Magnetic fields and chemical evolution in the disk of the Milky Way

An alternative non-infall model for the chemical evolution in the solar neighbourhood is proposed. The evolution of the disk is divided into two phases. In phaseA, the magnetic field and the gas viscosity produced an outward flux of gas, forming and maintaining the ring observed today. This flux balanced the star formation in the ring. The number of stars increased until the beginning of phaseB, during which stellar viscosity generated an inward flux of stars towards the inner disk, while the magnetic fields continued supplying gas to the ring. The combination of these two effects brought the ring to a quasi-steady state, with a constant mass of gas and stars which we assume has continued till the present. A coherent picture is obtained in which the observational restrictions are explained without introducing any arbitrary hypothesis. The inward flux of stars in phaseB has transported the metal-poor G-dwards to the inner region, thus explaining their absence in the solar neighbourhood.     

亮红外星系NGC 1614中多谱线示踪的致密分子气体

利用高空间分辨率的¹²CO(1-0)、¹³CO(1-0)、¹²CO(3-2)、¹²CO(6-5)、HCN(3-2)、\lk HCN(4-3)、 HCO⁺(3-2)和HCO⁺(4-3)分子谱线的ALMA (Atacama Large Millimeter/ submillimeter Array)归档数据, 来研究近邻亮红外星系NGC 1614的分子气体性质, 尤其是致密分子气体的性质. 在高分辨率分子气体谱线的积分强度图中, 在星系中心区域($<$ 1kpc)可以看到环状结构, 分子气体主要分布于星系中心区域, 核区分子气体含量较少. ¹²CO(1-0)显示出向南部、 北部以及东南部的延展结构, 高阶的CO ($J \ge$ 3, J为转振能级量子数)分子谱线和致密分子HCN、HCO⁺谱线显示, 较致密的分子气体主要集中于星系中心区域. HCN(4-3)/¹²CO(1-0)和HCN⁺(4-3)/¹²CO(1-0)积分强度比值图显示, 致密分子气体主要集中于中心区域的环状结构上. HCN/HCO⁺强度比值的分布变化表明星暴环的不同区域可能具有不同的激发条件. HCN/HCO⁺(4-3)强度比值分布在环的东、西部(sim0.44 pm 0.04)高于环的南、北部(sim0.35 pm 0.03). HCN/HCO⁺(3-2)强度比值较高的区域(sim0.38 pm 0.04)分布在HCN(3-2)峰值位置, 而环的西北、东南部强度比值相对较低(sim0.3 pm 0.03). 对于中心不同区域 HCN/HCO⁺比值变化的原因进行了讨论.     

Gas Flows and Star Formation in the Ringed Galaxy NGC 4736

We present high-resolution (∼5″) BIMA CO observations of the ringed galaxy NGC 4736, along with previously published VLA HI data (Braun, 1995). Strong CO emission is detected from the star-forming ring at r=45″ and in the central region, where a molecular bar is apparent. The azimuthally averaged gas surface density is still much less than the Toomre critical density within r=60″, despite the starburst conditions in the ring (gas depletion time ≲1Gyr). Both CO and HI velocity fields show strong departures from a circular rotating disc model. The velocity residuals are consistent with inflowing gas near the ends of the central bar, outflowing gas between the bar and the ring, and inflowing gas outside the ring. We propose that the high star formation efficiency in the ring results from gas being driven out towards the OLR of the bar and in towards the ILR of the larger oval distortion. However, the strong signature of inflow outside the ring is probably due in part to gas motion in elliptical orbits. This revised version was published online in July 2006 with corrections to the Cover Date.     

A study of the ultra-compact H-II region NGC 7538-IRS

From an analysis of VLBI observations of H₂CO and OH maser emission in the direction of the ultra-conpact HII region NGC 7538-IRS 1, the following model is proposed: The HII region is surrounded by a thick dusty shell which breaks open at the two poles and there is a bipolar outflow. Around it is a rotating gas/dust ring and matter falls from the ring onto the surface of the HII region. The whole system, HII region and the ring, moves with a sight line velocity of −61.0 km/s inside a large cloud which moves with a sight line velocity of −57 km/s. The H₂CO and OH masers occur near the poles of the HII region and within 0.2 R_HII of the surface. The positions of the H₂O maser and other line sources are discussed in term of this model.     

The effect of a central supermassive black hole on gas fuelling

When a supermassive black hole exists in the centre of a galaxy, an additional inner Lindblad resonance (ILR) exists inside the usual ILRs. We study gas dynamics in a weakly barred potential with a central supermassive black hole by using 2D numerical simulations, and we investigate the effect of the additional ILR on the fuelling of gas into nuclear starburst regions or active galactic nuclei (AGNs). Our numerical results show that strong trailing spiral shocks are formed at the resonance region, and that the gas in the shock region is rapidly fuelled into a central region and makes a nuclear gas ring. As a result, a large amount of gas is concentrated in the nuclear region beyond the ILR in a dynamical time-scale.     

The high-velocity molecular stream in W51B: a ring structure with compact H ii regions

We present ¹³ CO J  = 1 − 0 line observations of the H  ii region complex W51B located in the high-velocity (HV) stream. These observations reveal a filamentary and clumpy structure in the molecular gas. The mean local standard of rest (LSR) velocity ∼ + 65 km s⁻¹ of the molecular gas in this region is greater than the maximum velocities allowed by kinematic Galactic rotation curves. The size and mass of the molecular clouds are ∼ 48 × 17 pc² and ∼ 2.4 × 10⁵ M⊙ respectively. In a position–velocity diagram, molecular gas in the southern part comprises a redshifted ring structure with v _LSR=+ 60 to +73 km s⁻¹. The velocity gradient of this ring is ∼ 0.5 km s⁻¹ pc⁻¹, and the mass is ∼ 6.2 × 10⁴ M⊙. If we assume that the ring is expanding with a uniform velocity, the expansion velocity, radius and kinetic energy are ∼ 7 km s⁻¹, ∼ 13 pc and ∼ 3.0 × 10 ⁴⁹ erg respectively. The kinetic energy and mass spectrum of the ring could be explained by an expanding cylindrical cloud with a centrally condensed mass distribution. The locations of two compact H  ii regions, G49.0−0.3 and G48.9−0.3, coincide with the two molecular clumps in this ring. We discuss star formation, and the mechanism that produced the ring structure.     

Mass outflow and its consequences in the circumnuclear zones of galaxies

The distribution of hydrogen gas, atomic plus molecular, in the discs of spiral galaxies, takes two characteristic extreme alternative forms. In one, the density peaks at the nucleus, and falls radially monotonically and roughly exponentially with radius. In the other there is a hole in the gas distribution in the circumnuclear region. In this paper we examine the distributions of gas, and the kinematics in the central zones of a number of spirals which have been observed both spectroscopically and with photometric mapping. We find in addition to a ring structure in the gas, there is often measurable expansion with higher radial velocities occurring near the nucleus. Associated with the more expanded of these ring structures there appear annuli of younger stars with enhanced metallicities; and inside the gaseous ring older, less metallic populations. A strong correlation exists between the absence of central gas and the size of the nuclear bulge of a galaxy: Sa's and Sb's have lower ratios of gas to stars than Sc's and Sd's. We show that radially progressive bursts of star formation can account for a wide range of these observed phenomena and could be related to the presence ofliners in the interstellar medium close to the nucleus. The energetics and dynamical balance within the burst are considered in terms of three sources of outflow: supernovae, stellar winds, and radiation from massive stars, with the probability that all these mechanisms contribute to the collective phenomenon.     

Numerical modeling of ionian volcanic plumes with entrained particulates

Volcanic plumes on Jupiter's moon Io are modeled using the direct simulation Monte Carlo (DSMC) method. The modeled volcanic vent is interpreted as a “virtual” vent. A parametric study of the “virtual” vent gas temperature and velocity is performed to constrain the gas properties at the vent by observables, particularly the plume height and the surrounding condensate deposition ring radius. Also, the flow of refractory nano-size particulates entrained in the gas is modeled with “overlay” techniques which assume that the background gas flow is not altered by the particulates. The column density along the tangential line-of-sight and the shadow cast by the plume are calculated and compared with Voyager and Galileo images. The parametric study indicates that it is possible to obtain a unique solution for the vent temperature and velocity for a large plume like Pele. However, for a small Prometheus-type plume, several different possible combinations of vent temperature and velocity result in both the same shock height and peak deposition ring radius. Pele and Prometheus plume particulates are examined in detail. Encouraging matches with observations are obtained for each plume by varying both the gas and particle parameters. The calculated tangential gas column density of Pele agrees with that obtained from HST observations. An upper limit on the size of particles that track the gas flow well is found to be ∼10 nm, consistent with Voyager observations of Loki. While it is certainly possible for the plumes to contain refractory dust or pyroclastic particles, especially in the vent vicinity, we find that the conditions are favorable for SO₂ condensation into particles away from the vent vicinity for Prometheus. The shadow cast by Prometheus as seen in Galileo images is also reproduced by our simulation. A time averaged frost deposition profile is calculated for Prometheus in an effort to explain the multiple ring structure observed around the source region. However, this multiple ring structure may be better explained by the calculated deposition of entrained particles. The possibility of forming a dust cloud on Io is examined and, based on a lack of any such observed clouds, a subsolar frost temperature of less than 118 K is suggested.     

A spectroscopic study of the peculiar galaxy UGC 5600

We present our observations of the galaxy UGS 5600 with a long-slit spectrograph (UAGS) and a multipupil field spectrograph (MPFS) attached to the 6-m Special Astrophysical Observatory telescope. Radial-velocity fields of the stellar and gaseous components were constructed for the central region and inner ring of the galaxy. We proved the existence of two nearly orthogonal kinematic subsystems and conclude that UGC 5600 is a galaxy with an inner polar ring. In the circumnuclear region, we detected noncircular stellar motions and suspected the existence of a minibar. The emission lines are shown to originate in H II regions. We estimated the metallicity from the intensity ratio of the [N II]λ6583 and Hα lines to be nearly solar, which rules out the possibility that the polar ring was produced by the accretion of gas from a dwarf companion.     

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.     

Molecular Absorption in Cen a on Vlbi Scales

Centaurus A, the nearest AGN shows molecular absorption in the millimeter and radio regime. By observing the absorption with VLBI, we try to constrain the distribution of the gas, in particular whether it resides in the circumnuclear region. Analysis of VLBA observations in four OH and two H₂CO transitions is presented here, as well as molecular excitation models parameterized with distance from the AGN. We conclude that the gas is most likely associated with the tilted molecular ring structure observed before in molecular emission and IR continuum. The formaldehyde absorption shows small-scale absorption which requires a different distribution than the hydroxyl.     

Formation of planetary systems by successive contractions of the solar nebula

The theory discussed in the present paper is a solar nebula-type theory which assumes the initial existence of a big disk-shaped gas cloud in rotational motion around the Sun. At the outer edge of the gas cloud there is a steady loss of angular momentum, which is mainly caused by the diffusion induced by turbulence and shock waves. This leads to the formation of a doughnutshaped gas ring at the edge of the cloud, outside of which there is plasma in a state of partial corotation. The gas ring is then slowly shifted towards the Sun, whereby the grains of solid matter within the gas cloud are also transported and collected within the gas torus. During the contraction process the following two situations arise: First, due to the small amount of friction, the angular momentum of the inner part of the ring rapidly exceeds that of the outer part. Second, the angle between the orbits of the inner and outer part of the gas ring increases gradually. When, during contraction, a certain distance is covered, the gas ring turns over, i.e. there is a sudden interchange of the inner and outer parts of the gas ring, where two adjacent rings of solid matter (jet streams) are formed. Immediately after the turn-over process the speed of contraction is at first drastically reduced, but then the gas ring is shifted once more towards the Sun. This process is then repeated periodically. The planets originate from the outer rings of solid matter, which contain much more matter than their adjacent inner rings. The inclination between the inner and outer rings is roughly 5°. In particular, Mercury, the Moon, Titan as well as Triton result from the innermost rings of matter. Having gone through the formation process, most of the planets acquire a rotating gas disk out of which the regular satellites are also created by the same periodic contraction process (hetegonic principle). This theory is the first that can explain all noteworthy facts about our planetary system and the satellite systems in a qualitative yet conclusive way.     

Structure and kinematics of the interstellar medium in the star-forming region in the BCD galaxy VII Zw 403 (UGC 6456)

The structure and kinematics of ionized supershells in the star-forming region in the BCD galaxy VII Zw 403 (UGC 6456) are analyzed using observations with the SCORPIO focal reducer on the 6-m Special Astrophysical Observatory telescope in three modes: direct imaging (in the Hα, [O III], and [S II] lines), long-slit spectroscopy, and spectroscopy with a scanning Fabry-Perot interferometer. In addition to the previously known bright H II regions and the faint giant ring that surrounds the entire starforming region, many new faint diffuse and arc structures have been detected. A fine structure of the giant ring has been revealed. We do not confirm the previously detected expansion of the bright shells around young stellar associations with a velocity of 50–70 km s^?1. We have estimated their expansion velocities to be no higher than 15–20 km s^?1; the corresponding kinematic age, no younger than 3–4 Myr, agrees well with the age of the compact OB associations associated with them. We correlate the faint extended filamentary and diffuse regions of ionized gas identified almost in the entire central region of the galaxy and the giant H II ring with the older (10 Myr) stellar population of the most recent starburst. Weak high-velocity [O III] and Hα line wings (up to 300 km s^?1 from the line center) have been detected in the brightest H II region. Such velocities have been observed in the galaxy for the first time. The previously published Hα luminosity measurements for the galaxy are refined.     

Recurrent explosions in the nuclei of galaxies

High-velocity ejection of gas from the central region of galaxies is now an observationally established phenomenon. Such ejections have been attributed to some kind of activities in the nuclei of galaxies. It has been suggested that conditions leading to explosive events periodically prevail in the centre of galaxies causing recurrent explosions and driving the gas thereby outward with sufficiently high velocities. The magnitude of the ejection velocity and the amount of gas driven out will actually depend on the intensity of the activity at the centre. Remnants of recurrent activity have been discovered in the inner region of our Galaxy. The ‘3-kpc’ arm, the 2.4 kpc arm, the molecular ring at 270 pc and some other features are believed to have been caused by periodic activity at the centre of our Galaxy. We have outlined a model that can explain the recurrent explosions in the centre of a galaxy. The boundary of the nucleus of the Galaxy is considered here as a stationary shock front where high velocity gas coming from the outer regions impinges and gets heated and condensed. This condensed, hot gas then flows inwards by intense gravitational pull, but in course of its passage inward it loses its velocity due to radiation pressure and frictional retardation. A layer of dense, hot gas is therefore formed some distance (typically 0.001 pc) away from the centre where short radio and microwaves are trapped. As the density of gas in this layer is enhanced by the inflowing gas, shorter-wave radiation is trapped. The pressure of radiation therefore gradually builds up in the layer which ultimately overcomes the gravitational pull and the layer is blown off violently. The whole process may be completed over and over again at intervals of 10⁶–10⁷ yr.     

12CO, 13CO and HCN in the Barred Galaxy NGC 1530

Bars probably have a great importance in galactic evolution. The barred potential is able to concentrate large quantities of interstellar gas in the vicinity of the nucleus, feeding any nuclear activity, be it central starbursts or black hole accretion discs. The IRAM millimeter interferometer and 30 m telescope have allowed a precise analysis of the molecular gas in the bar and the nucleus of a typical barred spiral galaxy, NGC 1530. In this galaxy, I have detected CO(1→0) along two lanes that trace shocks in the molecular gas. In these lanes, the gas moves toward the centre of the galaxy, with typical in fall velocities of 100 km s^-1. I have shown in these shocks an anticorrelation between shear in the gas and star formation efficiency by comparing H^α and CO maps. I have also studied the centre of this galaxy at higher resolution in¹²CO(1→0), ¹²CO(2→1),¹³CO(1→0) and HCN(1→0). In the central region, the gas distribution is a ring or an unresolved spiral, surrounded by two curved arcs. The nuclear ring contains large amounts of dense gas traced by HCN and ¹³CO, and shows intense star formation, as indicated by the non-thermal centimetre continuum. The arcs, in contrast, are poor in dense gas and form few stars. This revised version was published online in July 2006 with corrections to the Cover Date.     

Observational evidence for AGN fueling: I. The case of NGC 6104—Merging with a companion

We investigate in detail the kinematics and morphology of the Seyfert galaxy NGC 6104 in order to identify the mechanism of gas transportation to the active galactic nucleus (AGN). Our observational data were obtained at the 6-m Special Astrophysical Observatory telescope with the MPFS integral-field spectrograph and the SCORPIO universal device in three modes: direct imaging, a scanning Fabry—Perot interferometer, and long-slit spectroscopy. Images from the HST archive were invoked to study the structure of the circumnuclear region. An analysis of deep images has shown for the first time that NGC 6104 is in the phase of active merging with a companion galaxy. We have been able to study the detailed picture of ionized gas motions up to galactocentric distances of 14 kpc and to construct the stellar velocity field for the inner region. The radial gas motions toward the AGN along the central bar play a significant role at galactocentric distances of 1–5 kpc. In addition, we have detected an outflow of ionized gas from the nucleus that presumably resulted from the intrusion of a radio jet into the ambient interstellar medium. Using diagnostic diagrams, we estimate the contributions from the AGN and star formation to the galactic gas ionization. We estimate the bar pattern speed by the Tremaine-Weinberg method and show that the inner ring observed in the galaxy’s images has a resonant nature. Two possible ring formation scenarios, before and during the interaction with a companion, are discussed.     

A radio study of neutral gas and dust in the radio galaxy 3C 293

We have observed broad H  i absorption in the radio galaxy 3C 293 using Multi-Element Radio Linked Interferometric Network (MERLIN) at 0.2-arcsec angular resolution and the Giant Meterwavelength Radio Telescope (GMRT) at arcsec resolution. Extensive H  i is found in absorption across the centre of this peculiar radio galaxy, allowing a detailed study of the dynamics of the neutral gas on linear scales down to ∼160 pc. In optical depth position–velocity diagrams across the central few kpc we detect a distinct velocity gradient of 179 km s⁻¹ arcsec⁻¹ associated with the broad absorption. This is interpreted as a ring of neutral gas rotating around the suspected position of the active galactic nucleus (AGN) . The radius of this high velocity gradient ring is found to be >0.74 arcsec (600 pc), implying an upper limit upon the enclosed mass of     , assuming a near edge-on disc with an inclination of i . The optical depth of H  i is mapped across the entire central region of 3C 293 showing enhancements of a factor of 4 in the areas that are co-spatial with dust lanes seen in Hubble Space Telescope ( HST ) imaging of this galaxy.     

Multifrequency study of the ring nebula SG 13

We investigate the morphology and kinematics of the interstellar medium in the environs of the open cluster Mrk 50, which includes the Wolf–Rayet star WR 157 and a number of early B-type stars. The analysis was performed using radio continuum images at 408 and 1420 MHz, and H  i 21-cm line data taken from the Canadian Galactic Plane Survey, molecular observations of the ¹²CO  ( J = 1 → 0)  line at 115 GHz from the Five College Radio Astronomy Observatory and available mid- and far-infrared (FIR) observations obtained with the Midcourse Space Experiment and IRAS satellites, respectively.
This study allowed the identification of the radio continuum and molecular counterpart of the ring nebula SG 13, while no neutral atomic structure was found to be associated. The nebula is also detected in the images in the mid- and FIR, showing the existence of dust well mixed with the ionized gas. We estimate the main physical parameters of the material linked to the nebula.
The interstellar gas distribution in the environs of Mrk 50 is compatible with a stellar wind bubble created by the mass loss from WR 157.
The distribution of young stellar object candidates in the region shows that the stellar formation activity may be present in the molecular shell that encircles the ring nebula.     

A photometric study of the double ring structure of NGC 4736

In this study we present photometric results for the galaxy NGC 4736: infrared and visible profiles. After a careful correction for the extinction within the galaxy based on measured neutral gas surface densities, we interpret the profiles in the individual bands and in colour indices, in terms of the radial distribution of stellar populations. We pick out the behaviour of the two rings, an inner ring some 40–50 arc sec from the nucleus, and an outer ring some 300 arc sec away. We show how the photometry allows us to make tentative physical inferences about the nature of these two structures, showing that the inner ring is connected with an outflow of gas observed via itsHii regions, and is probably the result of an axisymmetric starburst, while the outer ring is a site of star formation which appears to be further from the centre than the typical resonant structures associated with a density wave.