共查询到20条相似文献，搜索用时 31 毫秒
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R. C. Bolton C. J. Chandler G. Cotter T. J. Pearson G. G. Pooley A. C. S. Readhead J. M. Riley E. M. Waldram 《Monthly notices of the Royal Astronomical Society》2006,370(3):1556-1564
We present results from a 3-yr study of the 15-GHz variability of 51 9C sources. 48 of these sources make up a subsample of a larger one complete to 25 mJy in 9C, and as the sources are selected pseudo-randomly the results should be representative of the complete sample. 29 per cent of this subsample are found to be variable above the flux calibration uncertainties of ∼6 per cent. 50 per cent of the flat-spectrum objects are variable whilst none of the steep-spectrum objects or the objects with convex spectra peaking below 5 GHz are variable. Nine of the objects studied have convex spectra and peak frequencies above 5 GHz; eight of these were found to vary at 15 GHz, suggesting that the high-frequency peaking class in this sample is largely populated by objects with jets aligned close to the line of sight whose emission is dominated by beamed components. 相似文献
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Phillips Norris Ellingsen & Rayner 《Monthly notices of the Royal Astronomical Society》1998,294(2):265-271
We have used the Australia Telescope Compact Array (ATCA) to search for 6.7-GHz methanol maser emission towards 87 galaxies. We chose the target sources using several criteria, including far-IR luminosities and the presence of known OH megamasers. In addition, we searched for methanol masers in the nearby starburst galaxy NGC 253, making a full spectral-line synthesis image. No emission was detected in any of the galaxies, with detection limits ranging from 25 to 75 mJy. This is surprising, given the close association of OH and methanol masers in Galactic star formation regions, and significantly constrains models of OH megamaser emission. This absence of maser emission may be a result of low methanol abundances in molecular clouds in starburst galaxies. 相似文献
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Hopkins Mobasher Cram & Rowan-Robinson 《Monthly notices of the Royal Astronomical Society》1998,296(4):839-846
We present the first results from the Phoenix Deep Survey, a multiwavelength survey of a 2° diameter region. Observations in the radio continuum at 1.4 GHz carried out with the Australia Telescope Compact Array are described. The catalogue of over 1000 radio sources compiled from these observations is analysed, and the source counts are presented. We model the observational source counts using a two-population model and published luminosity functions for these populations. Upon invoking luminosity and density evolution, we find that a luminosity evolution model best fits the radio observations, consistent with earlier work. The redshift distribution of the two galaxy populations investigated is also modelled and discussed. 相似文献
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S. Goedhart † D. J. van der Walt A. J. Schutte ‡ 《Monthly notices of the Royal Astronomical Society》2000,315(2):316-324
Near-infrared photometry was performed on 56 southern 6.7-GHz methanol maser sources. A simple spherically symmetric model of the radiative transfer through a dust shell was developed and used to study the conditions in the dust cloud in which the masers are produced. The parameters investigated were the size of the cloud, the spectral type of the embedded star, the optical depth of the dust cloud and the dust density distribution. It was found that the infrared colours of the models have a complex dependence on the parameters and that no unique combination of parameter values explains the spectral energy distribution of any particular source. The model effectively reproduces the far-infrared ( IRAS ) colours but cannot simultaneously explain the near-infrared colours for any of the observed sources. 相似文献
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Phase-referenced observations of 13 star-forming regions in the 2 Π1/2 , J = 1/2 transition of rotationally excited OH at 4765 MHz have been carried out using MERLIN. Two of the regions were also observed at 4750 MHz and one at 4660 MHz. There were 10 maser detections at 4765 MHz and three non-detections. There were no detections at 4750 and 4660 MHz. The 4765-MHz masers have brightness temperatures of ∼107 K at MERLIN resolution (∼50 mas). Several cases of 4765-MHz masers overlapping in position and velocity with 1720- and 1665-MHz masers are reported. There are also isolated 4765-MHz masers with peak flux densities ≥30 times that of any ground-state counterpart. Most of the 4.7-GHz maser spots are unresolved at 50-mas angular resolution, but in four of the nearest sources the maser spots are resolved, indicating a characteristic size for 4765-MHz maser regions of ∼100 au. In W3(OH) we discovered that 20 per cent of the 4765-MHz emission comes from a narrow low-brightness filament that stretches north–south for ∼1.0 arcec (∼2200 au) between two previously known 4765-MHz maser spots. The filament appears in projection against the H ii region and has a brightness temperature of ∼4 × 105 K . There are matching absorption features in mainline transitions of highly excited OH. The filament may trace a shock front in a rotating disc. 相似文献
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E. Carretti S. Poppi W. Reich P. Reich E. Fürst G. Bernardi S. Cortiglioni C. Sbarra 《Monthly notices of the Royal Astronomical Society》2006,367(1):132-138
Polarized diffuse emission observations at 1.4 GHz in a high Galactic latitude area of the Northern celestial hemisphere are presented. The 3.2 × 3.2 deg2 field, centred at RA = 10h 58m , Dec. =+42°18' (B1950) , has Galactic coordinates l ∼ 172°, b ∼+63° and is located in the region selected as northern target of the Balloon-borne Radiometers for Sky Polarization Observations experiment. Observations have been performed with the Effelsberg 100-m telescope. We find that the angular power spectra of the E and B modes have slopes of β E =−1.79 ± 0.13 and β B =−1.74 ± 0.12 , respectively. Because of the very high Galactic latitude and the smooth emission, a weak Faraday rotation action is expected, which allows both a fair extrapolation to cosmic microwave background polarization (CMBP) frequencies and an estimate of the contamination by the Galactic synchrotron emission. We extrapolate the E -mode spectrum up to 32 GHz and confirm the possibility to safely detect the CMBP E -mode signal in the Ka band found in another low-emission region. Extrapolated up to 90 GHz, the Galactic synchrotron B mode looks to compete with the cosmic signal only for models with a tensor-to-scalar perturbation power ratio T / S < 0.001 , which is even lower than the T / S value of 0.01 found to be accessible in the only other high Galactic latitude area investigated to date. This suggests that values as low as T / S = 0.01 might be accessed at high Galactic latitudes. Such low-emission values can allow a significant redshift of the best frequency to detect the CMBP B mode, also reducing the contamination by Galactic dust, and opening interesting perspectives to investigate inflation models. 相似文献
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Johan van der Walt 《Monthly notices of the Royal Astronomical Society》2005,360(1):153-158
A statistical estimate of the number of 6.7-GHz methanol masers in the Milky Way and their lifetime is presented. The estimate is based on the currently known number of masers, a realistic correction for sensitivity effects and generally accepted Galactic star formation rates and initial mass functions. The analysis suggests that the minimum number of masers in the Galaxy is of the order of 850 while a more realistic estimate of the total number of masers is of the order of 1200 ± 84. The lifetime is estimated to be between 2.5 × 104 and 4.5 × 104 yr , with the variation being due to the use of different initial mass functions. The estimated lifetime agrees with that found from independent studies and agrees remarkably well with the time-scale for the chemical evolution of methanol in hot cores as well as with the dynamical time-scales of molecular outflows associated with high-mass star formation regions. It is shown that the hypothesis of the masers being associated with propagating planar shocks in cores or clumps results in lifetimes for the masers that are smaller by a factor of 2 or more compared with the lifetime of methanol masers as estimated here. 相似文献
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S. Goedhart D. J. van der Walt M. J. Gaylard 《Monthly notices of the Royal Astronomical Society》2002,335(1):125-141
Near-infrared (NIR) images at I , J , H and K bands were made of 12 southern 6.7-GHz methanol maser sources. Astrometry accurate to 0.5 arcsec was obtained. The positions of known H ii regions, water masers, hydroxyl masers, and mid- and far-infrared objects in the region are examined in order to try to determine the nature of the methanol maser source. Deeply embedded NIR sources were found close to seven out of 14 maser sites. In three cases, no NIR source, H ii region, water maser or hydroxyl maser could be found in likely association with the methanol masers. 相似文献
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M. J. Hardcastle L. W. Looney 《Monthly notices of the Royal Astronomical Society》2001,320(3):355-364
We present 85-GHz observations of the archetypal double-hotspot radio source 3C 20 made with the BIMA millimetre array. The resolution of BIMA allows us to separate the two components of the eastern hotspot. By comparing the BIMA observations with existing VLA data, we show that the spectra of the two hotspot components are very similar, despite the clear differences in their radio structure and their wide separation. We discuss the implications for models of double hotspot formation. Weak emission from the lobes of 3C 20 is detected at 85 GHz, at a level consistent with the predictions of standard spectral ageing models. 相似文献
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R. C. Bolton C. J. Chandler G. Cotter T. J. Pearson G. G. Pooley A. C. S. Readhead J. M. Riley E. M. Waldram 《Monthly notices of the Royal Astronomical Society》2006,367(1):323-330
In this paper, we present subarcsecond resolution observations of 36 compact sources from the 15h region of the 15-GHz 9th Cambridge survey. These sources all have previously measured simultaneous continuum radio spectra spanning 1.4–43 GHz and we classify each source by fitting a quadratic function to its spectrum. Using the Multi-Element Radio-Linked Interferometer Network and the Very Long Baseline Array, both at 5 GHz, we resolve all six steep-spectrum objects and four of the 13 flat-spectrum objects. However, none of the 16 objects with convex spectra peaking above 2.5 GHz is resolved even at <3-mas resolution. These results, in combination with the findings of a 15-GHz variability study, suggest that emission from the high-frequency peaking objects is affected by relativistic beaming, and that these objects are not necessarily as young as the synchrotron self-absorption interpretation of their peak frequencies would imply. 相似文献
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D. L. Blank J. I. Harnett P. A. Jones 《Monthly notices of the Royal Astronomical Society》2005,356(2):734-736
We have observed the type 1.5 Seyfert galaxy NGC 7213 with the Australian Long Baseline Array (LBA) at 8.4 GHz to discover whether this object has the high brightness temperature compact core suggested by low-frequency variability. Confirmation would support the hypothesis that radio-intermediate Seyfert galaxies have Doppler-boosted radio jets. Our observation confirms the existence of this core but with a flux density of almost a factor of 6 less than observed 12 yr earlier. Though few studies exist on the long-term radio variability of Seyferts, a decline of this magnitude does appear to be rare. 相似文献
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A. M. M. Scaife N. Hurley-Walker M. L. Davies P. J. Duffett-Smith F. Feroz K. J. B. Grainge D. A. Green M. P. Hobson T. Kaneko A. N. Lasenby G. G. Pooley R. D. E. Saunders P. F. Scott D. J. Titterington E. M. Waldram J. Zwart 《Monthly notices of the Royal Astronomical Society》2008,385(2):809-822
We present observations between 14.2 and 17.9 GHz of 16 Galactic H ii regions made with the Arcminute Microkelvin Imager. In conjunction with data from the literature at lower radio frequencies we investigate the possibility of a spinning dust component in the spectra of these objects. We conclude that there is no significant evidence for spinning dust towards these sources and measure an average spectral index of α= 0.15 ± 0.07 (where S ∝ν−α ) between 1.4 and 17.9 GHz for the sample. 相似文献
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A. D. Biggs E. Xanthopoulos I. W. A. Browne L. V. E. Koopmans C. D. Fassnacht 《Monthly notices of the Royal Astronomical Society》2000,318(1):73-76
The complex 10-component gravitational lens system B1933+503 has been monitored with the Very Large Array (VLA) during the period 1998 February–June with a view to measuring the time delay between the four compact components and hence to determine the Hubble parameter H 0 . Here we present the results of an 'A' configuration 8.4-GHz monitoring campaign that consists of 37 epochs with an average spacing of 2.8 d. The data have yielded light curves for the four flat-spectrum radio components (components 1, 3, 4 and 6). We observe only small flux density changes in the four flat-spectrum components that we do not believe are predominantly intrinsic to the source. Therefore the variations do not allow us to determine the independent time delays in this system. However, the data do allow us to accurately determine the flux density ratios between the four flat-spectrum components. These will prove important as modelling constraints and could prove crucial in future monitoring observations should these data show only a monotonic increase or decrease in the flux densities of the flat-spectrum components. 相似文献