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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. Source

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. Source

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. Source

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.. Source

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. Source

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