arXiv subjects

F. Galliano

Publications and source records attributed to F. Galliano.

At least 37 records · Page 2Linked to original sources

arXiv2020

Tracing the total molecular gas in galaxies: [CII] and the CO-dark gas

While the CO(1-0) transition is often used to deduce the total molecular hydrogen in galaxies, it is challenging to detect in low metallicity galaxies, in spite of the star formation taking place. In contrast, the [CII] 158 micron line is relatively bright, highlighting a potentially important reservoir of H2 that is not traced by CO(1-0), but residing in the C+ - emitting regions. We explore a method to quantify the total H2 mass (MH2) in galaxies and learn what parameters control the CO-dark gas reservoir. We present Cloudy grids of density, radiation field and metallicity in terms of observed quantities, such as [OI], [CI], CO(1-0), [CII], total infrared luminosity and the total MH2 and provide recipes based on these models to derive total MH2 mass estimates from observations. The models are applied to the Herschel Dwarf Galaxy Survey, extracting the total MH2 for each galaxy which is compared to the H2 determined from the observed CO(1-0) line. While the H2 traced by CO(1-0) can be negligible, the [CII] 158 micron line can trace the total H2. 70% to 100% of the total H2 mass is not traced by CO(1-0) in the dwarf galaxies, but is well-traced by [CII] 158 micron line. The CO-dark gas mass fraction correlates with the observed L[CII]/LCO(1-0) ratio. A conversion factor for [CII] luminosity to total H2 and a new CO-to-total-MH2 conversion factor, as a function of metallicity, is presented. A recipe is provided to quantify the total mass of H2 in galaxies, taking into account the CO and [CII] observations. Accounting for this CO-dark H2 gas, we find that the star forming dwarf galaxies now fall on the Schmidt-Kennicutt relation. Their star-forming efficiency is rather normal, since the reservoir from which they form stars is now more massive when introducing the [CII] measures of the total H2, compared to the little amount of H2 in the CO-emitting region.

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arXiv2020

High-resolution, 3D radiative transfer modelling IV. AGN-powered dust heating in NGC 1068

Dust emission, an important diagnostic of star formation and ISM mass throughout the Universe, can be powered by sources unrelated to ongoing star formation. In the framework of the DustPedia project we have set out to disentangle the radiation of the ongoing star formation from that of the older stellar populations. This is done through detailed, 3D radiative transfer simulations of face-on spiral galaxies. In this particular study, we focus on NGC 1068, which contains an active galactic nucleus (AGN). The effect of diffuse dust heating by AGN (beyond the torus) was so far only investigated for quasars. This additional dust heating source further contaminates the broadband fluxes on which classic galaxy modelling tools rely to derive physical properties. We aim to fit a realistic model to the observations of NGC 1068 and quantify the contribution of the several dust heating sources. Our model is able to reproduce the global spectral energy distribution of the galaxy. It matches the resolved optical and infrared images fairly well, but deviates in the UV and the submm. We find a strong wavelength dependency of AGN contamination to the broadband fluxes. It peaks in the MIR, drops in the FIR, but rises again at submm wavelengths. We quantify the contribution of the dust heating sources in each 3D dust cell and find a median value of 83% for the star formation component. The AGN contribution is measurable at the percentage level in the disc, but quickly increases in the inner few 100 pc, peaking above 90%. This is the first time the phenomenon of an AGN heating the diffuse dust beyond its torus is quantified in a nearby star-forming galaxy. NGC 1068 only contains a weak AGN, meaning this effect can be stronger in galaxies with a more luminous AGN. This could significantly impact the derived star formation rates and ISM masses for such systems.

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arXiv2019

A systematic metallicity study of DustPedia galaxies reveals evolution in the dust-to-metal ratios

Observations of evolution in the dust-to-metal ratio allow us to constrain the dominant dust processing mechanisms. In this work, we present a study of the dust-to-metal and dust-to-gas ratios in a sub-sample of ~500 DustPedia galaxies. Using literature and MUSE emission line fluxes, we derived gas-phase metallicities (oxygen abundances) for over 10000 individual regions and determine characteristic metallicities for each galaxy. We study how the relative dust, gas, and metal contents of galaxies evolve by using metallicity and gas fraction as proxies for evolutionary state. The global oxygen abundance and nitrogen-to-oxygen ratio are found to increase monotonically as galaxies evolve. Additionally, unevolved galaxies (gas fraction > 60%, metallicity 12 + log(O/H) < 8.2) have dust-to-metal ratios that are about a factor of 2.1 lower (a factor of six lower for galaxies with gas fraction > 80%) than the typical dust-to-metal ratio (Md/MZ ~ 0.214) for more evolved sources. However, for high gas fractions, the scatter is larger due to larger observational uncertainties as well as a potential dependence of the dust grain growth timescale and supernova dust yield on local conditions and star formation histories. We find chemical evolution models with a strong contribution from dust grain growth describe these observations reasonably well. The dust-to-metal ratio is also found to be lower for low stellar masses and high specific star formation rates (with the exception of some sources undergoing a starburst). Finally, the metallicity gradient correlates weakly with the HI-to-stellar mass ratio, the effective radius and the dust-to-stellar mass ratio, but not with stellar mass.

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arXiv2019

Predicting the global far-infrared SED of galaxies via machine learning techniques

Dust plays an important role in shaping a galaxy's spectral energy distribution (SED). It absorbs ultraviolet (UV) to near-infrared (NIR) radiation and re-emits this energy in the far-infrared (FIR). The FIR is essential to understand dust in galaxies. However, deep FIR observations require a space mission, none of which are still active today. We aim to infer the FIR emission across six Herschel bands, along with dust luminosity, mass, and effective temperature, based on the available UV to mid-infrared (MIR) observations. We also want to estimate the uncertainties of these predictions, compare our method to energy balance SED fitting, and determine possible limitations of the model. We propose a machine learning framework to predict the FIR fluxes from 14 UV-MIR broadband fluxes. We used a low redshift sample by combining DustPedia and H-ATLAS, and extracted Bayesian flux posteriors through SED fitting. We trained shallow neural networks to predict the far-infrared fluxes, uncertainties, and dust properties. We evaluated them on a test set using a root mean square error (RMSE) in log-space. Our results (RMSE = 0.19 dex) significantly outperform UV-MIR energy balance SED fitting (RMSE = 0.38 dex), and are inherently unbiased. We can identify when the predictions are off, for example when the input has large uncertainties on WISE 22, or when the input does not resemble the training set. The galaxies for which we have UV-FIR observations can be used as a blueprint for galaxies that lack FIR data. This results in a 'virtual FIR telescope', which can be applied to large optical-MIR galaxy samples. This helps bridge the gap until the next FIR mission.

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arXiv2019

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 the disc-averaged dust emissivity at 250 micrometres, and from this, the absorption cross section per H atom and per dust mass. The emissivity requires one assumption, which is the CO-to-H_2 conversion factor, and the dust temperature is additionally required for the absorption cross section per H atom; yet another constraint on the dust-to-hydrogen ratio D/H, depending on metallicity, is required for the absorption cross section dust mass. Results: We find epsilon(250) = 0.82 +/- 0.07 MJy sr^-1 (1E20 H cm^-2)^-1 for galaxies with 4 < F(250)/F(500) < 5. This depends only weakly on the adopted CO-to-H_2 conversion factor. The value is almost the same as that for the Milky Way at the same colour ratio. Instead, for F(250)/F(500) > 6, epsilon(250) 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 the absorption cross sections 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.

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arXiv2019

DustPedia - the relationships between stars, gas and dust for galaxies residing in different environments

We use a sub-set of the DustPedia galaxy sample (461 galaxies) to investigate the effect the environment has had on galaxies. We consider Virgo cluster and field samples and also assign a density contrast parameter to each galaxy, as defined by the local density of SDSS galaxies. We consider their chemical evolution (using M_{Dust}/M_{Baryon} and M_{Gas}/M_{Baryon}), their specific star formation rate (SFR/M_{Stars}), star formation efficiency (SFR/M_{Gas}), stars-to-dust mass ratio (M_{Stars}/M_{Dust}), gas-to-dust mass ratio (M_{Gas}/M_{Dust}) and the relationship between star formation rate per unit mass of dust and dust temperature (SFR/M_{Dust} and T_{Dust}). Late type galaxies (later than Sc) in all of the environments can be modelled using simple closed box chemical evolution and a simple star formation history (SFR(t) \propto t\exp{-t/τ}). For earlier type galaxies the physical mechanisms that give rise to their properties are clearly much more varied and require a more complicated model (mergers, gas in or outflow). However, we find little or no difference in the properties of galaxies of the same morphological type within the cluster, field or with different density contrasts. It appears that it is morphology, how and whenever this is laid down, and consistent internal physical processes that primarily determine the derived properties of galaxies in the DustPedia sample and not processes related to differences in the local environment.

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arXiv2019

The Herschel Dwarf Galaxy Survey: II. Physical conditions, origin of [CII] emission, and porosity of the multiphase low-metallicity ISM

The sensitive infrared telescopes, Spitzer and Herschel, have been used to target low-metallicity star-forming galaxies, allowing us to investigate the properties of their interstellar medium (ISM) in unprecedented detail. Interpretation of the observations in physical terms relies on careful modeling of those properties. We have employed a multiphase approach to model the ISM phases (HII region and photodissociation region) with the spectral synthesis code Cloudy. Our goal is to characterize the physical conditions (gas densities, radiation fields, etc.) in the ISM of the galaxies from the Herschel Dwarf Galaxy Survey. We are particularly interested in correlations between those physical conditions and metallicity or star-formation rate. Other key issues we have addressed are the contribution of different ISM phases to the total line emission, especially of the [CII]157um line, and the characterization of the porosity of the ISM. We find that the lower-metallicity galaxies of our sample tend to have higher ionization parameters and galaxies with higher specific star-formation rates have higher gas densities. The [CII] emission arises mainly from PDRs and the contribution from the ionized gas phases is small, typically less than 30% of the observed emission. We also find correlation - though with scatter - between metallicity and both the PDR covering factor and the fraction of [CII] from the ionized gas. Overall, the low metal abundances appear to be driving most of the changes in the ISM structure and conditions of these galaxies, and not the high specific star-formation rates. These results demonstrate in a quantitative way the increase of ISM porosity at low metallicity. Such porosity may be typical of galaxies in the young Universe.

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arXiv2019

Old and young stellar populations in DustPedia galaxies and their role in dust heating

Within the framework of the DustPedia project we investigate the properties of cosmic dust and its interaction with the stellar radiation (originating from different stellar populations) for 814 galaxies in the nearby Universe, all observed by the Herschel Space Observatory. We take advantage of the widely used galaxy SED fitting code CIGALE, properly adapted to include the state-of-the-art dust model THEMIS. Using the DustPedia photometry we determine the physical properties of the galaxies, such as, the dust and stellar mass, the star-formation rate, the bolometric luminosity as well as the unattenuated and the absorbed by dust stellar light, for both the old (> 200 Myr) and young (<= 200 Myr) stellar populations. We show how the mass of stars, dust, and atomic gas, as well as the star-formation rate and the dust temperature vary between galaxies of different morphologies and provide recipes to estimate these parameters given their Hubble stage (T). We find a mild correlation between the mass fraction of the small a-C(:H) grains with the specific star-formation rate. On average, young stars are very efficient in heating the dust, with absorption fractions reaching as high as ~77% of the total, unattenuated luminosity of this population. On the other hand, the maximum absorption fraction of old stars is ~24%. Dust heating in early-type galaxies is mainly due to old stars, up to a level of ~90%. Young stars progressively contribute more for `typical' spiral galaxies and they become the dominant source of dust heating for Sm type and irregular galaxies, donating up to ~60% of their luminosity to this purpose. Finally, we find a strong correlation of the dust heating fraction by young stars with morphology and the specific star-formation rate.

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arXiv2018

Dust emission profiles of DustPedia galaxies

Most radiative transfer models assume that dust in spiral galaxies is distributed exponentially. In this paper our goal is to verify this assumption by analysing the two-dimensional large-scale distribution of dust in galaxies from the DustPedia sample. For this purpose, we make use of Herschel imaging in five bands, from 100 to 500μm, in which the cold dust constituent is primarily traced and makes up the bulk of the dust mass in spiral galaxies. For a subsample of 320 disc galaxies, we successfully perform a simultaneous fitting with a single Sérsic model of the Herschel images in all five bands using the multiband modelling code GALFITM. We report that the Sérsic index $n$, which characterises the shape of the Sérsic profile, lies systematically below 1 in all Herschel bands and is almost constant with wavelength. The average value at 250μm is $0.67\pm0.37$ (187 galaxies are fitted with $n_{250}\leq0.75$, 87 galaxies have $0.75 1.25$). Most observed profiles exhibit a depletion in the inner region (at $r<0.3-0.4$ of the optical radius $r_{25}$ ) and are more or less exponential in the outer part. We also find breaks in the dust emission profiles at longer distances $(0.5-0.6)r_{25}$ which are associated with the breaks in the optical and near-infrared. We assume that the observed deficit of dust emission in the inner galaxy region is related to the depression in the radial profile of the HI surface density in the same region because the atomic gas reaches high enough surface densities there to be transformed into molecular gas. If a galaxy has a triggered star formation in the inner region (for example, because of a strong bar instability, which transfers the gas inwards to the centre, or a pseudobulge formation), no depletion or even an excess of dust emission in the centre is observed.

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arXiv2018

Modeling ionized gas in low-metallicity environments: the Local Group dwarf galaxy IC10

Our objective is to investigate the physical properties of the ionised gas of the low-metallicity dwarf galaxy, IC 10, at various spatial scales: from individual HII regions to the entire galaxy scale and examine whether diagnostics for integrated measurements introduce bias in the results. We modeled the ionised gas combining the mid- and far-infrared fine-structure cooling lines observed with Spitzer/IRS and Herschel/PACS, with the photoionisation code Cloudy. The free parameters of the models are the age of the stellar cluster, the density and the ionisation parameter of the ionised gas as well as the depth of the cloud. The latter is used to investigate the leakage of the ionising photons from the analysed regions of IC 10. We investigate HII regions in the main star-forming body, on scales of ~25 pc, three in the main star-forming region in the center of the galaxy and two on the first arc. We then consider larger sizes on the scale of ~200 pc. We find that most clumps have nearly identical properties, density ~10$^{2.} $ - 10$^{2.6}$ cm$^{-3}$, ionisation parameter between 10$^{-2.2}$ and 10$^{-1.6}$ and age of the stellar cluster ~5.5 Myr. All of them are matter-bounded regions, allowing ionising photons to leak. The relatively uniform physical properties of the clumps suggest a common origin for their star formation activity, which could be related to the feedback from stellar winds or supernovae of a previous generation of stars. The properties derived for ~200 pc size "zones" have similar properties as the HII regions they encompass, but with the larger regions tending to be more radiation-bounded. Finally, we investigate the fraction of [CII] 157.7 μm, [SiII] 34.8 μm, and [FeII] 25.9 μm, emission arising from the ionised gas phase and we find that most of the emission originates from the neutral gas, not from the ionised gas.

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arXiv2018

The fraction of bolometric luminosity absorbed by dust in DustPedia galaxies

We study the fraction of stellar radiation absorbed by dust, f_abs, in 814 galaxies of different morphological types. The targets constitute the vast majority (93%) of the DustPedia sample, including almost all large (optical diameter larger than 1'), nearby (v <= 3000 km/s) galaxies observed with the Herschel Space Observatory. For each object, we model the spectral energy distribution from the ultraviolet to the sub-millimetre using the dedicated, aperture-matched DustPedia photometry and the fitting code CIGALE. The value of f_abs is obtained from the total luminosity emitted by dust and from the bolometric luminosity, which are estimated by the fit. On average, 19% of the stellar radiation is absorbed by dust in DustPedia galaxies. The fraction rises to 25% if only late-type galaxies are considered. The dependence of f_abs on morphology, showing a peak for Sb-Sc galaxies, is weak; it reflects a stronger, yet broad, positive correlation with the bolometric luminosity, which is identified for late-type, disk-dominated, high-specific-star-formation rate, gas-rich objects. We find no variation of f_abs with inclination, at odds with radiative transfer models of edge-on galaxies. These results call for a self-consistent modelling of the evolution of the dust mass and geometry along the build-up of the stellar content. We also provide template spectral energy distributions in bins of morphology and luminosity and study the variation of f_abs with stellar mass and specific star formation rate. We confirm that the local Universe is missing the high f_abs}, luminous and actively star-forming objects necessary to explain the energy budget in observations of the extragalactic background light.

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arXiv2018

A Dust spectral energy distribution model with hierarchical Bayesian inference. I. Formalism & benchmarking

This article presents a new dust SED model, named HerBIE, aimed at eliminating the noise-induced correlations and large scatter obtained when performing least-squares fits. The originality of this code is to apply the hierarchical Bayesian approach to full dust models, including realistic optical properties, stochastic heating and the mixing of physical conditions in the observed regions. We test the performances of our model by applying it to synthetic observations. We explore the impact on the recovered parameters of several effects: signal-to-noise ratio, SED shape, sample size, the presence of intrinsic correlations, the wavelength coverage and the use of different SED model components. We show that this method is very efficient: the recovered parameters are consistently distributed around their true values. We do not find any clear bias, even for the most degenerate parameters, or with extreme signal-to-noise ratios.

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arXiv2017

Probing the baryon cycle of galaxies with SPICA mid- and far-infrared observations

The SPICA mid and far-infrared telescope will address fundamental issues in our understanding of star formation and ISM physics in galaxies. A particular hallmark of SPICA is the outstanding sensitivity enabled by the cold telescope, optimized detectors, and wide instantaneous bandwidth throughout the mid- and far-infrared. The spectroscopic, imaging and polarimetric observations that SPICA will be able to collect will help in clarifying the complex physical mechanisms which underlie the baryon cycle of galaxies. In particular: (i) The access to a large suite of atomic and ionic fine-structure lines for large samples of galaxies will shed light on the origin of the observed spread in star formation rates within and between galaxies. (ii) Observations of HD rotational lines (out to $\sim$10 Mpc) and fine structure lines such as [CII] 158 $μ$m (out to $\sim$100 Mpc) will clarify the main reservoirs of interstellar matter in galaxies, including phases where CO does not emit. (iii) Far-infrared spectroscopy of dust and ice features will address uncertainties in the mass and composition of dust in galaxies, and the contributions of supernovae to the interstellar dust budget will be quantified by photometry and monitoring of supernova remnants in nearby galaxies. (iv) Observations of far-infrared cooling lines such as [OI] 63 $μ$m from star-forming molecular clouds in our Galaxy will evaluate the importance of shocks to dissipate turbulent energy. The paper concludes with requirements for the telescope and instruments, and recommendations for the observing strategy.

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arXiv2017

Radial distribution of dust, stars, gas, and star-formation rate in DustPedia face-on galaxies

The purpose of this work is the characterization of the radial distribution of dust, stars, gas, and star-formation rate (SFR) in a sub-sample of 18 face-on spiral galaxies extracted from the DustPedia sample. This study is performed by exploiting the multi-wavelength, from UV to sub-mm bands, DustPedia database, in addition to molecular (12CO) and atomic (HI) gas maps and metallicity abundance information available in the literature. We fitted the surface brightness profiles of the tracers of dust and stars, the mass surface density profiles of dust, stars, molecular gas, and total gas, and the SFR surface density profiles with an exponential curve and derived their scale-lengths. We also developed a method to solve for the CO-to-H2 conversion factor (alpha_CO) per galaxy by using dust and gas mass profiles. Although each galaxy has its own peculiar behaviour, we identified a common trend of the exponential scale-lengths vs. wavelength. On average, the scale-lengths normalized to the B-band 25 mag/arcsec^2 radius decrease from UV to 70 micron, from 0.4 to 0.2, and then increase back up to 0.3 at 500 microns. The main result is that, on average, the dust mass surface density scale-length is about 1.8 times the stellar one derived from IRAC data and the 3.6 micron surface brightness, and close to that in the UV. We found a mild dependence of the scale-lengths on the Hubble stage T: the scale-lengths of the Herschel bands and the 3.6 micron scale-length tend to increase from earlier to later types, the scale-length at 70 micron tends to be smaller than that at longer sub-mm wavelength with ratios between longer sub-mm wavelengths and 70 micron that decrease with increasing T. The scale-length ratio of SFR and stars shows a weak increasing trend towards later types.

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arXiv2017

New ALMA constraints on the star-forming ISM at low metallicity: A 50 pc view of the blue compact dwarf galaxy SBS0335-052

Properties of the cold interstellar medium of low-metallicity galaxies are not well-known due to the faintness and extremely small scale on which emission is expected. We present deep ALMA band 6 (230GHz) observations of the nearby, low-metallicity (12 + log(O/H) = 7.25) blue compact dwarf galaxy SBS0335-052 at an unprecedented resolution of 0.2 arcsec (52 pc). The 12CO J=2-1 line is not detected and we report a 3-sigma upper limit of LCO(2-1) = 3.6x10^4 K km/s pc^2. Assuming that molecular gas is converted into stars with a given depletion time, ranging from 0.02 to 2 Gyr, we find lower limits on the CO-to-H2 conversion factor alpha_CO in the range 10^2-10^4 Msun pc^-2 (K km/s)^-1. The continuum emission is detected and resolved over the two main super star clusters. Re-analysis of the IR-radio spectral energy distribution suggests that the mm-fluxes are not only free-free emission but are most likely also associated with a cold dust component coincident with the position of the brightest cluster. With standard dust properties, we estimate its mass to be as large as 10^5 Msun. Both line and continuum results suggest the presence of a large cold gas reservoir unseen in CO even with ALMA.

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arXiv2017

Neutral gas heating by X-rays in primitive galaxies: Infrared observations of IZw18 with Herschel

(abridged) The dominant thermal mechanisms in the neutral interstellar medium, which acts as a star-forming gas reservoir, are uncertain in extremely metal-poor galaxies. Our objective is to identify the heating mechanisms in one such galaxy, IZw18, and assess the diagnostic value of fine-structure cooling lines. We also seek to constrain the mass of H$_2$, which, despite being an important catalyst and tracer of star formation, remains elusive in this object. Building on a previous photoionization model within a multi-sector topology, we provide additional constraints from the [CII] and [OI] lines and the dust mass recently measured with Herschel. The heating of the HI region appears to be mainly due to photoionization by radiation from a bright X-ray binary source, while photoelectric effect (PE) is negligible. The [CII] and [OI] lines imply an average X-ray luminosity of $4\times10^{40}$ erg s$^{-1}$, while the [NeV] upper limits bring strong constraints to the soft X-ray flux arising from the binary. A negligible amount of H$_2$ is predicted, but $\lesssim10^7$ M$_\odot$ of H$_2$ may be hidden in sufficiently dense clouds of order $\lesssim10$ pc in size. Regardless of the presence of significant amounts of H$_2$, [CII] and [OI] do not trace the so-called CO-dark gas, but the almost purely atomic medium. Although the [CII]+[OI]/TIR ratio is close to values found in more metal-rich sources, it cannot be safely used as a PE heating efficiency proxy. This ratio seems to be kept stable due to a correlation between the X-ray luminosity and the star-formation rate. We propose that X-ray heating is an important process in extremely metal-poor sources. The weak PE heating due to the low dust-to-gas ratio could be compensated for by the larger occurrence and power of X-ray binaries in low-metallicity galaxies. We speculate that X-ray heating may quench star formation.

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arXiv2016

DustPedia - A Definitive Study of Cosmic Dust in the Local Universe

The European Space Agency has invested heavily in two cornerstones missions; Herschel and Planck. The legacy data from these missions provides us with an unprecedented opportunity to study cosmic dust in galaxies so that we can answer fundamental questions about, for example: the origin of the chemical elements, physical processes in the interstellar medium (ISM), its effect on stellar radiation, its relation to star formation and how this relates to the cosmic far infrared background. In this paper we describe the DustPedia project, which is enabling us to develop tools and computer models that will help us relate observed cosmic dust emission to its physical properties (chemical composition, size distribution, temperature), to its origins (evolved stars, super novae, growth in the ISM) and the processes that destroy it (high energy collisions and shock heated gas). To carry out this research we will combine the Herschel/Planck data with that from other sources of data, providing observations at numerous wavelengths (< 41) across the spectral energy distribution, thus creating the DustPedia database. To maximise our spatial resolution and sensitivity to cosmic dust we limit our analysis to 4231 local galaxies (v < 3000 km/s) selected via their near infrared luminosity (stellar mass). To help us interpret the data we have developed a new physical model for dust (THEMIS), a new Bayesian method of fitting and interpreting spectral energy distributions (HerBIE) and a state-of-the-art Monte Carlo photon tracing radiative transfer model (SKIRT). In this the first of the DustPedia papers we describe the project objectives, data sets used and provide an insight into the new scientific methods we plan to implement.

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arXiv2016

A milestone toward understanding PDR properties in the extreme environment of LMC-30Dor

More complete knowledge of galaxy evolution requires understanding the process of star formation and interaction between the interstellar radiation field and the interstellar medium in galactic environments traversing a wide range of physical parameter space. Here we focus on the impact of massive star formation on the surrounding low metallicity ISM in 30 Doradus in the Large Magellanic Cloud. A low metal abundance, as is the case of some galaxies of the early universe, results in less ultra-violet shielding for the formation of the molecular gas necessary for star formation to proceed. The half-solar metallicity gas in this region is strongly irradiated by the super star cluster R136, making it an ideal laboratory to study the structure of the ISM in an extreme environment. Our spatially resolved study investigates the gas heating and cooling mechanisms, particularly in the photo-dissociation regions where the chemistry and thermal balance are regulated by far-ultraviolet photons (6 eV< hν<13.6 eV). We present Herschel observations of far-infrared fine-structure lines obtained with PACS and SPIRE/FTS. We have combined atomic fine-structure lines from Herschel and Spitzer observations with ground-based CO data to provide diagnostics on the properties and the structure of the gas by modeling it with the Meudon PDR code. We derive the spatial distribution of the radiation field, the pressure, the size, and the filling factor of the photodissociated gas and molecular clouds. We find a range of pressure of ~ 10^5 - 1.7x10^6 cm^{-3} K and a range of incident radiation field G_UV ~ 10^2 - 2.5x10^4 through PDR modeling. Assuming a plane-parallel geometry and a uniform medium, we find a total extinction of 1-3 mag , which correspond to a PDR cloud size of 0.2 to 3pc, with small CO depth scale of 0.06 to 0.5pc. We also determine the three dimensional structure of the gas. (Abridged)

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