Local stability of self-gravitating fluid disks made of two components in relative motion

Context. We consider a simple self-gravitating disk, made of two fluid components characterized by different effective thermal speeds and interacting with one another only through gravity; two-component models of this type have often been considered in order to estimate the impact of the cold interstellar medium on gravitational instabilities in star-dominated galaxy disks. Aims. This simple model allows us to produce a unified description of instabilities in non-viscous self-gravitating disks, some originating from Jeans collapse, and others from the relative motion between the two components. In particular, the model suggests that the small streaming velocity between the two components associated with the so-called asymmetric drift may be the origin of instability for suitable non-axisymmetric perturbations. Methods. The result is obtained by examining the properties of a local, linear dispersion relation for tightly wound density waves in such two-component model. The parameters characterizing the equilibrium model and the related dispersion relation allow us to recover as natural limits the cases, known in the literature, in which the relative drift between the two components is ignored. Results. Dynamically, the instability is similar to (although gentler than) that known to affect counter-rotating disks. However, in contrast to the instability induced by counter-rotation, which is a relatively rare phenomenon, the mechanism discussed in this paper is likely to be rather common in nature. Conclusions. We briefly indicate some consequences of the instability on the evolution of galaxy disks and possible applications to other astrophysical systems, in particular to protostellar disks and accretion disks.Comment: 9 pages, 5 figures, Astronomy & Astrophysics, in pres

Galaxy-Induced Transformation of Dark Matter Halos

We use N-body/gasdynamical LambdaCDM cosmological simulations to examine the effect of the assembly of a central galaxy on the shape and mass profile of its dark halo. Two series of simulations are compared; one that follows only the evolution of the dark matter component and a second one where a baryonic component is added. These simulations include radiative cooling but neglect star formation and feedback, leading most baryons to collect at the halo center in a disk which is too small and too massive when compared with typical spiral. This unrealistic model allows us, nevertheless, to gauge the maximum effect that galaxies may have in transforming their dark halos. We find that the shape of the halo becomes more axisymmetric: halos are transformed from triaxial into essentially oblate systems, with well-aligned isopotential contours of roughly constant flattening (c/a ~ 0.85). Halos always contract as a result of galaxy assembly, but the effect is substantially less pronounced than predicted by the "adiabatic contraction" hypothesis. The reduced contraction helps to reconcile LambdaCDM halos with constraints on the dark matter content inside the solar circle and should alleviate the long-standing difficulty of matching simultaneously the scaling properties of galaxy disks and the luminosity function. The halo contraction is also less pronounced than found in earlier simulations, a disagreement that suggests that halo contraction is not solely a function of the initial and final distribution of baryons. Not only how much baryonic mass has been deposited at the center of a halo matters, but also the mode of its deposition. It might prove impossible to predict the halo response without a detailed understanding of a galaxy's assembly history. (Abriged)Comment: 11 pages and 9 figure

The dynamics of stars around spiral arms

Spiral density wave theory attempts to describe the spiral pattern in spiral galaxies in terms of a long-lived wave structure with a constant pattern speed in order to avoid the winding dilemma. The pattern is consequently a rigidly rotating, long-lived feature. We run N-body simulations of a giant disc galaxy consisting of a pure stellar disc and a static dark matter halo, and find that the spiral arms are transient features whose pattern speeds decrease with radius in such a way that the pattern speed is almost equal to the rotation curve of the galaxy. We trace particle motion around the spiral arms. We show that particles from behind and in front of the spiral arm are drawn towards and join the arm. Particles move along the arm in the radial direction and we find a clear trend that they migrate towards the outer (inner) radii on the trailing (leading) side of the arm. Our simulations demonstrate that because the spiral arm feature is corotating, the particles continue to be accelerated (decelerated) by the spiral arm for long periods, which leads to strong and efficient migration, at all radii in the disc

Ring Star Formation Rates in Barred and Nonbarred Galaxies

Nonbarred ringed galaxies are relatively normal galaxies showing bright rings of star formation in spite of lacking a strong bar. This morphology is interesting because it is generally accepted that a typical ring forms when material collects near a resonance, set up by the pattern speed of a bar or bar-like perturbation. Our goal in this paper is to examine whether the ring star formation properties are related to the non-axisymmetric gravity potential in general. For this purpose, we obtained H{\alpha} emission line images and calculated the line fluxes and star formation rates (SFRs) for 16 nonbarred SA galaxies and four weakly barred SAB galaxies with rings. For comparison, we combine our observations with a re-analysis of previously published data on five SA, seven SAB, and 15 SB galaxies with rings, three of which are duplicates from our sample. With these data, we examine what role a bar may play in the star formation process in rings. Compared to barred ringed galaxies, we find that the inner ring SFRs and H{\alpha}+[N ii] equivalent widths in nonbarred ringed galaxies show a similar range and trend with absolute blue magnitude, revised Hubble type, and other parameters. On the whole, the star formation properties of inner rings, excluding the distribution of H ii regions, are independent of the ring shapes and the bar strength in our small samples. We confirm that the deprojected axis ratios of inner rings correlate with maximum relative gravitational force Q_g; however, if we consider all rings, a better correlation is found when local bar forcing at the radius of the ring, Q_r, is used. Individual cases are described and other correlations are discussed. By studying the physical properties of these galaxies, we hope to gain a better understanding of their placement in the scheme of the Hubble sequence and how they formed rings without the driving force of a bar.Comment: 55 pages; 21 figures and 9 tables. Article has been accepted for publication in the Astronomical Journa

The Swift/UVOT catalogue of NGC4321 star forming sources: A case against density wave theory

We study the star forming regions in the spiral galaxy NGC4321, taking advantage of the spatial resolution (2.5 arcsec FWHM) of the Swift/UVOT camera and the availability of three UV passbands in the region 1600-3000 A, in combination with optical and IR imaging from SDSS, KPNO/Ha and Spitzer/IRAC, to obtain a catalogue of 787 star forming regions out to three disc scale lengths. We determine the properties of the young stellar component and its relationship with the spiral arms. The Ha luminosities of the sources have a strong decreasing radial trend, suggesting more massive star forming regions in the central part of the galaxy. When segregated with respect to NUV-optical colour, blue sources have a significant excess of flux in the IR at 8 micron, revealing the contribution from PAHs, although the overall reddening of these sources stays below E(B-V)=0.2 mag. The distribution of distances to the spiral arms is compared for subsamples selected according to Ha luminosity, NUV-optical colour, or ages derived from a population synthesis model. An offset is expected between these subsamples as a function of radius if the pattern speed of the spiral arm were constant - as predicted by classic density wave theory. No significant offsets are found, favouring instead a mechanism where the pattern speed has a radial dependence.Comment: 12 pages, 11 figures, 4 tables. MNRAS, in pres

Grand Design and Flocculent Spirals in the Spitzer Survey of Stellar Structure in Galaxies (S4G)

Spiral arm properties of 46 galaxies in the Spitzer Survey of Stellar Structure in Galaxies (S4G) were measured at 3.6mu, where extinction is small and the old stars dominate. The sample includes flocculent, multiple arm, and grand design types with a wide range of Hubble and bar types. We find that most optically flocculent galaxies are also flocculent in the mid-IR because of star formation uncorrelated with stellar density waves, whereas multiple arm and grand design galaxies have underlying stellar waves. Arm-interarm contrasts increase from flocculent to multiple arm to grand design galaxies and with later Hubble types. Structure can be traced further out in the disk than in previous surveys. Some spirals peak at mid-radius while others continuously rise or fall, depending on Hubble and bar type. We find evidence for regular and symmetric modulations of the arm strength in NGC 4321. Bars tend to be long, high amplitude, and flat-profiled in early type spirals, with arm contrasts that decrease with radius beyond the end of the bar, and they tend to be short, low amplitude, and exponential-profiled in late Hubble types, with arm contrasts that are constant or increase with radius. Longer bars tend to have larger amplitudes and stronger arms.Comment: 31 pages, 14 figures, ApJ in pres

Tracing spiral density waves in M81

We use SPITZER IRAC 3.6 and 4.5micron near infrared data from the Spitzer Infrared Nearby Galaxies Survey (SINGS), optical B, V and I and 2MASS Ks band data to produce mass surface density maps of M81. The IRAC 3.6 and 4.5micron data, whilst dominated by emission from old stellar populations, is corrected for small-scale contamination by young stars and PAH emission. The I band data are used to produce a mass surface density map by a B-V colour-correction, following the method of Bell and de Jong. We fit a bulge and exponential disc to each mass map, and subtract these components to reveal the non-axisymmetric mass surface density. From the residual mass maps we are able to extract the amplitude and phase of the density wave, using azimuthal profiles. The response of the gas is observed via dust emission in the 8micron IRAC band, allowing a comparison between the phase of the stellar density wave and gas shock. The relationship between this angular offset and radius suggests that the spiral structure is reasonably long lived and allows the position of corotation to be determined.Comment: 15 pages, 17 figures, accepted for publication in MNRA

A multi-energy system optimisation software for advance process control using hypernetworks and a micro-service architecture

This paper describes a multi-energy system optimisation software, “Sustainable Energy Management System” (SEMS), developed as part of a Siemens, Greater London Authority and Royal Borough of Greenwich partnership in collaboration with the University of Nottingham, Nottingham Trent University and Imperial College London. The software was developed for application at a social housing estate in Greenwich, London, as part of the Borough’s efforts to retrofit the energy systems and building fabric of its housing stock. Its purpose is to balance energy across vectors and networks through day-ahead forecasting and optimisations that can be interpreted as control outputs for energy plant such as a water source heat pump, district heating pumps and values, power switchgear, gas boilers, a thermal store, electric vehicle chargers and a photovoltaic array. The optimisation objectives are to minimise greenhouse gas emissions and operational cost. The tool uses Hypernetwork Theory based orchestration coupled with a microservice architecture. The distributed nature of the design ensures flexibility and scalability. Currently, microservices have been programmed to forecast domestic heating demand, domestic electricity demand, electric vehicle demand, solar photovoltaic generation, ground temperature, and to run a day-ahead energy balance optimisation. This paper presents the results from both domestic heat and electricity demand forecasting, as well as the overall design and integration of the software with a physical system. The works build on that of O’Dwyer, et al. (2020) who developed a preliminary energy management software and digital twin. Their work acts as a foundation for this real-world commercialisation-ready program that integrates with physical assets

Simulations of the grand design galaxy M51: a case study for analysing tidally induced spiral structure

We present hydrodynamical models of the grand design spiral M51 (NGC 5194), and its interaction with its companion NGC 5195. Despite the simplicity of our models, our simulations capture the present day spiral structure of M51 remarkably well, and even reproduce details such as a kink along one spiral arm, and spiral arm bifurcations. We investigate the offset between the stellar and gaseous spiral arms, and find at most times (including the present day) there is no offset between the stars and gas to within our error bars. We also compare our simulations with recent observational analysis of M51. We compute the pattern speed versus radius, and like the observations, find no single global pattern speed. We also show that the spiral arms cannot be fitted well by logarithmic spirals. We interpret these findings as evidence that M51 does not exhibit a quasi-steady density wave, as would be predicted by density wave theory. The internal structure of M51 derives from the complicated and dynamical interaction with its companion, resulting in spiral arms showing considerable structure in the form of short-lived kinks and bifurcations. Rather than trying to model such galaxies in terms of global spiral modes with fixed pattern speeds, it is more realistic to start from a picture in which the spiral arms, while not being simple material arms, are the result of tidally induced kinematic density `waves' or density patterns, which wind up slowly over time.Comment: 23 pages, 20 figures, accepted for publication in MNRA

Spatial Diffusion of Stars in the Inner Galactic Bulge

Star formation in the inner few hundred pc of the Galactic bulge occurs in a flattened molecular layer called the central molecular zone (CMZ). Serabyn & Morris (1996) suggest that the star formation in the CMZ has been sustained for the lifetime of the Galaxy, and that the resulting agglomeration of stars formed in the CMZ has resulted in the prominent r^-2 stellar density cusp at the Galactic center having about the same physical extent as the CMZ. This "central cusp" is somewhat less flat than the CMZ; thus the population of stars formed in the CMZ appears to have diffused out to larger latitudes. We hypothesize that such vertical diffusion is driven by the scattering of stars off the giant molecular clouds (GMC) in the CMZ, and perform numerical simulations of the scattering between stars and GMCs in the presence of the non-axisymmetric background potential. The simulation results show that the time scale for an initially flattened stellar population to achieve an aspect ratio of the observed OH/IR stars in the inner bulge, 1 to 2 Gyr, agrees well with the estimated age of those OH/IR stars.Comment: To appear in ApJ, Jun 2001 issu