Overconsumption, outflows and the quenching of satellite galaxies

The baryon cycle of galaxies is a dynamic process involving the intake, consumption and ejection of vast quantities of gas. In contrast, the conventional picture of satellite galaxies has them methodically turning a large gas reservoir into stars until this reservoir is forcibly removed due to external ram pressure. This picture needs revision. Our modern understanding of the baryon cycle suggests that in some regimes the simple interruption of the fresh gas supply may quench satellite galaxies long before stripping events occur, a process we call overconsumption. We compile measurements from the literature of observed satellite quenching times at a range of redshifts to determine if satellites are principally quenched through orbit-based gas stripping events – either direct stripping of the disc (ram pressure stripping) or the extended gas halo (strangulation) – or from internally driven star formation outflows via overconsumption. These time-scales show significant deviations from the evolution expected for gas stripping mechanisms and suggest that either ram pressure stripping is much more efficient at high redshift, or that secular outflows quench satellites before orbit-based stripping occurs. Given the strong redshift evolution of star formation rates, at high redshift even moderate outflow rates will lead to extremely short delay times with the expectation that high-redshift (z > 1.5) satellites will be quenched almost immediately following the cessation of cosmological inflow. Observations of high-redshift satellites give an indirect but sensitive measure of the outflow rate, with current measurements suggesting that outflows are no larger than 2.5 times the star formation rate for galaxies with a stellar mass of 1010.5 M⊙

The metallicity of galactic winds

The abundance evolution of galaxies depends critically on the balance between the mixing of metals in their interstellar medium (ISM), the inflow of new gas and the outflow of enriched gas. We study these processes in gas columns perpendicular to a galactic disc using sub-parsec resolution simulations that track stellar ejecta with the flash code. We model a simplified ISM stirred and enriched by supernovae and their progenitors. We vary the density distribution of the gas column and integrate our results over an exponential disc to predict wind and ISM enrichment properties for disc galaxies. We find that winds from more massive galaxies are hotter and more highly enriched, in stark contrast to that which is often assumed in galaxy formation models. We use these findings in a simple model of galactic enrichment evolution, in which the metallicity of forming galaxies is the result of accretion of nearly pristine gas and outflow of enriched gas along an equilibrium sequence. We compare these predictions to the observed mass–metallicity relation, and demonstrate how the galaxy's gas fraction is a key controlling parameter. This explains the observed flattening of the mass–metallicity relation at higher stellar masses

The effect of baryons on redshift space distortions and cosmic density and velocity fields in the EAGLE simulation

We use the Evolution and Assembly of GaLaxies and their Environments (EAGLE) galaxy formation simulation to study the effects of baryons on the power spectrum of the total matter and dark matter distributions and on the velocity fields of dark matter and galaxies. On scales k ≳ 4 h Mpc−1 the effect of baryons on the amplitude of the total matter power spectrum is greater than 1 per cent. The back-reaction of baryons affects the density field of the dark matter at the level of ∼3 per cent on scales of 1 ≤ k/( h Mpc−1) ≤ 5. The dark matter velocity divergence power spectrum at k ≲ 0.5 h Mpc−1 is changed by less than 1 per cent. The 2D redshift space power spectrum is affected at the level of ∼6 per cent at |k|≳1hMpc−1|k|≳1hMpc−1 (for μ > 0.5), but for |k|≤0.4hMpc−1|k|≤0.4hMpc−1 it differs by less than 1 per cent. We report vanishingly small baryonic velocity bias for haloes: the peculiar velocities of haloes with M200 > 3 × 1011 M⊙ (hosting galaxies with M* > 109 M⊙) are affected at the level of at most 1 km s−1, which is negligible for 1 per cent-precision cosmology. We caution that since EAGLE overestimates cluster gas fractions it may also underestimate the impact of baryons, particularly for the total matter power spectrum. Nevertheless, our findings suggest that for theoretical modelling of redshift space distortions and galaxy velocity-based statistics, baryons and their back-reaction can be safely ignored at the current level of observational accuracy. However, we confirm that the modelling of the total matter power spectrum in weak lensing studies needs to include realistic galaxy formation physics in order to achieve the accuracy required in the precision cosmology era

The colour magnitude relation for galaxies in the Coma cluster

We present a new photometric catalogue of the Coma galaxy cluster in the Johnson U- and V- bands. We cover an area of 3360arcmin2 of sky, to a depth of V=20 mag in a 13 arcsec diameter aperture, and produce magnitudes for ~1400 extended objects in metric apertures from 8.8 to 26arcsec diameters. The mean internal RMS scatter in the photometry is 0.014 mag in V, and 0.026 mag in U, for V13 < 17 mag. We place new limits on the levels of scatter in the colour--magnitude relation (CMR) in the Coma cluster, and investigate how the slope and scatter of the CMR depends on galaxy morphology, luminosity and position within the cluster. As expected, the lowest levels of scatter are found in the elliptical galaxies, while the late type galaxies have the highest numbers of galaxies bluewards of the CMR. We investigate whether the slope of the CMR is an artifact of colour gradients within galaxies and, show that it persists when the colours are measured within a diameter that scales with galaxy size. Looking at the environmental dependence of the CMR, we find a trend of systematically bluer galaxy colours with increasing projected cluster-centric radius which we associate with a gradient in the mean galactic ages.Comment: 18 pages, 13 Figures. For associated data file, see ftp://ftp.sr.bham.ac.uk/pub/ale/ComaPhot

The link between galaxy and black hole growth in the eagle simulation

We investigate the connection between the star formation rate (SFR) of galaxies and their central black hole accretion rate (BHAR) using the EAGLE cosmological hydrodynamical simulation. We find, in striking concurrence with recent observational studies, that the 〈SFR〉–BHAR relation for an active galactic nucleus (AGN)-selected sample produces a relatively flat trend, whilst the 〈BHAR〉–SFR relation for an SFR-selected sample yields an approximately linear trend. These trends remain consistent with their instantaneous equivalents even when both SFR and BHAR are time averaged over a period of 100 Myr. There is no universal relationship between the two growth rates. Instead, SFR and BHAR evolve through distinct paths that depend strongly on the mass of the host dark matter halo. The galaxies hosted by haloes of mass M200 ≲ 1011.5 M⊙ grow steadily, yet black holes (BHs) in these systems hardly grow, yielding a lack of correlation between SFR and BHAR. As haloes grow through the mass range 1011.5 ≲ M200 ≲ 1012.5 M⊙ BHs undergo a rapid phase of non-linear growth. These systems yield a highly non-linear correlation between the SFR and BHAR, which are non-causally connected via the mass of the host halo. In massive haloes (M200 ≳ 1012.5 M⊙), both SFR and BHAR decline on average with a roughly constant scaling of SFR/BHAR ∼ 103. Given the complexity of the full SFR–BHAR plane built from multiple behaviours, and from the large dynamic range of BHARs, we find the primary driver of the different observed trends in the 〈SFR〉–BHAR and 〈BHAR〉–SFR relationships are due to sampling considerably different regions of this plane

A medieval multiverse?: Mathematical modelling of the thirteenth century universe of Robert Grosseteste

In his treatise on light, written about 1225, Robert Grosseteste describes a cosmological model in which the universe is created in a big-bang-like explosion and subsequent condensation. He postulates that the fundamental coupling of light and matter gives rises to the material body of the entire cosmos. Expansion is arrested when matter reaches a minimum density and subsequent emission of light from the outer region leads to compression and rarefaction of the inner bodily mass so as to create nine celestial spheres, with an imperfect residual core. In this paper, we reformulate the Latin description in terms of a modern mathematical model, teasing out consequences implicit in the text, but which the author would not have had the tools to explore. The equations which describe the coupling of light and matter are solved numerically, subjected to initial conditions and critical criteria consistent with the text. Formation of a universe with a non-infinite number of perfected spheres is extremely sensitive to the initial conditions, the intensity of the light and the transparency of these spheres. In this ‘medieval multiverse’, only a small range of opacity and initial density profiles leads to a stable universe with nine perfected spheres. As in current cosmological thinking, the existence of Grosseteste’s universe relies on a very special combination of fundamental parameters

SPHENIX: smoothed particle hydrodynamics for the next generation of galaxy formation simulations

Computational astrophysic

Music from the heavens - gravitational waves from supermassive black hole mergers in the EAGLE simulations

We estimate the expected event rate of gravitational wave signals from mergers of supermassive black holes that could be resolved by a space-based interferometer, such as the Evolved Laser Interferometer Space Antenna (eLISA), utilizing the reference cosmological hydrodynamical simulation from the EAGLE suite. These simulations assume a Lambda cold dark matter cosmogony with state-of-the-art subgrid models for radiative cooling, star formation, stellar mass loss, and feedback from stars and accreting black holes. They have been shown to reproduce the observed galaxy population with unprecedented fidelity. We combine the merger rates of supermassive black holes in EAGLE with the latest phenomenological waveform models to calculate the gravitational waves signals from the intrinsic parameters of the merging black holes. The EAGLE models predict ∼2 detections per year by a gravitational wave detector such as eLISA. We find that these signals are largely dominated by mergers between seed mass black holes merging at redshifts between z ∼ 2 and z ∼ 1. In order to investigate the dependence on the assumed black hole seed mass, we introduce an additional model with a black hole seed mass an order of magnitude smaller than in our reference model. We also consider a variation of the reference model where a prescription for the expected delays in the black hole merger time-scale has been included after their host galaxies merge. We find that the merger rate is similar in all models, but that the initial black hole seed mass could be distinguished through their detected gravitational waveforms. Hence, the characteristic gravitational wave signals detected by eLISA will provide profound insight into the origin of supermassive black holes and the initial mass distribution of black hole seeds

Supermassive black holes in the EAGLE Universe. Revealing the observables of their growth

We investigate the evolution of supermassive black holes in the ‘Evolution and Assembly of GaLaxies and their Environments’ (EAGLE) cosmological hydrodynamic simulations. The largest of the EAGLE volumes covers a (100 cMpc)3 and includes state-of-the-art physical models for star formation and black hole growth that depend only on local gas properties. We focus on the black hole mass function, Eddington ratio distribution and the implied duty cycle of nuclear activity. The simulation is broadly consistent with observational constraints on these quantities. In order to make a more direct comparison with observational data, we calculate the soft and hard X-ray luminosity functions of the active galactic nuclei (AGN). Between redshifts 0 and 1, the simulation is in agreement with data. At higher redshifts, the simulation tends to underpredict the luminosities of the brightest observed AGN. This may be due to the limited volume of the simulation, or a fundamental deficiency of the underlying model. It seems unlikely that additional unresolved variability can account for this difference. The simulation shows a similar ‘downsizing’ of the AGN population as seen in observational surveys