arXiv subjects

Shaun Cole

Publications and source records attributed to Shaun Cole.

At least 109 records · Page 6Linked to original sources

arXiv2017

The Pan-STARRS1 Medium-deep Survey: Star Formation Quenching in Group and Cluster Environments

We make use of a catalog of 1600 Pan-STARRS1 groups produced by the probability friends-of-friends algorithm to explore how the galaxy properties, i.e. the specific star formation rate (SSFR) and quiescent fraction, depend on stellar mass and group-centric radius. The work is the extension of Lin et al. (2014). In this work, powered by a stacking technique plus a background subtraction for contamination removal, a finer correction and more precise results are obtained than in our previous work. We find that while the quiescent fraction increases with decreasing group-centric radius the median SSFRs of star-forming galaxies in groups at fixed stellar mass drop slightly from the field toward the group center. This suggests that the major quenching process in groups is likely a fast mechanism. On the other hand, a reduction in SSFRs by ~0.2 dex is seen inside clusters as opposed to the field galaxies. If the reduction is attributed to the slow quenching effect, the slow quenching process acts dominantly in clusters. In addition, we also examine the density-color relation, where the density is defined by using a sixth-nearest neighbor approach. Comparing the quiescent fractions contributed from the density and radial effect, we find that the density effect dominates over the massive group or cluster galaxies, and the radial effect becomes more effective in less massive galaxies. The results support mergers and/or starvation as the main quenching mechanisms in the group environment, while harassment and/or starvation dominate in clusters.

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arXiv2017

What to expect from dynamical modelling of galactic haloes

Many dynamical models of the Milky Way halo require assumptions that the distribution function of a tracer population should be independent of time (i.e., a steady state distribution function) and that the underlying potential is spherical. We study the limitations of such modelling by applying a general dynamical model with minimal assumptions to a large sample of galactic haloes from cosmological $N$-body and hydrodynamical simulations. Using dark matter particles as dynamical tracers, we find that the systematic uncertainties in the measured mass and concentration parameters typically have an amplitude of 25% to 40%. When stars are used as tracers, however, the systematic uncertainties can be as large as a factor of $2-3$. The systematic uncertainties are not reduced by increasing the tracer sample size and vary stochastically from halo to halo. These systematic uncertainties are mostly driven by underestimated statistical noise caused by correlated phase-space structures that violate the steady state assumption. The number of independent phase-space structures inferred from the uncertainty level sets a limiting sample size beyond which a further increase no longer significantly improves the accuracy of dynamical inferences. The systematic uncertainty level is determined by the halo merger history, the shape and environment of the halo. Our conclusions apply generally to any spherical steady-state model.

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arXiv2017

Globular clusters vs dark matter haloes in strong lensing observations

Small distortions in the images of Einstein rings or giant arcs offer the exciting prospect of detecting dark matter haloes or subhaloes of mass below $10^9$M$_{\odot}$, most of which are too small to have made a visible galaxy. A very large number of such haloes are predicted to exist in the cold dark matter model of cosmogony; in contrast other models, such as warm dark matter, predict no haloes below a mass of this order which depends on the properties of the warm dark matter particle. Attempting to detect these small perturbers could therefore discriminate between different kinds of dark matter particles, and even rule out the cold dark matter model altogether. Globular clusters in the lens galaxy also induce distortions in the image which could, in principle, contaminate the test. Here, we investigate the population of globular clusters in six early type galaxies in the Virgo cluster. We find that the number density of globular clusters of mass $\sim10^6$M$_{\odot}$ is comparable to that of the dark matter perturbers (including subhaloes in the lens and haloes along the line-of-sight). We show that the very different degrees of mass concentration in globular clusters and dark matter haloes result in different lensing distortions. These are detectable with milli-arcsecond resolution imaging which can distinguish between globular cluster and dark matter halo signals.

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arXiv2017

A Lightcone Catalogue from the Millennium-XXL Simulation

Future galaxy surveys require realistic mock catalogues to understand and quantify systematics in order to make precise cosmological measurements. We present a halo lightcone catalogue and halo occupation distribution (HOD) galaxy catalogue built using the Millennium-XXL (MXXL) simulation. The halo catalogue covers the full sky, extending to z = 2 with a mass resolution of ~1e11 Msun/h . We use this to build a galaxy catalogue, which has an r-band magnitude limit of r < 20.0, with a median redshift of z~0.2. A Monte Carlo HOD method is used to assign galaxies to the halo lightcone catalogue, and we evolve the HODs to reproduce a target luminosity function; by construction, the luminosity function of galaxies in the mock is in agreement with the Sloan Digital Sky Survey (SDSS) at low redshifts and the Galaxy and Mass Assembly (GAMA) survey at high redshifts. A Monte Carlo method is used to assign a 0.1(g-r) colour to each galaxy, and the colour distribution of galaxies at different redshifts agrees with measurements from GAMA. The clustering of galaxies in the mock for galaxies in different magnitude and redshift bins is in good agreement with measurements from SDSS and GAMA, and the colour-dependent clustering is in reasonable agreement. We show that the baryon acoustic oscillation (BAO) can be measured in the mock catalogue, and the redshift space distortions (RSDs) are in agreement with measurements from SDSS, illustrating that this catalogue will be useful for upcoming surveys.

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arXiv2017

Predictions for the detection of Tidal Streams with Gaia using Great Circle Methods

The Gaia astrometric mission may offer an unprecedented opportunity to discover new tidal streams in the Galactic halo. To test this, we apply nGC3, a great-circle-cell count method that combines position and proper motion data to identify streams, to ten mock Gaia catalogues of K giants and RR Lyrae stars constructed from cosmological simulations of Milky Way analogues. We analyse two sets of simulations, one using a combination of $N$-body and semi-analytical methods which has extremely high resolution, the other using hydro-dynamical methods, which captures the dynamics of baryons, including the formation of an in situ halo. These ten realisations of plausible Galactic merger histories allow us to assess the potential for the recovery of tidal streams in different Milky Way formation scenarios. We include the Gaia~selection function and observational errors in these mock catalogues. We find that the nGC3 method has a well-defined detection boundary in the space of stream width and projected overdensity, that can be predicted based on direct observables alone. We predict that about 4-13 dwarf galaxy streams can be detected in a typical Milky Way-mass halo with Gaia+nGC3, with an estimated efficiency of $>$80\% inside the detection boundary. The progenitors of these streams are in the mass range of the classical dwarf galaxies and may have been accreted as early as redshift $\sim3$. Finally, we analyse how different possible extensions of the Gaia mission will improve the detection of tidal streams.

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arXiv2017

Revisiting HOD model assumptions: the impact of AGN feedback and assembly bias

The standard Halo Occupation Distribution (HOD) models were originally developed based on results from semi-analytic and hydrodynamical galaxy formation models. Those models have since progressed, in particular to include AGN feedback to match the galaxy luminosity function in a universe with the observed baryon fraction. AGN feedback affects the relationship between galaxy stellar mass and luminosity, in particular making the relationship non-monotonic. For matched number density samples, galaxies in luminosity-threshold samples occupy a different range of halo masses from those in stellar-mass-threshold samples. We find that the shapes of the HODs of luminosity-threshold samples are slightly more complicated in semi-analytic galaxy formation models that include AGN feedback than are assumed by standard HOD models. We also find that subhalo abundance matching (SHAM) does not preserve these non-standard shapes. We show that catalogues created using SHAM and the semi-analytic model Galform that have the same large-scale 2-point clustering by construction have different void probability functions (VPFs) in both real and redshift space. We find that these differences arise from the different HOD shapes, as opposed to assembly bias, which indicates that the VPF could be used to test the suitability of an HOD model with real data.

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arXiv2017

Blending bias impacts the host halo masses derived from a cross-correlation analysis of bright sub-millimetre galaxies

Placing bright sub-millimetre galaxies (SMGs) within the broader context of galaxy formation and evolution requires accurate measurements of their clustering, which can constrain the masses of their host dark matter halos. Recent work has shown that the clustering measurements of these galaxies may be affected by a `blending bias,' which results in the angular correlation function of the sources extracted from single-dish imaging surveys being boosted relative to that of the underlying galaxies. This is due to confusion introduced by the coarse angular resolution of the single-dish telescope and could lead to the inferred halo masses being significantly overestimated. We investigate the extent to which this bias affects the measurement of the correlation function of SMGs when it is derived via a cross-correlation with a more abundant galaxy population. We find that the blending bias is essentially the same as in the auto-correlation case and conclude that the best way to reduce its effects is to calculate the angular correlation function using SMGs in narrow redshift bins. Blending bias causes the inferred host halo masses of the SMGs to be overestimated by a factor of $\sim6$ when a redshift interval of $δz=3$ is used. However, this reduces to a factor of $\sim2$ for $δz=0.5$. The broadening of photometric redshift probability distributions with increasing redshift can therefore impart a mild halo `downsizing' effect onto the inferred host halo masses, though this trend is not as strong as seen in recent observational studies.

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arXiv2017

The far infra-red SEDs of main sequence and starburst galaxies

We compare observed far infra-red/sub-millimetre (FIR/sub-mm) galaxy spectral energy distributions (SEDs) of massive galaxies ($M_{\star}\gtrsim10^{10}$ $h^{-1}$M$_{\odot}$) derived through a stacking analysis with predictions from a new model of galaxy formation. The FIR SEDs of the model galaxies are calculated using a self-consistent model for the absorption and re-emission of radiation by interstellar dust based on radiative transfer calculations and global energy balance arguments. Galaxies are selected based on their position on the specific star formation rate (sSFR) - stellar mass ($M_{\star}$) plane. We identify a main sequence of star-forming galaxies in the model, i.e. a well defined relationship between sSFR and $M_\star$, up to redshift $z\sim6$. The scatter of this relationship evolves such that it is generally larger at higher stellar masses and higher redshifts. There is remarkable agreement between the predicted and observed average SEDs across a broad range of redshifts ($0.5\lesssim z\lesssim4$) for galaxies on the main sequence. However, the agreement is less good for starburst galaxies at $z\gtrsim2$, selected here to have elevated sSFRs$>10\times$ the main sequence value. We find that the predicted average SEDs are robust to changing the parameters of our dust model within physically plausible values. We also show that the dust temperature evolution of main sequence galaxies in the model is driven by star formation on the main sequence being more burst-dominated at higher redshifts.

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arXiv2016

Projection effects in the strong lensing study of subhaloes

The defining characteristic of the cold dark matter (CDM) hypothesis is the presence of a very large number of low-mass haloes, too small to have made a visible galaxy. Other hypotheses for the nature of the dark matter, such as warm dark matter (WDM), predict a much smaller number of such low-mass haloes. Strong lensing systems offer the possibility of detecting small-mass haloes through the distortions they induce in the lensed image. Here we show that the main contribution to the image distortions comes from haloes along the line of sight rather than subhaloes in the lens as has normally been assumed so far. These interlopers enhance the differences between the predictions of CDM and WDM models. We derive the total perturber mass function, including both subhaloes and interlopers, and show that measurements of approximately 20 strong lens systems with a detection limit of $M_{\rm low}=10^7 h^{-1} M_{\odot}$ would distinguish (at 3 sigma) between CDM and a WDM model consisting of 7 keV sterile neutrinos such as those required to explain the recently detected 3.5 keV X-ray emission line from the centres of galaxies and clusters.

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arXiv2016

The DESI Experiment Part I: Science,Targeting, and Survey Design

DESI (Dark Energy Spectroscopic Instrument) is a Stage IV ground-based dark energy experiment that will study baryon acoustic oscillations (BAO) and the growth of structure through redshift-space distortions with a wide-area galaxy and quasar redshift survey. To trace the underlying dark matter distribution, spectroscopic targets will be selected in four classes from imaging data. We will measure luminous red galaxies up to $z=1.0$. To probe the Universe out to even higher redshift, DESI will target bright [O II] emission line galaxies up to $z=1.7$. Quasars will be targeted both as direct tracers of the underlying dark matter distribution and, at higher redshifts ($ 2.1 < z < 3.5$), for the Ly-$α$ forest absorption features in their spectra, which will be used to trace the distribution of neutral hydrogen. When moonlight prevents efficient observations of the faint targets of the baseline survey, DESI will conduct a magnitude-limited Bright Galaxy Survey comprising approximately 10 million galaxies with a median $z\approx 0.2$. In total, more than 30 million galaxy and quasar redshifts will be obtained to measure the BAO feature and determine the matter power spectrum, including redshift space distortions.

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arXiv2016

The DESI Experiment Part II: Instrument Design

DESI (Dark Energy Spectropic Instrument) is a Stage IV ground-based dark energy experiment that will study baryon acoustic oscillations and the growth of structure through redshift-space distortions with a wide-area galaxy and quasar redshift survey. The DESI instrument is a robotically-actuated, fiber-fed spectrograph capable of taking up to 5,000 simultaneous spectra over a wavelength range from 360 nm to 980 nm. The fibers feed ten three-arm spectrographs with resolution $R= λ/Δλ$ between 2000 and 5500, depending on wavelength. The DESI instrument will be used to conduct a five-year survey designed to cover 14,000 deg$^2$. This powerful instrument will be installed at prime focus on the 4-m Mayall telescope in Kitt Peak, Arizona, along with a new optical corrector, which will provide a three-degree diameter field of view. The DESI collaboration will also deliver a spectroscopic pipeline and data management system to reduce and archive all data for eventual public use.

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arXiv2016

Satellite galaxies in semi-analytic models of galaxy formation with sterile neutrino dark matter

The sterile neutrino is a viable dark matter candidate that can be produced in the early Universe via non-equilibrium processes, and would therefore possess a highly non-thermal spectrum of primordial velocities. In this paper we analyse the process of structure formation with this class of dark matter particles. To this end we construct primordial dark matter power spectra as a function of the lepton asymmetry, $L_6$, that is present in the primordial plasma and leads to resonant sterile neutrino production. We compare these power spectra with those of thermally produced dark matter particles and show that resonantly produced sterile neutrinos are much colder than their thermal relic counterparts. We also demonstrate that the shape of these power spectra is not determined by the free-streaming scale alone. We then use the power spectra as an input for semi-analytic models of galaxy formation in order to predict the number of luminous satellite galaxies in a Milky Way-like halo. By assuming that the mass of the Milky Way halo must be no more than $2\times10^{12}M_{\odot}$ (the adopted upper bound based on current astronomical observations) we are able to constrain the value of $L_6$ for $M_s\le 8$~keV. We also show that the range of $L_6$ that is in best agreement with the 3.5~keV line (if produced by decays of 7~keV sterile neutrino) requires that the Milky Way halo has a mass no smaller than $1.5\times10^{12}M_{\odot}$. Finally, we compare the power spectra obtained by direct integration of the Boltzmann equations for a non-resonantly produced sterile neutrino with the fitting formula of Viel~et~al. and find that the latter significantly underestimates the power amplitude on scales relevant to satellite galaxies.

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arXiv2016

Modelling Galaxy Merger Timescales and Tidal Destruction

We present a model for the dynamical evolution of subhaloes based on an approach combining numerical and analytical methods. Our method is based on tracking subhaloes in an N-body simulation up to the last point that it can be resolved, and applying an analytic prescription for its merger timescale that takes dynamical friction and tidal disruption into account. When applied to cosmological N-body simulations with mass resolutions that differ by two orders of magnitude, the technique produces halo occupation distributions that agree to within 3%.

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arXiv2016

Gravitational redshift and asymmetric redshift-space distortions for stacked clusters

We derive the expression for the observed redshift in the weak field limit in the observer's past light cone, including all relativistic terms up to second order in velocity. We then apply it to compute the cluster-galaxy cross-correlation functions (CGCF) using N-body simulations. The CGCF is asymmetric along the line of sight (LOS) owing to the presence of the small second order terms such as the gravitational redshift (GRedshift). We identify two systematics in the modelling of the GRedshift signal in stacked clusters. First, it is affected by the morphology of dark matter haloes and the large-scale cosmic-web. The non-spherical distribution of galaxies around the central halo and the presence of neighbouring clusters systematically reduce the GRedshift signal. This bias is approximately 20% for $M_{\rm min}\simeq 10^{14} {\rm M_{\odot}}/h$, and is more than 50% for haloes with $M_{\rm min}\simeq 2\times 10^{13} {\rm M_{\odot}}/h$ at $r>$4 Mpc/$h$. Second, the best-fit gravitational redshift profiles as well as the profiles of all other relativistic terms are found to be significantly different in velocity space compared to their real space versions. We discuss some subtleties relating to these effects in velocity space. We also find that the S/N of the GRedshift signal increases with decreasing halo mass.

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arXiv2016

Achieving Convergence in Galaxy Formation Models by Augmenting N-body Merger Trees

Accurate modeling of galaxy formation in a hierarchical, cold dark matter universe requires the use of sufficiently high-resolution merger trees to obtain convergence in the predicted properties of galaxies. When semi-analytic galaxy formation models are applied to cosmological N-body simulation merger trees, it is often the case that those trees have insufficient resolution to give converged galaxy properties. We demonstrate a method to augment the resolution of N-body merger trees by grafting in branches of Monte Carlo merger trees with higher resolution, but which are consistent with the pre-existing branches in the N-body tree. We show that this approach leads to converged galaxy properties.

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arXiv2016

A unified multi-wavelength model of galaxy formation

We present a new version of the GALFORM semi-analytical model of galaxy formation. This brings together several previous developments of GALFORM into a single unified model, including a different initial mass function (IMF) in quiescent star formation and in starbursts, feedback from active galactic nuclei supressing gas cooling in massive halos, and a new empirical star formation law in galaxy disks based on their molecular gas content. In addition, we have updated the cosmology, introduced a more accurate treatment of dynamical friction acting on satellite galaxies, and updated the stellar population model. The new model is able to simultaneously explain both the observed evolution of the K-band luminosity function and stellar mass function, and the number counts and redshift distribution of sub-mm galaxies selected at 850 mu. This was not previously achieved by a single physical model within the LambdaCDM framework, but requires having an IMF in starbursts that is somewhat top-heavy. The new model is tested against a wide variety of observational data covering wavelengths from the far-UV to sub-mm, and redshifts from z=0 to z=6, and is found to be generally successful. These observations include the optical and near-IR luminosity functions, HI mass function, fraction of early type galaxies, Tully-Fisher, metallicity-luminosity and size-luminosity relations at z=0, as well as far-IR number counts, and far-UV luminosity functions at z ~ 3-6. [abridged]

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arXiv2016

Galaxies in the EAGLE hydrodynamical simulation and in the Durham and Munich semi-analytical models

We compare global predictions from the EAGLE hydrodynamical simulation, and two semi-analytic (SA) models of galaxy formation, L-GALAXIES and GALFORM. All three models include the key physical processes for the formation and evolution of galaxies and their parameters are calibrated against a small number of observables at $z\approx 0$. The two SA models have been applied to merger trees constructed from the EAGLE dark matter only simulation. We find that at $z\leq 2$, both the galaxy stellar mass functions for stellar masses $M_*<10^{10.5} M_{\odot}$ and the median specific star formation rates (sSFRs) in the three models agree to better than $0.4$~dex. The evolution of the sSFR predicted by the three models closely follows the mass assembly history of dark matter haloes. In both EAGLE and L-GALAXIES there are more central passive galaxies with $M_*<10^{9.5} M_{\odot}$ than in L-GALAXIES. This difference is related to galaxies that have entered and then left a larger halo and which are treated as satellites in GALFORM. In the range $0 10^{9.5} M_{\odot}$ differ in some instances by an order of magnitude, while the stellar mass-size relation in EAGLE is a factor of $\approx 2$ tighter than for the two SA models. Our results suggest the need for a revision of how SA models treat the effect of baryonic self-gravity on the underlying dark matter. The treatment of gas flows in the models needs to be revised based on detailed comparison with observations to understand in particular the evolution of the stellar mass-metallicity relation.

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arXiv2016

The Mass-Concentration-Redshift Relation of Cold and Warm Dark Matter Halos

We use a suite of cosmological simulations to study the mass-concentration-redshift relation, $c({\rm M},z)$, of dark matter halos. Our simulations include standard $Λ$-cold dark matter (CDM) models, and additional runs with truncated power spectra, consistent with a thermal warm dark matter (WDM) scenario. We find that the mass profiles of CDM and WDM halos are self-similar and well approximated by the Einasto profile. The $c({\rm M},z)$ relation of CDM halos is monotonic: concentrations decrease with increasing virial mass at fixed redshift, and decrease with increasing redshift at fixed mass. The mass accretion histories (MAHs) of CDM halos are also scale-free, and can be used to infer concentrations directly. These results do not apply to WDM halos: their MAHs are not scale-free because of the characteristic scale imposed by the power-spectrum suppression. Further, the WDM $c({\rm M},z)$ relation is non-monotonic: concentrations peak at a mass scale dictated by the truncation scale, and decrease at higher and lower masses. We show that the assembly history of a halo can still be used to infer its concentration, provided that the total mass of its progenitors is considered (the "collapsed mass history"; CMH), rather than just that of its main ancestor. This exploits the scale-free nature of CMHs to derive a simple scaling that reproduces the mass-concentration-redshift relation of both CDM and WDM halos over a vast range of halo masses and redshifts. Our model therefore provides a robust account of the mass, redshift, cosmology and power spectrum dependence of dark matter halo concentrations.

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