University of Groningen Bachelor Thesis in Astronomy Characterization of a simulated minor merger resembling Gaia-Enceladus Author: Supervisors: prof. dr. Amina Roy O. Y. Bos Helmi Helmer H. Koppelman Abstract The second Gaia data release (Gaia Collaboration et al., 2018a) has provided evidence for a merger event roughly 10 Gyr ago with an object named Gaia-Enceladus (Helmi et al., 2018). A previous simulation by Villalobos and Helmi (2008, 2009) produced a similar velocity distribution of halo stars near the Sun as seen in the Gaia data. In this bachelor thesis I continue this simulation for 10 Gyr in an attempt to characterize stars of Gaia-Enceladus, which accounts for a large fraction of the galactic halo and possibly triggered the formation of the thick disk. The aim of the thesis is to help interpret the observations and to provide means to find likely members of Gaia-Enceladus in the Gaia data, especially at large distances from the Sun. I find that the characteristic kinematics of the accreted stars are maintained through the extension of the simulation and still show similarities to the various features seen in the Gaia data. These features can be understood well when analyzing the angular momentum and eccentricity as they are correlated to the initial position of the stars, and to the mass loss history. July 2019 CONTENTS CONTENTS Contents 1 Introduction 1 1.1 Milky Way components . .1 1.2 Gaia mission . .1 1.3 Gaia-Enceladus . .1 1.4 Thick disk and halo formation . .2 1.5 Velocity . .2 1.6 Thesis description . .3 2 Simulations 4 2.1 Description . .4 2.2 Dark matter halo . .4 2.3 Host galaxy . .5 2.4 Satellite galaxy . .6 2.5 Numerical parameters . .6 2.6 Change of coordinate system . .7 3 Results 8 3.1 Morphology . .8 3.2 Velocity . 11 3.3 Angular momentum . 13 3.4 Eccentricity . 14 3.5 Eccentricity and angular momentum . 17 3.6 Metallicity . 20 4 Discussion 21 5 Summary and conclusion 22 1 Introduction 1.1 Milky Way components The Milky way consists of several distinct components, each having different properties. Figure 1 schematically shows each component in a side-on view of the Milky Way (Buser, 2000). The galactic center contains a supermassive black hole and mostly young giant stars. The galactic center is embedded in the galactic bulge. This is a nearly spherical collection of mainly old stars. Most stars in the Milky Way are located in the disk, which has a diameter of up to 60 kpc (L´opez- Corredoira et al., 2018). This disk can be subdivided into a thin disk and a thick disk, which show clear differences in origin. The thin disk contains a lot of gas and thus has, on average, younger stars compared to the thick disk. Lastly, the galactic halo surrounds the Milky Way and is extended in all directions. The galactic halo mainly contains old stars. The Milky Way is also thought to have a halo of dark matter extending much farther out. Figure 1: A diagram showing the different components of the Milky Way. 1.2 Gaia mission In late 2013, the European Space Agency launched the Gaia satellite. Its mission is to measure the three-dimensional positions and velocities of stars in the Milky Way and to determine their intrinsic properties (Gaia Collaboration et al., 2016). The second Gaia data release (DR2) contains the positions and apparent brightness of 1.7 billion stars (Gaia Collaboration et al., 2018a). More importantly for this thesis, the data release also includes the proper motions of 1:3 billion stars and line-of-sight velocities for 7 million stars. Alongside the kinematical information of these stars, the Gaia data release also measured colours for 1:3 billion stars and derived the intrinsic properties such as temperature, stellar radius and luminosity for a subset of these stars. However, the data release does not include the metallicity and abundances in the stars. For this it is necessary to combine the Gaia data set with other data sets such as APOGEE, RAVE and LAMOST. 1.3 Gaia-Enceladus DR2 also revealed that the Herzsprung-Russell diagram of stars in the stellar halo has two se- quences (Gaia Collaboration et al., 2018b); a red, metal rich sequence and a blue, metal poor sequence. This hints that the halo consists of two stellar populations of different origin. Koppel- man et al. (2018) has shown using kinematic data from DR2, that some of the stars in the solar 1 1.4 Thick disk and halo formation 1 INTRODUCTION neighbourhood are part of a large retrograde moving kinematic structure. The stars in this retro- grade structure trace the metal poor (blue) sequence found in Gaia Collaboration et al. (2018b). Helmi et al. (2018) proposed that the stars from this retrograde component are debris from an ob- ject that merged with the Milky Way about 10 Gyr ago, to which they refer to as Gaia-Enceladus. They state that the debris from Gaia-Enceladus dominates the inner halo. Furthermore, using isochrones they show that the members of Gaia-Enceladus span a range of ages of 10 - 13 Gyr. The results of Koppelman et al. (2018) agree well with Vincenzo et al. (2019) and Gallart et al. (2019), which put the median age of Gaia-Enceladus at 12:33 Gyr and 12:37 Gyr, respectively 1.4 Thick disk and halo formation If Gaia-Enceladus had indeed merged with a disk-like galaxy, it must have caused this disk to dynamically heat. Dynamical heating entails that gravitational interactions with stars of Gaia- Enceladus cause the stars from the disk to have a larger spread in velocity. This would thus result in a thicker disk. Hence, Gaia-Enceladus has been proposed to be at least partly responsible for the formation of the thick disk in the Milky Way (Helmi et al., 2018). We can even expect that some stars from the pre-existing disk were dynamically heated towards the halo. This is supported by Purcell et al. (2010) and Pillepich et al. (2015), which both use simulations to show that a considerable part of the pre-existing thick disk stars end up in the halo. Furthermore, Haywood et al. (2018) finds using observational data that the red, metal rich sequence could be the low rotational velocity tail of the old galactic disk. This further supports the hypothesis that Gaia-Enceladus caused the precursor of the thick disk to dynamically heat, causing some disk stars to end up in the halo. The newly accreted gas could also have lead to a burst of star formation. As Gallart et al. (2019) shows, the thick disk reached its peak in star formation about 9 Gyr ago, 4:5 Gyr after the formation of the first stars in the Milky Way. After a few gigayears, the gas would settle into the thin disk, where star formation still continues to this day (Vincenzo et al., 2019). 1.5 Velocity The kinematic properties of stars allow us to make distinctions between different populations of stars. For instance, the angular momentum of a star could be conserved over time, depending on the properties of the gravitational potential. The angular momentum of a star that we can currently measure could thus be related to the angular momentum it had during the time of the merger event. Since it is quite a plausible assumption that the stars from Gaia-Enceladus had, on average, different angular momenta, we could expect to see two distinct angular momentum distributions in the Gaia data. The velocities that stars in the solar neighbourhood have are related to their energy and angular momentum. Stars with negative angular momentum, for example, have negative vφ. Hence, if there is a net difference in the angular momentum of the stars from the disk and from Gaia- Enceladus, there should be a difference in vφ. Figure 2 (Helmi et al., 2018) shows the stellar velocity distribution in the solar vicinity retrieved from the second Gaia release (Gaia Collaboration et al., 2018a) and from a 5:1 mass ratio merger simulation performed by Villalobos and Helmi (2008, 2009). The figure shows that a large fraction of halo stars within the solar neighbourhood is part of a kinematic structure with a slightly retrograde motion (Vy < 0). The simulation, shown at 4 Gyr from the start of the simulation, shows similarities with the velocity distribution from the observations. The simulation shows that the stars from the satellite galaxy (shown in blue) consist of a main component with slightly retrograde movement and a highly retrograde component at Vy < −300 km=s, which is consistent with the findings based on Gaia DR2. 2 1.6 Thesis description 1 INTRODUCTION Figure 2: Velocity distribution of the stars in the solar vicinity (Helmi et al., 2018). The left panel shows the velocity diagram retrieved from the Gaia data. The blue stars indicate likely members of Gaia-Enceladus, selected by angular momentum and energy. The right panel shows the velocity distribution retrieved from the merger simulation of Villalobos and Helmi (2008, 2009) at t = 4 Gyr. Host stars are shown in black and satellite stars are shown in blue. The simulation was carried out on a smaller scale so the velocity diagram was scaled up by a factor 1.5 to resemble the Gaia data. 1.6 Thesis description In this thesis I carry out four N-body simulations, which model a minor galactic merger with a mass ratio of 5:1. The simulations continue from the simulations carried out by Villalobos and Helmi (2008, 2009).