Large-scale outflows in galaxies

We discuss massive outflows in galaxy bulges, particularly the ones driven by accretion episodes where the central supermassive black hole reaches the Eddington limit. We show that the quasar radiation field Compton-cools the wind shock until this reaches distances ∼1 kpc from the black hole, but becomes too dilute to do this at larger radii. Radiative processes cannot cool the shocked gas within the flow time at any radius. Outflows are therefore momentum driven at small radii (as required to explain the M—σ relation). At large radii, they are energy driven, contrary to recent claims. We solve analytically the motion of an energy-driven shell after the central source has turned off. This shows that the thermal energy in the shocked wind can drive further expansion for a time ∼10 times longer than the active time of the central source. Outflows observed at large radii with no active central source probably result from an earlier short (few Myr) active phase of this source

Simulating feedback from nuclear clusters: the impact of multiple sources

Nuclear star clusters (NCs) are found to exist in the centres of many galaxies and appear to follow scaling relations similar to those of supermassive black holes. Previous analytical work has suggested that such relations are a consequence of feedback-regulated growth. We explore this idea using high-resolution hydrodynamical simulations, focusing on the validity of the simplifying assumptions made in analytical models. In particular, we investigate feedback emanating from multiple stellar sources rather than from a single source, as is usually assumed, and show that collisions between shells of gas swept up by feedback leads to momentum cancellation and the formation of high-density clumps and filaments. This high-density material is resistant both to expulsion from the galaxy potential and to disruption by feedback; if it falls back on to the NC, we expect the gas to be available for further star formation or for feeding a central black hole. We also note that our results may have implications for the evolution of globular clusters and stellar clusters in high-redshift dark matter haloes

Feeding supermassive black holes through supersonic turbulence and ballistic accretion

It has long been recognized that the main obstacle to the accretion of gas on to supermassive black holes (SMBHs) is a large specific angular momentum. It is feared that the gas settles in a large-scale disc, and that accretion would then proceed too inefficiently to explain the masses of the observed SMBHs. Here we point out that, while the mean angular momentum in the bulge is very likely to be large, the deviations from the mean can also be significant. Indeed, cosmological simulations show that velocity and angular momentum fields of gas flows on to galaxies are very complex. Furthermore, inside bulges the gas velocity distribution can be further randomized by the velocity kicks due to feedback from star formation. We perform hydrodynamical simulations of gaseous rotating shells infalling on to an SMBH, attempting to quantify the importance of velocity dispersion in the gas at relatively large distances from the black hole. We implement this dispersion by means of a supersonic turbulent velocity spectrum. We find that, while in the purely rotating case the circularization process leads to efficient mixing of gases with different angular momenta, resulting in a low accretion rate, the inclusion of turbulence increases this accretion rate by up to several orders of magnitude. We show that this can be understood based on the notion of ‘ballistic’ accretion, whereby dense filaments, created by convergent turbulent flows, travel through the ambient gas largely unaffected by hydrodynamical drag. This prevents the efficient gas mixing that was found in the simulations without turbulence, and allows a fraction of gas to impact the innermost boundary of the simulations directly. Using the ballistic approximation, we derive a simple analytical formula that captures the numerical results to within a factor of a few. Rescaling our results to astrophysical bulges, we argue that this ‘ballistic’ mode of accretion could provide the SMBHs with sufficient fuel without the need to channel the gas via large-scale discs or bars. We therefore argue that star formation in bulges can be a strong catalyst for SMBH accretion

Modelling supermassive black hole growth: towards an improved sub-grid prescription

Accretion on to supermassive black holes (SMBHs) in galaxy formation simulations is frequently modelled by the Bondi–Hoyle formalism. Here we examine the validity of this approach analytically and numerically. We argue that the character of the flow where one evaluates the gas properties is unlikely to satisfy the simple Bondi–Hoyle model. Only in the specific case of hot virialized gas with zero angular momentum and negligible radiative cooling is the Bondi–Hoyle solution relevant. In the opposite extreme, where the gas is in a state of free‐fall at the evaluation radius due to efficient cooling and the dominant gravity of the surrounding halo, the Bondi–Hoyle formalism can be erroneous by orders of magnitude in either direction. This may impose artificial trends with halo mass in cosmological simulations by being wrong by different factors for different halo masses. We propose an expression for the sub‐grid accretion rate which interpolates between the free‐fall regime and the Bondi–Hoyle regime, therefore taking account of the contribution of the halo to the gas dynamics

Primordial globular clusters, X-ray binaries and cosmological reionization

Globular clusters are dense stellar systems that have typical ages of ∼13 billion years, implying that they formed during the early epochs of galaxy formation at redshifts of z≳ 6. Massive stars in newly formed or primordial globular clusters could have played an important role during the epoch of cosmological reionization (z≳ 6) as sources of energetic, neutral hydrogen ionizing UV photons. We investigate whether or not these stars could have been as important in death as sources of energetic X-ray photons as they were during their main sequence (MS) lives. Most massive stars are expected to form in binaries, and an appreciable fraction of these (as much as ∼30 per cent) will evolve into X-ray luminous (LX∼ 1038 erg s−1) high-mass X-ray binaries (HMXBs). These sources would have made a contribution to the X-ray background at z≳ 6. Using Monte Carlo models of a globular cluster, we estimate the total X-ray luminosity of a population of HMXBs. We compare and contrast this with the total UV luminosity of the massive stars during their MS lives. For reasonable estimates, we find that the bolometric luminosity of the cluster peaks at ∼1042 erg s−1 during the first few million years, but declines to ∼1041 erg s−1 after ∼5 Myr as the most massive stars evolve off the MS. From this time onwards, the total bolometric luminosity is dominated by HMXBs and falls gradually to ∼1040 erg s−1 after ∼50 Myr. Assuming a power-law spectral energy distribution for the HMXBs, we calculate the effective number of neutral hydrogen ionizations per HMXB and show that HMXBs can be as important as sources of ionizing radiation as massive stars. Finally, we discuss the implications of our results for modelling galaxy formation at high redshift and the prospects of using globular clusters as probes of reionization

Dark matter profiles and annihilation in dwarf spheroidal galaxies: Prospectives for present and futureγ-ray observatories - I. The classical dwarf spheroidal galaxies

Due to their large dynamical mass-to-light ratios, dwarf spheroidal galaxies (dSphs) are promising targets for the indirect detection of dark matter (DM) in γ-rays. We examine their detectability by present and future γ-ray observatories. The key innovative features of our analysis are as follows: (i) we take into account the angular size of the dSphs; while nearby objects have higher γ-ray flux, their larger angular extent can make them less attractive targets for background-dominated instruments; (ii) we derive DM profiles and the astrophysical J-factor (which parametrizes the expected γ-ray flux, independently of the choice of DM particle model) for the classical dSphs directly from photometric and kinematic data. We assume very little about the DM profile, modelling this as a smooth split-power-law distribution, with and without subclumps; (iii) we use a Markov chain Monte Carlo technique to marginalize over unknown parameters and determine the sensitivity of our derived J-factors to both model and measurement uncertainties; and (iv) we use simulated DM profiles to demonstrate that our J-factor determinations recover the correct solution within our quoted uncertainties

Discovery and Fine-Mapping of Glycaemic and Obesity-Related Trait Loci Using High-Density Imputation.

Reference panels from the 1000 Genomes (1000G) Project Consortium provide near complete coverage of common and low-frequency genetic variation with minor allele frequency ≥0.5% across European ancestry populations. Within the European Network for Genetic and Genomic Epidemiology (ENGAGE) Consortium, we have undertaken the first large-scale meta-analysis of genome-wide association studies (GWAS), supplemented by 1000G imputation, for four quantitative glycaemic and obesity-related traits, in up to 87,048 individuals of European ancestry. We identified two loci for body mass index (BMI) at genome-wide significance, and two for fasting glucose (FG), none of which has been previously reported in larger meta-analysis efforts to combine GWAS of European ancestry. Through conditional analysis, we also detected multiple distinct signals of association mapping to established loci for waist-hip ratio adjusted for BMI (RSPO3) and FG (GCK and G6PC2). The index variant for one association signal at the G6PC2 locus is a low-frequency coding allele, H177Y, which has recently been demonstrated to have a functional role in glucose regulation. Fine-mapping analyses revealed that the non-coding variants most likely to drive association signals at established and novel loci were enriched for overlap with enhancer elements, which for FG mapped to promoter and transcription factor binding sites in pancreatic islets, in particular. Our study demonstrates that 1000G imputation and genetic fine-mapping of common and low-frequency variant association signals at GWAS loci, integrated with genomic annotation in relevant tissues, can provide insight into the functional and regulatory mechanisms through which their effects on glycaemic and obesity-related traits are mediated

The Role of Adiposity in Cardiometabolic Traits: A Mendelian Randomization Analysis

BACKGROUND: The association between adiposity and cardiometabolic traits is well known from epidemiological studies. Whilst the causal relationship is clear for some of these traits, for others it is not. We aimed to determine whether adiposity is causally related to various cardiometabolic traits using the Mendelian randomization approach. METHODS AND FINDINGS: We used the adiposity-associated variant rs9939609 at the FTO locus as an instrumental variable (IV) for body mass index (BMI) in a Mendelian randomization design. Thirty-six population-based studies of individuals of European descent contributed to the analyses. Age- and sex-adjusted regression models were fitted to test for association between (i) rs9939609 and BMI (n  =  198,502), (ii) rs9939609 and 24 traits, and (iii) BMI and 24 traits. The causal effect of BMI on the outcome measures was quantified by IV estimators. The estimators were compared to the BMI-trait associations derived from the same individuals. In the IV analysis, we demonstrated novel evidence for a causal relationship between adiposity and incident heart failure (hazard ratio, 1.19 per BMI-unit increase; 95% CI, 1.03-1.39) and replicated earlier reports of a causal association with type 2 diabetes, metabolic syndrome, dyslipidemia, and hypertension (odds ratio for IV estimator, 1.1-1.4; all p < 0.05). For quantitative traits, our results provide novel evidence for a causal effect of adiposity on the liver enzymes alanine aminotransferase and gamma-glutamyl transferase and confirm previous reports of a causal effect of adiposity on systolic and diastolic blood pressure, fasting insulin, 2-h post-load glucose from the oral glucose tolerance test, C-reactive protein, triglycerides, and high-density lipoprotein cholesterol levels (all p < 0.05). The estimated causal effects were in agreement with traditional observational measures in all instances except for type 2 diabetes, where the causal estimate was larger than the observational estimate (p  =  0.001). CONCLUSIONS: We provide novel evidence for a causal relationship between adiposity and heart failure as well as between adiposity and increased liver enzymes

Sex-stratified genoSex-stratified Genome-wide Association Studies Including 270,000 Individuals Show Sexual Dimorphism in Genetic Loci for Anthropometric Traits

Given the anthropometric differences between men and women and previous evidence of sex-difference in genetic effects, we conducted a genome-wide search for sexually dimorphic associations with height, weight, body mass index, waist circumference, hip circumference, and waist-to-hip-ratio (133,723 individuals) and took forward 348 SNPs into follow-up (additional 137,052 individuals) in a total of 94 studies. Seven loci displayed significant sex-difference (FDR<5%), including four previously established (near GRB14/COBLL1, LYPLAL1/SLC30A10, VEGFA, ADAMTS9) and three novel anthropometric trait loci (near MAP3K1, HSD17B4, PPARG), all of which were genome-wide significant in women (P<5×10(-8)), but not in men. Sex-differences were apparent only for waist phenotypes, not for height, weight, BMI, or hip circumference. Moreover, we found no evidence for genetic effects with opposite directions in men versus women. The PPARG locus is of specific interest due to its role in diabetes genetics and therapy. Our results demonstrate the value of sex-specific GWAS to unravel the sexually dimorphic genetic underpinning of complex traits