arXiv subjects

Marco Spaans

Publications and source records attributed to Marco Spaans.

62 records · Page 4Linked to original sources

arXiv1998

Effective Star Formation Rates for Cosmological Applications

Effective star formation rates in tabular form are computed which yield a prescription for the star formation activity in model galaxies as a function of ambient density, metallicity, and stellar feedback. The effects of supernova explosions on the thermal balance of the Interstellar Medium (ISM) and the presence of a multi-phase ISM are explicitly included. The resulting grid of models can be implemented easily in N-body codes for the computation of star formation processes in merging galaxies and cosmological simulations.

astro-ph↗

arXiv1997

A Topological Formulation of the Standard Model

A topological theory for the interactions in Nature is presented. The theory derives from the cyclic properties of the topological manifold Q=2T^3 + 3S^1 x S^2 which has 23 intrinsic degrees of freedom, discrete Z_3 and Z_2 x Z_3 internal groups, an SU(5) gauge group, and an anomalous U(1) symmetry. These properties reproduce the standard model with a stable proton, a natural place for CP violation and doublet-triplet splitting. The equation of motion for the unified theory is derived and leads to a Higgs field. The thermodynamic properties of Q are discussed and yield a consistent amplitude for the cosmic microwave background fluctuations. The manifold Q possesses internal energy scales which are independent of the field theory defined on it, but which constrain the predicted mass hierarchy of such theories. In particular the electron and its neutrino are identified as ground states and their masses are predicted. The correct masses of quarks and the CKM mixing angles can be derived as well from these energy scales if one uses the anomalous U(1) symmetry. Furthermore, it is shown that if the Planck scale topology of the universe involves loops as fundamental objects, its spatial dimension is equal to three. The existence of the prime manifold T^3=S^1 x S^1 S^1 is then required for a dynamical universe, i.e. a universe which supports forces. Some links with M-theory are pointed out.

gr-qc↗

arXiv1997

Molecular Lines as Diagnostics of High Redshift Objects

Models are presented for CO rotational line emission by high redshift starburst galaxies. The influence of the cosmic microwave background on the thermal balance and the level populations of atomic and molecular species is explicitly included. Predictions are made for the observability of starburst galaxies through line and continuum emission between z=5 and z=30. It is found that the Millimeter Array could detect a starburst galaxy with ~10^5 Orion regions, corresponding to a star formation rate of about 30 Mo yr^{-1}, equally well at z=5 or z=30 due to the increasing cosmic microwave background temperature with redshift. Line emission is a potentially more powerful probe than dust continuum emission of very high redshift objects.

astro-ph↗

arXiv1997

Molecular Hydrogen in Diffuse Interstellar Clouds of Arbitrary Three-Dimensional Geometry

We have constructed three-dimensional models for the equilibrium abundance of molecular hydrogen within diffuse interstellar clouds of arbitrary geometry that are illuminated by ultraviolet radiation. The position-dependent photo- dissociation rate of H$_2$ within such clouds was computed using a 26-ray approximation to model the attenuation of the incident ultraviolet radiation field by dust and by H$_2$ line absorption. We have applied our modeling technique to the isolated diffuse cloud G236+39, assuming that the cloud has a constant density and that the thickness of the cloud along the line of sight is at every point proportional to the 100 um continuum intensity measured by IRAS. We find that our model can successfully account for observed variations in the ratio of 100 umu continuum intensity to HI column density, with larger values of that ratio occurring along lines of sight in which the molecular hydrogen fraction is expected to be largest. Using a standard chi^2 analysis to assess the goodness of fit of our models, we find (at the 60sigma level) that a three-dimensional model is more successful in matching the observational data than a one-dimensional model in which the geometrical extent of the cloud along the line of sight is assumed to be very much smaller than its extent in the plane-of-the-sky. If D is the distance to G236+39, and given standard assumptions about the rate of grain-catalysed H_2 formation, we find that the cloud has an extent along the line of sight that is 0.9+-0.1 times its mean extent projected onto the plane of the sky; a gas density of (53+-8)(100 pc/D) cm^-3; and is illuminated by a radiation field of (1.1+-0.2) (100 pc/D) times the mean interstellar radiation field estimated by Draine (1978). The derived 100 um emissivity per nucleon is (1.13+-0.06)x10^-20 MJy sr^-1 cm^2.

astro-ph↗

arXiv1997

Topological Dynamics and Grand Unified Theory

A formalism is presented to construct a non-perturbative Grand Unified Theory when gravitational Planck-scale phenomena are included. The fundamental object on the Planck scale is the three-torus T^3 from which the known properties of superstrings, such as the geometric action and duality, follow directly. The low energy theory is 11-dimensional and compactification to a Lorentzian four-manifold is an automatic feature of the unified model. In particular, the simply-connected K3 Calabi-Yau manifold follows naturally from the model and provides a direct link with M-theory. The high energy theory is formulated on a T^3 lattice with handles which exhibits the necessary symmetry groups for the standard model, and yields a consistent amplitude for the cosmic microwave background fluctuations. The equation of motion for the supersymmetric, unified theory is derived and leads to the Higgs field. This formulation predicts a remnant, scalar topological defect of mass < 9 m_Planck/46 from the Planck epoch, which is a candidate for dark matter. Finally, it is shown that if the universe is quantum-mechanical, its spatial dimension is equal to three and the laws of nature are Lorentz invariant when gravity can be neglected.

gr-qc↗

arXiv1997

Star Formation and Metal Production as a Function of Redshift: The Role of the Multi-Phase ISM

We present models of the cosmological star formation and metal production history of (proto-)galaxies with varying axis ratios. More massive and/or roughly spherical systems reach the threshold-metallicity for a transition to a multi-phase interstellar medium earlier than less massive, more flattened systems. Therefore, more flattened, lower-mass systems start to form stars actively at smaller redshifts. A natural explanation is found in the overall robustness of the interstellar medium against complete expulsion (blow-away) at high total masses, and in the prevention of metal enrichment in the outer regions due to axial outflow along the symmetry axis of a non-spherical proto-galaxy (blow-out). We suggest that the observed predominance of spheroidal systems observed at high redshift, e.g. in the Hubble Deep Field, is due to this effect: At z>2, roundish (proto-)galaxies with total (dark+baryonic) masses of ~10^11 M_o and/or the inner spheroidal cores of similarly massive flattened systems sustain a multi-phase interstellar medium, and therefore a high star-formation rate, whose magnitude depends on the fraction of baryonic matter in the systems. Conversely, the peak at z~1-2 in the observed cosmological metal production rate coincides with the epochs of star formation of lower mass spheroidals, as well as of massive proto-galactic disks.

astro-ph↗

arXiv1996

A Topological Extension of General Relativity

A set of algebraic equations for the topological properties of space-time is derived, and used to extend general relativity into the Planck domain. A unique basis set of three-dimensional prime manifolds is constructed which consists of $S^3$, $S^1\times S^2$, and $T^3$. The action of a loop algebra on these prime manifolds yields topological invariants which constrain the dynamics of the four-dimensional space-time manifold. An extended formulation of Mach's principle and Einstein's equivalence of inertial and gravitational mass is proposed which leads to the correct classical limit of the theory. It is found that the vacuum possesses four topological degrees of freedom corresponding to a lattice of three-tori. This structure for the quantum foam naturally leads to gauge groups O(n) and SU(n) for the fields, a boundary condition for the universe, and an initial state characterized by local thermal equilibrium. The current observational estimate of the cosmological constant is reproduced without fine-tuning and found to be proportional to the number of macroscopic black holes. The black hole entropy follows immediately from the theory and the quantum corrections to its Schwarzschild horizon are computed.

gr-qc↗

arXiv1996

Molecules at high Redshift: The Evolution of the Cool Phase of Protogalactic Disks

We study the formation of molecular hydrogen, after the epoch of re-ionization, in the context of canonical galaxy formation theory due to hierarchical clustering. There is an initial epoch of $H_2$ production in the gas phase through the $H^-$ route which ends at a redshift of order unity. Star formation in the protogalactic disks can become self-regulated. The process responsible for the feedback is heating of the gas by the internal stellar radiation field which can dominate the background radiation field at various epochs. It is possible to define a maximum star formation rate during this epoch. Plausible estimates give a rate of 0.2-2 Mo yr-1 for condensations corresponding to 1 sigma and 2 sigma initial density fluctuations. Therefore, the production of metals and dust proceeds slowly in this phase. This moderate epoch is terminated by a phase transition to a cold dense and warm neutral/ionized medium once the metals and dust have increased to a level Z=0.03-0.1 Z_o. Then: (1) atoms and molecules such as C, O and CO become abundant and cool the gas to below $300 K$ ; (2) the dust abundance has become sufficiently high to allow shielding of the molecular gas and; (3) molecular hydrogen formation can occur rapidly on grain surfaces. This phase transition occurs at a redshift of approximately 1.5, with a fiducial range of 1.2<z<2. The combination of feedback and a phase transition provides a natural resolution of the G-dwarf problem.

astro-ph↗