首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 203 毫秒
1.
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   

2.
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   

3.
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   

4.
  相似文献   

5.
  相似文献   

6.
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   

7.
The northern section of the molecular cloud complex NGC 6334 has been mapped in the CO and CS spectral line emission and in continuum emission at a wavelength of 1300 μm. Our observations highlight the two dominant sources, I and I(N), and a host of weaker sources. NGC 6334 I is associated with a cometary ultracompact H  ii region and a hot, compact core ≤10 arcsec in size. Mid-infrared and CH3OH observations indicate that it is also associated with at least two protostellar sources, each of which may drive a molecular outflow. For region I we confirm the extreme high-velocity outflow first discovered by Bachiller & Cernicharo and find that it is very energetic with a mechanical luminosity of 390 L⊙. A dynamical age for the outflow is ∼3000 yr. We also find a weaker outflow originating from the vicinity of NGC 6334 I. In CO and CS this outflow is quite prominent to the north-west, but much less so on the eastern side of I, where there is very little molecular gas. Spectral survey data show a molecular environment at position I which is rich in methanol, methyl formate and dimethyl ether, with lines ranging in energy up to 900 K above the ground state. NGC 6334 I(N) is more dense than I, but cooler, and has none of the high-excitation lines observed toward I. I(N) also has an associated outflow, but it is less energetic than the outflow from I. The fully sampled continuum map shows a network of filaments, voids and cores, many of which are likely to be sites of star formation. A striking feature is a narrow, linear ridge which defines the western boundary. It is unclear if there is a connection between this filament and the many potential sites of star formation, or if the filament existed prior to the star formation activity.  相似文献   

8.
Injecting phase calibration (PCAL) signals to the feed horn of the observation sys- tem and analyzing the output response signals of the spectrometer, we measured the working performance of a 4096-channel digital autocalibration spectrometer. The results demonstrate that the spectrometer has a fine working performance: (1) the channels are distributed uni- formly in the spectrometer; (2) line drift produces little effect on the observation results; (3) spectral resolution shows little changes with observation time. The distribution of the fre- quency resolution in an 80 MHz bandwidth was measured. A trial observation on the two molecular spectral lines of H2CO and H110α taken with this spectrometer is described.  相似文献   

9.
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   

10.
  相似文献   

11.
This investigation is aimed at clarifying the nature of the interstellar gas seen in absorption against bright O and B stars. Towards this end we have obtained for the first time HI absorption spectra towards radio sources very close to the lines of sight towards twenty five bright stars previously studied. In this paper we describe the selection criteria, the details regarding our observations, and finally present the absorption spectra. In the accompanying paper we analyse the results and draw conclusions.  相似文献   

12.
We present a new stellar evolution code and a set of results, demonstrating its capability at calculating full evolutionary tracks for a wide range of masses and metallicities. The code is fast and efficient, and is capable of following through all evolutionary phases, without interruption or human intervention. It is meant to be used also in the context of modelling the evolution of dense stellar systems, for performing live calculations for both normal star models and merger products.
The code is based on a fully implicit, adaptive-grid numerical scheme that solves simultaneously for structure, mesh and chemical composition. Full details are given for the treatment of convection, equation of state, opacity, nuclear reactions and mass loss.
Results of evolutionary calculations are shown for a solar model that matches the characteristics of the present sun to an accuracy of better than 1 per cent; a  1 M⊙  model for a wide range of metallicities; a series of models of stellar Populations I and II, for the mass range 0.25 to  64 M⊙  , followed from pre-main-sequence to a cool white dwarf or core collapse. An initial–final mass relationship is derived and compared with previous studies. Finally, we briefly address the evolution of non-canonical configurations, merger products of low-mass main-sequence parents.  相似文献   

13.
  相似文献   

14.
Polarized intensity and polarization angles are calculated from Stokes parameters Q and U in a nonlinear way. The statistical properties of polarized emission hold information about the structure of magnetic fields in a large range of scales, but the contributions of different stages of data processing to the statistical properties should first be understood. We use 1.4 GHz polarization data from the Effelsberg 100‐m telescope of emission in the Galactic plane, near the plane and far out of the plane. We analyze the probability distribution function and the wavelet spectrum of the original maps in Stokes parameters Q, U and corresponding PI. Then we apply absolute calibration (i.e. adding the large‐scale emission to the maps in Q and U), subtraction of polarized sources and subtraction of the positive bias in PI due to noise (“denoising”). We show how each procedure affects the statistical properties of the data. We find a complex behavior of the statistical properties for the different regions analyzed which depends largely on the intensity level of polarized emission. Absolute calibration changes the morphology of the polarized structures. The statistical properties change in a complex way: Compact sources in the field flatten the wavelet spectrum over a substantial range. Adding large‐scale emission does not change the spectral slopes in Q and U at small scales, but changes the PI spectrum in a complex way. “Denoising” significantly changes the p.d.f. of PI and raises the entire spectrum. The final spectra are flat in the Galactic plane due to magnetic structures in the ISM, but steeper at high Galactic latitude and in the anticenter. For a reliable study of the statistical properties of magnetic fields and turbulence in the ISM based on radio polarization observations, absolute calibration and source subtraction are required. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

15.
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   

16.
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   

17.
  相似文献   

18.
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   

19.
  相似文献   

20.
We have used the Mopra Telescope to search for glycine and the simple chiral molecule propylene oxide in the Sgr B2 (LMH) and Orion KL, in the 3-mm band. We have not detected either species, but have been able to put sensitive upper limits on the abundances of both molecules. The 3σ upper limits derived for glycine conformer I are  3.7 × 1014 cm−2  in both Orion-KL and Sgr B2 (LMH), comparable to the reported detections of conformer I by Kuan et al. However, as our values are 3σ upper limits rather than detections we conclude that this weighs against confirming the detection of Kuan et al. We find upper limits for the glycine II column density of  7.7 × 1012 cm−2  in both Orion-KL and Sgr B2 (LMH), in agreement with the results of Combes et al. The results presented here show that glycine conformer II is not present in the extended gas at the levels detected by Kuan et al. for conformer I. Our ATCA results have ruled out the detection of glycine (both conformers I and II) in the compact hot core of the LMH at the levels reported, so we conclude that it is unlikely that Kuan et al. have detected glycine in either Sgr B2 or Orion-KL. We find upper limits for propylene oxide abundance of  3.0 × 1014 cm−2  in Orion-KL and  6.7 × 1014 cm−2  in Sgr B2 (LMH). We have detected fourteen features in Sgr B2 and four features in Orion-KL which have not previously been reported in the interstellar medium, but have not been able to plausibly assign these transitions to any carrier.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号