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1.
Nearby interstellar clouds with high (|ν|≥10km s−1) random velocities although easily detected in NaI and CaII lines have hitherto not been detected (in emission or absorption) in the HI 21cm line. We describe here deep Giant Metrewave Radio Telescope (GMRT) HI absorption observations toward radio sources with small angular separation from bright O and B stars whose spectra reveal the presence of intervening high random velocity CaII absorbing clouds. In 5 out of the 14 directions searched we detect HI 21cm absorption features from these clouds. The mean optical depth of these detections is ∼0.09 and FWHM is ∼10km s−1, consistent with absorption arising from CNM clouds.  相似文献   

2.
We have carried out a sensitive high-latitude (|b| > 15°) HI 21 cm-line absorption survey towards 102 sources using the GMRT. With a 3σ detection limit in optical depth of ∼ 0.01, this is the most sensitive HI absorption survey. We detected 126 absorption features most of which also have corresponding HI emission features in the Leiden Dwingeloo Survey of Galactic neutral Hydrogen. The histogram of random velocities of the absorption features is well-fit by two Gaussians centered at V1sr ∼ 0 km s−1 with velocity dispersions of 7.6 ± 0.3 km s−1 and 21 ± 4 km s−1 respectively. About 20% of the HI absorption features form the larger velocity dispersion component. The HI absorption features forming the narrow Gaussian have a mean optical depth of 0.20 ± 0.19, a mean HI column density of (1.46 ± 1.03) × 1020 cm−2, and a mean spin temperature of 121 ± 69 K. These HI concentrations can be identified with the standard HI clouds in the cold neutral medium of the Galaxy. The HI absorption features forming the wider Gaussian have a mean optical depth of 0.04 ± 0.02, a mean HI column density of (4.3 ± 3.4) × 1019 cm−2, and a mean spin temperature of 125 ± 82 K. The HI column densities of these fast clouds decrease with their increasing random velocities. These fast clouds can be identified with a population of clouds detected so far only in optical absorption and in HI emission lines with a similar velocity dispersion. This population of fast clouds is likely to be in the lower Galactic Halo.  相似文献   

3.
Three-dimensional simulations of the disk-halo interaction show the formation of a thick HI and HII gas disk with different scale heights. The thick HI disk prevents the disk gas from expanding freely upwards, unless some highly energetic event such as chimneys occurs, whereas the thick HII disk acts as a disk-halo interaction region from where the hot ionized gas flows freely into the halo. The upflowing gas reaches the maximum height at z ∼ 9.3 ± 1 kpc becoming thermally unstable due to radiative losses, and condenses into HI clouds. Because the major fraction of the gas is gravitationally bound to the Galaxy, the cold gas returns to the disk. The descending clouds will have at some height high velocities. In a period of 200 Myr of fountain evolution, some 10 percent of the total number of clouds are HVCs. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

4.
We have modelled, for the cases of Milky Way and M31, the effects on the galactic discs, of the arrival at high velocity (≥150 km s−1) of giant HI clouds, with masses of up to 108M⊙. Predictions are compared with the detailed structure of the observed rotation curves for these two galaxies. The model explains the rises and falls observed at large distances from the centre of each galaxy, distributed with a degree of regularity in radius, in terms of a specific type of perturbations driven by the infall of the high velocity clouds (HVC's) arriving from the intracluster medium of the Local Group. The underlying rotation curve is explained conventionally via the distribution of the baryonic and dark matter components of the galaxy in question. This scenario, though tested here on the two major Local Group objects, is in principle applicable to galaxies undergoing minor mergers with subgalactic mass gas clouds.  相似文献   

5.
We have imaged a region of ∼ 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 ∼ 2′ (∼ 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 V1sr = −4 ± 15 km s{−1}. The mean full width at half maximum is 119 ± 42 km s−1. Such a wide line is absent in the spectra at positions beyond ∼ 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 ∼ 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.  相似文献   

6.
In the preceding paper (Paper I), we presented HI absorption spectra towards radio sources very close to the lines of sight towards twenty five bright stars against which optical absorption spectra had been obtained earlier, In this paper we analyse the results and draw some conclusions. To summarize briefly, in most cases we found HI absorption at velocities corresponding to the optical absorption features provided one restricted oneself to velocities ≲10 kms-1. At higher velocities we did not detect any HI absorption down to an optical depth limit of 0.1 (except in four cases which we attribute to gas in systematic motion rather than clouds in random motion). After discussing various scenarios, we suggest that this trend should perhaps be understood in terms of the high velocity interstellar clouds being accelerated, heated and ablated by expanding supernova remnants.  相似文献   

7.
Starting from Bosma (1981) paper, it was demonstrated by different authors that the observed shape of rotation curves of many spiral galaxies can be explained if to assume that the radial density distribution of the dark matter is correlated with the distribution of HI: the column densities of the dark matter and HI are proportional. We show that this correlation is obviously to be an artifact and can be explained by assuming that the gas surface density is approximately equal or in general proportional to the critical density for the local gravitational stability of gaseous layer.  相似文献   

8.
Massive stars have significant influence on the evolution of the interstellar medium. Bright rims, cometary morphology of clouds, as well as their motion are some examples of the influence of massive stars on nearby molecular clouds. The cometary clouds in the Gum-Vela region are very good examples. In an attempt to understand the kinematics of the clouds in such regions we have carried out CO line observations towards bright-rimmed clouds near the OB Association Cep OB2. The radial velocities of the clouds are consistent with an expansion of the system at ≈ 4kms?1 away from the dominant O6.5V star in the association, HD206267. We find the rocket mechanism to be the most likely cause for expansion as found for both the Gum-Vela and the Rosette globules. We conclude that such expanding motions are quite common in regions near massive stars and make a brief comparison of the Cepheus system with the Gum-Vela system.  相似文献   

9.
We examine a sample of 65 galaxies in the Virgo cluster with negative radial velocities relative to the Local Group. Some features of this sample are pointed out. All of these objects are positioned compactly within a virial zone of radius 6o in the cluster, but their centroid is displaced relative to the dynamic center of the cluster, M87, by 1o.1 to the northwest. The dwarf galaxies in this sample are clumped on a scale of ∼10' (50 kpc). The observed asymmetry in the distribution of the blueshifted galaxies may be caused by infall of a group of galaxies around M86 onto the main body of the cluster. We offer another attempt to explain this phenomenon, assuming a mutual tangential velocity of ∼300 km/s-1 between the Local Group and the Virgo cluster owing to their being repelled from the local cosmological void.  相似文献   

10.
It follows from observations that the clouds forming the Lα forest have a hierarchical structure. It is suggested that their structure is not only hierarchical, but also fractal. The relationship between the radial concentration of neutral hydrogen N(HI) and the concentration of clouds having the same value of N(HI), well known from observation, is explained on this basis. The fractal dimension of D = 2.5 ± 0.2 is estimated from observational data and calculations. Translated fromAstrofizika, Vol. 40, No. 1, pp. 29–37, February, 1997.  相似文献   

11.
We present a catalog of 520 most isolated nearby galaxies with angular velocities V LG < 3500 km/s, covering the entire sky. This population of “space orphans” makes up 4.8% among 10 900 galaxies with measured radial velocities. We describe the isolation criterion used to select our sample, called the “Local Orphan Galaxies”(LOG), and discuss their basic optical and HI properties. A half of the LOG catalog is occupied by the Sdm, Im and Ir morphological type galaxies without a bulge. The median ratio M gas/M star in the LOG galaxies exceeds 1. The distribution of the catalog galaxies on the sky looks uniform with some signatures of a weak clustering on the scale of about 0.5 Mpc. The LOG galaxies are located in the regions where the mean local density of matter is approximately 50 times lower than the mean global density. We indicate a number of LOG galaxies with distorted structures, which may be the consequence of interaction of isolated galaxies with massive dark objects.  相似文献   

12.
We consider a disk-like dark matter model for the Milky Way andcompare a few predictions with observations. The observed gaseousflaring for HI and molecular gas fits the model predictions indetail. The global HI distribution in the Milky Way needs to beexplained by a multiphase medium. The dark matter distribution in theGalactic halo is traced by a low density component of halogas. High-velocity clouds with distances up to ～ 50 kpc may beexplained as condensations which originate from instabilities withinthe gaseous halo. Our model explains also ‘beards’ and ‘forbiddenvelocities’ as observed in the rotation curves of externalgalaxies. A disk-like dark matter model is self-consistent and inexcellent agreement with observations.  相似文献   

13.
A new method of determination of distances to stellar associations is proposed. This method is based on the measurements of mean thicknesses of globules of the systems of dark globules connected with that association. It is shown that the method is in good agreement with other known methods. A new grouping of OB stars in Pup-CMa is found. It is shown that this grouping has properties characteristic for OB-associations: the stars of that grouping have similar distances, similar radial velocities, the grouping is connected with molecular clouds, with Herbig-Haro objects, and cometary nebulae. All these results are in favour of this grouping to be a new OB-association. We named that association OB-association Pup-CMa. The preliminary results of12CO observations of molecular clouds connected with that association are also given. The radial velocities of these clouds are in good agreement with the mean radial velocity of stars in the association in Pup-CMa. Translated from Astrofizika, Vol. 42, No. 2, pp. 179–188, April–June, 1999.  相似文献   

14.
The morphological and velocity structures in the gaseous (HI and CO) and stellar components of two interacting systems are examined. Both Arp 140 and Arp 104 reveal extended tidal tails in the HI. The Hα and FIR fluxes of Arp 140 yield similar SFR of ∼ 0.8 M⊙ yr-1. In contrast the Hα flux of Arp 104 yields a SFR of ∼ 0.05 M⊙ yr-1, ∼ 20 times smaller than that obtained from the FIR flux. Spectra were used to examine the changing velocity of atomic and molecular gas in NGC 5218 (Arp 104). The atomic and molecular gas were found to be dynamically similar with comparable velocities and velocity widths across the galaxy; consistent with the two phases responding similarly to the interaction, or enhanced HI to CO conversion in the centre of the galaxy. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   

15.
If the dwarf spheroidals are embedded in an extended cloud of dark matter then their density profiles can be reproduced by assuming a Maxwellian distribution of velocities for the constituent stars. The observed luminosity profiles of dwarf spheroidals imply densities for the dark matter in the range 10-26 to 10-25 g cm-3, and mass-to-luminosity ratios which are typically an order of magnitude greater than those of globular clusters. Neutrinos of mass ∼ 10 eV and (v) ∼ 1000 km s-1 can provide this requisite density for the background.  相似文献   

16.
We have investigated the effects of increasing optical depths on spectral lines formed in a rotating and expanding spherical shell. We have assumed a shell whose outer radius is 3 times the inner radius, with the radial optical depths equal to 10, 50, 100, 500. We have employed a constant velocity with no velocity gradients in the shell. The shell is assumed to be rotating with velocities varying as 1/ρ, whereρ is the perpendicular distance from the axis of rotation, implying the conservation of angular momentum. Two expansion (radial) velocities are treated: (1)V = 0 (static case) and (2)V = 10 mean thermal units. The maximum rotational velocities areV rot = 0, 5, 10 and 20. In the shell where there are no radial motions, we obtain symmetric lines with emission in the wings forV rot = 0 and 5 while forV rot ≥ 10 we obtain symmetric absorption lines. In the case of an expanding shell, we obtain lines with central emission.  相似文献   

17.
We present our recently developed 3-dimensional chemodynamical code for galaxy evolution. This code follows the evolution of different galactic components like stars, dark matter and different components of the interstellar medium (ISM), i.e. a diffuse gaseous phase and the molecular clouds. Stars and dark matter are treated as collisionless N-body systems. The ISM is numerically described by a smoothed particle hydrodynamics (SPH) approach for the diffuse gas and a sticky particle scheme for the molecular clouds. Additionally, the galactic components are coupled by several phase transitions like star formation, stellar death or condensation and evaporation processes within the ISM. As an example we show the dynamical and chemical evolution of a star forming dwarf galaxy with a total baryonic mass of 2 ċ 109 M⊙. After a moderate collapse phase the stars and the molecular clouds follow an exponential radial distribution, whereas the diffuse gas shows a central depression as a result of stellar feedback. The metallicities of the galactic components behave quite differently with respect to their temporal evolution as well as their radial distribution. Especially, the ISM is at no stage well mixed. This revised version was published online in September 2006 with corrections to the Cover Date.  相似文献   

18.
We present results from a Keck optical and near IR spectroscopic study of the giant emission line halos of the z>3 High Redshift Radio Galaxies (HiZRGs) 4C 41.17, 4C 60.07 and B2 0902+34. The outer regions of these halos show quiet kinematics with typical velocity dispersions of a few hundred km s−1 and velocity shears consistent with rotation. The inner regions contain shocked, clumpy cocoons of gas closely associated with the radio lobes with disturbed kinematics and expansion velocities and/or velocity dispersions >1000 km s−1. We also find evidence for the ejection of chemically enriched material in 4C 41.17 up to a distance of ∼60 kpc along the radio-axis. We infer that these HiZRGs are undergoing a final jet-induced phase of star formation with the ejection of most of their interstellar medium before evolving to become “red and dead” Elliptical galaxies.  相似文献   

19.
The 2MFGC catalog we have used contains 18020 galaxies selected from the extended objects in the 2MASS infrared sky survey as having apparent ratios of the axes b/a<0.3. Most of them are spiral galaxies of later morphological types whose disks are seen almost edge-on. The individual distances to the 2724 2MFGC galaxies with known rotation velocities and radial velocities are determined using a multiparameter infrared Tully-Fisher relation. A list of the distances and peculiar velocities of these galaxies is presented. The collective motion of the 2MFGC galaxies relative to the cosmic microwave background is characterized by a velocity V = 199 ± 37 km/s in the direction l = 304° ±11°, b = −8°±8°. Our list is currently the most representative and uniform sample for analyzing non-Hubble motions of galaxies on a scale of ∼100 Mpc. __________ Translated from Astrofizika, Vol. 49, No. 4, pp. 527–540 (November 2006).  相似文献   

20.
High resolution observations of interstellar Na Iabsorptions in the spectra of early-type stars in the second Galacticquadrant have been used to derive the spatial distribution of thenearby interstellar gas in the Galactic anticentre hemisphere. The datashow the presence, within the region explored and within the nearest150 pc, of eight diffuse interstellar clouds with LSR velocities in therange 15–55 km s-1. Molecular gas previously identified close tothe Sun by Trapero et al. (1995) and Trapero et al. (1996) is embeddedin one of the clouds. The motion of these clouds does not reflect anymotion of the gas away from the Scorpio-Centaurus association, showingthat the kinematics observed in the neighbourhood of the Sun byLallement and Bertin (1992) is restricted, in the second Galactic quadrant,to the nearest 50 pc.  相似文献   

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