Modelling the dust content of spiral galaxies: More dust mass vs. enhanced dust grain emissivity

We present detailed modelling of the spectral energy distribution (SED) of the spiral galaxies NGC 891, NGC 4013, and NGC 5907 in the far-infrared (FIR) and sub-millimeter (submm) wavelengths. The model takes into account the emission of the diffuse dust component, which is heated by the UV and optical radiation fields produced by the stars, as well as the emission produced locally in star forming HII complexes. The radiative transfer simulations of Xilouris et al. (1999) in the optical bands are used to constrain the stellar and dust geometrical parameters, as well as the total amount of dust. We find that the submm emission predicted by our model can not account for the observed fluxes at these wavelengths. We examine two cases, one having more dust embedded in a second thin disk and another allowing for an enhanced submillimeter emissivity of the dust grains. We argue that both cases can equally well reproduce the observed SED. The case of having more dust embedded in a second disk though, is not supported by the near-infrared observations and thus more realistic distributions of the dust (i.e., in spiral arms and clumps) have to be examined in order to better fit the surface brightness of each galaxy.Comment: To appear in the proceedings of: "The Spectral Energy Distribution of Gas-Rich Galaxies: Confronting Models with Data", Heidelberg, 4-8 Oct. 2004, eds. C.C. Popescu and R.J. Tuffs, AIP Conf. Ser., in pres

Is the Galactic submillimeter dust emissivity underestimated?

We present detailed modeling of the spectral energy distribution (SED) of the spiral galaxies NGC 891, NGC 4013, and NGC 5907 in the far-infrared and submm wavelengths. The model takes into account the emission produced by the diffuse dust and the star forming HII complexes. The dust mass is constrained by radiative transfer simulations in the optical (Xilouris et al. 1999). We find that the submm emission predicted by our model cannot account for the observed fluxes. Two scenarios may account for the "missing" submm flux. In the first scenario (Popescu et al. 2000), additional dust (to that derived from the optical, and associated with young stars) is embedded in the galaxy in the form of a thin disk and gives rise to additional submm emission. The other scenario investigates whether the average submm emissivity of the dust grains is higher than the values widely used in Galactic environments. In this case, the dust mass is equal to that derived from the optical observations, and the submm emissivity is treated as a free parameter calculated by fitting our model to the observed SED. We find the submm emissivity value to be ~3 times that often used for our Galaxy. While both scenarios reproduce the observed 850 micron surface brightness, the extra embedded dust model is not supported by the near infrared observations. We, thus, find that the enhanced dust submm emissivity scenario is the most plausible. [abridged]Comment: 12 pages, 10 figures, accepted for publication in Astronomy and Astrophysic

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.Comment: 17 pages, 12 figures (24 files), A&A in pres

Modelling the spectral energy distribution of galaxies. I. Radiation fields and grain heating in the edge-on spiral NGC891

We describe a new tool for the analysis of the UV to the sub-millimeter (sub-mm) spectral energy distribution (SED) of spiral galaxies. We use a consistent treatment of grain heating and emission, solve the radiation transfer problem for a finite disk and bulge, and self-consistently calculate the stochastic heating of grains placed in the resulting radiation field. We use this tool to analyse the well-studied nearby edge- on spiral galaxy NGC 891. First we investigate whether the old stellar population in NGC 891, along with a reasonable assumption about the young stellar population, can account for the heating of the dust and the observed far-infrared and sub- mm emission. The dust distribution is taken from the model of Xilouris et al. (1999), who used only optical and near-infrared observations to determine it. We have found that such a simple model cannot reproduce the SED of NGC 891, especially in the sub-mm range. It underestimates by a factor of 2–4 the observed sub-mm flux. A number of possible explanations exist for the missing sub-mm flux. We investigate a few of them and demonstrate that one can reproduce the observed SED in the far-infrared and the sub-mm quite well, as well as the observed radial profile at 850 µm. For the models calculated we give the relative proportion of the dust radiation powered by the old and young stellar popula- tions as a function of FIR/sub-mm wavelength. In all models we find that the dust is predominantly heated by the young stellar population

The Star Formation Law in a Multifractal ISM

The surface density of the star formation rate in different galaxies, as well as in different parts of a single galaxy, scales nonlinearly with the surface density of the total gas. This observationally established relation is known as the Kennicutt-Schmidt star formation law. The slope of the star formation law has been shown to change with the density of the gas against which the star formation rate is plotted. This dependence implies a nonlinear scaling between the dense gas and the total gas surface densities within galaxies. Here, we explore a possible interpretation of this scaling as a property of the geometry of the interstellar medium (ISM), and we find that it arises naturally if the topology of the ISM is multifractal. Under the additional assumption that, at very high densities, the star formation timescale is roughly constant, the star formation law itself can also be recovered as a consequence of the multifractal geometry of the ISM. The slope of the scaling depends on the width of the global probability density function (PDF), and is between 1.5 and 1.6 for wide PDFs relevant to high-mass systems, while it is higher for narrower PDFs appropriate for lower-mass dwarf galaxies, in agreement with observations.Comment: 8 pages, 4 figures, submitted to MNRA

Bayesian approach to SETI

The search for technosignatures from hypothetical galactic civilizations is going through a new phase of intense activity. For the first time, a significant fraction of the vast search space is expected to be sampled in the foreseeable future, potentially bringing informative data about the abundance of detectable extraterrestrial civilizations, or the lack thereof. Starting from the current state of ignorance about the galactic population of non-natural electromagnetic signals, we formulate a Bayesian statistical model to infer the mean number of radio signals crossing Earth, assuming either non-detection or the detection of signals in future surveys of the Galaxy. Under fairly noninformative priors, we find that not detecting signals within about $1$ kly from Earth, while suggesting the lack of galactic emitters or at best the scarcity thereof, is nonetheless still consistent with a probability exceeding $10$ \% that typically over $\sim 100$ signals could be crossing Earth, with radiated power analogous to that of the Arecibo radar, but coming from farther in the Milky Way. The existence in the Galaxy of potentially detectable Arecibo-like emitters can be reasonably ruled out only if all-sky surveys detect no such signals up to a radius of about $40$ kly, an endeavor requiring detector sensitivities thousands times higher than those of current telescopes. Conversely, finding even one Arecibo-like signal within $\sim 1000$ light years, a possibility within reach of current detectors, implies almost certainly that typically more than $\sim 100$ signals of comparable radiated power cross the Earth, yet to be discovered.Comment: Published in PNAS ahead of print October 1, 2018. Preprint has 13 pages, 7 figures + 7 pages of Supplementary Information with 5 figure

DART-RAY: a 3D ray-tracing radiative transfer code for calculating the propagation of light in dusty galaxies

We present DART-Ray, a new ray-tracing 3D dust radiative transfer (RT) code designed specifically to calculate radiation field energy density (RFED) distributions within dusty galaxy models with arbitrary geometries. In this paper, we introduce the basic algorithm implemented in . DART-Ray which is based on a pre-calculation of a lower limit for the RFED distribution. This pre-calculation allows us to estimate the extent of regions around the radiation sources within which these sources contribute significantly to the RFED. In this way, ray-tracing calculations can be restricted to take place only within these regions, thus substantially reducing the computational time compared to a complete ray-tracing RT calculation. Anisotropic scattering is included in the code and handled in a similar fashion. Furthermore, the code utilizes a Cartesian adaptive spatial grid and an iterative method has been implemented to optimize the angular densities of the rays originated from each emitting cell. In order to verify the accuracy of the RT calculations performed by DART-Ray, we present results of comparisons with solutions obtained using the dusty 1D RT code for a dust shell illuminated by a central point source and existing 2D RT calculations of disc galaxies with diffusely distributed stellar emission and dust opacity. Finally, we show the application of the code on a spiral galaxy model with logarithmic spiral arms in order to measure the effect of the spiral pattern on the attenuation and RFED. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society

Panchromatic radiation from galaxies as a probe of galaxy formation and evolution

I review work on modelling the infrared and submillimetre SEDs of galaxies. The underlying physical assumptions are discussed and spherically symmetric, axisymmetric, and 3-dimensional radiative transfer codes are reviewed. Models for galaxies with Spitzer IRS data and for galaxies in the Herschel-Hermes survey are discussed. Searches for high redshift infrared and submillimetre galaxies, the star formation history, the evolution of dust extinction, and constraints from source-counts, are briefly discussed.Comment: to be published in IAU Symposium 284 'The Spectral Energy Distribution of Galaxies', Preston 2012, eds. R.J.Tiffs and C.C.Popesc

Modeling the spectral energy distribution of galaxies. II. Disk opacity and star formation in 5 edge-on spirals

Using tools previously described and applied to the prototype galaxy NGC 891, we model the optical to far-infrared spectral energy distributions (SED) of four additional edge-on spiral galaxies, namely NGC 5907, NGC 4013, UGC 1082 and UGC 2048. Comparing the model predictions with IRAS and, where available, sub-millimeter and millimeter observations, we determine the respective roles of the old and young stellar populations in grain heating. In all cases, the young population dominates, with the contribution of the old stellar population being at most 40%, as previously found for NGC 891. After normalization to the disk area, the massive star-formation rate (SFR) derived using our SED modeling technique, which is primarily sensitive to the non-ionizing ultraviolet output from the young stellar population, lies in the range 7e-4 - 2e-2 M_sun * yr^-1 * kpc^-2. This is consistent with normalized SFRs derived for face-on galaxies of comparable surface gas densities from H_alpha observations. Though the most active star-forming galaxy of the five in absolute terms, NGC 891 is not an exceptional system in terms of its surface density in SFR.Comment: 9 pages, 3 figures, accepted for publication to A&