M. J. A. Oort’s research while affiliated with National Radio Astronomy Observatory and other places

Recent data in several wavebands on the peculiar galactic object G70.7+1 .2 is reviewed. A new VLA map clearly shows the radio shell. Possible interpretations of the object include supernova remnant, nova shell, and nebula resulting from mass loss from a giant or supergiant, although each of these interpretations has its own difficulties.

We present a deep 92cm survey down to 4.5mJy (5{sigma}) carried out with the Westerbork Synthesis Radio Telescope (WSRT) of a field in Lynx, previously observed at three other radio frequencies: 6, 21, and 49cm. The observations, totalising 4*12h were made in the redundancy mode, which makes it possible to achieve a high dynamic range. In total, 384 sources above a 5{sigma} peak flux limit were detected, of which 337 form a complete sample. (1 data file).

The authors present the extragalactic radio source count at 1.4 GHz, based on all high-latitude, high-declination (parts of) WSRT surveys published up to 1985. The main reason for the reevaluation of the source counts from these surveys is the recently obtained result on the angular sizes of sources below ≡10 mJy, which necessitates a rediscussion of so-called "resolution-corrections" that were applied in some of the earlier surveys. The authors also combine the resulting WSRT source count with those obtained from recent 1.4 GHz WSRT surveys down to a limit of ≡100 μJy, and with existing data at flux densities above ≡140 mJy. This combined count will serve as a very reliable integral constraint for model extrapolations of the spatial density of radio galaxies.

The paper presents a deep 92 cm survey down to 4.5 mJy (5 sigma) carried out with the Westerbork Synthesis Radio Telescope (WSRT) of a field in Lynx, previously observed at three other radio frequencies: 6, 21, and 49 cm. The observations were made in the redundancy mode, which makes it possible to achieve a high dynamic range. In total, 384 sources above a 5 sigma peak flux limit were detected, of which 337 form a complete sample. For the 92-cm complete sample, the spectral index distribution was determined as a function of 92 cm and 21 cm flux density. The fainter sources have significantly flatter spectra between 92 cm and 21 cm than the brighter sources, with a median spectral index that changes from about 0.7 above 25 mJy to about 0.55 below 25 mJy (at 21 cm). A similar decrease is seen as a function of 92 cm flux density. A much smaller effect is shown by the median spectral index between 49 and 21 cm. There is, therefore, clear evidence for a spectral turnover between 92 and 49 cm in a large fraction of the radio sources. From optical identifications it appears that the spectral turnover occurs primarily in the blue radio galaxies. The 327 MHz source counts above 4.75 mJy are presented.

The paper presents a set of various data ranging from radio to X-ray wavelengths, including optical spectroscopy and infrared spectrophotometry, of the galactic source G70.7 + 1.2 whose nature has recently been assessed contradictorily. The observations definitely confirm the presence of a spherical shock wave expanding in a very dense environment. This shock wave accelerates the relativistic electrons responsible for the shell structure of the radio emission. It is suggested that G70.7 + 1.2 could be a young and compact supernova remnant in a high density medium which absorbs part of the nonthermal radio emission, accounting for the turnover in the low frequency spectrum (absorption by thermal electrons). The lower than normal radio luminosity may indicate that the emission is only just turning on, although other possibilities are also likely. The central object, which dominates the near-infrared emission, could be a supergiant star ejecting an envelope. The interaction of ejected matter with the interstellar cloud would lead to the formation of a shock wave propagating in the cloud. G70.7 + 1.2 might also be interpreted as the remnant of a nova event.

The author presents high resolution (1arcsec) maps of 31 radio sources with total flux densities between 0.3 and 10 mJy, made with the VLA at 20 cm in the A-configuration. Seven sources were mapped individually with snapshots. These sources were selected from the 21 cm WSRT Leiden Berkeley Deep Survey (Windhorst et al., 1984) source list. The other 24 sources were detected in five fields situated in the area of the, at present, deepest Westerbork 21 cm survey (Oort, 1987). 25 sources are used to determine the median angular size in the flux density range between 0.4 and 1.0 mJy. The median angular size of these sources is smaller than 1arcsec, since more than half of the sources remain unresolved with the VLA A-array at 20 cm. All the high resolution information available on WSRT 21 cm deep survey radio sources is combined to determine the run of the median angular size below 50 mJy. It is found that θmed(S) decreases steadily toward lower flux densities, to under 1arcsec below 1 mJy.

The results of 6 cm observations of sources detected at 21 cm in the Leiden-Berkeley Deep Survey are used to study the radio spectra of blue radio galaxies. The mean spectral index between 6 and 21 cm of the present sources is found to be steep. The blue radio galaxies are shown to fall into two classes, those with radio morphologies similar to those of classical red radio galaxies, and those which are very compact. Both sets of characteristics can be interpreted as being typical of small optically thick nonthermal cores no more than a few parsec in size.

The study of radio source populations is a long term programme at Leiden Observatory, based primarily on surveys with the Westerbork Synthesis Radio Telescope. The first two references1) 2) and literature cited therein provide the overview of what was achieved since the early seventies. Oort’s thesis4) is the fourth one in this context and it enters new domains in terms of flux density range and angular size limits for statistically large samples.

Observations of the field Lynx 3A, obtained at 21 cm with 5sigma peak flux 85 microJy using the 3-km array of the Westerbork Synthesis Radio Telescope for 16 12-h periods during 1984, are reported. Details of the observations and the data reduction are given, and the results are presented in extensive tables and maps. A complete sample of 321 objects within the 0.5-deg attenuation radius is selected from the 349 objects detected above 85 microJy, and the 1.412-GHz source counts down to 100 microJy are found to be in good agreement with VLA surveys of similar depth. The median angular size of sources with total flux density between 350 microJy and 10 mJy is shown to be less than 10 arcsec.