Publications


Linking long- and short-term emission variability in pulsars

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 488 (2019) 5702-5712

PR Brook, A Karastergiou, S Johnston


Population estimates for electromagnetically distinguishable supermassive binary black holes

Astrophysical Journal American Astronomical Society 879 (2019) 110

JH Krolik, M Volonteri, Y Dubois, J Devriendt

Distinguishing the photon output of an accreting supermassive black hole binary system from that of a single supermassive black hole accreting at the same rate is intrinsically difficult because the majority of the light emerges from near the innermost stable orbits of the black holes. However, there are two possible signals that can distinctively mark binaries, both arising from the gap formed in circumbinary accretion flows inside approximately twice the binary separation. One of these is a "notch" cut into the thermal spectra of these systems in the IR/optical/UV, the other a periodically varying excess hard X-ray luminosity whose period is of order the binary orbital period. Using data from detailed galaxy evolution simulations, we estimate the distribution function in mass, mass ratio, and accretion rate for accreting supermassive binary black holes (SMBBHs) as a function of redshift and then transform this distribution function into predicted source counts for these two potential signals. At flux levels >~10−13 erg cm−2 s−1, there may be ~O(102) such systems in the sky, mostly in the redshift range 0.5 <~ z <~ 1. Roughly 10% should have periods short enough (<~5 yr) to detect the X-ray modulation; this is also the period range accessible to Pulsar Timing Array observations.


The energetics of starburst-driven outflows at z ∼ 1 from KMOS

Monthly Notices of the Royal Astronomical Society Oxford University Press 487 (2019) 381–393-

AM Swinbank, CM Harrison, AL Tiley, HL Johnson, I Smail, JP Stott, PN Best, M Bureau, RG Bower, A Bunker, M Cirasuolo, GE Magdis, M Jarvis, RM Sharples, D Sobral

We present an analysis of the gas outflow energetics from KMOS observations of ∼ 529 main-sequence star-forming galaxies at z ∼ 1 using broad, underlying H α and forbidden lines of [N II] and [S II]. Based on the stacked spectra for a sample with median star-formation rates and stellar masses of SFR = 7 M⊙   yr−1 and M⋆ = (1.0 ± 0.1) × 1010 M⊙, respectively, we derive a typical mass outflow rate of M˙wind = 1–4 M⊙ yr−1 and a mass loading of M˙wind / SFR = 0.2–0.4. By comparing the kinetic energy in the wind with the energy released by supernovae, we estimate a coupling efficiency between the star formation and wind energetics of ϵ ∼  0.03. The mass loading of the wind does not show a strong trend with star-formation rate over the range ∼ 2–20 M⊙ yr−1, although we identify a trend with stellar mass such that dM / dt / SFR ∝ M0.26±0.07⋆⁠. Finally, the line width of the broad H α increases with disc circular velocity with a sub-linear scaling relation FWHMbroad ∝ v0.21 ± 0.05. As a result of this behaviour, in the lowest mass galaxies (M⋆ ≲ 1010 M⊙), a significant fraction of the outflowing gas should have sufficient velocity to escape the gravitational potential of the halo whilst in the highest mass galaxies (M⋆ ≳ 1010 M⊙) most of the gas will be retained, flowing back on to the galaxy disc at later times.


The period-width relationship for radio pulsars revisited

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 485 (2019) 640-647

S Johnston, A Karastergiou


Black hole – Galaxy correlations in SIMBA

Monthly Notices of the Royal Astronomical Society Oxford University Press 487 (2019) 5764-5780

N Thomas, D Angles-Alcazar, R Dave, M Jarvis

We examine the co-evolution of galaxies and supermassive black holes in the simba cosmological hydrodynamic simulation. simba grows black holes via gravitational torque-limited accretion from cold gas and Bondi accretion from hot gas, while feedback from black holes is modelled in radiative and jet modes depending on the Eddington ratio (fEdd). simba shows generally good agreement with local studies of black hole properties, such as the black hole mass-stellar velocity dispersion (MBH-σ) relation, the black hole accretion rate versus star formation rate (BHAR-SFR), and the black hole mass function. MBH-σ evolves such that galaxies at a given MBH have higher σ at higher redshift, consistent with no evolution in MBH-M∗. For MBH ≤ 108 M⊙, fEdd is anticorrelated with MBH since the BHAR is approximately independent of MBH, while at higher masses fEdd-MBH flattens and has a larger scatter. BHAR versus SFR is invariant with redshift, but fEdd drops steadily with time at a given MBH, such that all but the most massive black holes are accreting in a radiatively efficient mode at z ≥ 2. The black hole mass function amplitude decreases with redshift and is locally dominated by quiescent galaxies for MBH > 108 M⊙, but for z≥ 1 star-forming galaxies dominate at all MBH. The z = 0 fEdd distribution is roughly lognormal with a peak at fEdd ≤ 0.01 as observed, shifting to higher fEdd at higher redshifts. Finally, we study the dependence of black hole properties with H i content and find that the correlation between gas content and SFR is modulated by black hole properties, such that higher SFR galaxies at a given gas content have smaller black holes with higher fEdd.


Understanding the radio beam of PSR J1136+1551 through its single pulses

Monthly Notices of the Royal Astronomical Society Oxford University Press 489 (2019) 310-324

L Oswald, S Johnston, A Karastergiou

The frequency widening of pulsar profiles is commonly attributed to lower frequencies being produced at greater heights above the surface of the pulsar; so-called radius-to-frequency mapping (RFM). The observer’s view of pulsar emission is a 1D cut through a 3D magnetosphere: we can only see that emission which points along our line of sight. However, by comparing the frequency evolution of many single pulses positioned at different phases, we can build up an understanding of the shape of the active emission region. We use single pulses observed with the Giant Metrewave Radio Telescope to investigate the emission region of PSR J1136+1551 and test RFM. Assuming that emission is produced tangential to the magnetic field lines and that each emission frequency corresponds to a single height, we simulate the single pulse profile evolution resulting from the canonical conal beam model and a fan beam model. Comparing the results of these simulations with the observations, we conclude that the emission region of PSR J1136+1551 is better described by the fan beam model. The diversity of profile widening behaviour observed for the single pulses can be explained by orthogonally polarized modes propagating along differing frequency-dependent paths in the magnetosphere.


The SAMI Galaxy Survey: First detection of a transition in spin orientation with respect to cosmic filaments in the stellar kinematics of galaxies

Monthly Notices of the Royal Astronomical Society Oxford University Press (2019)

C Welker, P Elahi, J Bryant, C Pichon, L Cortese, M Goodwin, JS Lawrence, Y Dubois, J Devriendt, J Bland-Hawthorn, J Van de Sande, C Lagos, D Obreschkow, SM Croom, S Sweet, A Lopez-Sanchez, A Medling

We present the first detection of mass dependent galactic spin alignments with local cosmic filaments with >2σ confidence using IFS kinematics. The 3D network of cosmic filaments is reconstructed on Mpc scales across GAMA fields using the cosmic web extractor DisPerSe. We assign field galaxies from the SAMI survey to their nearest filament segment in 3D and estimate the degree of alignment between SAMI galaxies’ kinematic spin axis and their nearest filament in projection. Low-mass galaxies align their spin with their nearest filament while higher mass counterparts are more likely to display an orthogonal orientation. The stellar transition mass from the first trend to the second is bracketed between 1010.4 M⊙ and 1010.9 M⊙, with hints of an increase with filament scale. Consistent signals are found in the Horizon-AGN cosmological hydrodynamic simulation. This supports a scenario of early angular momentum build-up in vorticity rich quadrants around filaments at low stellar mass followed by progressive flip of spins orthogonal to the cosmic filaments through mergers at high stellar mass. Conversely, we show that dark-matter only simulations post-processed with a semi-analytic model treatment of galaxy formation struggles to reproduce this alignment signal. This suggests that gas physics is key in enhancing the galaxy-filament alignment.


A fast radio burst with frequency-dependent polarization detected during Breakthrough Listen observations

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 486 (2019) 3636-3646

DC Price, G Foster, M Geyer, W van Straten, V Gajjar, G Hellbourg, A Karastergiou, EF Keane, APV Siemion, I Arcavi, R Bhat, M Caleb, S-W Chang, S Croft, D DeBoer, I de Pater, J Drew, JE Enriquez, W Farah, N Gizani, JA Green, H Isaacson, J Hickish, A Jameson, M Lebofsky, DHE MacMahon, A Moller, CA Onken, E Petroff, D Werthimer, C Wolf, SP Worden, YG Zhang


HORIZON-AGN virtual observatory – 2. Template-free estimates of galaxy properties from colours

Monthly Notices of the Royal Astronomical Society Oxford University Press 489 (2019) 4817–4835-

I Davidzon, C Laigle, PL Capak, O Ilbert, DC Masters, S Hemmati, N Apostolakos, J Coupon, SDL Torre, J Devriendt, Y Dubois, D Kashino, S Paltani, C Pichon

<p>Using the HORIZON-AGN hydrodynamical simulation and self-organizing maps (SOMs), we show how to compress the complex, high-dimensional data structure of a simulation into a 2D grid, which greatly facilitates the analysis of how galaxy observables are connected to intrinsic properties. We first verify the tight correlation between the observed 0.3–5 <em>μ</em>m broad-band colours of HORIZON-AGN galaxies and their high-resolution spectra. The correlation is found to extend to physical properties such as redshift, stellar mass, and star formation rate (SFR). This direct mapping from colour to physical parameter space still works after including photometric uncertainties that mimic the COSMOS survey. We then label the SOM grid with a simulated calibration sample to estimate redshift and SFR for COSMOS-like galaxies up to <em>z</em> ∼ 3. In comparison to state-of-the-art techniques based on synthetic templates, our method is comparable in performance but less biased at estimating redshifts, and significantly better at predicting SFRs. In particular, our ‘data-driven’ approach, in contrast to model libraries, intrinsically allows for the complexity of galaxy formation and can handle sample biases. We advocate that observations to calibrate this method should be one of the goals of next-generation galaxy surveys.</p>


Using sparse Gaussian processes for predicting robust inertial confinement fusion implosion yields

IEEE Transactions on Plasma Science IEEE (2019) 1-6

P Hatfield, I Almosallam, R Scott, S Rose, S Roberts, M Jarvis


Massive spheroids can form in single minor mergers

Monthly Notices of the Royal Astronomical Society Oxford University Press 489 (2019) 4679–4689-

RA Jackson, G Martin, S Kaviraj, C Laigle, J Devriendt, Y Dubois, C Pichon

<p>Understanding how rotationally supported discs transform into dispersion-dominated spheroids is central to our comprehension of galaxy evolution. Morphological transformation is largely merger-driven. While major mergers can efficiently create spheroids, recent work has highlighted the significant role of other processes, like minor mergers, in driving morphological change. Given their rich merger histories, spheroids typically exhibit large fractions of ‘<em>ex situ</em>’ stellar mass, i.e. mass that is accreted, via mergers, from external objects. This is particularly true for the most massive galaxies, whose stellar masses typically cannot be attained without a large number of mergers. Here, we explore an unusual population of extremely massive (<em>M</em><sub>*</sub> &gt; 10<sup>11</sup>M<sub>⊙</sub>) spheroids, in the Horizon-AGN simulation, which exhibit anomalously low <em>ex situ</em> mass fractions, indicating that they form without recourse to significant merging. These systems form in a single minor-merger event (with typical merger mass ratios of 0.11–0.33), with a specific orbital configuration, where the satellite orbit is virtually co-planar with the disc of the massive galaxy. The merger triggers a catastrophic change in morphology, over only a few hundred Myr, coupled with strong <em>in situ</em> star formation. While this channel produces a minority (∼5 per cent) of such galaxies, our study demonstrates that the formation of at least some of the <em>most massive</em> spheroids need not involve major mergers – or any significant merging at all – contrary to what is classically believed.</p>


A new sample of southern radio galaxies: host-galaxy masses and star-formation rates

Monthly Notices of the Royal Astronomical Society Oxford University Press 489 (2019) 3403-3411

T Marubini, M Jarvis, S Fine, K McAlpine, T Mauch, M Prescott


Radio source extraction with ProFound

Monthly Notices of the Royal Astronomical Society Oxford University Press 487 (2019) 3971-3989

M Jarvis, SP Driver, CL Hale, ASG Robotham, LJM Davies, I Heywood

In the current era of radio astronomy, continuum surveys observe a multitude of objects with complex morphologies and sizes, and are not limited to observing point sources. Typical radio source extraction software generates catalogues by using Gaussian components to form a model of the emission. This may not be well suited to complicated jet structures and extended emission, particularly in the era of interferometers with a high density of short baselines, which are sensitive to extended emission. In this paper, we investigate how the optically motivated source detection package ProFound (Robotham et al. 2018) may be used to model radio emission of both complicated and point-like radio sources. We use a combination of observations and simulations to investigate how ProFound compares to other source extractor packages used for radio surveys. We find that ProFound can accurately recover both the flux densities of simulated Gaussian sources as well as extended radio galaxies. ProFound can create models that trace the complicated nature of these extended galaxies, which we show is not necessarily the case with other source extraction software. Our work suggests that our knowledge of the emission from extended radio objects may be both over or under-estimated using traditional software. We suggest that ProFound offers a useful alternative to the fitting of Gaussian components for generating catalogues from current and future radio surveys. Furthermore, ProFound's multiwavelength capabilities will be useful in investigating radio sources in combination with multiwavelength data.


The LOFAR Tied-Array All-Sky Survey (LOTAAS): Survey overview and initial pulsar discoveries

ASTRONOMY & ASTROPHYSICS 626 (2019) ARTN A104

S Sanidas, S Cooper, CG Bassa, JWT Hessels, VI Kondratiev, D Michilli, BW Stappers, CM Tan, J van Leeuwen, L Cerrigone, RA Fallows, M Iacobelli, E Orru, RF Pizzo, A Shulevski, MC Toribio, S ter Veen, P Zucca, L Bondonneau, J-M Griessmeier, A Karastergiou, M Kramer, C Sobey


GREENBURST: a commensal fast radio burst search back-end for the Green Bank Telescope

Publications of the Astronomical Society of Australia Cambridge University Press 36 (2019) e032

MP Surnis, D Agarwal, X Pei, G Foster, A Karastergiou, G Golpayegani, RJ Maddalena, S White, W Armour, J Cobb, MA McLaughlin, DHE Macmahon, APV Siemion, D Werthimer, CJ Williams

We describe the design and deployment of GREENBURST, a commensal Fast Radio Burst (FRB) search system at the Green Bank Telescope. GREENBURST uses the dedicated L-band receiver tap to search over the 960$-$1920 MHz frequency range for pulses with dispersion measures out to $10^4$ pc cm$^{-3}$. Due to its unique design, GREENBURST will obtain data even when the L-band receiver is not being used for scheduled observing. This makes it a sensitive single pixel detector capable of reaching deeper in the radio sky. While single pulses from Galactic pulsars and rotating radio transients will be detectable in our observations, and will form part of the database we archive, the primary goal is to detect and study FRBs. Based on recent determinations of the all-sky rate, we predict that the system will detect approximately one FRB for every 2$-$3 months of continuous operation. The high sensitivity of GREENBURST means that it will also be able to probe the slope of the FRB source function, which is currently uncertain in this observing band.


Automatic selection of correlated double sampling timing parameters

Journal of Astronomical Telescopes, Instruments, and Systems 5 (2019)

DP Weatherill, I Shipsey, K Arndt, R Plackett, D Wood, K Metodiev, M Mironova, D Bortoletto, N Demetriou

© 2019 Society of Photo-Optical Instrumentation Engineers (SPIE). Correlated double sampling (CDS) is a process used in many charge-coupled device readout systems to cancel the reset noise component that would otherwise dominate. CDS processing typically consists of subtracting the integrated video signal during a "signal" period from that during a "reset" period. The response of this processing depends, therefore, on the shape of the video signal with respect to the integration bounds. In particular, the amount of noise appearing in the final image and the linearity of the pixel value with signal charge are affected by the choice of the CDS timing intervals. We use a digital CDS readout system which highly oversamples the video signal (as compared with the pixel rate) to reconstruct pixel values for different CDS timings using identical raw video signal data. We use this technique to develop insights into optimal strategy for selecting CDS timings both in the digital case (where the raw video signal may be available) and in the general case (where it is not). In particular, we show that the linearity of the CDS operation allows subtraction of the raw video signals of pixels in bias images from those in illuminated images to directly show the effects of CDS processing on the final (subtracted) pixel values.


Measuring the H I mass function below the detection threshold

Monthly Notices of the Royal Astronomical Society Oxford University Press 491 (2019) 1227–1242-

H Pan, X Kang, I Heywood, M Jarvis, N Maddox, BS Frank

We present a Bayesian stacking technique to directly measure the H i mass function (HIMF) and its evolution with redshift using galaxies formally below the nominal detection threshold. We generate galaxy samples over several sky areas given an assumed HIMF described by a Schechter function and simulate the H i emission lines with different levels of background noise to test the technique. We use Multinest to constrain the parameters of the HIMF in a broad redshift bin, demonstrating that the HIMF can be accurately reconstructed, using the simulated spectral cube far below the H i mass limit determined by the 5σ flux-density limit, i.e. down to MHI = 107.5 M⊙ over the redshift range 0 &lt; z &lt; 0.55 for this particular simulation, with a noise level similar to that expected for the MIGHTEE survey. We also find that the constraints on the parameters of the Schechter function, φ⋆, M⋆ and α can be reliably fit, becoming tighter as the background noise decreases as expected, although the constraints on the redshift evolution are not significantly affected. All the parameters become better constrained as the survey area increases. In summary, we provide an optimal method for estimating the H i mass at cosmological distances that allows us to constrain the H i mass function below the detection threshold in forthcoming H i surveys. This study is a first step towards the measurement of the HIMF at high (z &gt; 0.1) redshifts.


Extracting the global signal from 21-cm fluctuations: The multi-tracer approach

Monthly Notices of the Royal Astronomical Society Oxford University Press (2019)

A Fialkov, R Barkana, M Jarvis

The multi-tracer technique employs a ratio of densities of two differently biased galaxy samples that trace the same underlying matter density field, and was proposed to alleviate the cosmic variance problem. Here we propose a novel application of this approach, applying it to two different tracers one of which is the 21-cm signal of neutral hydrogen from the epochs of reionization and comic dawn. The second tracer is assumed to be a sample of high-redshift galaxies, but the approach can be generalized and applied to other high-redshift tracers. We show that the anisotropy of the ratio of the two density fields can be used to measure the sky-averaged 21-cm signal, probe the spectral energy distribution of radiative sources that drive this signal, and extract large-scale properties of the second tracer, e.g., the galaxy bias. Using simulated 21-cm maps and mock galaxy samples, we find that the method works well for an idealized galaxy survey. However, in the case of a more realistic galaxy survey which only probes highly biased luminous galaxies, the inevitable Poisson noise makes the reconstruction far more challenging. This difficulty can be mitigated with the greater sensitivity of future telescopes along with larger survey volumes.


A compact air cooling system for testing silicon detectors based on a vortex chiller

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT 940 (2019) 405-409

K Metodiev, L Vigani, R Plackett, K Arndt, D Wood, DP Weatherill, M Mironova, D Bortoletto, I Shipsey


Towards emulating cosmic shear data: revisiting the calibration of the shear measurements for the Kilo-Degree Survey

Astronomy and Astrophysics EDP Sciences 624 (2019) A92

H Hoekstra, L Miller, A Kannawadi, T Erben, AH Wright, M Viola, IF Conti, R Herbonnet, H Hildebrandt, C Heymans, K Kuijken, M Vakili

Exploiting the full statistical power of future cosmic shear surveys will necessitate improvements to the accuracy with which the gravitational lensing signal is measured. We present a framework for calibrating shear with image simulations that demonstrates the importance of including realistic correlations between galaxy morphology, size and more importantly, photometric redshifts. This realism is essential so that selection and shape measurement biases can be calibrated accurately for a tomographic cosmic shear analysis. We emulate Kilo-Degree Survey (KiDS) observations of the COSMOS field using morphological information from {\it Hubble} Space Telescope imaging, faithfully reproducing the measured galaxy properties from KiDS observations of the same field. We calibrate our shear measurements from lensfit, and find through a range of sensitivity tests that lensfit is robust and unbiased within the allowed 2 per cent tolerance of our study. Our results show that the calibration has to be performed by selecting the tomographic samples in the simulations, consistent with the actual cosmic shear analysis, because the joint distributions of galaxy properties are found to vary with redshift. Ignoring this redshift variation could result in misestimating the shear bias by an amount that exceeds the allowed tolerance. To improve the calibration for future cosmic shear analyses, it will be essential to also correctly account for the measurement of photometric redshifts, which requires simulating multi-band observations.

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