High Rydberg State Carbon Recombination Lines from Interstellar Clouds

We report observations of carbon recombination lines near 34.5 MHz (qunatum number n=578) and 325 MHz (n=272) made towards Cas A, the Galactic centre and about ten other directions in the galactic plane. Constraints on the physical conditions in the line forming regions are derived from these and other existing observations. The CII regions that produce the low-frequency lines are most likely associated with the neutral HI component of the ISM.Comment: 4 pages, 3 figures; Presented at the workshop on "New Perspects on the Interstellare Medium", Penticton, Canada, Aug 199

Low density ionized gas in the inner galaxy - Interpretation of recombination line observations at 325 MHz

The recent survey of H 272α recombination line (324.99 MHz) in the direction of 34 Hn regions, 12 SNRs and 6 regions of continuum minimum ('blank' regions) in the galactic plane is used to derive the properties of diffuse ionized gas in the inner Galaxy. The intensity of radio recombination lines at high frequencies is dominated by spontaneous emission in high-density gas and that at low frequencies (325 MHz) by stimulated emission in low-density gas. We have used this property to obtain the electron density in the gas in the direction of blank regions and SNRs, by combining the H 272 α measurements (preceeding paper) with the published data at higher frequencies. Further, we have imposed constraints on the electron temperature and pathlength through this gas using the observed high-frequency continuum emission, average interstellar electron density and geometry of the line-emitting regions. The derived properties of the gas are (i) electron density 0.5-6 cm-3, (ii) electron temperature 3000-8000 K and (iii) emission measures 500-3000 pc cm-6 The corresponding pathlengths are 50-200 pc. As the derived sizes of the low-density regions are small compared to the pathlength through the Galaxy, the low-frequency recombination lines cannot be considered as coming from a widely distributed component of the interstellar medium. The Hn regions studied in the above survey cannot themselves produce the H 272α lines detected towards them because of pressure broadening, optical depth, and beam dilution. However, the agreement in velocity of these lines with those seen at higher frequencies suggests that the low-frequency recombination lines arise in low-density envelopes of the Hn regions. Assuming that the temperature of the envelopes are similar to those of the cores and invoking geometrical considerations we find that these envelopes should have electron densities in the range 1-10 cm-3 and linear sizes of 30-300 pc in order to produce the observed H 272α lines

On the origin of the Galactic Ridge recombination lines

Radio recombination lines are known to be observable at positions along the galactic ridge which are free of discrete continuum sources. Based on the results of a recent survey of H272α lines it is shown that most of the observed galactic ridge recombination lines can be explained as emission from outer low-density envelopes of normal HII regions. The distribution of low-density ionized gas and discrete HII regions as a function of the distance from the galactic centre is also derived

A survey of recombination line emission from the galactic plane at 325 MHz

A survey of the H 272 α recombination line at 325 MHz has been made towards 53 directions in the galactic plane using the Ooty Radio Telescope (ORT). 34 of these directions correspond to well-known Hn regions, 12 to SNRs and 6 to 'blank' areas selected so that the 5 GHz continuum is a minimum over the telescope beam of 2° × 6 arcmin. Observing procedure and spectra of 47 sources towards which lines are detected are presented. Hydrogen recombination lines have been detected towards all the observed directions having l<40°. Carbon recombination lines are identified in 12 of the directions. The hydrogen line intensities are found to correlate well with the total continuum intensity (which includes the nonthermal galactic background) indicating that most of the lines arise due to stimulated emission by the background radiation. A preliminary discussion on the nature of the line-emitting regions is also presented

Discovery of a Non-Thermal Galactic Center Filament (G358.85+0.47) Parallel to the Galactic Plane

We report the discovery of a new non-thermal filament, G358.85+0.47, the ``Pelican'', located ~225 pc in projection from SgrA, and oriented parallel to the Galactic plane. VLA continuum observations at 20 cm reveal that this 7' (17.5 pc) structure bends at its northern extension and is comprised of parallel strands, most apparent at its ends. Observations at 6 and 3.6 cm reveal that the Pelican is a synchrotron-emitting source and is strongly linearly polarized over much of its extent. The spectral index of the filament changes from alpha(20/6)=-0.8 to alpha(6/3.6)=-1.5. The rotation measures exhibit a smooth gradient, with values ranging from -1000 rad/m2 to +500 rad/m2. The intrinsic magnetic field is well-aligned along the length of the filament. Based on these properties, we classify the Pelican as one of the non-thermal filaments unique to the Galactic center. Since these filaments (most of which are oriented perpendicular to the Galactic plane) are believed to trace the overall magnetic field in the inner Galaxy, the Pelican is the first detection of a component of this field parallel to the plane. The Pelican may thus mark a transition region of the magnetic field orientation in the inner kiloparsec of the Galaxy.Comment: 6 pages, 4 figures, to appear in ApJ Letters; Figs. 2 & 3 are color .ps files and best viewed in colo

Ionized gas towards galactic centre - Constraints from low-frequency recombination lines

Observations of the H272α recombination line towards the galactic centre show features near VLSR= 0, -50 and + 36 kms-1. We have combined the parameters of these features with the available H166α measurements to obtain the properties of the ionized gas present along the line of sight and also in the '3 kpc arm'. For the line-of-sight ionized gas we get an electron density around 7 cm-3 and a pathlength through it ~ 10-60 pc. The emission measure and the electron temperature are in the range 500-2900 pc cm-6 and 2000-6000 K. respectively. The ionized gas in the 3 kpc arm has an electron density of 30 cm-3 and extends over 9 pc along the line of sight if we assume an electron temperature of 104 K. Using the available upper limit to the intensity of the H351α recombination line, we show that the distributed ionized gas responsible for the dispersion of pulsar signals should have a temperature > 4500 K. and a minimum filling factor of 20 per cent. We also show that recombination lines from the 'warm ionized' gas proposed by McKee & Ostriker (1977) should be detectable in the frequency range 100-150 MHz towards the galactic centre with the sensitivity available at present

A search for Protocliisters at z = 3.3

We have used the Very Large Array to image a single field in a set of adjacent frequency bands around 333.0 MHz in an attempt to detect 21 cm emission from large scale H I inhomogeneities at a redshift of z = 3.3. Following the subtraction of continuum radio sources, the absence of any spectral signals apart from that expected due to the system thermal noise has been used to derive constraints on the evolutionary scenario leading to the formation of the present day clusters of galaxies. The observations rule out the existence of H I protoclusters at z = 3.3 with masses ≃3.5 ×1014 M⊙ in H I gas and space density exceeding (74 Mpc)-3. This indicates that the present day rich clusters of galaxies either formed as gaseous protocluster condensates prior to z = 3.3 or else they formed through the clustering of their constituent galaxies