Clumpy Dust Distribution Research Articles

Analytic models of dust temperature in high-redshift galaxies

ABSTRACT We investigate physical reasons for high-dust temperatures (Tdust ≳ 40K) observed in some high-redshift ($z$ > 5) galaxies using analytic models. We consider two models that can be treated analytically: the radiative transfer (RT) model, where a broad distribution of values for Tdust is considered, and the one-tempearture (one-T) model, which assumes uniform Tdust. These two extremes serve to bracket the most realistic scenario. We adopt the Kennicutt–Schmidt (KS) law to relate stellar radiation field to gas surface density, and vary the dust-to-gas ratio. As a consequence, our model is capable of predicting the relation between the surface density of star formation rate (ΣSFR) or dust mass (Σdust) and Tdust. We show that the high Tdust observed at $z$ ≳ 5 favour low dust-to-gas ratios (≲ 10−3). An enhanced star formation compared with the KS law gives an alternative explanation for the high Tdust. The dust temperatures are similar between the two (RT and one-T) models as long as we use ALMA Bands 6–8. We also examine the relation among ΣSFR, Σdust, and Tdust without assuming the KS law, and confirm the consistency with the actual observational data at $z$ > 5. In the one-T model, we also examine a clumpy dust distribution, which predicts lower Tdust because of the leakage of stellar radiation. This enhances the requirement of low-dust abundance or high-star formation efficiency to explain the observed high Tdust.

Dust in Clusters of Galaxies

The presence of dust in the intracluster medium (ICM) has been a long-standing problem that is still awaiting elucidation. Direct observational diagnostics are rather challenging (though not impossible) either because of a sparse distribution of dust in the intracluster space that makes extinction measurements difficult or because of a low surface brightness of infrared emission from dust. Complex indirect approaches are currently available that can overcome uncertainties and provide a reasonable understanding of the basic regulations of the physical state of dust in the ICM. Contrary to the common opinion that the hot ICM does not allow dust to survive and manifest, many sparse observational data either directly point out that dust exists in the intracluster space or its presence is consistent with the data. Highly divergent data in direct evidence and highly uncertain indirect indicators are often connected either with dust fragility in a hot environment, the possible compactness of spatial (clumpy) dust distribution in the ICM, or dynamical features of dust transport. The source of dust is obviously connected with galaxies, and it turns out that in most cases, dust is carried from galaxies into the ICM while being thermally and dynamically shielded against the hostile influence of high-energy ions. In this review, we briefly discuss related issues from observational and theoretical points of view, including the transport of dust into the ICM, and the associated destructive and protective mechanisms and their characteristic time scales.

The effect of the streaming instability on protoplanetary disc emission at millimetre wavelengths

ABSTRACT In this paper, we investigate whether overdensity formation via streaming instability is consistent with recent multiwavelength Atacama Large Millimeter Array (ALMA) observations in the Lupus star-forming region. We simulate the local action of streaming instability in 2D using the code athena, and examine the radiative properties at mm wavelengths of the resulting clumpy dust distribution by focusing on two observable quantities: the optically thick fraction ff (in ALMA band 6) and the spectral index α (in bands 3–7). By comparing the simulated distribution in the ff–α plane before and after the action of streaming instability, we observe that clump formation causes ff to drop, because of the suppression of emission from grains that end up in optically thick clumps. α, instead, can either increase or decline after the action of streaming instability; we use a simple toy model to demonstrate that this behaviour depends on the sizes of the grains whose emission is suppressed by being incorporated in optically thick clumps. In particular, the sign of evolution of α depends on whether grains near the opacity maximum at a few tenths of a mm end up in clumps. By comparing the simulation distributions before/after clump formation to the data distribution, we note that the action of streaming instability drives simulations towards the area of the plane where the data are located. We furthermore demonstrate that this behaviour is replicated in integrated disc models provided that the instability is operative over a region of the disc that contributes significantly to the total mm flux.

Polarization measurements of the polluted white dwarf G29-38

ABSTRACT We have made high-precision polarimetric observations of the polluted white dwarf G29-38 with the HIgh Precision Polarimetric Instrument 2. The observations were made at two different observatories – using the 8.1-m Gemini North Telescope and the 3.9-m Anglo-Australian Telescope – and are consistent with each other. After allowing for a small amount of interstellar polarization, the intrinsic linear polarization of the system is found to be 275.3 ± 31.9 parts per million at a position angle of 90.8 ± 3.8° in the SDSS g′ band. We compare the observed polarization with the predictions of circumstellar disc models. The measured polarization is small in the context of the models we develop, which only allows us to place limits on disc inclination and Bond albedo for optically thin disc geometries. In this case, either the inclination is near-face-on or the albedo is small – likely in the range 0.05–0.15 – which is in line with other debris disc measurements. A preliminary search for the effects of G29-38’s pulsations in the polarization signal produced inconsistent results. This may be caused by beating effects, indicate a clumpy dust distribution, or be a consequence of measurement systematics.

Interpreting the evolution of galaxy colours fromz = 8 to 5

We attempt to interpret existing data on the evolution of the UV luminosity function and UV colours, $\beta$, of galaxies at $5 \leq z \leq 8$, to improve our understanding of their dust content and ISM properties. To this aim, we post-process the results of a cosmological hydrodynamical simulation with a chemical evolution model, which includes dust formation by supernovae and intermediate mass stars, dust destruction in supernova shocks, and grain growth by accretion of gas-phase elements in dense gas. We find that observations require a steep, Small Magellanic Cloud-like extinction curve and a clumpy dust distribution, where stellar populations younger than 15 Myr are still embedded in their dusty natal clouds. Investigating the scatter in the colour distribution and stellar mass, we find that the observed trends can be explained by the presence of two populations: younger, less massive galaxies where dust enrichment is mainly due to stellar sources, and massive, more chemically evolved ones, where efficient grain growth provides the dominant contribution to the total dust mass. Computing the IR-excess - UV color relation we find that all but the dustiest model galaxies follow a relation shallower than the Meurer et al. (1999) one, usually adopted to correct the observed UV luminosities of high-$z$ galaxies for the effects of dust extinction. As a result, their total star formation rates might have been over-estimated. Our study illustrates the importance to incorporate a proper treatment of dust in simulations of high-$z$ galaxies, and that massive, dusty, UV-faint galaxies might have already appeared at $z \lesssim 7$.

THE GRAY EXTINCTION OF THE IONIZING CLUSTER IN NGC 3603 FROM ULTRAVIOLET TO OPTICAL WAVELENGTHS

ABSTRACT We use photometry in the F220W, F250W, F330W, F435W filters from the High Resolution Channel of the Advanced Camera for Surveys and photometry in the F555W, F675W, and F814W filters from the Wide Field and Planetary Camera 2 aboard the Hubble Space Telescope to derive individual stellar reddenings and extinctions for stars in the HD 97950 cluster in the giant H ii region NGC 3603. The mean line of sight reddening for about a hundred main-sequence member stars inside the cluster is E ( F 435 W − F 555 W ) = 1.33 ± 0.12 ?> mag. After correcting for foreground reddening, the total to selective extinction ratio is R F 555 W = 3.75 ± 0.87 ?> in the cluster. Within the standard deviation associated with E ( λ − F 555 W ) / E ( F 435 W − F 555 W ) ?> in each filter, the cluster extinction curve at ultraviolet wavelengths tends to be grayer than the average Galactic extinction laws from Cardelli et al. and Fitzpatrick. It is closer to the extinction law derived by Calzetti et al. for starburst galaxies, where the 0.2175 μm bump is absent. This indicates an anomalous extinction in the HD 97950 cluster, which may due to the clumpy dust distribution within the cluster, and the size of dust grains being larger than the average Galactic ISM.

Extinction and dust properties in a clumpy medium

(abridged) The dust content of the universe is primarily explored via its interaction with stellar photons, producing interstellar extinction. However, owing to the physical extension of the observing beam, observations may detect scattered photons, resulting in a change in the observed (or effective) extinction, depending on the spatial distribution of the dust and the resolution of the instrument. We investigate the influence of clumpy dust distributions on effective extinction toward embedded sources and those in the diffuse ISM. We use Monte Carlo radiative transfer to examine effective extinction for various geometries. By varying the number, optical depth and volume-filling factor of clumps in models of spherical shells and the diffuse ISM, we explore the evolution of extinction. Depending on the number of scattering events in the beam, the extinction curve steepens in homogeneous media and flattens in clumpy media. As a result, clumpy dust distributions can to reproduce extinction curves with arbitrary R_v, the effective ratio of total-to-selective extinction. The flattening `washes out' the 2175 \AA bump and shifts the peak to shorter wavelengths. The mean R_v of a shell correlates with the optical depth of an individual clump and the wavelength at which a clump becomes optically thick. Similar behaviour is seen for edge-on discs or tori. However, at grazing inclinations the combination of extinction and strong forward scattering results in chaotic behaviour. Caution is therefore advised when measuring extinction in, for example, AGN tori or toward SNIa or GRB afterglows. In face-on discs, the shape of the scattered continuum changes with clumpiness, however, unlike absorption features, individual features in the scattering cross-sections are preserved. Finally, we show that diffuse interstellar extinction is not modified by scattering on scales of a few kpc.

The dust geometric distribution in Seyfert 1 and Seyfert 2 galaxies, isolated and in interaction

We analyse the mid-infrared (MIR) spectra of 39 Seyfert galaxies observed with the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope. Our sample consists of 19 Seyfert type 1 (Sy1) galaxies, three intermediate Seyfert galaxies and 17 type 2 Seyfert (Sy2) galaxies in the nearby Universe. This sample was extracted from a larger sample, the circumgalactic environment of which was studied in a previous work by Dultzin-Hacyan et al., who found that Sy2 objects are in interaction more frequently than normal galaxies, while Sy1s are not. In this article, we study the silicate dust distribution that dominates the mid-infrared (MIR) spectra. This dust produces spectral features at 10 and 18 µm that are sensitive to the clumpiness of the dust. We measure the strength of the emission or absorption of the silicate features to find whether there is a correlation between the clumpiness of the circumnuclear dust and the active galactic nucleus (AGN) type and incidence of companions. All isolated Sy1 galaxies have clumpy dust distributions, whereas Sy1s with a close companion can have either clumpy or smooth distributions. In the case of Sy2 galaxies, those with one or more companions have mostly smooth dust distributions, whereas isolated Sy2s may have clumpy or smoothly distributed dust. Our result is at odds with the simplest formulation of the unified scheme for Seyferts and supports an evolutionary sequence where the influence of an interaction triggers a type 2 AGN, which evolves into a type 1.

A Lyα EMITTER WITH AN EXTREMELY LARGE REST-FRAME EQUIVALENT WIDTH OF ∼900 Å ATz= 6.5: A CANDIDATE POPULATION III-DOMINATED GALAXY?

We have identified a very interesting Ly-alpha emitter, whose Ly-alpha emission line has an extremely large observed equivalent width of EW_0=436^{+422}_{-149}A, which corresponds to an extraordinarily large intrinsic rest-frame equivalent width of EW_0^{int}=872^{+844}_{-298}A after the average intergalactic absorption correction. The object was spectroscopically confirmed to be a real Ly-alpha emitter by its apparent asymmetric Ly-alpha line profile detected at z=6.538. The continuum emission of the object was definitely detected in our deep z'-band image; thus, its EW_0 was reliably determined. Follow-up deep near-infrared spectroscopy revealed emission lines of neither He II lambda1640 as an apparent signature of Population III, nor C IV lambda1549 as a proof of active nucleus. No detection of short-lived He II lambda1640 line is not necessarily inconsistent with the interpretation that the underlying stellar population of the object is dominated by Population III. We found that the observed extremely large EW_0 of the Ly-alpha emission and the upper limit on the EW_0 of the He II lambda1640 emission can be explained by population synthesis models favoring a very young age less than 2-4Myr and massive metal-poor (Z<10^{-5}) or even metal-free stars. The observed large EW_0 of Ly-alpha is hardly explained by Population I/II synthesis models with Z>10^{-3}. However, we cannot conclusively rule out the possibility that this object is composed of a normal stellar population with a clumpy dust distribution, which could enhance the Ly-alpha EW_0, though its significance is still unclear.

Smooth and clumpy dust distributions in AGN: a direct comparison of two commonly explored infrared emission models

The geometry of dust distribution within the inner regions of active galactic nuclei (AGN) is still a debated issue and relates directly to the AGN unified scheme. Traditionally, models discussed in the literature assume one of the two distinct dust distributions in what is believed to be a toroidal region around the supermassive black holes: a continuous distribution, customarily referred to as smooth, and a concentration of dust in clumps or clouds, referred to as clumpy. In this paper we perform a thorough comparison between two of the most popular models in the literature, namely the smooth models by Fritz et al. and the clumpy models by Nenkova et al., in their common parameter space. Particular attention is paid to the silicate features at ∼9.7 and ∼18 μm, the width of the infrared bump, the near-infrared index and the luminosity at 12.3 μm, all previously reported as possible diagnostic tools to distinguish between the two dust distributions. We find that, due to different dust chemical compositions used in the two models, the behaviour of the silicate features at 9.7 and 18 μm is quite distinct between the two models. The width of the infrared bump and the peak of the infrared emission can take comparable values; their distributions do, however, vary. The near-infrared index is also quite different, due partly to the primary sources adopted by the two models. Models with matched parameters do not produce similar spectral energy distributions (SEDs) and virtually no random parameter combinations can result in seemingly identical SEDs.

Herschel/SPIRE observations of the dusty disk of NGC 4244

We present Herschel/SPIRE images at 250, 350, and 500 {\mu}m of NGC 4244, a typical low-mass, disk-only and edge-on spiral galaxy. The dust disk is clumpy and shows signs of truncation at the break radius of the stellar disk. This disk coincides with the densest part of the Hi disk. We compare the Spectral Energy Distribution, including the new SPIRE fluxes, to 3D radiative transfer models; a smooth model disk and a clumpy model with embedded heating. Each model requires a very high value for the dust scale-length (h(dust) = 2 - 5 h(stars)), higher dust masses than previous models of NGC 4244 (Md = 0.47 - 1.39 \times 10e7 Msun) and a face-on optical depth of {\tau}(V) = 0.4 - 1.12, in agreement with previous disk opacity studies. The vertical scales of stars and dust are similar. The clumpy model much better mimics the general morphology in the submm images and the general SED. The inferred gas-to-dust mass ratio is compatible with those of similar low-mass disks. The relatively large radial scale-length of the dust disk points to radial mixing of the dusty ISM within the stellar disk. The large vertical dust scale and the clumpy dust distribution of our SED model are both consistent with a scenario in which the vertical structure of the ISM is dictated by the balance of turbulence and self-gravity.

Modelling the fraction of Lyman break galaxies with strong Lyman emission at

We present theoretical results for the expected fraction of Lyman Break Galaxies (LBGs) to be detected as strong Lyman alpha emitters (LAEs) in the redshift range 5 < z < 7. We base our analysis on the 2-billion particle SPH simulation MareNostrum High-z Universe. We approximate galaxies as static dusty slabs with an additional clumpy dust distribution affecting stellar populations younger than 25 Myr. The model for the Lyman alpha escape fraction is based on the results of our Monte-Carlo radiative transfer code (CLARA) for a slab configuration. We also fix the transmission of Lyman alpha photons through the intergalactic medium to a constant value of 50% at all redshifts. From the results of this model we calculate xLya, the fraction of Lyman Break Galaxies with Lyman alpha equivalent width (EW) larger than 50 Angstrom. We find a remarkable agreement with observational data at 4.5 < z < 6. For bright (-22 < MUV < -20.5) and faint (-20.5 < MUV < -18.5) galaxies our model predicts xLya = 0.02 \pm 0.01 and xLya = 0.47 \pm 0.01 while observers report xLya = 0.08 \pm 0.02 and xLya = 0.47 \pm 0.16, respectively. Additional evolution of the extinction model at redshift z \sim 7, that decreases the intensity of transmitted Lyman alpha radiation by a factor of f_T = 0.4 as to match the LAE luminosity function at z \sim 6.5, naturally provides a good match for the recently reported xLya fractions at z > 6.3. Exploring different toy models for the Lyman alpha escape fraction, we show that a decreasing Lyman alpha escape fraction with increasing UV galaxy luminosity is a key element in our model to explain the of larger xLya fractions for fainter LBGs.

The effects of dust on the optical and infrared evolution of SN 2004et

We present an analysis of multi-epoch observations of the Type II-P supernova SN 2004et. New and archival optical spectra of SN 2004et are used to study the evolution of the Halpha and [O I] 6300A line profiles between days 259 and 646. Mid-infrared imaging was carried out between 2004 to 2010. We include Spitzer `warm' mission photometry at 3.6 and 4.5um obtained on days 1779, 1931 and 2151, along with ground-based and HST optical and near-infrared observations obtained between days 79 and 1803. Multi-wavelength light curves are presented, as well as optical-infrared spectral energy distributions (SEDs) for multiple epochs. Starting from about day 300, the optical light curves provide evidence for an increasing amount of circumstellar extinction attributable to newly formed dust, with the additional extinction reaching 0.8-1.5 magnitudes in the V-band by day 690. The overall SEDs were fitted with multiple blackbody components, in order to investigate the luminosity evolution of the supernova, and then with Monte Carlo radiative transfer models using smooth or clumpy dust distributions, in order to estimate how much new dust condensed in the ejecta. The luminosity evolution was consistent with the decay of 56Co in the ejecta up until about day 690, after which an additional emission source is required, in agreement with the findings of Kotak et al. (2009). Clumped dust density distributions consisting of 20% amorphous carbons and 80% silicates by mass were able to match the observed optical and infrared SEDs, with dust masses that increased from 8x10^{-5} Msun on day 300 to 1.5x10^{-3} Msun on day 690, still significantly lower than the values needed for core collapse supernovae to make a significant contribution to the dust enrichment of galaxies.

Lyα EMITTERS IN HIERARCHICAL GALAXY FORMATION. II. ULTRAVIOLET CONTINUUM LUMINOSITY FUNCTION AND EQUIVALENT WIDTH DISTRIBUTION

We present theoretical predictions of UV continuum luminosity function (UV LF) and Lya equivalent width (EW) distribution of Lyman alpha emitters (LAEs) in the framework of the hierarchical clustering model of galaxy formation. The model parameters about LAEs were determined by fitting to the observed Lya LF at z=5.7 in our previous study, and the fit indicates that extinction of Lya photons by dust is significantly less effective than that of UV continuum photons, implying clumpy dust distribution in interstellar medium. We then compare the predictions about UV LFs and EW distributions with a variety of observations at z ~ 3-6, allowing no more free parameters and paying careful attention to the selection conditions of LAEs in each survey. We find that the predicted UV LFs and EW distributions are in nice agreement with observed data, and especially, our model naturally reproduces the existence of large EW LAEs (> 240 A) without introducing Pop III stars or top-heavy initial mass function. We show that both the stellar population (young age and low metallicity) and extinction by clumpy dust are the keys to reproduce large EW LAEs. The evidence of EW enhancement by clumpy dust is further strengthened by the quantitative agreement between our model and recent observations about a positive correlation between EW and extinction. The observed trend that brighter LAEs in UV continuum tend to have smaller mean EW is also reproduced, and the clumpy dust is playing an important role again for this trend. We suggested in our previous study that the transmission of intergalactic medium for Lya emission rapidly decreases from z ~ 6 to 7 by the fitting to Lya LFs, and this evidence is quantitatively strengthened by the comparison with the UV LF and EW distribution at z ~ 6.6.

Near-infrared reddening of extra-galactic giant molecular clouds in a face-on geometry

Aims. We describe the near-infrared reddening signature of giant molecular clouds (GMCs) in external galaxies. In particular, we examine the E J-H and $ E_{H-K}$ color excesses and the effective extinction law observed in discrete GMC regions. We also study the effect of the relative scale height of the GMC distribution to the color excesses, and to the observed mass function of GMCs when the masses are derived using color excess as a linear estimator of mass. Methods. We performed Monte Carlo radiative transfer simulations with 3D models of stellar radiation and clumpy dust distributions, resembling a face-on geometry. The scattered light is included in the models, and near-infrared color maps were calculated from the simulated data. We performed the simulations with different scale heights of GMCs and compared the color excesses and attenuation of light in different geometries. We extracted GMCs from the simulated color maps and compared the mass functions to the input mass functions. Results. The effective near-infrared reddening law, i.e. the ratio $E_{J-H}/E_{H-K}$, has a value close to unity in GMC regions. The ratio depends significantly on the relative scale height of GMCs, ξ , and for ξ values 0.1...0.75, we find the typical ratios of 0.6...1.1. The effective extinction law turns out to be very flat in GMC regions. We find the ratios of apparent extinctions of $A_{H}^{\rm a}/A_{K}^{\rm a}$ = 1.35...1.55 and $A_{J}^{\rm a}/A_{H}^{\rm a}$ = 1.15. The effect of the scattered flux on the effective reddening law, as well as on the effective extinction law, is significant. Regarding the GMC mass function, we find no correlation between the input and observed slopes of the mass functions. Instead, the observed slope reflects the parameter ξ and the dynamical range of the mass function. As the observed slope depends on the geometric parameters, which are not known, it is not possible to constrain the slope of the mass function using this technique. We estimate that only a fraction of 10...20% of the total mass of GMCs is recovered, if the observed color excess values are transformed to masses using the Galactic reddening law. In the case of individual clouds, the fraction can vary between ~0...50%.

Determination of the mass function of extra-galactic GMCs via NIR color maps

The giant molecular clouds (GMCs) of external galaxies can be mapped with sub-arcsecond resolution using multiband observations in the near-infrared. However, the interpretation of the observed reddening and attenuation of light, and their transformation into physical quantities, is greatly hampered by the effects arising from the unknown geometry and the scattering of light by dust particles. We examine the relation between the observed near-infrared reddening and the column density of the dust clouds. In this paper we particularly assess the feasibility of deriving the mass function of GMCs from near-infrared color excess data. We perform Monte Carlo radiative transfer simulations with 3D models of stellar radiation and clumpy dust distributions. We include the scattered light in the models and calculate near-infrared color maps from the simulated data. The color maps are compared with the true line-of-sight density distributions of the models. We extract clumps from the color maps and compare the observed mass function to the true mass function. For the physical configuration chosen in this study, essentially a face-on geometry, the observed mass function is a non-trivial function of the true mass function with a large number of parameters affecting its exact form. The dynamical range of the observed mass function is confined to ~103.5 105.5 regardless of the dynamical range of the true mass function. The color maps are more sensitive in detecting the high-mass end of the mass function, and on average the masses of clouds are underestimated by a factor of ∼10 depending on the parameters describing the dust distribution. A significant fraction of clouds is expected to remain undetected at all masses. The simulations show that the cloud mass function derived from JHK color excess data using simple foreground screening geometry cannot be regarded as a one-to-one tracer of the underlying mass function.

Distribution of extinction and star formation in NGC 1569

We investigate spatial the distribution of the intrinsic extinction in the starburst dwarf galaxy NGC1569 creating an extinction map of the whole galaxy derived from the Halpha/Hbeta emission line ratio. We differentiate the extinction in the HII regions from the extinction of the diffuse gas. The intrinsic extinction shows considerable variations over the plane of the galaxy, from negligible extinction up to highest values of A(V)=0.8mag. The extinction map shows small scale clumpy structures possibly due to a clumpy dust distribution. We also identify in this map a shell structure, for which we establish a causal relation with the expanding gas structure produced by the stellar winds coming from the Super Star Clusters (SSC) in the center of the galaxy. The comparison of the spatial profiles of the extinction, dust and gaseous emissions crossing the border of the shell shows a layered structure; the peak of this Halpha distribution lying closest to the SSC A, followed outwards by the peak of the extinction and at a still larger distances by the bulk of the atomic gas. We suggest that the extinction shell has been produced by the SSCs and that it can be explained by the accumulation of dust at the border of this ionized gas structure.

The Starburst Nature of Lyman Break Galaxies: Testing Ultraviolet Extinction with X-Rays

We derive the bolometric–to–X-ray correlation for a local sample of normal and starburst galaxies and use it, in combination with several UV reddening schemes, to predict the 2–8 keV X-ray luminosity for a sample of 24 Lyman break galaxies in the Hubble Deep Field and Chandra Deep Field North. We find that the mean X-ray luminosity, as predicted from the Meurer UV reddening relation for starburst galaxies, agrees extremely well with the Brandt stacking analysis. This provides additional evidence that Lyman break galaxies can be considered as scaled-up local starbursts, and that the locally derived starburst UV reddening relation may be a reasonable tool for estimating the UV extinction at high redshift. Our analysis shows that the Lyman break sample cannot have far-IR to far-UV flux ratios similar to nearby ultraluminous infrared galaxies, since this would predict a mean X-ray luminosity 100 times larger than observed, as well as far-IR luminosities large enough to be detected in the submillimeter. We calculate the UV reddening expected from the Calzetti effective starburst attenuation curve and the radiative transfer models of Witt & Gordon for low-metallicity dust in a shell geometry with homogeneous or clumpy dust distributions and find that all are consistent with the observed X-ray emission. Finally, we show that the mean X-ray luminosity of the sample would be underpredicted by a factor of 6 if the far-UV is unattenuated by dust.

Seeing Galaxies through Thick and Thin. I. Optical Opacity Measures in Overlapping Galaxies

We describe the use of partially overlapping galaxies to provide direct measurements of the effective absorption in galaxy disks, independent of assumptions about internal disk structure. The nonoverlapping parts of the galaxies and symmetry considerations are used to reconstruct, via differential photometry, how much background galaxy light is lost in passing through the foreground disks. Extensive catalog searches and follow-up imaging yield ~15-25 nearby galaxy pairs suitable for varying degrees of our analysis; 11 of the best such examples are presented here. From these pairs, we find that interarm extinction is modest, declining from AB ~ 1 mag at 0.3R to essentially zero by R; the interarm dust has a scale length consistent with that of the disk starlight. In contrast, dust in spiral arms and resonance rings may be optically thick (AB > 2) at virtually any radius. Some disks have flatter extinction curves than the Galaxy, with AB/AI ≈ 1.6; this is probably the signature of clumpy dust distributions. Even though typical spirals are not optically thick throughout their disks, where they are optically thick is correlated with where they are most luminous: in spiral arms and inner disks. This correlation between absorption and emission regions may account for their apparent surface brightness being only mildly dependent on inclination, erroneously indicating that spirals are generally optically thick. Taken as an ensemble, the opacities of spiral galaxies may be just great enough to significantly affect QSO counts, though not enough to cause their high-redshift cutoff.

Effects of clumping on the observed properties of dusty galaxies

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.