The Ratio of H 2 Column Density to 12CO Intensity in the Vicinity of the Galactic Center

Sodroski, T. J.; Odegard, N.; Dwek, Eli; Hauser, M. G.; Franz, Bryan A.; Freedman, I.; Kelsall, T.; Wall, W. F.; Berriman, G. Bruce; Odenwald, Sten; Bennett, C. L.; Reach, W. T.; Weiland, J. L.

doi:10.1086/176297

Cited by 124 publications

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“…known although is generally thought to be considerably larger than the value in the solar neighbourhood, based, for example, on the ratio of CO line intensity to inferred H 2 column density in the central region of the Galaxy [12]. Even if one assumes that the z is an order of magnitude higher than in galactic diffuse clouds, values of L are surely significant fractions of the radius of the CMZ [9,10].…”

Section: In the Galactic Centre: Basic Results From 1997 To 2008mentioning

confidence: 99%

Exploring the central molecular zone of the Galaxy using spectroscopy of H ₃ ⁺ and CO

Geballe

2012

Phil. Trans. R. Soc. A.

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The central 400 parsecs of the Milky Way, a region known as the central molecular zone (CMZ), contains interstellar gas in a wide range of physical environments, from ultra-hot, rarified and highly ionized to warm, dense and molecular. The combination of infrared spectroscopy of H + 3 and CO is a powerful way to determine the basic properties of molecular interstellar gas, because the abundance ratio of H + 3 to CO in 'dense' clouds is quite different from that in 'diffuse' clouds. Moreover, the energy-level structure and the radiative properties of H + 3 combined with the unusually warm temperatures of molecular gas in the CMZ make H + 3 a unique probe of the physical conditions there. This paper describes how, using infrared absorption spectroscopy of H + 3 and CO, it has been discovered that a large fraction of the volume of the CMZ is filled with warm, diffuse and partially molecular gas moving at speeds of up to approximately 200 km sand that the mean cosmic ray ionization rate in the CMZ exceeds by roughly an order of magnitude values found in diffuse molecular clouds elsewhere in the Galaxy.

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Section: In the Galactic Centre: Basic Results From 1997 To 2008mentioning

confidence: 99%

Exploring the central molecular zone of the Galaxy using spectroscopy of H ₃ ⁺ and CO

Geballe

2012

Phil. Trans. R. Soc. A.

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“…In Fig. 15, following Strong et al (2004), we used the profiles given by Sodroski et al (1995) and by Sodroski et al (1997), as well as the profiles that those authors inferred from Israel (1997) and Boselli et al (2002). We moreover used the X CO radial profile, which can be found in the GALPROP package.…”

Section: The X Co Factormentioning

confidence: 99%

The GeV-TeV Galactic gamma-ray diffuse emission

Delahaye

Fiaßon

Pohl

et al. 2011

A&A

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Context. The Galactic γ-ray diffuse emission is currently observed in the GeV-TeV energy range with unprecedented accuracy by the Fermi satellite. Understanding this component is crucial because it provides a background to many different signals, such as extragalactic sources or annihilating dark matter. It is timely to reinvestigate how it is calculated and to assess the various uncertainties that are likely to affect the accuracy of the predictions. Aims. The Galactic γ-ray diffuse emission is mostly produced above a few GeV by the interactions of cosmic ray primaries impinging on the interstellar material. The theoretical error on that component is derived by exploring various potential sources of uncertainty. Particular attention is paid to cosmic ray propagation. Nuclear cross sections, the proton and helium fluxes at the Earth's position, the Galactic radial profile of supernova remnants, and the hydrogen distribution can also severely affect the signal. Methods. The propagation of cosmic ray species throughout the Galaxy is described in the framework of a semi-analytic two-zone diffusion/convection model. The γ-ray flux is reliably and quickly determined. This allows conversion of the constraints set by the boron-to-carbon data into a theoretical uncertainty on the diffuse emission. New deconvolutions of the HI and CO sky maps are also used to get the hydrogen distribution within the Galaxy. Results. The thickness of the cosmic ray diffusive halo is found to have a significant effect on the Galactic γ-ray diffuse emission, while the interplay between diffusion and convection has little influence on the signal. The uncertainties related to nuclear cross sections and to the primary cosmic ray fluxes at the Earth are significant. The radial distribution of supernova remnants along the Galactic plane turns out to be a key ingredient. As expected, the predictions are extremely sensitive to the spatial distribution of hydrogen within the Milky Way. Conclusions. Most of the sources of uncertainty are likely to be reduced in the near future. The stress should be put (i) on better determination of the thickness of the cosmic ray diffusive halo; and (ii) on refined observations of the radial profile of supernova remnants.

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“…The behaviour of X CO with radius has been the subject of several studies in the Milky Way and nearby galaxies (Wilson 1995;Sodroski et al 1995;Strong et al 2004). X CO is found to increase with increasing radius, perhaps in relation with decreasing metallicity.…”

Section: Radial Distributionsmentioning

confidence: 99%

Molecular Gas in the Andromeda Galaxy

Guèlin

Müller

Nieten

et al. 1997

Springer Proceedings in Physics

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Abstract. We present a new 12 CO(J=1-0)-line survey of the Andromeda galaxy, M 31, with the highest resolution to date (23 ′′ , or 85 pc along the major axis), observed On-the-Fly with the IRAM 30-m telescope. We mapped an area of about 2 • × 0.• 5 which was tightly sampled on a grid of 9′′ with a velocity resolution of 2.6 km s −1 . The r.m.s. noise in the velocity-integrated map is around 0.35 K km s −1 on the T mb -scale. Emission from the 12 CO(1-0) line is detected from galactocentric radius R = 3 kpc to R = 16 kpc, but peaks in intensity at R ∼ 10 kpc. Some clouds are visible beyond R = 16 kpc, the farthest of them at R = 19.4 kpc. The molecular gas traced by the (1-0) line is concentrated in narrow arm-like filaments, which often coincide with the dark dust lanes visible at optical wavelengths. The H  arms are broader and smoother than the molecular arms. Between R = 4 kpc and R = 12 kpc the brightest CO filaments and the darkest dust lanes define a two-armed spiral pattern that is well described by two logarithmic spirals with a constant pitch angle of 7• -8• . Except for some bridge-like structures between the arms, the interarm regions and the central bulge are free of emission at our sensitivity. The arm-interarm brightness ratio averaged over a length of 15 kpc along the western arms reaches about 20 compared to 4 for H  at an angular resolution of 45 ′′ . In several selected regions we also observed the 12 CO(2-1)-line on a finer grid. Towards the bright CO emission in our survey we find normal ratios of the (2-1)-to-(1-0) line intensities which are consistent with optically thick lines and thermal excitation of CO. We compare the (velocity-integrated) intensity distribution of CO with those of H , FIR at 175 µm and radio continuum, and interpret the CO data in terms of molecular gas column densities. For a constant conversion factor X CO , the molecular fraction of the neutral gas is enhanced in the spiral arms and decreases radially from 0.6 on the inner arms to 0.3 on the arms at R ≃ 10 kpc. We also compare the distributions of H , H 2 and total gas with that of the cold (16 K) dust traced at λ175 µm. The ratios N(H i)/I 175 and (N(H i) + 2N(H 2 ))/I 175 increase by a factor of ∼ 20 between the centre and R ≃ 14 kpc, whereas the ratio 2N(H 2 )/I 175 only increases by a factor of 4. For a constant value of X CO , this means that either the atomic and total gas-to-dust ratios increase by a factor of ∼ 20 or that the dust becomes colder towards larger radii. A strong variation of X CO with radius seems unlikely. The observed gradients affect the cross-correlations between gas and dust. In the radial range R = 8-14 kpc total gas and cold dust are well correlated; molecular gas is better correlated with cold dust than atomic gas. At smaller radii no significant correlations between gas and dust are found. The mass of the molecular gas in M 31 within a radius of 18 kpc is M(H 2 ) = 3.6 × 10 8 M ⊙ at the adopted distance of 780 kpc. This is 12% of the total neutral gas mass within this radius and 7%...

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The Ratio of H 2 Column Density to 12CO Intensity in the Vicinity of the Galactic Center

Cited by 124 publications

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Exploring the central molecular zone of the Galaxy using spectroscopy of H ₃ ⁺ and CO

Exploring the central molecular zone of the Galaxy using spectroscopy of H ₃ ⁺ and CO

The GeV-TeV Galactic gamma-ray diffuse emission

Molecular Gas in the Andromeda Galaxy

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The Ratio of H 2 Column Density to 12CO Intensity in the Vicinity of the Galactic Center

Cited by 124 publications

References 0 publications

Exploring the central molecular zone of the Galaxy using spectroscopy of H 3 + and CO

Exploring the central molecular zone of the Galaxy using spectroscopy of H 3 + and CO

The GeV-TeV Galactic gamma-ray diffuse emission

Molecular Gas in the Andromeda Galaxy

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Product

Resources

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Exploring the central molecular zone of the Galaxy using spectroscopy of H ₃ ⁺ and CO

Exploring the central molecular zone of the Galaxy using spectroscopy of H ₃ ⁺ and CO