An adaptive algorithm for N-body field expansions

An expansion of a density field or particle distribution in basis functions that solve the Poisson equation both provides an easily parallelized N-body force algorithm and simplifies perturbation theories. The expansion converges quickly and provides the highest computational advantage if the lowest order potential-density pair in the basis looks like the unperturbed galaxy or stellar system. Unfortunately, there are only a handful of such bases in the literature that limit this advantage. This paper presents an algorithm for deriving these bases to match a wide variety of galaxy models. The method is based on efficient numerical solution of the Sturm-Liouville equation and can be used for any geometry with a separable Laplacian. Two cases are described in detail. First, for the spherical case, the lowest order basis function pair may be chosen to be exactly that of the underlying model. The profile may be (1) cuspy or have a core and (2) truncated or of infinite extent. Second, the method yields a three-dimensional cylindrical basis appropriate for studying galactic disks. In this case, the vertical and radial bases are coupled; the lowest order radial part of the basis function can be chosen to match the underlying profile only in the disk plane. Practically, this basis is still a very good match to the overall disk profile and converges in a small number of terms. The ease of combining several bases makes this force solver ideally suited to multicomponent simulations, such as those of disks embedded in halos

The dynamics of tidal tails from massive satellites

We investigate the dynamical mechanisms responsible for producing tidal tails from dwarf satellites using N-body simulations. We describe the essential dynamical mechanisms and morphological consequences of tail production in satellites with masses greater than 0.0001 of the host halo virial mass. We identify two important dynamical coconspirators: (1) the points where the attractive force of the host halo and satellite are balanced (X-points) do not occur at equal distances from the satellite centre or at the same equipotential value for massive satellites, breaking the morphological symmetry of the leading and trailing tails and (2) the escaped ejecta in the leading (trailing) tail continues to be decelerated (accelerated) by the satellite\u27s gravity leading to large offsets of the ejecta orbits from the satellite orbit. The effect of the satellite\u27s self-gravity decreases only weakly with a decreasing ratio of satellite mass to host halo mass, proportional to (Ms/Mh)1/3, demonstrating the importance of these effects over a wide range of subhalo masses. Not only will the morphology of the leading and trailing tails for massive satellites be different, but the observed radial velocities of the tails will be displaced from that of the satellite orbit; both the displacement and the maximum radial velocity is proportional to satellite mass. If the tails are assumed to follow the progenitor satellite orbits, the tails from satellites with masses greater than 0.0001 of the host halo virial mass in a spherical halo will appear to indicate a flattened halo. Therefore, a constraint on the Milky Way halo shape using tidal streams requires mass-dependent modelling. Similarly, we compute the distribution of tail orbits both in Er–r−2 space and in E–Lz space, advocated for identifying satellite stream relics. The acceleration of ejecta by a massive satellite during escape spreads the velocity distribution and obscures the signature of a well-defined ‘moving group’ in phase space. Although these findings complicate the interpretation of stellar streams and moving groups, the intrinsic mass dependence provides additional leverage on both halo and progenitor satellite properties

3D MRI Data Curation for Deep Learning-based Brain Shift Simulation during Tumor Resection

https://openworks.mdanderson.org/sumexp21/1130/thumbnail.jp

Mechanical Thrombectomy in Acute Ischemic Stroke Patients Greater than 90 years of age experience in 26 patients in a Large Tertiary Care Center: Outcome comparison with younger patients

Introduction: Several independent randomized control trials have shown the superior efficacy of mechanical thrombectomy for acute ischemic stroke (AIS). However, the elderly has been underrepresented or excluded in these trials. In this study, we investigated the feasibility and safety of mechanical thrombectomy in patients with AIS aged 90 years or greater. Methods: A retrospective review of patients age 90 years or older presenting with AIS who underwent mechanical thrombectomy between 2010 and 2018. Results: Of total 453 patients with AIS, 5.74 % (26) were aged 90 or older, and 69.32 % (314) ranged from 60-89 years of age. Of all baseline characteristics between both groups, there is a significant difference in age, gender, body mass index (BMI), smoking, hyperlipidemia (HLD), atrial fibrillation, and diabetes mellitus. The mean NIHSS upon admission was higher in the nonagenarians (17 vs. 15). Similar proportions of both groups received tPA (57.69%, 15 vs. 42.68%, 134, p=0.14). There was no difference in peri & post-procedural complications, good TICI score (88.46%, 23 vs. 87.58%, 275, p=1.00), “good” mRS scores (34.62%, 4 vs. 49.36%, 155, p=0.40), and mortality (11.54%, 3 vs. 13.06%, 41, p= 0.82). Discussion: Age is one of the factors that affect functional outcome following mechanical thrombectomy. Advancements in catheter techniques, technical experience, and great outcomes with mechanical thrombectomy allow for pushing the envelope to deal with age as one of the factors, rather, than an exclusion criterion. Our results show that mechanical thrombectomy is safe and feasible in nonagenarians

Biomechanical Modeling of Brain Shift During Neurosurgery

https://openworks.mdanderson.org/sumexp22/1051/thumbnail.jp

The optical and near-infrared properties of galaxies. I. Luminosity and stellar mass functions

We use a large sample of galaxies from the Two Micron All Sky Survey (2MASS) and the Sloan Digital Sky Survey (SDSS) to calculate galaxy luminosity and stellar mass functions in the local universe. We estimate corrections for passband shifting and galaxy evolution, as well as present-day stellar mass-to-light (M/L) ratios, by fitting the optical-near-infrared galaxy data with simple models. Accounting for the 8% galaxy overdensity in the SDSS early data release region, the optical and near-infrared luminosity functions we construct for this sample agree with most recent literature optical and near-infrared determinations within the uncertainties. We argue that 2MASS is biased against low surface brightness galaxies and use SDSS plus our knowledge of stellar populations to estimate the true K-band luminosity function. This has a steeper faint end slope and a slightly higher overall luminosity density than the direct estimate. Furthermore, assuming a universally applicable stellar initial mass function (IMF), we find good agreement between the stellar mass function we derive from the 2MASS/SDSS data and that derived by Cole et al. The faint end slope for the stellar mass function is steeper than -1.1, reflecting the low stellar M/L ratios characteristic of low-mass galaxies. We estimate an upper limit to the stellar mass density in the local universe Ω*h = 2.0 ± 0.6 × 10-3 by assuming an IMF as rich in low-mass stars as allowed by observations of galaxy dynamics in the local universe. The stellar mass density may be lower than this value if a different IMF with fewer low-mass stars is assumed. Finally, we examine type-dependence in the optical and near-infrared luminosity functions and the stellar mass function. In agreement with previous work, we find that the characteristic luminosity or mass of early-type galaxies is larger than for later types, and the faint end slope is steeper for later types than for earlier types. Accounting for typing uncertainties, we estimate that at least half, and perhaps as much as 3/4, of the stellar mass in the universe is in early-type galaxies. As an aid to workers in the field, we present in an Appendix the relationship between model stellar M/L ratios and colors in SDSS/2MASS passbands, an updated discussion of near-infrared stellar M/L ratio estimates, and the volume-corrected distribution of g- and K-band stellar M/L ratios as a function of stellar mass

Homeostatic competition drives tumor growth and metastasis nucleation

We propose a mechanism for tumor growth emphasizing the role of homeostatic regulation and tissue stability. We show that competition between surface and bulk effects leads to the existence of a critical size that must be overcome by metastases to reach macroscopic sizes. This property can qualitatively explain the observed size distributions of metastases, while size-independent growth rates cannot account for clinical and experimental data. In addition, it potentially explains the observed preferential growth of metastases on tissue surfaces and membranes such as the pleural and peritoneal layers, suggests a mechanism underlying the seed and soil hypothesis introduced by Stephen Paget in 1889 and yields realistic values for metastatic inefficiency. We propose a number of key experiments to test these concepts. The homeostatic pressure as introduced in this work could constitute a quantitative, experimentally accessible measure for the metastatic potential of early malignant growths.Comment: 13 pages, 11 figures, to be published in the HFSP Journa

Do Observations Favour Galileon Over Quintessence?

We study the Galileon scalar field model arising as a decoupling limit of the Dvali-Gababdaze-Porrati (DGP) construction for the late time acceleration of the universe. The model has one extra Galileon correction term over and above the standard kinetic and potential energy terms for a canonical quintessence field. We aim to study whether the current observational data can distinguish between this Galileon field and the quintessence field. Our study shows the remarkable result that for potentials like linear, square or exponential, the data prefers the Galileon model over quintessence field. It confirms that the observable universe demands the inclusion of higher derivative Galileon corrections in the standard quintessence scalar field models.Comment: 5 pages, Revtex style, three eps figures, revised version with new comments added. Conclusion is unchange