Liberation of specific angular momentum through radiation and scattering in relativistic black-hole accretion discs

A key component of explaining the array of galaxies observed in the Universe is the feedback of active galactic nuclei, each powered by a massive black hole's accretion disc. For accretion to occur, angular momentum must be lost by that which is accreted. Electromagnetic radiation must offer some respite in this regard, the contribution for which is quantified in this paper, using solely general relativity, under the thin-disc regime. Herein, I calculate extremised situations where photons are entirely responsible for energy removal in the disc and then extend and relate this to the standard relativistic accretion disc outlined by Novikov & Thorne, which includes internal angular-momentum transport. While there is potential for the contribution of angular-momentum removal from photons to be >~1% out to ~10^4 Schwarzschild radii if the disc is irradiated and maximally liberated of angular momentum through inverse Compton scattering, it is more likely of order 10^2 Schwarzschild radii if thermal emission from the disc itself is stronger. The effect of radiation/scattering is stronger near the horizons of fast-spinning black holes, but, ultimately, other mechanisms must drive angular-momentum liberation/transport in accretion discs.Comment: 6 pages, 2 figures. Accepted for publication in PAS

Simulating the Role of Stellar Rotation in the Spectroscopic Effects of Differential Limb Magnification

Finite-source effects of gravitationally microlensed stars have been well discussed in the literature, but the role that stellar rotation plays has been neglected. A differential magnification map applied to a differentially Doppler-shifted surface alters the profiles of absorption lines, compromising their ordinarily symmetric nature. Herein, we assess the degree to which this finite-source effect of differential limb magnification (DLM), in combination with stellar rotation, alters spectroscopically derived stellar properties. To achieve this, we simulated a grid of high-magnification microlensing events using synthetic spectra. Our analysis shows that rotation of the source generates differences in the measured equivalent widths of absorption lines supplementary to DLM alone, but only of the order of a few percent. Using the wings of H alpha from the same simulated data, we confirmed the result of Johnson et al. (2010) that DLM alters measurements of effective temperature by < 100 K for dwarf stars, while showing rotation to bear no additional effect.Comment: Accepted for publication in PASA, 7 pages, 5 figures, 1 tabl

The relationship between cluster environment and molecular gas content of star-forming galaxies in the EAGLE simulation

We employ the EAGLE hydrodynamical simulation to uncover the relationship between cluster environment and $\rm H_2$ content of star-forming galaxies at redshifts spanning $0\leq z\leq 1$. To do so, we divide the star-forming sample into those that are bound to clusters and those that are not. We find that, at any given redshift, the galaxies in clusters generally have less $\rm H_2$ than their non-cluster counterparts with the same stellar mass (corresponding to an offset of $\lesssim 0.5$ dex), but this offset varies with stellar mass and is virtually absent at $M_\star\lesssim10^{9.3}~{\rm M}_\odot$. The $\rm H_2$ deficit in star-forming cluster galaxies can be traced back to a decline in their $\rm H_2$ content that commenced after first infall into a cluster, which occurred later than a typical cluster galaxy. Evolution of the full cluster population after infall is generally consistent with `slow-then-rapid' quenching, but galaxies with $M_\star\lesssim 10^{9.5}~{\rm M}_\odot$ exhibit rapid quenching. Unlike most cluster galaxies, star-forming ones were not pre-processed in groups prior to being accreted by clusters. For both of these cluster samples, the star formation efficiency remained oblivious to the infall. We track the particles associated with star-forming cluster galaxies and attribute the drop in $\rm H_2$ mass after infall to poor replenishment, depletion due to star formation, and stripping of $\rm H_2$ in cluster environments. These results provide predictions for future surveys, along with support and theoretical insights for existing molecular gas observations that suggest there is less $\rm H_2$ in cluster galaxies.Comment: Accepted for publication in MNRA

The Water Activity of Mars-relevant Multicomponent Brines: The Changing Influence of Perchlorate on Habitability over Time

Low water activity limits the habitability of aqueous environments, and salts present on Mars are known to reduce water activity. As environmental brines are not pure solutions of a single salt, predicting their water activity is difficult without direct measurement. Martian brines are likely complex and dominated by ions including sulfates and perchlorates, unlike typical terrestrial aqueous environments dominated by sodium chloride. We used the Pitzer model to predict the water activity of multicomponent brines and tested against laboratory-produced brines, including for the first time perchlorate salts that are known to exist on Mars. Our calculations match measurements of single-salt solutions and predict the water activity of multicomponent brines with an accuracy dependent on the quality of thermodynamic data available for a given ion combination. We tested the hypothesis that some salts will dominate the water activity, and therefore habitability, of multicomponent brines. Some salts, such as sodium and magnesium sulfates, did not strongly modulate the water activity of the solution, whereas others such as magnesium chloride and some perchlorates did. Applied to the history of Mars, the data suggest that sulfates and sodium chloride present in Noachian and early Hesperian environments would not have limited habitability. Perchlorates produced photochemically later in the Amazonian could impose a water activity limit at high concentrations that is not significantly changed by other salts. Overall we found that magnesium and calcium chlorides mixed with perchlorates can reach the lowest water activity values and therefore the lowest habitability of the brines tested

The Galaxy Number Density Profile of Haloes

More precise measurements of galaxy clustering will be provided by the next generation of galaxy surveys such as DESI, WALLABY and SKA. To utilize this information to improve our understanding of the Universe, we need to accurately model the distribution of galaxies in their host dark matter halos. In this work we present a new galaxy number density profile of haloes, which makes predictions for the positions of galaxies in the host halo, different to the widely adopted Navarro-Frenk-White (NFW) profile, since galaxies tend to be found more in the outskirts of halos (nearer the virial radius) than an NFW profile. The parameterised galaxy number density profile model of haloes is fit and tested using the DARKSAGE semi-analytic model of galaxy formation. We find that our galaxy number density profile model of haloes can accurately reproduce the halo occupation distribution and galaxy two-point correlation function of the DARKSAGE simulation. We also derive the analytic expressions for the circular velocity and gravitational potential energy for this profile model. We use the SDSS DR10 galaxy group catalogue to validate this galaxy number density profile model of haloes. Compared to the NFW profile, we find that our model more accurately predicts the positions of galaxies in their host halo and the galaxy two-point correlation function.Comment: 13 pages. 10 figures. Appear on Ap

Semi-Analytic Galaxy Evolution (SAGE): Model Calibration and Basic Results

This paper describes a new publicly available codebase for modelling galaxy formation in a cosmological context, the "Semi-Analytic Galaxy Evolution" model, or SAGE for short. SAGE is a significant update to that used in Croton et al. (2006) and has been rebuilt to be modular and customisable. The model will run on any N-body simulation whose trees are organised in a supported format and contain a minimum set of basic halo properties. In this work we present the baryonic prescriptions implemented in SAGE to describe the formation and evolution of galaxies, and their calibration for three N-body simulations: Millennium, Bolshoi, and GiggleZ. Updated physics include: gas accretion, ejection due to feedback, and reincorporation via the galactic fountain; a new gas cooling--radio mode active galactic nucleus (AGN) heating cycle; AGN feedback in the quasar mode; a new treatment of gas in satellite galaxies; and galaxy mergers, disruption, and the build-up of intra-cluster stars. Throughout, we show the results of a common default parameterization on each simulation, with a focus on the local galaxy population.Comment: 15 pages, 9 figures, accepted for publication in ApJS. SAGE is a publicly available codebase for modelling galaxy formation in a cosmological context, available at https://github.com/darrencroton/sage Questions and comments can be sent to Darren Croton: [email protected]