Change search
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Studies of molecular clouds at the Galactic centre
Stockholm University, Faculty of Science, Department of Astronomy. (Galaxer och kosmologi)
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 [en]
Galactic centre
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
URN: urn:nbn:se:su:diva-126752ISBN: 978-91-7649-356-4 (print)OAI: oai:DiVA.org:su-126752DiVA: diva2:903189
Public defence
2016-04-29, The Svedbergsalen (FD5), AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 13:00 (English)
Opponent
Supervisors
Available from: 2016-04-06 Created: 2016-02-15 Last updated: 2017-02-20Bibliographically approved
List of papers
1. 18-cm VLA observations of OH towards the Galactic Centre: Absorption and emission in the four ground-state OH lines
Open this publication in new window or tab >>18-cm VLA observations of OH towards the Galactic Centre: Absorption and emission in the four ground-state OH lines
2003 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 403, 1011-1021 p.Article in journal (Refereed) Published
Abstract [en]

The OH distribution in the Sgr A Complex has been observed in the 1612-, 1665-, 1667- and 1720-MHz OH transitions with the Very Large Array (VLA) in BnA configuration. Maps are presented with a channel velocity resolution of about 9 km s -1 and with angular resolutions of  . Some clear results are highlighted here, such as absorption from the Circumnuclear Disk (CND) and the OH-Streamer inside the CND near Sgr , strong absorption towards most of the eastern and western parts of the Sgr A East shell, lack of absorption towards both Sgr A West and the compact H II-regions to the east of Sgr A East, and double-lobed structure of the High Negative Velocity Gas (HNVG) oriented northeast and southwest of Sgr , and finally compact, point-like maser emission in all four transitions, in particular a 1720-MHz maser at -132 km s -1 in the CND as counterpart to a 1720-MHz maser at +132 km s -1 in the CND.

National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:su:diva-122685 (URN)10.1051/0004-6361:20030309 (DOI)
Available from: 2015-11-09 Created: 2015-11-09 Last updated: 2017-12-01Bibliographically approved
2. Hydroxyl, water, ammonia, carbon monoxide, and neutral carbon towards the Sagittarius A complex VLA, Odin, and SEST observations
Open this publication in new window or tab >>Hydroxyl, water, ammonia, carbon monoxide, and neutral carbon towards the Sagittarius A complex VLA, Odin, and SEST observations
Show others...
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.

Keyword
Galaxy: center, ISM: clouds, ISM: molecules, Galaxy: abundances
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:su:diva-92660 (URN)10.1051/0004-6361/201220471 (DOI)000320444200139 ()
Note

AuthorCount:7;

Available from: 2013-08-14 Created: 2013-08-14 Last updated: 2017-12-06Bibliographically approved
3. The OH-streamer in Sagittarius A revisited: Analysis of hydroxyl absorption within 10 pc from the Galactic centre
Open this publication in new window or tab >>The OH-streamer in Sagittarius A revisited: Analysis of hydroxyl absorption within 10 pc from the Galactic centre
2015 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 582, A118Article in journal (Refereed) Published
Abstract [en]

Aims. We study the structure and kinematics of the OH-streamer and the +80 km s(-1) cloud and their interactions with the circumnu-clear disk (CND) and with other molecular clouds in the vicinity of the Galactic centre (GC), and we map OH absorption at about 6 '' resolution at R <= 10 pc from the GC, with about 9 km s(-1) of velocity resolution. Methods. The VLA was used to map OH line absorption at the 1665 and 1667 MHz lambda doublet main lines of the (2)Pi(3/2) state towards the Sagittarius A complex. Results. Strong OH absorption was found in the OH-streamer, the southern streamer (SS), the +20, +50, and +80 km s(-1) molecular clouds, the molecular belt, the CND, the expanding molecular ring (EMR), and the high negative velocity gas (HNVG). The OH-streamer was found to comprise three parts, head, mid, and tail, and to interact with the SS/+20, +80 km s(-1) clouds and the CND. Optical depths and column densities divided by excitation temperatures have been calculated for the OH-streamer and the +80 km s(-1) cloud. Conclusions. The OH-streamer, the SS, the +20 and +80 km s(-1) clouds, and the CND are intimately related in position and velocity space. The OH-streamer was found to be a clumpy object stretching in projection from the inner radius of the CND at about 1.8 pc from Sgr A*towards and partly engulfing Sgr A*. As a side result of our data, a possible link between the near side of the EMR and the CND's southwest lobe was found. Additionally, we found OH absorption against all four of the previously known compact H II regions A-D, located east of Sgr A East, indicating their close association with the +50 km s(-1) cloud.

Keyword
Galaxy: center, ISM: individual objects: Sagittarius A, ISM: molecules, ISM: clouds
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:su:diva-122687 (URN)10.1051/0004-6361/201424426 (DOI)000363538500118 ()
Available from: 2015-11-09 Created: 2015-11-09 Last updated: 2017-12-01Bibliographically approved

Open Access in DiVA

fulltext(9920 kB)47 downloads
File information
File name FULLTEXT02.pdfFile size 9920 kBChecksum SHA-512
d6084a6ac189500b3edd8f884ed44d8908405cf6e0cb7ee84bde72db7a66c4e87501a106d7e3c377be173dee466b7e7bf03e895ab2711761fa909a421f1791c6
Type fulltextMimetype application/pdf

Search in DiVA

By author/editor
Karlsson, Roland
By organisation
Department of Astronomy
Astronomy, Astrophysics and Cosmology

Search outside of DiVA

GoogleGoogle Scholar
Total: 47 downloads
The number of downloads is the sum of all downloads of full texts. It may include eg previous versions that are now no longer available

isbn
urn-nbn

Altmetric score

isbn
urn-nbn
Total: 371 hits
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf