Dust and molecules in the Local Group galaxy NGC 6822

Israel, F. P.; Baas, F.; Rudy, R. J.; Skillman, Evan D.; Woodward, C. E.

doi:10.1051/0004-6361:20021464

Cited by 29 publications

(46 citation statements)

References 34 publications

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“…Earlier studies of GMCs in M 33 include Wilson et al (1997) and Rosolowsky et al (2003) and studies similar to our own of other Local Group galaxies have been made by e.g. Fukui et al (2008), Israel et al (2003), Leroy et al (2006), and Nieten et al (2006).…”

Section: Introductionsupporting

confidence: 65%

Molecular and atomic gas in the Local Group galaxy M 33

Gratier

Braine

Rodríguez-Fernández

et al. 2010

A&A

145

154

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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 HERM33ES 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 H i 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 . The H i 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 H i = 6± 2 M pc −2 deprojected 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 H 2 = 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.

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Section: Introductionsupporting

confidence: 65%

Molecular and atomic gas in the Local Group galaxy M 33

Gratier

Braine

Rodríguez-Fernández

et al. 2010

A&A

145

154

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“…The detection of molecular gas, however, is more difficult because of the intrinsic nature of H 2 , which accounts for the bulk the molecular gas mass. The most widespread method is to observe a rotational transition of the CO molecule which is a good tracer of molecular gas even though it is dependent on the temperature or the density of the gas (Maloney & Black 1988), and its metallicity (Wilson 1995;Israel 1997Israel , 2000Israel , 2005Barone et al 2000;Israel et al 2003;Strong et al 2004).…”

Section: Multi-wavelength Requirementsmentioning

confidence: 99%

Towards understanding the relation between the gas and the attenuation in galaxies at kpc scales

Boquien

Boselli

Buat

et al. 2013

A&A

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Context. Understanding the relation between the attenuation and the gas is fundamental for interpreting the appearance of galaxies. This can now be done down to a local scale in the local Universe, thanks to the high spatial resolution achievable from the farultraviolet (FUV) to the far-infrared (FIR). More importantly, this relation is also crucial for predicting the emission of galaxies. This is essential in semi-analytic models that link dark matter from large cosmological simulations, to the baryonic matter which we can observe directly. Aims. The aim of the present paper is to provide new and more detailed relations at the kpc scale between the gas surface density and the face-on optical depth directly calibrated on galaxies, in order to compute the attenuation not only for semi-analytic models, but also for observational models as new and upcoming radio observatories are able to trace gas ever farther away in the Universe. Methods. We have selected a sample of 4 nearby resolved galaxies and a sample of 27 unresolved galaxies from the Herschel Reference Survey and the Very Nearby Galaxies Survey, for which we have a large set of multi-wavelength data from the FUV to the FIR including metallicity gradients for resolved galaxies, along with radio HI and CO observations. For each pixel in resolved galaxies and for each galaxy in the unresolved sample, we compute the face-on optical depth from the attenuation determined with the CIGALE spectral energy distribution fitting code and an assumed geometry. We determine the gas surface density from HI and CO observations with a metallicity-dependent X CO factor. Results. We provide new, simple-to-use relations to determine the face-on optical depth from the gas surface density, taking the metallicity into account, which proves to be crucial for a proper estimate. The method used to determine the gas surface density or the face-on optical depth has little impact on the relations except for galaxies that have an inclination over 50• . Finally, we provide detailed instructions on how to efficiently compute the attenuation from the gas surface density taking into account possible information on the metallicity. Conclusions. Examination of the influence of these new relations on simulated FUV and IR luminosity functions shows a clear impact compared to older, more-frequently used relations, which in turn could affect the conclusions drawn from studies based on large-scale cosmological simulations.

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“…We have used so far the Galactic solar neighborhood value, but there is ample evidence that X CO varies in other galaxies as a function of physical conditions (e.g. Maloney & Black 1988;Israel 1997;Barone et al 2000;Boselli et al 2002;Israel et al 2003;Israel 2005;Bolatto et al 2008;Narayanan et al 2005;Leroy et al 2011;Shetty et al 2011;Narayanan et al 2012;Bell et al 2007). Thus, we consider an alternative conversion factor, which depends on the metallicity, and varies radially following the metallicity gradient along the disk:…”

Section: Radial Profilesmentioning

confidence: 99%

TheHerschelVirgo Cluster Survey

Pappalardo

Bianchi

Corbelli

et al. 2012

A&A

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Aims. We investigate the dust-to-gas mass ratio and the environmental effects on the various components of the interstellar medium for a spatially resolved sample of Virgo spirals. Methods. We have used the IRAM-30 m telescope to map over their full extent NGC 4189, NGC 4298, NGC 4388, and NGC 4299 in the 12 CO(1-0) and the 12 CO(2-1) lines. We observed the same lines in selected regions of NGC 4351, NGC 4294, and NGC 4424. The CO observations are combined with Herschel maps in 5 bands between 100-500 μm from the HeViCS survey, and with HI data from the VIVA survey, to obtain spatially resolved dust and gas distributions. We studied the environmental dependencies by adding to our sample eight galaxies with 12 CO(1-0) maps from the literature. Results. We estimate the integrated mass of molecular hydrogen for the galaxies observed in the CO lines. We find molecular-to-total gas mass fractions between 0.04 ≤ f mol ≤ 0.65, with the lowest values for the dimmest galaxy in the B-band. The integrated dust-togas ratio ranges between 0.011 and 0.004. For the 12 mapped galaxies we derive the radial distributions of the atomic gas, molecular gas, and dust. We also study the effect of different CO-to-H 2 conversion factors. Both the molecular gas and the dust distributions show steeper radial profiles for HI-deficient galaxies and the average dust-to-gas ratio for these galaxies increases or stays radially constant. On scales of ∼3 kpc, we find a strong correlation between the molecular gas and the 250 μm surface brightness that is tighter than average for non-deficient galaxies. The correlation becomes linear if we consider the total gas surface mass density. However, the inclusion of atomic hydrogen does not improve the statistical significance of the correlation. Conclusions. The environment can modify the distributions of molecules and dust within a galaxy, although these components are more tightly bound than the atomic gas.

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Dust and molecules in the Local Group galaxy NGC 6822

Cited by 29 publications

References 34 publications

Molecular and atomic gas in the Local Group galaxy M 33

Molecular and atomic gas in the Local Group galaxy M 33

Towards understanding the relation between the gas and the attenuation in galaxies at kpc scales

TheHerschelVirgo Cluster Survey

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