We model the infrared to submillimeter spectral energy distribution of 11 nearby galaxies of the KINGFISH sample using Spitzer and Herschel data and compare model extrapolations at 870um (using different fitting techniques) with LABOCA 870um observat
ions. We investigate how the differences between predictions and observations vary with model assumptions or environment. At global scales, we find that modified blackbody models using realistic cold emissivity indices (beta_c=2 or 1.5) are able to reproduce the 870um observed emission within the uncertainties for most of the sample. Low values (beta_c<1.3) would be required in NGC0337, NGC1512 and NGC7793. At local scales, we observe a systematic 870um excess when using beta_=2.0. The beta_c=1.5 or the Draine and Li (2007) models can reconcile predictions with observations in part of the disks. Some of the remaining excesses occur towards the centres and can be partly or fully accounted for by non-dust contributions such as CO(3-2) or, to a lesser extent, free-free or synchrotron emission. In three non-barred galaxies, the remaining excesses rather occur in the disk outskirts. This could be a sign of a flattening of the submm slope (and decrease of the effective emissivity index) with radius in these objects.
The Dwarf Galaxy Survey (DGS) program is studying low-metallicity galaxies using 230h of far-infrared (FIR) and submillimetre (submm) photometric and spectroscopic observations of the Herschel Space Observatory and draws to this a rich database of a
wide range of wavelengths tracing the dust, gas and stars. This sample of 50 galaxies includes the largest metallicity range achievable in the local Universe including the lowest metallicity (Z) galaxies, 1/50 Zsun, and spans 4 orders of magnitude in star formation rates. The survey is designed to get a handle on the physics of the interstellar medium (ISM) of low metallicity dwarf galaxies, especially on their dust and gas properties and the ISM heating and cooling processes. The DGS produces PACS and SPIRE maps of low-metallicity galaxies observed at 70, 100, 160, 250, 350, and 500 mic with the highest sensitivity achievable to date in the FIR and submm. The FIR fine-structure lines, [CII] 158 mic, [OI] 63 mic, [OI] 145 mic, [OIII] 88 mic, [NIII] 57 mic and [NII] 122 and 205 mic have also been observed with the aim of studying the gas cooling in the neutral and ionized phases. The SPIRE FTS observations include many CO lines (J=4-3 to J=13-12), [NII] 205 mic and [CI] lines at 370 and 609 mic. This paper describes the sample selection and global properties of the galaxies, the observing strategy as well as the vast ancillary database available to complement the Herschel observations. The scientific potential of the full DGS survey is described with some example results included.
We present new empirical calibrations to estimate resolved and integrated total infrared luminosities from Spitzer and Herschel bands used as monochromatic or combined tracers. We base our calibrations on resolved elements of nearby galaxies (3 to 30
Mpc) observed with Herschel. We perform a resolved SED modelling of these objects using the Draine and Li (2007) dust models and investigate the influence of the addition of SPIRE measurements in the estimation of LTIR. We find that using data up to 250 um leads to local LTIR values consistent with those obtained with a complete coverage (up to 500 um) within 10 per cent for most of our resolved elements. We then study the distribution of energy in the resolved SEDs of our galaxies. The bulk of energy (30-50 per cent) is contained in the (70-160 um) band. The (24-70 um) fraction decreases with increasing metallicity. The (160-1100 um) submillimeter band can account for up to 25 per cent of the LTIR in metal-rich galaxies. We investigate the correlation between TIR surface brightnesses/luminosities and monochromatic Spitzer and Herschel surface brightnesses/luminosities. The three PACS bands can be used as reliable monochromatic estimators of the LTIR, the 100 um band being the most reliable monochromatic tracer. There is also a strong correlation between the SPIRE 250um and LTIR, although with more scatter than for the PACS relations. We also study the ability of our monochromatic relations to reproduce integrated LTIR of nearby galaxies as well as LTIR of z=1-3 sources. Finally, we provide calibration coefficients that can be used to derive TIR surface brightnesses/luminosities from a combination of Spitzer and Herschel surface brightnesses/fluxes and analyse the associated uncertainties.
We present a study of the infrared/submm emission of the LMC star forming complex N158-N159-N160. Combining observations from the Spitzer Space Telescope (3.6-70um), the Herschel Space Observatory (100-500um) and LABOCA (870um) allows us to work at t
he best angular resolution available now for an extragalactic source. We observe a remarkably good correlation between SPIRE and LABOCA emission and resolve the low surface brightnesses emission. We use the Spitzer and Herschel data to perform a resolved Spectral Energy Distribution (SED) modelling of the complex. Using MBB, we derive a global emissivity index beta_c of 1.47. If beta cold is fixed to 1.5, we find an average temperature of 27K. We also apply the Galliano et al. (2011) modelling technique (and amorphous carbon to model carbon dust) to derive maps of the star formation rate, the mean starlight intensity, the fraction of PAHs or the dust mass surface density of the region. We observe that the PAH fraction strongly decreases in the HII regions. This decrease coincides with peaks in the mean radiation field intensity map. The dust surface densities follow the FIR distribution, with a total dust mass of 2.1x10^4 Msolar (2.8 times less than when using graphite grains) in the resolved elements we model. We find a non-negligible amount of dust in the molecular cloud N159 South (showing no massive SF). We also investigate the drivers of the Herschel/PACS and SPIRE submm colours as well as the variations in the gas-to-dust mass ratio (G/D) and the XCO conversion factor in the region N159. We finally model individual regions to analyse variations in the SED shape across the complex and the 870um emission in more details. No measurable submm excess emission at 870um seems to be detected in these regions.
We present the first Herschel PACS and SPIRE images of the low-metallicity galaxy NGC6822 observed from 70 to 500 mu and clearly resolve the HII regions with PACS and SPIRE. We find that the ratio 250/500 is dependent on the 24 mu surface brightness
in NGC6822, which would locally link the heating processes of the coldest phases of dust in the ISM to the star formation activity. We model the SEDs of some regions HII regions and less active regions across the galaxy and find that the SEDs of HII regions show warmer ranges of dust temperatures. We derive very high dust masses when graphite is used in our model to describe carbon dust. Using amorphous carbon, instead, requires less dust mass to account for submm emission due to its lower emissivity properties. This indicates that SED models including Herschel constraints may require different dust properties than commonly used.
