Radio continuum

Fig. 2. VLA detection of 3He+ in the PN J 320. We have modeled the radio continuum and line emission using the radiative transfer code NEBULA [1], assuming an expanding shell of ionized gas. The dashed line is the model including the H171 7 and 3He+ transitions. The solid line shows the observed spectrum and only includes the 3He+ transition. The model fits the data reasonably well even though the morphology is bipolar as indicated by the HST image [6]...

Te can be derived using the ratio of the two lines [O III] A4363 and [O III] A5007, which have very different excitation potentials. Other line ratios can also be used as temperature indicators in nebulae, such as [N ii] A5755/6584 and [S III] A6312/9532. The Balmer and Paschen jumps, the radio continuum and radio recombination lines also allow to estimate the electron temperature, but the measurements are more difficult. 

W cm ym" at = 23° and to decrease rather steeply to 8 X 10"11 W cm"2 m-1 at t - 75°. The longitude dependence is similar to that of the brightness temperature of radio continuum and shows humps at the same positions as was found for the intensity of 2.6 mm line of CO. This suggests the contribution of young objects to the infrared brightness. The isophotes of the infrared intensity are compared with that expected for models of the galaxy. 

Radio continuum observations together with IR-observations yield the following information ... 

Radio continuum observations are widely used for the interpretation of IR-observations. We therefore limit the review in Sect. II. to recent high frequency single dish observations (II.1) and aperture synthesis (SRT) observations (II.2). IR-observers probably are less familiar with radio recombination line observations and their application to the interpretation of IR-observations. Therefore more space is devoted in Sect. Ill to recent developments in this field. 

A special symposium on "HII regions and related topics" which summarizes radio observations of HII regions up to 1974 was held in Mittelberg, Austria, January 13 through 17, 1975. The proceedings of this symposium are published and will be referred to as (1975 Lect. Notes Phys. 42). In the following Sect. II (radio continuum) and III (radio recombination lines) we try to emphasize recent observational results not yet covered in these proceedings. 

For the interpretation of far IR observations of special interest are radio continuum observations with high angular resolution and at high frequencies, which qre reviewed in this section. 

R. Predmore, P. Wannier and myself. In particular, the luminous northern source observed by Emerson, Jennings, and Moorwood (1973) has no counterpart in the radio continuum maps, but instead is associated with an intense, extended cloud of molecular gas as evidenced by the CO emission. NGC 633 is a region where I hope future detailed studies of infrared and molecular emission can help pin down the interrelationship. 

Key words active galaxies - near-IR - radio continuum... 

Figure 9.3 A continuum of photon energies exists from radio waves through to y-rays. We call it a spectrum . The visible region extends from 350 to 700 nm...

Care although we give different names to these photons, in reality there exists a continuum of wavelengths from 10-11 m (for y-rays) through to hundreds of metres (for radio waves). 

In this section, we shall restrict ourselves to those aspects of radio astronomy which are relevant to the study of the rotational spectra of diatomic molecules. We will not deal with the study of continuum sources, with cosmology, or with the detailed structure, dynamics and chemistry of interstellar clouds. These are important parts of astrophysics, covered in many research articles, reviews and books [34, 35, 36]. We will describe the main features of the dishes which collect radiation (i.e. the telescope), the detectors and signal processing equipment, and the analysis of the spectra. Many of the microwave spectra of diatomic molecules are now used as important probes to... 

In radio astronomy multichannel or autocorrelation (Fourier) spectrometers are used which simultaneously cover the whole line profile. Consider a molecular cloud observed against a source of continuum radiation of a given brightness temperature. The continuum brightness temperature is the sum of the 2.7 °K isotropic background radiation Tbh of a continuum source (such as an HII region or a supernova remnant) which may be in the line of sight and located behind the molecular cloud. A specific molecular transition with optical... 

Fig. 20. Spectra of the 1 j 0 - 1 j j transitions for three isotopic species of formaldehyde seen in absorption against the radio source Sagittarius B2. The ordinate is the ratio of line/continuum antenna temperatures. The profiles are aligned at a velocity of 62 km/sec with respect to LSR. The center of the H141/3 recombination line for a velocity of 62 km/sec is indicated on the H213C16C) profile (from Gardner et al., 1971)...

Fig. 20 shows the observed interstellar molecular lines of various isotopic species of formaldehyde, H2CO, as detected by Gardner et al., 1971. This particular line, the lowest asymmetry-doublet transition 110 — lu, is seen in absorption in the continuum radiation of the strong radio source Sgr B2, which is located behind the molecular gas cloud. Frequency is plotted along the abscissa and the ordinate is intensity, expressed in the ratio of line-to-continuum antenna temperatures. For all three formaldehyde isotopes the continuum temperature is Tc T >b Tex- This is the case because the formaldehyde molecules are in approximate equilibrium with the microwave background... 

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