A 10-GHz Radio Continuum Survey [ CATS home ] [ Back to CATS list ] [ ftp ]

(erratum 1984PASJ...36..639 included)
(OCR by H.Andernach 1/96, to be proof-read)

A 10-GHz Radio Continuum Survey of the Galactic Plane Region. I. A Complex Region at l=21d-26d


Publ. Astron. Soc. Japan 36, 287-295 (1984)


This is the first of a series of papers presenting results of a 10-GHz radio continuum survey of the galactic plane region using the 45-m telescope at Nobeyama. An extensive study of a complex region at 21d<1<26d, |b|<1 has revealed the following remarkable features. A Crab-like SNR: Among many supernova remnants detected in the field, the SNR G24.7+0.6 shows an irregular morphology with a filled center and its peak flux spectrum between S and 10 GHz is flat. This fact suggests that this object may be a Crab-like SNR. H II Ring: Two ringlike orientations of H II regions of diameter 50' and 40' are found centered on G23.2+0.2 and G24.6+0.0, respectively. They may be physical associations on rings or shells with a diameter of about 100 pc at distances 7 and 9 kpc, respectively. A shock enhanced star formation is suggested for the origin of the rings. Weak Nonthermal Sources: A comparison with the Bonn S-GHz survey reveals a number of weak nonthermal sources near the galactic plane. Possible origins of the sources are discussed.

Key words: Galaxy; Hll regions; Radio continuum survey; Supernova remnants.

1. Introduction

A number of surveys of the galactic plane region have been made so far at various radio frequencies. Among them the Bonn survey at 5 GHz using the 100-m telescope (Altenhoff et al. 1978) provides maps with the highest angular resolution among the current surveys in the microwave range, revealing a number of interesting features near the galactic plane. A survey with the same angular resolution but at a higher frequency with a filled aperture antenna will certainly provide a powerful means to investigate the physical nature of the objects found in the Bonn survey.

For this reason we conducted a survey of the galactic plane region at 10 GHz using the 45-m radio telescope at the Nobeyama Radio Observatory (NRO). The angular resolution of 217 is almost comparable to that of the Bonn survey (2'6), so that the two surveys can be directly compared. It is known that the galactic radio emission is dominated by nonthermal components at frequencies lower than 5 GHz and by thermal components at higher frequencies (Hirabayashi 1974). The comparison will be therefore particularly useful to discriminate thermal radio sources from nonthermal sources and vice versa. In this paper we present a map of the 10-GHz total intensity distribution of the region at l=21d to 26d, which contains complexes of H n regions. We compare our results with the Bonn 5-GHz survey and discuss the nature of sources found in the region. A brief report of the results has been given by Sofue et al. (1983). A further report of the whole survey work will be given in a separate paper.

2. Observations

The observations were made in November, 1982 and March and June, 1983. The HPBW of the 45-m antenna was 2.70'+/-0.03' at the center frequency of 10.05 GHz. The bandwidth was 500 MHz. We used a cooled parametric amplifier combined with a Dicke-switching system referencing a cooled dummy load at 20-K stage. The system noise temperature was approximately 100 K. We used a circularly polarized feed system and detected one polarization component. The total intensity was obtained by assuming that the circular polarization in sources is negligible. The flux densities and the positions were calibrated using the radio sources 3C48 and 3C348 (Baars et al. 1977; Tabara et al. 1984). The conversion factor between the brightness temperature on the sky and the equivalent flux density per beam area was taken as Tb/S=0.47+/-0.05 K/Jy.

The region at l=21d-26d and b=-1d to +1d was mapped by several observations. The scans were made in the direction approximately perpendicular to the galactic plane. Both extreme sides at b = + 1d of each map were taken to be zero levels. All the maps were combined to get a final map covering the above region. The rms noise on the final map as calculated in an empty small region was about 20 mK in Tb. The data reduction was made using the radioastronomical reduction system at the NRO, a part of which contains the NOD2 reduction package described by Haslam (1974). Scanning effects, which are mainly caused by the weather condition, were removed by using the "pressing" method of Sofue and Reich (1979). The computations were made on a FAOoM M200 at NRO.

3. A Complex Region at 21d<l<26d

3.1 The Map and Source List
Results of the observations are shown in figure I in the form of a contour map of the total surface brightness. The numbers on the contours represent brightness in units of 21.8 mJy/2.7'-beam (=3.31 x 10^-22 W/m^2/Hz/sr=10.2 mK in Tb). We find a large number of discrete sources both compact and extended. Table I lists souroes with their positions, fluxes, and sizes. The sizes are full widths at half maximum when the source is fitted with a two-dimensional Gaussian brightness distribution using a Gaussian fitting program. Very extended and elongated sources are sometimes not fitted with the program and are not completely picked up. The column entry to the table is explained below: