Gas Dynamics in the Galactic Bar Region from N-Body and SPH Simulations

Self-consistent hybrid N-body and SPH simulations are used to give anew and coherent interpretation of the main features standing out from the HI and CO longitude–velocity observations within the Galactic bar. In particular, the traces of the gas associated with the Milky Way's dust-lanes can be reliably identified and the 3 kpc arm appears as a gaseous stream rather than a density wave. The bar and the gaseous nuclear ring in the simulations undergo synchronized and long-lived oscillations around the centre of mass, and the gas flow always remains non-stationary, suggesting a transient nature of the observed gas kinematics. This revised version was published online in July 2006 with corrections to the Cover Date.     

Kinematics and Mass Modelling of NGC 1068

We present the kinematics of the ionized gas over the inner 140″ (10 kpc) from observations with the HIFI Fabry-Perot interferometer. There is clear evidence for density wave streaming and bar-driven streaming across the field, with bi-symmetric arms that penetrate to within 200 pc of the nucleus. CO maps show linear structures along (although slightly offset from) the bar consistent with a strong shock. Along the spiral arms which encircle the bar, the H II regions lie downstream of the CO gas in the rest frame of the bar, as do the dust lanes, only if the gas outruns the stellar bar. As a first step towards understanding the details of the gas kinematics, and attempting to determine the mass inflow rate towards the nucleus, we build a mass model for the central disk constrained by near-infrared images. We plan to use this model as gravitational background potential for hydrodynamical simulations of the gas response to the bar. Comparing these with the data presented should enable us to constrain various quantities such as pattern speed, stellar mass-to-light ratio, central mass concentration, and gas fueling rate. This revised version was published online in July 2006 with corrections to the Cover Date.     

Star formation history in the central region of the barred galaxy NGC 7177

Using the method of two-dimensional spectroscopy, we have investigated the kinematics and distribution of the gas and stars at the center of the early-type spiral galaxy NGC 7177 with a mediumscale bar as well as the change in the mean age of the stellar population along the radius. A classical picture of radial gas inflow to the galactic center along the shock fronts delineated by dust concentration at the leading edges of the bar has been revealed. The gas inflow is observed down to a radius R = 1″.5−2″, where the gas flows at the inner Lindblad resonance concentrate in an azimuthally highly inhomogeneous nuclear star formation ring. The bar in NGC 7177 is shown to be thick in z coordinate—basically, it has already turned into a pseudo-bulge as a result of secular dynamical evolution. The mean stellar age inside the star formation ring, in the galactic nucleus, is old, ∼10 Gyr.Outside, at a distance R = 6″−8″ from the nucleus, the mean age of the stellar population is ∼2 Gyr. If we agree that the bar in NGC 7177 is old, then, obviously, the star formation ring has migrated radially inward in the last 1–2 Gyr, in accordance with the predictions of some dynamical models.     

Stellar Velocity Field in the Central Region of NGC 1068

We present the velocity field of the stars in the central 25″ × 22″ of NGC 1068 derived from 2-D spectroscopy of the Ca II triplet. A preliminary analysis provides evidence for two distinct stellar systems in the centre of NGC 1068. In the outer regions (say r > 10″), the mean stellar velocity field seems coupled to those of the ionized and molecular gas, indicating aproximately regular rotation with the kinematic minor axis at PA ~ 0°. However, in the inner region, the stars are rotating, whereas the ionized gas is outflowing in the NE-SW direction probably due to the effects of the nuclear activity, and the molecular gas is responding to the bar potential. This revised version was published online in July 2006 with corrections to the Cover Date.     

Kinematics of an Anaemic Cluster Galaxy

We present new HI and CO data including velocity information of NGC 4548, an anaemic galaxy in the Virgo Cluster. The distribution of the atomic gas shows a ring-like structure. The HI rotation curve is deduced and can be extrapolated inwards with the help of the CO data. The derived molecular fraction decreases continuously up to a radius of about 40″ showing a rather sharp transition between the molecular and the atomic disc. A three-dimensional visualization of the data cube permits us to detect a perturbation of the velocity field in the northern part of the galaxy. In a first approach we reconstruct the possible sites of HI emission with the help of a simple kinematical model in three dimensions. The reconstructed model shows a ridge which is believed to be due to ram-pressure stripping. In addition, we show first results of a dynamical model to simulate the interaction between the intra-cluster medium and the galaxy. This revised version was published online in July 2006 with corrections to the Cover Date.     

VLBI observations of high-opacity HI gas in NGC 5793

We report on observations, with sub-parsec resolution, of neutral hydrogen seen in absorption in the λ=21 cm line against the nucleus of the active spiral galaxy NGC 5793. The absorption line consists of three components separated in both location as well as velocity. We derive HI column densities of 2×10²² cm⁻² assuming a gas spin temperature of 100 K. For the first time we are able to reliably estimate the HI cloud sizes (≈15 pc) and atomic gas densities (≈200 cm⁻³). Our results suggest that the HI gas is not associated with the <10 pc region which presumably contains the H₂O masers, but it is more distant from the nucleus, and is probably associated with the r1 kpc gas seen in CO.     

Probing the physics of interacting galaxies

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.     

On-Going Galaxy Formation

We investigate the process of galaxy formation as can be observed in the only currently forming galaxies - the so-called Tidal Dwarf Galaxies, hereafter TDGs - through observations of the molecular gas detected via its CO (Carbon Monoxide) emission. These objects are formed of material torn off of the outer parts of a spiral disk due to tidal forces in a collision between two massive galaxies. Molecular gas is a key element in the galaxy formation process, providing the link between a cloud of gas and a bona fide galaxy. We have detected CO in 8 TDGs (Braine, Lisenfeld, Duc and Leon, 2000: Nature 403, 867; Braine, Duc, Lisenfeld, Charmandaris, Vallejo, Leon and Brinks: 2001, A&A 378, 51), with an overall detection rate of 80%, showing that molecular gas is abundant in TDGs, up to a few 10⁸ M _⊙. The CO emission coincides both spatially and kinematically with the HI emission, indicating that the molecular gas forms from the atomic hydrogen where the HI column density is high. A possible trend of more evolved TDGs having greater molecular gas masses is observed, in accord with the transformation of HI into H₂. Although TDGs share many of the properties of small irregulars, their CO luminosity is much greater (factor ∼ 100) than that of standard dwarf galaxies of comparable luminosity. This is most likely a consequence of the higher metallicity (≳sim 1/3 solar) of TDGs which makes CO a good tracer of molecular gas. This allows us to study star formation in environments ordinarily inaccessible due to the extreme difficulty of measuring the molecular gas mass. The star formation efficiency, measured by the CO luminosity per Hα flux, is the same in TDGs and full-sized spirals. CO is likely the best tracer of the dynamics of these objects because some fraction of the HI near the TDGs may be part of the tidal tail and not bound to the TDG. Although uncertainties are large for individual objects, as the geometry is unknown, our sample is now of eight detected objects and we find that the ‘dynamical’ masses of TDGs, estimated from the CO line widths, seem not to be greater than the ‘visible’ masses (HI + H₂ + a stellar component). Although higher spatial resolution CO (and HI) observations would help reduce the uncertainties, we find that TDGs require no dark matter, which would make them the only galaxy-sized systems where this is the case. Dark matter in spirals should then be in a halo and not a rotating disk. Most dwarf galaxies are dark matter-rich, implying that they are not of tidal origin. We provide strong evidence that TDGs are self-gravitating entities, implying that we are witnessing the ensemble of processes in galaxy formation: concentration of large amounts of gas in a bound object, condensation of the gas, which is atomic at this point, to form molecular gas and the subsequent star formation from the dense molecular component. This revised version was published online in September 2006 with corrections to the Cover Date.     

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.     

Methods and Results of an Optical Search for Extraterrestrial Civilizations

I summarize fully-sampled observations of the 3 mm emission from CO and HCN in the inner arcminute of NGC 1068. The CO emission is distributed in the form of a molecular bar, coincident with the infrared bar, from which emanate two spiral arms. A relatively weak concentration of CO line emission is found at the nucleus. By contrast, the HCN emission is strongly concentrated at the center, with relatively weak emission in the region of the star-forming arms. The ratio of HCN to CO integrated intensities is about 0.6 over the central r ≉ 175 pc and is the highest ratio measured in the center of any galaxy; the ratio reflects the high thermal pressure (TK ~ 50 K, n[H2] ~ 4 × 106 cm-3) in the few hundred parsecs surrounding the nucleus. The kinematics in the star-forming arms are well described by circular orbits, with ordered noncircular motions of < 30 km s-1 that may be attributed to spiral density wave streaming. Interior to the bar, noncircular motions dominate the gas kinematics. A model of the CO kinematics contrains any Inner Lindblad Resonance to be close to the location of the hundred-parsec scale HCN ‘disk’. At the nucleus, the spatially unresolved CO emission shows a triplet velocity structure characteristic of kinematically independent regions. This revised version was published online in July 2006 with corrections to the Cover Date.     

The starburst in the Wolf-Rayet nucleus of the Liner NGC 6764

We present high resolution millimeter, near-infrared, and optical data on the Wolf-Rayet nucleus of the Liner NGC 6764. The millimeter¹²CO(1-0) maps were obtained using the Nobeyama Millimeter Interferometer. Near-infrared images in the K-band continuum and the 2.12µm H₂, 2.06µm He I, 2.17µm Br, and 1.64µm [Fe II] lines were taken with the MPE imaging spectrometer FAST at the William Herschel Telescope on La Palma, Spain. The optical data were obtained at the 3.5m telescope on Calar Alto, Spain. The measurements indicate a strong concentration of molecular gas and a massive starburst at the nucleus of NGC 6764. The interferometric position velocity map of the nucleus shows the presence of distinct molecular cloud complexes with an apparently asymmetric velocity field shifted towards the blue with respect to the systemic velocity of 2420 km s^–1. The distribution of the 2.12µm H₂ line flux exhibits extensions approximately perpendicular to the bar which are in agreement with structural features in VLA radio maps and IRAM 30m maps of the¹²CO(2–1) line emission. This may represent evidence for combination of a nuclear outflow and a central oval distortion of gas predicted by gas dynamical calculations as a response to a bar like potential. A detailed investigation of the Wolf-Rayet-feature at 466 nm indicates that it is spatially extended on a scale of a few arcseconds.     

The warped gas and dust lane in NGC 3718

We present the first observations of molecular line emission in NGC 3718 with the IRAM 30m and the Plateau de Bure Interferometer. This galaxy is an excellent example for a strongly warped gas disk harboring an active galactic nucleus (AGN). An impressive dust lane is crossing the nucleus and a warp is developing into a polar ring. The molecular gas content is found to be typical of an elliptical galaxy with a relatively low molecular gas mass content (∼ 4 × 10⁸ M _⊙). The molecular gas distribution is found to warp from the inner disk together with the HI distribution. The CO data were also used to improve the kinematic modeling in the inner part of the galaxy, based on the so-called tilted ring-model. The nature of NGC 3718 is compared with its northern sky `twin' Centaurus A and the possible recent swallowing of a small-size gas-rich spiral is discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

A high galactic latitude HI 21 cm-line absorption survey using the GMRT: II. Results and interpretation

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 V_1sr ∼ 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.     

The BIMA Survey of Nearby Galaxies (BIMA SONG)

BIMA SONG is a systematic imaging study of the 3 mm CO J = 1 → 0molecular emission within the centres and discs of 44 nearby spiral galaxies on size scales of a few hundred parsecs (6-9"). The overall goal of the survey is to study the role of molecular gas in the evolution of spiral galaxies. To this end, BIMA SONG addresses 1) the distribution and physical conditions of the molecular gas in galactic discs and its relation to star formation, 2) the effects of a stellar bar on the kinematics of molecular gas, including the possible inflow of gas along a bar, and 3) the distribution and role of molecular gas in the central few hundred parsecs of active and quiescent galaxies. The source list includes all (except M33and M31) 44 galaxies of Hubble types Sa–Sd, with declinations δ >−20^°, visual magnitudes B < 11.0, velocities v _hel <2000 km s^-1, and inclinations i < 70^°. Beyond the specific scientific questions we will address, this survey will provide a unique database for astronomers who study galaxies at all wavelengths. 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.     

Modeling the Lyα radiation of high-redshift galaxies

The Lyα line emission of high-redshift galaxies depends on the density and temperature distribution of the gas, the kinematics and the dust content. We use a finite element method to model theLyα radiation of different 3D configurations considering complete frequency redistribution and the influence of velocity fields. Our results show that the central absorption feature of the double-peaked Lyα line profile observed in many radio galaxies with z=2-4 is probably the consequence of frequency redistribution rather than foreground absorption. The blue peak of the profile is enhanced for models with in fall motion and the red peak for models with outflow motion. In particular, we attempt to model the extendedLyα emission of high-redshift radio galaxies, where we consider results of corresponding hydrodynamical simulations to select possible model configurations. We find that Lyα photons scattered outside a jet-influenced low-density region are able to produce an extended Lyα halo. This revised version was published online in August 2006 with corrections to the Cover Date.     

A massive, dusty toroid with large grains in the pre-planetary nebula IRAS22036+5306

Using the Submillimeter Array (SMA), we have obtained high angular-resolution (∼1″) interferometric maps of the submillimeter (0.88 mm) continuum and CO J=3–2 line from IRAS 22036+5306 (I 22036), a bipolar pre-planetary nebula (PPN) with knotty jets discovered in our HST SNAPshot survey of young PPNe. In addition, we have obtained supporting lower-resolution (∼10″) 2.6 mm continuum and CO, ¹³CO J=1–0 observations with the Owens Valley Radio Observatory (OVRO) interferometer. We find an unresolved source of submillimeter (and millimeter-wave) continuum emission in I 22036, implying a very substantial mass (0.02–0.04M _⊙) of large (i.e., radius ≳1 mm), cold (≲50 K) dust grains associated with I 22036’s toroidal waist. The CO J=3–2 observations show the presence of a very fast (∼220 km s⁻¹), highly collimated, massive (0.03M _⊙) bipolar outflow with a very large scalar momentum (about 10³⁹ g cm s⁻¹), and the characteristic spatio-kinematic structure of bow-shocks at the tips of this outflow. The fast outflow in I 22036, as in most PPNe, cannot be driven by radiation pressure. The large mass of the torus suggests that it has most likely resulted from common-envelope evolution in a binary, however it remains to be seen whether or not the time-scales required for the growth of grains to millimeter sizes in the torus are commensurate with such a formation scenario. The presence of the torus should facilitate the formation of the accretion disk needed to launch the jet. We also find that the ¹³C/¹²C ratio in I 22036 is very high (0.16), close to the maximum value achieved in equilibrium CNO-nucleosynthesis (0.33). The combination of the high circumstellar mass (i.e., in the torus and an extended dust shell inferred from ISO far-infrared spectra) and the high ¹³C/¹²C ratio in I 22036 provides strong support for this object having evolved from a massive (≳4M _⊙) progenitor in which hot-bottom-burning has occurred.     

New Plateau de Bure observations of M 1–92; unveiling the core

M 1–92 is a very well studied bipolar pPN that can be considered an archetype of this type of sources; it shows a clear axial symmetry, along with the kinematics characteristic of this class of envelopes around post-AGB stars. We performed sub-arcsecond resolution observations of the J=2–1 rotational line of ¹³CO in M 1–92 with the new extended configurations of the IRAM Plateau de Bure interferometer, for studying the morphology and velocity field of the molecular gas better in the nebula, particularly in its central parts. We found that the equatorial structure dividing the two lobes is a thin flat disk, which expands radially with a velocity proportional to the distance to the central stellar system. The kinetic age of this equatorial flow is very similar to that measured in the two lobes, suggesting that the whole structure was formed as a result of a single event some 1200 yr ago, after which the nebula reached an expansion velocity field with axial symmetry. The small widths and velocity dispersion in the gas forming the lobe walls confirm that the acceleration responsible for the nebular shape could not last more than 100–120 yr. In view of the similarity to η Car, we speculate on the possibility that the whole nebula was formed as a result of a magneto-rotational explosion in a common-envelope system. The study of the possible importance of this mechanism in the context of global PNe and pPNe reshaping should be one on the fields in which future ALMA observations will make a crucial contribution. Based on observations carried out with the IRAM Plateau de Bure Interferometer. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain).     

The kinematics of Trapezium-type systems

Summary In this paper the results of the research of the stars proper motions Trapezium components are reported. They are: the galactic coordinates of the solar aprx and the Sun velocity (L _⊙=43±18°,B _⊙=+28±13°,V _⊙=13±4 km s⁻¹), the dispersion of peculiar velocities in the direction of the galactic coordinates for the above mentioned stars (σ _l =±11 km s⁻¹, σ _b =±7 km s⁻¹).The attained accuracy of the proper motions (±0.005″ yr⁻¹) is shown to be insufficient to the study of internal space motions in these systems. At present the work to increase the relative proper motions accuracy for multiple system components and to improve reductions from the relative to absolute proper motions, is being carried out in the Main Astronomical Observatory (Academy of Sciences of the Ukrainian SSR). The new catalogue of the AGK3 stars is composed now in the vicinity of the galactic equator in order to improve reductions from the relative to absolute proper motions. The r.m.s. errors of the proper motions, obtained in the AGK3 system, are ±0.005″ yr⁻¹.     

NGC 7479: A Case Study

The most salient features of the barred spiral galaxy NGC 7479 include the unusually strong and long bar, asymmetric spiral structure and peculiar dust lanes. The central, bar-dominated region has been robbed of neutral atomic gas. The neutral hydrogen kinematics of the strong western spiral arm are consistent with substantial non-circular motions. In contrast, the molecular gas is strongly concentrated in the nucleus and along the bar dust lanes. A molecular disc with near-circular motion is found in the nuclear area. Outside this component, the molecular gas has a strong radial velocity component consistent with inflow. The velocity gradients across the bar dust lanes show jumps of a few hundred km s^-1. A comparison of the dust/gas lane morphology between the observations and numerical simulations suggests that the corotation radius is at 1.1 times the bar length. I have modelled many of the peculiar morphological and kinematic features in numerical simulations of a minor merger. The predicted position of the merging companion matches the position of a bright clump in the bar with perturbed kinematics. This revised version was published online in July 2006 with corrections to the Cover Date.