Star cluster evolution in barred disc galaxies. I. Planar periodic orbits

The dynamical evolution of stellar clusters is driven to a large extent by their environment. Several studies so far have considered the effect of tidal fields and their variations, such as, e.g., from giant molecular clouds, galactic discs, or spiral arms. In this paper we will concentrate on a tidal field whose effects on star clusters have not yet been studied, namely that of bars. We present a set of direct N-body simulations of star clusters moving in an analytic potential representing a barred galaxy. We compare the evolution of the clusters moving both on different planar periodic orbits in the barred potential and on circular orbits in a potential obtained by axisymmetrising its mass distribution. We show that both the shape of the underlying orbit and its stability have strong impact on the cluster evolution as well as the morphology and orientation of the tidal tails and the sub-structures therein. We find that the dissolution time-scale of the cluster in our simulations is mainly determined by the tidal forcing along the orbit and, for a given tidal forcing, only very little by the exact shape of the gravitational potential in which the cluster is moving.Comment: 15 pages, 17 figures, 5 tables; accepted for publication in MNRAS. Complementary movies can be be found at this http URL http://lam.oamp.fr/research/dynamique-des-galaxies/scientific-results/star-cluster-evolution

The regeneration of stellar bars by tidal interactions. Numerical simulations of fly-by encounters

We study the regeneration of stellar bars triggered by a tidal interaction, using numerical simulations of either purely stellar or stellar+gas disc galaxies. We find that interactions which are sufficiently strong to regenerate the bar in the purely stellar models do not lead to a regeneration in the dissipative models, owing to the induced gas inflow in those models. In models in which the bar can be regenerated, we find a tight correlation between the strength and the pattern speed of the induced bar. This relation can be explained by a significant radial redistribution of angular momentum in the disc due to the interaction, similar to the processes and correlations found for isolated barred spirals. We furthermore show that the regenerated bars show the same dynamical properties as their isolated counterparts.Comment: 18 pages, 26 figures, accepted for publication in MNRA

Formation and Evolution of Black Holes in Galactic Nuclei and Star Clusters

We study the formation, growth, and co-evolution of single and multiple supermassive black holes (SMBHs) and compact objects like neutron stars, white dwarfs, and stellar mass black holes in galactic nuclei and star clusters, focusing on the role of stellar dynamics. In this paper we focus on one exemplary topic out of a wider range of work done, the study of orbital parameters of binary black holes in galactic nuclei (binding energy, eccentricity, relativistic coalescence) as a function of initial parameters. In some cases the classical evolution of black hole binaries in dense stellar systems drives them to surprisingly high eccentricities, which is very exciting for the emission of gravitational waves and relativistic orbit shrinkage. Such results are interesting to the emerging field of gravitational wave astronomy, in relation to a number of ground and space based instruments designed to measure gravitational waves from astrophysical sources (VIRGO, Geo600, LIGO, LISA). Our models self-consistently cover the entire range from Newtonian dynamics to the relativistic coalescence of SMBH binaries

Stellar kinematics in double-barred galaxies: the sigma-hollows

We present SAURON integral-field stellar velocity and velocity dispersion maps for four double-barred early-type galaxies: NGC2859, NGC3941, NGC4725 and NGC5850. The presence of the inner bar does not produce major changes in the line-of-sight velocity, but it appears to have an important effect in the stellar velocity dispersion maps: we find two sigma-hollows of amplitudes between 10 and 40 km/s on either side of the center, at the ends of the inner bars. We have performed numerical simulations to explain these features. Ruling out other possibilities, we conclude that the sigma-hollows are an effect of the contrast between two kinematically different components: the high velocity dispersion of the bulge and the more ordered motion (low velocity dispersion) of the inner bar.Comment: 5 pages, 2 figures. Accepted for publication in ApJ Letter

Changes in Waist Circumference and the Incidence of Diabetes in Middle-Aged Men and Women

Waist circumference (WC) is positively associated with diabetes, but the association with changes in WC (DWC) is less clear. We investigated the association between DWC and the subsequent risk of diabetes in middle-aged men and women, and evaluated the influence from concurrent changes in body mass index (DBMI).Data on 15,577 men and 20,066 women from the Danish Diet, Cancer and Health study were analyzed. Anthropometry was assessed in 1993–97 and 1999–02. Information on diabetes was obtained from The Danish National Diabetes Register. Hazard ratios (HR) were calculated from Cox' proportional hazard models with individuals considered at risk from 1999–02 until December 31 2006. During 5.4 years of follow-up, 1,027 and 876 new cases of diabetes occurred among men and women, respectively. WC was positively associated with diabetes in both sexes also with adjustment for covariates and BMI. DWC was positively associated with diabetes in women, but not in men (HR per 5 cm change = 1.09 (1.04∶1.15) in women, and 1.00 (0.94, 1.07) in men with adjustment for covariates, baseline WC, BMI and DBMI). Associations with DWC were not notably different in sub-groups stratified according to baseline WC or DBMI, or when individuals with diseases or diabetes occurring within the first years of follow-up were excluded.While this study confirmed that WC is positively associated with the risk of diabetes in middle-aged men and women, it surprisingly showed that changes in WC were not associated with the subsequent risk of diabetes in men, and only weakly positively associated with the risk of diabetes in women. Accordingly, these findings suggest that a reduction in WC may be a weak or insufficient or target for prevention of diabetes in middle-aged men and women

Variation of Bar Strength with Central Velocity Dispersion in Spiral Galaxies

We investigate the variation of bar strength with central velocity dispersion in a sample of barred spiral galaxies. The bar strength is characterized by $Q_g$, the maximal tangential perturbation associated with the bar, normalized by the mean axisymmetric force. It is derived from the galaxy potentials which are obtained using near-infrared images of the galaxies. However, $Q_g$ is sensitive to bulge mass. Hence we also estimated bar strengths from the relative Fourier intensity amplitude ($A_{2}$) of bars in near-infrared images. The central velocity dispersions were obtained from integral field spectroscopy observations of the velocity fields in the centers of these galaxies; it was normalized by the rotation curve amplitude obtained from HI line width for each galaxy. We found a correlation between bar strengths (both $Q_g$ and $A_{2}$) and the normalized central velocity dispersions in our sample. This suggests that bars weaken as their central components become kinematically hotter. This may have important implications for the secular evolution of barred galaxies.Comment: To appear in Ap&S

Interaction between Obesity and the NFKB1 - 94ins/delATTG Promoter Polymorphism in Relation to Incident Acute Coronary Syndrome: A Follow Up Study in Three Independent Cohorts

Introduction: The NF-κB transcription factor family regulates several genes encoding pro-inflammatory and anti-inflammatory proteins in adipose tissues and in atherosclerotic plaques. The deletion variant allele of the NFKB1 - 94ins/delATTG promoter polymorphism leads to lower transcript levels of the p50 subunit, and the variant allele has been associated with the risk of several inflammatory diseases as well as coronary heart disease where inflammation is important in the pathogenesis. The objective of this study was to explore the potential interaction between the NFKB1-94ins/delATTG promoter polymorphism and general, abdominal, and gluteofemoral obesity in relation to the risk of incident acute coronary syndrome (ACS) in three large independent cohorts. Methods and Results: The analyses were conducted in the Danish prospective study Diet, Cancer and Health and the two US based cohorts; Nurses’ Health Study and Health Professionals Follow-up Study. We conducted sex stratified analyses that included 1202 male and 708 female cases of incident ACS. We observed a positive association for general and abdominal obesity with risk of incident ACS, independent of genotype in both genders. Gluteofemoral obesity was negatively associated with ACS risk in women independent of genotype, whereas there was no clear association for men. We calculated the relative excess risk due to interaction (RERI) and observed a statistically significant excess risk among men jointly exposed to general or abdominal obesity and the variant allele of the NFKB1-94ATTG polymorphism, whereas there was a tendency towards sub-additivity for gluteofemoral obesity. The excess risks in all analyses were, however, small and could not clearly be demonstrated in women. Conclusion: The variant allele of the NFKB1-94ins/delATTG promoter polymorphism did not substantially modify the association between obesity and incident ACS

Boxy/peanut/X bulges, barlenses and the thick part of galactic bars: What are they and how did they form?

Bars have a complex three-dimensional shape. In particular their inner part is vertically much thicker than the parts further out. Viewed edge-on, the thick part of the bar is what is commonly known as a boxy-, peanut- or X- bulge and viewed face-on it is referred to as a barlens. These components are due to disc and bar instabilities and are composed of disc material. I review here their formation, evolution and dynamics, using simulations, orbital structure theory and comparisons to observations.Comment: 21 pages, 7 figures, invited review to appear in "Galactic Bulges", E. Laurikainen, R. Peletier, D. Gadotti, (eds.), Springe