Surface distribution of dark clouds in M31 absorption gradient along the major axis,I

The distribution of dust clouds in M31 was studied based on a photocopy of this galaxy taken from Hubble'sAtlas. The picture was divided into strips starting from the center of the galaxy on both sides along the major and the minor axis, respectively. The number of dark clouds per square kiloparsec was estimated as a function of the distance from the center of the galaxy along the major axis. If we assume the validity of the standard cloud model in M31, the filling factor introduced by the model was found to vary with distance from the center. It was found furthermore, that the filling factor introduced by the model may be an upper limit. The counts of dark clouds indicate a periodical change in the absorption gradient along the major axis with a very slight systematic variation assumed by the exponential part of the representation.     

The Amount of Dust and the Distance of NGC 253

The areas of 267 dark clouds in the galaxy NGC 253 were measured. It is shown that the size distribution of these clouds obeys the same exponential formula found for the galaxies NGC 3031, 5128, 5194, and 5457. From the apparent size distribution of the dark clouds the amount of dust and the distance of NGC 253 is found to 4.4 · 10⁷ M_⊙ and 2.06 Mpc, respectively.     

Absorption gradient in some galaxies,IV

The internal absorption and the absorption gradient along the major and minor axes are studied, following the approach outlined already in Papers I, II, and III. The sample consists of five spirals and one irregular. The surface distribution of dark clouds in these galaxies continues to assume periodicity as found in M31.Those results and the following ones constitute part of an M.Sc. Thesis, submitted to Cairo University by the junior author (S.S.A.).     

On the mass and density of the stellar disk of M33

The disk surface density of the nearby spiral galaxy M33 is estimated assuming that it is marginally stable against gravitational perturbations. For this purpose we used the radial profile of line-of-sight velocity dispersion of the disk planetary nebulae obtained by Ciardullo et al. (2004). The surface density profile we obtained is characterized by the radial scalelength which is close to the photometrical one and is in a good agreement with the rotation curve of M33 and with the mass-to-light ratio which follows from the stellar population model. However at the galactocentric distance r > 7 kpc the dynamical overheating of the disk remains quite possible. The thickness of the stellar disk of M33 should increase outwards from the center. The dark halo to total mass ratio is estimated as a function of r. The effective oxygen yield obtained in the frame of instantaneous recycling approximation using the disk surface density and the observed gradient of O/H increases with radius. It may indicate that the role of accretion of metalpoor gas in the chemical evolution of interstellar medium decreases outwards.     

Three-dimensional reddening map for stars from 2MASS photometry: The method and the first results

The first results of the construction of a three-dimensional reddening map for stars within 1600 pc of the Sun are presented. Analysis of the distribution of 70 million stars from the 2MASS catalog with the most accurate photometry on the (J-Ks)-Ks diagram supplemented with Monte Carlo simulations has shown that one of the maxima of this distribution corresponds to F-type dwarfs and subgiants with a mean absolute magnitude M _Ks = 2_⊙ ^m 5. The shift of this maximum toward large J-Ks with increasing Ks reflects the reddening of these stars with increasing heliocentric distance. The distribution of the sample of stars over Ks, l, and b cells with a statistically significant number of stars in each cell corresponds to their distribution over three-dimensional spatial cells. As a result, the reddening E(J-Ks) has been determined with an accuracy of 0_· ^m 03 for spatial cells with a side of 100 pc. All of the known large absorbing clouds within 1600 pc of the Sun have manifested themselves in the results obtained. The distances to the near and far edges of the clouds have been determined with a relative accuracy of 15%. The cases where unknown clouds are hidden behind known ones on the same line of sight have been found. The distance dependence of reddening is considered for various Galactic latitudes and longitudes. The absorbing matter of the Gould Belt is shown to manifest itself at latitudes up to 40° and within 600 pc of the Sun. The size and influence of the Gould Belt may have been underestimated thus far. The absorbing matter at latitudes up to 60° and within 1600 pc of the Sun has been found to be distributed predominantly in the first and second quadrants in the southern hemisphere and in the third and fourth quadrants in the northern hemisphere. The warping of the absorbing layer in the near Galaxy apparently manifests itself in this way. A nonrandom orientation of the clouds relative to the Sun is possible. The mass of the baryonic dark matter in solar neighborhoods can then be considerably larger than is generally believed.     

Absorption gradient in galaxies,V

The same procedure mentioned in Papers I, II, III for studying the surface distribution of dark clouds is used to study the same phenomenon in the galaxies NGC 253, NGC 2841, NGC 5321, NGC 5194, and NGC 5195 radially and angularly. The general form (sinusoidal distribution function) found earlier is still representative for the present results. Internal absorption was found to vary along these two variables (radially and angularly) in the same way as was found earlier for M31 (the papers cited at the end) which may lead to the assumption of universality of this phenomenon.     

Segregation of dust grains in dark clouds

Gravitational settling of dust grains in dark clouds has been considered. It has been shown that such a process gives rise to a modification of the grain size distribution. Starting with a simple model of uniform spherical cloud and normal interstellar grain size distribution for the dust we derive expressions for the modified grain size distribution function, average grain size and extinction as functions of distance from the cloud's center and the age of the cloud. The mean grain size increases towards the center of the cloud as does the extinction. Results of the numerical evaluation of these quantities have been discussed with their implications for the observations of anomalous reddening and polarization within dark clouds and Bok globules.     

Distance estimate of M31 based on the size distribution of Hii regions and dark clouds

The size distribution of dark clouds and of H II regions were used to derive a distance of 440±22 kpc for M31. Some galactic H II regions and dark clouds were combined with the same ingredients in M31 to study the mass spectrum of both phenomena. A gas to dust ratio of 10001 has been derived for M31. Radii of the classical H II regions as derived in this work using distance of 440 kpc agrees with the galactic classical H II region radii.     

Bars and the symmetry of star formation patterns in the discs of spiral galaxies

We present a test for the degree of symmetry in the distribution of the Hα brightness along the arms of a sample of spiral galaxies. The test consists of deriving the cross-correlation function of the Hα brightness as a function of curvilinear distance along pairs of opposed arms, after unfolding the arms geometrically. Our results reveal a significantly greater degree of symmetry in the non-barred population than in the barred. We derive parameters for both bar strength and bar ellipticity, and compare these with the derived cross-correlations to strengthen this conclusion. We suggest that density waves are a probable cause for the appearance of global, i.e. disc-wide, two-fold symmetry in spiral discs. Comparison with published work on abundance gradients in the discs of barred and non-barred galaxies indicates that, as for the abundances, mixing in the spiral disc as a result of the bar potential may well be responsible for our observation that stronger bars are related to reduced two-fold symmetry in the distribution of star-forming regions along the spiral arms.     

Interstellar extinction in the vicinity of the reflection nebula NGC 1333 in Perseus

The dependence of interstellar extinction on distance in the direction of a dark cloud around the reflection nebula NGC 1333 is determined on the basis of photoelectric Vilnius photometry and photometric classification of 78 stars. Two dust clouds are noted at distances 160 and 220 pc. The first one with mean extinction of 0.4 mag is concluded to belong to the Taurus cloud complex and the second cloud with mean extinction of 1.8 mag belongs to the chain of dark clouds and other young objects which is almost perpendicular to the spiral arm but lies 80 pc below the galactic plane. The star BD +30°549 which illuminates the NGC 1333 nebula is at distance 212 pc from the Sun. No extinction increase behind the Perseus cloud is detected.     

Observational Indicators of Formation Excitation of H2

Theoretical predictions by Farebrother et al. and Meijer et al. of rovibrational excitation probabilities in H₂ arising from formation by Eley-Rideal processes on a graphite surface are incorporated into a model of the chemistry and excitation of interstellar H₂. The model includes the usual radiative and collisional pumping of H₂ rotational and vibrational states, in addition to the formation processes. Predictions are made for HH₂ rovibrational emission line intensities for representative points in diffuse and in dark interstellar clouds. We find that – if all the interstellar HH₂ is formed by this Eley-Rideal process – then the consequences of formation pumping, as distinct from collisional and radiative pumping, should be clearly evident in both cases. In particular, we predict a clear spectral signature of this direct HH₂ formation process on graphite, distinct from radiative and collisional pumping; this signature should be evident in both diffuse and dark clouds; but the emissivity for dark clouds is predicted to be some 500 times greater than that in diffuse clouds in which the dense material may be embedded. An observational search for this signature in two dark cloud sources was made, but a preliminary analysis of the data did not yield a detection. The implications of and possible reasons for this preliminary conclusion are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

An iras atlas of ysos in nearby dark clouds

We are using the newIRAS Sky Survey Atlas to construct a catalog of Galactic dark clouds and their associated Young Stellar Objects (YSOs). FromIRAS 100 and 60µm data we calculate images of the 100µm optical depth distribution in the clouds. Using theIRAS Faint Source Catalog we identify sources which have colors indicative of YSOs and have positions that lie inside cloud boundaries in the optical depth images. Here we present some preliminary results of applying this analysis to the Chamaeleon dark cloud complex.     

Surface distribution of dark clouds in M31 absorption gradient along the minor axis and in theR andθ directions,II

Absorption and absorption gradient is studied along the minor axis of M31 and as a function of the angle measured from the minor axis with the center of the galaxy as origin. The radial distribution of these phenomena is studied also. The counts of dark clouds averaged over all in each case assumes periodical changes. In some cases systematic changes are shown also. The fraction of the volume occupied by these clouds is smaller by an order than that introduced by the standard cloud model.     

Quantum vacuum and dark matter

Recently, the gravitational polarization of the quantum vacuum was proposed as alternative to the dark matter paradigm. In the present paper we consider four benchmark measurements: the universality of the central surface density of galaxy dark matter haloes, the cored dark matter haloes in dwarf spheroidal galaxies, the non-existence of dark disks in spiral galaxies and distribution of dark matter after collision of clusters of galaxies (the Bullet cluster is a famous example). Only some of these phenomena (but not all of them) can (in principle) be explained by the dark matter and the theories of modified gravity. However, we argue that the framework of the gravitational polarization of the quantum vacuum allows the understanding of the totality of these phenomena.     

Can baryonic dark matter be solid hydrogen?

Some requirements are discussed for solid hydrogen formation in cold dark dense clouds in galaxies. If temperatures in the clouds are near the microwave background temperature of 2.7 K and molecular hydrogen densities are 3×10⁵ cm^–3 or higher, as suggested by recent observations, it may be possible for solid hydrogen objects to form. Comet size hydrogen solids could build from molecular hydrogen condensation on grains and by collisions. Heated primarily by cosmic rays, objects with 100 km radii could last billions of years. The larger objects may be detectable, in the future, by sensitive gravitational lensing or eclipsing observations. Other possibilities are discussed for future detection of the cold dark dense molecular hydrogen regions. In our model, helium is added along with the hydrogen to preserve the primordial helium to hydrogen mass ratio,Y _p , of the standard model. In the hot regions of the universe the solid hydrogen objects sublime and melt so our model predictsY _p =0.250, the same as other baryonic dark matter models with identical values of =0.1,H _o =50 and =6.8×10^–10. This value cannot be ruled out at present because of the large systematic uncertainties in the observed value of 0.232. In the cold dark regions where solid hydrogen objects exist, we predict thatY _p will be greater than 0.250. Observations are not yet sensitive enough to measure this ratio.     

The forest of QSO absorption lines and cosmological models with unstable dark matter

Indications on clustering features in the two-point correlation function of Ly α line separations in published high-resolution spectra of quasars are presented. They are in contradiction to the self-similar hierarchical clustering model, but they could be explained by a first generation of small-scale pancakes in a model with unstable dark matter. A characteristic scale of velocity dispersion Δv ≈ 150 km/s of absorption clouds within pancakes is described by corresponding coherent regions of positive correlation, while the structure adjacent on the lines of sight are separated by a comoving distance of (18 ± 6) Mpc (H₀ = 50 km/s Mpc). The appropriate coherence length of the fluctuation spectrum may result from the free streaming length of (unstable) neutrinos of (100 … 200) eV rest mass.     

NGC 3521: Stellar Counter-Rotation Induced by a Bar Component

The spiral galaxy NGC 3521 exhibits apparently normal kinematic properties of gas and stars along its major axis. However, the analysis of the LOSVD reveals strong asymmetries. A decomposition of the LOSVD data with a two-Gaussian component model shows two counter-rotating stellar components. The observed kinematic decoupling is interpreted as a projection effect induced by the presence of a bar component seen almost end on. The bar produces locally a greater concentration of retrograde stellar orbits but this does not relate to a specific counter-rotating population. The signatures of the bar are identified in the velocity field derived from long-slit spectra obtained along the major, minor and 45° intermediate axes and from R-band surface photometry. This revised version was published online in July 2006 with corrections to the Cover Date.     

Hydrodynamic and turbulent motions in the galactic disk

Statistics in absorption 21-cm data show two main types of clouds at low galactic latitudes: dense small clouds, many of them with molecular cores, with dispersions σ≈1.5 km s⁻¹ and large clouds forming the fine features of the spiral arms (the shingle like features) with a dispersion range α≈3–4 km s⁻¹. Sizes and dispersions of both types of clouds are compatible with the Kolmogorov law of turbulence: σ∞d ^1/3. The large clouds forming the shingle-like features can be considered as the largest clouds of a Kolmogorov spectrum (the initial vortices), or as the hydrodynamic features with minimum sizes in the Galaxy. In order to define hydrodynamic motions in the same sense as given by Ogrodnikov (1965) we use here the tensorial form of the Helmholtz theorem to obtain an approximation for the hydrodynamic motions depending on distances and seen from the local standard of rest:V _r ∞r. The intermediate range of sizes between turbulent motions and hydrodynamic motions is 100<d<300 pc which is also the range of sizes of the large clouds forming the fine features of the spiral arms. A classification on of motions in the Galaxy is postulated: (a) a basic rotation motion given by an smooth unperturbed curveΘ _b (R) associated to the old disk population. (b) Systematic motions of the spiral arms. (c) Systematic motions in the fine structure of the arms. For scale sizes smaller than these fine features one has turbulent motions according to the Kolmogorov law. The densities and sizes of the turbulent clouds behave asn _H ∝d ⁻² in a range of sizes 7 pc<d<300 pc. The obtained gas densities of the clouds are confirmed with the dust densities from photometric studies. The conditions for gravitational binding of the clouds are analyzed. Factors as the geometry and the magnetic field within the clouds increases the critic densities for gravitational binding. When we consider these factors we find that the wide component clouds have densities below such a critical value. The narrow component clouds have densities similar or above the critical value; but the real fraction of collapsing clouds remains unknown as far as the factor of geometry and the inner magnetic field of each cloud are not determinated.     

The statistical properties of cosmic gamma-ray bursts

Recent localizations of cosmic gamma-ray bursts (GBRs) are studied taking into account the estimations of their intensities. The increase of the avergage intensity of GRBs along galactic longitudesl=220–260° andl=40–80° is found. It may be associated with the Orion spiral arm and the central region of the Galaxy. Corresponding enhancement of the number of intense GRBs for directions along the Orion arm is also found. Astrophysical conclusions from this possible galaxy-scale distribution of GRBs are discussed.     

Dynamics of molecular clouds on the galactic scale

We have carried out an extensive investigation into the dynamics of the molecular clouds in the disk of the Galaxy. We have used both computational methods and physical arguments to try to understand how the ensemble of molecular clouds interacts, how the clouds are affected by the gravitational field of the Galaxy and also the circumstances under which they can aggregate into giant molecular clouds (GMC's).The dynamical model is three dimensional and consists of 120,000 spherical clouds, each having a mass of 10⁴ M . It allows for the mutual gravitation between clouds, up to a cut-off distance; when two clouds collide they rebound, with a specified coefficient of restitutione. We have also developed a physically more realistic model for a cloud, supported by a magnetic field, and used it to select a suitable range of values fore. Our first paper deals with the case of an axially symmetrical galaxy. The clouds are distributed initially in a disk extending 100 pc on either side of the Galactic plane. As it evolves the system of clouds loses energy, and the disk grows thinner at a rate which depends on the value ofe. GMC's start to form once the disk is thin enough. We believe this result to be valid more generally, and that it holds also in models with spiral structure.