arXiv subjects

Shaun Cole

Publications and source records attributed to Shaun Cole.

At least 91 records · Page 5Linked to original sources

arXiv2021

Characterising the target selection pipeline for the Dark Energy Spectroscopic Instrument Bright Galaxy Survey

We present the steps taken to produce a reliable and complete input galaxy catalogue for the Dark Energy Spectroscopic Instrument (DESI) Bright Galaxy Survey (BGS) using the photometric Legacy Survey DR8 DECam. We analyse some of the main issues faced in the selection of targets for the DESI BGS, such as star-galaxy separation, contamination by fragmented stars and bright galaxies. Our pipeline utilizes a new way to select BGS galaxies using Gaia photometry and we implement geometrical and photometric masks that reduce the number of spurious objects. The resulting catalogue is cross-matched with the Galaxy And Mass Assembly (GAMA) survey to assess the completeness of the galaxy catalogue and the performance of the target selection. We also validate the clustering of the sources in our BGS catalogue by comparing with mock catalogues and the Sloan Digital Sky Survey (SDSS) data. Finally, the robustness of the BGS selection criteria is assessed by quantifying the dependence of the target galaxy density on imaging and other properties. The largest systematic correlation we find is a 7 per cent suppression of the target density in regions of high stellar density.

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arXiv2021

Rosella: A mock catalogue from the P-Millennium simulation

The scientific exploitation of the Dark Energy Spectroscopic Instrument Bright Galaxy Survey (DESI BGS) data requires the construction of mocks with galaxy population properties closely mimicking those of the actual DESI BGS targets. We create a high fidelity mock galaxy catalogue, including information about galaxies and their host dark matter subhaloes. The mock catalogue uses subhalo abundance matching (SHAM) with scatter to populate the P-Millennium N-body simulation with galaxies at the median BGS redshift of ~ 0.2, using formation redshift information to assign (g-r) rest-frame colours. The mock provides information about r-band absolute magnitudes, (g-r) rest-frame colours, 3D positions and velocities of a complete sample of DESI BGS galaxies in a volume of (542 Mpc/h)^3, as well as the masses of host dark matter haloes. This P-Millennium DESI BGS mock catalogue is ideally suited for the tuning of approximate mocks unable to resolve subhaloes that DESI BGS galaxies reside in, to test for systematics in analysis pipelines and to interpret (non-cosmological focused) DESI BGS analysis.

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arXiv2020

Preliminary Target Selection for the DESI Bright Galaxy Survey (BGS)

The Dark Energy Spectroscopic Instrument (DESI) will execute a nearly magnitude-limited survey of low redshift galaxies ($0.05 \leq z \leq 0.4$, median $z \approx 0.2$). Clustering analyses of this Bright Galaxy Survey (BGS) will yield the most precise measurements to date of baryon acoustic oscillations and redshift-space distortions at low redshift. DESI BGS will comprise two target classes: (i) BRIGHT ($r<19.5$~mag), and (ii) FAINT ($19.5<r<20$~mag). Here we present a summary of the star-galaxy separation, and different photometric and geometrical masks, used in BGS to reduce the number of spurious targets. The selection results in a total density of $\sim 800$ objects/deg$^2$ for the BRIGHT and $\sim 600$ objects/deg$^2$ for the FAINT selections.A full characterization of the BGS selection can be found in Ruiz-Macias et al. (2020).

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arXiv2020

Baryon Acoustic Oscillations in the projected cross-correlation function between the eBOSS DR16 quasars and photometric galaxies from the DESI Legacy Imaging Surveys

We search for the Baryon Acoustic Oscillations in the projected cross-correlation function binned into transverse comoving radius between the SDSS-IV DR16 eBOSS quasars and a dense photometric sample of galaxies selected from the DESI Legacy Imaging Surveys. We estimate the density of the photometric sample of galaxies in this redshift range to be about 2900 deg$^{-2}$, which is deeper than the official DESI ELG selection, and the density of the spectroscopic sample is about 20 deg$^{-2}$. In order to mitigate the systematics related to the use of different imaging surveys close to the detection limit, we use a neural network approach that accounts for complex dependencies between the imaging attributes and the observed galaxy density. We find that we are limited by the depth of the imaging surveys which affects the density and purity of the photometric sample and its overlap in redshift with the quasar sample, which thus affects the performance of the method. When cross-correlating the photometric galaxies with quasars in $0.6 \leq z \leq 1.2$, the cross-correlation function can provide better constraints on the comoving angular distance, $D_{\rm M}$ (6\% precision) compared to the constraint on the spherically-averaged distance $D_{\rm V}$ (9\% precision) obtained from the auto-correlation. Although not yet competitive, this technique will benefit from the arrival of deeper photometric data from upcoming surveys which will enable it to go beyond the current limitations we have identified in this work.

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arXiv2020

To beta or not to beta: can higher-order Jeans analysis break the mass-anisotropy degeneracy in simulated dwarfs?

We test a non-parametric higher-order Jeans analysis method, GravSphere, on 32 simulated dwarf galaxies comparable to classical Local Group dwarfs like Fornax. The galaxies are selected from the APOSTLE suite of cosmological hydrodynamics simulations with Cold Dark Matter (CDM) and Self-Interacting Dark Matter (SIDM) models, allowing us to investigate cusps and cores in density distributions. We find that, for CDM dwarfs, the recovered enclosed mass profiles have a bias of no more than 10 per cent, with a 50 per cent scatter in the inner regions and a 20 per cent scatter near the half-light radius, consistent with standard mass estimators. The density profiles are also recovered with a bias of no more than 10 per cent and a scatter of 30 per cent in the inner regions. For SIDM dwarfs, the mass and density profiles are recovered within our 95 per cent confidence intervals, but are biased towards cuspy dark matter distributions. This is mainly due to a lack of sufficient constraints from the data. We explore the sources of scatter in the accuracy of the recovered profiles and suggest a $χ^2$ statistic to separate successful models from biased ones. Finally, we show that the uncertainties on the mass profiles obtained with GravSphere are smaller than those for comparable Jeans methods, and that they can be further improved if stronger priors, motivated by cosmological simulations, are placed on the velocity anisotropy. We conclude that GravSphere is a promising Jeans-based approach for modelling dark matter distributions in dwarf galaxies.

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arXiv2019

The distinct stellar metallicity populations of simulated Local Group dwarfs

A number of Local Group dwarf galaxies are known to have two spatially segregated stellar metallicity populations, a centrally concentrated metal-rich population and a more extended metal-poor population. In this work we discuss mechanisms that lead to the formation of two spatially segregated metallicity populations. Using a set of high-resolution hydrodynamical simulations of Local Group-like environments, we select a sample of satellite and field galaxies, spanning the stellar mass range $10^6-10^9$M$_{\odot}$, that exhibit bimodality in their metallicity distributions. Among those, we identify a subsample with a strong spatial segregation in the two populations. We find three distinct mechanisms for their formation. In field dwarfs and in a small fraction of satellites, a merger causes the metal-poor stars to migrate to larger radii and encourages the available gas to sink to the centre of the dwarf. Most of the gas is subsequently blown out of the halo through star formation feedback, but the remaining gas is consumed in the formation of a metal-rich population. In the exclusive case of satellites that have retained some of their gas at infall, it is the compression of this gas by ram pressure near pericentre that triggers the formation of metal-rich stars, whilst simultaneously preventing star formation at larger radii through stripping. Additionally, in a small number of field and satellite dwarfs, interactions with gaseous filaments and other galaxies can result in the formation of a metal-rich population. Regardless of the formation mechanism, a history of mergers typically enhances the spatial segregation.

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arXiv2019

Galaxy formation in the Planck Millennium: the atomic hydrogen content of dark matter halos

We present recalibrations of the GALFORM semi-analytical model of galaxy formation in a new N-body simulation with the Planck cosmology. The Planck Millennium simulation uses more than 128 billion particles to resolve the matter distribution in a cube of $800$ Mpc on a side, which contains more than 77 million dark matter haloes with mass greater than $2.12 \times 10^{9} h^{-1} {\rm M_{\odot}}$ at the present day. Only minor changes to a very small number of model parameters are required in the recalibration. We present predictions for the atomic hydrogen content (HI) of dark matter halos, which is a key input into the calculation of the HI intensity mapping signal expected from the large-scale structure of the Universe. We find that the HI mass $-$ halo mass relation displays a clear break at the halo mass above which AGN heating suppresses gas cooling, $\approx 3 \times 10^{11} h^{-1} M_{\rm \odot}$. Below this halo mass, the HI content of haloes is dominated by the central galaxy; above this mass it is the combined HI content of satellites that prevails. We find that the HI mass - halo mass relation changes little with redshift up to $z=3$. The bias of HI sources shows a scale dependence that gets more pronounced with increasing redshift.

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arXiv2019

The evolution of the UV-to-mm extragalactic background light: evidence for a top-heavy initial mass function?

We present predictions for the UV-to-mm extragalactic background light (EBL) from a recent version of the GALFORM semi-analytical model of galaxy formation which invokes a top-heavy stellar initial mass function (IMF) for galaxies undergoing dynamically-triggered bursts of star formation. We combine GALFORM with the GRASIL radiative transfer code for computing fully self-consistent UV-to-mm spectral energy distributions for each simulated galaxy, accounting for the absorption and re-emission of stellar radiation by interstellar dust. The predicted EBL is in near-perfect agreement with recent observations over the whole UV-to-mm spectrum, as is the evolution of the cosmic spectral energy distribution over the redshift range for which observations are available ($z\lesssim1$). We show that approximately 90~per~cent of the EBL is produced at $z<2$ although this shifts to higher redshifts for sub-mm wavelengths. We assess whether the top-heavy IMF in starbursts is necessary in order to reproduce the EBL at the same time as other key observables, and find that variant models with a universal solar-neighborhood IMF display poorer agreement with EBL observations over the whole UV-to-mm spectrum and fail to match the counts of galaxies in the sub-mm.

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arXiv2019

Correcting for Fibre Assignment Incompleteness in the DESI Bright Galaxy Survey

The Dark Energy Spectroscopic Instrument (DESI) Bright Galaxy Survey (BGS) will be a survey of bright, low redshift galaxies, which is planned to cover an area of ~14,000 sq deg in 3 passes. Each pass will cover the survey area with ~2000 pointings, each of area ~8 sq deg. The BGS is currently proposed to consist of a bright high priority sample to an r-band magnitude limit r ~ 19.5, with a fainter low priority sample to r ~ 20. The geometry of the DESI fibre positioners in the focal plane of the telescope affects the completeness of the survey, and has a non-trivial impact on clustering measurements. Using a BGS mock catalogue, we show that completeness due to fibre assignment primarily depends on the surface density of galaxies. Completeness is high (>95%) in low density regions, but very low (<10%) in the centre of massive clusters. We apply the pair inverse probability (PIP) weighting correction to clustering measurements from a BGS mock which has been through the fibre assignment algorithm. This method is only unbiased if it is possible to observe every galaxy pair. To facilitate this, we randomly promote a small fraction of the fainter sample to be high priority, and dither the set of tile positions by a small angle. We show that inverse pair weighting combined with angular upweighting provides an unbiased correction to galaxy clustering measurements for the complete 3 pass survey, and also after 1 pass, which is highly incomplete.

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arXiv2018

Halo concentrations from extended Press-Schechter merger histories

We apply the model relating halo concentration to formation history proposed by Ludlow et al. to merger trees generated using an algorithm based on excursion set theory. We find that while the model correctly predicts the median relation between halo concentration and mass, it underpredicts the scatter in concentration at fixed mass. Since the same model applied to N-body merger trees predicts the correct scatter, we postulate that the missing scatter is due to the lack of any environmental dependence in merger trees derived from excursion set theory. We show that a simple modification to the merger tree construction algorithm, which makes merger rates dependent on environment, can increase the scatter by the required amount, and simultaneously provide a qualitatively correct correlation between environment and formation epoch in the excursion set merger trees.

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arXiv2018

Aurigaia: mock Gaia DR2 stellar catalogues from the Auriga cosmological simulations

We present and analyse mock stellar catalogues that match the selection criteria and observables (including uncertainties) of the Gaia satellite data release 2 (DR2). The source are six cosmological high-resolution magneto-hydrodynamic $Λ$CDM zoom simulations of the formation of Milky Way analogues from the AURIGA project. Mock data are provided for stars with $V < 16$ mag, and $V < 20$ mag at $|b|>20$ degrees. The mock catalogues are made using two different methods: the public SNAPDRAGONS code, and a method based on that of Lowing et al. that preserves the phase-space distribution of the model stars. These publicly available catalogues contain 5-parameter astrometry, radial velocities, multi-band photometry, stellar parameters, dust extinction values, and uncertainties in all these quantities. In addition, we provide the gravitational potential and information on the origin of each star. By way of demonstration, we apply the mock catalogues to analyses of the young stellar disc and the stellar halo. We show that: i) the young outer stellar disc exhibits a flared distribution that is detectable in the height and vertical velocity distribution of A- and B-dwarf stars up to radii of ~15 kpc; and ii) the spin of the stellar halo out to 100 kpc can be accurately measured with Gaia DR2 RR Lyrae stars. These catalogues are well suited for comparisons with observations and should help to: i) develop and test analysis methods for the Gaia DR2 data; ii) gauge the limitations and biases of the data and iii) interpret the data in the light of theoretical predictions from realistic $ab$ $initio$ simulations of galaxy formation in the $Λ$CDM cosmological model.

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arXiv2018

What to expect from dynamical modelling of galactic haloes II: the spherical Jeans equation

The spherical Jeans equation (SJE) is widely used in dynamical modelling of the Milky Way (MW) halo potential. We use haloes and galaxies from the cosmological Millennium-II simulation and hydrodynamical APOSTLE simulations to investigate the performance of the SJE in recovering the underlying mass profiles of MW mass haloes. The best-fitting halo mass and concentration parameters scatter by 25% and 40% around their input values, respectively, when dark matter particles are used as tracers. This scatter becomes as large as a factor of 3 when using star particles instead. This is significantly larger than the estimated statistical uncertainty associated with the use of the SJE. The existence of correlated phase-space structures that violate the steady state assumption of the SJE as well as non-spherical geometries are the principal sources of the scatter. Binary haloes show larger scatter because they are more aspherical in shape and have a more perturbed dynamical state. Our results confirm the previous study of Wang et al. (2017) that the number of independent phase-space structures sets an intrinsic limiting precision on dynamical inferences based on the steady state assumption. Modelling with a radius-independent velocity anisotropy, or using tracers within a limited outer radius, result in significantly larger scatter, but the ensemble-averaged measurement over the whole halo sample is approximately unbiased.

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arXiv2017

The core-cusp problem: a matter of perspective

The existence of two kinematically and chemically distinct stellar subpopulations in the Sculptor and Fornax dwarf galaxies offers the opportunity to constrain the density profile of their matter haloes by measuring the mass contained within the well-separated half-light radii of the two metallicity subpopulations. Walker and Penarrubia have used this approach to argue that data for these galaxies are consistent with constant-density `cores' in their inner regions and rule out `cuspy' Navarro-Frenk-White (NFW) profiles with high statistical significance, particularly in the case of Sculptor. We test the validity of these claims using dwarf galaxies in the APOSTLE (A Project Of Simulating The Local Environment) Lambda cold dark matter cosmological hydrodynamics simulations of analogues of the Local Group. These galaxies all have NFW dark matter density profiles and a subset of them develop two distinct metallicity subpopulations reminiscent of Sculptor and Fornax. We apply a method analogous to that of Walker and Penarrubia to a sample of 53 simulated dwarfs and find that this procedure often leads to a statistically significant detection of a core in the profile when in reality there is a cusp. Although multiple factors contribute to these failures, the main cause is a violation of the assumption of spherical symmetry upon which the mass estimators are based. The stellar populations of the simulated dwarfs tend to be significantly elongated and, in several cases, the two metallicity populations have different asphericity and are misaligned. As a result, a wide range of slopes of the density profile are inferred depending on the angle from which the galaxy is viewed.

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arXiv2017

Predictions for deep galaxy surveys with JWST from $Λ$CDM

We present predictions for the outcome of deep galaxy surveys with the $James$ $Webb$ $Space$ $Telescope$ ($JWST$) obtained from a physical model of galaxy formation in $Λ$CDM. We use the latest version of the GALFORM model, embedded within a new ($800$ Mpc)$^{3}$ dark matter only simulation with a halo mass resolution of $M_{\rm halo}>2\times10^{9}$ $h^{-1}$ M$_{\odot}$. For computing full UV-to-mm galaxy spectral energy distributions, including the absorption and emission of radiation by dust, we use the spectrophotometric radiative transfer code GRASIL. The model is calibrated to reproduce a broad range of observational data at $z\lesssim6$, and we show here that it can also predict evolution of the rest-frame far-UV luminosity function for $7\lesssim z\lesssim10$ which is in good agreement with observations. We make predictions for the evolution of the luminosity function from $z=16$ to $z=0$ in all broadband filters on the Near InfraRed Camera (NIRCam) and Mid InfraRed Instrument (MIRI) on $JWST$ and present the resulting galaxy number counts and redshift distributions. Our fiducial model predicts that $\sim1$ galaxy per field of view will be observable at $z\sim11$ for a $10^4$ s exposure with NIRCam. A variant model, which produces a higher redshift of reionization in better agreement with $Planck$ data, predicts number densities of observable galaxies $\sim5\times$ greater at this redshift. Similar observations with MIRI are predicted not to detect any galaxies at $z\gtrsim6$. We also make predictions for the effect of different exposure times on the redshift distributions of galaxies observable with $JWST$, and for the angular sizes of galaxies in $JWST$ bands.

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arXiv2017

Comparing galaxy formation in semi-analytic models and hydrodynamical simulations

It is now possible for hydrodynamical simulations to reproduce a representative galaxy population. Accordingly, it is timely to assess critically some of the assumptions of traditional semi-analytic galaxy formation models. We use the Eagle simulations to assess assumptions built into the Galform semi-analytic model, focussing on those relating to baryon cycling, angular momentum and feedback. We show that the assumption in Galform that newly formed stars have the same specific angular momentum as the total disc leads to a significant overestimate of the total stellar specific angular momentum of disc galaxies. In Eagle, stars form preferentially out of low specific angular momentum gas in the interstellar medium (ISM) due to the assumed gas density threshold for stars to form, leading to more realistic galaxy sizes. We find that stellar mass assembly is similar between Galform and Eagle but that the evolution of gas properties is different, with various indications that the rate of baryon cycling in Eagle is slower than is assumed in Galform. Finally, by matching individual galaxies between Eagle and Galform, we find that an artificial dependence of AGN feedback and gas infall rates on halo mass doubling events in Galform drives most of the scatter in stellar mass between individual objects. Put together our results suggest that the Galform semi-analytic model can be significantly improved in light of recent advances.

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arXiv2017

Addressing the too big to fail problem with baryon physics and sterile neutrino dark matter

N-body dark matter simulations of structure formation in the $Λ$CDM model predict a population of subhalos within Galactic halos that have higher central densities than inferred for satellites of the Milky Way, a tension known as the `too big to fail' problem. Proposed solutions include baryonic effects, a smaller mass for the Milky Way halo, and warm dark matter. We test these three possibilities using a semi-analytic model of galaxy formation to generate luminosity functions for Milky Way halo-analogue satellite populations, the results of which are then coupled to the Jiang & van den Bosch model of subhalo stripping to predict the subhalo $V_\mathrm{max}$ functions for the 10 brightest satellites. We find that selecting the brightest satellites (as opposed to the most massive) and modelling the expulsion of gas by supernovae at early times increases the likelihood of generating the observed Milky Way satellite $V_\mathrm{max}$ function. The preferred halo mass is $6\times10^{11}M_{\odot}$, which has a 14 percent probability to host a $V_\mathrm{max}$ function like that of the Milky Way satellites. This probability is reduced to 8 percent for a $1.0\times10^{12}M_{\odot}$ halo and to 3 percent for a $1.4\times10^{12}M_{\odot}$ halo. We conclude that the Milky Way satellite $V_\mathrm{max}$ function is compatible with a CDM cosmology, as previously found by Sawala et al. using hydrodynamic simulations. Sterile neutrino-warm dark matter models achieve a higher degree of agreement with the observations, with a maximum 35 percent chance of generating the observed Milky Way satellite $V_\mathrm{max}$ function. However, more work is required to check that the semi-analytic stripping model is calibrated correctly in the sterile neutrino cosmology, and to check if our sterile neutrino models produce sufficient numbers of faint satellites.

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arXiv2017

Comparing semi-analytic particle tagging and hydrodynamical simulations of the Milky Way's stellar halo

Particle tagging is an efficient, but approximate, technique for using cosmological N-body simulations to model the phase-space evolution of the stellar populations predicted, for example, by a semi-analytic model of galaxy formation. We test the technique developed by Cooper et al. (which we call STINGS here) by comparing particle tags with stars in a smooth particle hydrodynamic (SPH) simulation. We focus on the spherically averaged density profile of stars accreted from satellite galaxies in a Milky Way (MW)-like system. The stellar profile in the SPH simulation can be recovered accurately by tagging dark matter (DM) particles in the same simulation according to a prescription based on the rank order of particle binding energy. Applying the same prescription to an N-body version of this simulation produces a density profile differing from that of the SPH simulation by <10 per cent on average between 1 and 200 kpc. This confirms that particle tagging can provide a faithful and robust approximation to a self-consistent hydrodynamical simulation in this regime (in contradiction to previous claims in the literature). We find only one systematic effect, likely due to the collisionless approximation, namely that massive satellites in the SPH simulation are disrupted somewhat earlier than their collisionless counterparts. In most cases this makes remarkably little difference to the spherically averaged distribution of their stellar debris. We conclude that, for galaxy formation models that do not predict strong baryonic effects on the present-day DM distribution of MW-like galaxies or their satellites, differences in stellar halo predictions associated with the treatment of star formation and feedback are much more important than those associated with the dynamical limitations of collisionless particle tagging.

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arXiv2017

HBT+: an improved code for finding subhalos and building merger trees in cosmological simulations

Dark matter subhalos are the remnants of (incomplete) halo mergers. Identifying them and establishing their evolutionary links in the form of merger trees is one of the most important applications of cosmological simulations. The Hierachical Bound-Tracing (HBT) code identifies halos as they form and tracks their evolution as they merge, simultaneously detecting subhalos and building their merger trees. Here we present a new implementation of this approach, HBT+, that is much faster, more user friendly, and more physically complete than the original code. Applying HBT+ to cosmological simulations we show that both the subhalo mass function and the peak-mass function are well fit by similar double-Schechter functions.The ratio between the two is highest at the high mass end, reflecting the resilience of massive subhalos that experience substantial dynamical friction but limited tidal stripping. The radial distribution of the most massive subhalos is more concentrated than the universal radial distribution of lower mass subhalos. Subhalo finders that work in configuration space tend to underestimate the masses of massive subhalos, an effect that is stronger in the host centre. This may explain, at least in part, the excess of massive subhalos in galaxy cluster centres inferred from recent lensing observations. We demonstrate that the peak-mass function is a powerful diagnostic of merger tree defects, and the merger trees constructed using HBT+ do not suffer from the missing or switched links that tend to afflict merger trees constructed from more conventional halo finders. We make the HBT+ code publicly available.

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