Showing papers by "Marco Spaans published in 2006"

Ly alpha radiation from collapsing protogalaxies. I. Characteristics of the emergent spectrum

Abstract: We present Monte Carlo calculations of Ly alpha radiative transfer through optically thick, spherically symmetric, collapsing gas clouds. These represent simplified models of protogalaxies in the process of their assembly. Such galaxies produce Ly alpha flux over an extended solid angle, either from a spatially extended Ly alpha emissivity, or from scattering effects, or both. We present a detailed study of the effect of the gas distribution and kinematics and of the Ly alpha emissivity profile on the emergent spectrum and surface brightness distribution. The emergent Ly alpha spectrum is typically double peaked and asymmetric. In practice, however, we find energy transfer from the infalling gas to the Ly alpha photons to significantly enhance the blue peak and to render the red peak, in most cases, undetectable. The resulting effective blueshift, combined with scattering in the intergalactic medium, renders extended Ly alpha emission from collapsing protogalaxies difficult to detect beyond redshift z greater than or similar to 4. We find a strong wavelength dependence of the slope of the surface brightness distribution (with preferential flattening at the red side of the line) to be a robust indication that Ly alpha photons are being generated (rather than just scattered) in a spatially extended, collapsing region around the galaxy. For self-ionized clouds whose effective Ly alpha optical depth is less than or similar to 10(3), infall and outflow models can produce nearly identical spectra and surface brightness distributions and are practically indistinguishable. The presence of cosmic abundance of deuterium may produce a detectable dip in the spectra of systems with moderate hydrogen column densities, N(H) = 10(18)-10(20) cm(-2). Finally, we present a new analytic solution for the emerging Ly alpha spectrum in the limiting case of a static uniform sphere, extending previous solutions for slabs.

Pregalactic Black Hole Formation with an Atomic Hydrogen Equation of State

Abstract: The polytropic equation of state of an atomic hydrogen gas is examined for primordial halos with baryonic masses of M-h similar to 10(7)-10(9) M-circle dot For roughly isothermal collapse around 10(4) K, we find that line trapping of Ly alpha (H I and He II) photons causes the polytropic exponent to stiffen to values significantly above unity Under the assumptions of zero H-2 abundance and very modest pollution by metals ( 10(4) K, that are attained when Ly alpha photons are trapped in the dense and massive halos that we consider An H-2-dissociating UV background is needed if positive feedback effects on H-2 formation from X-rays occur The black hole-to-baryon mass fraction is suggestively close to what is required for these intermediate-mass black holes, of mass M-BH similar to 10(4)-10(6) M-circle dot, to act as seeds for forming the supermassive black holes of mass similar to 0001M(spheroid) found in galaxies today

Lyα Radiation from Collapsing Protogalaxies. II. Observational Evidence for Gas Infall

Abstract: We model the spectra and surface brightness distributions for the Lyα 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 these spectra to incorporate the effect of subsequent resonant scattering in the IGM. Using these models, we interpret a number of recent observations of extended Lyα blobs (LABs) at high redshift. We suggest, based on the angular size, energetics, 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 Hα wavelengths should reveal the intrinsic Lyα emissivity and allow its separation from radiative transfer effects. We show that Lyα scattering by infalling gas can reproduce the observed spectrum of the Steidel et al. LAB2 as accurately as a recently proposed outflow model. Finally, we find similar evidence for infall in the spectra of pointlike Lyα emitters. The presence of scattering by the infalling gas implies that the intrinsic Lyα luminosities and derived quantities, such as the star formation rate, in these objects may have been underestimated by about an order of magnitude.

Irradiated ISM : Discriminating between cosmic rays and X-rays

Abstract: The interstellar medium (ISM) at the centers of active galaxies is exposed to a combination of cosmic-ray, far-ultraviolet (FUV), and X-ray radiation We apply photodissociation region (PDR) models to this ISM with both "normal" and highly elevated (5 × 10-15 s-1) cosmic-ray (CR) rates and compare the results to those obtained for X-ray dissociation regions (XDRs) Our existing PDR-XDR code is used to construct models over a 103-105 cm-3 density range and for 016-160 ergs s-1 cm-2 impingent fluxes We obtain larger high-J (J > 10) CO ratios in PDRs when we use the highly elevated CR rate, but these are always exceeded by the corresponding XDR ratios The [C I] 609 μm/13CO (2-1) line ratio is boosted by a factor of a few in PDRs with n ~ 103 cm-3 exposed to a high CR rate At higher densities, ratios become identical irrespective of CR flux, while XDRs always show elevated [C I] emission per CO column The HCN/CO and HCN/HCO+ line ratios, combined with high-J CO emission lines, are good diagnostics to distinguish between PDRs, under either low or high CR irradiation conditions, and XDRs Hence, the Heterodyne Instrument for the Far Infrared (HIFI) on the Herschel Space Observatory, which can detect these CO lines, will be crucial in the study of active galaxies

The Stellar Mass Spectrum in Warm and Dusty Gas: Deviations from Salpeter in the Galactic Centre and in Circum-Nuclear Starburst Regions

Abstract: Understanding the origin of stellar masses is a key problem in astrophysics. In the solar neighborhood, the mass distribution of stars follows a seemingly universal pattern. In the centre of the Milky Way, however, there are indications for strong deviations and the same may be true for the nuclei of distant starburst galaxies. Here we present the first numerical hydrodynamical calculations of stars formed in a molecular region with chemical and thermodynamic properties similar to those of warm and dusty circum-nuclear starburst regions. The resulting IMF is top-heavy with a peak at ~ 15 Msun, a sharp turn-down below 7 Msun and a power-law decline at high masses. We find a natural explanation for our results in terms of the temperature dependence of the Jeans mass, with collapse occuring at a temperature of ~ 100 K and an H2 density of a few times 10^5 cm^-3, and discuss possible implications for galaxy formation and evolution.

The effects of metallicity, radiation field and dust extinction on the charge state of PAHs in diffuse clouds: implications for the DIB carrier

Abstract: Context. The unidentified diffuse interstellar bands (DIB) are observed throughout the Galaxy, the Local Group and beyond. Their carriers are possibly related to complex carbonaceous gas-phase molecules, such as (cationic) polycyclic aromatic hydrocarbons and fullerenes. Aims. In order to reveal the identity of the DIB carrier we investigate the effects of metallicity, radiation field and extinction curve on the PAH charge state distribution, and thus the theoretical emergent PAH spectrum, in diffuse interstellar clouds. This behaviour can then be linked to that of the DIB carrier, thus giving insight into its identity. Methods. We use radiative transfer and chemical models to compute the physical and chemical conditions in diffuse clouds with Galactic and Magellanic Cloud types of interstellar dust and gas. Subsequently, the PAH charge state distributions throughout these clouds are determined. Results. We find that the fraction of PAH cations is much higher in the Magellanic Cloud environments than in the Milky Way, caused predominantly by the respective lower metallicities, and mitigated by the steeper UV extinction curve. The fraction of anions is much lower in a low metallicity environment. The predicted DIB strength of cationic PAH carriers is similar to that of the Milk Way for the LMC and 40% for the SMC due to the overall metallicity. Stronger DIBs could be expected in the Magellanic Clouds if they emanate from clouds that are exposed to an average interstellar radiation field that is significantly stronger than in the Milky Way, although photo-destruction processes could possibly reduce this effect, especially for the smaller PAHs. Our results show that the presence and absence of DIB carriers in the Magellanic Cloud lines of sight can be tied to the PAH charge balance which is driven by metallicity, UV radiation and dust extinction effects.

Line profiles of water for the photon dominated region and embedded sources in the S140 region

Abstract: H(2)O is a key ingredient in many interstellar environments, like photon dominated regions and star forming clouds. It plays an important role in the oxygen chemistry and can act as a coolant in dense interstellar clouds and shocks. Observations and modelling of water lines thus provide powerful diagnostics of the physical conditions in interstellar emission zones. A radiative transfer method for the treatment of molecular lines is presented. We apply this method to previous SWAS and ISO observations of water vapor in the source S140 in order to make models to plan for, and to interpret, HIFI data. Level populations are calculated with the use of a three-dimensional (multi-zone) escape probability method and with a long characteristic code that uses Monte-Carlo techniques with fixed directions. Homogeneous and inhomogeneous models are used to compute the differences between H(2)O line profiles across the S140 region. We find that when an outflow or infall velocity field with a gradient of a few km s(-1) is adopted, line profiles with a FWHM of 6 km s(-1) are found, in agreement with observations. Inhomogeneous models are favoured to produce a single-peaked line profile. When zooming in on smaller regions within the PDR, the shapes of the line profiles start to differ due to the different temperature and density distributions there. The embedded sources are traced by high excitation lines, e. g., 3(21)-2(21),3(03)-2(12),2(12)-1(01) and 2(20)- 1(11). The computed intensities are roughly consistent with existing ISO observations. Water emission in a PDR source like S140 requires a combination of a pure PDR and an embedded source in order to match the observations. Because of its good angular resolution, HIFI will be able to distinguish between a dense star forming region or a more diffuse gas component. It is therefore important for future observing programs to consider both in their predictions of the emission characteristics of water in these environments.

Pregalactic Black Hole Formation with an Atomic Hydrogen Equation of State

Abstract: The polytropic equation of state of an atomic hydrogen gas is examined for primordial halos with baryonic masses of M_h~10^7-10^9 Mo. For roughly isothermal collapse around 10^4 K, we find that line trapping of Lyman alpha (HI and HeII) photons causes the polytropic exponent to stiffen to values significantly above unity. Under the assumptions of zero H2 abundance and very modest pollution by metals (<10^-4 Solar), fragmentation is likely to be inhibited for such an equation of state. We argue on purely thermodynamic grounds that a single black hole of ~0.02-0.003M_h can form at the center of a halo for z=10-20 when the free-fall time is less than the time needed for a resonantly scattered Lyman alpha photon to escape from the halo. The absence of H2 follows naturally from the high, 10^4 K, temperatures that are attained when Lyman alpha photons are trapped in the dense and massive halos that we consider. An H2 dissociating UV background is needed if positive feedback effects on H2 formation from X-rays occur. The black hole to baryon mass fraction is suggestively close to what is required for these intermediate mass black holes, of mass M_BH~10^4-10^6 Mo, to act as seeds for forming the supermassive black holes of mass ~0.001M_spheroid found in galaxies today.

Diagnostics of active galaxies I. Modeling the infrared properties of dusty cores of starburst galaxies

Abstract: Aims. Despite extensive observations over the last decades, the central questions regarding the power source of the large IR luminosity of Ultra Luminous Infra Red Galaxies (ULIRGs), and their evolution, are still not fully answered. In this paper we will focus on massive star formation as a central engine and present an evolutionary model for these dust-enshrouded star formation regions. Methods. An evolutionary model was created using existing star formation and radiative transfer codes (STARBURST99, RADMC and RADICAL) as building blocks. The results of the simulations are compared to data from two IRAS catalogs. Results. From the simulations it is found that the dust surrounding the starburst region is made up from two components. There is a low optical depth (tau = 0.1, which corresponds to 0.1% of the total dust mass), hot (T similar to 400 K) non-grey component close to the starburst (scale size 10 pc) and a large scale, colder grey component (100 pc, 75 K) with a much larger column (tau = 10). The simulations also show that starburst galaxies can be powered by massive star formation. The parameters for this star forming region are difficult to determine, since the IR continuum luminosity is only sensitive to the total UV input. Therefore, there is a degeneracy between the total starburst mass and the initial mass function (IMF) slope. A less massive star formation with a shallower IMF will produce the same amount of OB stars and therefore the same amount of irradiating UV flux. Assuming the stars are formed according to a Salpeter IMF (Psi(M) proportional to M(-2.35)), the star formation region should produce 10(9) M(circle dot) of stars (either in one instantaneous burst, or in a continuous process) in order to produce enough IR radiation. Conclusions. Our models confirm that massive star formation is a valid power source for ULIRGs. In order to remove degeneracies and further determine the parameters of the physical environment also IR spectral features and molecular emissions need to be included.

Dust coagulation in protoplanetary disks: porosity matters

Abstract: Context: Sticking of colliding dust particles through van der Waals forces is the first stage in the grain growth process in protoplanetary disks, eventually leading to the formation of comets, asteroids and planets. A key aspect of the collisional evolution is the coupling between dust and gas motions, which depends on the internal structure (porosity) of aggregates. Aims: To quantify the importance of the internal structure on the collisional evolution of particles, and to create a new coagulation model to investigate the difference between porous and compact coagulation in the context of a turbulent protoplanetary disk. Methods: We have developed simple prescriptions for the collisional evolution of porosity of grain-aggregates in grain-grain collisions. Three regimes can then be distinguished: `hit-and-stick' at low velocities, with an increase in porosity; compaction at intermediate velocities, with a decrease of porosity; and fragmentation at high velocities. (..) Results: (..) We can discern three different stages in the particle growth process (..) We find that when compared to standard, compact models of coagulation, porous growth delays the onset of settling, because the surface area-to-mass ratio is higher, a consequence of the build-up of porosity during the initial stages. As a result, particles grow orders of magnitudes larger in mass before they rain-out to the mid-plane. Depending on the turbulent viscosity and on the position in the nebula, aggregates can grow to (porous) sizes of ~ 10 cm in a few thousand years. We also find that collisional energies are higher than in the limited PCA/CCA fractal models, thereby allowing aggregates to restructure. It is concluded that the microphysics of collisions plays a key role in the growth process.

Diagnostics of active galaxies: I. Modeling the infrared properties of dusty cores of starburst galaxies

Abstract: An evolutionary model of star formation in ULIRGs was created using existing star formation and radiative transfer codes (STARBURST99, RADMC and RADICAL) as building blocks. The results of the simulations are compared to data from two IRAS catalogs. From the simulations it is found that the dust surrounding the starburst region is made up from two components. There is a low optical depth (tau=0.1, which corresponds to 0.1% of the total dust mass), hot (T~400K) non-grey component close to the starburst (scale size 10pc) and a large scale, colder grey component (100pc, 75K) with a much larger column (tau=10). The simulations also show that starburst galaxies can be powered by massive star formation. The parameters for this star forming region are difficult to determine, since the IR continuum luminosity is only sensitive to the total UV input. Therefore, there is a degeneracy between the total starburst mass and the initial mass function (IMF) slope. A less massive star formation with a shallower IMF will produce the same amount of OB stars and therefore the same amount of irradiating UV flux. Assuming the stars are formed according to a Salpeter IMF (Psi(M) ~ M^-2.35), the star formation region should produce 10^9 Msun of stars (either in one instantaneous burst, or in a continuous process) in order to produce enough IR radiation.