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Exact timing measurements allow a determination of the phase shift between observations of a pulsar at different frequencies. It has become clear from these observations that a simple dipole magnetic field configuration can not explain the time lag observed for many pulsars between profiles at high frequencies (Kuz'min et al. 1986).
There are cases which might better be explained by a combination of dipole and quadrupole field components (Davies et al. 1984). We report in this paper on new pulsar time alignment observations of a number of pulsars at high and low frequencies which support the general picture outlined above.
Total-power and polarization observations of NGC 253 at 10.7 GHz have been performed with the 100-m MPIfR radio telescope. The observed arm/interarm polarization contrasts are discussed in the context of possible field configurations in spiral arms.
The four clumpy irregular galaxies Mkr 8, 296,297 and 325 have been observed by IRAS. All galaxies have been detected in at least two of the four detector bands. The ratios of the 100 to 60-m flux densities are comparable to those of HII regions or violently star forming galaxies. The average star formation rate in clumpy irregular galaxies is of the order of a few solar masses per year (based on their average far-infrared luminosity and a Hubble constant of 75 km s−1 Mpc−1.
We have mapped the edge-on galaxy NGC3628 in the 12CO(2−1) and 13CO(2−1) lines with the IRAM 30m MRT and in radio continuum at λ20 cm with the VLA. The 12CO(2−1) spectra were obtained out to a distance of 3 kpc east and 2 kpc west of the edge-on galaxy (resolution 0.4 kpc) with supplementary spectra ≃ 9 kpc east and west (1 arcmin ≃ 2 kpc at a distance of 6.7 Mpc). The resulting map shows a central peak of CO emission but also a multiple peaked structure from which we deduce a ringlike enhancement of molecular gas with a diameter of ≃ 400 pc and two outer maxima likely indicating spiral arms of the galaxy. The view of a molecular “ring” is supported by the fact that the maxima of the CO intensity are located close to the turnover radius where rigid rotation passes into differential rotation. Lesch et al. (1990) found for a number of galaxies with molecular rings that these rings can be formed by viscous accretion of molecular gas even at these radii. We find for the rigid rotating disk a molecular mass of m ≃ 108M⊙ assuming a conventional conversion factor. From our channelmaps we were able to distinguish between six unresolved clumps in the central region with comparable CO emissivity and masses of ≃ 106M⊙.
The rotation velocity of molecular gas in the halos of M82 and NGC4631 decreases with the height from the galactic plane. The slower rotation of halo gas can be explained if the gas is supplied from the central region of the galaxies due to some ejection.
We have mapped the southern edge-on spiral galaxy NGC 4945 in 12CO(1-0) and 12CO(2-1) with the SEST telescope. Additional 13CO spectra were also obtained. The intense nuclear CO emission is in agreement with the revised classification of this galaxy as an Sc object. NGC 4945 was for many years known to be a prototype of nuclear activity with detections of various molecular lines at cm-wavelengths, an OH kilomaser, a superluminous H2O maser, etc. (Whiteoak, 1986). A host of molecules (CN, CS, C2H, HCN, CH3OH, etc.) has recently been detected at mm-wavelengths in the nucleus of NGC 4945 (Henkel et al., 1990).
Ringlike structures (in Hα, CO and radio continuum) in central galactic regions have been observed in a number of objects. They seem to be a basic feature of galactic structure (for references see Dickman et al., 1988). These observations can be summarized as follows:
The typical mass of these rings is 108 − 109M⊙.
The gas distribution is smoothly decreasing from the ring into the innermost galactic regions. The rings appear always at the turnover radius where the galactic rotation turns from rigid to differential rotation.
The interacting edge-on galaxy NGC 4631 has been observed in the 12CO(J = 2 − 1) line emission using the IRAM 30-m telescope with a resolution of 13″ (330 pc). The molecular gas is strongly concentrated in a ring-like disk of 1 kpc radius, which is rotating rigidly. Line proriles show several velocity components, which are attributable to spiral arms. Although the J=2−1 to 1−0 transition line ratio indicates that the gas is generally opaque against the lines, we find some optically thinner regions, as well.
1) C18O(1−0) OBSERVATIONS. We observed 13 points of C18O(1−0) (resolution 22″) around the nucleus of M82 and obtained typical main beam brightness temperatures of 20-50 mK. The intensity distribution reveals a clear double–peaked structure with the maxima seperated by 25″. The relative strength of the peaks w.r.t. the emission from the center is consistent with the presence of two point–like sources located at the peak positions. We compared our data with the 17″ CO(1−0) observations of Nakai et al. (1987) and found a strong contrast for the CO(1−0)/C18O(1−0) ratio with high values (≃30) in the center and low values (≃15) at the peak positions. This result is consistent with that of Loiseau et al. (1990) who also found with 12″ resolution for the ratios of CO(2−1)/13CO(2−1) minima at the ring locations, although not as strong as we found for the CO(1−0)/C18O(1−0) ratio. This high contrast indicates the presence of opically thick gas within the molecular ring and supports the view that the main part of the star forming process occurs in the ring.
We have observed the early-type galaxy M104 (NGC 4594, Sombrero Galaxy) in the 12CO(1-0) transition with the 30-m IRAM telescope. We have detected CO in two positions which coincide with maxima of HI, dust and radio continuum. We failed to detect CO in the nuclear area of M104.
Radio continuum surveys give us the fundamental information about the distribution of the radio intensity across the sky. The radio waves originate in three fundamental emitting processes and are measured superposed in the antenna beam. At the lowest radio frequencies (below 10 GHz) the nonthermal emission process (synchrotron radiation) predominates. This radio emission comes from supernova remnants, from the “galactic background” (relativistic electrons diffusing in galactic magnetic fields), but also from extragalactic objects (galaxies, quasars, radio galaxies, etc.). In the frequency range 10 GHz < f < 300 GHz the thermal (free-free) process becomes dominant. In the Galaxy we see the HII regions along the galactic plane. In nearby galaxies we can identify the thermal emission in the disks. Above 300 GHz the dust is seen. This is the boundary region between radio and far-infrared radiation.
We have mapped the southern galaxies NGC 613, 1313, 1433, 1566, 1672, and 2442 in the 12CO(1-0) line with the SEST telescope. The sample bases on galaxies observed previously in radio continuum at the Molonglo Observatory (Harnett, 1985).
The ratio between the intensities of the various CO line transitions, with the additional information about the isotope abundance, should enable us to probe the optical depth in the molecular clouds in galaxies. There are, of course, problems in interpretation as a result of the unknown structure of the clouds, their distribution and temperature. These problems become particularly important since most of the present data are derived in the vicinity of nuclei of galaxies.
The structure of the Large Magellanic Cloud (LMC) has posed many questions and has led to many controversial discussions. Although classified as an 'irregular', repeated claims have been made suggesting that the LMC has spiral structure. Furthermore, this 'spiral' structure seems to originate in the 30-Doradus nebula which, as a result, was sometimes referred to as the 'nucleus' of the LMC. We summarize the recent data on the LMC and re-examine the HI data to point out that possibly different rotation centres exist for different components.
We report on the progress of our search for highly dispersed pulsars near the Galactic Center at 5 GHz using the 100-m radio telescope in Effelsberg. We also present key aspects of our new survey for millisecond pulsars at 21 cm in parts of the northern sky. This survey will greatly benefit from the L-band multibeam receiver and a new FFT-based backend which are currently under construction at the MPIfR.
Continuum observations of the southern extension of the radio Arc located near 1~0.2° have been carried out at λ20 and 6cm using the VLA in its hybrid B/C and C/D array configurations. A number of long and narrow filaments have been identified on the negative latitude side of the plane. Some of the filaments appear to extend continuously into the radio continuum Arc and suggesting strongly that they are associated physically with the Arc. Other filaments appear isolated and thus have characteristics similar to those of the radio “threads” which have been seen near the Galactic center. These new threads and filaments are highly polarized at λ6cm and show rotation measures which vary between 300 and 3000 rad m−2. The details present in the high-resolution images of this region strengthen the hypotheses that the large field strength is dynamically important and that the large-scale geometry of the magnetic field is poloidal near the Galactic center.
We have begun a long-range project to study southern galaxies using the radio telescopes at Parkes and Molonglo, the Siding Spring optical facilities and soon, the Australia Telescope. Here we present the results of polarization mapping at two wavelengths of the galaxies NGC 55, 253,4945, M 83 and the Circinus Galaxy.
This southern galaxy with its fairly regular pattern is well suited to study the influence of a bar onto the magnetic field in a galaxy. With its low inclination of 24° it provides a geometry similar to M 51. Also the magnetic field strength of 11±2 μG is in the same range.
We present new results from a number of deep radio polarization surveys of the Magellanic Clouds at 2.3 GHz, 4.75 GHz and 8.55 GHz. Extended linearly polarized radio emission has been found at 2.3 and 4.75 GHz from both galaxies.