Sacha Hony

The Herschel Reference Survey

The Herschel Reference Survey is a Herschel guaranteed time key project and will be a benchmark study of dust in the nearby universe. The survey will complement a number of other Herschel key projects including large cosmological surveys that trace dust in the distant universe. We will use Herschel to produce images of a statistically-complete sample of 323 galaxies at 250, 350, and 500 μm. The sample is volume-limited, containing sources with distances between 15 and 25 Mpc and flux limits in the K band to minimize the selection effects associated with dust and with young high-mass stars and to introduce a selection in stellar mass. The sample spans the whole range of morphological types (ellipticals to late-type spirals) and environments (from the field to the center of the Virgo Cluster) and as such will be useful for other purposes than our own. We plan to use the survey to investigate (i) the dust content of galaxies as a function of Hubble type, stellar mass, and environment; (ii) the connection between the dust content and composition and the other phases of the interstellar medium; and (iii) the origin and evolution of dust in galaxies. In this article, we describe the goals of the survey, the details of the sample and some of the auxiliary observing programs that we have started to collect complementary data. We also use the available multifrequency data to carry out an analysis of the statistical properties of the sample.

Probing the dust properties of galaxies up to submillimetre wavelengths - II. Dust-to-gas mass ratio trends with metallicity and the submm excess in dwarf galaxies

Aims. We are studying the effects of submm observations on the total dust mass and thus dust-to-gas mass ratio measurements. Methods. We gather a wide sample of galaxies that have been observed at submillimeter (submm) wavelengths to model their spectral energy distributions using submm observations (>160 μm) and then without submm observational constraints in order to quantify the error on the dust mass when submm data are not available. Our model does not make strong assumptions on the dust temperature distribution to precisely avoid submm biaises in the study. Our sample includes 52 galaxies observed at submm wavelengths. Out of these, 9 galaxies show an excess in submm which is not accounted for in our fiducial model, most of these galaxies being lowmetallicity dwarfs. We chose to add an independant very cold dust component (T = 10 K) to account for this excess. Results. We find that metal-rich galaxies modelled with submm data often show lower dust masses than when modelled without submm data. Indeed, these galaxies usually have dust SEDs that peaks at longer wavelengths and require constraints above 160 μ mt o correctly position the peak and sample the submillimeter slope of their SEDs and thus correctly cover the dust temperature distribution. On the other hand, some metal-poor dwarf galaxies modelled with submm data show higher dust masses than when modelled without submm data. Using submm constraints for the dust mass estimates, we find a tightened correlation of the dust-to-gas mass ratio with the metallicity of the galaxies. We also often find that when there is a submm excess present, it occurs preferentially in low-metallicity galaxies. We analyse the conditions for the presence of this excess and find a relation between the 160/850 μm ratio and the submm excess.

The Herschel Space Observatory view of dust in M81

We use Herschel Space Observatory data to place observational constraints on the peak and Rayleigh-Jeans slope of dust emission observed at 70–500 μm in the nearby spiral galaxy M81. We find that the ratios of wave bands between 160 and 500 μm are primarily dependent on radius but that the ratio of 70 to 160 μm emission shows no clear dependence on surface brightness or radius. These results along with analyses of the spectral energy distributions imply that the 160–500 μm emission traces 15–30 K dust heated by evolved stars in the bulge and disc whereas the 70 μm emission includes dust heated by the active galactic nucleus and young stars in star forming regions.

A Spitzer mid-infrared spectral survey of mass-losing carbon stars in the Large Magellanic Cloud

We present a Spitzer Space Telescopespectroscopic survey of mass-losing carbon stars (and one oxygen-rich star) in the Large Magellanic Cloud. The stars represent the superwind phase on the Asymptotic Giant Branch, which forms a major source of dust for the interstellar medium in galaxies. The spectra cover the wavelength range 5‐38� m. They show varying combinations of dust continuum, dust emission features (SiC, MgS) and molecular absorption bands (C2H2, HCN). A set of four narrow bands, dubbed the Manchester system, is used to define the infrared continuum for dusty carbon stars. The r elations between the continuum colours and the strength of the dust and molecular features are studied, and are compared to Galactic stars of similar colours. The circumstellar 7-� m C2H2 band is found to be stronger at lower metallicity, from a comparison of stars in the Galaxy, the LMC and the SMC. This is explained by dredge-up of carbon, causing higher C/O ratios at low metallicity (less O). A possible 10-� m absorption feature seen in our spectra may be due to C3. This band has also been identified with interstellar silicate or silicon-nitr ite dust. We investigate the strength and central wavelength of the SiC and MgS dust bands as function of colour and metallicity. The line-to-continuum ratio of these bands shows some indication of being lower at low metallicity. The MgS band is only seen at dust temperatures below 600 K. We discuss the selection of carbon versus oxygen-rich AGB stars using the J K vs. K A colours, and show that these colours are relatively insensitive to chemical type. Metal -poor carbon stars form amorphous carbon dust from self-produced carbon. This type of dust forms more readily in the presence of a higher C/O ratio. Low metallicity carbon dust may contain a smaller fraction of SiC and MgS constituents, which do depend on metallicity. The formation efficiency of oxygen-rich dust depends more strongly on metallicity. We suggest that in lower-metallicity environments, the dust input into the Interstellar Medium by AGB stars is efficient but may be strongly biassed towards carbonaceous dust, as compared to the Galaxy.

Probing the molecular interstellar medium of M82 with Herschel-SPIRE spectroscopy

We present the observations of the starburst galaxy M82 taken with the Herschel SPIRE Fourier-transform spectrometer. The spectrum (194-671 mu m) shows a prominent CO rotational ladder from J = 4-3 to 13-12 emitted by the central region of M82. The fundamental properties of the gas are well constrained by the high J lines observed for the first time. Radiative transfer modeling of these high-S/N (CO)-C-12 and (CO)-C-13 lines strongly indicates a very warm molecular gas component at similar to 500 K and pressure of similar to 3 x 10(6) K cm(-3), in good agreement with the H-2 rotational lines measurements from Spitzer and ISO. We suggest that this warm gas is heated by dissipation of turbulence in the interstellar medium (ISM) rather than X-rays or UV flux from the straburst. This paper illustrates the promise of the SPIRE FTS for the study of the ISM of nearby galaxies.

The SAGE‐Spec Spitzer Legacy programme: the life‐cycle of dust and gas in the Large Magellanic Cloud – Point source classification I

We present the classification of 197 point sources observed with the Infrared Spectrograph in the SAGE-Spec Legacy programme on the Spitzer Space Telescope. We introduce a decision-tree method of object classification based on infrared spectral features, continuum and spectral energy distribution shape, bolometric luminosity, cluster membership and variability information, which is used to classify the SAGE-Spec sample of point sources. The decision tree has a broad application to mid-infrared spectroscopic surveys, where supporting photometry and variability information are available. We use these classifications to make deductions about the stellar populations of the Large Magellanic Cloud and the success of photometric classification methods. We find 90 asymptotic giant branch (AGB) stars, 29 young stellar objects, 23 post-AGB objects, 19 red supergiants, eight stellar photospheres, seven background galaxies, seven planetary nebulae, two H_(II) regions and 12 other objects, seven of which remain unclassified.

The dust condensation sequence in red supergiant stars

Context. Red supergiant (RSG) stars exhibit significant mass loss by means of a slow, dense wind. They are often considered to be the more massive counterparts of Asymptotic Giant Branch (AGB) stars. While AGB mass loss is related to their strong pulsations, the RSG are often only weakly variable. This raises the question of whether their wind-driving mechanism and the dust composition of the wind are the same. Aims. We study the conditions at the base of the wind by determining the dust composition of a sample of RSG. The dust composition is assumed to be sensitive to the density, temperature, and acceleration at the base of the wind. We compare the derived dust composition with the composition measured in AGB star winds. Methods. We compile a sample of 27 RSG infrared spectra (ISO-SWS) and supplement these with photometric measurements to derive the full spectral energy distribution (SED). These data are modelled using a dust radiative-transfer code, taking into account the optical properties of the relevant candidate materials to search for correlations between mass-loss rate, density at the inner edge of the dust shell, and stellar parameters. Results. We find strong correlations between the dust composition, mass-loss rate, and the stellar luminosity, roughly in agreement with the theoretical dust condensation sequence. We identify the need for a continuous (near-)IR dust opacity and tentatively propose amorphous carbon, and we note significant differences with AGB star winds in terms of the presence of PAHs, absence of “the” 13 μm band, and a lack of strong water bands. Conclusions. Dust condensation in RSG is found to experience a freeze-out process that is similar to that in AGB stars. Together with the positive effect of the stellar luminosity on the mass-loss rate, this suggests that radiation pressure on dust grains is an important ingredient in the driving mechanism. Still, differences with AGB stars are manifold and thus the winds of RSG should be studied individually in further detail.

Spitzer spectroscopy of carbon stars in the Small Magellanic Cloud

We present Spitzer Space telescopespectroscopic observations of 14 carbon-rich AGB stars in the Small Magellanic Cloud. SiC dust is seen in most of the carbon-rich stars but it is weak compared to LMC stars. The SiC feature is strong only for stars with significant dust excess, opposite to what is observed for Galactic stars. We argue that in the SMC, SiC forms at lower temperature than graphite dust, whereas the reverse situation occurs in the Galaxy where SiC condenses at higher temperatures and forms first. Dust input into the interstellar medium by AGB stars consists mostly of carbonaceous dust, with little SiC or silicate dust. Only the two coolest stars show a 30-micron band due to MgS dust. We suggest that this is due to the fact that, in the SMC, mass-losing AGB stars generally have low circumstellar (dust) optical depth and therefore effective heating of dust by the central star does not allow temperatures below the 650 K necessary for MgS to exist as a solid. Gas phase C2H2 bands are stronger in the SMC than in the LMC or Galaxy. This is attributed to an increasing C/O ratio at low metallicity. We present a colour-colour diagram based on Spitzer IRAC and MIPS colours to discriminate between O- and C-rich stars. We show that AGB stars in the SMC become carbon stars early in the thermal-pulsing AGB evolution, and remain optically visible for � 6 × 10 5 yr. For the LMC, this lifetime is � 3 × 10 5 yr. The superwind phase traced with Spitzer lasts for � 10 4 yr. Spitzer spectra of a K supergiant and a compact HII region are also given.

An Overview of the Dwarf Galaxy Survey

The Dwarf Galaxy Survey (DGS) program is studying low-metallicity galaxies using 230 hr of farinfrared (FIR) and submillimetre (submm) photometric and spectroscopic observations of the Herschel Space Observatory and draws from 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 Z., and spans four 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 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 mu m with the highest sensitivity achievable to date in the FIR and submm. The FIR fine-structure lines, [CII] 158 mu m, [OI] 63 mu m, [OI] 145 mu m, [OIII] 88 mu m, [NIII] 57 mu m, and [NII] 122 and 205 mu m 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 mu m, and [CI] lines at 370 and 609 mu m. This paper describes the sample selection and global properties of the galaxies and 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.

Probing the dust properties of galaxies up to submillimetre wavelengths - I. The spectral energy distribution of dwarf galaxies using LABOCA

Aims. We study the dust properties of four low metallicity galaxies by modelling their spectral energy distributions. This modelling enables us to constrain the dust properties such as the mass, the temperature or the composition to characterise the global ISM properties in dwarf galaxies. Methods. We present 870 μm images of four low metallicity galaxies (NGC 1705, Haro 11, Mrk 1089 and UM311) observed with the Large APEX BOlometer CAmera (LABOCA) on the Atacama Pathfinder EXperiment (APEX) telescope. We modeled their spectral energy distributions combining the submm observations of LABOCA, 2MASS, IRAS, Spitzer photometric data, and the IRS data for Haro 11. Results. We found that the PAH mass abundance is very low in these galaxies, 5 to 50 times lower than the PAH mass fraction of our Galaxy. We also found that a significant mass of dust is revealed when using submm constraints compared to that measured with only mid-IR to far-IR observations extending only to 160 μm. For NGC 1705 and Haro 11, an excess in submillimeter wavelengths was detected when we used our standard dust SED model. We rerun our SED procedure adding a cold dust component (10 K) to better describe the high 870 μm flux derived from LABOCA observations, which significantly improves the fit. We found that at least 70% of the dust mass of these two galaxies can reside in a cold dust component. We also showed that the subsequent dust-to-gas mass ratios, considering HI and CO observations, can be strikingly high for Haro 11 in comparison with what is usually expected for these low-metallicity environments. Furthermore, we derived the star formation rate of our galaxies and compared them to the Schmidt law. Haro 11 falls anomalously far from the Schmidt relation. These results may suggest that a reservoir of hidden gas could be present in molecular form not traced by the current CO observations. While there can be a significant cold dust mass found in Haro 11, the SED peaks at exceptionally short wavelengths (36 μm), also highlighting the importance of the much warmer dust component heated by the massive star clusters in Haro 11. We also derived the total IR luminosities derived from our models and compared them with relations that derive this luminosity from Spitzer bands. We found that the Draine & Li (2007) formula compares well to our direct IR determinations.