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We present Herschel/PACS 100 and 160 μm integrated photometry for the 323 galaxies in the Herschel Reference Survey (HRS), a K-band, volume-limited sample of galaxies in the local Universe. Once combined with the Herschel/SPIRE observations already available, these data make the HRS the largest representative sample of nearby galaxies with homogeneous coverage across the 100-500 μm wavelength range. In this paper, we take advantage of this unique data set to investigate the properties and shape of the far-infrared/submillimetre spectral energy distribution in nearby galaxies. We show that, in the stellar mass range covered by the HRS (8 ≤ log (M*/M⊙) ≤ 12), the far-infrared/submillimetre colours are inconsistent with a single modified blackbody having the same dust emissivity index β for all galaxies. In particular, either β decreases or multiple temperature components are needed, when moving from metal-rich/gas-poor to metal-poor/gas-rich galaxies. We thus investigate how the dust temperature and mass obtained from a single modified blackbody depend on the assumptions made on β. We show that, while the correlations between dust temperature, galaxy structure and star formation rate are strongly model dependent, the dust mass scaling relations are much more reliable, and variations of β only change the strength of the observed trends. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

We examine the relation between polycyclic aromatic hydrocarbon (PAH) emission at 8 μm and far-infrared emission from hot dust grains at 24 μm and from large dust grains at 160 and 250 μm in the nearby spiral galaxies NGC 2403 and M83 using data from the Spitzer Space Telescope and Herschel Space Observatory. We find that the PAH emission in NGC 2403 is better correlated with emission at 250 μm from dust heated by the diffuse interstellar radiation field (ISRF) and that the 8/250-μm surface brightness ratio is well correlated with the stellar surface brightness as measured at 3.6 μm. This implies that the PAHs in NGC 2403 are intermixed with cold large dust grains in the diffuse interstellar medium (ISM) and that the PAHs are excited by the diffuse ISRF. In M83, the PAH emission appears more strongly correlated with 160 μm emission originating from large dust grains heated by star-forming regions. However, the PAH emission in M83 is low where the 24-μm emission peaks within star-forming regions, and enhancements in the 8/160-μm surface brightness ratios appear offset relative to the dust and the star-forming regions within the spiral arms. This suggests that the PAHs observed in the 8 μm band are not excited locally within star-forming regions but either by light escaping non-axisymmetrically from star-forming regions or locally by young, non-photoionizing stars that have migrated downstream from the spiral density waves. The results from just these two galaxies show that PAHs may be excited by different stellar populations in different spiral galaxies. [ABSTRACT FROM PUBLISHER]

We investigate the physical properties of the molecular and ionized gas, and their relationship to the star formation and dust properties in M?83, based on submillimeter imaging spectroscopy from within the central 3.5' (~4 kpc in diameter) around the starburst nucleus. The observations use the Fourier Transform Spectrometer (FTS) of the Spectral and Photometric Imaging REceiver (SPIRE) onboard the Herschel Space Observatory. The newly observed spectral lines include [CI] 370 μm, [CI] 609 μm, [NII] 205 μm, and CO transitions from J = 4-3 to J = 13-12. Combined with previously observed J = 1-0 to J = 3-2 transitions, the CO spectral line energy distributions are translated to spatially resolved physical parameters, column density of CO, N(CO), and molecular gas thermal pressure, Pth, with a non-local thermal equilibrium (non-LTE) radiative transfer model, RADEX. Our results show that there is a relationship between the spatially resolved intensities of [NII] 205 μm and the surface density of the star formation rate (SFR), ΣSFR. This relation, when compared to integrated properties of ultra-luminous infrared galaxies (ULIRGs), exhibits a different slope, because the [NII] 205 μm distribution is more extended than the SFR. The spatially resolved [CI] 370 μm, on the other hand, shows a generally linear relationship with ΣSFR and can potentially be a good SFR tracer. Compared with the dust properties derived from broad-band images, we find a positive trend between the emissivity of CO in the J = 1-0 transition with the average intensity of interstellar radiation field (ISRF), ⟨ U ⟩. This trend implies a decrease in the CO-to-H2 conversion factor, XCO, when ⟨ U ⟩ increases. We estimate the gas-to-dust mass ratios to be 77 ± 33 within the central 2 kpc and 93 ± 19 within the central 4 kpc of M?83, which implies a Galactic dust-to-metal mass ratio within the observed region of M?83. The estimated gas-depletion time for the M?83 nucleus is 1.13 ± 0.6 Gyr, which is shorter than the values for nearby spiral galaxies found in the literature (~2.35 Gyr), most likely due to the young nuclear starbursts. A linear relationship between Pth and the radiation pressure generated by ⟨ U ⟩, Prad, is found to be Pth ≈ 30 Prad, which signals that the ISRF alone is insufficient to sustain the observed CO transitions. The spatial distribution of Pth reveals a pressure gradient, which coincides with the observed propagation of starburst activities and the alignment of (possibly background) radio sources. We discover that the off-centered (from the optical nucleus) peak of the molecular gas volume density coincides well with a minimum in the relative aromatic feature strength, indicating a possible destruction of their carriers. We conclude that the observed CO transitions are most likely associated with mechanical heating processes that are directly or indirectly related to very recent nuclear starbursts. [ABSTRACT FROM AUTHOR]

Context. Dust reprocesses about half of the stellar radiation in galaxies. The thermal re-emission by dust of absorbed energy is considered to be driven merely by young stars so is often applied to tracing the star formation rate in galaxies. Recent studies have argued that the old stellar population might be responsible for a non-negligible fraction of the radiative dust heating. Aims. In this work, we aim to analyze the contribution of young (≲100 Myr) and old (~10 Gyr) stellar populations to radiative dust heating processes in the nearby grand-design spiral galaxy M51 using radiative transfer modeling. High-resolution 3D radiative transfer (RT) models are required to describe the complex morphologies of asymmetric spiral arms and clumpy star-forming regions and to model the propagation of light through a dusty medium. Methods. In this paper, we present a new technique developed to model the radiative transfer effects in nearby face-on galaxies. We construct a high-resolution 3D radiative transfer model with the Monte-Carlo code SKIRT to account for the absorption, scattering, and non-local thermal equilibrium (NLTE) emission of dust in M51. The 3D distribution of stars is derived from the 2D morphology observed in the IRAC 3.6 μm, GALEX FUV, Hμα and MIPS 24 μm wavebands, assuming an exponential vertical distribution with an appropriate scale height. The dust geometry is constrained through the far-ultraviolet (FUV) attenuation, which is derived from the observed total-infrared-to-far-ultraviolet luminosity ratio. The stellar luminosity, star formation rate, and dust mass have been scaled to reproduce the observed stellar spectral energy distribution (SED), FUV attenuation, and infrared SED. Results. The dust emission derived from RT calculations is consistent with far-infrared and submillimeter observations of M51, implying that the absorbed stellar energy is balanced by the thermal re-emission of dust. The young stars provide 63% of the energy for heating the dust responsible for the total infrared emission (8-1000 μm), while 37% of the dust emission is governed through heating by the evolved stellar population. In individual wavebands, the contribution from young stars to the dust heating dominates at all infrared wavebands but gradually decreases towards longer infrared and submillimeter wavebands for which the old stellar population becomes a non-negligible source of heating. Upon extrapolation of the results for M51, we present prescriptions for estimating the contribution of young stars to the global dust heating based on a tight correlation between the dust heating fraction and specific star formation rate. [ABSTRACT FROM AUTHOR]

Although it accounts only for a small fraction of the baryonic mass, dust has a profound impact on the physical processes at play in galaxies. Thus, to understand the evolution of galaxies, it is essential not only to characterize dust properties per se, but also in relation to global galaxy properties. To do so, we derive the dust properties of galaxies in a volume limited, K-band selected sample, the Herschel Reference Survey (HRS). We gather infrared photometric data from 8 m to 500 m from Spitzer, WISE, IRAS, and Herschel for all of the HRS galaxies. Draine & Li (2007, ApJ, 663, 866) models are fit to the data from which the stellar contribution has been carefully removed. We find that our photometric coverage is su cient to constrain all of the parameters of the Draine & Li models and that a strong constraint on the 20􀀀60 m range is mandatory to estimate the relative contribution of the photo-dissociation regions to the infrared spectral energy distribution (SED). The SED models tend to systematically underestimate the observed 500 m flux densities, especially for low-mass systems. We provide the output parameters for all of the galaxies, i.e., the minimum intensity of the interstellar radiation field, the fraction of polycyclic aromatic hydrocarbon (PAH), the relative contribution of PDR and evolved stellar population to the dust heating, the dust mass, and the infrared luminosity. For a subsample of gas-rich galaxies, we analyze the relations between these parameters and the main integrated properties of galaxies, such as stellar mass, star formation rate, infrared luminosity, metallicity, H and H-band surface brightness, and the far-ultraviolet attenuation. A good correlation between the fraction of PAH and the metallicity is found, implying a weakening of the PAH emission in galaxies with low metallicities and, thus, low stellar masses. The intensity of the diffuse interstellar radiation field and the H-band and Hα surface brightnesses are correlated, suggesting that the diffuse dust component is heated by both the young stars in star-forming regions and the diffuse evolved population. We use these results to provide a new set of infrared templates calibrated with Herschel observations on nearby galaxies and a mean SED template to provide the z = 0 reference for cosmological studies. For the same purpose, we place our sample on the S FR􀀀M* diagram. The templates are compared to the most popular infrared SED libraries, enlightening a large discrepancy between all of them in the 20􀀀100m range. [ABSTRACT FROM AUTHOR]

We investigate the connection between dust and gas in the nearby edge-on spiral galaxy NGC 891, a target of the Very Nearby Galaxies Survey. High resolution Herschel PACS and SPIRE 70, 100, 160, 250, 350, and 500 μm images are combined with JCMT SCUBA 850 μm observations to trace the far-infrared/submillimetre spectral energy distribution (SED). Maps of the Hi 21 cm line and CO(J = 3−2) emission trace the atomic and molecular hydrogen gas, respectively. We fit one-component modified blackbody models to the integrated SED, finding a global dust mass of (8.5 ± 2.0) × 107 M⊙ and an average temperature of 23 ± 2 K, consistent with results from previous far-infrared experiments. We also fit one-component modified blackbody models to pixel-by-pixel SEDs to produce maps of the dust mass and temperature. The dust mass distribution correlates with the total stellar population as traced by the 3.6 μm emission. The derived dust temperature, which ranges from approximately 17 to 24 K, is found to correlate with the 24 μm emission. Allowing the dust emissivity index to vary, we find an average value of β = 1.9±0.3.We confirm an inverse relation between the dust emissivity spectral index and dust temperature, but do not observe any variation of this relationship with vertical height from the mid-plane of the disc. A comparison of the dust properties with the gaseous components of the ISM reveals strong spatial correlations between the surface mass densities of dust (Σdust) and the molecular hydrogen (ΣH2 ) and total gas surface densities (Σgas). These observations reveal the presence of regions of dense, cold dust that are coincident with peaks in the gas distribution and are associated with a molecular ring. Furthermore, the observed asymmetries in the dust temperature, the H2-to-dust ratio and the total gas-to-dust ratio hint that an enhancement in the star formation rate may be the result of larger quantities of molecular gas available to fuel star formation in the NE compared to the SW. Whilst the asymmetry likely arises from dust obscuration due to the geometry of the line-of-sight projection of the spiral arms, we cannot exclude that there is also an enhancement in the star formation rate in the NE part of the disc. [ABSTRACT FROM AUTHOR]

Context. Observations of nearby starburst and spiral galaxies have revealed that molecular gas is the driver of star formation. However, some nearby low-metallicity dwarf galaxies are actively forming stars, but CO, the most common tracer of this reservoir, is faint, leaving us with a puzzle about how star formation proceeds in these environments. Aims. We aim to quantify the molecular gas reservoir in a subset of 6 galaxies from the Herschel Dwarf Galaxy Survey with newly acquired CO data and to link this reservoir to the observed star formation activity. Methods. We present CO(1-0), CO(2-1), and CO(3-2) observations obtained at the ATNF Mopra 22-m, APEX, and IRAM30-m telescopes, as well as [C ii] 157μm and [Oi] 63 μm observations obtained with the Herschel/PACS spectrometer in the 6 low-metallicity dwarf galaxies: Haro 11, Mrk 1089, Mrk 930, NGC4861, NGC625, and UM311.We derived their molecular gas masses from several methods, including using the CO-to-H2 conversion factor XCO (both Galactic and metallicity-scaled values) and dust measurements. The molecular and atomic gas reservoirs were compared to the star formation activity. We also constrained the physical conditions of the molecular clouds using the non-LTE code RADEX and the spectral synthesis code Cloudy. Results. We detect CO in 5 of the 6 galaxies, including first detections in Haro 11 (Z ∼ 0.4 Z☉), Mrk 930 (0.2 Z☉), and UM311 (0.5 Z☉), but CO remains undetected in NGC4861 (0.2 Z☉). The CO luminosities are low, while [C II] is bright in these galaxies, resulting in [C II]/CO(1-0) ⩾ 10 000. Our dwarf galaxies are in relatively good agreement with the Schmidt-Kennicutt relation for total gas. They show short molecular depletion timescales, even when considering metallicity-scaled XCO factors. Those galaxies are dominated by their Hi gas, except Haro 11, which has high star formation efficiency and is dominated by ionized and molecular gas. We determine the mass of each ISM phase in Haro 11 using Cloudy and estimate an equivalent XCO factor that is 10 times higher than the Galactic value. Overall, our results confirm the emerging picture that CO suffers from significant selective photodissociation in low-metallicity dwarf galaxies. [ABSTRACT FROM AUTHOR]

Aims. The goal of this paper is to analyse the behaviour of the gas-to-dust mass ratio (G/D) of local Universe galaxies over a wide metallicity range. We especially focus on the low-metallicity part of the G/D vs metallicity relation and investigate several explanations for the observed relation and scatter. Methods. We assembled a total of 126 galaxies, covering a 2 dex metallicity range and with 30% of the sample with 12 + log(O/H) ⩽ 8:0. We homogeneously determined the dust masses with a semi-empirical dust model including submm constraints. The atomic and molecular gas masses have been compiled from the literature. We used two XCO scenarios to estimate the molecular gas mass: the Galactic conversion factor, XCO,MW, and a XCO that depends on the metallicity XCO,Z (αZ-2). We modelled the observed trend of the G/D with metallicity using two simple power laws (slope of -1 and free) and a broken power law. Correlations with morphological type, stellar masses, star formation rates, and specific star formation rates are also discussed. We then compared the observed evolution of the G/D with predictions from several chemical evolution models and explored different physical explanations for the observed scatter in the G/D values. Results. We find that out of the five tested galactic parameters, metallicity is the main physical property of the galaxy driving the observed G/D. The G/D versus metallicity relation cannot be represented by a single power law with a slope of -1 over the whole metallicity range. The observed trend is steeper for metallicities lower than ~8.0. A large scatter is observed in the G/D values for a given metallicity: in metallicity bins of ~0.1 dex, the dispersion around the mean value is ~0.37 dex. On average, the broken power law reproduces the observed G/D best compared to the two power laws (slope of -1 or free) and provides estimates of the G/D that are accurate to a factor of 1.6. The good agreement of observed values of the G/D and its scatter with respect to metallicity with the predicted values of the three tested chemical evolution models allows us to infer that the scatter in the relation is intrinsic to galactic properties, reflecting the different star formation histories, dust destruction efficiencies, dust grain size distributions, and chemical compositions across the sample. Conclusions. Our results show that the chemical evolution of low-metallicity galaxies, traced by their G/D, strongly depends on their local internal conditions and individual histories. The large scatter in the observed G/D at a given metallicity reflects the impact of various processes occurring during the evolution of a galaxy. Despite the numerous degeneracies affecting them, disentangling these various processes is now the next step.q [ABSTRACT FROM AUTHOR]

Context. We present new photometric data from our Herschel guaranteed time key programme, the Dwarf Galaxy Survey (DGS), dedicated to the observation of the gas and dust in low-metallicity environments. A total of 48 dwarf galaxies were observed with the PACS and SPIRE instruments onboard the Herschel Space Observatory at 70, 100, 160, 250, 350, and 500 μm. Aims. The goal of this paper is to provide reliable far-infrared (FIR) photometry for the DGS sample and to analyse the FIR/submillimetre (submm) behaviour of the DGS galaxies. We focus on a systematic comparison of the derived FIR properties (FIR luminosity, LFIR, dust mass, Mdust, dust temperature, T, emissivity index, β) with more metal-rich galaxies and investigate the detection of a potential submm excess. Methods. The data reduction method is adapted for each galaxy in order to derive the most reliable photometry from the final maps. The derived PACS flux densities are compared with the Spitzer MIPS 70 and 160 μm bands. We use colour-colour diagrams to analyse the FIR/submm behaviour of the DGS galaxies and modified blackbody fitting procedures to determine their dust properties. To study the variation in these dust properties with metallicity, we also include galaxies from the Herschel KINGFISH sample, which contains more metal-rich environments, totalling 109 galaxies. Results. The location of the DGS galaxies on Herschel colour-colour diagrams highlights the differences in dust grain properties and/or global environments of low-metallicity dwarf galaxies. The dust in DGS galaxies is generally warmer than in KINGFISH galaxies (TDGS ~ 32 K and TKINGFISH ~ 23 K). The emissivity index, β, is ~1.7 in the DGS, however metallicity does not make a strong effect on β. The proportion of dust mass relative to stellar mass is lower in low-metallicity galaxies: Mdust/Mstar ~ 0:02% for the DGS versus 0.1% for KINGFISH. However, per unit dust mass, dwarf galaxies emit about six times more in the FIR/submm than higher metallicity galaxies. Out of the 22 DGS galaxies detected at 500 μμm, about 41% present an excess in the submm beyond the explanation of our dust SED model, and this excess can go up to 150% above the prediction from the model. The excess mainly appears in lower metallicity galaxies (12 + log(O/H) . 8.3), and the strongest excesses are detected in the most metal-poor galaxies. However, we also stress the need for observations longwards of the Herschel wavelengths to detect any submm excess appearing beyond 500 μm. [ABSTRACT FROM AUTHOR]

We present an integrated photometric spectral energy distribution (SED) of the Magellanic-type galaxy NGC 4449 from the far-ultraviolet (UV) to the submillimetre, including new observations acquired by the Herschel Space Observatory. We include integrated UV photometry from the Swift Ultraviolet and Optical Telescope using a measurement technique which is appropriate for extended sources with coincidence loss. In this paper, we examine the available multiwavelength data to infer a range of ages, metallicities and star formation rates for the underlying stellar populations, as well as the composition and the total mass of dust in NGC 4449. Our analysis of the global optical spectrum of NGC 4449 fitted using the spectral fitting code starlight suggests that the majority of stellar mass resides in old (≳1 Gyr old) and metal-poor (Z/Z⊙ ∼ 0.2) populations, with the first onset of star formation activity deduced to have taken place at an early epoch, approximately 12 Gyr ago. A simple chemical evolution model, suitable for a galaxy continuously forming stars, suggests a ratio of carbon to silicate dust mass comparable to that of the Large Magellanic Cloud over the inferred time-scales. We present an iterative scheme, which allows us to build an in-depth and multicomponent representation of NGC 4449 ‘bottom-up’, taking advantage of the broad capabilities of the photoionization and radiative transfer code mocassin (MOnte CArlo SimulationS of Ionized Nebulae). We fit the observed SED, the global ionization structure and the emission line intensities, and infer a recent star formation rate of 0.4 M⊙ yr− 1 and a total stellar mass of ≈ 1 × 109 M⊙ emitting with a bolometric luminosity of 5.7 × 109 L⊙. Our fits yield a total dust mass of 2.9 ± 0.5 × 106 M⊙ including 2 per cent attributed to polycyclic aromatic hydrocarbons. We deduce a dust to gas mass ratio of 1/190 within the modelled region. While we do not consider possible additional contributions from even colder dust, we note that including the extended H i envelope and the molecular gas is likely to bring the ratio down to as low as ∼1/800. [ABSTRACT FROM AUTHOR]

The article focuses on a study that compares the results for dust-to-stellar ratios and metallicity for two Herschel samples. The study is undertaken through the computation of stellar masses. The results of the study show that stellar masses are constantly estimated for both samples particularly in observed correlations.