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Rosenberg M.J.F.,Leiden University | Van Der Werf P.P.,Leiden University | Aalto S.,Chalmers University of Technology | Armus L.,California Institute of Technology | And 30 more authors.
Astrophysical Journal | Year: 2015

(Ultra) luminous infrared galaxies ((U)LIRGs) are objects characterized by their extreme infrared (8-1000 μm) luminosities (L⊙LIRG > 1011 L⊙ and L⊙ULIRG > 1012 L⊙). The Herschel Comprehensive ULIRG Emission Survey (PI: van der Werf) presents a representative flux-limited sample of 29 (U)LIRGs that spans the full luminosity range of these objects (1011 L⊙ ≤ L⊙IR ≤ 1013 L⊙). With the Herschel Space Observatory, we observe [C II] 157 μm, [O I] 63 μm, and [O I] 145 μm line emission with Photodetector Array Camera and Spectrometer, CO J = 4-3 through J = 13-12, [C I] 370 μm, and [C I] 609 μm with SPIRE, and low-J CO transitions with ground-based telescopes. The CO ladders of the sample are separated into three classes based on their excitation level. In 13 of the galaxies, the [O I] 63 μm emission line is self absorbed. Comparing the CO excitation to the InfraRed Astronomical Satellite 60/100 μm ratio and to far infrared luminosity, we find that the CO excitation is more correlated to the far infrared colors. We present cooling budgets for the galaxies and find fine-structure line flux deficits in the [C II], [Si II], [O I], and [C I] lines in the objects with the highest far IR fluxes, but do not observe this for CO 4 ≤ J upp ≤ 13. In order to study the heating of the molecular gas, we present a combination of three diagnostic quantities to help determine the dominant heating source. Using the CO excitation, the CO J = 1-0 linewidth, and the active galactic nucleus (AGN) contribution, we conclude that galaxies with large CO linewidths always have high-excitation CO ladders, and often low AGN contributions, suggesting that mechanical heating is important. © 2015. The American Astronomical Society. All rights reserved..

Kramer C.,Instituto Radioastronomia Milimetrica IRAM | Abreu-Vicente J.,Instituto Radioastronomia Milimetrica IRAM | Garcia-Burillo S.,Observatorio Astronomico Nacional OAN Observatorio de Madrid | Relano M.,University of Granada | And 11 more authors.
Astronomy and Astrophysics | Year: 2013

Aims. We aim to better understand the heating of gas by observing the prominent gas cooling line [C ii] at 158 μm in the low-metallicity environment of the Local Group spiral galaxy M 33 on scales of 280 pc. In particular, we describe the variation of the photoelectric heating efficiency with the galactic environment. Methods. In this study, we present [C ii] observations along the major axis of M 33 using the Infrared Space Observatory in combination with Herschel continuum maps, IRAM 30 m CO 2-1, and VLA H i data to study the variation in velocity integrated intensities. The ratio of [C ii] emission over the far-infrared continuum is used as a proxy for the heating efficiency, and models of photon-dominated regions are used to study the local physical densities, far-ultraviolet radiation fields, and average column densities of the molecular clouds. Results. The heating efficiency stays constant at 0.8% in the inner 4.5 kpc radius of the galaxy, where it increases to reach values of ~3% in the outskirts at about a 6 kpc radial distance. The rise of efficiency is explained in the framework of PDR models by lowered volume densities and FUV fields for optical extinctions of only a few magnitudes at constant metallicity. For the significant fraction of H i emission stemming from PDRs and for typical pressures found in the Galactic cold neutral medium (CNM) traced by H i emission, the CNM contributes ~15% to the observed [C ii] emission in the inner 2 kpc radius of M 33. The CNM contribution remains largely undetermined in the south, while positions between radial distances of 2 and 7.3 kpc in the north of M 33 show a contribution of ~40% ± 20%. © ESO, 2013.

Relano M.,University of Granada | Verley S.,University of Granada | Perez I.,University of Granada | Kramer C.,Instituto Radioastronomia Milimetrica IRAM | And 15 more authors.
Astronomy and Astrophysics | Year: 2013

Aims. Within the framework of the Herschel M 33 extended survey HerM33es and in combination with multi-wavelength data we study the spectral energy distribution (SED) of a set of H ii regions in the Local Group galaxy M 33 as a function of the morphology. We analyse the emission distribution in regions with different morphologies and present models to infer the Hα emission measure observed for H ii regions with well defined morphology. Methods. We present a catalogue of 119 H ii regions morphologically classified: 9 filled, 47 mixed, 36 shell, and 27 clear shell H ii regions. For each object we extracted the photometry at twelve available wavelength bands, covering a wide wavelength range from FUV-1516 Å (GALEX) to IR-250 μm (Herschel), and we obtained the SED for each object. We also obtained emission line profiles in vertical and horizontal directions across the regions to study the location of the stellar, ionised gas, and dust components. We constructed a simple geometrical model for the clear shell regions, whose properties allowed us to infer the electron density of these regions. Results. We find trends for the SEDs related to the morphology of the regions, showing that the star and gas-dust configuration affects the ratios of the emission in different bands. The mixed and filled regions show higher emission at 24 μm, corresponding to warm dust, than the shells and clear shells. This could be due to the proximity of the dust to the stellar clusters in the case of filled and mixed regions. The far-IR peak for shells and clear shells seems to be located towards longer wavelengths, indicating that the dust is colder for this type of object. The logarithmic 100 μm/70 μm ratio for filled and mixed regions remains constant over one order of magnitude in Hα and FUV surface brightness, while the shells and clear shells exhibit a wider range of values of almost two orders of magnitude. We derive dust masses and dust temperatures for each H ii region by fitting the individual SEDs with dust models proposed in the literature. The derived dust mass range is between 102-104 M· and the cold dust temperature spans Tcold ∼ 12-27 K. The spherical geometrical model proposed for the Hα clear shells is confirmed by the emission profile obtained from the observations and is used to infer the electron density within the envelope: the typical electron density is 0.7 ± 0.3 cm-3, while filled regions can reach values that are two to five times higher. © 2013 ESO.

Hermelo I.,Instituto Radioastronomia Milimetrica IRAM | Relano M.,University of Granada | Lisenfeld U.,University of Granada | Verley S.,University of Granada | And 5 more authors.
Astronomy and Astrophysics | Year: 2016

Context. Previous studies have shown the existence of an excess of emission at submillimeter (submm) and millimeter (mm) wavelengths in the spectral energy distribution (SED) of many low-metallicity galaxies. The so-called "submm excess", whose origin remains unknown, challenges our understanding of the dust properties in low-metallicity environments. Aims. The goal of the present study is to model separately the emission from the star forming (SF) component and the emission from the diffuse interstellar medium (ISM) in the nearby spiral galaxy M 33 in order to determine whether both components can be well fitted using radiation transfer models or whether there is an excess of submm emission associated with one or both of them. Methods. We decomposed the observed SED of M 33 into its SF and diffuse components. Mid-infrared (MIR) and far-infrared (FIR) fluxes were extracted from Spitzer and Herschel data. At submm and mm wavelengths, we used ground-based observations from APEX to measure the emission from the SF component and data from the Planck space telescope to estimate the diffuse emission. Both components were separately fitted using radiation transfer models based on standard dust properties (i.e., emissivity index β = 2) and a realistic geometry. The large number of previous studies helped us to estimate the thermal radio emission and to constrain an important part of the input parameters of the models. Both modeled SEDs were combined to build the global SED of M 33. In addition, the radiation field necessary to power the dust emission in our modeling was compared with observations from GALEX, Sloan, and Spitzer. Results. Our modeling is able to reproduce the observations at MIR and FIR wavelengths, but we found a strong excess of emission at submm and mm wavelengths where the model expectations severely underestimate the LABOCA and Planck fluxes. We also found that the ultraviolet (UV) radiation escaping the galaxy is 70% higher than the model predictions. From the total mass of dust derived from our modeling and the mass of atomic and molecular gas measured with the VLA and the IRAM 30 m telescope, we determined a gas-to-dust mass ratio Gdust ~ 100, significantly lower than the value expected from the subsolar metallicity of M 33. Conclusions. We discussed different hypotheses to explain the discrepancies found in our study (i.e., excess of emission at submm and mm wavelengths, deficit of UV attenuation, and abnormally low value of Gdust), concluding that different dust properties in M 33 is the most plausible explanation. © ESO, 2016.

Verley S.,University of Granada | Relano M.,University of Cambridge | Kramer C.,Instituto Radioastronomia Milimetrica IRAM | Xilouris E.M.,National institute for astrophysics | And 11 more authors.
Astronomy and Astrophysics | Year: 2010

Aims. Within the framework of the HERM33ES key program, using the high resolution and sensitivity of the Herschel photometric data, we study the compact emission in the Local Group spiral galaxy M 33 to investigate the nature of the compact SPIRE emission sources. We extracted a catalogue of sources at 250 μm in order to investigate the nature of this compact emission. Taking advantage of the unprecedented Herschel resolution at these wavelengths, we also focus on a more precise study of some striking Hα shells in the northern part of the galaxy. Methods. We present a catalogue of 159 compact emission sources in M 33 identified by SExtractor in the 250 μm SPIRE band that is the one that provides the best spatial resolution. We also measured fluxes at 24 μm and Hα for those 159 extracted sources. The morphological study of the shells also benefits from a multiwavelength approach including Hα, far-ultraviolet from GALEX, and infrared from both Spitzer IRAC 8 μm and MIPS 24 μm in order to make comparisons. Results. For the 159 compact sources selected at 250 μm, we find a very strong Pearson correlation coefficient with the MIPS 24 μm emission (r24 = 0.94) and a rather strong correlation with the Hα emission, although with more scatter (r H$r-{\alpha}$ = 0.83). The morphological study of the Hα shells shows a displacement between far-ultraviolet, Hα, and the SPIRE bands. The cool dust emission from SPIRE clearly delineates the Hα shell structures. Conclusions. The very strong link between the 250 μm compact emission and the 24 μm and Hα emissions, by recovering the star formation rate from standard recipes for H II regions, allows us to provide star formation rate calibrations based on the 250 μm compact emission alone. The different locations of the Hα and far-ultraviolet emissions with respect to the SPIRE cool dust emission leads to a dynamical age of a few Myr for the Hα shells and the associated cool dust. © 2010 ESO.

Nguyen-Lu'O'Ng Q.,University of Toronto | Motte F.,University Paris Diderot | Carlhoff P.,University of Cologne | Louvet F.,University Paris Diderot | And 23 more authors.
Astrophysical Journal | Year: 2013

The formation of high-mass stars is tightly linked to that of their parental clouds. Here, we focus on the high-density parts of W43, a molecular cloud undergoing an efficient event of star formation. Using a column density image derived from Herschel continuum maps, we identify two high-density filamentary clouds, called the W43-MM1 and W43-MM2 ridges. Both have gas masses of 2.1 × 104 M and 3.5 × 104 M above >10 and within areas of 6 and 14 pc2, respectively. The W43-MM1 and W43-MM2 ridges are structures that are coherent in velocity and gravitationally bound, despite their large velocity dispersion measured by the N2H+ (1-0) lines of the W43-HERO IRAM large program. Another intriguing result is that these ridges harbor widespread (10 pc2) bright SiO (2-1) emission, which we interpret to be the result of low-velocity shocks (≤10 km s-1). We measure a significant relationship between the SiO (2-1) luminosity and velocity extent and show that it distinguishes our observations from the high-velocity shocks associated with outflows. We use state-of-the-art shock models to demonstrate that a small percentage (10%) of Si atoms in low-velocity shocks, observed initially in gas phase or in grain mantles, can explain the observed SiO column density in the W43 ridges. The spatial and velocity overlaps between the ridges of high-density gas and the shocked SiO gas suggest that ridges could be forming via colliding flows driven by gravity and accompanied by low-velocity shocks. This mechanism may be the initial conditions for the formation of young massive clusters. © 2013. The American Astronomical Society. All rights reserved..

Braine J.,University of Bordeaux 1 | Braine J.,French National Center for Scientific Research | Gratier P.,IRAM | Kramer C.,Instituto Radioastronomia Milimetrica IRAM | And 8 more authors.
Astronomy and Astrophysics | Year: 2012

This work presents high spectral resolution observations of the [C ii] line at 158 μm, one of the major cooling lines of the interstellar medium, taken with the HIFI heterodyne spectrometer on the Herschel satellite. In BCLMP 691, an H ii region far north (3.3 kpc) in the disk of M 33, the [C ii] and CO line profiles show similar velocities within 0.5 km s -1, while the H i line velocities are systematically shifted towards lower rotation velocities by ~5 km s -1. Observed at the same 12′′angular resolution, the [C ii] lines are broader than those of CO by about 50% but narrower than the H i lines. The [C ii] line intensities also follow those of CO much better than those of H i. A weak shoulder on the [C ii] line suggests a marginal detection of the [ 13C ii] line, insufficient to constrain the [C ii] optical depth. The velocity coincidence of the CO and [C ii] lines and the morphology at optical/UV wavelengths indicate that the emission is coming from a molecular cloud behind the H ii region. The relative strength of [C ii] with respect to the FIR continuum emission is comparable to that observed in the Magellanic Clouds on similar linear scales but the CO emission relative to [C ii] is stronger in M 33. The [C ii] line to far-infrared continuum ratio suggests a photoelectric heating efficiency of 1.1%. The data, together with published models indicate a UV field G 0 ∼ 100 in units of the solar neighborhood value, a gas density n H∼1000 cm -3, and a gas temperature T ∼ 200 K. Adopting these values, we estimate the C + column density to be NC + ≈ 1.3×10 17cm -2. The [C ii] emission comes predominantly from the warm neutral region between the H ii region and the cool molecular cloud behind it. From published abundances, the inferred C + column corresponds to a hydrogen column density of N H∼2×10 21cm -2. The CO observations suggest that N H=2NH 2∼3.2×10 21cm -2 and 21 cm measurements, also at 12′′resolution, yield N H i≈ 1.2×10 21cm -2 within the [C ii] velocity range. Thus, some H 2 not detected in CO must be present, in agreement with earlier findings based on the SPIRE 250-500 μm emission. © 2012 ESO.

Gratier P.,French National Center for Scientific Research | Braine J.,French National Center for Scientific Research | Rodriguez-Fernandez N.J.,IRAM | Schuster K.F.,IRAM | And 6 more authors.
Astronomy and Astrophysics | Year: 2012

We present an analysis of a systematic CO(2-1) survey at 12′′ resolution covering most of the Local Group spiral M 33, which, at a distance of 840 kpc, is close enough for individual giant molecular clouds (GMCs) to be identified. The goal of this work is to study the properties of the GMCs in this subsolar metallicity galaxy. The CPROPS (Cloud PROPertieS) algorithm was used to identify 337 GMCs in M 33, the largest sample to date for an external galaxy. The sample is used to study the GMC luminosity function, or mass spectrum under the assumption of a constant N(H 2)/I CO ratio. We find that n(L)dL ∞ L -2.0 ± 0.1 for the entire sample. However, when the sample is divided into inner and outer disk samples, the exponent changes from 1.6 ± 0.2 in the center 2 kpc to 2.3 ± 0.2 for galactocentric distances larger than 2 kpc. On the basis of the emission in the FUV, Hα, 8 μm, and 24 μm bands, each cloud was classified in terms of its star-forming activity-no star formation or either embedded or exposed star formation (visible in FUV and Hα). At least one sixth of the clouds had no (massive) star formation, suggesting that the average time required for star formation to start is about one sixth of the total time for which the object is identifiable as a GMC. The clouds without star formation have significantly lower CO luminosities than those with star formation, whether embedded or exposed, a result that is presumably related to the lack of heating sources. Taking the cloud sample as a whole, the main non-trivial correlation is the decrease in cloud CO brightness (or luminosity) with galactocentric radius. The complete cloud catalog, including the CO and HI spectra and the CO contours overlaid on the FUV, Hα, 8 μm, and 24 μm images is presented in the appendix. © 2012 ESO.

Gratier P.,French National Center for Scientific Research | Braine J.,French National Center for Scientific Research | Rodriguez-Fernandez N.J.,IRAM | Schuster K.F.,IRAM | And 21 more authors.
Astronomy and Astrophysics | Year: 2010

We present high-resolution large-scale observations of the molecular and atomic gas in the Local Group galaxy M 33. The observations were carried out using the HEterodyne Receiver Array (HERA) at the 30 m IRAM telescope in the CO(2-1) line, achieving a resolution of 12″ × 2.6 km s -1, enabling individual giant molecular clouds (GMCs) to be resolved. The observed region is 650 square arcminutes mainly along the major axis and out to a radius of 8.5 kpc, and covers entirely the 2′ × 40′ radial strip observed with the HIFI and PACS Spectrometers as part of the HERM3 3ES Herschel key program. The achieved sensitivity in main-beam temperature is 20-50 mK at 2.6 km s -1 velocity resolution. The CO(2-1) luminosity of the observed region is 1.7 ± 0.1 × 10 7 K km s -1 pc 2 and is estimated to be 2.8 ± 0.3 × 10 7 K km s -1 pc 2 for the entire galaxy, corresponding to H 2 masses of 1.9 × 10 8 M ⊙ and 3.3 × 10 8 M ⊙ respectively (including He), calculated with N(H 2)/I CO(1-0) twice the Galactic value due to the half-solar metallicity of M 33. The HI 21 cm VLA archive observations were reduced, and the mosaic was imaged and cleaned using the multi-scale task in the CASA software package, yielding a series of datacubes with resolutions ranging from 5″ to 25&Prime The HI mass within a radius of 8.5 kpc is estimated to be 1.4 × 10 9 M ⊙. The azimuthally averaged CO surface brightness decreases exponentially with a scale length of 1.9 ± 0.1 kpc whereas the atomic gas surface density is constant at Σ HI = 6±2 M ⊙ pc -2eprojected to face-on. For an N(H 2)/I CO(1-0) conversion factor twice that of the Milky Way, the central kiloparsec H 2 surface density is Σ H2 = 8.5 ± 0.2 M ⊙ pc -2. The star formation rate per unit molecular gas (SF efficiency, the rate of transformation of molecular gas into stars), as traced by the ratio of CO to H α and FIR brightness, is constant with radius. The SFE, with a N(H 2)/I CO(1-0) factor twice galactic, appears 2-4 times greater than for large spiral galaxies. A morphological comparison of molecular and atomic gas with tracers of star formation is presented showing good agreement between these maps both in terms of peaks and holes. A few exceptions are noted. Several spectra, including those of a molecular cloud situated more than 8 kpc from the galaxy center, are presented. © ESO 2010.

Jeong I.-G.,Seoul National University | Koo B.-C.,Seoul National University | Cho W.-K.,Seoul National University | Kramer C.,Instituto Radioastronomia Milimetrica IRAM | And 3 more authors.
Astrophysical Journal | Year: 2013

We present the results of CO emission line observations toward the semicircular Galactic supernova remnant (SNR) 3C434.1 (G94.0+1.0). We mapped an area covering the whole SNR in the 12CO J = 1-0 emission line using the Seoul Radio Astronomy Observatory 6 m telescope and found a large molecular cloud superposed on the faint western part of the SNR. The cloud was elongated along the north-south direction and showed a very good spatial correlation with the radio features of the SNR. We carried out 12CO J = 2-1 line observations of this cloud using the Kölner Observatorium für Sub-Millimeter Astronomie 3 m telescope and found a region in which the 12CO J = 2-1 to J = 1-0 ratio was high (∼1.6). This higher excitation, together with the morphological relation, strongly suggested that the molecular cloud was interacting with the SNR. The systemic velocity of the molecular cloud (-13 km s-1) gave a kinematic distance of 3.0 kpc to the SNR-molecular cloud system. We derived the physical parameters of the SNR based on this new distance. We examined the variation of the radio spectral index over the remnant and found that it was flatter in the western part, wherein the SNR was interacting with the molecular cloud. We therefore propose that 3C434.1 is the remnant of a supernova explosion that occurred just outside the boundary of a relatively thin, sheet-like molecular cloud. We present a hydrodynamic model showing that its asymmetric radio morphology can result from its interaction with this blocking molecular cloud. © 2013. The American Astronomical Society. All rights reserved..

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