A 7 mm line survey of the shocked and disrupted molecular gas towards the W28 field TeV gamma-ray sources

Nicholas, B.; Rowell, G.; Burton, M. G.; Walsh, A. J.; Fukui, Y.; Kawamura, Akiko; Maxted, N.

doi:10.1111/j.1365-2966.2011.19688.x

Cited by 35 publications

(47 citation statements)

References 61 publications

(118 reference statements)

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“…robust mass estimates) in studies of dense gas cores towards TeV gamma-ray sources (e.g. Nicholas et al 2012), which can be used to probe the fundamental transport properties of cosmic rays in molecular clouds (e.g. Gabici & Aharonian 2007;Maxted et al 2012).…”

Section: Gamma Rays Cosmic Rays and Molecular Cloudsmentioning

confidence: 99%

The Mopra Southern Galactic Plane CO Survey

Burton

Braiding

Glueck

et al. 2013

Publ. Astron. Soc. Aust.

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We present the first results from a new carbon monoxide (CO) survey of the southern Galactic plane being conducted with the Mopra radio telescope in Australia. The 12 CO, 13 CO, and C 18 O J = 1-0 lines are being mapped over the l = 305 • -345 • , b = ±0.5 • portion of the fourth quadrant of the Galaxy, at 35 arcsec spatial and 0.1 km s −1 spectral resolution. The survey is being undertaken with two principal science objectives: (i) to determine where and how molecular clouds are forming in the Galaxy and (ii) to probe the connection between molecular clouds and the 'missing' gas inferred from gamma-ray observations. We describe the motivation for the survey, the instrumentation and observing techniques being applied, and the data reduction and analysis methodology. In this paper, we present the data from the first degree surveyed, l = 323 • -324 • , b = ±0.5 • . We compare the data to the previous CO survey of this region and present metrics quantifying the performance being achieved; the rms sensitivity per 0.1 km s −1 velocity channel is ß1.5 K for 12 CO and ß0.7 K for the other lines. We also present some results from the region surveyed, including line fluxes, column densities, molecular masses, 12 CO/ 13 CO line ratios, and 12 CO optical depths. We also examine how these quantities vary as a function of distance from the Sun when averaged over the 1 square degree survey area. Approximately 2 × 10 6 M of molecular gas is found along the G323 sightline, with an average H 2 number density of n H 2 ∼ 1 cm −3 within the Solar circle. The CO data cubes will be made publicly available as they are published.

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Section: Gamma Rays Cosmic Rays and Molecular Cloudsmentioning

confidence: 99%

The Mopra Southern Galactic Plane CO Survey

Burton

Braiding

Glueck

et al. 2013

Publ. Astron. Soc. Aust.

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“…However, CS has recently been shown to be present in shocked regions (e.g. Nicholas et al 2012). This is also indicated by the high temperature models which successfully fit many of the emission regions.…”

Section: Shocksmentioning

confidence: 78%

Mapping CS in starburst galaxies: Disentangling and characterising dense gas

Kelly¹,

Viti²,

Bayet³

et al. 2015

A&A

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Aims. We observe the dense gas tracer CS in two nearby starburst galaxies to determine how the conditions of the dense gas varies across the circumnuclear regions in starburst galaxies. Methods. Using the IRAM-30m telescope, we mapped the distribution of the CS(2−1) and CS(3−2) lines in the circumnuclear regions of the nearby starburst galaxies NGC 3079 and NGC 6946. We also detected formaldehyde (H 2 CO) and methanol (CH 3 OH) in both galaxies. We marginally detect the isotopologue C 34 S. Results. We calculate column densities under LTE conditions for CS and CH 3 OH. Using the detections accumulated here to guide our inputs, we link a time and depth dependent chemical model with a molecular line radiative transfer model; we reproduce the observations, showing how conditions where CS is present are likely to vary away from the galactic centres. Conclusions. Using the rotational diagram method for CH 3 OH, we obtain a lower limit temperature of 14 K. In addition to this, by comparing the chemical and radiative transfer models to observations, we determine the properties of the dense gas as traced by CS (and CH 3 OH). We also estimate the quantity of the dense gas. We find that, provided there are between 10 5 and 10 6 dense cores in our beam, for both target galaxies, emission of CS from warm (T = 100−400 K), dense (n(H 2 ) = 10 5−6 cm −3 ) cores, possibly with a high cosmic ray ionisation rate (ζ = 100 ζ 0 ) best describes conditions for our central pointing. In NGC 6946, conditions are generally cooler and/or less dense further from the centre, whereas in NGC 3079, conditions are more uniform. The inclusion of shocks allows for more efficient CS formation, which means that gas that is less dense by an order of magnitude is required to replicate observations in some cases.

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“…The W28 SNR is a prominent example of an association of partly resolved HESS γ-ray sources and dense, shocked molecular gas, as revealed by the molecular line observations of Nicholas et al (2011Nicholas et al ( , 2012. In particular, the cm-wave inversion transitions of ammonia (NH 3 ) show peak intensities that coincide with the positions of peaks in γ-ray emission and radio synchrotron emission.…”

Section: First Experimental Evidencementioning

confidence: 99%

“…5. Dense molecular gas associated with the W28 SNR shows evidence of heating and chemistry driven by shock waves (Nicholas et al 2011(Nicholas et al , 2012, but the high excitation of ammonia molecules in one molecular core can also be interpreted in terms of ion chemistry driven by an enhanced rate of ionization as outlined in the next subsection. A derivation of the ionization rate is given in Sect.…”

Section: First Experimental Evidencementioning

confidence: 99%

Cosmic-ray-induced ionization in molecular clouds adjacent to supernova remnants

Schuppan

Becker

Black

et al. 2012

A&A

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Context. Energetic gamma rays (GeV to TeV photon energy) have been detected toward several supernova remnants (SNRs) that are associated with molecular clouds. If the gamma rays are produced mainly by hadronic processes rather than leptonic processes like bremsstrahlung, then the flux of energetic cosmic ray nuclei (>1 GeV) required to produce the gamma rays can be inferred at the site where the particles are accelerated in SNR shocks. It is of great interest to understand the acceleration of the cosmic rays of lower energy (<1 GeV) that accompany the energetic component. These particles of lower energy are most effective in ionizing interstellar gas, which leaves an observable imprint on the interstellar ion chemistry. A correlation of energetic gamma radiation with enhanced interstellar ionization can thus be used to support the hadronic origin of the gamma rays and to constrain the acceleration of ionizing cosmic rays in SNR. Aims. We propose a method to test the hadronic origin of GeV gamma rays from SNRs associated with a molecular cloud. Methods. We use observational gamma ray data for each SNR known to be associated with a molecular cloud, modeling the observations to obtain the underlying proton spectrum under the assumption that the gamma rays are produced by pion decay. Assuming that the acceleration mechanism does not only produce high energy protons, but also low energy protons, this proton spectrum at the source is then used to calculate the ionization rate of the molecular cloud. Ionized molecular hydrogen triggers a chemical network forming molecular ions. The relaxation of these ions results in characteristic line emission, which can be predicted. Results. We show that the predicted ionization rate for at least two objects is more than an order of magnitude above Galactic average for molecular clouds, hinting at an enhanced formation rate of molecular ions. There will be interesting opportunities to measure crucial molecular ions in the infrared and submillimeter-wave parts of the spectrum.

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A 7 mm line survey of the shocked and disrupted molecular gas towards the W28 field TeV gamma-ray sources

Cited by 35 publications

References 61 publications

The Mopra Southern Galactic Plane CO Survey

The Mopra Southern Galactic Plane CO Survey

Mapping CS in starburst galaxies: Disentangling and characterising dense gas

Cosmic-ray-induced ionization in molecular clouds adjacent to supernova remnants

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