We present a test of different error estimators for two-point clustering statistics, appropriate for present and future large galaxy redshift surveys. Using an ensemble of very large dark matter ΛCDM ...N-body simulations, we compare internal error estimators (jackknife and bootstrap) to external ones (Monte Carlo realizations). For three-dimensional clustering statistics, we find that none of the internal error methods investigated is able to reproduce either accurately or robustly the errors of external estimators on 1 to 25 h−1 Mpc scales. The standard bootstrap overestimates the variance of ξ(s) by ∼40 per cent on all scales probed, but recovers, in a robust fashion, the principal eigenvectors of the underlying covariance matrix. The jackknife returns the correct variance on large scales, but significantly overestimates it on smaller scales. This scale dependence in the jackknife affects the recovered eigenvectors, which tend to disagree on small scales with the external estimates. Our results have important implications for fitting models to galaxy clustering measurements. For example, in a two-parameter fit to the projected correlation function, we find that the standard bootstrap systematically overestimates the 95 per cent confidence interval, while the jackknife method remains biased, but to a lesser extent. Ignoring the systematic bias, the scatter between realizations, for Gaussian statistics, implies that a 2σ confidence interval, as inferred from an internal estimator, corresponds in practice to anything from 1σ to 3σ. By oversampling the subvolumes, we find that it is possible, at least for the cases we consider, to obtain robust bootstrap variances and confidence intervals that agree with external error estimates. Our results are applicable to two-point statistics, like ξ(s) and wp(rp), measured in large redshift surveys, and show that the interpretation of clustering measurements with internally estimated errors should be treated with caution.
We present a theoretical model for the evolution of mass, angular momentum and size of galaxy discs and bulges, and we implement it into the semi-analytic galaxy formation code, Semi-Analytic Galaxy ...Evolution. The model follows both secular and violent evolutionary channels, including smooth accretion, disc instabilities, minor and major mergers. We find that the combination of our recipe with hierarchical clustering produces two distinct populations of bulges: merger-driven bulges, akin to classical bulges and ellipticals, and instability-driven bulges, akin to secular (or pseudo-)bulges. The model mostly reproduces the mass–size relation of gaseous and stellar discs, the evolution of the mass–size relation of ellipticals, the Faber–Jackson relation, and the magnitude–colour diagram of classical and secular bulges. The model predicts only a small overlap of merger-driven and instability-driven components in the same galaxy, and predicts different bulge types as a function of galaxy mass and disc fraction. Bulge type also affects the star formation rate and colour at a given luminosity. The model predicts a population of merger-driven red ellipticals that dominate both the low-mass and high-mass ends of the galaxy population, and span all dynamical ages; merger-driven bulges in disc galaxies are dynamically old and do not interfere with subsequent evolution of the star-forming component. Instability-driven bulges dominate the population at intermediate galaxy masses, especially thriving in massive discs. The model green valley is exclusively populated by instability-driven bulge hosts. Through the present implementation, the mass accretion history is perceivable in the galaxy structure, morphology and colours.
We have used high-resolution, Hubble Space Telescope, near-infrared imaging to conduct a detailed analysis of the morphological properties of the most massive galaxies at high redshift, modelling the ...WFC3/IR H
160-band images of the ≃200 galaxies in the CANDELS-UDS field with photometric redshifts 1 < z < 3, and stellar masses M
* > 1011 M⊙. We have explored the results of fitting single-Sérsic and bulge+disc models, and have investigated the additional errors and potential biases introduced by uncertainties in the background and the on-image point spread function. This approach has enabled us to obtain formally acceptable model fits to the WFC3/IR images of >90 per cent of the galaxies. Our results indicate that these massive galaxies at 1 < z < 3 lie both on and below the local size-mass relation, with a median effective radius of ∼2.6 kpc, a factor of ≃2.3 smaller than comparably massive local galaxies. Moreover, we find that bulge-dominated objects in particular show evidence for a growing bimodality in the size-mass relation with increasing redshift, and by z > 2 the compact bulges display effective radii a factor of ≃4 smaller than local ellipticals of comparable mass. These trends also appear to extend to the bulge components of disc-dominated galaxies. In addition, we find that, while such massive galaxies at low redshift are generally bulge-dominated, at redshifts 1 < z < 2 they are predominantly mixed bulge+disc systems, and by z > 2 they are mostly disc-dominated. The majority of the disc-dominated galaxies are actively forming stars, although this is also true for many of the bulge-dominated systems. Interestingly, however, while most of the quiescent galaxies are bulge-dominated, we find that a significant fraction (25-40 per cent) of the most quiescent galaxies, with specific star formation rates sSFR < 10−10 yr−1, have disc-dominated morphologies. Thus, while our results show that the massive galaxy population is undergoing dramatic changes at this crucial epoch, they also suggest that the physical mechanisms which quench star formation activity are not simply connected to those responsible for the morphological transformation of massive galaxies into present-day giant ellipticals.
The processes that trigger active galactic nuclei (AGN) remain poorly understood. While lower luminosity AGN may be triggered by minor disturbances to the host galaxy, stronger disturbances are ...likely required to trigger luminous AGN. Major wet mergers of galaxies are ideal environments for AGN triggering since they provide large gas supplies and galaxy scale torques. There is however little observational evidence for a strong connection between AGN and major mergers. We analyse the morphological properties of AGN host galaxies as a function of AGN and host galaxy luminosity and compare them to a carefully matched sample of control galaxies. AGN are X-ray selected in the redshift range 0.5 < z < 0.8 and have luminosities 41 log (L
X erg s−1) 44.5. 'Fake AGN' are simulated in the control galaxies by adding point sources with the magnitude of the matched AGN. We find that AGN host and control galaxies have comparable asymmetries, Sérsic indices and ellipticities at rest frame ∼950 nm. AGN host galaxies show neither higher average asymmetries nor higher fractions of very disturbed objects. There is no increase in the prevalence of merger signatures with AGN luminosity. At 95 per cent confidence we find that major mergers are responsible for <6 per cent of all AGN in our sample as well as <40 per cent of the highest luminosity AGN (log (L
X erg s−1) ∼ 43.5). Major mergers therefore either play only a very minor role in the triggering of AGN in the luminosity range studied or time delays are too long for merger features to remain visible.
The clustering of dark haloes depends not only on their mass but also on their assembly history, a dependence we term ‘assembly bias’. Using a galaxy formation model grafted on to the Millennium ...Simulation of the ΛCDM cosmogony, we study how assembly bias affects galaxy clustering. We compare the original simulation to ‘shuffled’ versions where the galaxy populations are randomly swapped among haloes of similar mass, thus isolating the effects of correlations between assembly history and environment at fixed mass. Such correlations are ignored in the halo occupation distribution models often used populate dark matter simulations with galaxies, but they are significant in our more realistic simulation. Assembly bias enhances two-point correlations by 10 per cent for galaxies with brighter than −17, but suppresses them by a similar amount for galaxies brighter than −20. When such samples are split by colour, assembly bias is 5 per cent stronger for red galaxies and 5 per cent weaker for blue ones. Halo central galaxies are differently affected by assembly bias than are galaxies of all types. It almost doubles the correlation amplitude for faint red central galaxies. Shuffling galaxies among haloes of fixed formation redshift or concentration in addition to fixed mass produces biases which are not much smaller than when mass alone is fixed. Assembly bias must reflect a correlation of environment with aspects of halo assembly which are not encoded in either of these parameters. It induces effects which could compromise precision measurements of cosmological parameters from large galaxy surveys.
We examine the properties of galaxies in the Galaxies and Mass Assembly (GAMA) survey located in voids with radii >10 h
−1 Mpc. Utilizing the GAMA equatorial survey, 592 void galaxies are identified ...out to z ≈ 0.1 brighter than M
r
= −18.4, our magnitude completeness limit. Using the W
Hα versus N ii/Hα (WHAN) line strength diagnostic diagram, we classify their spectra as star forming, AGN, or dominated by old stellar populations. For objects more massive than 5 × 109 M⊙, we identify a sample of 26 void galaxies with old stellar populations classed as passive and retired galaxies in the WHAN diagnostic diagram, else they lack any emission lines in their spectra. When matched to Wide-field Infrared Survey Explorer mid-IR photometry, these passive and retired galaxies exhibit a range of mid-IR colour, with a number of void galaxies exhibiting 4.6 − 12 colours inconsistent with completely quenched stellar populations, with a similar spread in colour seen for a randomly drawn non-void comparison sample. We hypothesize that a number of these galaxies host obscured star formation, else they are star forming outside of their central regions targeted for single-fibre spectroscopy. When matched to a randomly drawn sample of non-void galaxies, the void and non-void galaxies exhibit similar properties in terms of optical and mid-IR colour, morphology, and star formation activity, suggesting comparable mass assembly and quenching histories. A trend in mid-IR 4.6 − 12 colour is seen, such that both void and non-void galaxies with quenched/passive colours <1.5 typically have masses higher than 1010 M⊙, where internally driven processes play an increasingly important role in galaxy evolution.
We investigate the relationship between active galactic nucleus (AGN) activity and host galaxy properties using a sample of massive galaxies at z ∼ 2 in the Chandra Deep Field-South (CDFS). A sample ...of 268 galaxies with M
* > 1010.5 M⊙ at 1.4 < z < 3 are selected from Hubble Space Telescope wide field camera 3 (WFC3) H-band observations in CDFS taken as part of the cosmic assembly near-infrared deep extragalactic legacy survey (CANDELS) survey. We find that a large fraction (22.0 ± 2.5 per cent) are detected in the 4 Ms Chandra/Advanced CCD Image Spectrometer observations in the field, implying a high AGN content in these massive galaxies. To investigate further the relationship between these AGN and their hosts, we create four subsamples, based on their star formation rates (star-forming versus quiescent) and galaxy size (compact versus extended), following Barro et al. and perform X-ray spectral fitting. We find a clear effect whereby the AGN in compact galaxies – be they star forming or quiescent – show significantly higher luminosities and levels of obscuration than the AGN in extended galaxies. These results provide clear evidence for two modes of black hole growth in massive galaxies at high redshift. The dominant growth mode is a luminous, obscured phase which occurs overwhelmingly in compact galaxies while another lower luminosity, unobscured phase is predominantly seen in extended galaxies. Both modes could produce AGN feedback, with violent transformative feedback in the former and a gentler ‘maintenance mode’ produced by the latter.
We study the mean environment of galaxies in the DEEP2 Galaxy Redshift Survey as a function of rest-frame colour, luminosity, and O-ii 3727-Å equivalent width. The local galaxy overdensity for ...>14-000 galaxies at 0.75 < z < 1.35 is estimated using the projected third-nearest-neighbour surface density. Of the galaxy properties studied, mean environment is found to depend most strongly on galaxy colour; all major features of the correlation between mean overdensity and rest-frame colour observed in the local universe were already in place at z∼ 1. In contrast to local results, we find a substantial slope in the mean dependence of environment on luminosity for blue, star forming galaxies at z∼ 1, with brighter blue galaxies being found on average in regions of greater overdensity. We discuss the roles of galaxy clusters and groups in establishing the observed correlations between environment and galaxy properties at high redshift, and we also explore the evidence for a 'downsizing of quenching' from z∼ 1 to ∼0. Our results add weight to existing evidence that the mechanism(s) that result in star formation quenching are efficient in group environments as well as clusters. This work is the first of its kind at high redshift and represents the first in a series of papers addressing the role of environment in galaxy formation at 0 < z < 1.
We present new results for the three-point correlation function, ζ, measured as a function of scale, luminosity and colour from the final version of the 2dF Galaxy Redshift Survey (2dFGRS). The ...reduced three-point correlation function, Q3~ζ/ξ2, is estimated for different triangle shapes and sizes, employing a full covariance analysis. The form of Q3 is consistent with the expectations for the Λ cold dark matter model, confirming that the primary influence shaping the distribution of galaxies is gravitational instability acting on Gaussian primordial fluctuations. However, we find a clear offset in amplitude between Q3 for galaxies and the predictions for the dark matter. We are able to rule out the scenario in which galaxies are unbiased tracers of the mass at the 9σ level. On weakly non-linear scales, we can interpret our results in terms of galaxy bias parameters. We find a linear bias term that is consistent with unity, b1= 0.93+0.10-0.08 and a quadratic bias c2=b2/b1=-0.34+0.11-0.08. This is the first significant detection of a non-zero quadratic bias, indicating a small but important non-gravitational contribution to the three-point function. Our estimate of the linear bias from the three-point function is independent of the normalization of underlying density fluctuations, so we can combine this with the measurement of the power spectrum of 2dFGRS galaxies to constrain the amplitude of matter fluctuations. We find that the rms linear theory variance in spheres of radius 8 h−1 Mpc is σ8= 0.88+0.12-0.10, providing an independent confirmation of values derived from other techniques. On non-linear scales, where ξ > 1, we find that Q3 has a strong dependence on scale, colour and luminosity.
Recent observations have probed the formation histories of nearby elliptical galaxies by tracking correlations between the stellar population parameters, age and metallicity, and the structural ...parameters that enter the Fundamental Plane, size R
e, and velocity dispersion σ. These studies have found intriguing correlations between these four parameters. In this work, we make use of a semi-analytic model, based on halo merger trees extracted from the Bolshoi cosmological simulation, that predicts the structural properties of spheroid-dominated galaxies based on an analytic model that has been tested and calibrated against an extensive suite of hydrodynamic+N-body binary merger simulations. We predict the R
e, σ, luminosity, age, and metallicity of spheroid-dominated galaxies, enabling us to compare directly to observations. Our model predicts a strong correlation between age and σ for early-type galaxies, and no significant correlation between age and radius, in agreement with observations. In addition, we predict a strong correlation between metallicity and σ, and a weak correlation between metallicity and R
e, in qualitative agreement with observations. We find that the correlations with σ arise as a result of the strong link between σ and the galaxy's assembly time. Minor mergers produce a large change in radius while leaving σ nearly the same, which explains the weaker trends with radius.
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