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Nuclear rings in barred galaxies are sites of active star formation. We use hydrodynamic simulations to study the, temporal and spatial behavior of star formation occurring in nuclear rings of barred galaxies where radial gas, inflows are triggered solely by a bar potential. The star formation recipes include a density threshold, an efficiency, conversion of gas to star particles, and delayed momentum feedback via supernova explosions. We find that the, star formation rate (SFR) in a nuclear ring is roughly equal to the mass inflow rate to the ring, while it has a weak, dependence on the total gas mass in the ring. The SFR typically exhibits a strong primary burst followed by weak, secondary bursts before declining to very small values. The primary burst is associated with the rapid gas infall, to the ring due to the bar growth, while the secondary bursts are caused by re-infall of the ejected gas from the, primary burst. While star formation in observed rings persists episodically over a few Gyr, the duration of active, star formation in our models lasts for only about half of the bar growth time, suggesting that the bar potential, alone is unlikely to be responsible for gas supply to the rings. When the SFR is low, most star formation occurs at the contact points between the ring and the dust lanes, leading to an azimuthal age gradient of young star clusters. When the SFR is large, on the other hand, star formation is randomly distributed over the whole circumference, of the ring, resulting in no apparent azimuthal age gradient. Since the ring shrinks in size with time, star clusters, also exhibit a radial age gradient, with younger clusters found closer to the ring. The cluster mass function is well, described by a power law, with a slope depending on the SFR. Giant gas clouds in the rings have supersonic internal, velocity dispersions and are gravitationally bound. [ABSTRACT FROM AUTHOR]

The inner regions of barred galaxies contain substructures such as off-axis shocks, nuclear rings, and nuclear spirals. These substructures may affect star formation, and control the activity of a central black hole (BH) by determining the mass inflow rate. We investigate the formation and properties of such substructures using high-resolution, grid-based hydrodynamic simulations. The gaseous medium is assumed to be infinitesimally thin, isothermal, and non-self-gravitating. The stars and dark matter are represented by a static gravitational potential with four components: a stellar disk, a bulge, a central BH, and a bar. To investigate various galactic environments, we vary the gas sound speed, cs, as well as the mass of the central BH, MBH. Once the flow has reached a quasi-steady state, off-axis shocks tend to move closer to the bar major axis as cs increases. Nuclear rings shrink in size with increasing cs, but are independent of MBH, suggesting that the ring position is not determined by the Lindblad resonances. Rings in low-cs models are narrow since they are occupied largely by gas on x2-orbits and well decoupled from nuclear spirals, while they become broad because of large thermal perturbations in high-cs models. Nuclear spirals persist only when either cs is small or MBH is large; they would otherwise be destroyed completely by the ring material on eccentric orbits. The shape and strength of nuclear spirals depend sensitively on cs and MBH such that they are leading if both cs and MBH are small, weak trailing if cs is small and MBH is large, and strong trailing if both cs and MBH are large. While the mass inflow rate toward the nucleus is quite small in low-cs models because of the presence of a narrow nuclear ring, it becomes larger than 0.01 M⊙ yr-1 when cs is large, providing a potential explanation of nuclear activity in Seyfert galaxies. [ABSTRACT FROM AUTHOR]

We use vertically resolved numerical hydrodynamic simulations to study star formation and the interstellar medium (ISM) in galactic disks. We focus on outer-disk regions where diffuse HI dominates, with gas surface densities Σ = 3-20 M⊙ pc-2 and star-plus-dark matter volume densities ρsd = 0.003-0.5 M⊙ pc-3. Star formation occurs in very dense, self-gravitating clouds that form by mergers of smaller cold cloudlets. Turbulence, driven by momentum feedback from supernova events, destroys bound clouds and puffs up the disk vertically. Time-dependent radiative heating (FUV from recent star formation) offsets gas cooling. We use our simulations to test a new theory for self-regulated star formation. Consistent with this theory, the disks evolve to a state of vertical dynamical equilibrium and thermal equilibrium with both warm and cold phases. The range of star formation surface densities and midplane thermal pressures is ΣSFR ~ 10-4 to 10-2 M⊙ kpc-2 yr-1 and Pth/kB ~ 10² to 104 cm-3 K. In agreement with observations, turbulent velocity dispersions are ~7 km s-1 and the ratio of the total (effective) to thermal pressure is Ptot/Pth ~ 4-5, across this whole range (provided shielding is similar to the solar neighborhood). We show that ΣSFR is not well correlated with Σ alone, but rather with Σ√ρsd, because the vertical gravity from stars and dark matter dominates in outer disks. We also find that ΣSFR has a strong, nearly linear correlation with Ptot, which itself is within ~ 13% of the dynamical equilibrium estimate Ptot,DE. The quantitative relationships we find between ΣSFR and the turbulent and thermal pressures show that star formation is highly efficient for energy and momentum production, in contrast to the low efficiency of mass consumption. Star formation rates adjust until the ISM's energy and momentum losses are replenished by feedback within a dynamical time. [ABSTRACT FROM AUTHOR]

A fraction of dwarf galaxies in the Virgo Cluster contain disk features like bars and spiral arms. Using N-body simulations, we investigate the effects of tidal forces on the formation of such disk features in disk dwarf galaxies resembling VCC 856. We consider eight cluster–galaxy models in which disk dwarf galaxies with differing pericenter distance and spin orientation experience the tidal gravitational force of a Virgo-like Navarro–Frenk–White halo, as well as an additional eight galaxy–galaxy models in which two dwarf galaxies undergo tidal interactions with different strength. We find that the cluster tidal effect is moderate owing to the small galaxy size, making the bars form earlier by ∼1–1.5 Gyr compared to the cases in isolation. While the galactic halos significantly lose their mass within the virial radius due to the cluster tidal force, the mass of the stellar disks is nearly unchanged, suggesting that the inner regions of a disk–halo system are secured from the tidal force. The tidal forcing from either the cluster potential or a companion galaxy triggers the formation of two-armed spirals at early time before a bar develops. The tidally driven arms decay and wind with time, suggesting that they are kinematic density waves. In terms of the strength and pitch angle, the faint arms in VCC 856 are best matched with the arms in a marginally unstable galaxy produced by a distant tidal encounter with its neighbor ∼0.85 Gyr ago. [ABSTRACT FROM AUTHOR]

Gravitational interactions between a protoplanetary disk and its embedded planet are one of the formation mechanisms of gaps and rings found in recent ALMA observations. To quantify the gap properties measured in not only surface density but also rotational velocity profiles, we run two-dimensional hydrodynamic simulations of protoplanetary disks by varying three parameters: the mass ratio q of a planet to a central star, the ratio of the disk scale height hp to the orbital radius rp of the planet, and the viscosity parameter α. We find that the gap depth δΣ in the gas surface density depends on a single dimensionless parameter as , consistent with the previous results of Kanagawa et al. The gap depth δV in the rotational velocity is given by . The gap width, in both surface density and rotational velocity, has a minimum of about 4.7hp when the planet mass Mp is around the disk thermal mass Mth, while it increases in a power-law fashion as Mp/Mth increases or decreases from unity. This minimum in the gap width arises because spirals from sub-thermal planets have to propagate before they shock the disk gas and open a gap. We compare our relations for the gap depth and width with the previous results, and discuss their applicability to observations. [ABSTRACT FROM AUTHOR]

Using a suite of radiation hydrodynamic simulations of star cluster formation in turbulent clouds, we study the escape fraction of ionizing (Lyman continuum) and non-ionizing (FUV) radiation for a wide range of cloud masses and sizes. The escape fraction increases as H ii regions evolve and reaches unity within a few dynamical times. The cumulative escape fraction before the onset of the first supernova explosion is in the range 0.05–0.58; this is lower for higher initial cloud surface density, and higher for less massive and more compact clouds due to rapid destruction. Once H ii regions break out of their local environment, both ionizing and non-ionizing photons escape from clouds through fully ionized, low-density sight lines. Consequently, dust becomes the dominant absorber of ionizing radiation at late times, and the escape fraction of non-ionizing radiation is only slightly larger than that of ionizing radiation. The escape fraction is determined primarily by the mean and width σ of the optical-depth distribution in the large-scale cloud, increasing for smaller and/or larger σ. The escape fraction exceeds (sometimes by three orders of magnitude) the naive estimate due to the nonzero σ induced by turbulence. We present two simple methods to estimate, within ∼20%, the escape fraction of non-ionizing radiation using the observed dust optical depth in clouds projected on the plane of sky. We discuss implications of our results for observations, including inference of star formation rates in individual molecular clouds and accounting for diffuse ionized gas on galactic scales. [ABSTRACT FROM AUTHOR]

In this work, we present the analysis of the binary microlensing event OGLE-2018-BLG-0022 that is detected toward the Galactic bulge field. The dense and continuous coverage with the high-quality photometry data from ground-based observations combined with the space-based Spitzer observations of this long timescale event enables us to uniquely determine the masses M1 = 0.40 ± 0.05 M⊙ and M2 = 0.13 ± 0.01 M⊙ of the individual lens components. Because the lens-source relative parallax and the vector lens-source relative proper motion are unambiguously determined, we can likewise unambiguously predict the astrometric offset between the light centroid of the magnified images (as observed by the Gaia satellite) and the true position of the source. This prediction can be tested when the individual-epoch Gaia astrometric measurements are released. [ABSTRACT FROM AUTHOR]

We run self-consistent simulations of Milky Way-sized, isolated disk galaxies to study the formation and evolution of a stellar bar as well as a nuclear ring in the presence of gas. We consider two sets of models with cold or warm disks that differ in the radial velocity dispersions, and vary the gas fraction by fixing the total disk mass. A bar forms earlier and more strongly in the cold disks with larger , while gas progressively delays the bar formation in the warm disks. The bar formation enhances a central mass concentration, which in turn causes the bar to decay temporarily, after which it regrows in size and strength, eventually becoming stronger in models with smaller . Although all bars rotate fast in the beginning, they rapidly turn to slow rotators. In our models, only the gas-free, warm disk undergoes rapid buckling instability, while other disks thicken more gradually via vertical heating. The gas driven inward by the bar potential readily forms a star-forming nuclear ring. The ring is very small when it first forms and grows in size over time. The ring star formation rate is episodic and bursty due to feedback, and is well correlated with the mass inflow rate to the ring. Some expanding shells produced by star formation feedback are sheared out in the bar regions and collide with dust lanes to appear as filamentary interbar spurs. The bars and nuclear rings formed in our simulations have properties similar to those in the Milky Way. [ABSTRACT FROM AUTHOR]

We present the analysis of a binary microlensing event, KMT-2016-BLG-2052, for which the lensing-induced brightening of the source star lasted for two seasons. We determine the lens mass from the combined measurements of the microlens parallax, πE, and angular Einstein radius, θE. The measured mass indicates that the lens is a binary composed of M dwarfs with masses of M1 ∼ 0.34 M⊙ and M2 ∼ 0.17 M⊙. The measured relative lens-source proper motion of μ ∼ 3.9 mas yr−1 is smaller than ∼5 mas yr−1 of typical Galactic lensing events, while the estimated angular Einstein radius of θE ∼ 1.2 mas is substantially greater than the typical value of ∼0.5 mas. Therefore, it turns out that the long timescale of the event is caused by the combination of the slow μ and large θE rather than the heavy mass of the lens. From the simulation of Galactic lensing events with very long timescales (tE ≳ 100 days), we find that the probabilities that long timescale events are produced by lenses with masses ≥1.0 M⊙ and ≥3.0 M⊙ are ∼19% and 2.6%, respectively, indicating that events produced by heavy lenses comprise a minor fraction of long timescale events. The results indicate that it is essential to determine lens masses by measuring both πE and θE in order to firmly identify heavy stellar remnants, such as neutron stars and black holes. [ABSTRACT FROM AUTHOR]

Utilizing James Clark Maxwell Telescope 850 μm SCUPOL dust polarization data, we investigate the configuration of the magnetic (B) field in the circumnuclear disk (CND) of the Galactic center. The SCUPOL data show a highly improved polarization coverage and resolution compared with earlier 100 μm observations. The 850 μm data have a resolution and coverage similar to previous 350 μm polarimetry data. However, with a proper sampling on a 10″ grid, we find that the 850 μm data trace the morphological structures of the CND substantially better. Furthermore, because the 850 μm trace the field deeper into the material near SgrA*, they represent the highest-resolution submillimeter probe to date of the CND magnetic field. The observed B-field morphology is well described by a self-similar axisymmetric disk model where the radial infall velocity is one-quarter of the rotational velocity. A detailed comparison with higher-resolution interferometric maps from the Submillimeter Array further reveals that the B-field aligns with the neutral gas streamers connecting to the CND. Moreover, the innermost observed B-field structure also appears to trace and align with the mini-spiral located inside the CND. This suggests that there is one underlying B-field structure that is connecting the CND with its streamers and the inner mini-spiral. An estimate of βPlasma ≲ 1—based on the global B-field morphology that constrains the azimuthal-to-vertical field strength ratio of around 40 combined with a measurement of the azimuthal velocity indicates that the B-field appears dynamically significant toward the CND and also onwards to the inner mini-spiral. [ABSTRACT FROM AUTHOR]

UV radiation feedback from young massive stars plays a key role in the evolution of giant molecular clouds (GMCs) by photoevaporating and ejecting the surrounding gas. We conduct a suite of radiation hydrodynamic simulations of star cluster formation in marginally bound, turbulent GMCs, focusing on the effects of photoionization and radiation pressure on regulating the net star formation efficiency (SFE) and cloud lifetime. We find that the net SFE depends primarily on the initial gas surface density, Σ0, such that the SFE increases from 4% to 51% as Σ0 increases from 13 to . Cloud destruction occurs within 2–10 Myr after the onset of radiation feedback, or within 0.6–4.1 freefall times (increasing with Σ0). Photoevaporation dominates the mass loss in massive, low surface density clouds, but because most photons are absorbed in an ionization-bounded Strömgren volume, the photoevaporated gas fraction is proportional to the square root of the SFE. The measured momentum injection due to thermal and radiation pressure forces is proportional to , and the ejection of neutrals substantially contributes to the disruption of low mass and/or high surface density clouds. We present semi-analytic models for cloud dispersal mediated by photoevaporation and by dynamical mass ejection, and show that the predicted net SFE and mass loss efficiencies are consistent with the results of our numerical simulations. [ABSTRACT FROM AUTHOR]

We present an implementation of an adaptive ray-tracing (ART) module in the Athena hydrodynamics code that accurately and efficiently handles the radiative transfer involving multiple point sources on a three-dimensional Cartesian grid. We adopt a recently proposed parallel algorithm that uses nonblocking, asynchronous MPI communications to accelerate transport of rays across the computational domain. We validate our implementation through several standard test problems, including the propagation of radiation in vacuum and the expansions of various types of H ii regions. Additionally, scaling tests show that the cost of a full ray trace per source remains comparable to that of the hydrodynamics update on up to processors. To demonstrate application of our ART implementation, we perform a simulation of star cluster formation in a marginally bound, turbulent cloud, finding that its star formation efficiency is 12% when both radiation pressure forces and photoionization by UV radiation are treated. We directly compare the radiation forces computed from the ART scheme with those from the M1 closure relation. Although the ART and M1 schemes yield similar results on large scales, the latter is unable to resolve the radiation field accurately near individual point sources. [ABSTRACT FROM AUTHOR]

The interaction between a supermassive black hole (SMBH) and the surrounding material is of primary importance in modern astrophysics. The detection of the molecular 2 pc circumnuclear disk (CND) immediately around the Milky Way SMBH, SgrA*, provides a unique opportunity to study SMBH accretion at subparsec scales. Our new wide-field CS(J = 2 − 1) map toward the Galactic center (GC) reveals multiple dense molecular streamers that originated from the ambient clouds 20 pc further out, and that are connected to the central 2 pc of the CND. These dense gas streamers appear to carry gas directly toward the nuclear region and might be captured by the central potential. Our phase-plot analysis indicates that these streamers show a signature of rotation and inward radial motion with progressively higher velocities as the gas approaches the CND and finally ends up corotating with the CND. Our results might suggest a possible mechanism of gas feeding the CND from 20 pc around 2 pc in the GC. In this paper, we discuss the morphology and the kinematics of these streamers. As the nearest observable Galactic nucleus, this feeding process may have implications for understanding the processes in extragalactic nuclei. [ABSTRACT FROM AUTHOR]

We present a new face-on map of dense neutral atomic hydrogen ( i) gas in the outer Galaxy. Our map has been produced from the Leiden/Argentine/Bonn i 21 cm line all-sky survey by finding intensity maxima along every line of sight and then by projecting them on the Galactic plane. The resulting face-on map strikingly reveals the complex spiral structure beyond the solar circle, which is characterized by a mixture of distinct long arcs of i concentrations and numerous “interarm” features. The comparison with more conventional spiral tracers confirms the nature of those long arc structures as spiral arms. Our map shows that the i spiral structure in the outer Galaxy is well described by a four-arm spiral model (pitch angle of ) with some deviations, and gives a new insight into identifying i features associated with individual arms. [ABSTRACT FROM AUTHOR]

A fraction of early-type dwarf galaxies in the Virgo cluster have a disk component and even possess disk features such as bar, lens, and spiral arms. In this study, we construct 15 galaxy models that resemble VCC856, which is considered to be an infalling progenitor of disk dwarf galaxies, within observational error ranges, and use N-body simulations to study their long-term dynamical evolution in isolation as well as the formation of bar in them. We find that dwarf disk galaxies readily form bars unless they have an excessively concentrated halo or a hot disk. This suggests that infalling dwarf disk galaxies are intrinsically unstable to bar formation, even without any external perturbation, accounting for a population of barred dwarf galaxies in the outskirts of the Virgo cluster. The bars form earlier and stronger in galaxies with a lower fraction of counter-streaming motions, lower halo concentration, lower velocity anisotropy, and thinner disk. Similarly to normal disk galaxies, dwarf disk galaxies also undergo recurrent buckling instabilities. The first buckling instability tends to shorten the bar and to thicken the disk, and drives a dynamical transition in the bar pattern speed as well as mass inflow rate. In nine models, the bars regrow after the mild first buckling instability due to the efficient transfer of disk angular momentum to the halo, and are subject to recurrent buckling instabilities to turn into X-shaped bulges. [ABSTRACT FROM AUTHOR]

Nuclear rings at the centers of barred galaxies exhibit strong star formation activities. They are thought to undergo gravitational instability when they are sufficiently massive. We approximate them as rigidly rotating isothermal objects and investigate their gravitational instability. Using a self-consistent field method, we first construct their equilibrium sequences specified by two parameters: α corresponding to the thermal energy relative to gravitational potential energy, and measuring the ellipticity or ring thickness. Unlike in the incompressible case, not all values of yield an isothermal equilibrium, and the range of for such equilibria shrinks with decreasing α. The density distributions in the meridional plane are steeper for smaller α, and well approximated by those of infinite cylinders for slender rings. We also calculate the dispersion relations of non-axisymmetric modes in rigidly rotating slender rings with angular frequency Ω0 and central density . Rings with smaller α are found more unstable with a larger unstable range of the azimuthal mode number. The instability is completely suppressed by rotation when Ω0 exceeds the critical value. The critical angular frequency is found to be almost constant at for α ≳ 0.01 and increases rapidly for smaller α. We apply our results to a sample of observed star-forming rings and confirm that rings without a noticeable azimuthal age gradient of young star clusters are indeed gravitationally unstable. [ABSTRACT FROM AUTHOR]

Dynamical expansion of H ii regions around star clusters plays a key role in dispersing the surrounding dense gas and therefore in limiting the efficiency of star formation in molecular clouds. We use a semianalytic method and numerical simulations to explore expansion of spherical dusty H ii regions and surrounding neutral shells and the resulting cloud disruption. Our model for shell expansion adopts the static solutions of Draine for dusty H ii regions and considers the contact outward forces on the shell due to radiation and thermal pressures, as well as the inward gravity from the central star and the shell itself. We show that the internal structure we adopt and the shell evolution from the semianalytic approach are in good agreement with the results of numerical simulations. Strong radiation pressure in the interior controls the shell expansion indirectly by enhancing the density and pressure at the ionization front. We calculate the minimum star formation efficiency required for cloud disruption as a function of the cloud’s total mass and mean surface density. Within the adopted spherical geometry, we find that typical giant molecular clouds in normal disk galaxies have , with comparable gas and radiation pressure effects on shell expansion. Massive cluster-forming clumps require a significantly higher efficiency of for disruption, produced mainly by radiation-driven expansion. The disruption time is typically of the order of a free-fall timescale, suggesting that the cloud disruption occurs rapidly once a sufficiently luminous H ii region is formed. We also discuss limitations of the spherical idealization. [ABSTRACT FROM AUTHOR]

It has been suggested that the wiggle instability (WI) of spiral shocks in a galactic disk is responsible for the formation of gaseous feathers observed in grand-design spiral galaxies. We perform both a linear stability analysis and numerical simulations to investigate the effect of magnetic fields on the WI. The disk is assumed to be infinitesimally thin, isothermal, and non-self-gravitating. We control the strengths of magnetic fields and spiral-arm forcing using the dimensionless parameters β and , respectively. By solving the perturbation equations as a boundary-eigenvalue problem, we obtain dispersion relations of the WI for various values of and and 10%. We find that the WI arising from the accumulation of potential vorticity at disturbed shocks is suppressed, albeit not completely, by magnetic fields. The stabilizing effect of magnetic fields is not from the perturbed fields but from the unperturbed fields that reduce the density compression factor in the background shocks. When and or and β ∼ 5–10, the most unstable mode has a wavelength of ∼0.1–0.2 times the arm-to-arm separation, which appears consistent with a mean spacing of observed feathers. [ABSTRACT FROM AUTHOR]

Using self-consistent three-dimensional (3D) N-body simulations, we investigate the physical properties of nonaxisymmetric features in a disk galaxy created by a tidal interaction with its companion. The primary galaxy consists of a stellar disk, a bulge, and a live halo, corresponding to Milky-Way-type galaxies, while the companion is represented by a halo alone. We vary the companion mass and the pericenter distance to explore situations with differing tidal strength parameterized by either the relative tidal force P or the relative imparted momentum S. We find that the formation of a tidal tail in the outer parts requires or . A stronger interaction results in a stronger, less wound tail that forms earlier. Similarly, a stronger tidal forcing produces stronger, more loosely wound spiral arms in the inner parts. The arms are approximately logarithmic in shape, with both amplitude and pitch angle decaying with time. The derived pattern speed decreases with radius and is close to the curve at late time, with Ω and κ denoting the angular and epicycle frequencies, respectively. This suggests that the tidally induced spiral arms are most likely kinematic density waves weakly modified by self-gravity. Compared to the razor-thin counterparts, arms in the 3D models are weaker, have a smaller pitch angle, and wind and decay more rapidly. The 3D density structure of the arms is well described by the concentrated and sinusoidal models when the arms are in the nonlinear and linear regimes, respectively. We demonstrate that dynamical friction between interacting galaxies transfers the orbital angular momentum of one galaxy to the spin angular momentum of the companion halo. [ABSTRACT FROM AUTHOR]

Dust lanes, nuclear rings, and nuclear spirals are typical gas structures in the inner region of barred galaxies. Their shapes and properties are linked to the physical parameters of the host galaxy. We use high-resolution hydrodynamical simulations to study 2D gas flows in simple barred galaxy models. The nuclear rings formed in our simulations can be divided into two groups: one group is nearly round and the other is highly elongated. We find that roundish rings may not form when the bar pattern speed is too high or the bulge central density is too low. We also study the periodic orbits in our galaxy models, and find that the concept of inner Lindblad resonance (ILR) may be generalized by the extent of orbits. All roundish nuclear rings in our simulations settle in the range of orbits (or ILRs). However, knowing the resonances is insufficient to pin down the exact location of these nuclear rings. We suggest that the backbone of round nuclear rings is the orbital family, i.e., round nuclear rings are allowed only in the radial range of orbits. A round nuclear ring forms exactly at the radius where the residual angular momentum of infalling gas balances the centrifugal force, which can be described by a parameter measured from the rotation curve. The gravitational torque on gas in high pattern speed models is larger, leading to a smaller ring size than in the low pattern speed models. Our result may have important implications for using nuclear rings to measure the parameters of real barred galaxies with 2D gas kinematics. [ABSTRACT FROM AUTHOR]