Fitting the integrated Spectral Energy Distributions of Galaxies

Fitting the spectral energy distributions (SEDs) of galaxies is an almost universally used technique that has matured significantly in the last decade. Model predictions and fitting procedures have improved significantly over this time, attempting to keep up with the vastly increased volume and quality of available data. We review here the field of SED fitting, describing the modelling of ultraviolet to infrared galaxy SEDs, the creation of multiwavelength data sets, and the methods used to fit model SEDs to observed galaxy data sets. We touch upon the achievements and challenges in the major ingredients of SED fitting, with a special emphasis on describing the interplay between the quality of the available data, the quality of the available models, and the best fitting technique to use in order to obtain a realistic measurement as well as realistic uncertainties. We conclude that SED fitting can be used effectively to derive a range of physical properties of galaxies, such as redshift, stellar masses, star formation rates, dust masses, and metallicities, with care taken not to over-interpret the available data. Yet there still exist many issues such as estimating the age of the oldest stars in a galaxy, finer details ofdust properties and dust-star geometry, and the influences of poorly understood, luminous stellar types and phases. The challenge for the coming years will be to improve both the models and the observational data sets to resolve these uncertainties. The present review will be made available on an interactive, moderated web page (sedfitting.org), where the community can access and change the text. The intention is to expand the text and keep it up to date over the coming years.Comment: 54 pages, 26 figures, Accepted for publication in Astrophysics & Space Scienc

Gas and dust cooling along the major axis of M 33 (HerM33es). Herschel/PACS [C II] and [O I] observations

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Modelling the dust content of spiral galaxies towards an understanding of the dust opacity problem

We compare optical and near-infrared surface photometry of seven edge-on spiral galaxies with corresponding surface photometry calculated from a realistic model of spiral galaxies which takes into account both absorption and scattering by the interstellar dust. For the stars and the dust in the disc we use exponential distributions in both directions, radially and perpendicular to the plane of the disc, while the de Vaucouleurs (R1/4) profile is used for the description of the bulge. The effect of the spiral structure in the galactic discs is also examined and it is found that the simple exponential disc model is able to describe quite accurately the real galaxy. From this analysis we were able to obtain some general conclusions, the most significant of which are: 1) The face-on central optical depth is less than unity in all optical bands, indicating that typical spiral galaxies like those we have modelled would be completely transparent if they were to be seen face on. 2) The dust scale height is about half that of the stars, which means that dust is more concentrated near the plane of the disc. 3) The dust scale length is about 1.4 times larger than that of the stars and dust is more radially extended than the stars. 4) The gas-to-dust mass ratio calculated, with the dust mass derived from the model, is close to the value derived for our Galaxy. 5) The derived extinction law matches the Galactic extinction law quite well, indicating a universal dust behaviour

Tracing the molecular gas in distant submillimetre galaxies via CO(1-0) imaging with the Expanded Very Large Array

We report the results of a pilot study with the EVLA of 12CO J=1-0 emission from four SMGs at z=2.2-2.5, each with an existing detection of CO J=3-2. Using the EVLA's most compact configuration we detect strong, broad J=1-0 line emission from all of our targets. The median line width ratio, sigma(1-0)/sigma(3-2) = 1.15 +/- 0.06, suggests that the J=1-0 is more spatially extended than the J=3-2 emission, a situation confirmed by our maps which reveal velocity structure in several cases and typical sizes of ~16 kpc FWHM. The median Tb ratio is r(3-2/1-0) = 0.55 +/- 0.05, noting that our value may be biased high because of the J=3-2-based sample selection. Naively, this suggests gas masses ~2x higher than estimates made using higher-J transitions of CO, with the discrepency due to the difference in assumed Tb ratio. We also estimate masses using the 12CO J=1-0 line and the observed global Tb ratios, assuming standard underlying Tb ratios as well as a limiting SFE, i.e. without calling upon X(CO). Using this new method, we find a median molecular gas mass of (2.5 +/- 0.8) x 10^10 Msun, with a plausible range stretching 3x higher. Even larger masses cannot be ruled out, but are not favoured by dynamical constraints: the median dynamical mass for our sample is (2.3 +/- 1.4) x 10^11 Msun. We examine the Schmidt-Kennicutt relation for all the distant galaxy populations for which CO J=1-0 or J=2-1 data are available, finding small systematic differences. These have previously been interpreted as evidence for different modes of star formation, but we argue that these differences are to be expected, given the still considerable uncertainties. Finally, we discuss the morass of degeneracies surrounding molecular gas mass estimates, the possibilities for breaking them, and the future prospects for imaging and studying cold, quiescent molecular gas at high redshifts [abridged].Comment: 13 pages, 2 figures, in press at MNRA

The physical properties of local (U)LIRGs: A comparison with nearby early- And late-type galaxies

Aims. In order to pinpoint the place of the (ultra-) luminous infrared galaxies ((U)LIRGs) in the local Universe, we examine the properties of a sample of 67 such nearby systems and compare them with those of 268 early- and 542 late-type, well studied, galaxies from the DustPedia database. Methods. We made use of multi-wavelength photometric data (from the ultra-violet to the sub-millimetre), culled from the literature, and the CIGALE spectral energy distribution fitting code to extract the physical parameters of each system. The median spectral energy distributions as well as the values of the derived parameters were compared to those of the local early- and late-type galaxies. In addition to that, (U)LIRGs were divided into seven classes, according to the merging stage of each system, and variations in the derived parameters were investigated. Results. (U)LIRGs occupy the 'high-end' on the dust mass, stellar mass, and star-formation rate (SFR) plane in the local Universe with median values of 5.2 × 107 Mpdbl, 6.3 × 1010 Mpdbl, and 52 Mpdblyr-1, respectively. The median value of the dust temperature in (U)LIRGs is 32 K, which is higher compared to both the early-type (28 K) and the late-type (22 K) galaxies. The dust emission in PDR regions in (U)LIRGs is 11.7% of the total dust luminosity, which is significantly higher than early-type (1.6%) and late-type (5.2%) galaxies. Small differences in the derived parameters are seen for the seven merging classes of our sample of (U)LIRGs with the most evident one being on the SFR, where in systems in late merging stages ('M3' and 'M4') the median SFR reaches up to 99 Mpdblyr-1compared to 26 Mpdblyr-1for the isolated ones. In contrast to the local early-and late-type galaxies where the old stars are the dominant source of the stellar emission, the young stars in (U)LIRGs contribute with 64% of their luminosity to the total stellar luminosity. The fraction of the stellar luminosity absorbed by the dust is extremely high in (U)LIRGs (78%) compared to 7% and 25% in early- and late-type galaxies, respectively. The fraction of the stellar luminosity used to heat up the dust grains is very high in (U)LIRGs, for both stellar components (92% and 56% for the young and the old stellar populations, respectively) while 74% of the dust emission comes from the young stars. © ESO 2021

The Complex Interplay of Dust and Star Light in Spiral Galaxy Discs

Interstellar dust grains efficiently absorb and scatter UV and optical radiation in galaxies, and therefore can significantly affect the apparent structure of spiral galaxies. We discuss the effect of dust attenuation on the observed structural properties of bulges and discs. We also present some first results on modelling the dust content of edge-on spiral galaxies using both optical and Herschel far-infrared data. Both of these results demonstrate the complex interplay of dust and star light in spiral galaxies.Comment: 6 pages, 2 figures, to appear in "Galaxies and their Masks", eds. D. L. Block, K. C. Freeman & I. Puerari, Springer (New York

NELIOTA: The wide-field, high-cadence, lunar monitoring system at the prime focus of the Kryoneri telescope

We present the technical specifications and first results of the ESA-funded, lunar monitoring project "NELIOTA" (NEO Lunar Impacts and Optical TrAnsients) at the National Observatory of Athens, which aims to determine the size-frequency distribution of small near-Earth objects (NEOs) via detection of impact flashes on the surface of the Moon. For the purposes of this project a twin camera instrument was specially designed and installed at the 1.2 m Kryoneri telescope utilizing the fast-frame capabilities of scientific Complementary Metal-Oxide Semiconductor detectors (sCMOS). The system provides a wide field-of-view (17.0′ × 14.4′) and simultaneous observations in two photometric bands (R and I), reaching limiting magnitudes of 18.7 mag in 10 s in both bands at a 2.5 signal-to-noise ratio (S/N) level. This makes it a unique instrument that can be used for the detection of NEO impacts on the Moon, as well as for any astronomy projects that demand high-cadence multicolor observations. The wide field-of-view ensures that a large portion of the Moon is observed, while the simultaneous, high-cadence, monitoring in two photometric bands makes possible the determination of the temperatures of the impacts on the Moon's surface and the validation of the impact flashes from a single site. Considering the varying background level on the Moon's surface we demonstrate that the NELIOTA system can detect NEO impact flashes at a 2.5 S/N level of ∼12.4 mag in the I-band and R-band for observations made at low lunar phases (∼0.1). We report 31 NEO impact flashes detected during the first year of the NELIOTA campaign. The faintest flash was at 11.24 mag in the R-band (about two magnitudes fainter than ever observed before) at lunar phase 0.32. Our observations suggest a detection rate of 1.96×10-7 events km-2 h-1. © 2018 ESO

High-resolution, 3D radiative transfer modelling: II. The early-type spiral galaxy M 81

Context. Interstellar dust absorbs stellar light very efficiently, thus shaping the energy output of galaxies. Studying the impact of different stellar populations on the dust heating continues to be a challenge because it requires decoupling the relative geometry of stars and dust and also involves complex processes such as scattering and non-local dust heating. Aims. We aim to constrain the relative distribution of dust and stellar populations in the spiral galaxy M 81 and create a realistic model of the radiation field that adequately describes the observations. By investigating the dust-starlight interaction on local scales, we want to quantify the contribution of young and old stellar populations to the dust heating. We aim to standardise the setup and model selection of such inverse radiative transfer simulations so these can be used for comparable modelling of other nearby galaxies. Methods. We present a semi-automated radiative transfer modelling pipeline that implements necessary steps such as the geometric model construction and the normalisation of the components through an optimisation routine. We used the Monte Carlo radiative transfer code SKIRT to calculate a self-consistent, panchromatic model of the interstellar radiation field. By looking at different stellar populations independently, we were able to quantify to what extent different stellar age populations contribute to the heating of dust. Our method takes into account the effects of non-local heating. Results. We obtained a realistic 3D radiative transfer model of the face-on galaxy M 81. We find that only 50.2% of the dust heating can be attributed to young stellar populations (≲ 100  Myr). We confirm that there is a tight correlation between the specific star formation rate and the heating fraction by young stellar populations, both in sky projections and in 3D, which is also found for radiative transfer models of M 31 and M 51. Conclusions. We conclude that old stellar populations can be a major contributor to the heating of dust. In M 81, old stellar populations are the dominant heating agent in the central regions, contributing to half of the absorbed radiation. Regions of higher star formation do not correspond to the highest dust temperatures. On the contrary, it is the dominant bulge which is most efficient in heating the dust. The approach we present here can immediately be applied to other galaxies. It does contain a number of caveats, which we discuss in detail. © ESO 2020

Dust emissivity and absorption cross section in DustPedia late-type galaxies

Aims. We compare the far-infrared to sub-millimetre dust emission properties measured in high Galactic latitude cirrus with those determined in a sample of 204 late-type DustPedia galaxies. The aim is to verify if it is appropriate to use Milky Way dust properties to derive dust masses in external galaxies. Methods. We used Herschel observations and atomic and molecular gas masses to estimate ϵ (250 μm), the disc-averaged dust emissivity at 250 μm, and from this, the absorption cross section per H atom σ(250 μm) and per dust mass κ(250 μm). The emissivity ϵ (250 μm) requires one assumption, which is the CO-to-H2 conversion factor, and the dust temperature is additionally required for σ(250 μm); yet another constraint on the dust-to-hydrogen ratio D/H, depending on metallicity, is required for κ(250 μm). Results. We find ϵ (250 μm) = 0.82 ± 0.07 MJy sr-1 (1020 H cm-2)-1 for galaxies with 4 < F(250 μm)/F(500 μm) < 5. This depends only weakly on the adopted CO-to-H2 conversion factor. The value is almost the same as that for the Milky Way at the same colour ratio. Instead, for F(250 μm)/F(500 μm) > 6, ϵ (250 μm) is lower than predicted by its dependence on the heating conditions. The reduction suggests a variation in dust emission properties for spirals of earlier type, higher metallicity, and with a higher fraction of molecular gas. When the standard emission properties of Galactic cirrus are used for these galaxies, their dust masses might be underestimated by up to a factor of two. Values for σ(250 μm) and κ(250 μm) at the Milky Way metallicity are also close to those of the cirrus. Mild trends of the absorption cross sections with metallicity are found, although the results depend on the assumptions made. © ESO 2019