Dr Matthew Orkney

Postgraduate Research Student
Mphys in Astrophysics
Maximum of 10am - 6pm

Academic and research departments

Astrophysics Research Group.


My research project


James Alvey, Nashwan Sabti, Victoria Tiki, Diego Blas, Kyrylo Bondarenko, Alexey Boyarsky, Miguel Escudero, Malcolm Fairbairn, Matthew Orkney, Justin Read (2021)New constraints on the mass of fermionic dark matter from dwarf spheroidal galaxies, In: Monthly notices of the Royal Astronomical Society501(1)pp. 1188-1201 Oxford Univ Press

Dwarf spheroidal galaxies are excellent systems to probe the nature of fermionic dark matter due to their high observed dark matter phase-space density. In this work, we review, revise, and improve upon previous phase-space considerations to obtain lower bounds on the mass of fermionic dark matter particles. The refinement in the results compared to previous works is realized particularly due to a significantly improved Jeans analysis of the galaxies. We discuss two methods to obtain phase-space bounds on the dark matter mass, one model-independent bound based on Pauli's principle, and the other derived from an application of Liouville's theorem. As benchmark examples for the latter case, we derive constraints for thermally decoupled particles and (non-)resonantly produced sterile neutrinos. Using the Pauli principle, we report a model-independent lower bound of m >= 0.18 keV at 68 per cent CL and m >= 0.13 keV at 95 per cent CL. For relativistically decoupled thermal relics, this bound is strengthened to m >= 0.59 keV at 68 per cent CL and m >= 0.41 keV at 95 per cent CL, while for non-resonantly produced sterile neutrinos the constraint is m >= 2.80 keV at 68 per cent CL and m >= 1.74 keV at 95 per cent CL. Finally, the phase-space bounds on resonantly produced sterile neutrinos are compared with complementary limits from X-ray, Lyman alpha, and big bang nucleosynthesis observations.

Andrew Pontzen, Martin P. Rey, Corentin Cadiou, Oscar Agertz, Romain Teyssier, Justin Read, Matthew D. A. Orkney (2021)EDGE: a new approach to suppressing numerical diffusion in adaptive mesh simulations of galaxy formation, In: Monthly notices of the Royal Astronomical Society501(2)pp. 1755-1765 Oxford Univ Press

We introduce a new method to mitigate numerical diffusion in adaptive mesh refinement (AMR) simulations of cosmological galaxy formation, and study its impact on a simulated dwarf galaxy as part of the 'EDGE' project. The target galaxy has a maximum circular velocity of 21 km s(-1) but evolves in a region that is moving at up to 90 km s(-1) relative to the hydrodynamic grid. In the absence of any mitigation, diffusion softens the filaments feeding our galaxy. As a result, gas is unphysically held in the circumgalactic medium around the galaxy for 320 Myr, delaying the onset of star formation until cooling and collapse eventually triggers an initial starburst at z = 9. Using genetic modification, we produce 'velocity-zeroed' initial conditions in which the grid-relative streaming is strongly suppressed; by design, the change does not significantly modify the large-scale structure or dark matter accretion history. The resulting simulation recovers a more physical, gradual onset of star formation starting at z = 17. While the final stellar masses are nearly consistent (4.8 x 10(6) M-circle dot and 4.4 x 10(6) M-circle dot for unmodified and velocity-zeroed, respectively), the dynamical and morphological structure of the z = 0 dwarf galaxies are markedly different due to the contrasting histories. Our approach to diffusion suppression is suitable for any AMR zoom cosmological galaxy formation simulations, and is especially recommended for those of small galaxies at high redshift.

Matthew D. A. Orkney, Justin Read, Martin P. Rey, Imran Nasim, Andrew Pontzen, Oscar Agertz, Stacy Y. Kim, Maxime Delorme, Walter Dehnen (2021)EDGE: two routes to dark matter core formation in ultra-faint dwarfs, In: Monthly notices of the Royal Astronomical Society504(3)pp. 3509-3522 Oxford Univ Press

In the standard Lambda cold dark matter paradigm, pure dark matter simulations predict dwarf galaxies should inhabit dark matter haloes with a centrally diverging density 'cusp'. This is in conflict with observations that typically favour a constant density 'core'. We investigate this 'cusp-core problem' in 'ultra-faint' dwarf galaxies simulated as part of the 'Engineering Dwarfs at Galaxy formation's Edge' project. We find, similarly to previous work, that gravitational potential fluctuations within the central region of the simulated dwarfs kinematically heat the dark matter particles, lowering the dwarfs' central dark matter density. However, these fluctuations are not exclusively caused by gas inflow/outflow, but also by impulsive heating from minor mergers. We use the genetic modification approach on one of our dwarf's initial conditions to show how a delayed assembly history leads to more late minor mergers and, correspondingly, more dark matter heating. This provides a mechanism by which even ultra-faint dwarfs (M-star < 10(5) M-circle dot), in which star formation was fully quenched at high redshift, can have their central dark matter density lowered over time. In contrast, we find that late major mergers can regenerate a central dark matter cusp, if the merging galaxy had sufficiently little star formation. The combination of these effects leads us to predict significant stochasticity in the central dark matter density slopes of the smallest dwarfs, driven by their unique star formation and mass assembly histories.

Martin P. Rey, Andrew Pontzen, Oscar Agertz, Matthew D. A. Orkney, Justin Read, Amelie Saintonge, Stacy Y. Kim, Payel Das (2022)EDGE: What shapes the relationship between H i and stellar observables in faint dwarf galaxies?, In: Monthly notices of the Royal Astronomical Society511(4)pp. 5672-5681 Oxford Univ Press

We show how the interplay between feedback and mass-growth histories introduces scatter in the relationship between stellar and neutral gas properties of field faint dwarf galaxies (M-*less than or similar to 10(6) M-circle dot). Across a suite of cosmological, high-resolution zoomed simulations, we find that dwarf galaxies of stellar masses 10(5)

Matthew D A Orkney, Justin I Read, Oscar Agertz, Andrew Pontzen, Martin P Rey, Alex Goater, Ethan Taylor, Stacy Y Kim, Maxime Delorme (2022)EDGE: the puzzling ellipticity of Eridanus II’s star cluster and its implications for dark matter at the heart of an ultra-faint dwarf, In: Monthly notices of the Royal Astronomical Society515(1)pp. 185-200 Oxford University Press

ABSTRACT The Eridanus II (EriII) ‘ultra-faint’ dwarf has a large (15 pc) and low-mass (4.3 × 103 M⊙) star cluster (SC) offset from its centre by 23 ± 3 pc in projection. Its size and offset are naturally explained if EriII has a central dark matter core, but such a core may be challenging to explain in a ΛCDM cosmology. In this paper, we revisit the survival and evolution of EriII’s SC, focusing for the first time on its puzzlingly large ellipticity ($0.31^{+0.05}_{-0.06}$). We perform a suite of 960 direct N-body simulations of SCs, orbiting within a range of spherical background potentials fit to ultra-faint dwarf (UFD) galaxy simulations. We find only two scenarios that come close to explaining EriII’s SC. In the first scenario, EriII has a low-density dark matter core (of size ${\sim}70\, \text{pc}$ and density $\lesssim 2\times 10^8\, \text{M}_{\odot }\, \text{kpc}^{-3}$). In this model, the high ellipticity of EriII’s SC is set at birth, with the lack of tidal forces in the core allowing its ellipticity to remain frozen for long times. In the second scenario, EriII’s SC orbits in a partial core, with its high ellipticity owing to its imminent tidal destruction. However, this latter model struggles to reproduce the large size of EriII’s SC, and it predicts substantial tidal tails around EriII’s SC that should have already been seen in the data. This leads us to favour the cored model. We discuss potential caveats to these findings, and the implications of the cored model for galaxy formation and the nature of dark matter.

M D A Orkney, J I Read, J A Petts, M Gieles (2019)Globular clusters as probes of dark matter cusp-core transformations, In: Monthly Notices of the Royal Astronomical Society488(3)pp. 2977-2988 Oxford University Press (OUP)

Bursty star formation in dwarf galaxies can slowly transform a steep dark matter cusp into a constant density core. We explore the possibility that globular clusters (GCs) retain a dynamical memory of this transformation. To test this, we use the NBODY6DF code to simulate the dynamical evolution of GCs, including stellar evolution, orbiting in static and time-varying potentials for a Hubble time. We find that GCs orbiting within a cored dark matter halo, or within a halo that has undergone a cusp-core transformation, grow to a size that is substantially larger (Reff ˃ 10 pc) than those in a static cusped dark matter halo. They also produce much less tidal debris. We find that the cleanest signal of an historic cusp-core transformation is the presence of large GCs with tidal debris. However, the effect is small and will be challenging to observe in real galaxies. Finally, we qualitatively compare our simulated GCs with the observed GC populations in the Fornax, NGC 6822, IKN, and Sagittarius dwarf galaxies. We find that the GCs in these dwarf galaxies are systematically larger (⟨Reff⟩ ≃ 7.8 pc), and have substantially more scatter in their sizes than in situ metal-rich GCs in the Milky Way and young massive star clusters forming in M83 (⟨Reff⟩ ≃ 2.5 pc). We show that the size, scatter, and survival of GCs in dwarf galaxies are all consistent with them having evolved in a constant density core, or a potential that has undergone a cusp-core transformation, but not in a dark matter cusp.

Martin P. Rey, Andrew Pontzen, Oscar Agertz, Matthew Orkney, Justin Read, Amélie Saintonge, Christian Pedersen (2019)EDGE: The Origin of Scatter in Ultra-faint Dwarf Stellar Masses and Surface Brightnesses, In: Astrophysical Journal Letters886(1)L3 The American Astronomical Society

We demonstrate how the least luminous galaxies in the universe, ultra-faint dwarf galaxies, are sensitive to their dynamical mass at the time of cosmic reionization. We select a low-mass (~ ´ 1.5 10 M☉ 9 ) dark matter halo from a cosmological volume, and perform zoom hydrodynamical simulations with multiple alternative histories using “genetically modified” initial conditions. Earlier-forming ultra-faints have higher stellar mass today, due to a longer period of star formation before their quenching by reionization. Our histories all converge to the same final dynamical mass, demonstrating the existence of extended scatter (1 dex) in stellar masses at fixed halo mass due to the diversity of possible histories. One of our variants builds less than 2% of its final dynamical mass before reionization, rapidly quenching in situ star formation. The bulk of its final stellar mass is later grown by dry mergers, depositing stars in the galaxy’s outskirts and hence expanding its effective radius. This mechanism constitutes a new formation scenario for highly diffuse (r1 2 ~ 820 pc, ~ - 32 mag arcsec 2 ), metal-poor ([Fe H 2.9 ] = - ), ultra-faint (V = -5.7) dwarf galaxies within the reach of next-generation low surface brightness surveys.

Oscar Agertz, Andrew Pontzen, Justin Read, Martin P Rey, Matthew Orkney, Joakim Rosdahl, Romain Teyssier, Robbert Verbeke, Michael Kretschmer, Sarah Nickerson (2020)EDGE: the mass–metallicity relation as a critical test of galaxy formation physics, In: Monthly Notices of the Royal Astronomical Society491(2)pp. 1656-1672 Oxford University Press

We introduce the 'Engineering Dwarfs at Galaxy Formation's Edge' (EDGE) project to study the cosmological formation and evolution of the smallest galaxies in the Universe. In this first paper, we explore the effects of resolution and sub-grid physics on a single low-mass halo (M_halo=109{ M}_☉), simulated to redshift z = 0 at a mass and spatial resolution of ̃ 20{ M}_☉ and ∼3 pc. We consider different star formation prescriptions, supernova feedback strengths, and on-the-fly radiative transfer (RT). We show that RT changes the mode of galactic self-regulation at this halo mass, suppressing star formation by causing the interstellar and circumgalactic gas to remain predominantly warm (∼104 K) even before cosmic reionization. By contrast, without RT, star formation regulation occurs only through starbursts and their associated vigorous galactic outflows. In spite of this difference, the entire simulation suite (with the exception of models without any feedback) matches observed dwarf galaxy sizes, velocity dispersions, V-band magnitudes, and dynamical mass-to-light-ratios. This is because such structural scaling relations are predominantly set by the host dark matter halo, with the remaining model-to-model variation being smaller than the observational scatter. We find that only the stellar mass-metallicity relation differentiates the galaxy formation models. Explosive feedback ejects more metals from the dwarf, leading to a lower metallicity at a fixed stellar mass. We conclude that the stellar mass-metallicity relation of the very smallest galaxies provides a unique constraint on galaxy formation physics.