GMRT and VLA observations at 49cm and 20cm of the HII region near l=24.4d, b=0.1d

We report multifrequency radio continuum and hydrogen radio recombination line observations of HII regions near l=24.8d b=0.1d using the Giant Metrewave Radio Telescope (GMRT) at 1.28 GHz (n=172), 0.61 GHz (n=220) and the Very Large Array (VLA) at 1.42 GHz (n=166). The region consists of a large number of resolved HII regions and a few compact HII regions as seen in our continuum maps, many of which have associated infrared (IR) point sources. The largest HII region at l=24.83d and b=0.1d is a few arcmins in size and has a shell-type morphology. It is a massive HII region enclosing ~ 550 solar mass with a linear size of 7 pc and an rms electron density of ~ 110 cm^-3 at a kinematic distance of 6 kpc. The required ionization can be provided by a single star of spectral type O5.5. We also report detection of hydrogen recombination lines from the HII region at l=24.83d and b=0.1d at all observed frequencies near Vlsr=100 km/s. We model the observed integrated line flux density as arising in the diffuse HII region and find that the best fitting model has an electron density comparable to that derived from the continuum. We also report detection of hydrogen recombination lines from two other HII regions in the field.Comment: 14 pages, 8 Figures. Uses JAA style file. Accepted for publication in Journal of Astrophysics and Astronomy. High resolution figures (fig 1a, fig 1b and fig 2b) can be downloaded from http://www.ncra.tifr.res.in/~ngk/G2

A new technique to improve RFI suppression in radio interferometers

Radio interferometric observations are less susceptible to radio frequency interference (RFI) than single dish observations. This is primarily due to : (1)fringe-frequency averaging at the correlator output and (2) bandwidth decorrelation of broadband RFI. Here, we propose a new technique to improve RFI suppression of interferometers by replacing the fringe-frequency averaging process with a different filtering process. In the digital implementation of the correlator, such a filter should have cutoff frequencies $< 10^{-6}$ times the frequency at which the baseband signals are sampled. We show that filters with such cutoff frequencies and attenuation $>$ 40 dB at frequencies above the cutoff frequency can be realized using multirate filtering techniques. Simulation of a two element interferometer system with correlator using multirate filters shows that the RFI suppression at the output of the correlator can be improved by 40 dB or more compared to correlators using a simple averaging process.Comment: 12 pages, 7 figures; Invited talk given at IVS Symposium in Korea -- New Technologies in VLBI, Korea, Nov 2002; to appear in the conference proceedings (Added answers to the questions during the discussion session

Hydrogen recombination lines near 327 MHz. I. distribution of low-density ionized gas in the galactic disk

We present the results of a low-resolution (2&#176; &#215; 2&#176;) survey of radio recombination lines (RRLs) near 327 MHz in the Galactic plane made with the Ooty Radio Telescope (ORT). Although the angular resolution is coarse, these observations represent the first contiguous survey of low-frequency RRL emission in the longitude range l = 330&#176;-0&#176;-89&#176; (inner Galaxy). Hydrogen RRLs were detected in almost all directions in the inner Galaxy and carbon lines in several positions. In the outer Galaxy (l = 172&#176;-252&#176;), an unbiased set of 14 positions were observed and lines were detected toward three of them. To study the extent of the ionized gas above the Galactic disk, we have observed RRLs along the Galactic latitude at two specific longitudes (l = 0&#176;.0 and 13&#176;.9). RRLs were detected up to b = &#177;3&#176;. The l-&#957; diagram and the radial distribution, obtained from RRL emission near 327 MHz, show good similarity with that of RRL emission near 1.4 GHz, "intense" 12CO emission and to some extent with the RRLs observed near 3 cm from normal H II region. These distributions are distinctly different from those of H&#945; and H I emission from the Galactic disk. Based on a comparison of the radial distribution of different components in the Galactic disk, we conclude that the diffuse RRL emission is associated with star-forming regions and possibly with a low-density component of known H II regions in the inner Galaxy

Hydrogen recombination lines near 327 MHz. III. physical properties and origin of the low-density ionized gas in the inner galaxy

We present constraints on the physical properties of the ionized gas responsible for hydrogen radio recombination lines (RRLs) near 327 MHz detected in a recent Galactic plane survey made with the Ooty Radio Telescope. To obtain these constraints, we combined the data at 327 MHz with previously published RRL observations near 1.4 GHz. The density of the ionized gas is well constrained and is in the range of 1 to 10 cm-3. The data implies upper limits to the temperature and size of the line emitting regions of ~12,000 K and ~500 pc, respectively. Assuming an electron temperature of 7000 K, the derived path lengths of the line emitting region are in the range of 20 to 200 pc. The derived properties of the ionized gas responsible for the RRL emission near 327 MHz suggest that most of the [N II] 205 &#181;m emission and a considerable fraction of the [C II] 158 &#181;m emission observed in the Galactic plane by the COBE satellite could also originate in the same gas. The H&#945; emission from these ionized gases is mostly undetected in the existing H&#945; surveys because of large interstellar extinction. About 50% of the free-free absorption of the Galactic nonthermal radiation observed at frequencies less than 100 MHz can be accounted for by the same ionized gas. We also discuss the origin of this low-density ionized gas in the inner Galaxy. The derived low line-of-sight filling factor (&lt;1%) for this ionized gas indicates that it does not form a pervasive medium. On the basis of the similarity of the distribution of this gas in the Galactic disk with that of the star-forming regions and the range of derived physical properties, we support the earlier suggestion that the low-frequency RRL emission originates from low-density ionized gas, which forms envelopes of normal H II regions

Magnetic fields at the periphery of UCHII regions from carbon recombination line observations

Several indirect evidences indicate a magnetic origin for the non-thermal width of spectral lines observed toward molecular clouds. In this letter, I suggest that the origin of the non-thermal width of carbon recombination lines (CRLs) observed from photo-dissociation regions (PDRs) near ultra-compact \HII\ regions is magnetic and that the magnitude of the line width is an estimate of the \alfven speed. The magnetic field strengths estimated based on this suggestion compare well with those measured toward molecular clouds with densities similar to PDR densities. I conclude that multi-frequency CRL observations have the potential to form a new tool to determine the field strength near star forming regions.Comment: To appear in ApJ Letter

SIGGMA: A Survey of Ionized Gas in the Galaxy, Made with the Arecibo Telescope

A Survey of Ionized Gas in the Galaxy, made with the Arecibo telescope (SIGGMA) uses the Arecibo L-band Feed Array (ALFA) to fully sample the Galactic plane (30 < l < 75 and -2 < b < 2; 175 < l < 207 and -2 < b < 1) observable with the telescope in radio recombination lines (RRLs). Processed data sets are being produced in the form of data cubes of 2 degree (along l) x 4 degree (along b) x 151 (number of channels), archived and made public. The 151 channels cover a velocity range of 600 km/s and the velocity resolution of the survey changes from 4.2 km/s to 5.1 km/s from the lowest frequency channel to the highest frequency channel, respectively.RRL maps with 3.4 arcmin resolution and line flux density sensitivity of 0.5 mJy will enable us to identify new HII regions, measure their electron temperatures, study the physics of photodissociation regions (PDRs) with carbon RRLs, and investigate the origin of the extended low density medium (ELDM). Twelve Hn{\alpha} lines fall within the 300 MHz bandpass of ALFA; they are resampled to a common velocity resolution to improve the signal-to-noise ratio (SN) by a factor of 3 or more and preserve the line width. SIGGMA will produce the most sensitive fully sampled RRL survey to date. Here we discuss the observing and data reduction techniques in detail. A test observation toward the HII region complex S255/S257 has detected Hn{\alpha} and Cn{\alpha} lines with SN>10