ALBERT

Description: We investigate the clustering of Lyman-break galaxies (LBGs) at z  ~ 4. Using the hierarchical galaxy formation model GALFORM, we predict, for the first time using a semi-analytical model with feedback from active galactic nuclei (AGN), the angular correlation function (ACF) of LBGs and find agreement within 3 with new measurements of the ACF from surveys including the Hubble eXtreme Deep Field (XDF) and Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) field. Our simulations confirm the conclusion reached using independent models that although the predicted ACFs reproduce the trend of increased clustering with luminosity, the dependence is less strong than observed. We find that for the detection limits of the XDF field, central LBGs at z  ~ 4 predominantly reside in haloes of mass ~10 11 –10 12 h –1 M and that satellites reside in larger haloes of mass ~10 12 –10 13 h –1 M . The model predicts fewer bright satellite LBGs at z  ~ 4 than is inferred from measurements of the ACF at small scales. By analysing the halo occupation distribution (HOD) predicted by the model, we find evidence that AGN feedback affects the HOD of central LBGs in massive haloes. This is a new high-redshift test of this important feedback mechanism. We investigate the effect of photometric errors in the observations on the ACF predictions. We find that the observational uncertainty in the galaxy luminosity reduces the clustering amplitude and that this effect increases towards faint galaxies, particularly on small scales. To compare properties of model with observed LBGs, this uncertainty must be considered.

Description: Gravitationally lensed galaxies with magnification μ  10–100 are routinely detected at high redshifts, but magnifications significantly higher than this are hampered by a combination of low probability and large source sizes. Magnifications of μ ~ 1000 may none the less be relevant in the case of intrinsically small, high-redshift objects with very high number densities. Here, we explore the prospects of detecting compact (10 pc), high-redshift ( z   7) Population III star clusters at such extreme magnifications in large-area surveys with planned telescopes like Euclid , Wide Field Infrared Survey Telescope and Wide-field Imaging Surveyor for High-redshift ( WISH ). We find that the planned WISH 100 deg 2 ultradeep survey may be able to detect a small number of such objects, provided that the total stellar mass of these star clusters is 10 4 M . If candidates for such lensed Population III star clusters are found, follow-up spectroscopy of the surrounding nebula with the James Webb Space Telescope or ground-based Extremely Large Telescopes should be able to confirm the Population III nature of these objects. Multiband photometry of these objects with the James Webb Space Telescope also has the potential to confirm that the stellar initial mass function in these Population III star clusters is top-heavy, as supported by current simulations.

Description: Detections of Lyman-break galaxies (LBGs) at high-redshift are affected by gravitational lensing induced by foreground deflectors not only in galaxy clusters, but also in blank fields. We quantify the impact of strong magnification in the samples of B 435 , V 606 , i 775 and z 850 & Y 105 dropouts (4 z 8) observed in the eXtreme Deep Field (XDF) and the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) fields by investigating the proximity of dropouts to foreground objects. We find that ~6 per cent of bright z ~ 7 LBGs ( $m_{H_{160}} 〈 26$ ) have been strongly lensed (μ 〉 2) by foreground objects. This fraction decreases from ~3.5 per cent at z ~ 6 to ~1.5 per cent at z ~ 4. Since the observed fraction of strongly lensed LBGs is a function of the shape of the luminosity function (LF), it can be used to derive Schechter parameters, α and M * , independently from galaxy number counts. Our magnification bias analysis yields Schechter-function parameters in close agreement with those determined from galaxy counts albeit with larger uncertainties. Extrapolation of our analysis to z 8 suggests that surveys with JWST , WFIRST and Euclid should find excess LBGs at the bright end, over an intrinsic exponential cutoff. Finally, we highlight how the magnification bias measurement near the XDF detection limit can be used to probe the population of galaxies beyond this limit. Preliminary results suggest that the magnification bias at M UV ~ –18 is weaker than expected if α –1.7 extends well below the current detection limits. This could imply a flattening of the LF at M UV –16.5. However, selection effects and completeness estimates are difficult to quantify precisely. Thus, we do not rule out a steep LF extending to M UV –15.

Description: It is widely believed that globular clusters evolve over many two-body relaxation times towards a state of energy equipartition, so that velocity dispersion scales with stellar mass as   m – with  = 0.5. We show here that this is incorrect, using a suite of direct N -body simulations with a variety of realistic initial mass functions and initial conditions. No simulated system ever reaches a state close to equipartition. Near the centre, the luminous main-sequence stars reach a maximum max 0.15 ± 0.03. At large times, all radial bins convergence on an asymptotic value 0.08 ± 0.02. The development of this ‘partial equipartition’ is strikingly similar across our simulations, despite the range of different initial conditions employed. Compact remnants tend to have higher than main-sequence stars (but still  〈 0.5), due to their steeper (evolved) mass function. The presence of an intermediate-mass black hole (IMBH) decreases , consistent with our previous findings of a quenching of mass segregation under these conditions. All these results can be understood as a consequence of the Spitzer instability for two-component systems, extended by Vishniac to a continuous mass spectrum. Mass segregation (the tendency of heavier stars to sink towards the core) has often been studied observationally, but energy equipartition has not. Due to the advent of high-quality proper motion data sets from the Hubble Space Telescope , it is now possible to measure for real clusters. Detailed data-model comparisons open up a new observational window on globular cluster dynamics and evolution. A first comparison of our simulations to observations of Omega Cen yields good agreement, supporting the view that globular clusters are not generally in energy equipartition. Modelling techniques that assume equipartition by construction (e.g. multi-mass Michie–King models) are approximate at best.

Description: We predict Lyman α (Lyα) luminosity functions (LFs) of Lyα-selected galaxies (Lyα emitters, or LAEs) at z = 3–6 using the phenomenological model. This model combines observed UV-LFs of Lyman-break galaxies (LBGs, or drop-out galaxies), with constraints on their distribution of Lyα line strengths as a function of UV-luminosity and redshift. Our analysis shows that while Lyα LFs of LAEs are generally not Schechter functions, these provide a good description over the luminosity range of log 10 ( L α /erg s –1 ) = 41–44. Motivated by this result, we predict Schechter function parameters at z = 3–6. Our analysis further shows that (i) the faint-end slope of the Lyα LF is steeper than that of the UV-LF of LBGs, (with a median α Lyα 〈 –2.0 at z 4), and (ii) a turnover in the Lyα LF of LAEs at Lyα luminosities 10 40 L α 10 41  erg s –1 may signal a flattening of UV-LF of LBGs at –12 〉 M UV 〉 –14. We discuss the implications of these results – which can be tested directly with upcoming surveys – for the Epoch of Reionization.

Description: We have identified 274 M-type brown dwarfs in the Hubble Space Telescope 's Wide Field Camera 3 pure parallel fields from the Brightest of Reionizing Galaxies (BoRG) survey for high-redshift galaxies. These are near-infrared observations with multiple lines of sight out of our Milky Way. Using these observed M-type brown dwarfs, we fitted a Galactic disc and halo model with a Markov chain Monte Carlo analysis. This model worked best with the scalelength of the disc fixed at h = 2.6 kpc. For the scaleheight of the disc, we found $z_0 = 0.29^{+0.02}_{-0.019}$  kpc and for the central number density, $\rho _0 = 0.29^{+0.20}_{-0.13}$ # pc –3 . For the halo, we derived a flattening parameter = 0.45 ± 0.04 and a power-law index p = 2.4 ± 0.07. We found the fraction of M-type brown dwarfs in the local density that belong to the halo to be f h = 0.0075 $^{+0.0025}_{-0.0019}$ . We found no correlation between subtype of M-dwarf and any model parameters. The total number of M-type brown dwarfs in the disc and halo was determined to be $58.2^{+9.81}_{-6.70} \times 10^{9}$ . We found an upper limit for the fraction of M-type brown dwarfs in the halo of 7 $^{+5}_{-4}$ per cent. The upper limit for the total Galactic disc mass in M-dwarfs is $4.34^{+0.73}_{-0.5}\times 10^{9}$ M , assuming all M-type brown dwarfs have a mass of 80  M J .

Description: We employ cosmological hydrodynamical simulations to investigate models in which the supermassive black holes powering luminous z  ~ 6 quasars (QSOs) grow from massive seeds. We simulate 18 regions with densities ranging from the mean cosmic density to the highest peaks in the Millennium simulation volume. Only in the most massive haloes situated in the most overdense regions, can black holes grow to masses up to 10 9 M by z  ~ 6 without invoking super-Eddington accretion. Accretion on to the most massive black holes becomes limited by thermal active galactic nucleus (AGN) feedback by z  ~ 9–8 with further growth proceeding in short Eddington-limited bursts. Our modelling suggests that current flux-limited surveys of QSOs at high redshift preferentially detect objects at their peak luminosity and therefore miss a substantial population of QSOs powered by similarly massive black holes but with low accretion rates. To test whether the required host halo masses are consistent with the observed galaxy environments of z  ~ 6 QSOs, we produce realistic rest-frame UV images of our simulated galaxies. Without strong stellar feedback, our simulations predict numbers of bright galaxies larger than observed by a factor of 10 or more. Supernova-driven galactic winds reduce the predicted numbers to a level consistent with observations indicating that stellar feedback was already very efficient at high redshifts. We further investigate the effect of thermal AGN feedback on the surrounding gas. AGN outflows are highly anisotropic and mostly energy driven, pushing gas at 1000 km s –1 out to tens of kpc consistently with observations. The spatially extended thermal X-ray emission around bright QSOs is powered by these outflows and is an important diagnostic of the mechanism whereby AGN feedback energy couples to surrounding gas.

Description: We present a detailed analysis of an individual case of gravitational lensing of a z  ~ 8 Lyman-break galaxy (LBG) in a blank field, identified in Hubble Space Telescope imaging obtained as part of the Brightest of Reionizing Galaxies survey. To investigate the close proximity of the bright ( m AB  = 25.8) Y 098 -dropout to a small group of foreground galaxies, we obtained deep spectroscopy of the dropout and two foreground galaxies using VLT/X-Shooter. We detect H α, H β, [O  iii ] and [O  ii ] emission in the brightest two foreground galaxies (unresolved at the natural seeing of 0.8 arcsec), placing the pair at z  = 1.327. We can rule out emission lines contributing all of the observed broad-band flux in H 160 band at 70, allowing us to exclude the z  ~ 8 candidate as a low-redshift interloper with broad-band photometry dominated by strong emission lines. The foreground galaxy pair lies at the peak of the luminosity, redshift and separation distributions for deflectors of strongly lensed z  ~ 8 objects, and we make a marginal detection of a demagnified secondary image in the deepest ( J 125 ) filter. We show that the configuration can be accurately modelled by a singular isothermal ellipsoidal deflector and a Sérsic source magnified by a factor of μ = 4.3 ± 0.2. The reconstructed source in the best-fitting model is consistent with luminosities and morphologies of z  ~ 8 LBGs in the literature. The lens model yields a group mass of 9.62 ± 0.31  x  10 11 M and a stellar mass-to-light ratio for the brightest deflector galaxy of $M_{\star }/L_{B}=2.3^{+0.8}_{-0.6} \,\mathrm{M}_{{\odot }}/\mathrm{L}_{{\odot }}$ within its effective radius. The foreground galaxies’ redshifts would make this one of the few strong lensing deflectors discovered at z  〉 1. Deeper imaging would allow for confirmation of the existence of the secondary image and elongation in the primary image, verifying multiple imaging and producing more robust estimations of the image magnifications.