Origin of Galactic and Extragalactic Magnetic Fields

A variety of observations suggest that magnetic fields are present in all galaxies and galaxy clusters. These fields are characterized by a modest strength (10^{-7}-10^{-5} G) and huge spatial scale (~Mpc). It is generally assumed that magnetic fields in spiral galaxies arise from the combined action of differential rotation and helical turbulence, a process known as the alpha-omega dynamo. However fundamental questions concerning the nature of the dynamo as well as the origin of the seed fields necessary to prime it remain unclear. Moreover, the standard alpha-omega dynamo does not explain the existence of magnetic fields in elliptical galaxies and clusters. The author summarizes what is known observationally about magnetic fields in galaxies, clusters, superclusters, and beyond. He then reviews the standard dynamo paradigm, the challenges that have been leveled against it, and several alternative scenarios. He concludes with a discussion of astrophysical and early Universe candidates for seed fields.Comment: 67 pages, 17 figures, accepted for publication in Reviews of Modern Physic

Search for CO in Atomic Hydrogen Clouds

The relationship between dense molecular clouds and diffuse clouds, as well as the mechanisms connected with the formation of molecules in diffuse clouds, may be studied using HI 21-cm line observations and molecular line observations in the same directions. For this purpose we previously studied the OH 18-cm main lines (Kazès et al., 1977) and the 2.6-mm CO lines (Crovisier and Kazès, 1977) in directions where strong 21-cm absorption features had been detected in the Nancay survey (Crovisier et al., 1978). Liszt and Burton (1979) also measured CO lines toward 19 directions observed in the Arecibo 21-cm emission/absorption survey (Dickey et al., 1978). This paper presents preliminary results of a more comprehensive search for 12CO in directions previously studied in the Nancay survey.</jats:p

A statistical study of local interstellar matter based on the Nancay 21-cm absorption survey

The survey of 21-cm galactic absorption towards 819 extragalactic sources, recently carried out with the Nancay radio telescope (Crovisier et al. 1978) provides a sample of absorbing neutral hydrogen clouds which is an order of magnitude larger than those resulting from other available surveys (Hughes et al. 1971, Radhakrishnan et al. 1972, Lazareff 1975, and Dickey et al. 1978). A statistical study of this sample offers insight into the properties of local interstellar matter.</jats:p

Measurement of Magnetic-Field Strengths in Molecular Clouds: Detection of OH Line Zeeman Splitting

We report here the first results of an extended program to measure magnetic-field strengths in interstellar molecular clouds. The very large radio telescope located near Nancay, France, has been used to measure the Stokes-parameter I and V spectra of the 1665 and 1667 MHz lines of OH in emission and in absorption from extended (non-masing) molecular clouds. Signals in the V spectra are produced by Zeeman splitting of the spectral lines; we derive magnetic-field strengths or limits from these data.</jats:p

A

We present observations of the 12CO(J=1-0) line at 2.6 mm of 65 galaxies located in the Coma supercluster region: 33 actually belong to the Coma supercluster while 32 are either foreground or background objects. These data have been obtained using the NRAO 12 m telescope at Kitt Peak (United States), and for four galaxies, using the IRAM 30 m telescope at Pico Veleta (Spain). Out of these 65 galaxies, 54 had never been observed in the CO(1-0) line; 49 have been detected by us, of which 37 are new detections. We give molecular gas masses deduced from the CO line integrated intensities, and upper limits for the 16 undetected objects, computed with a Galactic conversion factor N(${\rm H}_2$) = 2.3 1020 I(CO) and H0 = 75 km/s/Mpc

The Magnetic Field in the Perseus Molecular Cloud Complex

In Figure 1, we present a map of the polarization of background starlight in the Perseus region (Goodman, Bastien, Myers, and Menard 1989) superposed on contours of integrated 13CO emission (Bachiller and Cernicharo 1986). The polarization vectors map the plane-of-the-sky field (B⊥), assuming as usual that the observed polarization is the result of selective extinction by magnetically aligned dust grains associated with the molecular clouds between the observer and background stars (e.g. Dolginov 1989).</jats:p