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Hydroxyl, water, ammonia, carbon monoxide, and neutral carbon towards the Sagittarius A complex VLA, Odin, and SEST observations
Stockholm University, Faculty of Science, Department of Astronomy.
Stockholm University, Faculty of Science, Department of Astronomy.
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2013 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 554, A141Article in journal (Refereed) Published
Abstract [en]

Aims. The Sagittarius A complex in the Galactic centre comprises an ensemble of molecular clouds of different species with a variety of geometrical and kinematic properties. This work aims to study molecular abundances, morphology, and kinematics by comparing hydroxyl, water, carbon monoxide, ammonia, and atomic carbon and some of their isotopologues, in the +50 km s(-1) cloud, the circumnuclear disk (CND), the +20 km s(-1) cloud, the expanding molecular ring and the line-of-sight spiral arm features, including the Local/Sgr arm, the -30 km s(-1) arm, and the 3-kpc arm. Methods. We observed the +50 km s(-1) cloud, the CND and the +20 km s(-1) cloud, and other selected positions at the Galactic centre with the VLA, and the Odin satellite. The VLA was used to map the 1665 and 1667 MHz OH lambda doublet main lines of the (H-2(3/2)) state, and the Odin satellite was used to map the 557 GHz H2O (1(10)-1(01)) line as well as to observe the 548 GHz (H2O)-O-18 (1(10)-1(01)) line, the 572 GHz NH3 (1(0)-0(0)) line, the 576 GHz CO J = 5-4 line and the 492 GHz C-I (P-3(1)-P-3(0)) line. Furthermore, the SEST was used to map a 4'.5 x 6' region of the SgrAcomplex in the 220 GHz (CO)-O-18 J = 2-1 line. Results. Strong OH absorption, H2O emission and absorption lines were seen at all observed positions, and the (H2O)-O-18 line was detected in absorption towards the +50 km s(-1) cloud, the CND, the +20 km s(-1)cloud, the expanding molecular ring, and the 3-kpc arm. Strong CO J = 5-4, (CO)-O-18 J = 2-1, and neutral carbon C-I emissions were seen towards the +50 and +20 km s(-1) clouds. NH3 was only detected in weak absorption originating in the line-of-sight spiral arm features. The abundances of OH and H2O in the +50 and +20 km s(-1) clouds reflect the different physical environments in the clouds, where shocks and star formation prevail in the +50 km s(-1) cloud and giving rise to a higher rate of H2O production there than in the +20 km s(-1) cloud. In the CND, cloud collisions and shocks are frequent, and the CND is also subject to intense UV-radiation emanating from the supermassive black hole and the central star cluster. The CND is rich in (HO)-O-2 and OH, and these abundances are considerably higher than in the +50 and +20 km s(-1) clouds. We compare our estimated abundances of OH, H2O, and NH3 with similar and differing results for some other sources available in the literature. As compared to the quiescent cloud values of a few x 10(-9), or lower, the H2O abundance is markedly enhanced in the front sides of the Sgr A molecular cloud cores, (2-7) x 10(-8), as observed in absorption, and highest in the CND. A similar abundance enhancement is seen in OH. The likely explanation is PDR chemistry including grain surface reactions, and perhaps also the influence of shocks. In the redward high-velocity line wings of the +50 and +20 km s(-1) clouds and the CND, the H2O abundances are estimated to be (1-6) x 10(-6) or higher, i.e., similar to the water abundances in outflows of the Orion KL and DR21 molecular clouds, which are said to be caused by the combined action of shock desorption from icy grain mantles and high-temperature, gas-phase shock chemistry.

Place, publisher, year, edition, pages
2013. Vol. 554, A141
Keyword [en]
Galaxy: center, ISM: clouds, ISM: molecules, Galaxy: abundances
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
URN: urn:nbn:se:su:diva-92660DOI: 10.1051/0004-6361/201220471ISI: 000320444200139OAI: oai:DiVA.org:su-92660DiVA: diva2:640783
Note

AuthorCount:7;

Available from: 2013-08-14 Created: 2013-08-14 Last updated: 2017-12-06Bibliographically approved
In thesis
1. Studies of molecular clouds at the Galactic centre
Open this publication in new window or tab >>Studies of molecular clouds at the Galactic centre
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Interstellar molecular clouds play an essential role in the Universe. Such clouds are invoked for the production and destruction of stars, galaxies and gas and also for energy transport in galaxies. The Galaxy, or the Milky Way, is a large spiral galaxy, with a central bar structure, that harbours a few hundred billion stars and large amounts of gas and dust. At the centre of the Galaxy, a 4 million solar mass supermassive black hole resides, surrounded by a dense core of millions of stars, as well as molecular and dust clouds. The Galactic centre (GC) is hidden by gas and dust, such that only astronomical observations of radio-, infrared-, X-rays and gamma-rays are available for a gathering of information at the centre. In this work, I have studied neutral molecular clouds in absorption at the innermost 50 light years from the centre with the Karl Jansky Very Large Array Observatory in New Mexico in the USA, and with data from observations with the Swedish-ESO Submillimetre Telescope in Chile, and also from the orbital observatory Odin. I have detected a new stream-like feature of gas that seems to link a previously known ring of gas clouds (the CND) and the GC. Moreover, the hypothesis of feeding the CND from an outside cloud is supported by this work. Contemporary discussions in the literature that the central bar structure would act as a pump of material inwards from the spiral arms towards the GC via molecular clouds are also suggested by the data. A number of maser sources have been observed and some of those are shown to reside at shock fronts or anticipated regions of collisions between molecular clouds or at star forming regions. Unusually high water abundance was detected at the south-west part of the CND, indicative of shocks and strong turbulence. Moreover, I have produced high-resolution spectral line maps of hydroxyl (OH) absorption intensity in the four main transition lines of OH at 1612, 1720, 1665 and 1667 MHz, as well as apparent opacity and position-velocity maps of the GC region.

Place, publisher, year, edition, pages
Stockholm: Department of Astronomy, Stockholm University, 2016
Keyword
Galactic centre
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:su:diva-126752 (URN)978-91-7649-356-4 (ISBN)
Public defence
2016-04-29, The Svedbergsalen (FD5), AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 13:00 (English)
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Available from: 2016-04-06 Created: 2016-02-15 Last updated: 2017-02-20Bibliographically approved

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