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1.
We present the results of a search for the ground-state hyperfine transition of the OH radical near 53 MHz using the National Mesosphere–Stratosphere–Troposphere (MST) Radar Facility at Gadanki, India. The observed position was G48.4−1.4 near the Galactic plane. The OH line is not detected. We place a 3σ upper limit for the line flux density at 39 Jy from our observations. We also did not detect recombination lines (RLs) of carbon, which were within the frequency range of our observations. The 3σ upper limit of 20 Jy obtained for the flux density of carbon RLs, along with observations at 34.5 and 327 MHz, are used to constrain the physical properties of the line-forming region. Our upper limit is consistent with the line emission expected from a partially ionized region with electron temperature, density and path lengths in the range 20–300 K, 0.03–0.3 cm−3 and 0.1–170 pc, respectively.  相似文献   

2.
We report high-spectral-resolution Australia Telescope Compact Array (ATCA) H  i 21-cm observations resulting in the detection of the warm neutral medium (WNM) of the Galaxy in absorption against two extragalactic radio sources, PKS 1814−637 and PKS 0407−658. The two lines of sight were selected on the basis of the simplicity of their absorption profiles and the strength of the background sources; the high velocity resolution of the spectra then enabled us to estimate the kinetic temperatures of the absorbing gas by fitting multiple Gaussians to the absorption profiles. Four separate WNM components were detected towards the two sources, with peak optical depths  τmax= (1.0 ± 0.08) × 10−2, (1.4 ± 0.2) × 10−3, (2.2 ± 0.5) × 10−3  and  (3.4 ± 0.5) × 10−3  and kinetic temperatures   T k= 3127 ± 300, 3694 ± 1595, 3500 ± 1354  and  2165 ± 608 K  , respectively. All four components were thus found to have temperatures in the thermally unstable range  500 < T k < 5000 K  ; this suggests that thermal equilibrium has not been reached throughout the WNM.  相似文献   

3.
We continue with our previous work on statistics of velocity centroids, to retrieve information about the scaling properties of an underlying turbulent velocity field from spectroscopic observations. We use synthetic data sets with extreme effects of velocity–density correlations that we create artificially, which also have a non-Gaussian distribution of fluctuations. We confirm that centroids can be used to obtain the scaling properties of the turbulent velocity when the ratio of the density dispersion to the mean density is less than unity, regardless of velocity–density correlations and non-Gaussianity. It was found that extreme velocity–density correlations can distort the statistics of velocity centroids, impeding the recovery of the turbulent velocity spectral index from centroids. We show that such correlations introduce high-order moments to the maps of centroids, which we disregarded in previous work, but that they are only important when the density dispersion is large in comparison with the mean density. It was also found that non-Gaussian velocity and/or density distort the statistics of centroids too, but to a lower degree than extreme cross-correlations.  相似文献   

4.
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5.
During the past decade or so, measurements of Galactic H  i absorption using VLBI against extragalactic sources, as well as multi-epoch observations in pulsar directions, have detected small-scale transverse variations corresponding to tens of au at the distance of the absorbing matter. Hitherto these measurements have been interpreted as small-scale structure in the H  i distribution with densities n H  i ∼104–105 cm−3, orders of magnitude greater than those of the pc-scale structure. Naturally, it is difficult to imagine how such structures could exist in equilibrium with other components of the ISM.
In this paper we show that structure on all scales contributes to the differences on neighbouring lines of sight, and that the observed differences can be accounted for by a natural extension of the distribution of irregularities in the distribution of H  i opacities at larger scales, using a single power law. This, in our opinion, should put an end to the decades-long puzzle of the so-called small-scale structure in H  i and other species in the Galaxy.  相似文献   

6.
We present the results of a survey, the deepest to date, for H  i emission at 21 cm and OH emission at 18 cm (lines at 1612, 1665, 1667 and 1720 MHz) in the direction towards the Galactic globular clusters M15, M2, NGC 6934, NGC 7006 and Pal 13. The aim is to measure the amount of hydrogen in the intracluster medium, and to find OH masers in the circumstellar envelopes of globular cluster red giants. We present a tentative detection of  0.3 M⊙  of neutral hydrogen in M15 and possible detections of neutral hydrogen in M2 and Pal 13. We derive upper limits to the neutral hydrogen content of NGC 6934 and 7006. No OH emission is detected. We also present deep H  i data of the northern tip of the Magellanic Stream behind Pal 13.  相似文献   

7.
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8.
High spatial resolution radio continuum and 6.67-GHz methanol spectral line data are presented for methanol masers previously detected by Walsh et al. (1997). Methanol maser and/or radio continuum emission is found in 364 cases towards IRAS -selected regions. For those sources with methanol maser emission, relative positions have been obtained to an accuracy of typically 0.05 arcsec, with absolute positions accurate to around 1 arcsec. Maps of selected sources are provided. The intensity of the maser emission does not seem to depend on the presence of a continuum source. The coincidence of water and methanol maser positions in some regions suggests there is overlap in the requirements for methanol and water maser emission to be observable. However, there is a striking difference between the general proximity of methanol and water masers to both cometary and irregularly shaped ultracompact (UC) H  ii regions, indicating that, in other cases, there must be differing environments conducive to stimulating their emission. We show that the methanol maser is most likely present before an observable UC H  ii region is formed around a massive star and is quickly destroyed as the UC H  ii region evolves. There are 36 out of 97 maser sites that are linearly extended. The hypothesis that the maser emission is found in a circumstellar disc is not inconsistent with these 36 maser sites, but is unlikely. It cannot, however, account for all other maser sites. An alternative model which uses shocks to create the masing spots can more readily reproduce the maser spot distributions.  相似文献   

9.
We have measured 21-cm absorption and emission spectra in the direction of a further seven southern pulsars with the Parkes telescope to derive their kinematic distances and to study the interstellar medium. For the first time we have successfully obtained H  i absorption measurements for PSRs J1602–5100, J1740–3015 and J1745–3040. We have also significantly improved the sensitivity and resolution on PSRs J1600–5044, J1752–2806 and J1825–0935, the spectra of which have previously been measured, and have corrected an error in the published distance to PSR J1824–1945.
Since the Frail & Weisberg summary of pulsar distances in 1990, a further 23 pulsars now have measured H  i distances, mainly through the efforts of the current group. We discuss the Taylor & Cordes electron density model in light of these new measurements and find that, although the model towards the Galactic Centre appears good, the line of sight through the Carina spiral arm is poorly fitted by the model.  相似文献   

10.
We present theoretically established values of the CO-to-H2 and C-to-H2 conversion factors that may be used to estimate the gas masses of external galaxies. We consider four distinct galaxy types, represented by M51, NGC 6946, M82 and SMC N27. The physical parameters that best represent the conditions within the molecular clouds in each of the galaxy types are estimated using a χ2 analysis of several observed atomic fine structure and CO rotational lines. This analysis is explored over a wide range of density, radiation field, extinction and other relevant parameters. Using these estimated physical conditions in methods that we have previously established, CO-to-H2 conversion factors are then computed for CO transitions up to J = 9 → 8. For the conventional CO(1–0) transition, the computed conversion factor varies significantly below and above the canonical value for the Milky Way in the four galaxy types considered. Since atomic carbon emission is now frequently used as a probe of external galaxies, we also present, for the first time, the C-to-H2 conversion factor for this emission in the four galaxy types considered.  相似文献   

11.
A reliable estimate of the molecular gas content in galaxies plays a crucial role in determining their dynamical and star-forming properties. However, H2, the dominant molecular species, is difficult to observe directly, particularly in the regions where most molecular gas is thought to reside. Its mass is therefore commonly inferred by assuming a direct proportionality with the integrated intensity of the  12CO( J = 1 → 0)  emission line, using a CO-to-H2 conversion factor, X . Although a canonical value for X is used extensively in such estimates, there is increasing evidence, both theoretical and observational, that the conversion factor may vary by over an order of magnitude under conditions different from those of the local neighbourhood. In an effort to understand the influence of changing environmental conditions on the conversion factor, we derive theoretical estimates of X for a wide range of physical parameters using a photon-dominated region (PDR) time-dependent chemical model, benchmarking key results against those of an independent PDR code to ensure reliability. Based on these results, the sensitivity of the X factor to change in each physical parameter is interpreted in terms of the chemistry and physical processes within the cloud. In addition to confirming previous observationally derived trends, we find that the time-dependence of the chemistry, often neglected in such models, has a considerable influence on the value of the conversion factor.  相似文献   

12.
We have obtained a spectrum of the Na D lines of the Seyfert galaxy Mrk 595 with a signal-to-noise ratio (S/N) of about 30. The importance of this sightline is that this galaxy lies in a direction close to the highest column densities of the Cohen high-velocity (HV) stream, and hence represents the best opportunity of detecting this stream in interstellar absorption, as previous searches towards stellar targets up to a distance of 600 pc have not been successful. The H  I column density in the direction of this galaxy implies that the HV gas should be detectable in absorption in the Na D lines. No such detections are made (at the 3σ level) and we consider possible explanations. Hence previous non-detections towards stars do not necessarily mean that the HV gas is more distant than the stars, and we emphasize that the distance of this high-velocity cloud (HVC), which is a key parameter to the understanding of the nature and origin of this feature, remains unknown and indeed may be difficult to establish.  相似文献   

13.
We present single-dish Arecibo 21-cm H  i observations, covering a 0675×0625 RA–Dec. grid, of the intermediate-velocity cloud (IVC) centred upon the M15 globular cluster. The velocity and positional structure of the IVC gas at V LSR=70 km s−1 are investigated; it is found to be clumpy and has a peak surface density N H  i ∼8×1019 cm−2. Additionally, we have performed a long H  i integration towards HD 203664, a Galactic halo star some 31 from M15, in which optical IVC absorption has previously been detected. No H  i with a velocity exceeding 60 km s−1 was found to a brightness temperature limit of 0.05 K. However, additional pointings did detect IVC gas approximately mid-way between HD 203664 and M15. Finally, we present both Arecibo H  i pointings and low-resolution spectra in the Ca  ii H and K lines towards 15 field stars in the general field towards M15, in an attempt to obtain the distance to the IVC. Intermediate-velocity H  i is detected towards seven sightlines. Stellar spectral types are derived for 12 of the sample. Assuming that these stars lie on the main sequence, their distances are estimated to lie in the range 150≤ d ≤1350 pc. No Ca  ii absorption is observed, either because the IVC is further away than ∼1350 pc or more likely because the gas along these sightlines is of too low a density to be detected by the current observations.  相似文献   

14.
We have mapped 16 molecular clouds toward a new OB association in the Pup-CMa region to derive their physical properties. The observations were carried out in the 12CO (J = 1 – 0) line with the Southern millimetre-wave Telescope at Cerro Tololo, Chile. Distances have been determined kinematically using the rotation curve of Brand with R⊙ = 8.5 kpc and V⊙ = 220 km/s. Masses have been derived adopting a CO luminosity to H2 conversion factor X = 3.8 . 1020 molecules cm-2 (K km/s)-1. The observed mean radial velocity of the clouds is comparable with the mean radial velocity of stars composing an OB association in Pup-CMa; it is in favor of the close connection of clouds with these stars. __________ Published in Astrofizika, Vol. 48, No. 4, pp. 491–501 (October–December, 2005).  相似文献   

15.
The Effelsberg‐Bonn H I survey (EBHIS) comprises an all‐sky survey north of Dec = –5° of the Milky Way and the local volume out to a red‐shift of z ≃ 0.07. Using state of the art Field Programmable Gate Array (FPGA) spectrometers it is feasible to cover the 100 MHz bandwidth with 16.384 spectral channels. High speed storage of H I spectra allows us to minimize the degradation by Radio Frequency Interference (RFI) signals. Regular EBHIS survey observations started during the winter season 2008/2009 after extensive system evaluation and verification tests. Until today, we surveyed about 8000 square degrees, focusing during the first all‐sky coverage of the Sloan‐Digital Sky Survey (SDSS) area and the northern extension of the Magellanic stream. The first whole sky coverage will be finished in 2011. Already this first coverage will reach the same sensitivity level as the Parkes Milky Way (GASS) and extragalactic surveys (HIPASS). EBHIS data will be calibrated, stray‐radiation corrected and freely accessible for the scientific community via a webinterface. In this paper we demonstrate the scientific data quality and explore the expected harvest of this new all‐sky survey (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

16.
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17.
We present multimolecular line maps obtained with the Mopra telescope towards the southern giant molecular cloud (GMC) complex G333, associated with the H  ii region RCW 106. We have characterized the GMC by decomposing the 3D data cubes with gaussclumps , and investigated spatial correlations among different molecules with principal component analysis (PCA). We find no correlation between clump size and linewidth, but a strong correlation between emission luminosity and linewidth. PCA classifies molecules into high- and low-density tracers, and reveals that HCO+ and N2H+ are anticorrelated.  相似文献   

18.
A southern hemisphere survey of methanol emission sources has been carried out using the Australia Telescope National Facility Mopra millimetre telescope. 85 sources, the majority of them masers, have been detected in the 80−71 A+ transition of methanol at 95 GHz. Together with a similar northern hemisphere survey, this completes the search for 95-GHz methanol emission from the Galactic plane. The previously found correlation between intensities of methanol emission at 44 and 95 GHz is confirmed here with the larger sample of sources. The results of large velocity gradient statistical equilibrium calculations confirm the classification of these sources as class I methanol masers pumped through collisional excitation.  相似文献   

19.
A southern hemisphere survey of methanol emission sources in two millimetre-wave transitions has been carried out using the ATNF Mopra millimetre telescope. 16 emission sources have been detected in the 31–40 A+ transition of methanol at 107 GHz, including six new sources exhibiting class II methanol maser emission features. Combining these results with the similar northern hemisphere survey, a total of eleven 107-GHz methanol masers have been detected. A survey of the methanol emission in the 00–1−1 E transition at 108 GHz has resulted in the detection of 16 sources; one of them showing maser characteristics. This is the first methanol maser detected at 108 GHz, presumably of class II. The results of large velocity gradient statistical equilibrium calculations confirm the classification of these new sources as class II methanol masers.  相似文献   

20.
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