arXiv subjects

W. M. Goss

Publications and source records attributed to W. M. Goss.

At least 109 records · Page 6Linked to original sources

arXiv2005

Multi-wavelength carbon recombination line observations with the VLA toward an UCHII region in W48: Physical properties and kinematics of neutral material

Using the Very Large Array (VLA) the C76$α$ and C53$α$ recombination lines (RLs) have been detected toward the ultra-compact \HII\ region (UCHII region) G35.20$-$1.74. We also obtained upper limits to the carbon RLs at 6 cm (C110$α$ & C111$α$) and 3.6 cm (C92$α$) wavelengths with the VLA. In addition, continuum images of the W48A complex (which includes G35.20$-$1.74) are made with angular resolutions in the range 14\arcsec to 2\arcsec. Modeling the multi-wavelength line and continuum data has provided the physical properties of the UCHII region and the photodissociation region (PDR) responsible for the carbon RL emission. The gas pressure in the PDR, estimated using the derived physical properties, is at least four times larger than that in the UCHII region. The dominance of stimulated emission of carbon RLs near 2 cm, as implied by our models, is used to study the relative motion of the PDR with respect to the molecular cloud and ionized gas. Our results from the kinematical study are consistent with a pressure-confined UCHII region with the ionizing star moving with respect to the molecular cloud. However, based on the existing data, other models to explain the extended lifetime and morphology of UCHII regions cannot be ruled out.

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arXiv2005

Radio Recombination Lines from the Starburst Galaxy NGC 3256

We have detected the radio recombination lines H91-alpha and H92-alpha with rest frequencies of 8.6 GHz and 8.3 GHz from the starburst nucleus NGC 3256 at an angular resolution of 16.4" x 9.6" using the Australia Telescope Compact Array and at an angular resolution of 12.0" x 2.9" using the VLA. The line was detected at ~ 1 mJy/beam peak with a width of 160 km/s with the ATCA and at ~ 0.5 mJy/beam peak with a width of 114 km/s with the VLA. Modelling the line emitting region as a collection of HII regions, we derive constraints on the required number of HII regions, their temperature, density, and distribution. We find that a collection of 10 to 300 HII regions with temperatures of 5000 K, densities of 1000 cm^-3 to 5000 cm^-3 and diameters of 15 pc produced good matches to the line and continuum emmission. The Lyman continuum production rate required to maintain the ionization is 2 x 10^52 s^-1 to 6 x 10^53 s^-1, which requires 600 to 17000 O5 stars to be produced in the starburst.

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arXiv2005

The neutral gas in the environs of the Geminga gamma-ray pulsar

We present a high-resolution (24 arcsec) study of the HI interstellar gas distribution around the radio-quiet neutron star Geminga. Based on Very Large Array (VLA) and MPIfR Effelsberg telescope data, we analyzed a 40' x 40' field around Geminga. These observations have revealed the presence of a neutral gas shell, 0.4 pc in radius, with an associated HI mass of 0.8 Msun, which surrounds Geminga at a radial velocity compatible with the kinematical distance of the neutron star. In addition, morphological agreement is observed between the internal face of the HI shell and the brightest structure of Geminga's tail observed in X-rays.We explore the possibility that this morphological agreement is the result of a physical association.

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arXiv2005

An 8.5 GHz Arecibo survey of Carbon Recombination Lines toward Ultra-compact \HII regions: Physical properties of dense molecular material

We report here on a survey of carbon recombination lines (RLs) near 8.5 GHz toward 17 ultra-compact \HII regions (\UCHII s). Carbon RLs are detected in 11 directions, indicating the presence of dense photodissociation regions (PDRs) associated with the \UCHII s. In this paper, we show that the carbon RLs provide important, complementary information on the kinematics and physical properties of the ambient medium near \UCHII s. Non-LTE models for the carbon line forming region are developed, assuming that the PDRs surround the \UCHII s, and we constrained the model parameters by multi-frequency RL data. Modeling shows that carbon RL emission near 8.5 GHz is dominated by stimulated emission and hence we preferentially observe the PDR material that is in front of the \UCHII continuum. We find that the relative motion between ionized gas and the associated PDR is about half that estimated earlier, and has an RMS velocity difference of 3.3 \kms. Our models also give estimates for the PDR density and pressure. We found that the neutral density of PDRs is typically $>$ 5 $\times$ 10$^5$ \cmthree and \UCHII s can be embedded in regions with high ambient pressure. Our results are consistent with a pressure confined \HII region model where the stars are moving relative to the cloud core. Other models cannot be ruled out, however. Interestingly, in most cases, the PDR pressure is an order of magnitude larger than the pressure of the ionized gas. Further investigation is needed to understand this large pressure difference.

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arXiv2004

The simultaneous VLA observations of Sgr A* from 90 to 0.7 cm

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 $μ$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).

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arXiv2004

Protostars and Outflows in the NGC7538 - IRS9 Cloud Core

New high resolution observations of HCO+ J=1-0, H13CN J=1-0, SO 2,2 - 1,1, and continuum with BIMA at 3.4 mm show that the NGC7538 - IRS9 cloud core is a site of active ongoing star formation. Our observations reveal at least three young bipolar molecular outflows, all ~ 10,000 -- 20,000 years old. IRS9 drives a bipolar, extreme high velocity outflow observed nearly pole on. South of IRS9 we find a cold, protostellar condensation with a size of ~ 14" x 6" with a mass > 250 Msun. This is the center of one of the outflows and shows deep, red-shifted self absorption in HCO+, suggesting that there is a protostar embedded in the core, still in a phase of active accretion. This source is not detected in the far infrared, suggesting that the luminosity < 10^4 Lsun; yet the mass of the outflow is ~ 60 Msun. The red-shifted HCO+ self-absorption profiles observed toward the southern protostar and IRS9 predict accretion rates of a few times 10^-4 to 10^-3 Msun/yr. Deep VLA continuum observations at 3.6 cm show that IRS9 coincides with a faint thermal VLA source, but no other young star in the IRS9 region has any detectable free-free emission at a level of ~ 60 microJy at 3.6 cm. The HCO+ abundance is significantly enhanced in the hot IRS9 outflow. A direct comparison of mass estimates from HCO+ and CO for the well-characterized red-shifted IRS9 outflow predicts an HCO+ enhancement of more than a factor of 30, or [HCO+/H2] >= 6 10^-8.

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arXiv2004

Multi-density model of the ionised gas in NGC 253 using radio recombination lines

We have imaged the H92alpha (8.3 GHz), H75alpha (15 GHz), and H166alpha (1.4 GHz) Radio Recombination Lines (RRLs) from NGC 253 at resolutions of 4.5 pc (0.4"), 2.5 pc (0.2") and 53 pc (4.5") respectively. The H92alpha line arises from individual compact sources, most of which possess radio continuum counterparts. The line widths range from ~200 km/s for the sources near the radio nucleus to 70-100 km/s for the extranuclear ones. These lines are emitted by gas at a density ~10000 /cc. The remainder of the cm-wave RRLs arise in lower density gas (~500 /cc) with a higher area filling factor and with ten times highermass. A third component of higher density gas (>10000 /cc) is required to explain the mm-wave RRLs.

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arXiv2004

Radio Recombination Lines from Starbursts: NGC 3256, NGC 4945 and the Circinus Galaxy

A renewed attempt to detect radio recombination lines from external galaxies has resulted in the measurement of lines from several bright starburst galaxies. The lines are produced by hydrogen ionized by young, high-mass stars and are diagnostic of the conditions and gas dynamics in the starburst regions without problems of dust obscuration. We present here detections of the lines H91alpha and H92alpha near 8.6 GHz from the starburst nuclei in NGC 3256, NGC 4945, and the Circinus galaxy using the ATCA and VLA. Modelling the line emitting region as a collection of H II regions, we derive the required number of H II regions, their temperature, density, and distribution.

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arXiv2004

The OH (1720 MHz) Supernova Remnant Masers in W28: MERLIN and VLBA Polarization Observations

Full-polarization MERLIN and VLBA observations of the 1720 MHz maser emission from the OH molecule in the supernova remnant W28 are presented. Zeeman splitting (|B| \approx 0.75 mG) has been directly resolved between right- and left-circularly polarized spectra for the first time. Linear-polarization position angles and circular-polarization Zeeman splitting observed in the maser emission permit interpretation of a comprehensive picture of the magnetic field at the supernova/molecular cloud interface marked by the masers. We find the post-SNR-shock magnetic field to be well-ordered over the ~1 pc covered by the maser region (compared with the approximately 30-pc diameter of the entire SNR) and well-aligned with the shock front that is traced by synchrotron radiation and molecular emission. Based on MERLIN data having a resolution of 200 mas and VLBA data with a resolution of 15 mas, the masers are measured to have deconvolved angular sizes of 90 to 350 mas (225 to 875 AU) with compact cores 20 mas (50 AU) in size, consistent with theoretical expectation and previous observations.

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arXiv2004

On the Origin of the Wide HI Absorption Line Toward Sgr A*

We have imaged a region of about 5' extent surrounding Sgr A* in the HI 21 cm-line absorption using the Very Large Array. A Gaussian decomposition of the optical depth spectra at positions within about 2' (approx. 5 pc at 8.5 kpc) of Sgr A* detects a wide line underlying the many narrow absorption lines. The wide line has a mean peak optical depth of 0.32 +/- 0.12 centered at a mean velocity of V(lsr) = -4 +/- 15 km/s. The mean full width at half maximum is 119 +/- 42 km/s. Such a wide line is absent in the spectra at positions beyond about 2' from Sgr A*. The position-velocity diagrams in optical depth reveal that the wide line originates in various components of the circumnuclear disk (radius approx. 1.3') surrounding Sgr A*. These components contribute to the optical depth of the wide line in different velocity ranges. The position-velocity diagrams do not reveal any diffuse feature which could be attributed to a large number of HI clouds along the line of sight to Sgr A*. Consequently, the wide line has no implications either to a global population of shocked HI clouds in the Galaxy or to the energetics of the interstellar medium as was earlier thought.

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arXiv2004

VLA H92 Alpha and H53 Alpha Radio Recombination Line Observations of M82

We present high angular resolution (0.6'') observations made with the VLA of the radio continuum at 8.3 and 43 GHz as well as H92 Alpha and H53 Alpha radio recombination lines from the nearby (3 Mpc) starburst galaxy M82. In the continuum we report 19 newly identified sources at 8.3 GHz and 5 at 43 GHz that were unknown previously. The spatial distribution of the H92 Alpha line is inhomogeneous; we identify 27 features. The line and continuum emission are modeled using a collection of HII regions at different distances from the nucleus assuming a single-density component and two-density components. The high-density component has a density of 4 X 10^{4} cm^{-3}. However, the bulk of the ionization is in regions with densities which are typically a factor 10 lower. The gas kinematics, using the H92 Alpha line, confirms the presence of steep velocity gradient (26 km s^{-1} arcsec^{-1}) in the nuclear region. As this steep gradient is observed not only on the major axis but also at large distances along a band of PA of 150 degrees, the interpretation in terms of x2 orbits elongated along the minor axis of the bar, which would be observed at an angle close to the inclination of the main disk, seems inadequate. Ad-hoc radial motions must be introduced to reproduce the pattern of the velocity field. Different families of orbits are indicated as we detect a signature in the kinematics at the transition between the two plateaus observed in the NIR light distribution. The H92 Alpha line also reveals the base of the outflow where the injection towards the halo on the Northern side occurs. The kinematical pattern suggests a connection between the gas flowing in the plane of M82 towards the center; this behavior most likely originates due to the presence of a bar and the outflow out of the plane.

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arXiv2004

Water Masers in W49 North and Sagittarius B2

Using the Very Large Array (VLA) of the National Radio Astronomy Observatory in the A and B configurations, we have obtained simultaneous high resolution observations of both the 22 GHz water maser lines as well as the 22 GHz continuum for the H II regions W49N and Sagittarius B2. The angular resolution of both observations is ~0.1", which at the distance of W49N (11.4 kpc; Gwinn, Moran, & Reid 1992) and Sgr B2 (8.5 kpc) corresponds to a physical size of <1000 AU in both sources. The velocity coverage for W49N is +/- 435 km/s; positions for 316 water maser components were obtained. The velocity coverage for Sgr B2 is -40 to +120 km/s; positions for 68 maser components were determined in Sgr B2 Main, 79 in Sgr B2 North, 14 in Sgr B2 Mid-North, and 17 in Sgr B2 South, for a total of 178 water maser positions in Sgr B2. The cross calibration scheme of Reid & Menten (1990, 1997) was used. Using this procedure, high dynamic range continuum images were obtained with accurate registration (rms ~0.01") of the continuum and maser positions. A detailed comparison between H II components and maser positions for both Sgr B2 and W49N is presented. In Sgr B2 Main, the water masers are predominantly located at the outside edge of the high-frequency continuum, lending support to the proposal that entrainment by stellar winds may play an important role in water maser emission.

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arXiv2004

OH (1720 MHz) Maser Search Toward the Large Magellanic Cloud

We have carried out a sensitive search for OH (1720 MHz) masers in the Large Magellanic Cloud (LMC) toward five regions using the Australia Telescope Compact Array (ATCA). Our source list includes the 30 Doradus region, and four supernova remnants (SNRs): N44, N49, N120, and N132D. These data have a typical resolution of ~8 arcsec and rms noise levels of 5-10 mJy/beam. We have detected OH (1720 MHz) masers in the NE part of 30 Doradus and toward the SNR N49. The OH (1720 MHz) maser emission in 30 Doradus is coincident with a cluster of young stars known as ``Knot 1'', and is almost certainly of the star formation variety. Our spectral resolution (0.68 km/s) is insufficient to detect the Zeeman effect from the strongest (320 mJy/beam) of the 30 Doradus OH (1720 MHz) masers, leading to an upper limit to the field strength of 6 mG. The weak OH (1720 MHz) maser emission (35 mJy/beam) detected toward the LMC SNR N49 is located just west of a previously identified CO cloud, and are indicative of an interaction between the SNR and the molecular cloud. Although the statistics are low, the detection rate seems consistent with that seen for Galactic star forming regions and SNR type OH (1720 MHz) masers -- both of which are low.

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arXiv2004

Detection of the Intrinsic Size of Sagittarius A* through Closure Amplitude Imaging (includes Online Supporting Material)

We have detected the intrinsic size of Sagittarius A*, the Galactic Center radio source associated with a supermassive black hole, showing that the short-wavelength radio emission arises from very near the event horizon of the black hole. Radio observations with the Very Long Baseline Array show that the source has a size of $24 \pm 2$ Schwarzschild radii at 7 mm wavelength. In one of eight 7-mm epochs we also detect an increase in the intrinsic size of $60^{+25}_{-17}$%. These observations place a lower limit to the mass density of Sgr A* of $1.4\times10^{4}$ solar masses per cubic astronomical unit.

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arXiv2004

The Variability of Sagittarius A* at Centimeter Wavelengths

We present the results of a 3.3-year project to monitor the flux density of Sagittarius A* at 2.0, 1.3, and 0.7 cm with the VLA. The fully calibrated light curves for Sgr A* at all three wavelengths are presented. Typical errors in the flux density are 6.1%, 6.2%, and 9.2% at 2.0, 1.3, and 0.7 cm, respectively. There is preliminary evidence for a bimodal distribution of flux densities, which may indicate the existence of two distinct states of accretion onto the supermassive black hole. At 1.3 and 0.7 cm, there is a tail in the distribution towards high flux densities. Significant variability is detected at all three wavelengths, with the largest amplitude variations occurring at 0.7 cm. The rms deviation of the flux density of Sgr A* is 0.13, 0.16, and 0.21 Jy at 2.0, 1.3, and 0.7 cm, respectively. During much of this monitoring campaign, Sgr A* appeared to be relatively quiescent compared to results from previous campaigns. At no point during the monitoring campaign did the flux density of Sgr A* more than double its mean value. The mean spectral index of Sgr A* is alpha=0.20+/-0.01, with a standard deviation of 0.14. The spectral index appears to depend linearly on the observed flux density at 0.7 cm with a steeper index observed during outbursts. This correlation is consistent with the expectation for outbursts that are self-absorbed at wavelengths of 0.7 cm or longer and inconsistent with the effects of simple models for interstellar scintillation. Much of the variability of Sgr A*, including possible time lags between flux density changes at the different wavelengths, appears to occur on time scales less than the time resolution of our observations (8 days). Future observations should focus on the evolution of the flux density on these time scales.

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arXiv2004

A Very Large Array 3.6cm continuum survey of Galactic Wolf-Rayet stars

We report the results of a survey of radio continuum emission of Galactic Wolf-Rayet stars north of declination -46 degrees. The observations were obtained at 8.46 GHz (3.6cm) using the Very Large Array (VLA), with an angular resolution of about 6 x 9 arcsec and typical rms noise of 0.04 mJy/beam. Our survey of 34 WR stars resulted in 15 definite and 5 probable detections, 13 of these for the first time at radio wavelengths. All detections are unresolved. Time variations in flux are confirmed in the cases of WR98a, WR104, WR105 and WR125. WR79a and WR89 are also variable in flux and we suspect they are also non-thermal emitters. Thus, of our sample 20-30% of the detected stars are non-thermal emiters. Average mass loss rates determinations obtained excluding definite and suspected non-thermal cases give similar values for WN (all subtypes) and WC5-7 stars, while a lower value was obtained for WC8-9 stars. Uncertainties in stellar distances largely contribute to the observed scatter in mass loss rates. Upper limits to the mass loss rates were obtained in cases of undetected sources or for sources which probably show additional non-thermal emission.

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arXiv2004

A Radio Outburst Nearly Coincident with the Large X-ray Flare from Sgr A* on 2002-10-03

A large radio outburst from Sgr A* was observed during the VLA weekly monitoring program at 2 cm, 1.3 cm and 7 mm, nearly coincident with the brightest X-ray flare detected to date with the XMM-Newton X-ray Observatory on 2002-10-03. The flux density of 1.9$\pm$0.2 Jy measured at 7 mm exceeds the mean value (1.00$\pm$0.01 Jy) by a factor of $\sim 2$, one of the two highest increases observed during the past three years (June 2000-October 2003), while less significant increases in flux densities were observed at 1.3 cm and 2 cm. The radio observation started 13.5 hrs after the onset of the X-ray flare (which had occurred over a 45 min duration) and continued for 1.3 hrs. During the observation, there was no significant ($<3σ$) change in the radio flux densities at all the three wavelengths, indicating that the radio outburst varied on a timescale of $>1$hr. A spectral index of $α=2.4^{+0.3}_{-0.6}$ (${\rm S \proptoν^α}$) was derived for the outburst component, consistent with an optically thick nonthermal synchrotron source. These results suggest that energetic electrons responsible for the radio outburst might be produced via a process associated with the X-ray flare, then transported to large radii, producing the observed radio outburst. The observation is the first evidence for a correlated variation in the radio and X-ray emissions from Sgr A*.

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arXiv2003

Detection of Sagittarius A* at 330 MHz with the Very Large Array

We report the detection of Sagittarius A*, the radio source associated with our Galaxy's central massive black hole, at 330 MHz with the Very Large Array. Implications for the spectrum and emission processes of Sagittarius A* are discussed and several hypothetical geometries of the central region are considered.

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