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Matthew Orkney

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

Academic and research departments

Astrophysics Research Group.

My research project

My publications


Orkney M D A, Read J I, Petts J A, Gieles M (2019) Globular clusters as probes of dark matter cusp-core transformations, Monthly Notices of the Royal Astronomical Society 488 (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 à 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* C 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* C 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.
Rey Martin P., Pontzen Andrew, Agertz Oscar, Orkney Matthew, Read Justin, Saintonge Amélie, Pedersen Christian (2019) EDGE: The Origin of Scatter in Ultra-faint Dwarf Stellar Masses and Surface Brightnesses, Astrophysical Journal Letters 886 (1) 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 (ý1 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 (MV = -5.7) dwarf galaxies within the reach of next-generation low surface
brightness surveys.
Agertz Oscar, Pontzen Andrew, Read Justin, Rey Martin P, Orkney Matthew, Rosdahl Joakim, Teyssier Romain, Verbeke Robbert, Kretschmer Michael, Nickerson Sarah (2019) EDGE: the mass?metallicity relation as a critical test of galaxy formation physics, Monthly Notices of the Royal Astronomical Society 491 (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