some of the effects of radio lobes on protogalactic clouds are also considered. At redshifts above 2, the number of radio galaxies (RGs) is much larger than it is in the local universe, and their lobes may well have filled a large fraction of the web of baryonic matter that is still forming galaxies at that epoch. The overpressures in those lobes can trigger extensive star formation on galactic scales and also may have major implications for the spreading of magnetic fields and metals through the intergalactic medium.     

Chemodynamical Modelling of Galaxy Formation and Evolution

We present our recently developed 3-dimensional chemodynamical code for galaxy evolution. This code follows the evolution of different galactic components like stars, dark matter and different components of the interstellar medium (ISM), i.e. a diffuse gaseous phase and the molecular clouds. Stars and dark matter are treated as collisionless N-body systems. The ISM is numerically described by a smoothed particle hydrodynamics (SPH) approach for the diffuse gas and a sticky particle scheme for the molecular clouds. Additionally, the galactic components are coupled by several phase transitions like star formation, stellar death or condensation and evaporation processes within the ISM. As an example we show the dynamical and chemical evolution of a star forming dwarf galaxy with a total baryonic mass of 2 ċ 10⁹ M_⊙. After a moderate collapse phase the stars and the molecular clouds follow an exponential radial distribution, whereas the diffuse gas shows a central depression as a result of stellar feedback. The metallicities of the galactic components behave quite differently with respect to their temporal evolution as well as their radial distribution. Especially, the ISM is at no stage well mixed. This revised version was published online in September 2006 with corrections to the Cover Date.     

Collisional baryonic dark matter haloes

If dark haloes are composed of dense gas clouds, as has recently been inferred, then collisions between clouds lead to galaxy evolution. Collisions introduce a core in an initially singular dark matter distribution, and can thus help to reconcile scale-free initial conditions – such as are found in simulations – with observed haloes, which have cores. A pseudo-Tully–Fisher relation, between halo circular speed and visible mass (not luminosity), emerges naturally from the model: M _vis∝ V ^7/2.
Published data conform astonishingly well to this theoretical prediction. For our sample of galaxies, the mass–velocity relationship has much less scatter than the Tully–Fisher relation, and holds as well for dwarf galaxies (where diffuse gas makes a sizeable contribution to the total visible mass) as it does for giants. It seems very likely that this visible-mass/velocity relationship is the underlying physical basis for the Tully–Fisher relation, and this discovery in turn suggests that the dark matter is both baryonic and collisional.     

Generalized inflation with a gravitational wave background

We propose a -inflation model that explains a significant part of the COBE signal by primordial cosmic gravitational waves. The primordial density perturbations fulfil both the constraints of large-scale microwave background and galaxy cluster normalization. The model is tested against the galaxy cluster power spectrum and the high-multipole angular cosmic microwave background anisotropy.     

Non-GUT baryogenesis and large-scale structure of the Universe

We discuss a mechanism for producing baryon density perturbations during the inflationary stage, and study the evolution of the baryon charge density distribution in the framework of the low-temperature baryogenesis scenario. This mechanism may be important for large-scale structure formation in the Universe and, in particular, may be essential for understanding the existence of a characteristic scale of 130  h ⁻¹ Mpc (comoving size) in the distribution of the visible matter.
A detailed analysis shows that both the observed very large scale of the visible matter distribution in the Universe and the observed baryon asymmetry value could naturally appear as a result of the evolution of a complex scalar field condensate, formed at the inflationary stage.
Moreover, according to our model, the visible part of the Universe at present may consist of baryonic and antibaryonic regions, sufficiently separated, so that annihilation radiation is not observed.     

Collapse of primordial clouds and the formation of a central core

The collapse of rotating clouds is investigated using three-dimensional self-gravitating hydrodynamical simulations. We take into account the detailed non-equilibrium chemical reactions for primordial gas that consists of pure hydrogen. The parameters of the collapse and the condition of the fragmentation are compared with those of isothermal clouds. It is shown that the geometrical flatness of the central region of the disc is a good indicator for predicting whether the clouds fragment or not. If the flatness is greater than the critical value, ∼ 4π, a cloud fragments into filaments and blobs. On the other hand, if the flatness is smaller than the critical value, fragmentation is not expected before the central core formation even if the cooling is efficient and the total mass becomes much greater than the local Jeans mass at the centre. The critical mass is found to be 3 × 10⁶ M_⊙ for a typical initial condition. If the initial cloud mass is smaller than this critical value, fragmentation before the central core formation is not expected. For a typically estimated first collapsing cosmological baryonic object, M ≲ 10⁶ M_⊙, central core formation is expected and will have a significant effect on the later evolution of the whole system. This revised version was published online in July 2006 with corrections to the Cover Date.     

Cosmological implications and structure formation from a time varying vacuum

We study the dynamics of the Friedmann–Lemaitre–Robertson–Walker (FLRW) flat cosmological models in which the vacuum energy varies with time,  Λ( t )  . In this model, we find that the main cosmological functions such as the scale factor of the universe and the Hubble flow are defined in terms of exponential functions. Applying a joint likelihood analysis of the recent Type Ia supernovae data, the cosmic microwave background shift parameter and the baryonic acoustic oscillations traced by the Sloan Digital Sky Survey (SDSS) galaxies, we place tight constraints on the main cosmological parameters of the  Λ( t )  scenario. Also, we compare the  Λ( t )  model with the traditional Λ cosmology and we find that the former model provides a Hubble expansion which compares well with that of the Λ cosmology. However, the  Λ( t )  scenario predicts stronger small scale dynamics, which implies a faster growth rate of perturbations with respect to the usual Λ cosmology, despite the fact that they share the same equation of state parameter. In this framework, we find that galaxy clusters in the  Λ( t )  model appear to form earlier than in the Λ model.     

On the galaxy stellar mass function, the mass–metallicity relation and the implied baryonic mass function

A comparison between published field galaxy stellar mass functions (GSMFs) shows that the cosmic stellar mass density is in the range 4–8 per cent of the baryon density (assuming  Ω_b= 0.045  ). There remain significant sources of uncertainty for the dust correction and underlying stellar mass-to-light ratio even assuming a reasonable universal stellar initial mass function. We determine the   z < 0.05  GSMF using the New York University Value-Added Galaxy Catalog sample of 49 968 galaxies derived from the Sloan Digital Sky Survey and various estimates of stellar mass. The GSMF shows clear evidence for a low-mass upturn and is fitted with a double Schechter function that has  α₂≃−1.6  . At masses below  ∼10^8.5 M_⊙  , the GSMF may be significantly incomplete because of missing low-surface-brightness galaxies. One interpretation of the stellar mass–metallicity relation is that it is primarily caused by a lower fraction of available baryons converted to stars in low-mass galaxies. Using this principle, we determine a simple relationship between baryonic mass and stellar mass and present an 'implied baryonic mass function'. This function has a faint-end slope,  α₂≃−1.9  . Thus, we find evidence that the slope of the low-mass end of the galaxy mass function could plausibly be as steep as the halo mass function. We illustrate the relationship between halo baryonic mass function → galaxy baryonic mass function → GSMF. This demonstrates the requirement for peak galaxy formation efficiency at baryonic masses  ∼10¹¹ M_⊙  corresponding to a minimum in feedback effects. The baryonic-infall efficiency may have levelled off at lower masses.     

Collapse and fragmentation of rotating, prolate clouds

We present the results of collapse calculations for uniformly rotating, prolate clouds performed using the numerical method: smoothed particle hydrodynamics (SPH). The clouds considered are isothermal, prolate spheroids with different axial ratios ( a/b ), and with different values of β, the ratio of the rotational to gravitational energy. Small density perturbations are added to the clouds, and different initial perturbation spectra are studied. All of the clouds considered are strongly unstable to gravitational contraction, and so collapse to form a spindle configuration. Such a linear structure is unstable to fragmentation, so that the clouds break up into a number of subcondensations. The long-term evolution of the system is then determined by the angular momentum possessed by these fragments.
It is found that a number of the calculations performed result in the formation of orbitally stable binary systems, composed of two rotationally supported discs in orbit about their common centre of mass. Tidal interactions during closest approach, close three-body interactions and the continued accretion of material with high specific angular momentum are all found to increase the orbital separation during these calculations, ensuring that the systems do not merge at later times. The calculations are therefore relevant to the problem of binary star formation, though the systems produced tend to have large orbital separations and periods. One of the strong points of the models presented, however, is their ability to produce systems with a range of mass ratios and orbital eccentricities, without the explicit inclusion of biases in the initial conditions.     

Cloud formation along mountain ridges on Titan

Cassini radar passes have shown a number of mountain ranges on Titan. Radar data covering approximately one quarter of Titan's surface places mountains in primarily equatorial regions with the mean height of about 900 m. The flow of air over topographic features can both trigger and enhance cloud formation. Orographically induced clouds near terrestrial mountain ranges include shallow wave clouds produced from upslope flow as well as precipitating stratus and cumulus type clouds; mountains can provide the perturbations needed to trigger convective clouds. The Titan regional atmospheric modeling system (TRAMS) has been used to explore a number of convective cloud properties and is now used to report on clouds formed when a mountain peak is placed within the model domain. Using a range of heights and surface winds compatible with Cassini/Huygens data, constraints can be placed on the scenarios in which clouds can be expected to form. Given sufficiently humid conditions (at least 50% humidity), convection is triggered. For drier environments similar to the Huygens landing site, short-lived, optically thin clouds form from air rising upslope. Precipitation is also seen in the cases of the convective clouds, which could have implications for the eroded appearance of Titan's mountains.     

Redistributing hot gas around galaxies: do cool clouds signal a solution to the overcooling problem?

We present a pair of high-resolution smoothed particle hydrodynamics simulations that explore the evolution and cooling behaviour of hot gas around Milky Way size galaxies. The simulations contain the same total baryonic mass and are identical other than their initial gas density distributions. The first is initialized with a low-entropy hot gas halo that traces the cuspy profile of the dark matter, and the second is initialized with a high-entropy hot halo with a cored density profile as might be expected in models with pre-heating feedback. Galaxy formation proceeds in dramatically different fashion depending on the initial setup. While the low-entropy halo cools rapidly, primarily from the central region, the high-entropy halo is quasi-stable for  ∼4 Gyr  and eventually cools via the fragmentation and infall of clouds from ∼100 kpc distances. The low-entropy halo's X-ray surface brightness is ∼100 times brighter than current limits and the resultant disc galaxy contains more than half of the system's baryons. The high-entropy halo has an X-ray brightness that is in line with observations, an extended distribution of pressure-confined clouds reminiscent of observed populations and a final disc galaxy that has half the mass and ∼50 per cent more specific angular momentum than the disc formed in the low-entropy simulation. The final high-entropy system retains the majority of its baryons in a low-density hot halo. The hot halo harbours a trace population of cool, mostly ionized, pressure-confined clouds that contain ∼10 per cent of the halo's baryons after 10 Gyr of cooling. The covering fraction for H  i and Mg  ii absorption clouds in the high-entropy halo is ∼0.4 and ∼0.6, respectively, although most of the mass that fuels disc growth is ionized, and hence would be under counted in H  i surveys.     

The effect of cosmological background dynamics on the spherical collapse in MOND

The effect of background dynamics of the universe on formation of large scale structures in the framework of Modified Newtonian Dynamics (MOND) is investigated. A spherical collapse model is used for modeling the formation of the structures. This study is done in two extreme cases: (i) assuming a universe with a low-density baryonic matter without any cold dark matter and dark energy; (ii) a dark energy dominated universe with baryonic matter, without cold dark matter. We show that for the case (ii) the structures virialize at lower redshifts with larger radii compared to the low-density background universe. The dark energy slow downs the collapse of the structures. We show that our results are compatible with recent simulations of the structure formation in MOND.     

Observations of the development of striations in large barium ion clouds

Striations are observed to develop within large (6–352 kg releases) barium ion clouds in a two-stage process. First the clouds split into sheets commencing at the trailing edge of the cloud. Then distortions or pinching effects within the individual sheets cause the formation of field-aligned raylike structures. In the clouds observed, the individual sheets were 200–1000 m in thickness and were spaced 700–2000 m apart. Quasi-sinusoidal waviness of spatially periodic thickenings exhibited a wavelength typically 700–1000 m. When rod-like structures appeared, these were typically 200–400 m dia and were spaced along the pre-existing sheet at 700–1000 m on centers.     

Bursts of star formation caused by disk-halo mass exchange

It is well known that galaxies accumulating large quantities of gas undergo violent bursts of star formation. This is believed to be due to tidal interactions of galaxies leading to the infall of gas into their central regions. Bursts of star formation in this scenario are transitory phenomena and can be induced only by external sources.However, in some cases there is no direct evidence of tidal interactions in starburst galaxies.We discuss another possibility of bursting phenomena in galaxies connected with nonlinear feedback processes in mass-exchange between components of star-forming region. We consider a three-component model including cold clouds, warm gas and massive stars and take into account the delay processes in the transformation of hot gas ejected by massive stars and evaporated from cold phase, into the warm phase. Self-regulating mechanism of phase transition of small clouds into warm gas due to heating radiation of massive stars is also taken into account.The analysis of stability of the system shows that it could be unstable even in case of a small efficiency in the birth of massive stars. The evolution of unstable nonlinear perturbations leads to the development of self-sustained nonlinear oscillations of star formation.