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Register a new userVLA observations of 6-cm excited OH
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The VLA was used to determine precise positions for 4765-MHz OH maser emission sources toward star-forming regions which had been observed about seven months earlier with the Effelsberg 100-meter telescope. The observations were successful for K3-50, DR21EX, W75N, and W49A. No line was detected toward S255: this line had decreased to less than 5 per cent of the flux density observed only seven months earlier. The time-variability of the observed features during the past 30 years is summarised. In addition, to compare with the Effelsberg observations, the 4750-MHz and 4660-MHz lines were observed in W49A. These lines were found to originate primarily from an extended region which is distinguished as an exceptional collection of compact continuum components as well as by being the dynamical centre of the very powerful H_2 O outflow.
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Using the Very Large Array (VLA), we observed all three of the 6-cm lines of the doublet Pi 1/2, J=1/2 state of OH with sub-arcsecond resolution (about 0.4 arcsec) in W49A. While the spatial distribution and the range in velocities of the 6-cm lines are similar to those of the ground state (18-cm) OH lines, a large fraction of the total emission in all three 6-cm lines has large linewidths (about 5 -- 10 km/s) and is spatially-extended, very unlike typical ground state OH masers which typically are point-like at VLA resolutions and have linewidths less than 1 km/s. We find brightness temperatures of 5900 K, 4700 K, and greater than 730 K for the 4660-MHz, 4750-MHz, and 4765-MHz lines, respectively. We conclude that these are indeed maser lines. However, the gains are about 0.3, again very unlike the 18-cm lines which have gains greater than 10000. We compare the excited state OH emission with that from other molecules observed with comparable angular resolution to estimate physical conditions in the regions emitting the peculiar, low-gain maser lines. We also comment on the relationship with the 18-cm masers
We have obtained a deep radio image with the Very Large Array at 6 cm in the Lockman Hole. The noise level in the central part of the field is about 11 microJy. From these data we have extracted a catalogue of 63 radio sources. The analysis of the radio spectral index suggests a flattening of the average radio spectra and an increase of the population of flat spectrum radio sources in the faintest flux bin. Cross correlation with the ROSAT/XMM X-ray sources list yields 13 reliable radio/X-ray associations, corresponding to about 21 per cent of the radio sample. Most of these associations (8 out of 13) are classified as Type II AGN. Using optical CCD (V and I) and K band data we found an optical identification for 58 of the 63 radio sources. This corresponds to an identification rate of about 92 per cent, one of the highest percentages so far available. From the analysis of the colour-colour diagram and of the radio flux - optical magnitude diagram we have been able to select a subsample of radio sources whose optical counterparts are likely to be high redshift (z>0.5) early-type galaxies, hosting an Active Galactic Nucleus responsible of the radio activity. We also find evidence that at these faint radio limits a large fraction (about 60 per cent) of the faintest optical counterparts (i.e. sources in the magnitude range 22.5<I<24.5 mag) of the radio sources are Extremely Red Objects (EROs) with I-K>4.
We present a spectrum of Sgr A* observed simultaneously on June 17, 2003 at wavelengths from 90 to 0.7 cm with the VLA. In the spectrum, we also include the measurements of Sgr A* observed on the same day with the GMRT at 49 cm, the SMA at 0.89 mm and the Keck II at 3.8 $mu$m. The spectrum at the centimeter wavelengths suggests the presence of a break wavelength at 3.8 cm (8 GHz). The spectral index is alpha=0.43+-0.03 (propto nu(alpha}) at 3.8 cm and shorter wavelengths. The spectrum between 3.8 cm and 49 cm can be described by a power law with spectral index of alpha=0.10+-0.03$. We detected Sgr A* with 0.22+-0.06 Jy at 90 cm, suggesting a sharp decrease in flux density at the wavelengths longer than 49 cm. The best fit to the spectrum at the wavelengths longer than lambda_b appears to be consistent with free-free absorption by a screen of ionized gas with a turnover wavelength at nu(tau_ff}=1) = 100 cm (300 MHz). This turnover wavelength appears to be three times longer than that of 30 cm (1 GHz) as suggested by Davies et al. (1976) based on the observations in 1994 and 1995. Our analysis suggests that stellar winds from the massive stars near Sgr A* could modulate the flux density at the wavelengths longer than 30 cm (or frequencies below 1 GHz).
We describe the results of a mJy-sensitivity, VLA survey of roughly 1 square degree near the Galactic center at 6 and 20 cm. Catalogs of compact and filamentary structures are given and compared to previous surveys of the region. Eight of the unusual, nonthermal radio filaments are detected in 6 cm polarized emission; three of these are the first such detections, confirming their nonthermal nature. This survey found emission from a filament at (l,b)=(359.1,0.75), or a projected distance from Sgr A* of 200 pc, greatly extending the latitude range observed with such features. There is also new evidence for spatial gradients in the 6/20 cm spectral indices of some filaments and we discuss models for these gradients. In studying compact sources, the combination of spectral index and polarization information allows us to identify pulsar candidates and compact HII regions in the survey. There is also some evidence that the flux measurements of compact sources may be affected by electron scattering from the interstellar medium in the cent ral few hundred parsecs of the Galaxy.
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