ALBERT

Description: This is the first of a series of papers on the Infrared Database of Extragalactic Observables from Spitzer (IDEOS). In this work, we describe the identification of optical counterparts of the infrared sources detected in Spitzer Infrared Spectrograph (IRS) observations, and the acquisition and validation of redshifts. The IDEOS sample includes all the spectra from the Cornell Atlas of Spitzer /IRS Sources (CASSIS) of galaxies beyond the Local Group. Optical counterparts were identified from correlation of the extraction coordinates with the NASA Extragalactic Database (NED). To confirm the optical association and validate NED redshifts, we measure redshifts with unprecedented accuracy on the IRS spectra (( z /(1+ z )) ~ 0.0011) by using an improved version of the maximum combined pseudo-likelihood method (MCPL). We perform a multistage verification of redshifts that considers alternate NED redshifts, the MCPL redshift, and visual inspection of the IRS spectrum. The statistics is as follows: the IDEOS sample contains 3361 galaxies at redshift 0 〈 z 〈 6.42 (mean: 0.48, median: 0.14). We confirm the default NED redshift for 2429 sources and identify 124 with incorrect NED redshifts. We obtain IRS-based redshifts for 568 IDEOS sources without optical spectroscopic redshifts, including 228 with no previous redshift measurements. We provide the entire IDEOS redshift catalogue in machine-readable formats. The catalogue condenses our compilation and verification effort, and includes our final evaluation on the most likely redshift for each source, its origin, and reliability estimates.

Description: We investigate star formation and dust heating in the compact far-infrared (FIR) bright sources detected in the Herschel maps of M83. We use the source extraction code getsources to detect and extract sources in the FIR, as well as their photometry in the mid-infrared and Hα. By performing infrared spectral energy distribution fitting and applying an Hα-based star formation rate (SFR) calibration, we derive the dust masses and temperatures, SFRs, gas masses and star formation efficiencies (SFEs). The detected sources lie exclusively on the spiral arms and represent giant molecular associations, with gas masses and sizes of 10 6 –10 8  M and 200–300 pc, respectively. The inferred parameters show little to no radial dependence and there is only a weak correlation between the SFRs and gas masses, which suggests that more massive clouds are less efficient at forming stars. Dust heating is mainly due to local star formation. However, although the sources are not optically thick, the total intrinsic young stellar population luminosity can almost completely account for the dust luminosity. This suggests that other radiation sources also contribute to the dust heating and approximately compensate for the unabsorbed fraction of UV light.

Description: We combine Spitzer and Herschel data of the star-forming region N11 in the Large Magellanic Cloud (LMC) to produce detailed maps of the dust properties in the complex and study their variations with the interstellar-medium conditions. We also compare Atacama Pathfinder EXperiment/Large APEX Bolometer Camera (APEX/LABOCA) 870 μm observations with our model predictions in order to decompose the 870 μm emission into dust and non-dust [free–free emission and CO(3–2) line] contributions. We find that in N11, the 870 μm can be fully accounted for by these three components. The dust surface density map of N11 is combined with H  i and CO observations to study local variations in the gas-to-dust mass ratios. Our analysis leads to values lower than those expected from the LMC low-metallicity as well as to a decrease of the gas-to-dust mass ratio with the dust surface density. We explore potential hypotheses that could explain the low ‘observed’ gas-to-dust mass ratios (variations in the X CO factor, presence of CO-dark gas or of optically thick H  i or variations in the dust abundance in the dense regions). We finally decompose the local spectral energy distributions (SEDs) using a principal component analysis (i.e. with no a priori assumption on the dust composition in the complex). Our results lead to a promising decomposition of the local SEDs in various dust components (hot, warm, cold) coherent with that expected for the region. Further analysis on a larger sample of galaxies will follow in order to understand how unique this decomposition is or how it evolves from one environment to another.

Description: 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.

Description: The mid-infrared (MIR) spectra observed with the Spitzer Infrared Spectrograph (IRS) provide a valuable data set for untangling the physical processes and conditions within galaxies. This paper presents the first attempt to blindly learn fundamental spectral components of MIR galaxy spectra, using non-negative matrix factorization (NMF). NMF is a recently developed multivariate technique shown to be successful in blind source separation problems. Unlike the more popular multivariate analysis technique, principal component analysis, NMF imposes the condition that weights and spectral components are non-negative. This more closely resembles the physical process of emission in the MIR, resulting in physically intuitive components. By applying NMF to galaxy spectra in the Cornell Atlas of Spitzer/IRS sources, we find similar components amongst different NMF sets. These similar components include two for active galactic nucleus (AGN) emission and one for star formation. The first AGN component is dominated by fine structure emission lines and hot dust, the second by broad silicate emission at 10 and 18 μm. The star formation component contains all the polycyclic aromatic hydrocarbon features and molecular hydrogen lines. Other components include rising continuums at longer wavelengths, indicative of colder grey-body dust emission. We show an NMF set with seven components can reconstruct the general spectral shape of a wide variety of objects, though struggle to fit the varying strength of emission lines. We also show that the seven components can be used to separate out different types of objects. We model this separation with Gaussian mixtures modelling and use the result to provide a classification tool. We also show that the NMF components can be used to separate out the emission from AGN and star formation regions and define a new star formation/AGN diagnostic which is consistent with all MIR diagnostics already in use but has the advantage that it can be applied to MIR spectra with low signal-to-noise ratio or with limited spectral range. The seven NMF components and code for classification are available at https://github.com/pdh21/NMF_software/ .

Description: We present Herschel photometry and spectroscopy, carried out as part of the Herschel ultraluminous infrared galaxy (ULIRG) survey, and a model for the infrared to submillimetre emission of the ULIRG IRAS 08572+3915. This source shows one of the deepest known silicate absorption features and no polycyclic aromatic hydrocarbon emission. The model suggests that this object is powered by an active galactic nucleus (AGN) with a fairly smooth torus viewed almost edge-on and a very young starburst. According to our model, the AGN contributes about 90 per cent of the total luminosity of 1.1 x 10 13 L , which is about a factor of 5 higher than previous estimates. The large correction of the luminosity is due to the anisotropy of the emission of the best-fitting torus. Similar corrections may be necessary for other local and high- z analogues. This correction implies that IRAS 08572+3915 at a redshift of 0.058 35 may be the nearest hyperluminous infrared galaxy and probably the most luminous infrared galaxy in the local ( z  〈 0.2) Universe. IRAS 08572+3915 shows a low ratio of [C ii ] to IR luminosity (log L [C ii ] / L IR 〈 –3.8) and a [O i ]63 μm to [C ii ]158 μm line ratio of about 1 that supports the model presented in this Letter.

Description: NGC 1569 has some of the most vigorous star formation among nearby galaxies. It hosts two super star clusters (SSCs) and has a higher star formation rate (SFR) per unit area than other starburst dwarf galaxies. Extended emission beyond the galaxy's optical body is observed in warm and hot ionized and atomic hydrogen gas; a cavity surrounds the SSCs. We aim to understand the impact of the massive star formation on the surrounding interstellar medium in NGC 1569 through a study of its stellar and dust properties. We use Herschel and ancillary multiwavelength observations, from the ultraviolet to the submillimetre regime, to construct its spectral energy distribution, which we model with magphys on ~300 pc scales at the Spectral and Photometric Imaging Receiver (SPIRE) 250 μm resolution. The multiwavelength morphology shows low levels of dust emission in the cavity, and a concentration of several dust knots in its periphery. The extended emission seen in the ionized and neutral hydrogen observations is also present in the far-infrared emission. The dust mass is higher in the periphery of the cavity, driven by ongoing star formation and dust emission knots. The SFR is highest in the central region, while the specific SFR is more sensitive to the ongoing star formation. The region encompassing the cavity and SSCs contains only 12 per cent of the dust mass of the central starburst, in accord with other tracers of the interstellar medium. The gas-to-dust mass ratio is lower in the cavity and fluctuates to higher values in its periphery.

Description: We present the detection and analysis of molecular hydrogen emission towards ten interstellar regions in the Large Magellanic Cloud. We examined low-resolution infrared spectral maps of 12 regions obtained with the Spitzer infrared spectrograph (IRS). The pure rotational 0–0 transitions of H 2 at 28.2 and 17.1 μm are detected in the IRS spectra for 10 regions. The higher level transitions are mostly upper limit measurements except for three regions, where a 3 detection threshold is achieved for lines at 12.2 and 8.6 μm. The excitation diagrams of the detected H 2 transitions are used to determine the warm H 2 gas column density and temperature. The single-temperature fits through the lower transition lines give temperatures in the range 86–137 K. The bulk of the excited H 2 gas is found at these temperatures and contributes ~5–17 per cent to the total gas mass. We find a tight correlation of the H 2 surface brightness with polycyclic aromatic hydrocarbon and total infrared emission, which is a clear indication of photoelectric heating in photodissociation regions. We find the excitation of H 2 by this process is equally efficient in both atomic- and molecular-dominated regions. We also present the correlation of the warm H 2 physical conditions with dust properties. The warm H 2 mass fraction and excitation temperature show positive correlations with the average starlight intensity, again supporting H 2 excitation in photodissociation regions.

Description: 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 x 10 9 M emitting with a bolometric luminosity of 5.7 x 10 9 L . Our fits yield a total dust mass of 2.9 ± 0.5 x 10 6 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.

Description: Low-metallicity galaxies exhibit different properties of the interstellar medium (ISM) compared to nearby spiral galaxies. Obtaining a resolved inventory of the various gas and dust components of massive star-forming regions and diffuse ISM is necessary to understand how those differences are driven. We present a study of the infrared/submillimetre (submm) emission of the massive star-forming complex N158–N159–N160 located in the Large Magellanic Cloud (LMC). Combining observations from the Spitzer Space Telescope (3.6–70 μm), the Herschel Space Observatory (100–500 μm) and Large APEX Bolometer Camera (LABOCA) (on Atacama Pathfinder EXperiment, 870 μm) allows us to work at the best angular resolution available now for an extragalactic source (a few parsec for the LMC). We observe a remarkably good correlation between the Herschel Spectral and Photometric Imaging Receiver (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 modified blackbodies, we derive an average ‘effective’ emissivity index of the cold dust component β c of 1.47 across the complex. If β c is fixed to 1.5, we find an average temperature of ~27 K (maximum of ~32 K in N160). We also apply the Galliano et al. SED modelling technique (using amorphous carbon to model carbon dust) to derive maps of the star formation rate, the grain temperature, the mean starlight intensity, the fraction of polycyclic aromatic hydrocarbons (PAH) or the dust mass surface density of the region. We observe that the PAH fraction strongly decreases in the H  ii regions we study. This decrease coincides with peaks in the mean radiation field intensity map. The dust surface densities follow the far-infrared distribution, with a total dust mass of 2.1  x 10 4 M (2.8 times less than if carbon dust was modelled by standard graphite grains) in the resolved elements we model. We also find a non-negligible amount of dust in the region called ‘N159 South’, a molecular cloud that does not show massive star formation. We also investigate the drivers of the Herschel /PACS (Photodetector Array Camera and Spectrometer) and SPIRE submm colours and find that the submm ratios correlate strongly with the radiation field intensity and with the near and mid-IR surface brightnesses equally well. Comparing our dust map to H  i and CO observations in N159, we then investigate variations in the gas-to-dust mass ratio (G/D) and the CO-to-H 2 conversion factor X CO . A mean value of G/D~356 is derived when using X CO  = 7 x 10 20 H 2  cm –2 (K km s –1 ) –1 . If a constant G/D across N159 is assumed, we derive a X CO conversion factor of 5.4 x 10 20 H 2  cm –2 (K km s –1 ) –1 . We finally model individual regions to analyse variations in the SED shape across the complex and the 870 μm emission in more details. No measurable submm excess emission at 870 μm seems to be detected in these regions.