Model of outgrowths in the spiral galaxies NGC 4921 and NGC 7049 and the origin of spiral arms
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O R I G I N A L A RT I C L E
Model of outgrowths in the spiral galaxies NGC 4921 and NGC 7049 and the origin of spiral arms Per Carlqvist
Received: 15 May 2012 / Accepted: 1 September 2012 / Published online: 18 October 2012 © Springer Science+Business Media Dordrecht 2012
Abstract NGC 4921 and 7049 are two spiral galaxies presenting narrow, distinct dust features. A detailed study of the morphology of those features has been carried out using Hubble Space Telescope archival images. NGC 4921 shows a few but well-defined dust arms midway to its centre while NGC 7049 displays many more dusty features, mainly collected within a ring-shaped formation. Numerous dark and filamentary structures, called outgrowths, are found to protrude from the dusty arms in both galaxies. The outgrowths point both outwards and inwards in the galaxies. Mostly they are found to be V-shaped or Y-shaped with the branches connected to dark arm filaments. Often the stem of the Y appears to consist of intertwined filaments. Remarkably, the outgrowths show considerable similarities to elephant trunks in H II regions. A model of the outgrowths, based on magnetized filaments, is proposed. The model provides explanations of both the shapes and orientations of the outgrowths. Most important, it can also give an account for their intertwined structures. It is found that the longest outgrowths are confusingly similar to dusty spiral arms. This suggests that some of the outgrowths can develop into such arms. The time-scale of the development is estimated to be on the order of the rotation period of the arms or shorter. Similar processes may also take place in other spiral galaxies. If so, the
Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the Data Archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with programs ID 10842 and ID 9427. P. Carlqvist () Space and Plasma Physics, School of Electrical Engineering, Royal Institute of Technology, 100 44 Stockholm, Sweden e-mail: [email protected]
model of the outgrowths can offer a new approach to the old winding problem of spiral arms. Keywords Spiral arms · Feathers · Spurs · Molecular clouds · H II regions · Elephant trunks · Mammoth trunks · Magnetic fields · Double helix · NGC 4921 · NGC 7049 1 Introduction Galaxies containing dusty features can sometimes display remarkable and unexpected shapes. An illustration of that is the giant, elliptical galaxy NGC 1316 in Fornax showing a conspicuous web of dusty filaments (Evans 1949; Shklovskii and Cholopov 1952; de Vaucouleurs 1953; Schweizer 1980; Grillmair et al. 1999). It is believed that the galaxy is a merger, which has captured one or more gas-rich galaxies during the last three billion years (Schweizer 1980; Horellou et al. 2001; Goudfrooij et al. 2004). An intriguing feature of NGC 1316 is constituted by a great number of dark structures protruding from the du
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