The spiral structure in the inner parts of the Galaxy

The spiral structure of the inner parts of the Galaxy is studied using 21 cm line data and stellar data. To study the neutral hydrogen distribution in the galactic layer a parameter =(dV/dr) proportional to the mean densities is calculated using a first approximation for the velocity gradients due to differential rotation.The obtained distribution (R, Z) shows spiral features completely consistent with the early star distribution and with the Hii regions. The corrugation effect of the galactic layer is observed in all the studied zones in neutral hydrogen and in the distribution of the OB stars in the Carina zone.The pattern obtained indicates four spiral arms for the inner parts of the Galaxy, three of which are identified also in the stellar data (arms -I, -II, and -III) and the more distant -IV in Hii regions.The local arm according to the stellar data of Kilkennyet al. forms a feature completely similar to the arms -I and -II and there are no indications that this arm is a special material branch between two main spiral arms as has been supposed in order to conciliate the neutral hydrogen pattern with the stellar distribution.The pitch angles for the spiral arms are approximately 13°–17°.The observed wave form distribution of the hydrogen cloud layer is completely consistent with the theoretical predictions of Nelson (1976) but there are no indications of such an effect in the intercloud hydrogen. The corrugated cloud layer has a width of 100 pc, a wave amplitude of 70 pc, and a wavelength which grows with the galactic center distance (approx. 2 kpc in the zones next to the galactic nucleus and 2.6–3.0 kpc in the zones next to the Sun). To each wavelength correspond two spiral arms. The spiral features in our Galaxy show characteristics quite similar to the features in the Andromeda nebula, not only in the component materials (neutral hydrogen, Hii regions and possibly also dust and stars) but also in their kinematics.     

Local galactic field of forces and the interstellar matter

The density distributions of the two main components in interstellar hydrogen are calculated using 21 cm line data from the Berkeley Survey and the Pulkovo Survey. The narrow, dense component (state I of neutral hydrogen) has a Gaussianz-distribution with a scale-height of 50 pc in the local zones (the galactic disk). For the wide, tenuous component (hydrogen in state II) we postulate a distribution valid in the zones where such a material predominates (70 pc?z? 350 pc the galactic stratum) i.e., $$n_H \left( z \right) = n_H \left( 0 \right)exp \left( { - \left( {z/300{\text{ }}pc} \right)^{3/2} } \right).$$ Similar components are found in the dust distribution and in the available stellar data reaching sufficiently highz-altitudes. The scale-heights depend on the stellar type: the stratum in M III stars is considerably wider than in A stars (500–700 pc against 300 pc). The gas to dust ratio is approximately the same in both components: 0.66 atom cm^?3 mag^?1 kpc in the galactic plane. A third state of the gas is postulated associating it the observed free electron stratum at a scale-height of 660 pc (hydrogen fully ionized at high temperatures). The ratio between the observed dispersions in neutral hydrogen (thermal width plus turbulence) and the total dispersions corresponding to the real inner energies in the medium is obtained by a comparison with the dispersion distribution σ(z) of the different stellar types associated with the disk and the stratum $$\sigma ^2 \left( {total} \right) = \sigma ^2 \left( {21{\text{ cm line}}} \right) \cdot {\text{ }}Q^2 ,$$ from which we graphically obtainedQ ²=2.9 ± 0.3, although that number could be lower in the densest parts of the spiral arms. Its dependence on magnetic field and cosmic rays is analysed, indicating equipartition of the different energy components in the interstellar medium and consistency with the observed values of the magnetic field: i.e., fluctuations with an average of ～ 3 μG (associated with the disk) in a homogeneous background of ～ 1 μG (associated with the stratum). A minimum and maximumK _z-force are obtained assuming extreme conditions for the total density distribution (gas plus stars). TheK _z-force obtained from the interstellar gas in its different states using approximations of the Boltzmann equation is a reasonable intermediate case between maximum and minimumK _z. The mass density obtained in the galactic plane is 0.20±0.05M _⊙ pc^?3, and the results indicate that the galactic disk is somewhat narrower and denser than has usually been believed. The effects of wave-like distributions of matter in thez-coordinate are analysed in relation with theK _z-force, and comparisons with theoretical results are performed. A qualitative model for the galactic field of force is postulated together with a classification of the different zones of the Galaxy according to their observed ranges in velocity dispersions and the behaviour of the potential well at differentz-altitudes. The disk containing at least two-thirds of the total mass atz<100 pc, the stratum containing one-third or less of the total mass atz≤600–800 pc, and the halo at higherz-altitudes with a small fraction of such a mass which is difficult to evaluate.     

The galactic centre (invited review)

Recent observations of the galactic centre at radio and infrared wavelengths are reviewed. Particular emphasis is placed on the new large scale radio structures — the galactic centre lobe, and the filamentary ribbon structures in the radio arc — which have just been reported. New observations of the two parsec ring of molecular hydrogen, and the relationship of the ring to the interior distribution of ionised gas are discussed. The kinematis and distribution of neutral material, shocked molecular hydrogen and plasma in the inner few parsecs of the nucleus are considered in terms of a central luminous mass-losing object, together with rotation and infall of the neutral material.Paper presented at the IAU Third Asian-Pacific Regional Meeting, held in Kyoto, Japan, between 30 September–6 October, 1984.     

An anomalous velocity neutral hydrogen structure near the galactic center

An extensive concentration of neutral hydrogen has been observed in the fourth galactic quadrant, with a mean radial velocity of +44 km s^?1 referred to the local standard of rest. At a distance ofR kpc from the Sun this structure would contain 2.5×10⁴ R ² solar masses of neutral hydrogen. Five possible interpretations of this extensive concentration are considered: (1) part of the shell of a nearby explosive event; (2) a distant spiral arm of the Galaxy; (3) an extragalactic object; (4) material falling into our Galaxy; (5) gas expelled from the galactic center. Arguments are offered against the first three possibilities.     

Galactic perturbations on nearly-parabolic cometary orbits

The effect of galactic perturbations on long-period comet orbits is examined via numerical and analytical means. Relations are found between a comet's initial perihelion position and the positions of succeeding perihelia. It was found that the galactic effects were strongest on the comets initially at galactic latitudes close to 40°. In such cases the galactic perturbations caused the orbit to become almost circular before becoming nearly parabolic again. This effect allows comets with semimajor axes of about 25 000 AU to make only a few passages through the inner solar system in a time interval of 10⁹yr. Thus the galactic field is an important factor in the evolution of long-period comet orbits. The observed distribution of perihelia of long-period comets indicates that galactic effects have been active.     

On the spiral structure in the galactic distribution of co clouds

The CO distribution in the Galaxy is investigated through an analysis of longitude-velocity diagrams of CO emission lines for the two longitude ranges 20°<l<80° and 105°<l<140°. For the kinematics of the Galaxy we adopt the three typical models; the circular rotation, the linear density waves, and the galactic shock waves. It is shown that the distributions and kinematics of CO clouds are consistent with the predictions of the density wave model and the galactic shock model, and that the observed data of CO emissions do not contradict with the claim that the CO clouds form spiral arms.     

Simulation of the apparent anomalies of the galactic structure

In the maps of the galactic structure based on the kinematical method, several systematic heliocentric anomalies are found: in the northern galactic hemisphere the spiral arms are more tightly wound and the extent of neutral hydrogen is smaller than in the southern hemisphere; with separate rotation curves for the north and the south the arms become anomalously circular with a consequent discrepancy to the stellar distribution; there are straight portions in the arms pointing towards the Sun, as well as systematic strong curvatures and knee-like features; the inner arms affect the structure of the outer arms; with the northern rotation model, Hii-regions and Hi avoid the southern tangential circle; in the rear of the Galaxy, at symmetric longitudes, enhanced Hi-densities are found; the Perseus arm is displaced atl=180°. All of these anomalies can be explained with a simple model involving a non-velocity redshift field within the Galaxy, with an enhancement within the spiral arms. This is demonstrated by numerical simulations of the structural anomalies. Reducing the redshift effect from the kinematic data, the Galaxy's structure and kinematics appear symmetric. The significance of the result for the redshift problem is discussed.     

Are Damped Lyα Systems Large Galactic Discs?

The hypothesis that damped Lyα systems (DLAs) are large galactic discs (Milky Way sized) is tested by confronting predictions of models of the formation and evolution of (large) disc galaxies with observations, in particular the zinc abundance distribution with neutral hydrogen column density found for DLAs. A pronounced mismatch is found, strongly hinting that the majority of DLAs may not be large galactic discs. This revised version was published online in July 2006 with corrections to the Cover Date.     

The HI halo of spiral galaxies

A deep H I survey with the VLA of the spiral galaxy NGC 2403 has revealed the existence of a thick, low density layer of neutral gas surrounding the thin ‘cold’ disk. This layer has a mean rotation velocity 25–50 km s^-1 lower than that of the disk and a 10–20 km s^-1inflow towards the centre of the galaxy. In the central parts there are velocity differences from rotation of up to 150 km s^-1.Chandra observations of NGC 2403 show a diffuse, hot X-ray emitting gas component with a temperature of a few 10⁶ K. These results point at galactic fountain type of flows between disk and halo. ‘Halo’ gas with similar characteristics has also been observed in other spiral galaxies(e.g. NGC 6946, NGC 891). Such gas is probably similar to the IVCs and to some of the HVCs of the Milky Way.     

Neutral hydrogen absorption observations of the central region of NGC 5929

We present 0.15-arcsec (25-pc) resolution MERLIN observations of neutral hydrogen absorption detected towards the nuclear region of the type 2 Seyfert galaxy NGC 5929. Absorption is detected only towards the north-eastern radio component with a column density of (6.5 ± 0.6) × 10²¹ cm⁻². Based on comparison with an HST WFPC2 continuum image, we propose that the absorption is caused by a 1.5-arcsec structure of neutral gas and dust offset 0.3 arcsec south-east of the nucleus and running NE–SW. A separate cloud of dust is apparent 1.5 arcsec to the south-west of the nucleus in the HST image. A comparison of the centroid velocity (2358 ± 5 km s⁻¹) and full width at half-maximum (43 ± 6 km s⁻¹) of the absorbing gas with previous [O  III ] observations suggests that both the neutral and ionized gas are undergoing galactic rotation towards the observer in the north-east and away from the observer in the south-west. The main structure is consistent with an inclined ring of gas and dust encircling the active galactic nucleus (AGN); alternatively it may be a bar or inner spiral arm. We do not detect neutral hydrogen absorption or dust obscuration against the radio nucleus (column density < 3.1 × 10²¹ cm⁻²) expected by a torus of neutral gas and dust in unified models of AGNs for a type 2 Seyfert galaxy.     

Column Density Distribution of Galactic Lyα AbsorptionSystems

A simple consistency argument for hypothesis of the galactic halo origin of the Lyα forest absorption lines is advanced, based on the recent determination of column-density vs. impact parameter relation for the low-redshift gaseous galactic haloes. It is shown that observations of neutral hydrogen absorption around luminous galaxies are consistent with the index of the power-law column density distribution derived from statistical analyses of large samples of high-redshift Lyα forest lines.PACS: 98.62.Ra, 98.62.Gq, 98.80.Es This revised version was published online in July 2006 with corrections to the Cover Date.     

Vertical distribution of Galactic disc stars and gas constrained by a molecular cloud complex

We investigate the dynamical effects of a molecular cloud complex with a mass ∼ 10⁷ M_⊙ and a size ∼ a few 100 pc on the vertical distribution of stars and atomic hydrogen gas in a spiral galactic disc. Such massive complexes have now been observed in a number of spiral galaxies. The extended mass distribution in a complex, with an average mass density 6 times higher than the Oort limit, is shown to dominate the local gravitational field. This results in a significant redistribution or clustering of the surrounding disc components towards the mid-plane, with a resulting decrease in their vertical scaleheights.
The modified, self-consistent stellar density distribution is obtained by solving the combined Poisson equation and the force equation along the z -direction for an isothermal stellar disc on which the complex is imposed. The effect of the complex is strongest at its centre, where the stellar mid-plane density increases by a factor of 2.6 and the vertical scaleheight decreases by a factor of 3.4 compared with the undisturbed stellar disc. A surprising result is the large radial distance of ∼ 500 pc from the complex centre over which the complex influences the disc; this is due to the extended mass distribution in a complex. The complex has a comparable effect on the vertical distribution of the atomic hydrogen gas in the galactic disc. This 'pinching' or constraining effect should be detectable in the nearby spiral galaxies, as for example has been done for NGC 2403 by Sicking. Thus the gravitational field of a complex results in local corrugations of the stellar and H  i vertical scaleheights, and the galactic disc potential is highly non-uniform on scales of the intercomplex separation of ∼ 1 kpc.     

The two basic components of the interstellar neutral hydrogen and its relation with the galactic structure

The two basic components of the neutral hydrogen, cool dense clouds merged in a hotter tenuous medium, are studied using 21 cm absorption data of the Parkes Survey. The mean parameters obtained for the typical clouds next to the galactic plane are τ_p = 1.7, velocity half-width=3.3 km s^?1. Their temperatures areT _sc ≥40 K with a meanT _sc =63±12 K and the obtained hot gas density isn _HH=(0.15±0.05) atom cm^?3. Theoretical analysis following Giovanelli and Brown (1973) reveals that the pressure equilibrium condition (n _HH+2n _e )·T _SH≈n _HC·T _sc is compatible with the quoted values if it is assumed that the cosmic abundances in the interstellar medium are below the adopted normal solar abundance. This lack of heavy elements suggests accretion to grains which is consistent with the observed narrow concentration of the dark matter on the galactic layer (≤100 pc halfwidth). The same pressure condition leads to a mean cool cloud density ofn _HC～30 atom cm^?3 and a hot gas temperature ofT _SH～10 500 K. Comparison with data from Hii regions suggests that the cool clouds are somewhat denser and less extensive than such regions. An explanation for it is the expansion that the Hii regions went through in their origin. Comparison with 21 cm emission data shows that the cloud galactic layer is only about a quarter as thick as the hot gas layer. All the present results suggest that only such clouds can be spatially related with the typical I population associated with the spiral structure.     

Neutral Gas in Starbursts and Active Galactic Nuclei

High angular resolution decimetric observations of neutral gas in active galactic nuclei and starburst galaxies are reviewed. The neutral gas is mostly observed via atomic hydrogen absorption, or via maser emission from the hydroxyl radical (OH). The role of these observations in investigating the properties of neutral gas associated with the proposed dusty torus in AGN is discussed, together with the dynamical constraints to the mass of possible black holes in starburst galaxies.     

Neutral hydrogen absorption observations of the central region of NGC 3628

Using MERLIN with 0.2-arcsec resolution we have observed neutral hydrogen absorption against the central region of the starburst galaxy NGC 3628. The central region resolves into ∼16 continuum components at 1.4 GHz. From comparison with published 15-GHz data, we infer that these components are supernova remnants, although three components may be consistent with a weak active galactic nucleus. Neutral hydrogen absorption is seen against the continuum emission with column densities ∼10²² cm⁻². The absorption appears to be from two separate absorbing structures. Assuming a simple morphology, the main velocity structure can be attributed to a ring of neutral gas with a radius 130 pc rotating around a central starburst with a velocity gradient of 1270 km s⁻¹ kpc⁻¹. From simple assumptions, the mass interior to this ring is 0.9 × 10⁹ M_⊙. The second absorption structure may represent outflow from the starburst region or a large-scale galactic structure. Alternatively the absorption structure may be non-axisymmetric, such as a bar.     

Large-scale flow of interstellar gas in galactic spiral waves: Effects of thermal balance and self-gravitation

Using the recent observational data on atomic and molecular hydrogen in the Galaxy, we analyse the dynamics of the interstellar gas in a spiral density wave. Within the framework of Marochniket al.'s (1972) model of the galactic spiral structure, the gas flow is obtained, with self-gravitation and thermal processes taken into account.It is shown that: (1) the self-gravitation of gas does not practically affect the galactic shock if the dominant contribution into the gas density comes from atomic hydrogen; (2) the effects of self-gravitation could be essential for both the gas flow and the stellar spiral wave only if the density contribution of H₂ exceeded several times that ofHi, with molecular hydrogen as a continuous medium having the isothermal equation of state; (3) however, regardless of the estimates of H₂ abundance in the Galaxy, its reaction to the density wave is weak, since it forms a collisionless system not dragged by the intercloud gas.It has been found that, if we allow for thermal processes in the interstellar medium, new types of gas flow can develop alongside with a previously-known continuous flow and galactic shock. They are: (1) galactic shock with the phase transition leading to the formation of dense cold clouds; (2) a three-phase flow where hot cavities and dense cold clouds coexist with an initial, moderately dense and cold phase; (3) an accretion wave which is a specific type of nonlinear wave with an amplitude of 11/2 orders of magnitude larger than that of the isothermal galactic shock appearing under the same conditions, but without heating and cooling.     

宇宙线分布、氢分布和宇宙γ射线

本文利用几种典型的银河系宇宙线分布律和星际氢分布律计算单漏模式和双漏模式中的弥散宇宙γ射线谱。结果表明,几种典型的宇宙线分布中,李惕碚的分布律优于其他作者的分布律;星际氢分子数量的取值应当比Gordon值除以1.7更小;只要适当地选择宇宙线分布和氢分布就可得到与观测γ谱相近的理论谱,宇宙线分布和氢分布均可在一定范围里选取。     

Massive disks in low surface brightness galaxies

An update of the set of low surface brightness galaxies is presented which can be used to set constraints on the otherwise ambiguous decompositions of their rotation curves into contributions due to the various components of the galaxies. The selected galaxies show all clear spiral structure and arguments of density wave theory of galactic spiral arms are used to estimate the masses of the galactic disks. Again these estimates seem to indicate that the disks of low surface brightness galaxies might be much more massive than currently thought. This puzzling result contradicts stellar population synthesis models. This would mean also that low surface brightness galaxies are not dominated by dark matter in their inner parts. This revised version was published online in August 2006 with corrections to the Cover Date.     

Persistent evidence of a jovian mass solar companion in the Oort cloud

We present updated dynamical and statistical analyses of outer Oort cloud cometary evidence suggesting that the Sun has a wide-binary jovian mass companion. The results support a conjecture that there exists a companion of mass ≈ orbiting in the innermost region of the outer Oort cloud. Our most restrictive prediction is that the orientation angles of the orbit plane in galactic coordinates are centered on Ω, the galactic longitude of the ascending node = 319° and i, the galactic inclination = 103° (or the opposite direction) with an uncertainty in the orbit normal direction subtending <2% of the sky. Such a companion could also have produced the detached Kuiper Belt object Sedna. If the object exists, the absence of similar evidence in the inner Oort cloud implies that common beliefs about the origin of observed inner Oort cloud comets must be reconsidered. Evidence of the putative companion would have been recorded by the Wide-field Infrared Survey Explorer (WISE) which has completed its primary mission and is continuing on secondary objectives.     

Rotation curve of galaxies by the force induced by mass of moving particles

Many attempts have been made to explain the flat rotation curve of spiral galaxies regardless of distance from the center, in disagreement with the Newtonian prediction that this speed should diminish as the inverse square of distance. One explanation for this discrepancy is that the galaxy is embedded in dark matter, which interacts with baryonic matter only gravitationally. Many studies have focused on finding the distribution of this dark matter that fits well with observed data, but it is by definition undetectable by current technology, and must therefore remain hypothetical. Instead of dark matter, we propose a novel force, named mirinae force, generated by the mass of relatively-moving particles, and demonstrate that this force explains the rotation curve and evolution of a galaxy in which some of the inner mass of the supermassive black hole at the galactic center is assumed to be revolving at a relativistic speed. The calculation yielded important results that support the existence of mirinae force and validate the proposed model: First, the mirinae force explains why most of the matter is in the galactic disk and in circular motion which is similar to that of particles in a cyclotron. Second, the mirinae force explains well both the flat rotation curve and the varied slope of the rotation curve observed in spiral galaxies. Third, at the flat velocity of 220 km/s, the inner mass of the Milky Way calculated by using the proposed model is 6.0×10¹¹ M _⊙, which is very close to 5.5×10¹¹ M _⊙ (r<50 kpc, including Leo I) estimated by using the latest kinematic information.