Gaseous Bar Research Articles

Detecting a disc bending wave in a barred-spiral galaxy at redshift 4.4

ABSTRACT The recent discovery of barred spiral galaxies in the early Universe (z > 2) poses questions of how these structures form and how they influence galaxy evolution in the early Universe. In this study, we investigate the morphology and kinematics of the far-infrared (FIR) continuum and [C ii] emission in BRI1335-0417 at z ≈ 4.4 from ALMA observations. The variations in position angle and ellipticity of the isophotes show the characteristic signature of a barred galaxy. The bar, $3.3^{+0.2}_{-0.2}$ kpc long in radius and bridging the previously identified two-armed spiral, is evident in both [C ii] and FIR images, driving the galaxy’s rapid evolution by channelling gas towards the nucleus. Fourier analysis of the [C ii] velocity field reveals an unambiguous kinematic m = 2 mode with a line-of-sight velocity amplitude of up to ∼30–40 km s−1; a plausible explanation is the disc’s vertical bending mode triggered by external perturbation, which presumably induced the high star formation rate and the bar/spiral structure. The bar identified in [C ii] and FIR images of the gas-rich disc galaxy (≳ 70 per cent of the total mass within radius R ≈ 2.2 disc scale lengths) suggests a new perspective of early bar formation in high redshift gas-rich galaxies – a gravitationally unstable gas-rich disc creating a star-forming gaseous bar, rather than a stellar bar emerging from a pre-existing stellar disc. This may explain the prevalent bar-like structures seen in FIR images of high-redshift submillimeter galaxies.

The Smallest Scale of Hierarchy Survey (SSH) – II. Extended star formation and bar-like features in the dwarf galaxy NGC 3741: recent merger or ongoing gas accretion?

ABSTRACTUsing Large Binocular Telescope deep imaging data from the Smallest Scale of Hierarchy Survey (SSH) and archival Hubble Space Telescope data, we reveal the presence of two elongated stellar features contiguous to a bar-like stellar structure in the inner regions of the dwarf irregular galaxy NGC 3741. These structures are dominated by stars younger than a few hundred Myr and collectively are about twice as extended as the old stellar component. These properties are very unusual for dwarf galaxies in the nearby Universe and difficult to explain by hydro-dynamical simulations. From the analysis of archival 21-cm observations, we find that the young stellar ‘bar’ coincides with an H i high-density region proposed by previous studies to be a purely gaseous bar; we furthermore confirm radial motions of a few km s−1, compatible with an inflow/outflow, and derive a steeply rising rotation curve and high H i surface density at the centre, indicating a very concentrated mass distribution. We propose that the peculiar properties of the stellar and gaseous components of NGC 3741 may be explained by a recent merger or ongoing gas accretion from the intergalactic medium, which caused gas inflows towards the galaxy centre and triggered star formation a few hundred Myr ago. This event may explain the young and extended stellar features, the bar-like structure, the very extended H i disc and the central H i spiral arms. The high central H i density and the steeply rising rotation curve suggest that NGC 3741 may be the progenitor or the descendant of a starburst dwarf.

Molecular line emission in NGC 4945, imaged with ALMA

NGC 4945 is one of the nearest (D ≈ 3.8 Mpc; 1″ ≈ 19 pc) starburst galaxies. To investigate the structure, dynamics, and composition of the dense nuclear gas of this galaxy, ALMA band 3 (λ ≈ 3−4 mm) observations were carried out with ≈2″ resolution. Three HCN and two HCO+ isotopologues, CS, C3H2, SiO, HCO, and CH3C2H were measured. Spectral line imaging demonstrates the presence of a rotating nuclear disk of projected size 10″ × 2″ reaching out to a galactocentric radius of r ≈ 100 pc with position angle PA = 45° ± 2°, inclination i = 75° ± 2° and an unresolved bright central core of size ≲2″. The continuum source, representing mostly free-free radiation from star forming regions, is more compact than the nuclear disk by a linear factor of two but shows the same position angle and is centered 0.′′39 ± 0.′′14 northeast of the nuclear accretion disk defined by H2O maser emission. Near the systemic velocity but outside the nuclear disk, both HCN J = 1 → 0 and CS J = 2 → 1 delineate molecular arms of length ≳15″ (≳285 pc) on opposite sides of the dynamical center. These are connected by a (deprojected) ≈ 0.6 kpc sized molecular bridge, likely a dense gaseous bar seen almost ends-on, shifting gas from the front and back side into the nuclear disk. Modeling this nuclear disk located farther inside (r ≲100 pc) with tilted rings provides a good fit by inferring a coplanar outflow reaching a characteristic deprojected velocity of ≈50 km s−1. All our molecular lines, with the notable exception of CH3 C2H, show significant absorption near the systemic velocity (≈571 km s−1), within the range ≈500–660 km s−1. Apparently, only molecular transitions with low critical H2 density (ncrit ≲ 104 cm−3) do not show absorption. The velocity field of the nuclear disk, derived from CH3 C2H, provides evidence for rigid rotation in the inner few arcseconds and a dynamical mass of Mtot = (2.1 ± 0.2) × 108 M⊙ inside a galactocentric radius of 2.′′45 (≈45 pc), with a significantly flattened rotation curve farther out. Velocity integrated line intensity maps with most pronounced absorption show molecular peak positions up to ≈1.′′5 (≈30 pc) southwest of the continuum peak, presumably due to absorption, which appears to be most severe slightly northeast of the nuclear maser disk. A nitrogen isotope ratio of 14N/15N ≈ 200–450 is estimated. This range of values is much higher then previously reported on a tentative basis. Therefore, because 15N is less abundant than expected, the question for strong 15N enrichment by massive star ejecta in starbursts still remains to be settled.

ASTE CO(3-2) Observations of the Southern Barred Spiral Galaxy NGC 986: a Large Gaseous Bar Filled with a Dense Molecular Medium

Abstract We present CO(3-2) emission observations toward the 3′$\times$3′ (or 20kpc$\times$20kpc at a distance of 23Mpc) region of the southern barred spiral galaxy NGC 986 using the Atacama Submillimeter Telescope Experiment (ASTE). This effort is a part of our on-going extragalactic CO(3-2) imaging project, ADIoS (ASTE Dense gas Imaging of Spiral galaxies). Our CO(3-2) image revealed the presence of a large (the major axis is 14kpc in total length) gaseous bar filled with a dense molecular medium along the dark lanes observed in optical images. This is the largest “dense-gas rich bar” known to date. The dense gas bar, discovered in NGC 986, could be a huge reservoir of possible “fuel” for future starbursts in the central region, and we suggest that star formation in the central region of NGC 986 could still be in a growing phase. We found a good spatial coincidence between the overall distributions of dense molecular gas traced by CO(3-2) and massive star formation depicted by H$\alpha $. The global CO(3-2) luminosity, $L’_{\rm CO(3-2)}$, of NGC 986 was determined to be (5.4$\pm$1.1)$\times$10$^8$Kkms$^{-1}$pc$^2$. The CO(3-2)$/$CO(1-0) integrated intensity ratio was found to be 0.60$\pm$0.13 at a spatial resolution of 44$”$ or 5kpc, and the CO(3-2)$/$CO(2-1) ratio was 0.67$\pm$0.14 at a beam size of $\sim $25$”$ or $\sim $2.8kpc. These line ratios suggest moderate excitation conditions of CO lines ($n_{\rm H_2}$$\sim $10$^{3-4}$cm$^{-3}$) in a few kiloparsec region of central NGC 986.

On the Relevance of the Tremaine‐Weinberg Method Applied to an Hα Velocity Field: Pattern Speed Determination in M100 (NGC 4321)

The relevance of the Tremaine-Weinberg (TW) method is tested to measure the bar, spiral and inner structure pattern speeds using a gaseous velocity field. The TW method is applied to various simulated barred galaxies in order to demonstrate its validity in seven different configurations, including star formation or/and dark matter halo. The reliability of the different physical processes involved and of the various observational parameters are also tested. The simulations show that the TW method could be applied to the gaseous velocity fields to get a good estimate of the bar pattern speed, under the condition that regions of shocks are avoided and measurements are confined to regions where the gaseous bar is well formed. We successfully apply the TW method to the \\ha velocity field of the Virgo Cluster Galaxy M100 (NGC 4321) and derive pattern speeds of 55+/-5 km/s/kpc for the nuclear structure, 30+/-2 km/s/kpc for the bar and 20+/-1 km/s/kpc for the spiral pattern, in full agreement with published determinations using the same method or alternative ones.

Populating Stellar Orbits inside a Rotating, Gaseous Bar

In an effort to understand better the formation and evolution of barred galaxies, we have examined the properties of equatorial orbits in the effective potential of one specific model of a rapidly rotating, steady state gasdynamical bar that has been constructed via a self-consistent hydrodynamical simulation. At a given value of the Jacobi constant, roughly half of all test particles (stars) that are injected into the equatorial plane of this potential follow quasi-ergodic orbits; most regular prograde orbits have an overall bow tie shape; and some trace out trajectories that resemble the x4 family of regular, retrograde orbits. The bow tie orbits appear to be related to the 4/1 orbit family discussed by Contopoulous in 1988, but particles moving along a bow tie orbit pass very close to the center of the bar twice each orbit. Unlike the barlike configurations that previously have been constructed using dissipationless, N-body simulation techniques, the effective potential of our gasdynamical bar is very shallow and generally does not support the x1 family of orbits. If primordial galaxies evolve to a rapidly rotating barlike configuration before a significant amount of star formation has taken place and then stars form from the gas that makes up the bar, the initial stellar distribution function should consist of orbits that are (1) supported by the gaseous barlike potential and (2) restricted to have initial conditions dictated by the gasdynamics of the bar. With this restriction hypothesis in mind, we propose that stellar dynamical systems that form from gaseous bars will have characteristics that differ significantly from systems that form from a bisymmetric instability in an initially axisymmetric stellar system. Since bow tie orbits are preferred over x1 orbits, for example, such systems should have a more boxy or peanut shape when seen face-on; there will be a mechanism for funneling material more directly into the center of the galaxy; and, near the galaxy center, stars may appear to move along retrograde trajectories.

Mid-Infrared observations of NGC 1068 with the Infrared Space Observatory

We report on Mid-Infrared (MIR) observations of the Seyfert 2 galaxy NGC 1068, obtained with ISOCAM in low-resolution spectro-imaging mode. The spatial resolution (around 5 arcsec) allows us to disentangle the circumnuclear starburst regions from the emission of the active galactic nucleus (AGN). The global spatial distribution of the Unidentified Infrared Bands (UIBs) is similar to the cold dust component, traced by the 450 microns emission and the gaseous component obtained from the CO(1-0) map. However, a shift between the maximum of the UIB and 450 microns emission is clearly seen in our maps. The UIBs in the MIR (5-16 microns) originate almost exclusively from the starburst regions in the galactic disk with an emission peaking at the extremity of the stellar/ gaseous bar at a distance of 1 kpc from the AGN. The spectrum of the nucleus is characterized over the whole 5-16 microns range by a strong continuum which can be fitted with a power law of index -1.7. Moreover, the high [NeIII]/[NeII] ratio (>2.5) in the nuclear region argues for a hard radiation field from the AGN. Observations indicate that the AGN in NGC 1068 contributes less than 5% to the total integrated UIB emission even though its hot dust continuum contributes as much as 75% to the total MIR flux. On the contrary, the nuclear contribution to the cold dust emission decreases considerably at submillimeter wavelengths and does not represent more than 25% of the total integrated emission at 450 microns.

Star formation and AGN fuelling

We present new observational data that tackle the issues of the star formation in Seyfert galaxies, the obscuration and fuelling mechanisms of active galactic nuclei, and the connection between these phenomena. New ISOCAM mid-IR images of nearby Seyfert 2 galaxies confirm that these systems are characterized by enhanced star forming activity. In barred systems the star forming activity occurs preferentially along the bar, indicating that these bars have formed recently and are still in the process of transporting gas towards the center. New X-ray data indicate that the gaseous column density absorbing Sy2 nuclei is a function of the bar strength, therefore indicating that stellar bars play an important role in obscuring AGNs. We speculate that non-axisymmetric disturbances (interaction/bars) both enhance the star forming activity in host galaxy and drive gas into the nuclear region to obscure the AGN, thus making the observed starburst-Sy2 connection. On smaller scales (10–100 pc), we report the discovery of a nuclear gaseous bar in the nearby Sy2 Circinus galaxy. The molecular gas kinematics indicates that this bar causes the gas to flow into the nuclear 10 pc. In the nuclear 10 pc we detect a young nuclear stellar cluster. We show that the post-main-sequence mass loss of this young nuclear stellar population could account for the fuelling of the active nucleus.

A neutral hydrogen study of the barred spiral galaxy NGC 3319

Neutral hydrogen line observations of the late-type barred spiral galaxy NGC 3319 are presented. The distribution and kinematics of the galaxy are studied using the Very Large Array,1 The Very Large Array is an instrument of the National Radio Astronomy Observatory, which is operated by Associated Universities, Inc., under cooperative agreement with the National Science Foundation. with spatial resolutions between 11 and 50 arcsec, and a channel separation of 10.33 km s−1. As is typical for late-type galaxies, NGC 3319 is rich in H i, with a gaseous bar and spiral features. Several large, low-density regions are present, and the H i spiral structure is distorted, especially in the south. The gas distribution is asymmetric and extends significantly further to the south-east due to a long, off-centre tail. Non-circular motions caused by the bar, spiral structure and low-density regions are present in the radial velocity field, complicating the rotation curve analysis. These non-axisymmetric structures cause the values of the position angle and inclination derived from the velocity field to vary across the disc. In addition, beyond a radius of 180 arcsec, the velocity field is severely perturbed on the approaching (southern) side of the galaxy, and the disc becomes non-planar. However, the galaxy does not show the typical ‘integral sign’ shape of a warped system. We detect a small system approximately 11 arcmin (40 kpc) south of the centre of NGC 3319. It is seen in eight velocity channels and is coincident with a small, resolved object in the Palomar Sky Survey. A tidal interaction between this object and NGC 3319 is the most likely cause of the distorted spiral structure, the H i tail, and the velocity perturbations found in the southern half of the galaxy. Infalling tidal debris from such an event may account for the large, low-density regions found in the disc, several of which show kinematic evidence that suggest they are expanding superstructures. We are unable to determine the corotation radius from the H i velocity field using kinematic methods, but a break in the slope of the abundance gradient places it near the end of the bar. Star formation zones, as determined from an Hα image of the galaxy, are aligned with H i peaks and, except in the outermost optical disc, occur when the local H i density is in excess of 1.5×1021 cm−2.

6.4. Formation and evolution of gaseous bars

Although numerous gaseous bars are observed, they seem to be much less common than stellar bars. This might be due to an observational bias, but gaseous bars could also be difficult to form and/or have short lifetimes. Gas morphology is strongly dependent on the resolution and the considered scale. A true bar-like shape (Maffei 2) should not be confused with either ring-like plus open arms (IC 342), or “twin peaks” (NGC 3351) morphologies. Typical observed gaseous bar characteristics are 2ag ~ 1 kpc, and Mg ~ 0.1–0.3Mdyn inside a few 100 pc. Below, three mechanisms of gaseous bar formation, as well as their evolution, are briefly discussed.

High-Resolution CO and H i Observations of the Interacting Galaxy NGC 3627

view Abstract Citations (50) References (38) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS High-Resolution CO and H i Observations of the Interacting Galaxy NGC 3627 Zhang, Xiaolei ; Wright, Melvyn ; Alexander, Paul Abstract A nearby normal spiral galaxy, NGC 3627, which is a member of the interacting group the Leo Triplet, is mapped in the CO 1-0 transition for its central 4.5 kpc region using the Berkeley-Illinois-Maryland Association (BIMA) interferometer array with 7" resolution, and in the 21 cm emission of neutral hydrogen for the entire galaxy using the Very Large Array (VLA) with 30" resolution. The combined CO and H I data provide new information, both on the history of the past encounter of NGC 3627 with its companion galaxy NGC 3628 and on the subsequent dynamical evolution of NGC 3627 as a result of this tidal interaction. In particular, the morphological and kinematic information indicates that the gravitational torque experienced by NGC 3627 during the close encounter triggered a sequence of dynamical processes, including the formation of prominent spiral structures, the central concentration of both the stellar and gas mass, the formation of two widely separated and outwardly located inner Lindblad resonances, and the formation of a gaseous bar inside the inner resonance. These processes in coordination allow the continuous and efficient radial mass accretion across the entire galactic disk. The observational result in the current work provides a detailed picture of a nearby interacting galaxy which is very likely in the process of evolving into a nuclear active galaxy. It also suggests one of the possible mechanisms for the formation of successive instabilities in postinteraction galaxies, which could very efficiently channel the interstellar medium into the center of the galaxy to fuel nuclear starburst and Seyfert activities. Publication: The Astrophysical Journal Pub Date: November 1993 DOI: 10.1086/173374 Bibcode: 1993ApJ...418..100Z Keywords: GALAXIES: EVOLUTION; GALAXIES: INDIVIDUAL NGC NUMBER: NGC 3627; GALAXIES: INTERACTIONS; GALAXIES: ISM; GALAXIES: STRUCTURE full text sources ADS | data products SIMBAD (2) NED (1)