Marco Spaans

Models for Dusty Lyα Emitters at High Redshift

Models are presented for the Lyα 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 Lyα 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 H II regions that surround the ionizing OB stars. Good agreement is found with recent observations, as well as with previous nondetections. Although a detailed determination of the individual model parameters is precluded by uncertainties, we find that (1) the dust content in primordial galaxies builds up in no more than ~5 × 108 yr, (2) the galactic H II regions are inhomogeneous with a cloud-covering factor of order unity, and (3) the overall star formation efficiency is at least ~5%. It is predicted that future observations can detect these Lyα galaxies up to redshifts of ~8. If the universe is reionized at zr 8, the corresponding decline in the number of Lyα emitters at z zr could prove to be a useful probe of the reionization epoch.

Formation of Cavities, Filaments, and Clumps by the Nonlinear 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 Cloud (LMC). Our numerical model of star formation allows us to follow the evolution of the blast waves due to supernovae in the inhomogenous, multiphase, and turbulent-like media self-consistently. Formation of kiloparsec-scale inhomogeneity, such as cavities as seen in the observed H I map of the LMC, is suppressed by frequent supernovae (the average supernova rate for the whole disk is ~0.001 yr-1). However, the supernova explosions are necessary for the hot component (Tg > 106-107 K). Position-velocity maps show that kiloparsec-scale shells/arcs formed through nonlinear evolution in a model without stellar energy feedback have kinematics similar to explosive phenomena, such as supernovae. We also find that dense clumps and filamentary structure are formed as a natural consequence of the nonlinear evolution of the multiphase interstellar medium (ISM). Although the ISM on a small scale looks turbulent-like and transient, the global structure of the ISM is quasi-stable. In the quasi-stable phase, the volume filling factor of the hot, warm, and cold components are ~0.2, ~0.6, and ~0.2, respectively. We compare observations of H I and molecular gas of the LMC with the numerically obtained H I and CO brightness temperature distributions. The morphology and statistical properties of the numerical H I and CO maps are discussed. We find that the cloud mass spectra of our models represent a power-law shape, but their slopes change between models with and without the stellar energy injection. We also find that the slope depends on the threshold brightness temperature of CO.

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. We find that the stiffness of the equation of state strongly depends on all of 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.

Cosmological Evolution of Dwarf Galaxies: The Influence of Star Formation and the Multiphase Interstellar Medium

A model is developed to explain the cosmological evolution of dwarf galaxies. The population of small galaxies is found to evolve rapidly for z < 1, which provides a natural explanation for the evolution observed in the galaxy luminosity function. A tail is found in the redshift distribution of the faint blue excess that can extend to a redshift of 2. The star formation history is followed in detail for these objects. Constraints on the metallicity are identified for which stars are formed with much higher efficiency in a multiphase interstellar medium than in massive galaxies. Blue dwarf galaxies at the current epoch are identified with this starburst mode. The collapse of 1 and 2 σ perturbations of the initial density fluctuation spectrum is followed using the extended standard hierarchical clustering formalism. The collapse of these perturbations is normally associated with the formation of dwarf galaxies. These objects have shallow gravitational potential wells, and their evolution strongly depends upon the cooling time of the gas. The latter is determined by the ionization and chemical equilibrium of the gas in the presence of the intergalactic and local stellar radiation fields. The latter generally dominates and creates a feedback mechanism that regulates the evolutionary timescale. To improve upon previous models, essential new astrophysical ingredients are incorporated, such as a more detailed description of the physical processes regulating the multiphase structure of the interstellar medium in dwarf galaxies and the effects of evolution in the galaxys metallicity on the formation of stars in molecular clouds. It is found that for a low star formation rate of 0.1 M☉ yr-1, the cooling time of interstellar gas is longer than the local Hubble time until z ~ 1. At this epoch, a two-phase medium makes the dwarf interstellar medium less fragile against supernova explosions, and the volume filling factor of the hot phase (107 K) becomes of order unity. The resulting X-ray luminosity is consistent with observations of nearby dwarf galaxies if exchange processes between the hot and cold phases of the interstellar medium (i.e., evaporation and ablation of clouds due the supernova blast wave) are included. The tenuous halo gas has a temperature in excess of the escape velocity and exhibits very high ionization stages. The dark matter halo potential (M/L ~ 102) influences the loss rate of metals and the star formation history. It is found that if the metallicity is between 0.01 and 0.1 Z☉ then a period of rapid star formation, ~1-3 M☉ yr-1, can be excited for z ~ 1. The physical reason for this starburst mode in our model is that the free-fall time exceeds the ambipolar diffusion timescale for clouds. Consequently, molecular clouds are not magnetically supported and star formation can proceed much more efficiently compared with more massive and metal-rich galaxies. Our results are consistent with the redshift distribution of dwarf galaxies observed in the Hubble Deep Field. In particular, a rapidly evolving dwarf population can account for the excess number counts of faint galaxies: the blue excess. Predictions are made for the color evolution of the old stellar population in these systems, observable with the Near-Infrared Camera and Multiobject Spectrometer (NICMOS) on the Hubble Space Telescope in the near future.

Photon heating of envelopes around young stellar objects : an explanation for CO J=6-5 emission

We propose that the narrow 12CO and 13CO J = 6-5 emission observed toward many low-mass young stellar objects is produced in molecular material in the circumstellar envelope, which is heated by the 10,000 K radiation field generated in the inner part of the accretion disk. Ultraviolet photons traveling through the biconical cavity evacuated by the bipolar outflow are scattered by dust grains present in the low-density material in the cavity. These photons are not energetic enough to photodissociate H2 and CO, but can heat the envelope surrounding the cavity. The temperature structure and the CO excitation of this photon-dominated region are computed using two-dimensional Monte Carlo methods. It is found that the material is heated up to a few hundred K close to the cavity wall, and that the observed low-velocity mid-J CO emission can be well explained by our model for a wide range in envelope density and stellar luminosity. Emergent CO spectra are compared to observations of the embedded low-mass YSO IRAS 04361+2547 (TMR-1).

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

We study the formation of molecular hydrogen, after the epoch of reionization, in the context of canonical galaxy formation theory due to hierarchical clustering. There is an initial epoch of H2 production in the gas phase through the H- route that ends at a redshift of order unity. We assume that the fundamental units in the gas phase of protogalaxies during this epoch are similar to diffuse clouds found in our own Galaxy, and we restrict our attention to protogalactic disks, although some of our analysis applies to multiphase halo gas. Giant molecular clouds are not formed until lower redshifts. Star formation in the protogalactic disks can become self-regulated. The process responsible for the feedback is the heating of the gas by the internal stellar radiation field that can dominate the background radiation field at various epochs. If the gas is heated to above 2000-3000 K, the hydrogen molecules are collisionally dissociated, and we assume that in their absence the star formation process is strongly suppressed because of insufficient cooling. As we demonstrate by the analysis of phase diagrams, the H2-induced cool phase disappears. A priori, the cool phase with molecular hydrogen cooling can only achieve temperatures ≥300 K. Consequently, it is possible to define a maximum star formation rate during this epoch. Plausible estimates give a rate of 0.2-2 M☉ yr-1 for condensations corresponding to 1 σ and 2 σ initial density fluctuations. For more massive structures, this limit is relaxed and in agreement with observations of high-redshift galaxies. 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☉. 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, and initiates the rapid formation of molecular species, giant molecular clouds, and stars. Consequently, the delayed initiation of the cold phase in the interstellar medium of protostellar disks at a metallicity of Z 0.1 Z☉ is a plausible physical reason why the formation phase of the stellar disks of the bulk of the galaxies occurs only at a redshift of order unity. The combination of feedback and a phase transition provides a natural resolution of the G-dwarf problem.

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 kiloparsecs of a low surface brightness galaxy. This molecular gas would be at temperatures of ~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.

Far-infrared and submillimeter observations and physical models of the reflection nebula Cederblad 201

Infrared Space Observatory (ISO) [C II] 158 μm, [O I] 63 μm, and H2 9 and 17 μm observations are presented of the reflection nebula Ced 201, which is a photon-dominated region (PDR) 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 μm, 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 that 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 photoelectric heating rate is estimated from these models and is used to constrain the total abundance of polycyclic aromatic hydrocarbons and small grains. Observations of the pure rotational H2 lines with ISO, in particular the S(3) line, indicate the presence of a small amount of very warm ~330 K molecular gas. This gas cannot be accommodated by the presented models.

Hydrogen Recombination Line Masers at the Epochs of Recombination and Reionization

We model the physical properties of masing hydrogen recombination lines at zrc ~ 1000. We find that if small-scale, strongly overdense regions exist, then maser action is possible in this phase of cosmological evolution owing to the expansion of the universe and the low ambient temperature at decoupling. Partial-line absorption of nα ~ 60 maser radiation can occur at the epoch of reionization (zri ~ 10-50). The paths along which maser amplification is sustained during the epoch of recombination are assumed to be associated with the strong density perturbations required for the development of ionizing sources at zri. The combined spectrum can be observed at around 10-100 MHz (z = 0) and provides information on the power spectrum, H0, q0, and the evolution of H II regions during the epoch of reionization. The flux level is ~40 μJy, depending on the magnitude of the perturbation, with brightness temperatures of greater than 107 K. The Square Kilometer Array (SKAI) would have the sensitivity to observe these features. During reionization nα ~ 120, maser lines can also be produced along the edges of the early H II regions. Observations of these lines require telescopes that are able to operate below 80 MHz with a sensitivity of 1 μJy at an angular size of ~1. Although these requirements offer many challenges for the development of suitable ground-based and space-based telescopes, we indicate in this paper that such observations can be very important to understanding the physics and cosmology of these early epochs. Line observations of this sort have the potential to probe the complicated physical processes occurring in the nonlinear regime that is not directly accessible to continuum instruments such as COBE and its successors MAP and the Planck Surveyor.

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

We have constructed three-dimensional models for the equilibrium abundance of molecular hydrogen in diffuse interstellar clouds of arbitrary geometry that are illuminated by ultraviolet radiation. The position-dependent photodissociation rate of H2 in such clouds was computed with a 26 ray approximation to model the attenuation of the incident ultraviolet radiation field by dust and by H2 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 μm continuum intensity measured by IRAS. We find that our model can successfully account for observed variations in the ratio of 100 μm continuum intensity to H I column density, with larger values of that ratio occurring along lines of sight in which the molecular hydrogen fraction is expected to be the largest. Using a standard χ2 analysis to assess the goodness of fit of our models, we find (at the 60 σ 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 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-catalyzed H2 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 and a gas density of 53 ± 8 (100 pc/D) H nuclei cm-3 and is illuminated by a radiation field of 1.1 ± 0.2 (100 pc/D) times the mean interstellar radiation field. The derived 100 μm emissivity per nucleon is 1.13 ± 0.06 × 10-20 MJy sr-1 cm2.