MNRAS 487, 4424–4456 (2019) doi:10.1093/mnras/stz1563 Advance Access publication 2019 June 12 NIHAO XVI: the properties and evolution of kinematically selected discs, bulges, and stellar haloes Aura Obreja , 1,2‹ Aaron A. Dutton , 2 Andrea V. Macci` o , 2,3 Benjamin Moster, 1,4 Tobias Buck , 5 Glenn van den Ven , 6,7 Liang Wang, 8 Gregory S. Stinson 3 and Ling Zhu 9 1 University Observatory Munich, Scheinerstraße 1, D-81679 Munich, Germany 2 New York University Abu Dhabi, PO Box 129188, Saadiyat Island, Abu Dhabi, UAE 3 Max-Planck-Institut f¨ ur Astronomie, K¨ onigstuhl 17, D-69117 Heidelberg, Germany 4 Max-Planck-Institut f¨ ur Astrophysik, Karl-Schwarzschild Straße 1, D-85748 Garching, Germany 5 Leibniz-Institut f ¨ ur Astrophysik Potsdam (AIP), An der Sternwarte 16, D-14482 Potsdam, Germany 6 European Southern Observatory, Karl-Schwarzschild-Str. 2, D-85748 Garching bei M¨ unchen, Germany 7 Department of Astrophysics, University of Vienna, T¨ urkenschanzstrasse 17, A-1180 Vienna, Austria 8 University of Western Australia, Crawley, WA 6009, Australia 9 Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, China Accepted 2019 June 2. Received 2019 May 8; in original form 2018 April 18 ABSTRACT We use 25 simulated galaxies from the NIHAO project to define and characterize a variety of kinematic stellar structures: thin and thick discs, large-scale single discs, classical and pseudo- bulges, spheroids, inner discs, and stellar haloes. These structures have masses, spins, shapes, and rotational support in good agreement with theoretical expectations and observational data. Above a dark matter halo mass of 2.5 × 10 11 M ⊙ , all galaxies have a classical bulge and 70 percent have a thin and thick disc. The kinematic (thin) discs follow a power-law relation between angular momentum and stellar mass J ∗ = 3.4M 1.26±0.06 ∗ , in very good agreement with the prediction based on the empirical stellar-to-halo-mass relation in the same mass range, and show a strong correlation between maximum ‘observed’ rotation velocity and dark matter halo circular velocity v c = 6.4v 0.64±0.04 max . Tracing back in time these structures’ progenitors, we find all of them to lose a fraction 1 − f j of their maximum angular momentum. Thin discs are significantly better at retaining their high-redshift spins (f j ∼ 0.70) than thick ones (f j ∼ 0.40). Stellar haloes have their progenitor baryons assembled the latest (z 1/2 ∼ 1.1) and over the longest time-scales (τ ∼ 6.2 Gyr), and have the smallest fraction of stars born in situ (f in situ = 0.35 ± 0.14). All other structures have 1.5  z 1/2  3, τ = 4 ± 2 Gyr, and f in situ  0.9. Key words: methods: numerical – galaxies: fundamental parameters – galaxies: kinematics and dynamics – galaxies: stellar content – galaxies: structure. 1 INTRODUCTION From the pioneering work of Hubble in the 1920s, a large effort has been put into the discovery and classification of galaxies, which constitute the luminous tracers of the Universe’s elementary blocks, the dark matter haloes. For both historical and practical reasons, the most widely used galaxy classifications rely on photometry. Among the first, de Vaucouleurs (1959), van Houten (1961), S´ ersic (1963), Freeman (1970), Yoshizawa & Wakamatsu (1975), and Simien (1989) used fits of azimuthally averaged luminosity profiles of galaxies to distinguish between highly centrally concentrated ⋆ E-mail: obreja@usm.lmu.de ellipticals and extended fainter spirals (Sandage 1961). With the re- cent observations with ever-increasing spatial resolution, it became feasible to use the complete 2D photometric data to derive structural parameters of galaxies, both at high and low redshifts (e.g. van der Wel et al. 2012; Salo et al. 2015; M´ endez-Abreu et al. 2017). The Hubble classification typically relies on 2D or 1D two- components fits: one function describing the inner region (a de Vaucouleurs or a more general S´ ersic) and the other characterizing the outer region (an exponential, a low-index S´ ersic, or a truncated disc). The integral luminosity of the inner function divided by the total galaxy luminosity is known as the bulge-to-total ratio (B/T), and is the main parameter defining the location of a galaxy in the Hubble diagram. With the development of new instruments able to C  2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society Downloaded from https://academic.oup.com/mnras/article/487/3/4424/5514351 by guest on 30 November 2022