# Dr Maxime Delorme

Scientific programmer
+44 (0)1483 689291
15 BC 03

Astrophysics Research Group.

### Publications

Alexandra L. Gregory, Michelle L. M. Collins, Denis Erkal, Erik Tollerud, Maxime Delorme, Lewis Hill, David J. Sand, Jay Strader, Beth Willman (2020)Uncovering the Orbit of the Hercules Dwarf Galaxy, In: Monthly Notices of the Royal Astronomical Society Oxford University Press (OUP)

We present new chemo{kinematics of the Hercules dwarf galaxy based on Keck II{ DEIMOS spectroscopy. Our 21 conrmed members, including 9 newly con- rmed members, have a systemic velocity of vHerc = 46:4 1:3 kms

A Rojas-Nino, Justin Read, A Aguilar, Maxime Delorme (2016)An efficient positive potential-density pair expansion for modelling galaxies, In: Monthly Notices of the Royal Astronomical Society459(3)pp. 3349-3355 Oxford University Press

We present a novel positive potential-density pair expansion for modelling galaxies, based on the Miyamoto–Nagai disc. By using three sets of such discs, each one of them aligned along each symmetry axis, we are able to reconstruct a broad range of potentials that correspond to density profiles from exponential discs to 3D power-law models with varying triaxiality (henceforth simply ‘twisted’ models). We increase the efficiency of our expansion by allowing the scalelength parameter of each disc to be negative. We show that, for suitable priors on the scalelength and scaleheight parameters, these ‘MNn discs’ (Miyamoto–Nagai negative) have just one negative density minimum. This allows us to ensure global positivity by demanding that the total density at the global minimum is positive. We find that at better than 10 per cent accuracy in our density reconstruction, we can represent a radial and vertical exponential disc over 0.1–10 scalelengths/scaleheights with four MNn discs; a Navarro, Frenk and White (NFW) profile over 0.1–10 scalelengths with four MNn discs; and a twisted triaxial NFW profile with three MNn discs per symmetry axis. Our expansion is efficient, fully analytic, and well suited to reproducing the density distribution and gravitational potential of galaxies from discs to ellipsoids.

Imran Tariq Nasim, Alessia Gualandris, Justin I Read, Fabio Antonini, Walter Dehnen, Maxime Delorme (2021)Formation of the largest galactic cores through binary scouring and gravitational wave recoil, In: Monthly notices of the Royal Astronomical Society502(4)pp. 4794-4814

Massive elliptical galaxies are typically observed to have central cores in their projected radial light profiles. Such cores have long been thought to form through ‘binary scouring’ as supermassive black holes (SMBHs), brought in through mergers, form a hard binary and eject stars from the galactic centre. However, the most massive cores, like the $\sim 3{\, \mathrm{kpc}}$ core in A2261-BCG, remain challenging to explain in this way. In this paper, we run a suite of dry galaxy merger simulations to explore three different scenarios for central core formation in massive elliptical galaxies: ‘binary scouring’, ‘tidal deposition’, and ‘gravitational wave (GW) induced recoil’. Using the griffin code, we self-consistently model the stars, dark matter, and SMBHs in our merging galaxies, following the SMBH dynamics through to the formation of a hard binary. We find that we can only explain the large surface brightness core of A2261-BCG with a combination of a major merger that produces a small $\sim 1{\, \mathrm{kpc}}$ core through binary scouring, followed by the subsequent GW recoil of its SMBH that acts to grow the core size. Key predictions of this scenario are an offset SMBH surrounded by a compact cluster of bound stars and a non-divergent central density profile. We show that the bright ‘knots’ observed in the core region of A2261-BCG are best explained as stalled perturbers resulting from minor mergers, though the brightest may also represent ejected SMBHs surrounded by a stellar cloak of bound stars.

Filippo Contenta, Eduardo Balbinot, James Petts, Justin Read, Mark Gieles, Michelle Collins, Jorge Peñarrubia, Maxime Delorme, Alessia Gualandris (2018)Probing dark matter with star clusters: a dark matter core in the ultra-faint dwarf Eridanus II, In: Monthly Notices of the Royal Astronomical Society Oxford University Press (OUP)

We present a new technique to probe the central dark matter (DM) density profile of galaxies that harnesses both the survival and observed properties of star clusters. As a first application, we apply our method to the ultra-faint' dwarf Eridanus II (Eri II) that has a lone star cluster ~45 pc from its centre. Using a grid of collisional N-body simulations, incorporating the effects of stellar evolution, external tides and dynamical friction, we show that a DM core for Eri II naturally reproduces the size and the projected position of its star cluster. By contrast, a dense cusped galaxy requires the cluster to lie implausibly far from the centre of Eri II (>1 kpc), with a high inclination orbit that must be observed at a particular orbital phase. Our results imply that either a cold DM cusp was heated up' at the centre of Eri II by bursty star formation, or we are seeing an evidence for physics beyond cold DM.

Imran Nasim, Alessia Gualandris, Justin Read, Walter Dehnen, Maxime Delorme, Fabio Antonini (2020)Defeating stochasticity: coalescence timescales of massive black holes in galaxy mergers, In: Monthly Notices of the Royal Astronomical Society Oxford University Press

The coalescence of massive black hole binaries (BHBs) in galactic mergers is the primary source of gravitational waves (GWs) at low frequencies. Current estimates of GW detection rates for the Laser Interferometer Space Antenna and the Pulsar Timing Array vary by three orders of magnitude. To understand this variation, we simulate the merger of equal-mass, eccentric, galaxy pairs with central massive black holes and shallow inner density cusps. We model the formation and hardening of a central BHB using the Fast Multiple Method as a force solver, which features a O¹Nº scaling with the number N of particles and obtains results equivalent to direct-summation simulations. At N 5105, typical for contemporary studies, the eccentricity of the BHBs can vary significantly for different random realisations of the same initial condition, resulting in a substantial variation of the merger timescale. This scatter owes to the stochasticity of stellar encounters with the BHB and decreases with increasing N. We estimate that N 107 within the stellar half-light radius suffices to reduce the scatter in the merger timescale to 10%. Our results suggest that at least some of the uncertainty in low-frequency GW rates owes to insufficient numerical resolution.

Janet Preston, Michelle L M Collins, Rodrigo A Ibata, Erik J Tollerud, R Michael Rich, Ana Bonaca, Alan W McConnachie, Dougal Mackey, Geraint F Lewis, Nicolas F Martin, Jorge Peñarrubia, Scott C Chapman, Maxime Delorme (2019)A Dwarf Disrupting - Andromeda XXVII and the North West Stream, In: Monthly Notices of the Royal Astronomical Society

We present a kinematic and spectroscopic analysis of 38 red giant branch stars, in 7 fields, spanning the dwarf spheroidal galaxy Andromeda XXVII and the upper segment of the North West Stream. Both features are located in the outer halo of the Andromeda galaxy at a projected radius of 50-80 kpc, with the stream extending for ∼3° on the sky. Our data is obtained as part of the PAndAS survey and enables us to confirm that Andromeda XXVII’s heliocentric distance is 827 ± 47 kpc and spectroscopic metallicity is -2.1+0.4−0.5⁠. We also re-derive Andromeda XXVII’s kinematic properties, measuring a systemic velocity = -526.1+10.0−11.0 kms−1 and a velocity dispersion that we find to be non-Gaussian but for which we derive a formal value of 27.0+2.2−3.9 kms−1. In the upper segment of the North West Stream we measure mean values for the metallicity = -1.8±0.4, systemic velocity = -519.4 ±4.0 kms−1 and velocity dispersion = 10.0±4.0 kms−1. We also detect a velocity gradient of 1.7±0.3 kms−1 kpc−1 on an infall trajectory towards M31. With a similar gradient, acting in the same direction, in the lower segment we suggest that the North West Stream is not a single structure. As the properties of the upper segment of the North West Stream and Andromeda XXVII are consistent within 90% confidence limits, it is likely that the two are related and plausible that Andromeda XXVII is the progenitor of this stream.