Imperial College London


Faculty of Natural SciencesDepartment of Physics

Professor of Astrophysics and Statistics



+44 (0)20 7594 7878d.mortlock Website




1018ABlackett LaboratorySouth Kensington Campus





Publication Type

101 results found

Alsing J, Peiris H, Mortlock D, Leja J, Leistedt Bet al., 2023, Forward Modeling of Galaxy Populations for Cosmological Redshift Distribution Inference, ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES, Vol: 264, ISSN: 0067-0049

Journal article

Leistedt B, Alsing J, Peiris H, Mortlock D, Leja Jet al., 2023, Hierarchical Bayesian inference of photometric redshifts with stellar population synthesis models, Astrophysical Journal Supplement Series, Vol: 264, Pages: 1-12, ISSN: 0067-0049

We present a Bayesian hierarchical framework to analyze photometric galaxy survey data with stellar population synthesis (SPS) models. Our method couples robust modeling of spectral energy distributions with a population model and a noise model to characterize the statistical properties of the galaxy populations and real observations, respectively. By self-consistently inferring all model parameters, from high-level hyperparameters to SPS parameters of individual galaxies, one can separate sources of bias and uncertainty in the data. We demonstrate the strengths and flexibility of this approach by deriving accurate photometric redshifts for a sample of spectroscopically confirmed galaxies in the COSMOS field, all with 26-band photometry and spectroscopic redshifts. We achieve a performance competitive with publicly released photometric redshift catalogs based on the same data. Prior to this work, this approach was computationally intractable in practice due to the heavy computational load of SPS model calls; we overcome this challenge by the addition of neural emulators. We find that the largest photometric residuals are associated with poor calibration for emission-line luminosities and thus build a framework to mitigate these effects. This combination of physics-based modeling accelerated with machine learning paves the path toward meeting the stringent requirements on the accuracy of photometric redshift estimation imposed by upcoming cosmological surveys. The approach also has the potential to create new links between cosmology and galaxy evolution through the analysis of photometric data sets.

Journal article

Capel F, Burgess JM, Mortlock DJ, Padovani Pet al., 2022, Assessing coincident neutrino detections using population models, Astronomy and Astrophysics, Vol: 668, ISSN: 0004-6361

Several tentative associations between high-energy neutrinos and astrophysical sources have been recently reported, but a conclusive identification of these potential neutrino emitters remains challenging. We explore the use of Monte Carlo simulations of source populations to gain deeper insight into the physical implications of proposed individual source- neutrino associations. In particular, we focus on the IC170922A- TXS 0506+056 observation. Assuming a null model, we find a 7.6% chance of mistakenly identifying coincidences between γ-ray flares from blazars and neutrino alerts in 10-year surveys. We confirm that a blazar- neutrino connection based on the γ-ray flux is required to find a low chance coincidence probability and, therefore, a significant IC170922A- TXS 0506+056 association. We then assume this blazar- neutrino connection for the whole population and find that the ratio of neutrino to γ-ray fluxes must be ≲10-2 in order not to overproduce the total number of neutrino alerts seen by IceCube. For the IC170922A- TXS 0506+056 association to make sense, we must either accept this low flux ratio or suppose that only some rare sub-population of blazars is capable of high-energy neutrino production. For example, if we consider neutrino production only in blazar flares, we expect the flux ratio of between 10-3 and 10-1 to be consistent with a single coincident observation of a neutrino alert and flaring γ-ray blazar. These constraints should be interpreted in the context of the likelihood models used to find the IC170922A- TXS 0506+056 association, which assumes a fixed power-law neutrino spectrum of E-2.13 for all blazars.

Journal article

O'Riordan CM, Warren SJ, Mortlock DJ, 2021, Galaxy mass profiles from strong lensing III: The two-dimensional broken power-law model, Monthly Notices of the Royal Astronomical Society, Vol: 501, Pages: 3687-3694, ISSN: 0035-8711

When modelling strong gravitational lenses, i.e., where there are multipleimages of the same source, the most widely used parameterisation for the massprofile in the lens galaxy is the singular power-law model $\rho(r)\proptor^{-\gamma}$. This model may be insufficiently flexible for very accurate work,for example measuring the Hubble constant based on time delays between multipleimages. Here we derive the lensing properties - deflection angle, shear, andmagnification - of a more adaptable model where the projected mass surfacedensity is parameterised as a continuous two-dimensional broken power-law(2DBPL). This elliptical 2DBPL model is characterised by power-law slopes$t_1$, $t_2$ either side of the break radius $\theta_\mathrm{B}$. The key tothe 2DBPL model is the derivation of the lensing properties of the truncatedpower law (TPL) model, where the surface density is a power law out to thetruncation radius $\theta_\mathrm{T}$ and zero beyond. This TPL model is alsouseful by itself. We create mock observations of lensing by a TPL profile wherethe images form outside the truncation radius, so there is no mass in theannulus covered by the images. We then show that the slope of the profileinterior to the images may be accurately recovered for lenses of moderateellipticity. This demonstrates that the widely-held notion that lensingmeasures the slope of the mass profile in the annulus of the images, and isinsensitive to the mass distribution at radii interior to the images, isincorrect.

Journal article

Feeney SM, Peiris HV, Nissanke SM, Mortlock DJet al., 2021, Prospects for measuring the Hubble constant with neutron-star–black-hole mergers, Physical Review Letters, Vol: 126, ISSN: 0031-9007

Gravitational wave (GW) and electromagnetic (EM) observations of neutron-star-black-hole (NSBH) mergers can provide precise local measurements of the Hubble constant (H0), ideal for resolving the current H0 tension. We perform end-to-end analyses of realistic populations of simulated NSBHs, incorporating both GW and EM selection for the first time. We show that NSBHs could achieve unbiased 1.5%–2.4% precision H0 estimates by 2030. The achievable precision is strongly affected by the details of spin precession and tidal disruption, highlighting the need for improved modeling of NSBH mergers.

Journal article

Porqueres N, Heavens A, Mortlock D, Lavaux Get al., 2021, Bayesian forward modelling of cosmic shear data, Monthly Notices of the Royal Astronomical Society, Vol: 502, Pages: 3035-3044, ISSN: 0035-8711

We present a Bayesian hierarchical modelling approach to infer the cosmic matter density field, and the lensing and the matter power spectra, from cosmic shear data. This method uses a physical model of cosmic structure formation to infer physically plausible cosmic structures, which accounts for the non-Gaussian features of the gravitationally evolved matter distribution and light-cone effects. We test and validate our framework with realistic simulated shear data, demonstrating that the method recovers the unbiased matter distribution and the correct lensing and matter power spectrum. While the cosmology is fixed in this test, and the method employs a prior power spectrum, we demonstrate that the lensing results are sensitive to the true power spectrum when this differs from the prior. In this case, the density field samples are generated with a power spectrum that deviates from the prior, and the method recovers the true lensing power spectrum. The method also recovers the matter power spectrum across the sky, but as currently implemented, it cannot determine the radial power since isotropy is not imposed. In summary, our method provides physically plausible inference of the dark matter distribution from cosmic shear data, allowing us to extract information beyond the two-point statistics and exploiting the full information content of the cosmological fields.

Journal article

Barnett R, Warren SJ, Cross NJG, Mortlock DJ, Fan X, Wang F, Hewett PCet al., 2020, A complete search for redshift z>6.5 quasars in the VIKING survey, Monthly Notices of the Royal Astronomical Society, Vol: 501, Pages: 1663-1676, ISSN: 0035-8711

We present the results of a new, deeper, and complete search forhigh-redshift $6.5<z<9.3$ quasars over 977deg$^2$ of the VISTA Kilo-DegreeInfrared Galaxy (VIKING) survey. This exploits a new list-driven datasetproviding photometry in all bands ZYJHKs, for all sources detected by VIKING in$J$. We use the Bayesian model comparison (BMC) selection method of Mortlock etal., producing a ranked list of just 21 candidates. The sources ranked 1, 2, 3and 5 are the four known $z>6.5$ quasars in this field. Additional observationsof the other 17 candidates, primarily DESI Legacy Survey photometry and ESOFORS2 spectroscopy, confirm that none is a quasar. This is the first completesample from the VIKING survey, and we provide the computed selection function.We include a detailed comparison of the BMC method against two other selectionmethods: colour cuts and minimum-$\chi^2$ SED fitting. We find that: i) BMCproduces eight times fewer false positives than colour cuts, while alsoreaching 0.3 mag. deeper, ii) the minimum-$\chi^2$ SED fitting method isextremely efficient but reaches 0.7 mag. less deep than the BMC method, andselects only one of the four known quasars. We show that BMC candidates,rejected because their photometric SEDs have high $\chi^2$ values, includebright examples of galaxies with very strong [OIII]$\lambda\lambda$4959,5007emission in the $Y$ band, identified in fainter surveys by Matsuoka et al. Thisis a potential contaminant population in Euclid searches for faint $z>7$quasars, not previously accounted for, and that requires bettercharacterisation.

Journal article

ORiordan CM, Warren SJ, Mortlock DJ, 2020, Galaxy mass profiles from strong lensing II: The elliptical power-law model, Monthly Notices of the Royal Astronomical Society, Vol: 496, Pages: 3424-3435, ISSN: 0035-8711

We present a systematic analysis of the constraints σγ on the mass profile slope γ obtainable when fitting a singular power-law ellipsoid model to a typical strong lensing observation of an extended source. These results extend our previous analysis of circular systems, Paper I. We draw our results from 676 mock observations covering a range of image configurations, each created with a fixed signal to noise ratio S = 100 in the images. We analyse the results using a combination of theory and a simplified model which identifies the contribution to the constraints of the individual fluxes and positions in each of the lensed images. The main results are: 1. For any lens ellipticity, the constraints σγ for two image systems are well described by the results of Paper I, transformed to elliptical coordinates; 2. We derive an analytical expression for σγ for systems with the source aligned with the axis of the lens; 3. For both two-image systems and aligned systems, σγ is limited by the flux uncertainties; 4. The constraints for off-axis four-image systems are a factor of two to eight better, depending on source size, than for two-image systems, and improve with increasing lens ellipticity. We show that the constraints on γ in these systems derive from the complementary positional information of the images alone, without using flux. The complementarity improves as the offset of the source from the axis increases, such that the best constraints σγ < 0.01, for S = 100, occur when the source approaches the caustic.

Journal article

Alsing J, Peiris H, Leja J, Hahn C, Tojeiro R, Mortlock D, Leistedt B, Johnson BD, Conroy Cet al., 2020, SPECULATOR: Emulating stellar population synthesis for fast and accurate galaxy spectra and photometry, Astrophysical Journal Supplement Series, Vol: 249, ISSN: 0067-0049

We present SPECULATOR - a fast, accurate, and flexible framework foremulating stellar population synthesis (SPS) models for predicting galaxyspectra and photometry. For emulating spectra, we use principal componentanalysis to construct a set of basis functions, and neural networks to learnthe basis coefficients as a function of the SPS model parameters. Forphotometry, we parameterize the magnitudes (for the filters of interest) as afunction of SPS parameters by a neural network. The resulting emulators areable to predict spectra and photometry under both simple and complicated SPSmodel parameterizations to percent-level accuracy, giving a factor of$10^3$-$10^4$ speed up over direct SPS computation. They havereadily-computable derivatives, making them amenable to gradient-basedinference and optimization methods. The emulators are also straightforward tocall from a GPU, giving an additional order-of-magnitude speed-up. Rapid SPScomputations delivered by emulation offers a massive reduction in thecomputational resources required to infer the physical properties of galaxiesfrom observed spectra or photometry and simulate galaxy populations under SPSmodels, whilst maintaining the accuracy required for a range of applications.

Journal article

Capel F, Mortlock D, Finley C, 2020, Bayesian constraints on the astrophysical neutrino source population from IceCube data, Physical Review D: Particles, Fields, Gravitation and Cosmology, Vol: 101, Pages: 123017 – 1-123017 – 21, ISSN: 1550-2368

We present constraints on an astrophysical population of neutrino sources imposed by recent data from the IceCube neutrino observatory. By using the IceCube point source search method to model the detection of sources, our detection criterion is more sensitive than using the observation of high-energy neutrino multiplets for source identification. We frame the problem as a Bayesian hierarchical model to connect the high-level population parameters to the IceCube data, allowing us to consistently account for all relevant sources of uncertainty in our model assumptions. Our results show that sources with a local density of n0≳10−7  Mpc−3 and luminosity L≲1043  erg s−1 are the most likely candidates, but that populations of rare sources with n0≃10−9  Mpc−3 and L≃1045  erg s−1 can still be consistent with the IceCube observations. We demonstrate that these conclusions are strongly dependent on the source evolution considered, for which we consider a wide range of models. In doing so, we present realistic, model-independent constraints on the population parameters that reflect our current state of knowledge from astrophysical neutrino observations. We also use our framework to investigate constraints in the case of possible source detections and future instrument upgrades. Our approach is flexible and can be used to model specific source cases and extended to include multimessenger information.

Journal article

Acuner Z, Ryde F, Pe'er A, Mortlock D, Ahlgren Bet al., 2020, The fraction of gamma-ray bursts with an observed photospheric emission episode, ASTROPHYSICAL JOURNAL, Vol: 893, Pages: 1-17, ISSN: 0004-637X

There is no complete description of the emission physics during the prompt phase in gamma-ray bursts. Spectral analyses, however, indicate that many spectra are narrower than what is expected for nonthermal emission models. Here, we reanalyze the sample of 37 bursts in Yu et al. by fitting the narrowest time-resolved spectrum in each burst. We perform a model comparison between photospheric and synchrotron emission models based on Bayesian evidence. We compare the shapes of the narrowest expected spectra: emission from the photosphere in a non-dissipative flow and slow cooled synchrotron emission from a narrow electron distribution. We find that the photospheric spectral shape is preferred by 54% ± 8% of the spectra (20/37), while 38% ± 8% of the spectra (14/37) prefer the synchrotron spectral shape; three spectra are inconclusive. We hence conclude that GRB spectra are indeed very narrow and that more than half of the bursts have a photospheric emission episode. We also find that a third of all analyzed spectra, not only prefer, but are also compatible with a non-dissipative photosphere, confirming previous similar findings. Furthermore, we notice that the spectra that prefer the photospheric model all have low-energy power-law indices α gsim −0.5. This means that α is a good estimator for which model is preferred by the data. Finally, we argue that the spectra that statistically prefer the synchrotron model could equally as well be caused by subphotospheric dissipation. If that is the case, photospheric emission during the early, prompt phase would be even more dominant.

Journal article

Mortlock DJ, Feeney SM, Peiris HV, Williamson AR, Nissanke SMet al., 2019, Unbiased Hubble constant estimation from binary neutron star mergers, Physical Review D: Particles, Fields, Gravitation and Cosmology, Vol: 100, ISSN: 1550-2368

Gravitational-wave (GW) observations of binary neutron star (BNS) mergers can be used to measure luminosity distances and hence, when coupled with estimates for the mergers’ host redshifts, infer the Hubble constant H0. These observations are, however, affected by GW measurement noise, uncertainties in host redshifts and peculiar velocities, and are potentially biased by selection effects and the misspecification of the cosmological model or the BNS population. The estimation of H0 from samples of BNS mergers with optical counterparts is tested here by using a phenomenological model for the GW strains that captures both the data-driven event selection and the distance-inclination degeneracy, while being simple enough to facilitate large numbers of simulations. A rigorous Bayesian approach to analyzing the data from such simulated BNS merger samples is shown to yield results that are unbiased, have the appropriate uncertainties, and are robust to model misspecification. Applying such methods to a sample of N≃50 BNS merger events, as LIGO+Virgo could produce in the next ∼5  years, should yield robust and accurate Hubble constant estimates that are precise to a level of ≲2  km s−1 Mpc−1, sufficient to reliably resolve the current tension between local and cosmological measurements of H0.

Journal article

Barnett R, Warren SJ, Mortlock DJ, Cuby J-G, Conselice C, Hewett PC, Willott CJet al., 2019, Euclid preparation. V. Predicted yield of redshift 7<z<9 quasars from the wide survey, Astronomy & Astrophysics, Vol: 631, ISSN: 0004-6361

We provide predictions of the yield of 7 < z < 9 quasars from the Euclid wide survey, updating the calculation presented in theEuclid Red Book in several ways. We account for revisions to the Euclid near-infrared filter wavelengths; we adopt steeper ratesof decline of the quasar luminosity function (QLF; Φ) with redshift, Φ ∝ 10k(z−6), k = −0.72, and a further steeper rate of decline,k = −0.92; we use better models of the contaminating populations (MLT dwarfs and compact early-type galaxies); and we make useof an improved Bayesian selection method, compared to the colour cuts used for the Red Book calculation, allowing the identificationof fainter quasars, down to JAB ∼ 23. Quasars at z > 8 may be selected from Euclid OY JH photometry alone, but selection overthe redshift interval 7 < z < 8 is greatly improved by the addition of z-band data from, e.g., Pan-STARRS and LSST. We calculatepredicted quasar yields for the assumed values of the rate of decline of the QLF beyond z = 6. If the decline of the QLF acceleratesbeyond z = 6, with k = −0.92, Euclid should nevertheless find over 100 quasars with 7.0 < z < 7.5, and ∼ 25 quasars beyond thecurrent record of z = 7.5, including ∼ 8 beyond z = 8.0. The first Euclid quasars at z > 7.5 should be found in the DR1 data release,expected in 2024. It will be possible to determine the bright-end slope of the QLF, 7 < z < 8, M1450 < −25, using 8 m class telescopesto confirm candidates, but follow-up with JWST or E-ELT will be required to measure the faint-end slope. Contamination of thecandidate lists is predicted to be modest even at JAB ∼ 23. The precision with which k can be determined over 7 < z < 8 depends onthe value of k, but assuming k = −0.72 it can be measured to a 1σ uncertainty of 0.07.

Journal article

Ahlgren B, Larrson J, Valan V, Mortlock D, Ryde F, Asaf Pet al., 2019, Investigating subphotospheric dissipation in gamma-ray bursts using joint Fermi-Swift observations, Astronomy and Astrophysics, Vol: 880, ISSN: 0004-6361

The jet photosphere has been proposed as the origin for the gamma-ray burst (GRB) prompt emis-sion. In many such models, characteristic features in the spectra appear below the energy range of theFermiGBM detectors, so joint fits with X-ray data are important in order to assess the photosphericscenario. Here we consider a particular photospheric model which assumes localized subphotosphericdissipation by internal shocks in a non-magnetized outflow. We investigate it using Bayesian inferenceand a sample of 8 GRBs with known redshifts which are observed simultaneously withFermiGBM andSwiftXRT. This provides us with an energy range of 0.3 keV to 40 MeV and much tighter parameterconstraints. We analyze 32 spectra and find that 16 are well described by the model. We also findthat the estimates of the bulk Lorentz factor, Γ, and the fireball luminosity,L0,52, decrease while thefraction of dissipated energy,εd, increase in the joint fits compared to GBM only fits. These changesare caused by a small excess of counts in the XRT data, relative to the model predictions from fits toGBM only data. The fact that our limited implementation of the physical scenario yields 50% acceptedspectra is promising, and we discuss possible model revisions in the light of the new data. Specifically,we argue that the inclusion of significant magnetization, as well as removing the assumption of internalshocks, will provide better fits at low energies.

Journal article

O'Riordan CM, Warren SJ, Mortlock DJ, 2019, Galaxy mass profiles from strong lensing I: the circular power-law model, Monthly Notices of the Royal Astronomical Society, ISSN: 0035-8711

In this series of papers we develop a formalism for constraining mass profiles in strong gravitational lenses with extended images, using fluxes in addition to positional information. We start in this paper with a circular power-law profile and show that the slope γ is uniquely determined by only two observables: the flux ratio f1/f2 and the image position ratio θ1/θ2 of the two images. We derive an analytic expression relating these two observables to the slope, a result which does not depend on the Einstein angle or the structure or brightness of the source. We then find an expression for the uncertainty on the slope σγ that depends only on the position ratio θ1/θ2 and the total S/N in the images. For example, in a system with position ratio θ1/θ2=0.5, S/N =100 and γ=2 we find that γ is constrained to a precision of ±0.03. We then test these results against a series of mock observations. We invert the images and fit an 11 parameter model, including ellipticity and position angle for both lens and source and measure the uncertainty on γ. We find agreement with the theoretical estimate for all mock observations. In future papers we will examine the radial range of the galaxy over which the constraint on the slope applies, and extend the analysis to elliptical lenses.

Journal article

Widmark A, Mortlock DJ, Peiris HV, 2019, A Bayesian model for inferring properties of the local white dwarf population in astrometric and photometric surveys, Monthly Notices of the Royal Astronomical Society, Vol: 485, Pages: 179-188, ISSN: 0035-8711

The Gaia mission is providing precise astrometry for an unprecedented number of white dwarfs (WDs), encoding information on stellar evolution, Type Ia supernovae progenitor scenarios, and the star formation and dynamical history of the Milky Way. With such a large data set, it is possible to infer properties of the WD population using only astrometric and photometric informations. We demonstrate a framework to accomplish this using a mock data set with Sloan Digital Sky Survey ugriz photometry and Gaia astrometric information. Our technique utilizes a Bayesian hierarchical model for inferring properties of a WD population while also taking into account all observational errors of individual objects, as well as selection and incompleteness effects. We demonstrate that photometry alone can constrain the WD population’s distributions of temperature, surface gravity, and atmospheric composition, and that astrometric information significantly improves determination of the WD surface gravity distribution. We also discuss the possibility of identifying unresolved binary WDs using only photometric and astrometric informations.

Journal article

Capel F, Mortlock DJ, 2019, Impact of using the ultrahigh-energy cosmic ray arrival energies to constrain source associations, Monthly Notices of the Royal Astronomical Society, Vol: 484, Pages: 2324-2340, ISSN: 0035-8711

We present a Bayesian hierarchical model which enables a joint fit of the ultrahigh-energy cosmic ray (UHECR) energy spectrum and arrival directions within the context of a physical model for the UHECR phenomenology. In this way, possible associations with astrophysical source populations can be assessed in a physically and statistically principled manner. The importance of including the UHECR energy data and detection effects is demonstrated through simulation studies, showing that the effective GZK horizon is significantly extended for typical reconstruction uncertainties. We also verify the ability of the model to fit and recover physical parameters from crpropa 3 simulations. Finally, the model is used to assess the fraction of the publicly available data set of 231 UHECRs detected by the Pierre Auger Observatory which are associated with the Fermi-LAT 2FHL catalogue, a set of starburst galaxies, and Swift-BAT hard X-ray sources. We find association fractions of 9.5+2.4−5.9⁠, 22.7+6.6−12.4⁠, and 22.8+6.6−8.0 per cent for the 2FHL, starburst galaxies, and Swift-BAT catalogues respectively.

Journal article

Ball WT, Rozanov EV, Alsing J, Marsh DR, Tummon F, Mortlock DJ, Kinnison D, Haigh JDet al., 2019, The upper stratospheric solar cycle ozone response, Geophysical Research Letters, Vol: 46, Pages: 1831-1841, ISSN: 0094-8276

The solar cycle (SC) stratospheric ozone response is thought to influence surface weather and climate. To understand the chain of processes and ensure climate models adequately represent them, it is important to detect and quantify an accurate SC ozone response from observations. Chemistry climate models (CCMs) and observations display a range of upper stratosphere (1–10 hPa) zonally averaged spatial responses; this and the recommended data set for comparison remains disputed. Recent data-merging advancements have led to more robust observational data. Using these data, we show that the observed SC signal exhibits an upper stratosphere U-shaped spatial structure with lobes emanating from the tropics (5–10 hPa) to high altitudes at midlatitudes (1–3 hPa). We confirm this using two independent chemistry climate models in specified dynamics mode and an idealized timeslice experiment. We recommend the BASIC v2 ozone composite to best represent historical upper stratospheric solar variability, and that those based on SBUV alone should not be used.

Journal article

Feeney SM, Peiris HV, Williamson AR, Nissanke SM, Mortlock DJ, Alsing J, Scolnic Det al., 2019, Prospects for resolving the Hubble constant tension with standard sirens, Physical Review Letters, Vol: 122, ISSN: 0031-9007

The Hubble constant ($H_0$) estimated from the local Cepheid-supernova (SN)distance ladder is in 3-$\sigma$ tension with the value extrapolated fromcosmic microwave background (CMB) data assuming the standard cosmologicalmodel. Whether this tension represents new physics or systematic effects is thesubject of intense debate. Here, we investigate how new, independent $H_0$estimates can arbitrate this tension, assessing whether the measurements areconsistent with being derived from the same model using the posteriorpredictive distribution (PPD). We show that, with existing data, the inversedistance ladder formed from BOSS baryon acoustic oscillation measurements andthe Pantheon SN sample yields an $H_0$ posterior near-identical to the PlanckCMB measurement. The observed local distance ladder value is a very unlikelydraw from the resulting PPD. Turning to the future, we find that a sample of$\sim50$ binary neutron star "standard sirens" (detectable within the nextdecade) will be able to adjudicate between the local and CMB estimates.

Journal article

Argyle JJ, Mendez-Abreu J, Wild V, Mortlock DJet al., 2018, Bayesian bulge-disc decomposition of galaxy images, Monthly Notices of the Royal Astronomical Society, Vol: 479, Pages: 3076-3093, ISSN: 0035-8711

We introduce phi, a fully Bayesian Markov chain Monte Carlo algorithm designed for the structural decomposition of galaxy images. phi uses a triple layer approach to effectively and efficiently explore the complex parameter space. Combining this with the use of priors to prevent non-physical models, phi offers a number of significant advantages for estimating surface brightness profile parameters over traditional optimization algorithms. We apply phi to a sample of synthetic galaxies with Sloan Digital Sky Survey (SDSS)-like image properties to investigate the effect of galaxy properties on our ability to recover unbiased and well-constrained structural parameters. In two-component bulge+disc galaxies, we find that the bulge structural parameters are recovered less well than those of the disc, particularly when the bulge contributes a lower fraction to the luminosity, or is barely resolved with respect to the pixel scale or point spread function (PSF). There are few systematic biases, apart from for bulge+disc galaxies with large bulge Sérsic parameter, n. On application to SDSS images, we find good agreement with other codes, when run on the same images with the same masks, weights, and PSF. Again, we find that bulge parameters are the most difficult to constrain robustly. Finally, we explore the use of a Bayesian information criterion method for deciding whether a galaxy has one or two components.

Journal article

Burgess JM, Yu HF, Greiner J, Mortlock DJet al., 2018, Awakening the BALROG: bayesian location reconstruction of GRBs, Monthly Notices of the Royal Astronomical Society, Vol: 476, Pages: 1427-1444, ISSN: 0035-8711

The accurate spatial location of gamma-ray bursts (GRBs) is crucial for both accurately characterizing their spectra and follow-up observations by other instruments. The Fermi Gamma-ray Burst Monitor (GBM) has the largest field of view for detecting GRBs as it views the entire unocculted sky, but as a non-imaging instrument it relies on the relative count rates observed in each of its 14 detectors to localize transients. Improving its ability to accurately locate GRBs and other transients is vital to the paradigm of multimessenger astronomy, including the electromagnetic follow-up of gravitational wave signals. Here we present the BAyesian Location Reconstruction Of GRBs (balrog) method for localizing and characterizing GBM transients. Our approach eliminates the systematics of previous approaches by simultaneously fitting for the location and spectrum of a source. It also correctly incorporates the uncertainties in the location of a transient into the spectral parameters and produces reliable positional uncertainties for both well-localized sources and those for which the GBM data cannot effectively constrain the position. While computationally expensive, balrog can be implemented to enable quick follow-up of all GBM transient signals. Also, we identify possible response problems that require attention and caution when using standard, public GBM detector response matrices. Finally, we examine the effects of including the uncertainty in location on the spectral parameters of GRB 080916C. We find that spectral parameters change and no extra components are required when these effects are included in contrast to when we use a fixed location. This finding has the potential to alter both the GRB spectral catalogues and the reported spectral composition of some well-known GRBs.

Journal article

Feeney SM, Mortlock DJ, Dalmasso N, 2018, Clarifying the Hubble constant tension with a Bayesian hierarchical model of the local distance ladder, Monthly Notices of the Royal Astronomical Society, Vol: 476, Pages: 3861-3882, ISSN: 0035-8711

Estimates of the Hubble constant, H0, from the local distance ladder and from the cosmic microwave background (CMB) are discrepant at the ∼3-σ level, indicating a potential issue with the standard ΛCDM cosmology. A probabilistic (i.e., Bayesian) interpretation of this tension requires a model comparison calculation, which in turn depends strongly on the tails of the H₀ likelihoods. Evaluating the tails of the local H₀ likelihood requires the use of non-Gaussian distributions to faithfully represent anchor likelihoods and outliers, and simultaneous fitting of the complete distance ladder dataset to ensure correct uncertainty propagation. We have hence developed a Bayesian hierarchical model of the full distance ladder that does not rely on Gaussian distributions and allows outliers to be modelled without arbitrary data cuts. Marginalizing over the full ∼3000-parameter joint posterior distribution we find H0 = (72.72 ± 1.67) km s¯¹ Mpc¯¹ when applied to the outlier-cleaned Riess et al. (2016) data, and (73.15 ± 1.78) km s¯¹ Mpc¯¹ with SN outliers reintroduced (the pre-cut Cepheid dataset is not available). Using our precise evaluation of the tails of the H0 likelihood, we apply Bayesian model comparison to assess the evidence for deviation from ΛCDM given the distance-ladder and CMB data. The odds against ΛCDM are at worst ∼10:1 when considering the Planck Collaboration (2016b) data, regardless of outlier treatment, considerably less dramatic than naïvely implied by the 2.8-σ discrepancy. These odds become ∼60:1 when an approximation to the more-discrepant Planck Collaboration (2016c) likelihood is included.

Journal article

Ball WT, Alsing J, Mortlock DJ, Staehelin J, Haigh JD, Peter T, Tummon F, Stubi R, Stenke A, Anderson J, Bourassa A, Davis SM, Degenstein D, Frith S, Froidevaux L, Roth C, Sofieva V, Wang R, Wild J, Yu P, Ziemke JR, Rozanov EVet al., 2018, Evidence for a continuous decline in lower stratospheric ozone offsetting ozone layer recovery, Atmospheric Chemistry and Physics, Vol: 18, Pages: 1379-1394, ISSN: 1680-7316

Ozone forms in the Earth's atmosphere from the photodissociation of molecular oxygen, primarily in the tropical stratosphere. It is then transported to the extratropics by the Brewer–Dobson circulation (BDC), forming a protective "ozone layer" around the globe. Human emissions of halogen-containing ozone-depleting substances (hODSs) led to a decline in stratospheric ozone until they were banned by the Montreal Protocol, and since 1998 ozone in the upper stratosphere is rising again, likely the recovery from halogen-induced losses. Total column measurements of ozone between the Earth's surface and the top of the atmosphere indicate that the ozone layer has stopped declining across the globe, but no clear increase has been observed at latitudes between 60° S and 60° N outside the polar regions (60–90°). Here we report evidence from multiple satellite measurements that ozone in the lower stratosphere between 60° S and 60° N has indeed continued to decline since 1998. We find that, even though upper stratospheric ozone is recovering, the continuing downward trend in the lower stratosphere prevails, resulting in a downward trend in stratospheric column ozone between 60° S and 60° N. We find that total column ozone between 60° S and 60° N appears not to have decreased only because of increases in tropospheric column ozone that compensate for the stratospheric decreases. The reasons for the continued reduction of lower stratospheric ozone are not clear; models do not reproduce these trends, and thus the causes now urgently need to be established.

Journal article

Ball WT, Alsing J, Mortlock DJ, Rozanov EV, Tummon F, Haigh JDet al., 2017, Reconciling differences in stratospheric ozone composites, Atmospheric Chemistry and Physics, Vol: 17, Pages: 12269-12302, ISSN: 1680-7316

Observations of stratospheric ozone from multipleinstruments now span three decades; combining these intocomposite datasets allows long-term ozone trends to be estimated.Recently, several ozone composites have been published,but trends disagree by latitude and altitude, even betweencomposites built upon the same instrument data. Weconfirm that the main causes of differences in decadal trendestimates lie in (i) steps in the composite time series when theinstrument source data changes and (ii) artificial sub-decadaltrends in the underlying instrument data. These artefacts introducefeatures that can alias with regressors in multiple linearregression (MLR) analysis; both can lead to inaccuratetrend estimates. Here, we aim to remove these artefacts usingBayesian methods to infer the underlying ozone time seriesfrom a set of composites by building a joint-likelihoodfunction using a Gaussian-mixture density to model outliersintroduced by data artefacts, together with a data-driven prioron ozone variability that incorporates knowledge of problemsduring instrument operation. We apply this Bayesianself-calibration approach to stratospheric ozone in 10◦ bandsfrom 60◦ S to 60◦ N and from 46 to 1 hPa (∼ 21–48 km) for1985–2012. There are two main outcomes: (i) we independentlyidentify and confirm many of the data problems previouslyidentified, but which remain unaccounted for in existingcomposites; (ii) we construct an ozone composite, withuncertainties, that is free from most of these problems – wecall this the BAyeSian Integrated and Consolidated (BASIC)composite. To analyse the new BASIC composite, we usedynamical linear modelling (DLM), which provides a morerobust estimate of long-term changes through Bayesian inferencethan MLR. BASIC and DLM, together, provide astep forward in improving estimates of decadal trends. Ourresults indicate a significant recovery of ozone since 1998 inthe upper stratosphere, of both northern and southern midlatitudes,in all f

Journal article

Bosman SEI, Becker GD, Haehnelt MG, Hewett PC, McMahon RG, Mortlock DJ, Simpson C, Venemans BPet al., 2017, A deep search for metals near redshift 7: the line of sight towards ULAS J1120+0641, Monthly Notices of the Royal Astronomical Society, Vol: 470, Pages: 1919-1934, ISSN: 0035-8711

We present a search for metal absorption line systems at the highest redshifts to date using a deep (30 h) Very Large Telescope/X-Shooter spectrum of the z = 7.084 quasi-stellar object (QSO) ULAS J1120+0641. We detect seven intervening systems at z > 5.5, with the highest redshift system being a C iv absorber at z = 6.51. We find tentative evidence that the mass density of C iv remains flat or declines with redshift at z < 6, while the number density of C ii systems remains relatively flat over 5 < z < 7. These trends are broadly consistent with models of chemical enrichment by star formation-driven winds that include a softening of the ultraviolet background towards higher redshifts. We find a larger number of weak (Wrest < 0.3 Å) Mg ii systems over 5.9 < z < 7.0 than predicted by a power-law fit to the number density of stronger systems. This is consistent with trends in the number density of weak Mg ii systems at z ≲ 2.5, and suggests that the mechanisms that create these absorbers are already in place at z ∼ 7. Finally, we investigate the associated narrow Si iv, C iv and N v absorbers located near the QSO redshift, and find that at least one component shows evidence of partial covering of the continuum source.

Journal article

Barnett R, Warren SJ, Becker GD, Mortlock DJ, Hewett PC, McMahon RG, Simpson C, Venemans BPet al., 2017, Observations of the Lyman series forest towards the redshift 7.1 quasar ULAS J1120+0641, Astronomy and Astrophysics: a European journal, Vol: 601, ISSN: 0004-6361

We present a 30 h integration Very Large Telescope X-shooter spectrum of the Lyman series forest towards the z = 7.084 quasar ULAS J1120+0641. The only detected transmission at S/N > 5 is confined to seven narrow spikes in the Lyα forest, over the redshift range 5.858 <z < 6.122, just longward of the wavelength of the onset of the Lyβ forest. There is also a possible detection of one further unresolved spike in the Lyβ forest at z = 6.854, with S/N = 4.5. We also present revised Hubble Space Telescope F814W photometry of the source. The summed flux from the transmission spikes is in agreement with the F814W photometry, so all the transmission in the Lyman series forest may have been detected. There is a Gunn-Peterson (GP) trough in the Lyα forest from z = 6.122 all the way to the quasar near zone at z = 7.04. The trough, of comoving length 240 h-1 Mpc, is over twice as long as the next longest known GP trough. We combine the spectroscopic and photometric results to constrain the evolution of the Lyα effective optical depth (τGPeff) with redshift, extending a similar analysis by Simpson et al. We find τGPeff ∝ (1 + z)ξ where ξ = 11.2+ 0.4-0.6, for z > 5.5. The data nevertheless provide only a weak limit on the volume-weighted intergalactic medium (IGM) hydrogen neutral fraction at z ~ 6.5, xH i > 10-4, similar to limits at redshift z ~ 6 from less distant quasars. The new observations cannot extend measurements of the neutral fraction of the IGM to higher values because absorption in the Lyα forest is already saturated near z ~ 6. For higher neutral fractions, other methods such as measuring the red damping wing of the IGM will be required.

Journal article

Venemans BP, Walter F, Decarli R, Bañados E, Hodge J, Hewett P, McMahon RG, Mortlock DJ, Simpson Cet al., 2017, The compact, ∼1 kpc host galaxy of a quasar at a redshift of 7.1, The Astrophysical Journal: an international review of astronomy and astronomical physics, Vol: 837, Pages: 146-146, ISSN: 0004-637X

We present Atacama Large Millimeter/submillimeter Array (ALMA) observations of the [C ii] fine-structure line and the underlying far-infrared (FIR) dust continuum emission in J1120+0641, the most distant quasar currently known ($z=7.1$). We also present observations targeting the CO(2–1), CO(7–6), and [C i] 369 μm lines in the same source obtained at the Very Large Array and Plateau de Bure Interferometer. We find a [C ii] line flux of ${F}_{[{\rm{C}}{\rm{II}}]}=1.11\pm 0.10$ Jy $\mathrm{km}\,{{\rm{s}}}^{-1}$ and a continuum flux density of ${S}_{227\mathrm{GHz}}=0.53\pm 0.04$ mJy beam−1, consistent with previous unresolved measurements. No other source is detected in continuum or [C ii] emission in the field covered by ALMA (~ 25''). At the resolution of our ALMA observations (0farcs23, or 1.2 kpc, a factor of ~70 smaller beam area compared to previous measurements), we find that the majority of the emission is very compact: a high fraction (~80%) of the total line and continuum flux is associated with a region 1–1.5 kpc in diameter. The remaining ~20% of the emission is distributed over a larger area with radius lesssim4 kpc. The [C ii] emission does not exhibit ordered motion on kiloparsec scales: applying the virial theorem yields an upper limit on the dynamical mass of the host galaxy of $(4.3\pm 0.9)\times {10}^{10}$ ${M}_{\odot }$, only ~20 × higher than the central black hole (BH). The other targeted lines (CO(2–1), CO(7–6), and [C i]) are not detected, but the limits of the line ratios with respect to the [C ii] emission imply that the heating in the quasar host is dominated by star formation, and not by the accreting BH. The star formation rate (SFR) implied by the FIR continuum is 105–340 ${M}_{\odot }\,{\mathrm{yr}}^{-1}$, with a resulting SFR surface density of ~100–350 ${M}_{\odot }\,{\mathrm{yr}}^{-1}$ kpc−2, well below the value for Eddington-accretion-limited star formation.

Journal article

Ade PAR, Aghanim N, Arnaud M, Aumont J, Baccigalupi C, Banday AJ, Barreiro RB, Bartolo N, Battaner E, Benabed K, Benoit-Levy A, Bernard J-P, Bersanelli M, Bielewicz P, Bonaldi A, Bonavera L, Bond JR, Borrill J, Bouchet FR, Boulanger F, Burigana C, Butler RC, Calabrese E, Catalano A, Chiang HC, Christensen PR, Clements DL, Colombo LPL, Couchot F, Coulais A, Crill BP, Curto A, Cuttaia F, Danese L, Davies RD, Davis RJ, de Bernardis P, de Rosa A, de Zotti G, Delabrouille J, Dickinson C, Diego JM, Dole H, Dore O, Douspis M, Ducout A, Dupac X, Elsner F, Ensslin TA, Eriksen HK, Falgarone E, Finelli F, Flores-Cacho I, Frailis M, Fraisse AA, Franceschi E, Galeotta S, Galli S, Ganga K, Giard M, Giraud-Heraud Y, Gjerlow E, Gonzalez-Nuevo J, Gorski KM, Gregorio A, Gruppuso A, Gudmundsson JE, Hansen FK, Harrison DL, Helou G, Hernandez-Monteagudo C, Herranz D, Hildebrandt SR, Hivon E, Hobson M, Hornstrup A, Hovest W, Huffenberger KM, Hurier G, Jaffe AH, Jaffe TR, Keihanen E, Keskitalo R, Kisner TS, Kneissl R, Knoche J, Kunz M, Kurki-Suonio H, Lagache G, Lamarre J-M, Lasenby A, Lattanzi M, Lawrence CR, Leonardi R, Levrier F, Liguori M, Lilje PB, Linden-Vornle M, Lopez-Caniego M, Lubin PM, Macias-Perez JF, Maffei B, Maggio G, Maino D, Mandolesi N, Mangilli A, Maris M, Martin PG, Martinez-Gonzalez E, Masi S, Matarrese S, Melchiorri A, Mennella A, Migliaccio M, Mitra S, Miville-Deschenes M-A, Moneti A, Montier L, Morgante G, Mortlock D, Munshi D, Murphy JA, Nati F, Natoli P, Nesvadba NPH, Noviello F, Novikov D, Novikov I, Oxborrow CA, Pagano L, Pajot F, Paoletti D, Partridge B, Pasian F, Pearson TJ, Perdereau O, Perotto L, Pettorino V, Piacentini F, Piat M, Plaszczynski S, Pointecouteau E, Polenta G, Pratt GW, Prunet S, Puget J-L, Rachen JP, Reinecke M, Remazeilles M, Renault C, Renzi A, Ristorcelli I, Rocha G, Rosset C, Rossetti M, Roudier G, Rubino-Martin JA, Rusholme B, Sandri M, Santos D, Savelainen M, Savini G, Scott D, Spencer LD, Stolyarov V, Stompor R, Sudiwala R, Sunyaev Ret al., 2016, Planck intermediate results XXXIX. The Planck list of high-redshift source candidates, Publisher: EDP SCIENCES S A

Working paper

Leistedt B, Mortlock DJ, Peiris HV, 2016, Hierarchical Bayesian inference of galaxy redshift distributions from photometric surveys, Monthly Notices of the Royal Astronomical Society, Vol: 460, Pages: 4258-4267, ISSN: 1365-2966

Accurately characterizing the redshift distributions of galaxies is essential for analysing deep photometric surveys and testing cosmological models. We present a technique to simultaneously infer redshift distributions and individual redshifts from photometric galaxy catalogues. Our model constructs a piecewise constant representation (effectively a histogram) of the distribution of galaxy types and redshifts, the parameters of which are efficiently inferred from noisy photometric flux measurements. This approach can be seen as a generalization of template-fitting photometric redshift methods and relies on a library of spectral templates to relate the photometric fluxes of individual galaxies to their redshifts. We illustrate this technique on simulated galaxy survey data, and demonstrate that it delivers correct posterior distributions on the underlying type and redshift distributions, as well as on the individual types and redshifts of galaxies. We show that even with uninformative priors, large photometric errors and parameter degeneracies, the redshift and type distributions can be recovered robustly thanks to the hierarchical nature of the model, which is not possible with common photometric redshift estimation techniques. As a result, redshift uncertainties can be fully propagated in cosmological analyses for the first time, fulfilling an essential requirement for the current and future generations of surveys.

Journal article

Khanin A, Mortlock DJ, 2016, A Bayesian analysis of the 69 highest energy cosmic rays detected by the Pierre Auger Observatory, Monthly Notices of the Royal Astronomical Society, Vol: 460, Pages: 2765-2778, ISSN: 0035-8711

The origins of ultrahigh energy cosmic rays (UHECRs) remain an open question. Several attempts have been made to cross-correlate the arrival directions of the UHECRs with catalogues of potential sources, but no definite conclusion has been reached. We report a Bayesiananalysis of the 69 events, from the Pierre Auger Observatory (PAO), that aims to determine the fraction of the UHECRs that originate from known AGNs in the Veron-Cety & Verson (VCV) catalogue, as well as AGNs detected with the Swift Burst Alert Telescope (Swift-BAT),galaxies from the 2MASS Redshift Survey (2MRS), and an additional volume-limited sample of 17 nearby AGNs. The study makes use of a multilevel Bayesian model of UHECR injection, propagation and detection. We find that for reasonable ranges of prior parameters theBayes factors disfavour a purely isotropic model. For fiducial values of the model parameters, we report 68 per cent credible intervals for the fraction of source originating UHECRs of 0.09+0.05 −0.04, 0.25+0.09 −0.08, 0.24+0.12 −0.10, and 0.08+0.04 −0.03 for the VCV, Swift-BAT and 2MRS catalogues, and the sample of 17 AGNs, respectively.

Journal article

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