arXiv subjects

Andrea Ferrara

Publications and source records attributed to Andrea Ferrara.

At least 253 records · Page 14Linked to original sources

arXiv1999

The Escape of Ionizing Photons from OB Associations in Disk Galaxies: Radiation Transfer Through Superbubbles

By solving the time-dependent radiation transfer problem of stellar radiation through evolving superbubbles within a smoothly varying HI distribution, we estimate the fraction of ionizing photons emitted by OB associations that escapes the HI disk of our Galaxy into the halo and intergalactic medium (IGM). We consider both coeval star-formation and a Gaussian star-formation history with a time spread sigma_t = 2 Myr. We consider both a uniform H I distribution and a two-phase (cloud/intercloud) model, with a negligible filling factor of hot gas. We find that the shells of the expanding superbubbles quickly trap or attenuate the ionizing flux, so that most of the escaping radiation escapes shortly after the formation of the superbubble. For the coeval star-formation history, the total fraction of Lyman Continuum photons that escape both sides of the disk in the solar vicinity is f_esc approx 0.15 +/- 0.05. For the Gaussian star formation history, f_esc approx 0.06 +/- 0.03, a value roughly a factor of two lower than the results of Dove & Shull (1994), where superbubbles were not considered. For a local production rate of ionizing photons Psi_LyC = 4.95 X 10^7 cm^{-2} s^{-1}, the flux escaping the disk is Phi_LyC approx (1.5-3.0) X 10^6 cm^{-2} s^{-1} for coeval and Gaussian star formation, comparable to the flux required to sustain the Reynolds layer.

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arXiv1999

Effects of Clumping on the Observed Properties of Dusty Galaxies

We present Monte Carlo radiative transfer simulations for spiral galaxies modelled as a stellar disk and a two-phase clumpy dust distribution. We divide the volume occupied by the dust into a three-dimensional grid and assign each cell a clump or smooth medium status. Cell dimension, clump dust mass and spatial distribution are derived from the observed properties of Giant Molecular Clouds and molecular gas in the Galaxy. We produce models for several values of the optical depth and fraction of the interstellar medium residing in clumps. As a general result, clumpy models are less opaque than the corresponding homogeneous models. For the adopted parameters, the increase in the fraction of energy that escapes the disk is moderate, resulting in surface brightness profiles that are less than one magnitude brighter than those of the homogeneous models. The effects of clumping are larger for edge-on views of the disk. This is in contrast with previous preliminary results for clumping in the literature. We show how differences arise from the different parametrisation and clump distribution adopted. We also consider models in which a fraction of the stellar radiation is emitted within the clumps. In this case, galaxies are less transparent than in the case when only dust is clumped. The opacity can be even higher than in the homogeneous case, depending on the fraction of embedded stellar emission. We point out the implications of the results for the determination of the opacity and dust mass of spiral galaxies.

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arXiv1999

Intergalactic H_2 Photodissociation and the Soft UV Background Produced by Population III Objects

We study the effects of the ionizing and dissociating photons produced by PopIII objects on the surrounding intergalactic medium. We find that the typical size of a H_2 photodissociated region, R_d ~ 1-5 kpc, is smaller than the mean distance between sources at z ~ 20-30, but larger than the ionized region by a factor depending on the detailed properties of the emission spectrum. This implies that clearing of intergalactic H_2 occurs before reionization of the universe is complete. In the same redshift range, the soft-UV background in the Lyman-Werner bands, when the intergalactic H and H_2 opacity is included, is found to be J_LW ~ 1d-30 - 1d-27 erg cm^{-2} s^{-1} Hz^{-1}. This value is well below the threshold required for the negative feedback of PopIII objects on the subsequent galaxy formation to be effective in that redshift range.

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arXiv1998

The impact of population III objects on the early universe

We study the effects of the ionizing and dissociating photons produced by Pop III objects on the surrounding intergalactic medium. We find that the typical size of a H_2 photodissociated region is smaller than the mean distance between sources at z=20-30, but larger than the ionized region. This implies that clearing of the intergalactic H_2 occurs before reionization of the universe is complete. In the same redshift range, the soft-UV background in the Lyman- Werner bands, when the intergalactic H and H_2 opacity is included, is found to be J_LW = 10^-28 - 10^-26 erg cm^-2 s^-1 Hz^-1. This value is well below the treshold required for the negative feedback of Pop III objects on the subsequent galaxy formation to be effective. We have combined these semi-analytical results with high-resolution N-body simulations, to study the topological structure of photoionization and photodissociation and the evolution of the H^+ and H_2 filling factor.

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arXiv1998

Magnetohydrodynamics of Cloud Collisions in a Multi-phase Interstellar Medium

We extend previous studies of the physics of interstellar cloud collisions by beginning investigation of the role of magnetic fields through 2D magnetohydrodynamic (MHD) numerical simulations. We study head-on collisions between equal mass, mildly supersonic diffuse clouds. We include a moderate magnetic field and two limiting field geometries, with the field lines parallel (aligned) and perpendicular (transverse) to the colliding cloud motion. We explore both adiabatic and radiative cases, as well as symmetric and asymmetric ones. We also compute collisions between clouds evolved through prior motion in the intercloud medium and compare with unevolved cases. We find that: In the (i) aligned case, adiabatic collisions, like their HD counterparts, are very disruptive, independent of the cloud symmetry. However, when radiative processes are taken into account, partial coalescence takes place even in the asymmetric case, unlike the HD calculations. In the (ii) transverse case, collisions between initially adjacent unevolved clouds are almost unaffected by magnetic fields. However, the interaction with the magnetized intercloud gas during the pre-collision evolution produces a region of very high magnetic energy in front of the cloud. In collisions between evolved clouds with transverse field geometry, this region acts like a ``bumper'', preventing direct contact between the clouds, and eventually reverses their motion. The ``elasticity'', defined as the ratio of the final to the initial kinetic energy of each cloud, is about 0.5-0.6 in the cases we considered. This behavior is found both in adiabatic and radiative cases.

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arXiv1998

Starburst-driven Mass Loss from Dwarf Galaxies: Efficiency and Metal Ejection

We model the effects of repeated supernova explosions from starbursts in dwarf galaxies on the interstellar medium of these galaxies, taking into account the gravitational potential of their dominant dark matter haloes. We explore supernova rates from one every 30,000 yr to one every 3 million yr, equivalent to steady mechanical luminosities of L=0.1-10 x 10^38 ergs/s, occurring in dwarf galaxies with gas masses M_g=10^6 - 10^9 solar masses. We address in detail, both analytically and numerically, the following three questions: 1. When do the supernova ejecta blow out of the disk of the galaxy? 2. When blowout occurs, what fraction of the interstellar gas is blown away, escaping the potential of the galactic halo? 3. What happens to the metals ejected from the massive stars of the starburst? Are they retained or blown away? We give quantitative results for when blowout will or will not occur in galaxies with 10^6 \leq M_g \leq 10^9 solar masses. Surprisingly, we find that the mass ejection efficiency is very low for galaxies with mass M_g \geq 10^7 solar masses. Only galaxies with M_g \leq 10^6 solar masses have their interstellar gas blown away, and then virtually independently of L. On the other hand, metals from the supernova ejecta are accelerated to velocities larger than the escape speed from the galaxy far more easily than the gas. We find that for L_38=1, only about 30% of the metals are retained by a 10^9 solar mass galaxy, and virtually none by smaller galaxies. We discuss the implications of our results for the evolution, metallicity and observational properties of dwarf galaxies.

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arXiv1998

Detecting Pop III Objects with NGST

We discuss some aspects concerning the formation and the impact of the first luminous structures in the universe (PopIIIs), with particular emphasis on their feedback effects on subsequent galaxy formation. We argue that supernovae in these objects might provide, if detected, important constraints for cosmological models. Some of these constraints come from the gravitational magnification of the supernova flux due to the intervening cosmological matter distribution which results in different predictions, for example, on rates, magnification probability, detection limits and number counts of distant supernovae. We show that NGST can tremendously contribute to such a study due to its advanced technological capabilities.

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arXiv1998

Pop III Objects and their Relationship with Dwarf Galaxies

In this paper we briefly review some of the most recent developments concerning the formation of the first luminous objects in the universe, to which we refer to as Pop III objects, and their observable effects. In particular, we try to critically discuss the possible connection between them and dwarf spheroidal galaxies. We come to the conclusion that, if the properties of Pop IIIs are correctly predicted by hierarchical cosmological models, their identification with present day dwarf galaxies presents several difficulties.

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arXiv1998

Gravitational Magnification of Pop III Supernovae in Hierarchical Cosmological Models: NGST Perspectives

We study the gravitational lensing magnification produced by the intervening cosmological matter distribution, as deduced from three different hierarchical models (SCDM, LCDM, CHDM) on very high redshift sources, particularly supernovae in protogalactic (Pop III) objects. By means of ray-shooting numerical simulations we find that caustics are more intense and concentrated in SCDM models. The magnification probability function presents a moderate degree of evolution up to $z\approx 5$ (CHDM) and $z\approx 7$ (SCDM/LCDM). All models predict that statistically large magnifications, $μ\simgt 20$ are achievable, with a probability of the order of a fraction of percent, the SCDM model being the most efficient magnifier. All cosmologies predict that above $z\approx 4$ there is a 10% chance to get magnifications larger than 3. We have explored the observational perspectives for Pop III SNe detection with NGST when gravitational magnification is taken into account. We find that NGST should be able to detect and confirm spectroscopically Type II SNe up to a redshift of $z \approx 4$ in the J band (for $T_{SN}=25000$ K); this limit could be increased up to $z\approx 9$ in the K band, allowing for a relatively moderate magnification. Possibly promising strategies to discriminate among cosmological models using their GL magnification predictions and very high-$z$ SNe are sketched. Finally, we outline and discuss the limitations of our study.

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arXiv1998

Hydrodynamics of Cloud Collisions in 2D: The Fate of Clouds in a Multi-phase Medium

We have studied head-on collisions between equal-mass, mildly supersonic (Mach number 1.5) HI clouds, in a standard Two-phase ISM (T_cl = 74 K, n_cl = 22 cm^-3, χ= 100). We explore the role of various factors, including the radiative cooling parameter η= t_rad/t_coll (t_coll=R_c/v_c), evolutionary modifications on the cloud structure (by colliding clouds ``evolved'' through independent motion within the intercloud medium (WIM)), and the symmetry of the problem (by colliding initially identical clouds, evolved to different ages before impact). The presence of bow shocks and ram pressure from material in the cloud wake, developed during such evolution through the WIM, significantly alters these interactions with respect to the standard case of non-evolved clouds. In general, in our adiabatic collisions the clouds are disrupted and convert their gas into a few low density contrast clumps. By contrast, for symmetric radiative cases we find that the two clouds coalesce, with almost all the initial kinetic energy radiated away. On the other hand, for both adiabatic and radiative collisions, asymmetric collisions have a much greater tendency to disrupt the two clouds. Fragmentation of the clouds may occur, and instabilities are in general enhanced. In addition, radiative cooling is less efficient in our asymmetric interactions, so that those parts of the clouds that initially seem to merge are more likely to re-expand and fade into the WIM. Since the majority of real cloud collisions should be asymmetric for one reason or another, we conclude that most gasdynamical diffuse cloud collisions will be disruptive, at least in the absence of significant self-gravity or of a significant magnetic field.

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arXiv1998

The Positive Feedback of Pop III Objects on Galaxy Formation

We study the formation of molecular hydrogen in cooling gas behind shocks produced during the blow-away process thought to occur in the first collapsed, luminous (Pop III) objects in the early universe. We find that for a wide range of physical parameters the $H_2$ fraction is $f \approx 6 \times 10^{-3}$. The $H_2$ mass produced in such explosions can exceed the amount of relic $H_2$ destroyed inside the photodissociation region surrounding a given Pop III. We conclude that these first objects, differently from the suggestion of Haiman et al 1997, might have a net positive feedback on subsequent galactic formation. We discuss the effects of radiation and the implications of our results for the soft-UV background.

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arXiv1998

The First Structures in the Universe: Pop III Objects

We review some of the possible observable effects (reionization, feedback on galaxy formation, supernovae and metal enrichment) of the first collapsed, luminous (Pop III) objects in the universe. We show that supernovae in these objects should be considerably magnified by the intervening cosmological matter distribution; the implications of this process are briefly discussed, anticipating some of the results of Marri & Ferrara (1998).

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arXiv1997

On the Dust Extinction in High-z Galaxies and the Case of Extremely Red Objects

We present the preliminary results of Monte Carlo simulations aimed to investigate the effects of realistic dust extinction (absorption + scattering) on the colours of high-z galaxies. In this paper, we concentrate on the case of spheroidal galaxies, and we obtain attenuation curves in the range 0.1-1.2$μ$m for different dust spatial distributions, and for a range of values of the dust optical depth, geometrical thickness and inclination of the dust disk. We find that the resultant curves are strongly dependent on the dust geometrical distribution and optical depth. A serendipitous finding is that the strength of the 2200 Åabsorption feature depends not only on the optical depth, but also on the dust geometrical distribution. As a first application, we test our results on two high-z galaxies with extremely red colours (HR10 and HR14) in order to infer clues on their ages, dust content and dust spatial distribution. We confirm that HR10 must be a very dusty galaxy, and we suggest that its stellar component should be strongly embedded in the dust in order to reproduce the observed extremely red colours.

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arXiv1997

Probing ISM Models with H-alpha observations

I review the capabilities of H$α$ observations to constrain some aspects of the current models of the interstellar medium. In particular, it is shown that turbulence is a necessary ingredient of any viable model, since most of the energy produced by supernova explosions and ionizing radiation is stored in kinetic form in the ISM. Various forms of turbulent energy dissipation, including cloud collisions, are analyzed. Two additional aspects, concerning the existence of galactic fountains and their relation with High Velocity Clouds, and the extended ionized layer of spiral galaxies are discussed; some crucial experiments are suggested.

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arXiv1997

Dust in Hot Environments: Giant Dusty Galactic Halos

I review some of the evidences for dust in the Local Bubble and in galactic halos and show that a general mechanism based on radiation pressure is capable of evacuating dust grains from regions dominated by massive star energy input and thus originate huge dusty halos. A Monte Carlo/particle model has been developed to study the dust dynamics above HII chimneys and the results, among other findings, show that dust can travel several kpc away from the plane of the parent galaxy. The cosmological implications of extragalactic dust are briefly outlined.

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arXiv1997

Ly-alpha Clouds Associated with Pop III Objects in Cold Dark Matter Models

We present a semi-analytical model to test the hypothesis that Ly-alpha QSO absorption lines originate in gaseous halos produced by multiple supernova explosions occurring in Pop III objects in a CDM cosmological scenario hoping to assess the validity of CDM models and/or constrain their parameters. The preliminary results indicate that the range of $N_{HI}$, redshift distribution and metallicity of clouds are well reproduced by CDM if they are associated with galaxy halos or groups. Firm conclusions on clouds with $N_{HI}\le 10^{14} cm^{-2}$ need a more refined study.

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arXiv1997

Energy Dissipation in Interstellar Cloud Collisions

We present a study of the kinetic energy dissipation in interstellar cloud collisions. The main aim is to understand the dependence of the elasticity (defined as the ratio of the final to the initial kinetic energy of the clouds) on the velocity and mass ratio of the colliding clouds, magnetic field strength, and gas metallicity for head-on collisions. The problem has been studied both analytically and via numerical simulations. We have derived handy analytical relationships that well approximate the analogous numerical results. The main findings of this work are: (i) the kinetic energy dissipation in cloud collisions is minimum (i.e. the collision elasticity is maximum) for a cloud relative velocity $v_r \simeq 30 km s^{-1}$; (ii) the above minimum value is proportional $Z L_c^2$, where $Z$ is the metallicity and $L_c$ is the cloud size: the larger is $Z L_c^2$ the more dissipative (i.e. inelastic) the collision will be; (iii) in general, we find that the energy dissipation decreases when the magnetic field strength, and mass ratio of the clouds are increased and the metallicity is decreased, respectively. We briefly discuss the relevance of this study to the global structure of the interstellar medium and to galaxy formation and evolution.

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arXiv1997

The survival of interstellar clouds against Kelvin-Helmholtz instabilities

We consider the stability of clouds surrounded by a hotter confining medium with respect to which they are in motion, against Kelvin-Helmholtz instabilities (KHI). In the presence of cooling, sound waves are damped by dissipation. Whenever cooling times are shorter than sound crossing times, as they are in the normal interstellar medium, this implies that the instability generated at the interface of the two media cannot propagate far from the interface itself. To study how this influences the overall stability, first we derive an analytic dispersion relation for cooling media, separated by a shear layer. The inclusion of dissipation does not heal the instability, but it is shown that only a small volume around the interface is affected, the perturbation decaying exponentially with distance from the surface; this is confirmed by numerical simulations. Numerical simulations of spherical clouds moving in a surrounding intercloud medium by which they are pressure confined show that these clouds develop a core/halo structure, with a turbulent halo, and a core in laminar flow nearly unscathed by the KHI. The related and previously reported ``champagne effect'', whereby clouds seem to explode from their top sides, is cured by the inclusion of radiative losses.

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