arXiv subjects

Simone Bianchi

Publications and source records attributed to Simone Bianchi.

70 records · Page 4Linked to original sources

arXiv2010

A decline and fall in the future of Italian Astronomy?

On May 27th 2010, the Italian astronomical community learned with concern that the National Institute for Astrophysics (INAF) was going to be suppressed, and that its employees were going to be transferred to the National Research Council (CNR). It was not clear if this applied to all employees (i.e. also to researchers hired on short-term contracts), and how this was going to happen in practice. In this letter, we give a brief historical overview of INAF and present a short chronicle of the few eventful days that followed. Starting from this example, we then comment on the current situation and prospects of astronomical research in Italy.

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arXiv2009

Stellar sources of dust in the high redshift Universe

With the aim of investigating whether stellar sources can account for the >10^8 Msun dust masses inferred from mm/sub-mm observations of samples of 5<z<6.4 quasars,we develop a chemical evolution model which follows the evolution of metals and dust on the stellar characteristic lifetimes, taking into account dust destruction mechanisms.Using a grid of stellar dust yields as a function of the initial mass and metallicity over the range 1-40 Msun and 0-1 Zsun,we show that the role of AGB stars in cosmic dust evolution at high redshift might have been over-looked.We apply the chemical evolution model with dust to the host galaxy of the most distant quasar at z=6.4, SDSS J1148+5251.Given the current uncertainties on the star formation history of the host galaxy, we have considered two models: (i) a star formation history obtained in a numerical simulation by Li et al.(2007) which predicts that a large stellar bulge is already formed at z=6.4,and (ii) a constant star formation rate of 1000 Msun/yr, as suggested by the observations if most of the FIR luminosity is due to young stars.The total mass of dust predicted at z=6.4 by the first model is 2x10^8Msun,within the range of values inferred by observations,with a substantial contribution (80%) of AGB-dust.When a constant star formation rate is adopted,the contribution of AGB-dust decreases to 50% but the total mass of dust formed is a factor 2 smaller.Both models predict a rapid enrichment of the ISM with metals and a relatively mild evolution of the carbon abundance,in agreement with observational constraints. This supports the idea that stellar sources can account for the dust observed but show that the contribution of AGB stars to dust production cannot be neglected, even at the most extreme redshifts currently accessible to observations.

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arXiv2008

Dust Extinction and Emission in a Clumpy Galactic Disk. An Application of the Radiative Transfer Code TRADING

AIMS: I present the Monte Carlo radiative transfer code TRADING (Transfer of RAdiation through Dust In Galaxies). The code computes self-consistently the extinction of radiation in a dusty medium (including absorption and scattering) and the dust emission. METHODS: A binary-tree adaptive grid is used for the description of the dust distribution. Dust radiation is computed at thermal equilibrium or under stochastic heating condition, for a distribution of grains of different radii and materials. The code is applied to the case of a clumpy galactic disk, including both diffuse dust and a distribution of spherical clouds modelled on the GMCs of local galaxies. Diffuse and localised sources of starlight are used, with independent spectra. RESULTS: A model is provided for the edge-on galaxy NGC 891. The SED of the galaxy from the UV to the submm/mm range can be well reproduced by: a bulge/disk configuration of old stars together with an extended dust disk, as suggested by the analysis of optical/near-infrared images; a clumpy dust distribution of the same mass as the diffuse dust disk, together with a UV emitting component, half of which in the form of a diffuse disk and half in sources embedded in clouds. In total, it is found that about 35% of the bolometric radiation is absorbed (and emitted) by dust; and that absorption of starlight from the old population contributes to about 60% of the dust emission. A significant component of the dust emission from clouds is due to absorption of diffuse radiation. Radial profiles of dust emission in a clumpy disk are almost independent of the wavelength, with the exception of the wavelength range on the Wien side of the thermal equilibrium peak.

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arXiv2007

The dust distribution in edge-on galaxies. Radiative transfer fits of V and K'-band images

Aims: I have analyzed a sample of seven nearby edge-on galaxies observed in the V and K'-band, in order to infer the properties of the dust distribution. Methods: A radiative transfer model, including scattering, have been used to decompose each image into a stellar disk, a bulge, and a dust disk. The parameters describing the distributions have been obtained through standard X^2 minimization techniques. Results: The dust disks fitted to the V-band images are consistent with previous work in literature: the radial scalelength of dust is larger than that for stars (h_d/h_s ~ 1.5); the dust disk has a smaller vertical scalelength than the stellar (z_d/z_s ~ 1/3); the dust disk is almost transparent when seen face-on (central, face-on, optical depth tau_0 =0.5-1.5). Faster radiative transfer models which neglect scattering can produce equivalent fits, with changes in the derived parameters within the accuracy of full fits including scattering. In the K'-band, no trace is found of a second, massive, dust disk which has been invoked to explain observations of dust emission in the submillimeter. I discuss the model degeneracies and the effect of complex structures on the fitted distributions. In particular, most bulges in the sample show a box/peanuts morphology with large residuals; two lower-inclination galaxies show a dust ring distribution, which could be the cause for the large fitted dust scalelengths.

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arXiv2005

The optical-to-radio spectral energy distributions of low-metallicity blue compact dwarf galaxies

We present global spectral energy distributions (SEDs) from 0.3$μ$m to 90cm for a sample of seven low-metallicity blue compact dwarf galaxies (BCDs). In addition to data compiled from the literature, we report new SCUBA data for the galaxies in the sample, including the two most metal-poor star-forming galaxies known, IZw18 and SBS0335-052. The standard starburst templates, M82 and Arp220, do not give a good approximation to any of the BCD SEDs in our sample. Several SEDs are instead characterized by: (i) approximately flat radio spectra suggesting dominant thermal processes; (ii) far-infrared (FIR) spectra which peak at or shortward of 60 $μ$m; (iii) a significant warm dust component and absence of Aromatic Features in Emission (or PAHs) in the mid-infrared (MIR). The SEDs of low-metallicity BCDs do not follow ``canonical'' scaling relations, and the ``standard'' star-formation indicators based on radio continuum, FIR, and MIR luminosities can be discrepant by factors of $\simgt10$. We present new models of the dust SEDs from 5$μ$m to 1cm, and derive infrared luminosities, dust distributions, temperatures, and masses. The observed SEDs and dust models are interpreted in terms of the active/passive regimes of star formation and trends with metallicity. Finally, we discuss the implications of our results on the $z\simgt6$ starburst populations which will be detected by forthcoming submm and radio facilities.

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arXiv2005

IGM metal enrichment through dust sputtering

We study the motion of dust grains into the Intergalactic Medium (IGM) around redshift z=3, to test the hypothesis that grains can efficiently pollute the gas with metals through sputtering. We use the results available in the literature for radiation-driven dust ejection from galaxies as initial conditions, and follow the motion onward. Via this mechanism, grains are ejected into the IGM with velocities >100 km/s; as they move supersonically, grains can be efficiently eroded by non-thermal sputtering. However, Coulomb and collisional drag forces effectively reduce the charged grain velocity. Up-to-date sputtering yields for graphite and silicate (olivine) grains have been derived using the code TRIM, for which we provide analytic fits. After training our method on a homogeneous density case, we analyze the grain motion and sputtering in the IGM density field as derived from a LambdaCDM cosmological simulation at z = 3.27. We found that only large (a >~ 0.1-um) grains can travel up to considerable distances (few times 100 kpc physical) before being stopped. Resulting metallicities show a well defined trend with overdensity delta. The maximum metallicities are reached for 10<delta<100 (corresponding to systems, in QSO absorption spectra, with 14.5 < log N_HI < 16). However the distribution of sputtered metals is very inhomogeneous, with only a small fraction of the IGM volume polluted by dust sputtering (filling factors of 18 per cent for Si and 6 per cent for C). For the adopted size distribution, grains are never completely destroyed; nevertheless, the extinction and gas photo-electric heating effects due to this population of intergalactic grains are well below current detection limits.

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arXiv2003

Ultraviolet Background Radiation from Cosmic Structure Formation

We calculate the contribution to the ultraviolet background (UVB) from thermal emission from gas shock heated by cosmic structure formation. Our main calculation is based on an updated version of Press-Schechter theory. It is consistent with a more empirical estimate based on the observed properties of galaxies and the observed cosmic star formation history. Thermal UVB emission is characterized by a hard spectrum extending well beyond 4 Ry. The bulk of the radiation is produced by objects in the mass range 10^11-13 M_solar, i.e. large galaxies and small groups. We compute a composite UVB spectrum due to QSO, stellar and thermal components. The ratio of the UVB intensities at the H and He Lyman limits increases from 60 at z=2 to more than 300 at z=6. A comparison of the resulting photoionization rates to the observed Gunn-Peterson effect at high redshifts constrains the escape fraction of ionizing photons from galaxies to be less than a few percent. Near 1 Ry, thermal and stellar emission are comparable amounting to about 10 %, 20 % and 35 % of the total flux at redshifts of 3, 4.5 and higher, respectively. However, near the ionization threshold for HeII, the thermal contribution is much stronger. It is comparable to the QSO intensity already at redshift ~3 and dominates at redshifts above 4. Thermal photons alone are enough to produce and sustain HeII reionization already at z~6. We discuss the possible implications of our results for the thermal history of the intergalactic medium, in particular for HeII reionization.

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arXiv2003

Radiative Transfer in Spiral Galaxies

The internal dust extinction in spiral galaxies can affect our understanding of their structure and morphology, as well as our perception of the distant universe in the background. The intrinsic properties of the stellar and dust components can be studied by comparing the observed appearance of a dusty spiral in the optical with a radiative transfer model. The absorbed starlight is re-radiated by dust in the FIR/sub-mm. Thus, observations at long wavelengths allow to put further constraints on the properties of the dust distribution. I present our Monte Carlo simulations for dust extinction and emission in galactic disks, discuss the basic ingredients and limitations and compare them with other solutions for the radiative transfer available in the literature. I review the current picture of dusty disks that emerges from radiative transfer studies.

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arXiv2001

The contribution of galaxies to the UV ionising background and the evolution of the Lyman forest

We have modelled the evolution of the number of Ly_alpha absorbers with redshift, resulting from the evolution of the ionising background and the Hubble expansion. The contribution of quasars (QSOs) and galaxies to the HI-ionising UV background has been estimated. The QSOs emissivity is derived from recent fits of their luminosity function. The galaxy emissivity is computed using a stellar population synthesis model, with a star-formation history scaled on observations of faint galaxies at lambda <= 1500 A. We allow for three values of the fraction of ionising photons that can escape the interstellar medium, f_esc=0.05, 0.1 and 0.4. The Intergalactic Medium is modelled as made of purely-absorbing clouds with the distribution in redshift and column density obtained from QSOs absorption lines. For the adopted values of f_esc, the contribution of galaxies to the ionising UV background is comparable or greater than that of QSOs. Accounting for the contribution of clouds to the UV emission, all models with f_esc <~ 0.1 provide an ionising flux compatible with local and high-z determination, including those with a pure QSOs background. The observed z ~ 1 break in the evolution can be better explained by a dominant contribution from galaxies. We find that models in Lambda-cosmology with Omega_matter=0.3, Omega_Lambda=0.7 describe the flat absorbers evolution for z <~ 1.0 better than models for Omega_matter=1.0.

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arXiv1999

Modelling the Far-Infrared emission in spyral galaxies

This thesis presents an original model for the FIR emission in spirals, developed from an existing radiative transfer code (Bianchi, Ferrara & Giovanardi, 1996, ApJ, 465, 127). The model's main features are: a complete treatment of multiple scattering within geometries appropriate for spirals; a full coverage of spectral range of stellar emission; the use of empirically determined dust properties (some of which are derived in the present work); the production of maps of the dust temperature distribution, together with simulated optical, FIR and sub-millimetre images. The model has been applied to observations of stellar and dust emission in the galaxy NGC 6946. It is found that optically thick models (central face-on optrical depth tau_V \~ 5) are necessary to explain the observed FIR output. For such models, almost 30-40% of the intrinsic starlight is absorbed. The observed ratio of FIR and optical scalelengths can be explained if the dust distribution is more extended than the stellar. However, because of the less steep gradients of optical emission in optically thick cases, a very extended dust distribution is needed (alpha_d ~ 3 alpha_s). The distribution of atomic gas in NGC 6946 has a similar extent. I discuss the approximations in the modelling (mainly the use of smooth distributions against the observed clumpiness of the interstellar medium) and the implications of the results.

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arXiv1999

ISO observations of spirals: modelling the FIR emission

ISO observations at 200 micron have modified our view of the dust component in spiral galaxies. For a sample of seven resolved spirals we have retrieved a mean temperature of 20K, about 10K lower than previous estimates based on IRAS data at shorter wavelengths. Because of the steep dependence of far-infrared emission on the dust temperature, the dust masses inferred from ISO fluxes are a factor of 10 higher than those derived from IRAS data only, leading to gas-to-dust ratios close to the value observed in the Galaxy. The scale-length of the 200 micron emission is larger than for the IRAS 100 micron emission, with colder dust at larger distances from the galactic centre, as expected if the interstellar radiation field is the main source of dust heating. The 200 micron scale-length is also larger than the optical, for all the galaxies in the sample. This suggests that the dust distribution is more extended than that of the stars.A model of the dust heating is needed to derive the parameters of the dust distribution from the FIR emission. Therefore, we have adapted an existing radiative transfer code to deal with dust emission. Simulated maps of the temperature distribution within the dust disk and of the dust emission at any wavelength can be produced. The stellar spectral energy distribution is derived from observations in the ultraviolet, optical and near infrared. The parameters of the dust distribution (scale-lengths and optical depth) are chosen to reproduce the observed characteristics of the FIR emission, i.e. the shape of the spectrum, the flux and the spatial distribution. We describe the application of the model to one of the galaxies in the sample, NGC 6946.

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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

Dust Emissivity in the Far-Infrared

We have derived the dust emissivity in the Far-Infrared (FIR) using data available in the literature. We use two wavelength dependences derived from spectra of Galactic FIR emission (Reach et al. 1995). A value for the emissivity, normalised to the extinction efficiency in the V band, has been retrieved from maps of Galactic FIR emission, dust temperature and extinction (Schlegel et al. 1998). Our results are similar to other measurements in the Galaxy but only marginally consistent with the widely quoted values of Hildebrand (1983) derived on one reflection nebula. The discrepancy with measurements on other reflection nebulae (Casey 1991) is higher and suggests a different grain composition in these environments with respect to the diffuse interstellar medium. We measure dust masses for a sample of six spiral galaxies with FIR observations and obtain gas-to-dust ratios close to the Galactic value.

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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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arXiv1996

The Effect of Light Scattering by Dust in Galactic Halos on Emission Line Ratios

We present results from Monte Carlo simulations describing the radiation transfer of $Hα$ line emission, produced both by HII regions in the disk and in the diffuse ionized gas (DIG), through the dust layer of the galaxy NGC891. This allows us to calculate the amount of light originating in the HII regions of the disk and scattered by dust at high $z$, and compare it with the emission produced by recombinations in the DIG. The cuts of photometric and polarimetric maps along the $z$-axis show that scattered light from HII regions is still 10\% of that of the DIG at $z\sim 600$~pc, whereas the the degree of linear polarization is small ($<1$\%). The importance of these results for the determination of intrinsic emission line ratios is emphasized, and the significance and possible implications of dust at high $z$ are discussed.

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arXiv1995

Monte Carlo simulations of dusty spiral galaxies: extinction and polarization properties

We present Monte Carlo simulations of dusty spiral galaxies, modelled as bulge + disk systems, aimed to study their extinction and polarization properties. The extinction parameters (absorption and scattering) of dust grains are calculated from Mie's theory for a full distribution of sizes and materials; the radiation transfer is carried on for the four Stokes parameters. Photometric and polarimetric maps of galaxies of different optical depths, inclinations and bulge-to-total ratios have been produced in the B and I bandpasses. As expected, the effect of scattering is to reduce substantially the extinction for a given optical depth, in particular for what concerns the obscuration of bright bulge cores. For the same reason, scattering reduces also the reddening, as evaluated from B-I maps. On the other hand the bluing directly due to forward scattering is hardly appreciable. Radial color gradients are often found. A comparison with ``sandwich'' models shows that they fail dramatically to reproduce the extinction - optical depth relation. The degree of linear polarization produced by scattering is usually of the order of a few percent; it increases with optical depth, and with inclination (less than 80 degrees). The polarization pattern is always perpendicular to the major axis, unless the dust distribution is drastically modified. There is little local correlation between extinction and polarization degree and there is a trend of increasing polarization from the B to the I band. We discuss implications and relevance of the results for studies of the structure and morphology of spiral galaxies and of their interstellar medium.

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