Bar Pattern Research Articles

Pattern speed evolution of barred galaxies in TNG50

Context. Galactic bars are found in the majority of disc galaxies. They rotate nearly rigidly with an angular frequency called pattern speed. In idealised simulations, the bar pattern speed generally decreases with time due to dynamical friction exerted by the dark-matter halo, while cold gas can reduce or even reverse this trend. Aims. We want to understand how different galaxy properties affect the evolution of the bar pattern speed in more realistic situations, including ongoing star formation, mass infall, active galactic nucleus (AGN) feedback, and galaxy interactions. Methods. We traced the pattern-speed evolution of simulated bars in the TNG50-1 cosmological simulations. Results. Simulated bars with an initially high pattern speed and a subsequent rapid slowdown are more likely found in more massive galaxies. Lower mass galaxies, on the other hand, preferentially host bars that start at relatively low pattern speeds and retain the same value until the end of the simulation. More massive simulated barred galaxies are also more affected by the AGN-feedback model, which very efficiently removes the cold gas that could have prevented the slowdown. Conclusions. We find that bars grow and strengthen with slowdown, in agreement with higher resolution simulations. We find that strong correlations between the bar slowdown rate and galaxy mass weaken considerably when we use dimensionless measures to quantify the slowdown. In TNG50, the AGN-feedback prescription amplifies the mass dependence. Turned around, this provides an interesting statistic to constrain sub-grid physics by bar growth and slowdown.

Equilibrium dynamical models in the inner region of the Large Magellanic Cloud based on Gaia DR3 kinematics

Context. The Large Magellanic Cloud (LMC) contains complex dynamics driven by both internal and external processes. The external forces are due to tidal interactions with the Small Magellanic Cloud and the Milky Way, while internally its dynamics mainly depend on the stellar, gas, and dark matter mass distributions. Despite this complexity, simple physical models often provide valuable insights into the primary driving factors. Aims. We used Gaia Data Release 3 (DR3) to explore how well equilibrium dynamical models based on the Jeans equations and the Schwarzschild orbit superposition method are able to describe the LMC’s five-dimensional phase-space distribution and line-of-sight (LOS) velocity distribution, respectively. In the Schwarzschild model, we incorporated a triaxial bar component for the first time and derived the LMC’s bar pattern speed. Methods. We fit comprehensive Jeans dynamical models to all Gaia DR3 stars with proper motion and LOS velocity measurements found in the footprint of the VISTA near-infrared survey of the Magellanic System using a discrete maximum likelihood approach. These models are very efficient at discriminating genuine LMC member stars from Milky Way foreground stars and background galaxies. They constrain the shape, orientation, and enclosed mass of the galaxy under the assumption of axisymmetry. We used the Jeans model results as a stepping stone to more complex two-component Schwarzschild models, which include an axisymmetric disc and a co-centric triaxial bar, which we fit to the LMC Gaia DR3 LOS velocity field using a χ2 minimisation approach. Results. The Jeans models describe the rotation and velocity dispersion of the LMC disc well, and we find an inclination angle of θ = 25.5° ±0.2°, line of nodes orientation of ψ = 124° ±0.4°, and an intrinsic thickness of the disc of q0d = b/a = 0.23 ± 0.01 (minor to major axis ratio). However, bound to axisymmetry, these models fail to properly describe the kinematics in the central region of the galaxy dominated by the LMC bar. We used the derived disc orientation and the Gaia DR3 density image of the LMC to obtain the intrinsic shape of the bar. Using these two components as input to our Schwarzschild models, we performed orbit integration and weighting in a rotating reference frame fixed to the bar, deriving an independent measurement of the LMC bar pattern speed of Ω = 11 ± 4 km s−1 kpc−1. Both the Jeans and Schwarzschild models predict the same enclosed mass distribution within a radius of 6.2 kpc of ∼ 1.4 × 1010 M⊙.

Stellar Bars Form Dark Matter Counterparts in TNG50

Dark matter (DM) bars that shadow stellar bars have been previously shown to form in idealized simulations of isolated disk galaxies, but have yet to be studied in a fully cosmological context. In this work, we analyze a population of disk galaxies within the TNG50 simulation to determine the characteristics of their dark bars. We estimate bar strength and orientation using both the in-plane Fourier A 2 density moments and the quadrupolar coefficients of the spherical harmonic basis function expansions of the density. We additionally present two novel methods for measuring the bar pattern speed Ω p and rotation axis orientation using these coefficients, and apply them to one sample galaxy located in a TNG50 subbox. Consistent with isolated simulations, DM bars are shorter than their stellar counterparts and are 75% weaker in A 2. DM bars dominate the shape of the inner halo potential and are apparent in the time series of quadrupolar coefficients. In our selected subbox galaxy, the stellar and dark bars remain co-aligned throughout the last 8 Gyr and have identical Ω p . Pattern speed Ω p evolves considerably over the last 8 Gyr, consistent with torques on the bars due to dynamical friction and gas accretion, and is seen to increase following a merger at t lb = 1.5 Gyr. Rather than remaining static in time, the bar rotation axis displays both precession and nutation possibly caused by torques outside the plane of rotation. We find that the shape of the stellar and DM mass distributions are tightly correlated with Ω p .

The redshift evolution of galactic bar pattern speed in TNG50

In this paper, the redshift evolution of the galactic bar properties, like the bar length, pattern speed, and bar fraction, has been investigated for simulated galaxies at stellar masses of M* > 1010 M⊙ in the cosmological magnetohydrodynamical simulation TNG50. We focus on the redshift evolution of the bar pattern speeds and the fast bar tension. We show that the median value of the pattern speed of the bars increases as the redshift grows. On the other hand, although the median value of the bar length increases with time, the ratio between the corotation radius and the bar radius - namely, the 𝓡 = RCR/Rbar parameter - increases as well. In other words, the corotation radius increases with a higher rate than the bar length. This directly means that galactic bars slow down with time, or equivalently as the redshift declines. We discuss the possible mechanisms that reduce the pattern speeds in TNG50. We demonstrate that while mergers can have a significant impact on a galaxy’s pattern speed, they do not play a crucial role in the overall evolution of mean pattern speed within the redshift range ɀ ≤ 1.0. Furthermore, we show that the 𝓡 parameter does not correlate with the gas fraction. Consequently, the existence of gas in TNG50 does not alleviate the fast bar tension. We show that the mean value of the pattern speed, computed for all the galaxies irrespective of their mass, at ɀ = 1.0 is Ωp = 70.98 ± 2.34 km s−1 kpc−1 and reduces to Ωp = 33.65 ± 1.07 km s−1 kpc−1 at ɀ = 0.0. This is a direct prediction by TNG50 that bars at ɀ = 1.0 rotate faster by a factor of ~2 compared to bars at ɀ = 0.0.

Schwarzschild modelling of barred s0 galaxy NGC 4371

ABSTRACT We apply the barred Schwarzschild method developed by Tahmasebzadeh et al. (2022) to a barred S0 galaxy, NGC 4371, observed by IFU instruments from the TIMER and ATLAS3D projects. We construct the gravitational potential by combining a fixed black hole mass, a spherical dark matter halo, and stellar mass distribution deprojected from 3.6 μm S$^4$G image considering an axisymmetric disc and a triaxial bar. We independently modelled kinematic data from TIMER and ATLAS3D. Both models fit the data remarkably well. We find a consistent bar pattern speed from the two sets of models with $\Omega _{\rm p} = 23.6 \pm 2.8 \, \mathrm{km \, s^{-1} \, kpc^{-1} }$ and $\Omega _{\rm p} = 22.4 \pm 3.5 \, \mathrm{km \, s^{-1} \, kpc^{-1} }$, respectively. The dimensionless bar rotation parameter is determined to be $\mathcal {R} \equiv R_{\rm cor}/R_{\rm bar}=1.88 \pm 0.37$, indicating a likely slow bar in NGC 4371. Additionally, our model predicts a high amount of dark matter within the bar region ($M_{\rm DM}/ M_{\rm total}$$\sim 0.51 \pm 0.06$), which, aligned with the predictions of cosmological simulations, indicates that fast bars are generally found in baryon-dominated discs. Based on the best-fitting model, we further decompose the galaxy into multiple 3D orbital structures, including a BP/X bar, a classical bulge, a nuclear disc, and a main disc. The BP/X bar is not perfectly included in the input 3D density model, but BP/X-supporting orbits are picked through the fitting to the kinematic data. This is the first time a real barred galaxy has been modelled utilizing the Schwarzschild method including a 3D bar.

Effects of Halo Spin on the Formation and Evolution of Bars in Disk Galaxies

The spin of dark halos has been shown to significantly affect bar formation and evolution in disk galaxies. To understand the physical role of halo spin in bar formation, we run N-body simulations of isolated, Milky Way–sized galaxies by varying the halo spin parameter in the range −0.16 ≤ λ ≤ 0.16 and the bulge mass. We find that our adopted halo alone is subject to swing amplification of an m = 2 nonaxisymmetric mode rotating in the same sense as the halo, which assists or inhibits the bar formation in a disk depending on its sense of rotation. The m = 2 mode in the disk, growing via swing amplification, interacts constructively (destructively) with the m = 2 mode in the prograde (retrograde) halo, promoting (delaying) bar formation. A bar grows by losing its angular momentum primarily to a halo. Since the halo particles inside (outside) the corotation resonance with the bar can emit (absorb) angular momentum to (from) the bar, the bar pattern speed decays more slowly for larger λ > 0, while it decreases relatively fast almost independent of λ ≤ 0. Models with a strong bar develop a boxy peanut-shaped bulge. In models without a bulge, this occurs rapidly via buckling instability, while bars with a bulge thicken gradually without undergoing buckling instability. Among the models considered in the present work, the bar in the λ = 0.06 model with a bulge of 10% of the disk mass best describes the Milky Way in terms of its length and pattern speed.

KRATOS: A large suite of N-body simulations to interpret the stellar kinematics of LMC-like discs

Context. The Large and Small Magellanic Clouds (LMC and SMC, respectively) are the brightest satellites of the Milky Way (MW), and for the last thousand million years they have been interacting with one another. As observations only provide a static picture of the entire process, numerical simulations are used to interpret the present-day observational properties of these kinds of systems, and most of them have been focused on attempting to recreate the neutral gas distribution and characteristics through hydrodynamical simulations. Aims. We present KRATOS, a comprehensive suite of 28 open-access pure N-body simulations of isolated and interacting LMC-like galaxies designed for studying the formation of substructures in their discs after interaction with an SMC-mass galaxy. The primary objective of this paper is to provide theoretical models that help us to interpret the formation of general structures in an LMC-like galaxy under various tidal interaction scenarios. This is the first paper of a series dedicated to the analysis of this complex interaction. Methods. Simulations are grouped into 11 sets of up to three configurations, with each set containing (1) a control model of an isolated LMC-like galaxy; (2) a model that contains the interaction with an SMC-mass galaxy, and (3) a model where both an SMC-mass and a MW-mass galaxy may interact with the LMC-like galaxy (the most realistic model). In each simulation, we analysed the orbital history between the three galaxies and examined the morphological and kinematic features of the LMC-like disc galaxy throughout the interaction. This includes investigating the disc scale height and velocity maps. When a bar was found to develop, we characterised its strength, length, off-centredness, and pattern speed. Results. The diverse outcomes found in the KRATOS simulations, including the presence of bars, warped discs, and various spiral arm shapes, demonstrate the opportunities they offer to explore a range of LMC-like galaxy morphologies. These morphologies directly correspond to distinct disc kinematic maps, making them well-suited for a first-order interpretation of the LMC’s kinematic maps. From the simulations, we note that tidal interactions can: boost the disc scale height; both destroy and create bars; and naturally explain the off-centre stellar bars. The bar length and pattern speed of long-lived bars are not appreciably altered by the interaction. Conclusions. The high spatial, temporal, and mass resolution used in the KRATOS simulations has been shown to be appropriate for the purpose of interpreting the internal kinematics of LMC-like discs, as evidenced by the first scientific results presented in this work.

Radial halo substructure in harmony with the Galactic bar

ABSTRACT Overdensities in the radial phase space $(r,v_{\rm r})$ of the Milky Way’s halo have previously been associated with the phase-mixed debris of a highly radial merger event such as Gaia Sausage–Enceladus. We present and test an alternative theory in which the overdense ‘chevrons’ are instead composed of stars trapped in resonances with the Galactic bar. We develop an analytic model of resonant orbits in the isochrone potential, and complement this with a test particle simulation of a stellar halo in a realistic barred Milky Way potential. These models are used to predict the appearance of action space $(J_\phi ,J_{\rm r})$ and radial phase space in the Solar neighbourhood. They are able to reproduce almost all salient features of the observed chevrons. In particular, both the analytic model and simulation predict that the chevrons are more prominent at $v_{\rm r}\ \lt\ 0$ when viewed near the Sun, as is observed by Gaia. This is inconsistent with formation by an ancient merger event. We also associate individual chevrons with specific resonances. At a bar pattern speed of $\Omega _\mathrm{b}=35$ km s$^{-1}$ kpc$^{-1}$, the two most prominent prograde chevrons align very closely with the corotation and outer Lindblad resonances. The former can be viewed as a highly eccentric extension of the Hercules stream. Finally, our model predicts that the $v_{\rm r}$ asymmetry changes sign as a function of Galactic radius and azimuth, and we find evidence that this is indeed the case in the Milky Way.

Nonlinear finite element modelling of the bond behavior of near-surface mounted Fe-SMA bars

Strengthening of concrete members with near-surface-mounted (NSM) iron-based shape memory alloy (Fe-SMA) bars can increase the load-carrying capacity and reduce deflections/crack widths owing to the unique self-prestressing ability of the Fe-SMA. However, the bond behavior of Fe-SMA bars is critical for the effectiveness of the NSM strengthening method, as inadequate bond strength can result in premature bond failure, rendering the strengthening technique ineffective. This study presents the results of a non-linear finite element (FE) model that was developed and validated to simulate the bond behavior and failure patterns of NSM Fe-SMA bars grouted into concrete. The variable parameters of the study included the effect of the compressive strength of the grout and the concrete block, the effect of cover depth and the activation of the Fe-SMA bar on the bond behavior. Investigations showed that the bond resistance and failure patterns of the NSM bars were significantly affected by the compressive strength of the concrete block and grout. The results showed that increasing the compressive strength of the grout by two times resulted in a 35 % increase in bond strength while increasing the compressive strength of the concrete block by three times resulted in a 30 % increase in bond strength. Additionally, the load transfer between the grout and concrete block was strongly influenced by the grout's compressive strength, where a higher strength of the grout resulted in a stiffer bond and increased contribution of the concrete block to resisting loads. Similarly, increasing the cover depth by four times from 5 mm to 20 mm increased the bond strength by more than twice and ultimately led to a change in failure mode from a splitting crack in the grout to a splitting crack in the concrete block. The results of the study also show that prestressing the short NSM bond with Fe-SMA can induce an initial bar slip in the range of 0.025 to 0.14 mm for recovery stresses of 100 MPa to 350 MPa.

Experimental studies on bond behaviour of steel rebar with different rib patterns in concrete

The bond between concrete and steel rebar is one of the most crucial parameters in the construction of reinforced concrete structures. The load-bearing capacity and serviceability of these structures depend heavily on bond strength. The rib pattern of the rebar plays a significant role in enhancing bond behaviour. The impact of different rebar patterns on bond studies in the concrete mix is found to be limited. In the study, six concrete mixes were prepared and tested. Three of these mixes incorporated fly ash in varied amounts to partially replace cement, while the other three mixes had no fly ash. Three different rib patterns of steel bars were embedded into these six grades of concrete. The pull-out tests were used to assess the bond strengths of various combinations in accordance with RILEM recommendations. The findings show that the ribs in rebars significantly increase mechanical interlocking whereas the presence of fly ash improves chemical adhesion in bond behaviour. Particularly, pattern 2 rebar (transverse rib thickness, 0.797 mm; transverse rib width, 2.06 mm; rib spacing (inner to inner), 6.428 mm; longitudinal rib thickness, 1.04 mm; longitudinal rib width, 2.312 mm; rib inclination angle, 66.39o; nominal diameter, 16 mm; length of transverse rib, 25 mm) exhibited notably superior bond strength, reaching a maximum of 17.21 MPa in M30 grade concrete in which 95 kg/m³ of fly ash was used as a replacement for cement.

PSF and MTF from a bar pattern in digital mammography

Background. The MTF has difficulties being determined (according to the provisions of the IEC standards) in the hospital setting due to the lack of resources. Purpose. The objective of this work is to propose a quantitative method for obtaining the point spread function (PSF) and the modulation transfer function (MTF) of a digital mammography system from an image of a bar pattern. Methods. The method is based on the measurement of the contrast transfer function (CTF) of the system over the image of the bar pattern. In addition, a theoretical model for the PSF is proposed, from which the theoretical CTF of the system is obtained by means of convolution with a square wave (mathematical simulation of the bar pattern). Through an iterative process, the free parameters of the PSF model are varied until the experimental CTF coincides with the one calculated by convolution. Once the PSF of the system is obtained, we calculate the MTF by means of its Fourier transform. The MTF calculated from the model PSF have been compared with those calculated from an image of a 65 μm diameter gold wire using an oversampling process. Results. The CTF has been calculated for three digital mammographic systems (DMS 1, DMS 2 and DMS 3), no differences of more than 5 % were found with the CTF obtained with the PSF model. The comparison of the MTF shows us the goodness of the PSF model. Conclusions. The proposed method for obtaining PSF and MTF is a simple and accessible method, which does not require a complex configuration or the use of phantoms that are difficult to access in the hospital world. In addition, it can be used to calculate other magnitudes of interest such as the normalized noise power spectrum (NNPS) and the detection quantum efficiency (DQE).

Wide-incident-angle, polarization-independent broadband-absorption metastructure without external resistive elements by using a trapezoidal structure

The absorption of electromagnetic waves in a broadband frequency range with polarization insensitivity and incidence-angle independence is greatly needed in modern technology applications. Many structures based on metamaterials have been suggested for addressing these requirements; these structures were complex multilayer structures or used special materials or external electric components, such as resistive ones. In this paper, we present a metasurface structure that was fabricated simply by employing the standard printed-circuit-board technique but provides a high absorption above 90% in a broadband frequency range from 12.35 to 14.65 GHz. The metasurface consisted of structural unit cells of 4 symmetric substructures assembled with a metallic bar pattern, which induced broadband absorption by using a planar resistive interaction in the pattern without a real resistive component. The analysis, simulation, and measurement results showed that the metasurface was also polarization insensitive and still maintained an absorption above 90% at incident angles up to 45°. The suggested metasurface plays a role in the fundamental design and can also be used to design absorbers at different frequency ranges. Furthermore, further enhancement of the absorption performance is achieved by improved design and fabrication.

Turning The Tide: Live-Bed Scale Experiments Of Bar-Dominated Estuaries And Effects Of Dredging On Intertidal Habitat

Many large sand-dominated estuaries and tidal basins have complex channel and bar patterns, wherein a continuous deep channel suitable for shipping is rarely naturally present, which requires dredging. The intertidal area has important, often protected habitats for macro-benthic species, wader birds and other species. The question is to what degree channel and shoal dimensions and the amount of intertidal area are affected by dredging and disposal for access to ports. While numerical modelling is conducted for these systems with some success, complementary scale experiments with live beds have challenged the fields of coastal engineering and coastal morphodynamics for over a century. Compared to scale models for rivers, the scale issues in tidal experiments are more complicated and very few attempts have been pursued. For example, Reynolds (1889) conducted the first scale experiments of an estuary by a periodic sea level fluctuation, but found that even fine sand was hard to mobilize, while the bed surface was dominated by ripples rather than bars. Similarly, Tambroni et al. (2005) carefully scaled sediment mobility in a converging channel and obtained large-scale morphological patterns with gentle bars nearly overwhelmed by ripples. Stefanon et al. (2010) present a careful attempt at erosive tidal channel network development in a scale model with low-density sediment, which showed unexplained, over-large scour holes at channel junctions. The key issues for tidal experiments are to obtain sufficient sediment mobility (expressed as Shields or Rouse number) and to avoid over-large ripples and scours, and to this end classic river scale modelling is inadequate. Here we assess whether our recent innovation solves these scale issues. The objective of this abstract is to elucidate the scaling of experiments that enable study of morphodynamic estuaries in many aspects including effects of sediment management by dredging.

The bar pattern speed of the Large Magellanic Cloud

Context. The internal kinematics of the Large Magellanic Cloud (LMC) have been studied in unprecedented depth thanks to the excellent quality of the Gaia mission data, thus revealing the disc’s non-axisymmetric structure. Aims. We seek to constrain the LMC bar pattern speed using the astrometric and spectroscopic data from the Gaia mission. Methods. We applied three methods to evaluate the bar pattern speed by measuring it via: the Tremaine-Weinberg (TW) method, the Dehnen method, and a bisymmetric velocity (BV) model. These methods provide additional information on the bar properties, such as the corotation radius as well as the bar length and strength. We tested the validity of the methods with numerical simulations. Results. A wide range of pattern speeds are inferred by the TW method, owing to a strong dependency on the orientation of the galaxy frame and the viewing angle of the bar perturbation. The simulated bar pattern speeds (corotation radii, respectively) are well recovered by the Dehnen method (BV model). Applied to the LMC data, the Dehnen method finds a pattern speed of Ωp = −1.0 ± 0.5 km s−1 kpc−1, thus corresponding to a bar that barely rotates and is only slightly counter-rotating with respect to the LMC disc. The BV method gives a LMC bar corotation radius of Rc = 4.20 ± 0.25 kpc, corresponding to a pattern speed of Ωp = 18.5−1.1+1.2 km s−1 kpc−1. Conclusions. It is not possible to determine which global value best represents an LMC bar pattern speed with the TW method, due to the strong variation with the orientation of the reference frame. The non-rotating bar from the Dehnen method would be at odds with the structure and kinematics of the LMC disc. The BV method result is consistent with previous estimates and gives a bar corotation-to-length ratio of 1.8 ± 0.1, suggesting that the LMC is hosting a slow bar.

Bar-driven Gas Dynamics of M31

The large-scale gaseous shocks in the bulge of M31 can be naturally explained by a rotating stellar bar. We use gas dynamical models to provide an independent measurement of the bar pattern speed in M31. The gravitational potentials of our simulations are from a set of made-to-measure models constrained by stellar photometry and kinematics. If the inclination of the gas disk is fixed at i = 77°, we find that a low pattern speed of 16–20 km s−1 kpc−1 is needed to match the observed position and amplitude of the shock features, as shock positions are too close to the bar major axis in high Ω b models. The pattern speed can increase to 20–30 km s−1 kpc−1 if the inner gas disk has a slightly smaller inclination angle compared with the outer one. Including subgrid physics such as star formation and stellar feedback has minor effects on the shock amplitude, and does not change the shock position significantly. If the inner gas disk is allowed to follow a varying inclination similar to the H i and ionized gas observations, the gas models with a pattern speed of 38 km s−1 kpc−1, which is consistent with stellar-dynamical models, can match both the shock features and the central gas features.

Counting Nemo: anemonefish Amphiprion ocellaris identify species by number of white bars.

The brilliant colors of coral reef fish have received much research attention. This is well exemplified by anemonefish, which have distinct white bar patterns and inhabit host anemones and defend them as a territory. The 28 described species have between 0 and 3 white bars present, which has been suggested to be important for species recognition. In the present study, we found that Amphiprion ocellaris (a species that displays three white bars) hatched and reared in aquaria, when faced with an intruder fish, attacked their own species more frequently than other species of intruding anemonefish. Additionally, we explicitly tested whether this species could distinguish models with different numbers of bars. For this, 120 individuals of A. ocellaris were presented with four different models (no bars, and 1, 2 and 3 bars) and we compared whether the frequency of aggressive behavior towards the model differed according to the number of bars. The frequency of aggressive behavior toward the 3-bar model was the same as against living A. ocellaris, and was higher than towards any of the other models. We conclude that A. ocellaris use the number of white bars as a cue to identify and attack only competitors that might use the same host. We considered this as an important behavior for efficient host defense.

Globular clusters and bar: captured or not captured?

ABSTRACT Studies of the dynamics of globular clusters assume different values of bar parameters (mass, velocity, and size) and analyse the results of orbit classifications over the range of the chosen values. It is also a usual thing that a spherical bulge component is converted into the bar to obtain a non-axisymmetric potential from an axisymmetric one. The choice of bar parameters and the way the bar is converted from the bulge introduce systematics into the orbit classifications that we explore in this study. We integrate orbits of 30 bulge globular clusters residing in the inner area of the Galaxy (R ≲ 5 kpc) backwards in time for three different potentials, two of which are obtained by fitting the rotation curve, and one is taken from the surrogate N-body model representing our Galaxy. We analyse each orbit in terms of dominant frequencies obtained from its coordinate spectra. We find that the bar pattern speed is a key factor in orbital classification. With an increase of the bar pattern speed, frequencies deviate more and more from the ‘bar’ frequency ratio 2:1. The bar-to-bulge mass ratio (assuming the total mass of the bar plus the bulge is fixed) and size of the bar play a smaller role. We also find that, in the N-body potential, the fraction of orbits that follow the bar is higher than in those obtained from fitting the rotation curve.

Non-Isocentric Geometry for Next-Generation Tomosynthesis With Super-Resolution.

Our lab at the University of Pennsylvania (UPenn) is investigating novel designs for digital breast tomosynthesis. We built a next-generation tomosynthesis system with a non-isocentric geometry (superior-to-inferior detector motion). This paper examines four metrics of image quality affected by this design. First, aliasing was analyzed in reconstructions prepared with smaller pixelation than the detector. Aliasing was assessed with a theoretical model of r -factor, a metric calculating amplitudes of alias signal relative to input signal in the Fourier transform of the reconstruction of a sinusoidal object. Aliasing was also assessed experimentally with a bar pattern (illustrating spatial variations in aliasing) and 360°-star pattern (illustrating directional anisotropies in aliasing). Second, the point spread function (PSF) was modeled in the direction perpendicular to the detector to assess out-of-plane blurring. Third, power spectra were analyzed in an anthropomorphic phantom developed by UPenn and manufactured by Computerized Imaging Reference Systems (CIRS), Inc. (Norfolk, VA). Finally, calcifications were analyzed in the CIRS Model 020 BR3D Breast Imaging Phantom in terms of signal-to-noise ratio (SNR); i.e., mean calcification signal relative to background-tissue noise. Image quality was generally superior in the non-isocentric geometry: Aliasing artifacts were suppressed in both theoretical and experimental reconstructions prepared with smaller pixelation than the detector. PSF width was also reduced at most positions. Anatomic noise was reduced. Finally, SNR in calcification detection was improved. (A potential trade-off of smaller-pixel reconstructions was reduced SNR; however, SNR was still improved by the detector-motion acquisition.) In conclusion, the non-isocentric geometry improved image quality in several ways.

Recovering the gravitational potential in a rotating frame: Deep Potential applied to a simulated barred galaxy

ABSTRACT Stellar kinematics provides a window into the gravitational field, and therefore into the distribution of all mass, including dark matter. Deep Potential is a method for determining the gravitational potential from a snapshot of stellar positions in phase space, using mathematical tools borrowed from deep learning to model the distribution function and solve the Collisionless Boltzmann equation. In this work, we extend the Deep Potential method to rotating systems, and then demonstrate that it can accurately recover the gravitational potential, density distribution, and pattern speed of a simulated barred disc galaxy, using only a frozen snapshot of the stellar velocities. We demonstrate that we are able to recover the bar pattern speed to within $15 \,\rm {per\, cent}$ in our simulated galaxy using stars in a 4 kpc subvolume centred on a Solar-like position, and to within $20 \,\rm{per\,cent}$ in a 2 kpc subvolume. In addition, by subtracting the mock ‘observed’ stellar density from the recovered total density, we are able to infer the radial profile of the dark matter density in our simulated galaxy. This extension of Deep Potential is an important step in allowing its application to the Milky Way, which has rotating features, such as a central bar and spiral arms, and may moreover provide a new method of determining the pattern speed of the Milky Way bar.

The dynamical state of bars in cluster dwarf galaxies: the cases of NGC 4483 and NGC 4516

ABSTRACT Dwarf barred galaxies are the perfect candidates for hosting slowly rotating bars. They are common in dense environments and have a relatively shallow potential well, making them prone to heating by interactions. When an interaction induces bar formation, the bar should rotate slowly. They reside in massive and centrally concentrated dark matter haloes, which slow down the bar rotation through dynamical friction. While predictions suggest that slow bars should be common, measurements of bar pattern speed, using the Tremaine–Weinberg method, show that bars are mostly fast in the local Universe. We present a photometric and kinematic characterization of bars hosted by two dwarf galaxies in the Virgo Cluster, NGC 4483, and NGC 4516. We derive the bar length and strength using the Next Generation Virgo Survey imaging and the circular velocity, bar pattern speed, and rotation rate using spectroscopy from the Multi-Unit Spectroscopic Explorer. Including the previously studied galaxy IC 3167, we compare the bar properties of the three dwarf galaxies with those of their massive counterparts from literature. Bars in the dwarf galaxies are shorter and weaker, and rotate slightly slower with respect to those in massive galaxies. This could be due to a different bar formation mechanism and/or to a large dark matter fraction in the centre of dwarf galaxies. We show that it is possible to push the application of the Tremaine–Weinberg method to the galaxy low-mass regime.