The relation betweenthe morphology and kinematics ofgalaxies in the EAGLE simulation

We investigate the connection between the morphology and kinematics of the stellar component of 𝟒𝟏𝟓𝟓 central galaxies with mass M⋆ > 𝟏𝟎𝟗.𝟓M⊙ in the EAGLE simulation. Wecompare 4 kinematic diagnostics commonly used to describe simulated galaxies, and find good consistency between them. We model the structure of galaxies as ellipsoids andquantify morphology via their minor to major axes ratios. Flattened galaxies typically exhibit greater rotational support, but there is significant scatter in the relationship betweenmorphological and kinematical diagnostics, such that kinematically-similar galaxies can exhibit a broad range of morphologies. The scatter in the relationship between the ellipticityand the ratio of the rotation and dispersion velocities correlates with the anisotropy of the stellar velocity dispersion, with more flattened galaxies exhibiting less dispersion along theminor axis than in the plane defined by the intermediate and major axes. Analysis of the dark matter-only simulation starting from the same initial conditions reveals that the velocitydispersion anisotropy of EAGLE galaxies is significantly correlated with the intrinsic morphology (i.e. that which emerges in the absence of gas-dynamical processes) of the host halo.

Referenceso Binney, James; Tremaine, Scott; Galactic Dynamics: Second Edition; 2008 (1st edition in 1987), ISBN 978-0-691-13026-2 (HB) Published by Princeton University Press, Princeton, NJ USA, 2008o Schaye, Joop; Crain, Robert A.; et al. The EAGLE project: simulating the evolution and assembly of galaxies and their environments; 01/2015, Monthly Notices of the Royal Astronomical Society, Volume 446, Issue 1, p.521-554o Crain, Robert A.; Schaye, Joop; et al. The EAGLE simulations of galaxy formation: calibration of subgrid physics and model variations; 06/2015, Monthly Notices of the Royal Astronomical Society, Volume 450, Issue 2, p.1937-1961o Schaller, Matthieu; et al. Baryon effects on the internal structure of ΛCDM haloes in the EAGLE simulations; 08/2015, Monthly Notices of the Royal Astronomical Society, Volume 451, Issue 2, p.1247-1267o Zavala, Jesús; et al. The link between the assembly of the inner dark matter halo and the angular momentum evolution of galaxies in the EAGLE simulation; 08/2016, Monthly Notices of the Royal Astronomical Society, Volume 460, Issue 4, p.4466-4482o Lagos, Claudia del P.; et al. Angular momentum evolution of galaxies in EAGLE; 02/2017, Monthly Notices of the Royal Astronomical Society, Volume 464, Issue 4, p.3850-3870o Trayford, James W.; et al. Optical colours and spectral indices of z = 0.1 eagle galaxies with the 3D dust radiative transfer code skirt; 09/2017, Monthly Notices of the Royal Astronomical Society, Volume 470, Issue 1, p.771-799o Correa, Camila A.; et al. The relation between galaxy morphology and colour in the EAGLE simulation; 11/2017, Monthly Notices of the Royal Astronomical Society: Letters, Volume 472, Issue 1, p.L45-L49

Adrien C. R. Thob, Robert A. Crain, Ian G. McCarthyAstrophysics Research Institute, Liverpool John Moores University

Byrom Street, Liverpool, L3 3AF, UKEmail: A.Thob@2015.ljmu.ac.uk, Website: http://www.astro.ljmu.ac.uk/~astathob/,

I - KINEMATICS DESCRIPTIONKinematical properties of our sample, as characterised by 4 commonly used diagnostics:

the spin parameter 𝝀⋆, the median orbital circularity ഥ𝝃i, the fraction of kinetic energy 𝜿co and the rotation-to-dispersion ratio 𝒗rot/𝝈o.

▪ 𝝀⋆, ഥ𝝃i, 𝜿co, and 𝒗rot/𝝈o quantify the degree of rotation-support ⇒ higher values represent strongerrotational dynamics.

▪ The strong correlation shown in the left and top panels justify their interchangeable use:• hereafter, we use 𝒗rot/𝝈o as it is analogous to measurements derived from spectroscopy,• also can be derived from the virial tensor theorem as 𝜹, the velocity dispersion anisotropy.

▪ 𝜹 quantifies the in-plane-to-vertical anisotropy ⇒ higher values denote less vertical and/or more in-planedispersion.

▪ Its significant impact on kinematics diversity is shown bottom right in the case of the 𝜿co - 𝒗rot/𝝈orelation.

III - MORPHO-KINEMATICS RELATIONRelationship between 𝒗rot/𝝈o and 𝝐 with respect to 4 populations: the full sample, its blue or red sub-

populations, and a selection with only oblate rotators & spheroids.

▪ Full sample shows good correspondence between high 𝒗rot/𝝈o & high 𝝐, that loses significance with lowervalues as similar 𝒗rot/𝝈o show more 𝝐 diversity.

▪ Blue galaxies span the entire relationship, being composed of the most rotational & flattened systems aswell as highly dispersive configurations.

▪ Red galaxies are generally spheroidal and dispersion supported.▪ Excluding triaxial/prolate & rotationally mis-aligned systems mostly excludes dispersion-dominated systems.

II - MORPHOLOGIES DIVERSITYDistribution and visualization of our sample in the space defined by 2 morphological parameters,

assuming galaxies to be ellipsoid-shaped : the triaxiality 𝑻 and the ellipticity 𝝐.

▪ 𝝐 quantifies the flattening degree of the galaxies ⇒ small values represent near-spheroidal systems and largevalues correspond to flattened structures.• With higher 𝝐, 𝑻 identifies 3 possible flattened configurations: Oblate disc-shaped, prolate cigar-shaped and

completely triaxial systems respectively at low, high or mid 𝑻.• Most galaxies are oblate ellipsoids of diverse flatness.

▪ Background face-on & edge-on mock u,g,r images provide a visual impression of various (𝝐, 𝑻)-configurations.• Orientations were defined relative to the axis of rotation: this equates to the minor axis for relaxed

oblate/triaxial systems, but is often mis-aligned in prolate systems.• Some show evidence of tidal disturbance and/or merger remnants ⇒ they may be induced by interacting

neighbouring galaxies.

IV – ANISOTROPIC DISPERSION & ORIGINSPhysics behind the width along 𝝐 in 𝒗rot/𝝈o-vs-𝝐 for the selected oblate rotators & spheroids

using the dispersion anisotropy 𝜹 and its relation with the dark matter flatness 𝝐 dm

▪ At fixed rotation-support 𝒗rot/𝝈o, larger ellipticity 𝝐 correspond to greater dispersion anisotropy 𝜹.• Stronger anisotropy correspond to more flattened systems• As the stars show strong vertical dispersion, systems appear thicker

▪ Anisotropy deviation 𝜹 − ෩𝜹 [𝒗rot/𝝈o] at fixed 𝒗rot/𝝈o correlate (red median) with the dark matter flatness𝝐 dm with a 𝟏𝝈 shift from lowest to highest range• Stronger anisotropy form in naturally “flatter” dark matter halos

▪ Matching each galaxy to their twin halo in a Dark Matter Only (DMO) sister simulation with identical initial

conditions, 𝝐 dmDMO represents the DMO dark matter structure flatness ( 𝝐 dm

Ref for the Reference one).

• 𝝐 dmDMO (blue median) appears systematically higher than 𝝐 dm

Ref ⇒ ordinary matter and gas-dynamical

processes act to make halos more spheroidal

• 𝝐 dmDMO also appears to mildly correlate with 𝜹 − ෩𝜹 [𝒗rot/𝝈o] ⇒ dark matter structures could cause the

build up of dispersion anisotropy