arXiv subjects

Marco Spaans

Publications and source records attributed to Marco Spaans.

At least 55 records · Page 3Linked to original sources

arXiv2006

Lyman Alpha Radiation From Collapsing Protogalaxies I: Characteristics of the Emergent Spectrum

We present Monte Carlo calculations of Lyman alpha (Lya) radiative transfer through collapsing gas clouds, representing proto-galaxies that are caught in the process of their assembly. Such galaxies produce Lya flux over an extended solid angle from a spatially extended Lya emissivity and/or from scattering effects. We study the effect of the gas distribution and kinematics, and of the Lya emissivity profile, on the emergent spectrum and surface brightness distribution. The emergent Lya spectrum is typically double-peaked and asymmetric. In practice, the blue peak is significantly enhanced and the red peak, in most cases, will be undetectable. The resulting effective blueshift, combined with scattering in the intergalactic medium, will render extended Lya emission from collapsing protogalaxies difficult to detect beyond redshift z=4. The surface brightness distribution is typically flat, and a strong wavelength dependence of its slope (with preferential flattening at the red side of the line) would be a robust indication that Lya photons are being generated (rather than just scattered) in a spatially extended region around the galaxy. We also find that for self-ionized clouds whose effective Lya optical depth is less than 10^3, infall and outflow models can produce nearly identical spectra and surface brightness distributions, and are difficult to distinguish from one another. The presence of deuterium with a cosmic abundance may produce a narrow but detectable dip in the spectra of systems with moderate hydrogen column densities, in the range 10^18-10^20 cm^-2. Finally, we present a new analytic solution for the emerging Lya spectrum in the limiting case of a static uniform sphere, extending previous solutions for static plane-parallel slabs.

astro-ph↗

arXiv2006

Lyman Alpha Radiation From Collapsing Protogalaxies II: Observational Evidence for Gas Infall

We model the spectra and surface brightness distributions for the Lyman alpha (Lya) radiation expected from protogalaxies that are caught in the early stages of their assembly. We use the results of a companion paper to characterize the radiation emerging from spherically collapsing gas clouds. We then modify the intrinsic spectra to incorporate the effect of subsequent resonant scattering in the intergalactic medium (IGM). Using these models, we interpret a number of recent observations of extended Lya blobs (LABs) at high redshift. We suggest, based on the angular size, energetics, as well as the relatively shallow surface brightness profiles, and double-peaked spectra, that several of these LABs may be associated with collapsing protogalaxies. We suggest two follow-up observations to diagnose the presence of gas infall. High S/N spectra of LABs should reveal a preferential flattening of the surface brightness profile at the red side of the line. Complementary imaging of the blobs at redshifted Balmer alpha wavelengths should reveal the intrinsic Lya emissivity and allow its separation from radiative transfer effects. We show that Lya scattering by infalling gas can reproduce the observed spectrum of Steidel et al's LAB2 as accurately as a recently proposed outflow model. Finally, we find similar evidence for infall in the spectra of point-like Lyman alpha emitters. The presence of scattering by the infalling gas implies that the intrinsic Lya luminosities, and derived quantities, such as the star-formation rate, in these objects may have been underestimated by about an order of magnitude.

astro-ph↗

arXiv2005

The Polytropic Equation of State of Primordial Gas Clouds

The polytropic equation of state (EOS) of primordial gas clouds with modest enrichment is computed, motivated by the recent observations of very Fe-deficient stars, [Fe/H]~10^{-3.5}-10^{-5}, such as HE 0107-5240 and CS 29498-043. These stars are overabundant, relative to Fe, in C and O. We assume that the observed abundances of species like C, O, Si and Fe are representative of the gas from which the currently observed metal-deficient stars formed. Under this assumption, we find that this primordial metal abundance pattern has profound consequences for the thermal balance and chemical composition of the gas, and hence for the EOS of the parental cloud. The polytopic EOS is soft for low, [O/H]<10^{-3}, oxygen abundances, but stiffens to a polytropic index gamma large than unity for [O/H]>10^{-2} due to the large opacity in the CO and H2O cooling lines. It is further found that a regulating role is played by the presence and temperature of the dust, even when the overall carbon abundance is only [C/H]~10^{-2}. When the dust is warmer than the gas, a region with gamma~1.2 results around a density of ~10^4 cm^{-3}. When the dust is colder than the gas, a region with gamma~0.8 is found for a density of ~10^6 cm^{-3}. Implications for the primordial initial mass function (IMF) as well as the IMF in starburst galaxies, where the metallicity is super-solar, are explored and related to processes that influence the temperature of the ambient dust.

astro-ph↗

arXiv2005

Diagnostics of irradiated gas in galaxy nuclei. I: A Far-ultraviolet and X-ray dominated region code

We present a far-ultraviolet (PDR) and an X-ray dominated region (XDR) code. We include and discuss thermal and chemical processes that pertain to irradiated gas. An elaborate chemical network is used and a careful treatment of PAHs and H2 formation, destruction and excitation is included. For both codes we calculate four depth-dependent models for different densities and radiation fields, relevant to conditions in starburst galaxies and active galactic nuclei. A detailed comparison between PDR and XDR physics is made for total gas column densities between ~10^20 and ~10^25 cm^-2. We show cumulative line intensities for a number of fine-structure lines (e.g., [CII], [OI], [CI], [SiII], [FeII]), as well as cumulative column densities and column density ratios for a number of species (e.g., CO/H2, CO/C, HCO+/HCN, HNC/HCN). The comparison between the results for the PDRs and XDRs shows that column density ratios are almost constant up to N_H=10^22 cm^-2 for XDRs, unlike those in PDRs. For example, CO/C in PDRs changes over four orders of magnitude from the edge to N_H=10^22 cm^-2. The CO/C and CO/H2 ratios are lower in XDRs at low column densities and rise at N_H > 10^23 cm^-2. At most column densities N_H > 10^21.5 cm^-2, HNC/HCN ratios are lower in XDRs too, but they show a more moderate increase at higher N_H.

astro-ph↗

arXiv2003

Chemical Composition of the Early Universe

A prediction of standard inflationary cosmology is that the elemental composition of the medium out of which the earliest stars and galaxies condensed consisted primarily of hydrogen and helium 4He with small admixtures of deuterium, lithium 7Li, and 3He. The most red-shifted quasars, galaxies, and Ly-alpha absorbers currently observed, however, all exhibit at least some admixture of heavier elements, as do the most ancient stars in the Galaxy. Here we examine ways in which the abundance of these same elements, if present before the epoch of population III formation, might be observationally established or ruled out.

astro-ph↗

arXiv2001

CO emission from high redshift

Future observations with ALMA will be able to detect star-forming primordial gal axies, and perhaps even their dwarf spheroidal satellites, in CO emission lines at redshifts approaching 10. These observations will compliment other tools designed to study the dawn of galaxy formation, such as NGST and FIRST.

astro-ph↗

arXiv2000

The Abundance and Emission of H2O and O2 in Clumpy Molecular Clouds

Recent observations with the Submillimeter Wave Astronomy Satellite indicate abundances of gaseous H2O and O2 in dense molecular clouds which are significantly lower than found in standard homogeneous chemistry models. We present here results for the thermal and chemical balance of inhomogeneous molecular clouds exposed to ultraviolet radiation in which the abundances of H2O and O2 are computed for various density distributions, radiation field strengths and geometries. It is found that an inhomogeneous density distribution lowers the column densities of H2O and O2 compared to the homogeneous case by more than an order of magnitude at the same A_V. O2 is particularly sensitive to the penetrating ultraviolet radiation, more so than H2O. The S140 and rho Oph clouds are studied as relevant test cases of star-forming and quiescent regions. The SWAS results of S140 can be accommodated naturally in a clumpy model with mean density of 2x10^3 cm-3 and enhancement I_UV=140 compared with the average interstellar radiation field, in agreement with observations of [CI] and 13CO of this cloud. Additional radiative transfer computations suggest that this diffuse H2O component is warm, ~60-90 K, and can account for the bulk of the 1_10-1_01 line emission observed by SWAS. The rho Oph model yields consistent O2 abundances but too much H2O, even for [C]/[O]=0.94, if I_UV<10 respectively <40 for a mean density of 10^3 respectively 10^4 cm-3. It is concluded that enhanced photodissociation in clumpy regions can explain the low H2O and O2 abundances and emissivities found in the large SWAS beam for extended molecular clouds, but that additional freeze-out of oxygen onto grains is needed in dense cold cores.

astro-ph↗

arXiv2000

On the Topological Nature of Fundamental Interactions

A thought experiment is proposed to unify quantum mechanics and general relativity. The central paradigm is that space-time {\it topology} is ultimately responsible for the Heisenberg uncertaintly principle. It is found that Plankian space-time exhibits a complicated, but also definite, multiply connected character. In this framework, an analysis of the interactions in Nature is presented. I. The Universal ground state of the constructed theory derives from the properties of the topological manifold $Q=2T^3\oplus 3S^1\times S^2$, which has 23 intrinsic degrees of freedom, discrete $Z_3$ and $Z_2\times Z_3$ internal groups, an SU(5) gauge group, and leads to a U(1) symmetry on a lattice. The structure of $Q$ provides a unique equation motion for the mass-energy and particle rest mass wave functions. In its excited state the Universe is characterized by a lattice of three-tori, $L(T^3)$. The topological identifications present in this structure, a direct reflection of the Heisenberg uncertainty principle, provide the boundary conditions for solutions to the equation of motion, and suggest an interpretation for the conceptually difficult concept of quantum mechanical entanglement. II. In the second half of the paper the (observable) properties of $Q$ and $L(T^3)$ are investigated. One reproduces the standard model, and the theory naturally contains a Higgs field with possible inflation. The electron and its neutrino are identified as particle ground states and their masses, together with those of all other known particles, are predicted. A mass of $m_{\rm H}=131.6$ GeV is found for the Higgs boson. [Abridged]

gr-qc↗

arXiv2000

Formation of Cavities, Filaments, and Clumps by the Non-linear Development of Thermal and Gravitational Instabilities in the Interstellar Medium under Stellar Feedback

Based on our high resolution, two-dimensional hydrodynamical simulations, we propose that large cavities may be formed by the nonlinear development of the combined thermal and gravitational instabilities, without need for stellar energy injection in a galaxy modeling the Large Magellanic Clouds (LMC). Our numerical model of the star formation allows us to follow the evolution of the blastwaves due to supernovae in the inhomogeous, multi-phase, and turbulent-like media self-consistently. Formation of kpc-scale inhomogeneity, such as cavities, observed HI map of the LMC, is suppressed by frequent supernovae (average supernova rate for the whole disk is ~0.001/yr). However the supernova explosions are necessary for the hot component (T_g > 10^{6-7} K). Position-velocity maps show that kpc-scale shells/arcs formed through the nonlinear evolution in a model without stellar energy feedback has similar kinematics to explosional phenomena, such as supernovae. We also find that dense clumps and filamentary structure are formed due to a natural consequence of the non-linear evolution of the multi-phage ISM. Although the ISM in a small scale looks turbulent-like and transient, the global structure of the ISM is quasi-stable. We compare the observations of HI and molecular gas of the LMC with the numerically obtained HI and CO map. The morphology and statistical properties of the numerical HI and CO maps are discussed.

astro-ph↗

arXiv2000

Lyman Alpha Cooling Radiation from High-Redshift Halos

The baryons inside high-redshift halos with virial temperatures above T=10^4 K cool radiatively as they condense inside dark matter potential wells. We show that the release of the gravitational binding energy, over the halo assembly time-scale, results in a significant and detectable Ly alpha flux. At the limiting line flux of approximately 10^-19 erg/s/cm^2/asec^2 of the Next Generation Space Telescope, several sufficiently massive halos, with velocity dispersions above sigma=120 km/s, would be visible per 4'x4' field. The halos would have characteristic angular sizes of approximately 10 asec, would be detectable in a broad-band survey out to redshifts z=6-8, and would provide a direct probe of galaxies in the process of forming. They may be accompanied by HeII Ly alpha emission at the 10 percent level, but remain undetectable at other wavelengths. Our predictions are in good agreement with the recent finding of two Ly alpha ``blobs'' at z=3.1 by Steidel et al. (1999).

astro-ph↗

arXiv2000

The Polytropic Equation of State of Interstellar Gas Clouds

Models are presented for the polytropic equation of state of self-gravitating, quiescent interstellar gas clouds. A detailed analysis, including chemistry, thermal balance, and radiative transfer, is performed for the physical state of the gas as a function of density, metallicity, velocity field, and background radiation field. It is found that the stiffness of the equation of state strongly depends on all these physical parameters, and the adiabatic index varies between 0.2-1.4. The implications for star formation, in particular at high redshift and in starburst galaxies, and the initial stellar mass function are discussed.

astro-ph↗

arXiv1998

Models for High-Redshift Lyman Alpha Emitters

We present models for dusty high-redshift Lyman alpha emitting galaxies by combining the Press-Schechter formalism with a treatment of inhomogeneous dust distribution inside galaxies. These models reproduce the surface density of emitters inferred from recent observations, and also agree with previous non-detections. Although a detailed determination of the individual model parameters is precluded by uncertainties, we find that (i) the dust content of primordial galaxies builds up in no more than 5x10^8 years, (ii) the galactic HII regions are inhomogeneous with a cloud covering factor of order unity, and (iii) the overall star formation efficiency is at least 5 percent. Future observations should be able to detect Lyman alpha galaxies upto redshifts of about z=8. If the universe is reionized at z(reion)<8, the corresponding decline in the number of Lyman alpha emitters at z>z(reion) could prove to be a useful probe of the reionization epoch.

astro-ph↗

arXiv1998

Observational Tests to Discern the Topology of Planckian Space-Time

Observational diagnostics are constructed which reflect the underlying topology of Planckian space-time, and are directly related to phenomena on much larger scales. Specific predictions are made for the masses of elementary particles, coupling constants, quark confinement, black hole states, and the cosmological constant. In particular, a mass of 131.6 GeV is found for the Higgs boson. The main presented result is a discrete spectral signature at inverse integer multiples of the zero point frequency $ν_0=857.3588$ MHz. That is, each photon of frequency $ν_0m$, for an integer $m$, is paired with an otherwise identical photon $ν_0/m$, produced by the vacuum, but not vice versa. The reader interested in the latter result only, should proceed to Section 6 after the Introduction.

hep-th↗

arXiv1998

Far-Infrared and Sub-Millimeter Observations and Physical Models of the Reflection Nebula Ced 201

ISO [C II] 158 micron, [O I] 63 micron, and H_2 9 and 17 micron observations are presented of the reflection nebula Ced 201, which is a photon-dominated region illuminated by a B9.5 star with a color temperature of 10,000 K (a cool PDR). In combination with ground based [C I] 609 micron, CO, 13CO, CS and HCO+ data, the carbon budget and physical structure of the reflection nebula are constrained. The obtained data set is the first one to contain all important cooling lines of a cool PDR, and allows a comparison to be made with classical PDRs. To this effect one- and three-dimensional PDR models are presented which incorporate the physical characteristics of the source, and are aimed at understanding the dominant heating processes of the cloud. The contribution of very small grains to the photo-electric heating rate is estimated from these models and used to constrain the total abundance of PAHs and small grains. Observations of the pure rotational H_2 lines with ISO, in particular the S(3) line, indicate the presence of a small amount of very warm, approximately 330 K, molecular gas. This gas cannot be accommodated by the presented models.

astro-ph↗

arXiv1998

The Molecular ISM in Low Surface Brightness Disk Galaxies

We present models for the interstellar medium in disk galaxies. In particular, we investigate whether the ISM in low surface brightness galaxies can support a significant fraction of molecular gas given their low metallicity and surface density. It is found that the abundance and line brightness of CO in LSB galaxies is small and typically below current observational limits. Still, depending on physical details of the ISM, the fraction of gas in the form of molecular hydrogen can be significant in the inner few kpc of a low surface brightness galaxy. This molecular gas would be at temperatures of $\sim$ 30 -- 50 K, rather higher than in high surface brightness galaxies. These results may help explain the star forming properties and inferred evolutionary history of LSB galaxies.

astro-ph↗

arXiv1998

Models for Dusty Lyman alpha Emitters at High Redshift

Models are presented for the Lyman alpha emission of dusty high-redshift galaxies by combining the Press-Schechter formalism with a treatment of the inhomogeneous dust distribution inside galaxies. It is found that the amount of Lyman alpha radiation escaping from the galaxies strongly depends on the time over which the dust is produced through stellar activity, and on the ambient inhomogeneity of the HII regions that surround the ionizing OB stars. Good agreement is found with recent observations, as well as previous non-detections. Our models indicate that the dust content builds up in no more than approximately 5x10^8 yr, the galactic HII regions are inhomogeneous with a cloud covering factor of order unity, and the overall star formation efficiency is at least about 5%. It is predicted that future observations can detect these Lyman alpha galaxies upto redshifts of about 8.

astro-ph↗

arXiv1998

Towards a Topological Formulation of Fundamental Interactions

A thought experiment is formulated to unify quantum mechanics and general relativity in a topological manner. An analysis of the interactions in Nature is then presented. The universal ground state of the constructed theory derives from the cyclic properties ($S^1$ homotopy) of the topological manifold $Q=2T^3\oplus 3S^1\times S^2$ which has 23 intrinsic degrees of freedom, discrete $Z_3$ and $Z_2\times Z_3$ internal groups, an SU(5) or SO(10) gauge group, and leads to an anomalous U(1) symmetry on a lattice. These properties can in principle reproduce the standard model with a stable proton. The general equation of motion for the unified theory is derived up to the Planck energy and leads to a Higgs field with possible inflation. 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 particle ground states and their masses are predicted. The mass of the electron agrees very well with observations. A heuristic argument for the occurrence and magnitude of CP violation is given. Future extensions of the presented framework are discussed.

hep-th↗

arXiv1998

Search for Interstellar Water in the Translucent Molecular Cloud toward HD 154368

We report an upper limit of 9 x 10^{12} cm-2 on the column density of water in the translucent cloud along the line of sight toward HD 154368. This result is based upon a search for the C-X band of water near 1240 Åcarried out using the Goddard High Resolution Spectrograph of the Hubble Space Telescope. Our observational limit on the water abundance together with detailed chemical models of translucent clouds and previous measurements of OH along the line of sight constrain the branching ratio in the dissociative recombination of H_3O+ to form water. We find at the $3σ$ level that no more than 30% of dissociative recombinations of H_3O+ can lead to H_2O. The observed spectrum also yielded high-resolution observations of the Mg II doublet at 1239.9 Åand 1240.4 Å, allowing the velocity structure of the dominant ionization state of magnesium to be studied along the line of sight. The Mg II spectrum is consistent with GHRS observations at lower spectral resolution that were obtained previously but allow an additional velocity component to be identified.

astro-ph↗