| dc.description.abstract | This thesis describes the results of an observational study of partially ionized gas in the Galaxy using low-frequency radio recombination lines of carbon, which arise due to electronic transitions at large quantum numbers (150 < n < 600).
Background and Motivation
Radio recombination lines are powerful tools for studying the ionized component of the interstellar medium (ISM). Different types of ionized regions in the Galaxy give rise to detectable recombination lines of various atomic species such as hydrogen, helium, and carbon.
• Hydrogen and helium lines are generally detected from hot (T? ? 10? K) fully ionized clouds known as H?II regions.
• Carbon lines, on the other hand, arise in cooler (50–500 K) partially ionized regions.
Although extensive studies of hot, ionized regions using hydrogen and helium recombination lines have been carried out, observations of carbon recombination lines from partially ionized gas are relatively few. Carbon lines at relatively high frequencies (> 1 GHz) have been detected from partially ionized gas surrounding strong H?II regions and are reasonably well studied.
Discovery of Low-Frequency Carbon Lines
In 1980, Konovalenko & Sodin discovered a new type of carbon recombination line at a very low frequency (~26 MHz) in the direction of the strong radio source Cas A. This line was observed in absorption and corresponded to the transition between n = 631 and n = 632 in carbon. Subsequently, many other transitions at higher (n ~ 800) and lower quantum numbers (n ~ 160) have been observed in this direction.
These low-frequency carbon recombination lines exhibit interesting properties:
• Line width increases sharply towards lower frequencies (higher n) due to pressure and radiation broadening.
• Lines turn over into emission at frequencies above 150 MHz (n < 350) due to level population inversion and consequent stimulated emission.
Interpretation of these lines suggests that they arise in one of the major constituents of the ISM-either atomic clouds, molecular clouds, or possibly both. In these largely neutral components, carbon, due to its lower ionization potential (11.4 eV), can be ionized by background ultraviolet radiation.
Significance and Previous Observations
Since these low-frequency carbon recombination lines originate in a major ISM component, they are potentially widely detectable and serve as useful diagnostics of physical conditions in the ISM. Searches for these lines in other directions in the Galactic plane have met with success:
• Near 25 MHz, carbon lines have been detected in absorption from directions such as G75+00, NGC2024, S140, L1407, and DR21.
• Towards the Galactic center, lines have been detected in absorption at 75 MHz and in emission near 328 MHz.
• Similarly, towards M16, absorption lines at 69 & 80 MHz and an emission line near 325 MHz have been detected.
A recent survey at 76 MHz using the Parkes radio telescope detected carbon recombination lines in absorption from all observed directions with Galactic longitude < 20°. In total, more than 20 new detections were made, indicating that ionized carbon is fairly widespread in the inner Galaxy and that these lines are detectable at low frequencies with existing instruments.
Observations at 34.5 MHz and Instrumentation
In this thesis, extensive observations of carbon recombination lines near 34.5 MHz (n ~ 580) were undertaken using the low-frequency ‘T’-shaped dipole array located at Gauribidanur, 80 km north of Bangalore. Since these were the first major spectral line observations using this radio telescope, a new spectrometer with associated hardware and software was developed for this purpose.
Using this instrument, we searched for carbon recombination lines at 34.5 MHz from about 35 positions, most in the Galactic plane and a few against specific sources where low-frequency lines had previously been detected. Carbon lines in absorption were detected from ten positions, eight in the first quadrant and one in the fourth quadrant of the Galaxy.
The positive results include the first detection of a broad Voigt-shaped line profile from the direction of Cas A. This profile results from pressure and/or radiation broadening, and in this thesis, we derive constraints on the line-forming region from the observed line parameters.
Complementary Observations at 328 MHz
To complement the absorption line detections made with the Gauribidanur telescope, a subset of these directions was observed at 328 MHz (n ~ 270) using the Ooty Radio Telescope, where carbon recombination lines in emission were detected from all positions with Galactic longitude < 20°. An emission line at 328 MHz was also detected from Cas A.
At low frequencies, Cas A is an extremely strong continuum source, so the effective angular resolution is determined by its angular size (~5'). For other Galactic-plane directions, the angular resolution is determined by the telescope beam. At low frequencies, angular resolutions are generally poor (beam size ~ several degrees), and cloud sizes are likely smaller than the beam size, leading to beam dilution effects that significantly modify line strengths. Consequently, models fitting the observed data must account for this effect.
To obtain constraints on the sizes of line-forming regions, observations with the Ooty Radio Telescope were made with two different angular resolutions: 2° × 6' and 2° × 2°. Furthermore, we attempted to image one of the Galactic-plane positions in recombination lines.
High-Resolution Observations and Modeling of Carbon Line-Forming Regions
Carbon recombination lines with high angular resolution (~5') near 330 MHz were observed using the Very Large Array (VLA), an aperture synthesis instrument located in the USA. The methods, data analysis, and results of all these observations are described in this thesis.
We combined our observed recombination-line data towards various directions in the Galactic plane with previous results at other frequencies to model the carbon line-forming regions. Two types of models were considered:
1. Cold Gas Model (T? < 20 K):
In this model, the carbon line regions are assumed to be associated with molecular clouds, which may not be in pressure equilibrium with the interstellar medium.
2. Warm Gas Model (50 < T? < 300 K):
Here, the observed carbon lines are considered to originate in the neutral H?I component, which is in rough pressure equilibrium with other atomic components of the interstellar medium.
In both models, the effect of a dielectronic-like recombination process on the level populations of high quantum number states of carbon was included. In this process, an energetic (~100 K) free electron recombines to a high quantum number level in carbon with simultaneous excitation of a core electron from the fine-structure state ²P?/? to ²P?/?. Since these fine-structure levels are separated by ~92 K, this process is more effective in the neutral H?I component with temperatures around 100 K.
The physical parameters obtained from modeling favor the association of the carbon line region with neutral H?I gas in the Galaxy. However, the origin of these lines in molecular gas cannot be entirely ruled out. A range of physical conditions is probable depending on the cloud size.
Observations Towards Cas A and Spatial Distribution Studies
The only direction extensively investigated in low-frequency recombination lines of carbon is towards Cas A. The observed dependence of optical depth on quantum number seems to rule out the origin of these lines in cold (~20 K) molecular clouds and favors association with warmer (~100 K) H?I gas.
If the spatial distribution of carbon recombination lines over the face of Cas A can be compared with that of the H?I 21-cm line (tracing atomic gas) and the ¹²CO line (tracing molecular gas), further knowledge of the possible association of the carbon line region with neutral gas components can be obtained. For this purpose:
• We imaged Cas A in the C270? line emission near 332 MHz using the VLA with an angular resolution of ~25".
• The ¹²CO line emission with a resolution of ~1' was obtained using the 10.4-m mm-wave telescope at Raman Research Institute, Bangalore.
• The distribution of H?I emission was obtained from published data.
A comparison of recombination line distribution with H?I distributions showed good correspondence between C270? and H?I distributions, and rather poor correlation between ¹²CO and C270? distributions. These observations support models in which the line-forming regions are associated with H?I gas.
Study of Partially Ionized Gas Adjacent to W3A
Lastly, we investigated the partially ionized gas adjacent to the well-known Galactic H?II region W3A, using radio recombination lines of carbon and hydrogen near 1420 MHz. High angular resolution, high-sensitivity images of W3A were obtained in C168? and H168? lines using the VLA.
The H168? line is found to be a combination of:
• A broad component (>20 km s?¹) attributable to the fully ionized hot H?II region.
• A narrow component (<10 km s?¹) arising in the partially ionized gas surrounding the H?II region.
The intense carbon and hydrogen line emission is a result of stimulated emission by the background thermal region. A comparison of the narrow H168? and C168? distributions across the continuum source shows that the two line-forming regions, although sharing some overlap, are not entirely coextensive. We have constrained the physical properties of these regions from the observed line parameters.
The parameters of the broad hydrogen line and the continuum emission detected towards W3A were combined with other data to obtain constraints on the temperature, density, and clumpiness in the H?II region.
Summary
In summary, this thesis presents:
• An extensive observational study of low-frequency carbon recombination lines from partially ionized gas associated with a widely distributed component of the interstellar medium.
• A limited study of carbon and hydrogen recombination lines at higher frequencies from the partially ionized medium adjacent to a well-known H?II region. | |
