ALBERT

Description: Several dedicated surveys focusing on early-type galaxies (ETGs) reveal that significant fractions of them are detectable in all interstellar medium phases studied to date. We select ETGs from the Herschel Reference Survey that have both far-infrared Herschel and either H  i or CO detection (or both). We derive their star formation rates (SFRs), stellar masses and dust masses via modelling their spectral energy distributions. We combine these with literature information on their atomic and molecular gas properties, in order to relate their star formation, total gas mass and dust mass on global scales. The ETGs deviate from the dust mass–SFR relation and the Schmidt–Kennicutt relation that SDSS star-forming galaxies define: compared to SDSS galaxies, ETGs have more dust at the same SFR, or less SFR at the same dust mass. When placing them in the M * –SFR plane, ETGs show a much lower specific SFR as compared to normal star-forming galaxies. ETGs show a large scatter compared to the Schmidt–Kennicutt relation found locally within our Galaxy, extending to lower SFRs and gas mass surface densities. Using an ETG's SFR and the Schmidt–Kennicutt law to predict its gas mass leads to an underestimate. ETGs have similar observed-gas-to-modelled-dust mass ratios to star-forming galaxies of the same stellar mass, as well as they exhibit a similar scatter.

Description: We present a line survey of the ultraluminous infrared galaxy Arp 220, taken with the newly installed SEPIA (Swedish-European Southern Observatory PI receiver for APEX) Band 5 instrument on APEX (Atacama Pathfinder Experiment). We illustrate the capacity of SEPIA to detect the 183.3 GHz H 2 O 3 1,3 –2 2,0 line against the atmospheric H 2 O absorption feature. We confirm the previous detection of the HCN(2–1) line, and detect new transitions of standard dense gas tracers such as HNC(2–1), HCO + (2–1), CS(4–3), C 34 S(4–3) and HC 3 N(20–19). We also detect HCN(2–1) v 2 = 1 and the 193.5 GHz methanol (4–3) group for the first time. The absence of time variations in the megamaser water line compared to previous observations seems to rule out an AGN nuclear origin for the line. It could, on the contrary, favour a thermal origin instead, but also possibly be a sign that the megamaser emission is associated with star-forming cores washed out in the beam. We finally discuss how the new transitions of HCN, HNC and HCO + refine our knowledge of the interstellar medium physical conditions in Arp 220.

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: We present the results of an analysis of archival 21 cm (H i ) data of the blue compact dwarf galaxy Haro 11 (ESO 350-IG038). Observations were obtained at the Very Large Array, and the presence of a compact absorption feature near the optical centre of the galaxy has been detected. The central location of the absorption feature coincides with the centre of the continuum background of the galaxy, as well as with the location of knot B. The absorption feature yields an H i mass in the range of 3–10 x 10 8 M , corresponding to spin temperatures from 91 K to 200 K, respectively. The absence of H i seen in emission places an upper limit of 1.7 10 9 M on the mass. To our knowledge this is the first example of a dwarf galaxy that shows H i absorption from its own background continuum. The continuum emission from the galaxy is also used to determine star formation rates, namely 6.85 ± 0.05 M yr –1 (for a stellar mass range of 5 M 〈 M 〈 100 M ), or 32.8 ± 0.2 M yr –1 (for an extended range of 0.1 M 〈 M 〈 100 M ).

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: Dwarf spheroidal galaxies are among the most numerous galaxy population in the Universe, but their main formation and evolution channels are still not well understood. The three dwarf spheroidal satellites (NGC 147, NGC 185, and NGC 205) of the Andromeda galaxy are characterized by very different interstellar medium properties, which might suggest them being at different galaxy evolutionary stages. While the dust content of NGC 205 has been studied in detail in an earlier work, we present new Herschel dust continuum observations of NGC 147 and NGC 185. The non-detection of NGC 147 in Herschel SPIRE maps puts a strong constraint on its dust mass (≤128 $^{+124}_{-68}$ M ). For NGC 185, we derive a total dust mass M d = 5.1±1.0  x  10 3 M , which is a factor of ~2–3 higher than that derived from ISO and Spitzer observations and confirms the need for longer wavelength observations to trace more massive cold dust reservoirs. We, furthermore, estimate the dust production by asymptotic giant branch (AGB) stars and supernovae (SNe). For NGC 147, the upper limit on the dust mass is consistent with expectations of the material injected by the evolved stellar population. In NGC 185 and NGC 205, the observed dust content is one order of magnitude higher compared to the estimated dust production by AGBs and SNe. Efficient grain growth, and potentially longer dust survival times (3–6 Gyr) are required to account for their current dust content. Our study confirms the importance of grain growth in the gas phase to account for the current dust reservoir in galaxies.

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.