ALBERT

Description: We present a sub-100 pc-scale analysis of the CO molecular gas emission and kinematics of the gravitational lens system SDP.81 at redshift 3.042 using Atacama Large Millimetre/submillimetre Array (ALMA) science verification data and a visibility-plane lens reconstruction technique. We find clear evidence for an excitation-dependent structure in the unlensed molecular gas distribution, with emission in CO (5–4) being significantly more diffuse and structured than in CO (8–7). The intrinsic line luminosity ratio is r 8–7/5–4  = 0.30 ± 0.04, which is consistent with other low-excitation starbursts at z  ~ 3. An analysis of the velocity fields shows evidence for a star-forming disc with multiple velocity components that is consistent with a merger/post-coalescence merger scenario, and a dynamical mass of M (〈1.56 kpc) = 1.6 ± 0.6  x  10 10 M . Source reconstructions from ALMA and the Hubble Space Telescope show that the stellar component is offset from the molecular gas and dust components. Together with Karl G. Jansky Very Large Array CO (1–0) data, they provide corroborative evidence for a complex ~2 kpc-scale starburst that is embedded within a larger ~15 kpc structure.

Description: We examine the circular velocity profiles of galaxies in cold dark matter (CDM) cosmological hydrodynamical simulations from the EAGLE and LOCAL GROUPS projects and compare them with a compilation of observed rotation curves of galaxies spanning a wide range in mass. The shape of the circular velocity profiles of simulated galaxies varies systematically as a function of galaxy mass, but shows remarkably little variation at fixed maximum circular velocity. This is especially true for low-mass dark-matter-dominated systems, reflecting the expected similarity of the underlying CDM haloes. This is at odds with observed dwarf galaxies, which show a large diversity of rotation curve shapes, even at fixed maximum rotation speed. Some dwarfs have rotation curves that agree well with simulations, others do not. The latter are systems where the inferred mass enclosed in the inner regions is much lower than expected for CDM haloes and include many galaxies where previous work claims the presence of a constant density ‘core’. The ‘cusp versus core’ issue is thus better characterized as an ‘inner mass deficit’ problem than as a density slope mismatch. For several galaxies, the magnitude of this inner mass deficit is well in excess of that reported in recent simulations where cores result from baryon-induced fluctuations in the gravitational potential. We conclude that one or more of the following statements must be true: (i) the dark matter is more complex than envisaged by any current model; (ii) current simulations fail to reproduce the diversity in the effects of baryons on the inner regions of dwarf galaxies; and/or (iii) the mass profiles of ‘inner mass deficit’ galaxies inferred from kinematic data are incorrect.

Description: We use cosmological hydrodynamical simulations of the APOSTLE project along with high-quality rotation curve observations to examine the fraction of baryons in CDM haloes that collect into galaxies. This ‘galaxy formation efficiency’ correlates strongly and with little scatter with halo mass, dropping steadily towards dwarf galaxies. The baryonic mass of a galaxy may thus be used to place a lower limit on total halo mass and, consequently, on its asymptotic maximum circular velocity. A number of observed dwarfs seem to violate this constraint, having baryonic masses up to 10 times higher than expected from their rotation speeds, or, alternatively, rotating at only half the speed expected for their mass. Taking the data at face value, either these systems have formed galaxies with extraordinary efficiency – highly unlikely given their shallow potential wells – or their dark matter content is much lower than expected from CDM haloes. This ‘missing dark matter’ is reminiscent of the inner mass deficit of galaxies with slowly rising rotation curves, but cannot be explained away by star formation-induced ‘cores’ in the dark mass profile, since the anomalous deficit applies to regions larger than the luminous galaxies themselves. We argue that explaining the structure of these galaxies would require either substantial modification of the standard CDM paradigm or else significant revision to the uncertainties in their inferred mass profiles, which should be much larger than reported. Systematic errors in inclination may provide a simple resolution to what would otherwise be a rather intractable problem for the current paradigm.

Description: We present results from 13 cosmological simulations that explore the parameter space of the ‘Evolution and Assembly of GaLaxies and their Environments’ (EAGLE) simulation project. Four of the simulations follow the evolution of a periodic cube L  = 50 cMpc on a side, and each employs a different subgrid model of the energetic feedback associated with star formation. The relevant parameters were adjusted so that the simulations each reproduce the observed galaxy stellar mass function at z  = 0.1. Three of the simulations fail to form disc galaxies as extended as observed, and we show analytically that this is a consequence of numerical radiative losses that reduce the efficiency of stellar feedback in high-density gas. Such losses are greatly reduced in the fourth simulation – the EAGLE reference model – by injecting more energy in higher density gas. This model produces galaxies with the observed size distribution, and also reproduces many galaxy scaling relations. In the remaining nine simulations, a single parameter or process of the reference model was varied at a time. We find that the properties of galaxies with stellar mass   M * (the ‘knee’ of the galaxy stellar mass function) are largely governed by feedback associated with star formation, while those of more massive galaxies are also controlled by feedback from accretion on to their central black holes. Both processes must be efficient in order to reproduce the observed galaxy population. In general, simulations that have been calibrated to reproduce the low-redshift galaxy stellar mass function will still not form realistic galaxies, but the additional requirement that galaxy sizes be acceptable leads to agreement with a large range of observables.

Description: We examine a sample of ~250 000 ‘locally brightest galaxies’ selected from the Sloan Digital Sky Survey to be central galaxies within their dark matter haloes. We stack the X-ray emission from these haloes, as a function of the stellar mass of the central galaxy, using data from the ROSAT All-Sky Survey. We detect emission across almost our entire sample, including emission which we attribute to hot gas around galaxies spanning a range of 1.2 dex in stellar mass (corresponding to nearly two orders of magnitude in halo mass) down to M *  = 10 10.8 M ( M 500   10 12.6 M ). Over this range, the X-ray luminosity can be fit by a power law, either of stellar mass or of halo mass. From this, we infer a single unified scaling relation between mass and L X which applies for galaxies, groups, and clusters. This relation has a slope steeper than expected for self-similarity, showing the importance of non-gravitational heating. Assuming this non-gravitational heating is predominately due to AGN feedback, the lack of a break in the relation shows that AGN feedback is tightly self-regulated and fairly gentle, in agreement with the predictions of recent high-resolution simulations. Our relation is consistent with established measurements of the L X – L K relation for elliptical galaxies as well as the L X – M 500 relation for optically selected galaxy clusters. However, our L X – M 500 relation has a normalization more than a factor of 2 below most previous relations based on X-ray-selected cluster samples. We argue that optical selection offers a less biased view of the L X – M 500 relation for mass-selected clusters.

Description: We use weak gravitational lensing to measure mean mass profiles around locally brightest galaxies (LBGs). These are selected from the Seventh Data Release of the Sloan Digital Sky Survey spectroscopic and photometric catalogues to be brighter than any neighbour projected within 1.0 Mpc and differing in redshift by 〈1000 km s –1 . Most (〉83 per cent) are expected to be the central galaxies of their dark matter haloes. Previous stacking analyses have used this LBG sample to measure mean Sunyaev–Zeldovich flux and mean X-ray luminosity as a function of LBG stellar mass. In both cases, a simulation of the formation of the galaxy population was used to estimate effective halo mass for LBGs of given stellar mass, allowing the derivation of scaling relations between the gas properties of haloes and their mass. By comparing results from a variety of simulations to our lensing data, we show that this procedure has significant model dependence reflecting: (i) the failure of any given simulation to reproduce observed galaxy abundances exactly; (ii) a dependence on the cosmology underlying the simulation; and (iii) a dependence on the details of how galaxies populate haloes. We use our lensing results to recalibrate the scaling relations, eliminating most of this model dependence and explicitly accounting both for residual modelling uncertainties and for observational uncertainties in the lensing results. The resulting scaling relations link the mean gas properties of dark haloes to their mass over an unprecedentedly wide range, 10 12.5 〈 M 500 /M 〈 10 14.5 , and should fairly and robustly represent the full halo population.

Description: We have updated the Munich galaxy formation model to the Planck first-year cosmology, while modifying the treatment of baryonic processes to reproduce recent data on the abundance and passive fractions of galaxies from z  = 3 down to z  = 0. Matching these more extensive and more precise observational results requires us to delay the reincorporation of wind ejecta, to lower the surface density threshold for turning cold gas into stars, to eliminate ram-pressure stripping in haloes less massive than ${\sim }10^{14}{\rm \, M_{{\odot }}}$ , and to modify our model for radio mode feedback. These changes cure the most obvious failings of our previous models, namely the overly early formation of low-mass galaxies and the overly large fraction of them that are passive at late times. The new model is calibrated to reproduce the observed evolution both of the stellar mass function and of the distribution of star formation rate at each stellar mass. Massive galaxies (log M * /M  ≥ 11.0) assemble most of their mass before z  = 1 and are predominantly old and passive at z  = 0, while lower mass galaxies assemble later and, for log M * /M  ≤ 9.5, are still predominantly blue and star forming at z  = 0. This phenomenological but physically based model allows the observations to be interpreted in terms of the efficiency of the various processes that control the formation and evolution of galaxies as a function of their stellar mass, gas content, environment and time.

Description: We simulate the phase-space distribution of stellar mass in nine massive cold dark matter galaxy clusters by applying the semi-analytic particle tagging method of Cooper et al. to the Phoenix suite of high-resolution N -body simulations ( M 200   7.5–33  x  10 14 M ). The resulting surface brightness (SB) profiles of brightest cluster galaxies (BCGs) match well to observations. On average, stars formed in galaxies accreted by the BCG account for 90 per cent of its total mass (the remainder is formed in situ ). In circular BCG-centred apertures, the superposition of multiple debris clouds (each 10 per cent of the total BCG mass) from different progenitors can result in an extensive outer diffuse component, qualitatively similar to a ‘cD envelope'. These clouds typically originate from tidal stripping at z   1 and comprise both streams and the extended envelopes of other massive galaxies in the cluster. Stars at very low SB contribute a significant fraction of the total cluster stellar mass budget: in the central 1 Mpc 2 of a z  ~ 0.15 cluster imaged at SDSS-like resolution, our fiducial model predicts 80–95 per cent of stellar mass below a SB of μ V  ~ 26.5 mag arcsec –2 is associated with accreted stars in the envelope of the BCG. The ratio of BCG stellar mass (including this diffuse component) to total cluster stellar mass is ~30 per cent.

Description: As a result of their internal dynamical coherence, thin stellar streams formed by disrupting globular clusters (GCs) can act as detectors of dark matter (DM) substructure in the Galactic halo. Perturbations induced by close flybys amplify into detectable density gaps, providing a probe both of the abundance and of the masses of DM subhaloes. Here, we use N -body simulations to show that the Galactic population of giant molecular clouds (GMCs) can also produce gaps (and clumps) in GC streams, and so may confuse the detection of DM subhaloes. We explore the cases of streams analogous to the observed Palomar 5 and GD1 systems, quantifying the expected incidence of structure caused by GMC perturbations. Deep observations should detect such disturbances regardless of the substructure content of the Milky Way's halo. Detailed modelling will be needed to demonstrate that any detected gaps or clumps were produced by DM subhaloes rather than by molecular clouds.