A New Interpretation of the Bipolar HII Region S106 from HCN J = 3 - 2 Mapping Observations

The first mapping observations of the bipolar HII region S106 in HCN J = 3 - 2 line were made by KOSMA submillimeter telescope in April, 2004. The results show that there is a bipolar outflow centered on the high-mass star S106 IRS4 and that the flat structure of molecular cloud core is perpendicular to the axis of the outflow. This image roughly corresponds to the optical image where a dark lane bisects the bipolar HII region. Together with the optical, infrared and radio data, we conclude that the central UC HII region and molecular outflow formed before the     

Accumulation of mass in accelerating self-gravitating gas layers

The two-dimensional motions of a plane layer of self-gravitating gas accelerated by the pressure difference on both sides of the layer have been simulated numerically. The change in the shape of the layer surface has been found to depend significantly on the ratio of gravity and the pressure difference. In the region where the forces of gravitational attraction between the particles of the medium are greater than or of the order of the pressure difference on the layer sides, the structure of the condensations is such that the differences in inhomogeneity scales in the longitudinal and transverse directions with respect to the unperturbed motion decrease significantly compared to the Rayleigh-Taylor instability. Allowance for the self-gravity leads to a considerable increase in the maximum density in the layer. It is shown that the instability of the shell formed during the propagation of the ionization-shock front can be responsible for the emergence of an inhomogeneous structure of the HII region RCW 82, with the instability growth time being less than the estimated age of the HII region.     

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.     

NGC 7538—IRS1致密HⅡ区研究

本文在分析研究NGC7538-IRS1致密HⅡ区H_2CO和OH脉泽辐射VLBI观测结果的基础上,指出该HⅡ区合理的模型是:HⅡ区表面为厚的尘埃层包围,尘埃层两极已被突破,并形成双极流;HⅡ区外面有一个环形转动气体-尘埃云,存在由环向HⅡ区表面的物质下落;包括环和HⅡ区在内的整个系统视向速度为-61km/s,该系统居于视向速度为-57km/s的更大分子云中。H_2CO和OH脉泽发生在HⅡ区两极附近离HⅡ区表面小于0.2R_(HⅡ)的区域内。利用上述模型,还讨论了H_2O脉泽及其他分子吸收线和发射线的发生区域。     

A Kinematical Study of the NGC 7538 IRS 1 Region

We present high angular resolution images of both NH3(1,1)and (2,2) lines toward NGC 7538 IRS 1.The density and velocity-position plots have been used to study the interaction among the outflows,winds and their environment.For the first time we have found an expanding half-shell of molecular gas around the HⅡ region associated with IRS 1,which may be produced by the interaction of the bipolar outflows and the winds originating in IRS 1-3,and optical HⅡ region NGC 7538 with ambient molecular gas.     

Are HVCs Produced in Galactic Fountains?

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.     

Bubbles, feedback and the intracluster medium: three-dimensional hydrodynamic simulations

We use a three-dimensional hydrodynamical code to simulate the effect of energy injection on cooling flows in the intracluster medium. Specifically, we compare a simulation of a 10¹⁵-M_⊙ cluster with radiative cooling only with a second simulation in which thermal energy is injected 31 kpc off-centre, over 64 kpc³ at a rate of     for 50 Myr. The heat injection forms a hot, low-density bubble which quickly rises, dragging behind it material from the cluster core. The rising bubble pushes with it a shell of gas which expands and cools. We find the appearance of the bubble in X-ray temperature and luminosity to be in good qualitative agreement with recent Chandra observations of cluster cores. Toward the end of the simulation, at 600 Myr, the displaced gas begins to fall back toward the core, and the subsequent turbulence is very efficient at mixing the low- and high-entropy gas. The result is that the cooling flow is disrupted for up to ∼ 50 Myr after the injection of energy ceases. Thus this mechanism provides a very efficient method for regulating cooling flows, if the injection events occur with a 1:1 duty cycle.     

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.     

Photoionising shocks in SNRs and AGN

A high velocity radiative shock, or one moving into high-metallicity gas, provides an efficient means to generate a strong local UV photon field. The optical emission from the shock and precursor region is dominated by the photoionised gas, rather than by the cooling region, and the total optical + UV emission scales as the mechanical energy flux through the shock. In this paper, such models are applied to oxygen-rich supernova remnants and AGN. For AGN, the degree of magnetic support in the post-shock gas is an important parameter. LINER and cooling flow spectra can be understood as resulting from high velocity shocks without precursors, while Seyfert 1.5–2 galaxy emission line ratios result from high velocity shocks with their photoionised precursor HII regions. This model explains the problem of the high electron temperatures observed in both classes of object.     

The Luminosity Function and Size Distribution of HII Regions in NGC 1365

We present the HII region luminosity function and size distribution obtained for the HII regions in NGC 1365. The diameters were corrected for the effects of seeing using a Wiener image restoration procedure. The luminosities and sizes were calculated using the Cepheid distance of 18±2 Mpc, to NGC 1365. We find good correlation between the H_α luminosities and other properties, such as absolute magnitudes and sizes of the HII regions within this galaxy. We find that the HII region luminosity function can be fitted by a power law of slope α=–2.29$plusmn;0.11, which is consistent with what is expected for its Hubble type. Also, the size distribution of the HII regions can be represented by an exponential form. This revised version was published online in July 2006 with corrections to the Cover Date.     

A Closer View of the Nucleus of NGC 4314

We present a high-resolution kinematical study of the ionized gas in the circumnuclear region of NGC 4314. Our spectra reveal the presence of a central structure (apparently a ring or torus) located at ∼ 2 arcsec from the nucleus, which we identify as the shocked interface between the nuclear bar and the gas inside the HII region ring. This revised version was published online in July 2006 with corrections to the Cover Date.     

Nonisothermal Wave Motions in the HII Regions

The propagation of the linear and nonlinear waves in the HII regions is considered. The relaxation processes to the temperature and ionization fraction are taken into account. It is shown, that the essential damping of the waves takes place. The motion at long wavelengths is not isothermal and the velocity perturbations should be accompanied with the noticeable temperature variations. Instability of the traveling waves in the partially ionized hydrogen is possible. This effect may be important in the gas layer between ionization and shock fronts. It is found out, that the nonlinear waves can be generated by the ionization-shock front. This revised version was published online in July 2006 with corrections to the Cover Date.     

Uplifted cool gas and heating by mixing in cooling flows

We analyze our earlier three-dimensional hydrodynamical numerical simulation of jet-inflated bubbles in cooling flow clusters, and find that dense gas that was not heated by the jets' activity and that resides around the hot jet-inflated bubbles can be identified as uplifted gas as observed in some clusters.During the build up of the dense gas around the hot bubble, mixing of hot bubble gas with other regions of the intracluster medium(ICM) heats the ICM. The vortices that mix the ICM with the hot bubble gas also excite shock waves, sound waves and turbulence. Sound waves, shocks, turbulence and uplifted gas might be easier to detect than the mixing process and hence attract more attention, but we argue that the contributions of these processes to the heating of the ICM do not add up to the level of contribution of the mixing-heating process.     

Hα emission from late type be stars

We show here that the Hα flux from late type Be stars can be explained as emission from an HII region formed in the gas envelope around the Be star, by the UV flux emitted by a helium star binary companion. We also discuss the observability of the helium star companions.     

Gemini GMOS/integral field unit spectroscopy of NGC 1569 – II. Mapping the roots of the galactic outflow

We present a set of four Gemini-North Multi-Object Spectrograph/integral field unit (IFU) observations of the central disturbed regions of the dwarf irregular starburst galaxy NGC 1569, surrounding the well-known superstar clusters A and B. This continues on directly from a companion paper, in which we describe the data reduction and analysis techniques employed and present the analysis of one of the IFU pointings. By decomposing the emission-line profiles across the IFU fields, we map out the properties of each individual component identified and identify a number of relationships and correlations that allow us to investigate in detail the state of the ionized interstellar medium (ISM). Our observations support and expand on the main findings from the analysis of the first IFU position, where we conclude that a broad (≲400 km s⁻¹) component underlying the bright nebular emission lines is produced in a turbulent mixing layer on the surface of cool gas knots, set up by the impact of the fast-flowing cluster winds. We discuss the kinematic, electron-density and excitation maps of each region in detail and compare our results to previous studies. Our analysis reveals a very complex environment with many overlapping and superimposed components, including dissolving gas knots, rapidly expanding shocked shells and embedded ionizing sources, but no evidence for organized bulk motions. We conclude that the four IFU positions presented here lie well within the starburst region where energy is injected, and, from the lack of substantial ordered gas flows, within the quasi-hydrostatic zone of the wind interior to the sonic point. The net outflow occurs at radii beyond 100–200 pc, but our data imply that mass-loading of the hot ISM is active even at the roots of the wind.     

Nonlinear Dynamics of Ionization Fronts in HII Regions

Hydrodynamic instability of an accelerating ionization front (IF) is investigated with 2D hydrodynamic simulations, including absorption of incident photoionizing photons, recombination in the HII region, and radiative molecular cooling. When the amplitude of the perturbation is large enough, nonlinear dynamics of the IF triggered by the separation of the IF from the cloud surface is observed. This causes the second harmonic of the imposed perturbation to appear on the cloud surfaces, whereas the perturbation in density of ablated gas in the HII region remains largely single mode. This mismatch of modes between the IF and the density perturbation in the HII region prevents the strong stabilization effect seen in the linear regime. Large growth of the perturbation caused by Rayleigh-Taylor-like instability is observed late in time.     

GMRT and VLA observations at 49 cm and 20 cm of the HII region near <Emphasis Type="Italic">l</Emphasis> = 24.8°, <Emphasis Type="Italic">b</Emphasis> = 0.1°

We report multi-frequency radio continuum and hydrogen radio recombination line observations of HII regions near l = 24.8°, b = 0.1° using the Giant Metrewave Radio Telescope (GMRT) at 1.28 GHz (n = 172), 0.61 GHz (n = 220) and the Very Large Array (VLA) at 1.42 GHz (n = 166). The region consists of a large number of resolved HII regions and a few compact HII regions as seen in our continuum maps, many of which have associated infrared (IR) point sources. The largest HII region at l = 24.83° and b = 0.1° is a few arcmins in size and has a shell-type morphology. It is a massive HII region enclosing ∼550 M_⊙ with a linear size of 7 pc and an rms electron density of ∼110 cm⁻³ at a kinematic distance of 6 kpc. The required ionization can be provided by a single star of spectral type O5.5. We also report detection of hydrogen recombination lines from the HII region at l = 24.83° and b = 0.1° at all observed frequencies near V _lsr = 100 km s⁻¹. We model the observed integrated line flux density as arising in the diffuse HII region and find that the best fitting model has an electron density comparable to that derived from the continuum. We also report detection of hydrogen recombination lines from two other HII regions in the field.     

A Search for Warm Neutral Gas Associated with the Giant HII Region NGC 604

The characteristics of the halo gas and its interaction with the galactic disk in spiral galaxies are poorly known; this is particularly true for the warm neutral gas associated with HVCs and galactic chimneys. The detection of absorption features such as the NaI D or the CaII (H,K)lines is instrumental to study its detailed physical properties but requires very long integration times. In this work very deep optical spectra of NGC 604, the brightest giant HII region among the nearby spirals, are presented. The detection of two absorption components at LSR velocities -255 km/s and −20 km/s respectively, is reported; the first component is associated with the HII region. The inferred line width after deconvolution is 155 km/s; this large width is produced by the blending of the multiple absorption components produced by the diverse sources of internal motion (expanding shells and general turbulence). The radial velocity of the CaII absorption is slightly larger than the measured in the HII emission lines suggesting a possible flow of gas into the halo above the young star cluster. The large ratio x = W _λ (NaID_2)/W _λ (CaIIK) = 0.7indicates the probable presence of shocks which release Ca from the dust grains into the gas phase. The lower velocity component most likely trace galactic gas. This revised version was published online in July 2006 with corrections to the Cover Date.     

Internal motions of HII regions and giant HII regions

We report new echelle observations of the kinematics of 30 HII regions in the LMC, including the 30 Doradus giant HII region. All of the HII regions possess supersonic velocity dispersions, which can be attributed to a combination of turbulent motions and discrete velocity splitting produced by stellar winds and/or embedded supernova remnants (SNRs). The core of 30 Dor is unique, with a complex velocity structure that parallels its chaotic optical morphology. We use our calibrated echelle data to measure the physical properties and energetic requirements of these velocity structures. The most spectacular structures in 30 Dor are several fast expanding shells, which appear to be produced at least partially by SNRs.     

Study of the HII regions in the spiral galaxy NGC6384

The galaxy NGC6384 has been observed with an IPCS through Hα and [NII] narrow-band interference filters for direct imagery with the 2.6-m Byurakan telescope. We studied the main physical parameters of the identified 98 HII regions, their diameter and luminosity functions, as well the [NII]/Hα ratio distribution. The integrated distribution function of the HII region diameters can be well fitted by the exponential function. The characteristic diameter has the value (Do = 217 pc) predicted for a galaxy of its measured luminosity. The luminosity function of HII regions has a double power law profile with relatively shallow slope at low luminosities (α = −0.4), an abrupt turnover at logL(Hα) ≈ 38.75, and sharper slope at higher luminosities (α = −2.3). The correlation between the luminosity and diameter of HII regions confirms that in general they are constant density, radiation-bound systems. [NII]/Hα ratio data for the HII regions show that there is a negative radial gradient of [NII]/Hα. In the central region of the galaxy, nitrogen abundance is higher than in the periphery. The properties of the HII region population of this AGN galaxy do not differ significantly from the properties of the HII region population of the “normal” galaxies. Reexamining the location of the type Ia SN 1971L in the galaxy, we confirm that it lies on the spiral arm at about 8″.6 far from the closest HII region N 53 (F81). Such a location can be taken as proof that the progenitor of this SN does not belong to an old, evolved stellar population. Published in Astrofizika, Vol. 50, No. 4, pp. 519–533 (November 2007).