Face-on Optical Depth Research Articles

Radiative transfer in disc galaxies – V. The accuracy of the

We investigate the accuracy of an approximate radiative transfer technique that was first proposed by Kylafis & Bahcall (hereafter the KB approximation) and has been popular in modelling dusty late-type galaxies. We compare realistic galaxy models calculated with the KB approximation with those of a three-dimensional Monte Carlo radiative transfer code SKIRT. The SKIRT code fully takes into account of the contribution of multiple scattering whereas the KB approximation calculates only single scattered intensity and multiple scattering components are approximated. We find that the KB approximation gives fairly accurate results if optically thin, face-on galaxies are considered. However, for highly inclined ($i \gtrsim 85^{\circ}$) and/or optically thick (central face-on optical depth $\gtrsim1$) galaxy models, the approximation can give rise to substantial errors, sometimes, up to $\gtrsim 40\%$. Moreover, it is also found that the KB approximation is not always physical, sometimes producing infinite intensities at lines of sight with high optical depth in edge-on galaxy models. There is no "simple recipe" to correct the errors of the KB approximation that is universally applicable to any galaxy models. Therefore, it is recommended that the full radiative transfer calculation be used, even though it's slower than the KB approximation.

ULTRAVIOLET RADIATIVE TRANSFER MODELING OF NEARBY GALAXIES WITH EXTRAPLANAR DUSTS

In order to examine their relation to the host galaxy, the extraplanar dust of six nearby galaxies are modeled, employing a three dimensional Monte Carlo radiative transfer code. The targets are from the highly-inclined galaxies that show dust-scattered ultraviolet halos, and the archival Galaxy Evolution Explorer FUV band images were fitted with the model. The observed images are in general well reproduced by two dust layers and one light-source layer, whose vertical and radial distributions have exponential profiles. We obtained several important physical parameters, such as star formation rate (SFR_UV), face-on optical depth, and scale-heights. Three galaxies (NGC 891, NGC 3628, and UGC 11794) show clear evidence for the existence of extraplanar dust layer. However, it is found that the rest three targets (IC 5249, NGC 24, and NGC 4173) do not necessarily need a thick dust disk to model the ultraviolet (UV) halo, because its contribution is too small and the UV halo may be caused by the wing part of the GALEX point spread function. This indicates that the galaxy samples reported to have UV halos may be contaminated by galaxies with negligible extraplanar (halo) dust. The galaxies showing evidence of the extraplanar dust layer fall within a narrow range on the scatter plots between physical parameters such as SFR_UV and extraplanar dust mass. Several mechanisms possible to produce the extraplanar dust are discussed. We also found a hint that the extraplanar dust scale-height might not be much different from the polycyclic aromatic hydrocarbon emission characteristic height.

THE PSEUDO-ZODI PROBLEM FOR EDGE-ON PLANETARY SYSTEMS

Future direct observations of extrasolar Earth-sized planets in the habitable zone could be hampered by a worrisome source of noise, starlight-reflecting exozodiacal dust. Mid-infrared surveys are currently underway to constrain the amount of exozodiacal dust in the habitable zones around nearby stars. However, at visible wavelengths another source of dust, invisible to these surveys, may dominate over exozodiacal dust. For systems observed near edge-on, a cloud of dust with face-on optical depth 10^-7 beyond ~5 AU can mimic the surface brightness of a cloud of exozodiacal dust with equal optical depth if the dust grains are sufficiently forward-scattering. We posit that dust migrating inward from cold debris belts via Poynting-Robertson drag could produce this "pseudo-zodiacal" effect, potentially making it ~50% as common as exozodiacal clouds. We place constraints on the disk radii and scattering phase function required to produce the effect.

MONTE-CARLO RADIATIVE TRANSFER MODEL OF THE DIFFUSE GALACTIC LIGHT

Monte-Carlo radiative models of the diffuse Galactic light (DGL) in our Galaxy are calculated using the dust radiative transfer code MoCafe, which is three-dimensional and takes full account of multiple scattering. The code is recently updated to use a fast voxel traversal algorithm, which has dramatically increased the computing speed. The radiative transfer models are calculated with the generally accepted dust scale-height of 0.1 kpc. The stellar scale-heights are assumed to be 0.1 or 0.35 kpc, appropriate for far-ultraviolet (FUV) and optical wavelengths, respectively. The face-on optical depth, measured perpendicular to the Galactic plane, is also varied from 0.2 to 0.6, suitable to the optical to FUV wavelengths, respectively. We find that the DGL at high Galactic latitudes is mostly due to backward or large-angle scattering of starlight originating from the local stars within a radial distance of r < 0.5 kpc from the Earth. On the other hand, the DGL measured in the Galactic plane is mostly due to stars at a distance range that corresponds to an optical depth of <TEX>$${\sim_\sim}$$</TEX> 1 measured from the Earth. Therefore, the low-latitude DGL at the FUV wavelength band would be mostly caused by the stars located at a distance of <TEX>$r{\leq}0.5$</TEX> kpc and the optical DGL near the Galactic plane mainly originates from stars within a distance range of <TEX>$1{\leq}r{\leq}2kpc$</TEX>. We also calculate the radiative transfer models in a clumpy two-phase medium. The clumpy two-phase models provide lower intensities at high Galactic latitudes compared to the uniform density models, because of the lower effective optical depth in clumpy media. However, no significant difference in the intensity at the Galactic plane is found.

The distribution of interstellar dust in CALIFA edge-on galaxies via oligochromatic radiative transfer fitting

We investigate the amount and spatial distribution of interstellar dust in edge-on spiral galaxies, using detailed radiative transfer modeling of a homogeneous sample of 12 galaxies selected from the CALIFA survey. Our automated fitting routine, FitSKIRT, was first validated against artificial data. This is done by simultaneously reproducing the SDSS $g$-, $r$-, $i$- and $z$-band observations of a toy model in order to combine the information present in the different bands. We show that this combined, oligochromatic fitting, has clear advantages over standard monochromatic fitting especially regarding constraints on the dust properties. We model all galaxies in our sample using a three-component model, consisting of a double exponential disc to describe the stellar and dust discs and using a S\'ersic profile to describe the central bulge. The full model contains 19 free parameters, and we are able to constrain all these parameters to a satisfactory level of accuracy without human intervention or strong boundary conditions. Apart from two galaxies, the entire sample can be accurately reproduced by our model. We find that the dust disc is about 75% more extended but only half as high as the stellar disc. The average face-on optical depth in the V-band is $0.76$ and the spread of $0.60$ within our sample is quite substantial, which indicates that some spiral galaxies are relatively opaque even when seen face-on.

DIFFUSE EXTRAPLANAR DUST IN NGC 891

We report the detection of vertically extended far-ultraviolet (FUV) and near-UV emissions in an edge-on spiral galaxy NGC 891, which we interpret as being due to dust-scattered starlight. Three-dimensional radiative transfer models are used to investigate the content of the extraplanar dust that is required to explain the UV emission. The UV halos are well reproduced by a radiative transfer model with two exponential dust disks, one with a scaleheight of $\approx$ 0.2 - 0.25 kpc and the other with a scaleheight of $\approx$ 1.2 - 2.0 kpc. The central face-on optical depth of the geometrically thick disk is found to be $\tau_{thick} \approx$ 0.3 - 0.5 at B-band. The results indicate that the dust mass at |z| > 2 kpc is $\approx$ 3 - 5% of the total dust mass, which accord well with the recent Herschel sub-millimeter observation. Our results, together with the recent discovery of the UV halos in other edge-on galaxies, suggest the widespread existence of the geometrically thick dust layer above the galactic plane in spirals.

Towards understanding the relation between the gas and the attenuation in galaxies at kpc scales

[abridged] Aims. The aim of the present paper is to provide new and more detailed relations at the kpc scale between the gas surface density and the face-on optical depth directly calibrated on galaxies, in order to compute the attenuation not only for semi-analytic models but also observationally as new and upcoming radio observatories are able to trace gas ever farther in the Universe. Methods. We have selected a sample of 4 nearby resolved galaxies and a sample of 27 unresolved galaxies from the Herschel Reference Survey and the Very Nearby Galaxies Survey, for which we have a large set of multi-wavelength data from the FUV to the FIR including metallicity gradients for resolved galaxies, along with radio HI and CO observations. For each pixel in resolved galaxies and for each galaxy in the unresolved sample, we compute the face-on optical depth from the attenuation determined with the CIGALE SED fitting code and an assumed geometry. We determine the gas surface density from HI and CO observations with a metallicity-dependent XCO factor. Results. We provide new, simple to use, relations to determine the face-on optical depth from the gas surface density, taking the metallicity into account, which proves to be crucial for a proper estimate. The method used to determine the gas surface density or the face-on optical depth has little impact on the relations except for galaxies that have an inclination over 50d. Finally, we provide detailed instructions on how to compute the attenuation practically from the gas surface density taking into account possible information on the metallicity. Conclusions. Examination of the influence of these new relations on simulated FUV and IR luminosity functions shows a clear impact compared to older oft-used relations, which in turn could affect the conclusions drawn from studies based on large scale cosmological simulations.

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.

COLLISIONAL GROOMING MODELS OF THE KUIPER BELT DUST CLOUD

We modeled the 3-D structure of the Kuiper Belt dust cloud at four different dust production rates, incorporating both planet-dust interactions and grain-grain collisions using the collisional grooming algorithm. Simulated images of a model with a face-on optical depth of ~10^-4 primarily show an azimuthally-symmetric ring at 40-47 AU in submillimeter and infrared wavelengths; this ring is associated with the cold classical Kuiper Belt. For models with lower optical depths (10^-6 and 10^-7), synthetic infrared images show that the ring widens and a gap opens in the ring at the location of of Neptune; this feature is caused by trapping of dust grains in Neptune's mean motion resonances. At low optical depths, a secondary ring also appears associated with the hole cleared in the center of the disk by Saturn. Our simulations, which incorporate 25 different grain sizes, illustrate that grain-grain collisions are important in sculpting today's Kuiper Belt dust, and probably other aspects of the Solar System dust complex; collisions erase all signs of azimuthal asymmetry from the submillimeter image of the disk at every dust level we considered. The model images switch from being dominated by resonantly-trapped small grains ("transport dominated") to being dominated by the birth ring ("collision dominated") when the optical depth reaches a critical value of tau ~ v/c, where v is the local Keplerian speed.

An evolutionary disc model of the edge-on galaxy NGC 5907

We present a physical model that explains the two disparate observational facts: 1) the exponential vertical disc structure in the optical and NIR of the non-obscured part of the stellar disc and 2) the enhanced FIR/submm luminosity by about a factor of four near the obscured mid-plane, which requires additional dust and also stellar light to heat the dust component. We use multi-band photometry in U, B, V, R, and I- band combined with radiative transfer through a dust component to fit simultaneously the vertical surface-brightness and colour index profiles in all bands adopting a reasonable star formation history and dynamical heating function. The final disc model reproduces the surface-brightness profiles in all bands with a moderately declining star formation rate and a slowly starting heating function for young stars. The total dust mass is 57 million solar masses as required from the FIR/submm measurements. Without a recent star burst we find in the midplane an excess of 5.2-, 4.0-, and 3.0-times more stellar light in the U-, B-, and V-band, respectively. The corresponding stellar mass-to-light ratios are 0.91 in V- and 1.0 in R-band. The central face-on optical depth in V-band is 0.81 and the radial scale length of the dust is 40% larger than that of the stellar disc. Evolutionary disc models are a powerful method to understand the vertical structure of edge-on galaxies. Insights to the star formation history and the dynamical evolution of stellar discs can be gained. FIR/submm observations are necessary to restrict the parameter space for the models.

A Comparative Study of the Spatial Distribution of Ultraviolet and Far-Infrared Fluxes from M101

The total ultraviolet (UV) flux (from 1412 to 2718 A) of M101 is compared on a pixel-to-pixel basis with the total far-infrared (FIR) flux (from 60 to 170 μm) using the maps of the galaxy taken by the Galaxy Evolution Explorer (GALEX) in the near-UV and far-UV and by the Infrared Space Observatory (ISO) at 60, 100, and 170 μm. The main result of this investigation is the discovery of a tight dependence of the FIR/UV ratio on radius, with values monotonically decreasing from ~4 in the nuclear region to nearly zero toward the edge of the optical disk. Although the tightness of this dependence is in part attributable to resolution effects, the result is consistent with the presence of a large-scale distribution of diffuse dust having a face-on optical depth that decreases with radius and that dominates over the more localized variations in opacity between the arm and interarm regions. We also find a trend for the FIR/UV ratio of taking on higher values in the regions of diffuse interarm emission than in the spiral-arm regions, at a given radius. This is interpreted quantitatively in terms of the escape probability of UV photons from spiral arms and their subsequent scattering in the interarm regions, and in terms of the larger relative contribution of optical photons to the heating of the dust in the interarm regions.

Dust masses and star formation in bright IRAS galaxies

We address the problem of modeling the far-infrared (FIR) spectrum and deriving the star-formation rate (SFR) and the dust mass of spiral galaxies. We use the realistic physical model of Popescu et al. ([CITE]) to describe the overall ultra-violet (UV), optical and FIR spectral energy distribution (SED) of a spiral galaxy. The model takes into account the 3-dimensional old and young stellar distributions in the bulge and the disk of a galaxy, together with the dust geometry. The geometrical characteristics of the galaxy and the intrinsic optical and near-infrared spectra are determined by the galaxy's observed K-band photometry. The UV part of the spectrum is assumed to be proportional to the SFR through the use of population synthesis models. By solving the radiative transfer equation, we are able to determine the absorbed energy, the dust temperature and the resulting FIR spectrum. The model has only three free parameters: SFR, dust mass, and the fraction of the UV radiation which is absorbed locally by dense dust in the HII regions. Using this model, we are able to fit well the FIR spectra of 62 bright IRAS galaxies from the “SCUBA Local Universe Galaxy Survey" of Dunne et al. ([CITE]). As a result, we are able to determine, among others, their SFR and dust mass. We find that, on average, the SFR (in absolute units), the star-formation efficiency, the SFR surface density and the ratio of FIR luminosity over the total intrinsic luminosity, are larger than the respective values of typical spiral galaxies of the same morphological type. We also find that the mean gas-to-dust mass ratio is close to the Galactic value, while the average central face-on optical depth of these galaxies in the V band is 2.3. Finally, we find a strong correlation between SFR or dust mass and observed FIR quantities like total FIR luminosity or FIR luminosity at 100 and 850 . These correlations yield well-defined relations, which can be used to determine a spiral galaxy's SFR and dust-mass content from FIR observations.

Radiatively driven mass accretion on to galactic nuclei by circumnuclear starbursts

We examine the physical processes of radiatively driven mass accretion on to galactic nuclei, owing to intensive radiation from circumnuclear starbursts. The radiation from a starburst not only causes the inner gas disc to contract via radition flux force, but also extracts angular momentum owing to relativistic radiation drag, thereby inducing an avalanche of the surface layer of the disc. To analyse such a mechanism, the radiation–hydrodynamical equations are solved, including the effects of the radiation drag force as well as the radiation flux force. As a result, it is found that the mass accretion rate owing to the radiative avalanche is given by M˙ (r)= η (L*/c2)(r/R)2 (ΔR/R)(1 − e−τ) at radius r, where the efficiency η ranges from 0.2 up to 1, L* and R are respectively the bolometric luminosity and the radius of the starburst ring, ΔR is the extent of the emission regions, and τ is the face-on optical depth of the disc. In an optically thick regime, the rate depends upon neither the optical depth nor the surface mass density distribution of the disc. The present radiatively driven mass accretion may provide a physical mechanism which enables mass accretion from 100-pc scales down to ∼ parsec scales, and it may eventually be linked to advection-dominated viscous accretion on to a massive black hole. The radiation–hydrodynamical and self-gravitational instabilities of the disc are briefly discussed. In particular, the radiative acceleration possibly builds up a dusty wall, which ‘shades’ the nucleus in edge-on views. This provides another version of the model for the formation of an obscuring torus.

The average optical depth of disc galaxies

The average optical depth of disc galaxies is calculated assuming that their infrared output originates from the dust reprocessing of stellar light. Essentially, the luminosity functions at both optical (B-band) and FIR (IRAS) wavelengths are integrated to give a local luminosity density in the optical and FIR. This method was first proposed by Saunders et al., who found a B-band optical depth of tau_B=0.26. In this paper we reconsider the method and employ more realistic models. We conclude that on average a B-band photon originates from a region with an optical depth of tau_B~0.9, a value that indicates that galaxies have high rather than low optical depths. Adjusting the FIR luminosity for flux at wavelengths longer than those detected by IRAS could increase this value further (to tau_B>1). Interpreting this, a galaxy with the dust distributed in a similar manner to the stars would have a central face-on optical depth of tau_B>2.5, and this `typical' galaxy would become optically thin at ~1 scalelength from the centre.

Internal absorption in spiral galaxies

A sample of 15 000 faint spiral galaxies is used for the statistical determination of internal absorption by investigating the projected surface brightness. Analyses with different disc models yield small absorption values. For a more realistic three-component model, however, a central face-on optical depth of τ 0 =7 is obtained, which is in agreement with suggestions that spiral galaxies are not optically thin