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.Comment: A&A accepted, 22 pages, including 13 pages of "Online Material", 16 figure

Vindicating single-T modified blackbody fits to Herschel SEDs (Research Note)

I show here that the bulk of the dust mass in a galaxy can be equivalently estimated from: i) the full spectral energy distribution of dust emission, using the approach of Draine & Lee (2007) that includes a distribution of dust grains and a range of interstellar radiation field intensities; ii) the emission in the wavelength range 100um <= lambda <= 500um (covered by the Herschel Space Observatory), by fitting to the data a simpler single temperature modified blackbody. Recent claims on the contrary (Dale et al. 2012) should be interpreted as a caveat to use in the simpler fits an absorption cross section which is consistent both in the normalization and in the spectral index beta with that of the full dust model. I also show that the dust mass does not depend significantly on the choice of beta, if both the dust mass and the absorption cross section are derived with the same assumption on beta.Comment: 5 pages, 2 figures, accepted for publication on A&

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.Comment: 43 pages, 13 Postscript figures, Latex AAS manuscript substyle, Ap. J. Accepte

Where was Mean Solar Time first adopted?

It is usually stated in the literature that Geneva was the first city to adopt mean solar time, in 1780, followed by London (or the whole of England) in 1792, Berlin in 1810 and Paris in 1816. In this short paper I will partially revise this statement, using primary references when available, and provide dates for a few other European cities. Although no exact date was found for the first public use of mean time, the primacy seems to belong to England, followed by Geneva in 1778-1779 (for horologists), Berlin in 1810, Geneva in 1821 (for public clocks), Vienna in 1823, Paris in 1826, Rome in 1847, Turin in 1849, and Milan, Bologna and Florence in 1860.Comment: 5 pages. Text changed to match the published versio

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.Comment: 5 pages, 1 ps file, A&A letter accepte

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.Comment: 11 pages, 8 figures, MNRAS accepted, also available at http://www.arcetri.astro.it/~sbianchi/clumping.htm

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.Comment: 6 pages, 5 figures. Contribution to the proceedings of the workshop "ISO Beyond Point Sources" held at VILSPA 14-17 September 199

Bidirectional deep-readout echo state networks

We propose a deep architecture for the classification of multivariate time series. By means of a recurrent and untrained reservoir we generate a vectorial representation that embeds temporal relationships in the data. To improve the memorization capability, we implement a bidirectional reservoir, whose last state captures also past dependencies in the input. We apply dimensionality reduction to the final reservoir states to obtain compressed fixed size representations of the time series. These are subsequently fed into a deep feedforward network trained to perform the final classification. We test our architecture on benchmark datasets and on a real-world use-case of blood samples classification. Results show that our method performs better than a standard echo state network and, at the same time, achieves results comparable to a fully-trained recurrent network, but with a faster training