S. Bianchi

An Atlas of Monte Carlo Models of Dust Extinction in Galaxies for Cosmological Applications

We present an extensive study of the radiative transfer in dusty galaxies based on Monte Carlo simulations. The main output of these simulations are the attenuation curves

DEEP SUBMILLIMETER IMAGES OF NGC 891—COLD DUST AT LARGER GALACTIC RADII

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Effects of clumping on the observed properties of dusty galaxies

(i.e. the ratio between the observed, dust extinguished, total intensity to the intrinsic unextinguished one of the galaxy as a function of wavelength). We have explored the dependence of

The Low Surface Brightness Extent of the Fornax Cluster

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ISO Long Wavelength Spectrograph Observations of Cold Dust in Galaxies

on a conspicuous set of quantities (Hubble type, inclination, dust optical thickness, dust distribution and extinction properties) for a large wavelength interval, ranging from 1250\AA to the K band, thus finally providing a comprehensive atlas of dust extinction in galaxies, which is electronically available. This study is particularly suitable for inclusion into galaxy formation evolution models and to directly interpret observational data on high redshift galaxies.

SCUBA imaging of the NGC 7331 dust ring

We present images in the 450 and 850 μm continuum of the edge-on galaxy NGC 891. These measurements, carried out with the recently commissioned Submillimeter Common-User Bolometer Array, provide the deepest images yet of a nearby galaxy in the submillimeter wave band. We detect dust emission from 2/3 of the optical disk and confirm the presence of dust chimneys escaping from the main absorption layer up to z-heights of nearly 2 kpc. A comparison between the submillimeter surface brightness along the major axis with that corresponding to the IRAS 60 and 100 μm filters implies that large amounts of cold dust (~15 K) are present in the disk (in fact, an order of magnitude more grain material than the warm dust detected by IRAS). These cold grains predominate at larger galactic radii.

Deep submillimeter images of NGC 7331; dust at the periphery of spiral disks

We present Monte Carlo radiative-transfer simulations for spiral galaxies modelled as a stellar disc 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 disc 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 disc. This is in contrast with previous preliminary results for clumping in the literature. We show how differences arise from the different parametrization 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.

On the dust extinction in high-z galaxies and the case of extremely red objects

We have used a large format CCD camera to survey the nearby Fornax cluster and its immediate environment for low-luminosity low surface brightness galaxies. Recent observations indicate that these are the most dark matter–dominated galaxies known, and so they are likely to be a good tracer of the dark matter in clusters. We have identified large numbers of these galaxies consistent with a steep faint-end slope of the luminosity function (α ≈ -2) down to MB ≈ -12. These galaxies contribute almost the same amount to the total cluster light as the brighter galaxies, and they have a spatial extent that is some 4 times larger. They satisfy two of the important predictions of N-body hierarchical simulations of structure formation using dark halos. The luminosity (mass?) function is steep, and the mass distribution is more extended than that defined by the brighter galaxies. We also find a large concentration of low surface brightness galaxies around the nearby galaxy NGC 1291.

Mapping the submillimeter spiral wave in NGC 6946

We describe observations of five nearby galaxies obtained using the Long Wavelength Spectrograph (LWS) on the Infrared Space Observatory (ISO). We observed five galaxies, using spectrograph apertures positioned at the galactic center and in the outskirts of the disk, to compare the spectral energy distribution of the emitting dust at different positions. The central spectra are typical of those inferred previously from IRAS data; peaking at about 100 μm with estimated dust temperatures of T(d) = 30-35 K. However, there is a rapid change in the spectral energy distribution with increasing galactocentric distance. In the outer regions the spectra are flat, or still rising, out to 197μm, indicating a predominantly cold dust component with T(d) < 20 K. In the central regions, the inferred cold dust component is 2-10 times more massive than the warm dust, and it increasingly dominates the mass and spectral energy distribution in the outer regions. We discuss reasons for believing that emission beyond ≈150 μm in disk galaxies is associated with a separate component of dust with an extended distribution that may be associated with a possible molecular halo.

Cold dust in nearby galaxies: implications for observing the high-redshift universe

We present observations of the spiral galaxy NGC 7331 using the Submillimetre Common User Bolometer Array (SCUBA) on the James Clark Maxwell Telescope. We have detected a dust ring of 45 arcsec radius (3.3 kpc) at wavelengths of 450 and 850 μm. The dust ring is in good correspondence with other observations of the ring in the mid-infrared (MIR), CO and radio continuum, suggesting that the observed dust is associated with molecular gas and star formation. A B − K colour map shows an analogous ring structure with an asymmetry about the major axis, consistent with the extinction being produced by a dust ring. The derived temperature of the dust lies between 16 and 31 K and the gas-to-dust ratio lies between 150 and 570, depending on the assumed dust emission efficiency index (β = 1.5 or 2).