Secondary Bar Research Articles

Gas flow models in the Milky Way embedded bars

The gas distribution and dynamics in the inner Galaxy present many unknowns as the origin of the asymmetry of the longitude-velocity (lv) diagram of the Central Molecular Zone (CMZ). On the other hand, there are recent evidences in the stellar component of the presence of a nuclear bar that could be slightly lopsided. Our goal is to characterize the nuclear bar observed in 2MASS maps and to study the gas dynamics in the inner Milky Way taking into account this secondary bar. We have derived a realistic mass distribution by fitting 2MASS star counts maps with three components (disk, bulge and nuclear bar) and we have simulated the gas dynamics, in the deduced gravitational potential, using a sticky-particles code. Our simulations of the gas dynamics reproduce successfully the main characteristics of the Milky Way for a bulge orientation of 20-35 deg with respect to the Sun-Galactic Center (GC) line and a pattern speed of 30-40 km/s/kpc. In our models the Galactic Molecular Ring (GMR) is not an actual ring but the inner parts of the spiral arms, while the 3-kpc arm and its far side counterpart are lateral arms that contour the bar. Our simulations reproduce, for the first time, the parallelogram shape of the lv-diagram of the CMZ as the gas response to the nuclear bar. This bar should be oriented by an angle of ~60-75 deg with respect to the Sun-GC line and its mass amounts to (2-5.5)10e9 Msun. We show that the observed asymmetry of the CMZ cannot be due to lopsidedness of the nuclear bar as suggested by the 2MASS maps. We do not find clear evidences of lopsidedness in the stellar potential. We propose that the observed asymmetry of the central gas layer can be due to the infalling of gas into the CMZ in the l=1.3-complex

THE BAR PATTERN SPEED OF NGC 1433 ESTIMATED VIA STICKY-PARTICLE SIMULATIONS

We present detailed numerical simulations of NGC 1433, an intermediate-type barred spiral showing strong morphological features including a secondary bar, nuclear ring, inner ring, outer pseudoring, and two striking, detached spiral arcs known as ``plumes.'' This galaxy is an ideal candidate for recreating the observed morphology through dynamical models and determining the pattern speed. We derived a gravitational potential from an $H$-band image of the galaxy and simulated the behavior of a two-dimensional disk of 100,000 inelastically colliding gas particles. We find that the closest matching morphology between a $B$-band image and a simulation occurs with a pattern speed of 0.89 km s$^{-1}$ arcsec$^{-1}$ $\pm$ 5-10%. We also determine that the ratio of corotation radius to the average published bar radius is 1.7 $\pm$ 0.3, with the ambiguity in the bar radius being the largest contributor to the error.

Tests of the Radial Tremaine‐Weinberg Method

At the intersection of galactic dynamics, evolution and global structure, issues such as the relation between bars and spirals and the persistence of spiral patterns can be addressed through the characterization of the angular speeds of the patterns and their possible radial variation. The Radial Tremaine-Weinberg (TWR) Method, a generalized version of the Tremaine-Weinberg method for observationally determining a single, constant pattern speed, allows the pattern speed to vary arbitrarily with radius. Here, we perform tests of the TWR method with regularization on several simulated galaxy data sets. The regularization is employed as a means of smoothing intrinsically noisy solutions, as well as for testing model solutions of different radial dependence (e.g. constant, linear or quadratic). We test these facilities in studies of individual simulations, and demonstrate successful measurement of both bar and spiral pattern speeds in a single disk, secondary bar pattern speeds, and spiral winding (in the first application of a TW calculation to a spiral simulation). We also explore the major sources of error in the calculation and find uncertainty in the major axis position angle most dominant. In all cases, the method is able to extract pattern speed solutions where discernible patterns exist to within 20% of the known values, suggesting that the TWR method should be a valuable tool in the area of galactic dynamics. For utility, we also discuss the caveats in, and compile a prescription for, applications to real galaxies.

Structural behaviour of three-pile caps subjected to axial compressive loading

This work presents a comparative analysis about the behaviour of pile caps supported by 3 piles subjected to axial loading. Piles with 20 cm and 30 cm diameters were analysed. The main reinforcement was maintained in all the specimens, however, the arrangement of the secondary reinforcement varied. The main reinforcement consisted of steel bars connecting the piles. The secondary reinforcement was made up of: (a) bars going through the piles and through the projection of the column, (b) bars forming a network, and (c) vertical and horizontal stirrups. The main objective was the observation of the pile cap behaviour regarding the cracks and the modes of rupture. The real scale specimens were subjected to experimental tests until failure by rupture. Instruments were placed with the aim to obtain the displacement of the bases, the strains in the main and secondary reinforcement bars, in the compression struts, in the lower and upper nodal zones and in the sides of the caps. None of the caps reached failure by rupture with a load less than 1.12 times the theoretical load. The specimens ruptured due to the cracking of the compression strut and/or the yielding of the reinforcement bars in one direction.

Induced Nested Galactic Bars inside Assembling Dark Matter Halos

We investigate the formation and evolution of nested bar systems in disk galaxies in a cosmological setting by following the development of an isolated dark matter (DM) and baryon density perturbation. The disks form within the assembling triaxial DM halos, and the feedback from the stellar evolution is accounted for in terms of supernovae and OB stellar winds. Focusing on a representative model, we show the formation of an oval disk and of a first generation of nested bars with characteristic subkiloparsec sizes and a few kiloparsec sizes. The system evolves through successive dynamical couplings and decouplings, forcing the gas inward, and settles in a state of resonant coupling. The inflow rate can support a broad range of activity within the central kiloparsec, from quasar to Seyfert types, supplemented by vigorous star formation as a by-product. The initial bar formation is triggered in response to the tidal torques from the triaxial DM halo, which acts as a finite perturbation. This first generation of bars does not survive for more than 4-5 Gyr; by that time, the secondary bar has totally dissolved, while the primary one has very substantially weakened, reduced to a fat oval. This evolution is largely due to chaos introduced by the interaction of the multiple nonaxisymmetric components.

Long-lived Double-barred Galaxies from Pseudobulges

A large fraction of barred galaxies host secondary bars that are embedded in their large-scale primary counterparts. These are common also in gas poor early-type barred galaxies. The evolution of such double-barred galaxies is still not well understood, partly because of a lack of realistic $N$-body models with which to study them. Here we report a new mechanism for generating such systems, namely the presence of rotating pseudo-bulges. We demonstate with high mass and force resolution collisionless $N$-body simulations that long-lived secondary bars can form spontaneously without requiring gas, contrary to previous claims. We find that secondary bars rotate faster than primary ones. The rotation is not, however, rigid: the secondary bars pulsate, with their amplitude and pattern speed oscillating as they rotate through the primary bars. This self-consistent study supports previous work based on orbital analysis in the potential of two rigidly rotating bars. The pulsating nature of secondary bars may have important implications for understanding the central region of double-barred galaxies.

Structure of the Galactic Bulge: Is the Milky Way a Double-barred Galaxy?

Using the data of the IRSF/SIRIUS infrared survey along the Galactic plane (| l | ≤ 10.°5 at b = +1°), we find a distinct structure, which is probably a secondary bar, inside the primary bar of our Galaxy. The apparent magnitude peak of Bulge red clump stars changes continuously from KS ∼ 13.5 (l = −10°) to KS ∼ 12.3 (l = +10°), and this can be explained by the bar structure of the Galactic Bulge. However, the apparent magnitude changes by only ∼ 0.1 mag over the central 8°, and this indicates that there is a distinct structure inside the primary bar. In the process of the distance derivation, we have used the infrared extinction law in the J, H, and KS bands toward the Galactic center newly determined from our survey.

Evidence of a Counterrotating Core in the Large Magellanic Cloud

Stellar radial velocity data in the central region of the Large Magellanic Cloud (LMC) are used to estimate its radial velocity curve along various position angles (P.A.'s), including the line of nodes (LON). The central part of the radial velocity profile, along the LON, shows a V-shaped profile—a clear indication of counterrotation. The counterrotating region and the secondary bar have similar location and P.A. The origin of the counterrotating core could be internal (a secondary bar) or external (accretion). To explain the observed velocity profile, we propose the existence of two disks in the inner LMC, with one counterrotating. This two-disk model is found to match the H I velocities as well. Two disks with different LONs and velocity profiles can create regions that are kinematically and spatially separated. Their predicted locations are found to match the observed locations where the H I clouds are found to have two velocities.

Dynamical Decoupling of Nested Bars: Self-gravitating Gaseous Nuclear Bars

A substantial fraction of barred galaxies host additional nuclear bars that tumble with pattern speeds exceeding those of the large-scale (primary) stellar bars. We have investigated the mechanism of formation and dynamical decoupling in such nested bars that include gaseous (secondary) nuclear bars within the full-size galactic disks, hosting a double inner Lindblad resonance. Becoming increasingly massive and self-gravitating, the nuclear bars lose internal (circulation) angular momentum to the primary bars and increase their strength. Developing chaos within these bars triggers a rapid gas collapse—bar contraction. During this time period, the secondary bar pattern speed Ωs ~ a-1, where a stands for the bar size. As a result, Ωs increases dramatically until a new equilibrium is reached (if at all), while the gas specific angular momentum decreases—demonstrating the dynamical decoupling of nested bars. Viscosity, and therefore the gas presence, appears to be a necessary condition for the prograde decoupling of nested bars. This process maintains an inflow rate of ~1 M☉ yr-1 over ~108 yr across the central 200 pc and has important implications for fueling the nuclear starbursts and active galactic nuclei.

The use of ballclip attachments with an implant-supported primary-secondary bar overdenture.

The use of ballclip attachments with an implant-supported primary-secondary bar overdenture.

BUDDA: A New Two‐dimensional Bulge/Disk Decomposition Code for Detailed Structural Analysis of Galaxies

We present BUDDA (Bulge/Disk Decomposition Analysis), a new code devoted to perform a two-dimensional bulge/disk decomposition directly from the images of galaxies. The bulge component is fitted with a generalized Sérsic profile, whereas disks have an exponential profile. No other components are included. Bars and other substructures, like lenses, rings, inner bars, and inner disks, are studied with the residual images obtained through the subtraction of bulges and disks from the original images. This means that a detailed structural analysis of galaxies may be performed with a small number of parameters, and substructures may be directly studied with no a priori assumptions. As has been already shown by several studies, two-dimensional fitting is much more reliable than one-dimensional profile fitting. Moreover, our code has been thoroughly tested with artificial data, and we demonstrate it to be an accurate tool for determining structural parameters of galaxies. We also show that our code is useful in various kinds of studies, including galaxies of, e.g., different morphological types, and inclinations, which also may be observed at different spatial resolutions. Thus, the code has a broader range of potential applications than most of the previous codes, which are developed to tackle specific problems. To illustrate its usefulness, we present the results obtained with a sample of 51 mostly early-type galaxies (but covering the whole Hubble sequence). These results show some of the applications in which the code may be used: the determination of parameters for fundamental plane and structural studies, quantitative morphological classification of galaxies, and the identification and study of hidden substructures. We have determined the structural parameters of the galaxies in our sample and found many examples of hidden inner disks in ellipticals, secondary bars, nuclear rings and dust lanes in lenticulars and spirals, and also wrong morphological classification cases. We now make BUDDA generally available to the astronomical community.

Structure and kinematics of candidate double-barred galaxies

Results of optical and NIR spectral and photometric observations of a sample of candidate double-barred galaxies are presented. Velocity fields and velocity dispersion maps of stars and ionized gas, continuum and emission-line images were constructed from integral-field spectroscopy observations carried out at the 6m telescope (BTA) of SAO RAS, with the MPFS spectrograph and the scanning Fabry-Perot Interferometer. NGC2681 was also observed with the long-slit spectrograph of the BTA. Optical and NIR images were obtained at the BTA and at the 2.1m telescope (OAN, M\'exico). High-resolution images were retrieved from the HST data archive. Morphological and kinematic features of all 13 sample objects are described in detail. Attention is focused on the interpretation of observed non-circular motions of gas and stars in circumnuclear (one kiloparsec- scale) regions. We have shown first of all that these motions are caused by the gravitational potential of a large-scale bar. NGC3368 and NGC3786 have nuclear bars only, their isophotal twist at larger radii being connected with the bright spiral arms. Three cases of inner polar disks in our sample (NGC2681, NGC3368 and NGC5850) are considered. We found ionized-gas counter-rotation in the central kiloparsec of the lenticular galaxy NGC3945. Seven galaxies (NGC470, NGC2273, NGC2681, NGC3945, NGC5566, NGC5905, and NGC6951) have inner mini-disks nested in large-scale bars. Minispiral structures occur often in these nuclear disks. It is interesting that the majority of the observed, morphological and kinematical, features in the sample galaxies can be explained without the secondary bar hypothesis. Thus we suggest that a dynamically independent secondary bar is a rarer phenomenon than follows from isophotal analysis of the images only.

The role of bars

Secular evolution and fueling is driven by bars in spiral galaxies, and the related dynamical processes are reviewed. It is shown that gravity torques dominate over viscous torques, and produce gas infall to the center. In this infall, the bar wave accepts the angular momentum, which produces its destruction. In the end, a central mass concentration is built, which also contributes to this destruction. While gas can be stalled at ILR for a while, secondary bars then take over the fueling. In a galaxy life-time, several bar episodes can successively develop. Through external gas accretion, disks are replenished by intermittence, between two bar phases. While the general trend for a galaxy is to evolve towards early-types, its morphology can oscillate, and turn back transiently to later-types.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Evidence of a Misaligned Secondary Bar in the Large Magellanic Cloud

Evidence of a misaligned secondary bar, within the primary bar of the Large Magellanic Cloud (LMC), is presented. The density distribution and the dereddened mean magnitudes (I0) of the red clump stars in the bar obtained from the Optical Gravitational Lensing Experiment II data are used for this study. The bar region that predominantly showed a wavy pattern in the line of sight in a recent paper by Subramaniam was located. These points in the X-Z plane delineate an S-shaped pattern, clearly indicating a misaligned bar. This feature is statistically significant and does not depend on the considered value of I0 for the LMC center. The rest of the bar region was not found to show the warp or the wavy pattern. The secondary bar is found to be considerably elongated in the Z-direction, with an inclination of 665 ± 09, whereas the undisturbed part of the primary bar is found to have an inclination of 151 ± 27, such that the eastern sides are closer to us with respect to the western sides of both the bars. The P.A.maj of the secondary bar is found to be 1084 ± 73. The streaming motions found in the H I velocity map close to the LMC center could be caused by the secondary bar. The recent star formation and the gas distribution in LMC could be driven by the misaligned secondary bar.

Double-barred galaxies

I present a catalog of 67 barred galaxies which contain distinct, elliptical stellar structures inside their bars. Fifty of these are double-barred galaxies: a small-scale, inner or secondary bar is embedded within a large-scale, outer or primary bar. I provide homogenized measurements of the sizes, ellipticities, and orientations of both inner and outer bars, along with global parameters for the galaxies. The other 17 are classified as inner-disk galaxies, where a large-scale bar harbors an inner elliptical structure which is aligned with the galaxy's outer disk. Four of the double-barred galaxies also possess inner disks, located in between the inner and outer bars. While the inner-disk classification is ad-hoc – and undoubtedly includes some inner bars with chance alignments (five such probable cases are identified) – there is good evidence that inner disks form a statistically distinct population, and that at least some are indeed disks rather than bars. In addition, I list 36 galaxies which may be double-barred, but for which current observations are ambiguous or incomplete, and another 23 galaxies which have been previously suggested as potentially being double-barred, but which are probably not. False double-bar identifications are usually due to features such as nuclear rings and spirals being misclassified as bars; I provide some illustrated examples of how this can happen. A detailed statistical analysis of the general population of double-bar and inner-disk galaxies, as represented by this catalog, will be presented in a companion paper.

Direct Confirmation of Two Pattern Speeds in the Double-barred Galaxy NGC 2950

We present the surface photometry and stellar kinematics of NGC 2950, which is a nearby and undisturbed SB0 galaxy hosting two nested stellar bars. We use the Tremaine-Weinberg method to measure the pattern speed of the primary bar. This also permits us to establish directly and for the first time that the two nested bars are rotating with different pattern speeds and, in particular, that the rotation frequency of the secondary bar is higher than that of the primary one.

The Nuclear Gasdynamics and Star Formation of NGC 7469

We report interferometric radio CO 2-1 and HCN 1-0 observations at resolutions of 07 and 20, respectively, and 0085 resolution adaptive optics K-band spectroscopy, including H2 1-0 S(1) line emission and CO 2-0 stellar absorption, of the inner few arcseconds of NGC 7469. The CO 2-1 map shows a ring of molecular clouds (which in general lie outside the compact knots seen in K-band images) and a bright extended nucleus, with a bar or pair of spiral arms between them. The dynamical structure of both the radio CO 2-1 and the K-band H2 1-0 S(1) lines at their different resolutions can be reproduced using a single axisymmetric mass model comprising three components: a broad disk, a ring 4''-5'' across, and an extended nucleus, which we interpret as an inner nuclear ring about 05 across. The velocity residuals between the model and the data have a standard deviation of 25 km s-1, and no noncircular motions faster than this are seen, although this may be because in some cases a secondary bar is not expected to cause gas inflow. From the dynamical mass and estimates of the stellar mass we find that the CO-to-H2 conversion is 0.4-0.8 times that for the Milky Way, following the trend to small factors that has been reported for intense star-forming environments. The central H2 1-0 S(1) morphology has a strong peak at the nucleus, but this does not trace the mass distribution; the rotation curves indicate that there is no strong nuclear mass concentration. The origins of the 1-0 S(1) emission are instead likely to lie in X-ray and UV irradiation of gas by the active galactic nucleus rather than via processes associated with star formation. Using the 2.3 ?m stellar CO 2-0 band head absorption and the slope of the continuum, we have directly resolved the nuclear star cluster to be 015-020 across and find that it is asymmetric. This cluster has an age of less than about 60 Myr and contributes 20%-30% of the nuclear K-band light and about 10% of the nuclear bolometric luminosity. Within a radius of ~4'' gas contributes more than half the total mass, but in the nucleus, within a radius of 01, it is likely that most of the mass is due instead to stars.

Long-lived Double-barred Galaxies: Critical Mass and Length Scales

A substantial fraction of disk galaxies is double-barred. We analyze the dynamical stability of such nested bar systems by means of Liapunov exponents, by fixing a generic model and varying the inner (secondary) bar mass. We show that there exists a critical mass below which the secondary bar cannot sustain its own orbital structure and above which it progressively destroys the outer (primary) bar-supporting orbits. In this critical state, a large fraction of the trajectories (regular and chaotic) is aligned with either bar, suggesting the plausibility of long-lived dynamical states when secondary-to-primary bar mass ratio is of the order of a few percent. Qualitatively similar results are obtained by varying the size of the secondary bar, within certain limits, while keeping its mass constant. In both cases, an important role appears to be played by chaotic trajectories that are trapped around (especially) the primary bar for long periods of time.

An Imaging Survey of Early‐Type Barred Galaxies

This paper presents the results of a high-resolution imaging survey, using both ground-based and Hubble Space Telescope images, of a complete sample of nearby barred S0-Sa galaxies in the field, with a particular emphasis on identifying and measuring central structures within the bars: secondary bars, inner disks, nuclear rings and spirals, and off-plane dust. A discussion of the frequency and statistical properties of the various types of inner structures has already been published. Here we present the data for the individual galaxies and measurements of their bars and inner structures. We set out the methods we use to find and measure these structures, and how we discriminate between them. In particular, we discuss some of the deficiencies of ellipse fitting of the isophotes, which by itself cannot always distinguish between bars, rings, spirals, and dust, and which can produce erroneous measurements of bar sizes and orientations.

Morphology and kinematics of spiral galaxies with and without AGNs

The analysis of a sample of 30 isolated spiral galaxies with and without an Active Galactic Nucleus (AGN) results in active and control galaxies having similar bulge and disk properties. Secondary bars also seem to be indistinguishable. Circumnuclear kinematics are also analyzed to disentangle what kind of mechanisms are taking place in the circumnuclear regions. From long slit spectra of a subsample of isolated active and non-active spiral galaxies in the Hα emission line region, the shape and properties of the rotation curves are obtained together with the line ratios used to estimate the metallicity of all the detected HII regions. Kinematics and metallicities are also studied in comparison with the properties determined for a family of 111 spiral galaxies with a well determined environmental status, ranging from isolation to close, though not disruptive, interaction with satellites or companions. The stellar kinematics are also studied in some cases, with as main result the detection of a stellar velocity dispersion drop in the centers of four Seyfert galaxies. Together with previous findings, our results from the comparison between active and non-active isolated galaxies point to AGNs being an episode in the life of galaxies, provided that the fuel is available and the accumulation mechanisms are at work.