Galaxy tilting in the era of Gaia

Abstract

Determining the orientation of galaxies with respect to their dark matter halo is vital in understanding effects such as gravitational lensing. Disc galaxies, like the Milky Way, can be orientated simply by using the angular momentum of the stellar disc. However, this angular momentum is not constant. The change in direction of the angular momentum vector, with respect to time, can be considered as a tilting rate. The tilting direction provides an indication of the angular momentum reaching the disc. Both gas and satellite accretion can deliver misaligned angular momentum directly to the disc. It has also been argued that torques imposed by the dark matter halo can also change the angular momentum of the disc.
This thesis presents the tilting rates for galaxies evolving in two Λ cold dark matter cosmological hydrodynamic simulations. All of the galaxies comparable with the Milky Way have tilting rates higher than Gaia’s detection limit of 0.28◦ Gyr−1 (Perryman et al., 2014).
Debattista et al. (2015) found that red galaxies tend to be aligned with the minor axis of their dark matter halo, whereas, blue galaxies tend to have random orientations. Observationally, similar trends are found, for example between the alignment distribution of the brightest satellite galaxies and the major axes of their host groups (Li et al., 2013). This thesis finds a very strong correlation between the specific star formation rate and the tilting rate, using the state-of-the-art Numerical Investigation of One Hundred Astrophysical Objects (NIHAO) suite of cosmological zoom-in simulations. Galaxies with higher star formation rates tilt faster and therefore are likely to be perturbed from any stable orientation, between the disc and the host halo. Moreover, for the predominantly blue galaxies within the NIHAO suite, there is no preferential orientation, with respect to the dark matter halo.
The local environment provides a reservoir of angular momentum available to the disc. For both of the cosmological simulations presented, the normalized local overdensity was compared to the tilting rate of the disc, finding a strong correlation at R = 6 Mpc. On the other hand, no correlations are found between the shape of the dark matter halo, and the tilting direction of the stellar disc, contrary to previous claims that the torques imposed by the halo will drive the tilting of the stellar disc (e.g. Yurin and Springel, 2015).
Five of the NIHAO galaxies are looked at in greater detail, comparing tilting direction of the stellar disc to the angular momentum of various features and interactions. For two of the galaxies, the tilting direction is dominated by interactions/mergers with satellites. The remaining three are all driven by the infall of cool gas with misaligned angular momentum compared to that of the hot gas corona and the stellar disc. By the time the cool gas reaches the disc its angular momentum has been torqued by the hydrodynamical forces imposed by the hot gas and has angular momentum aligned with the hot gas. The same process has been proposed for forming warps in cosmological simulations (Roškar et al., 2010), and this thesis also finds that in this case the cool gas disc is misaligned in the direction of tilting.