João A. S. Amarante
Our Galaxy, the Milky Way, experienced several merger events which left their imprints on the stellar halo. In particular, it has been shown that when the Galaxy was still young, in its first billion years, a major merger happened and, likely, perturbed its disc-shape structure. Nonetheless, in order to fully understand the effects of such event, we need to know the chemical and dynamical characteristics of the young Milky Way, i.e., before the major merger event. For this purpose, we use state-of-art simulations to aid us explore formation scenarios of Milky Way-like galaxies. First we explore the evolution of isolated simulation galaxies with different star formation prescriptions and find evidence which supports a clumpy phase of star formation in the young Galaxy. For instance, kinematic and chemical properties of stars near the Sun, commonly associated with the effects of a merger, can be explained by a clumpy formation scenario. This scenario is supported by observations, with the Hubble Space Telescope, of very distant galaxies with high dense clumpy regions, while the Universe was still young. Secondly, we study the effects of a merger in a clumpy galaxy and study the full evolution of the disc in a clumpy+merger scenario. We explore several parameters of the merger event and investigate which scenarios better explain the current structure of Milky Way recently observed with the ESA space telescope Gaia.
Page 1 – Right Figure – An artist impression of the ESA’s Gaia satellite mapping our Galaxy. The background image shows the Milky Way as observed from Earth. We can observe thousands of stars and in the central parts the interstellar dust obscures the star's visual light and creates opaque/dark regions. Thanks to the exquisite Gaia data, we now have strong evidence that our Galaxy experienced a major merger event, named the Gaia-Enceladus-Sausage (GES), during its first billion years (Belokurov et al., 2018, Helmi et al., 2018). It has been suggested that this merger event created the thick disc and a population called ‘‘the Splash”.
Page 2 – Top Figure – We made use of a state-of-art simulation, and showed that an isolated galaxy, with an early clumpy star formation, can form a Splash like population without invoking a merger event (Amarante et al. 2020). Left panel shows the simulation, at 1 billion years, as it would be observed by the Hubble Space Telescope without observational noise. The remaining panels convolve these images to different resolution and add observational noise. Although the image is blurred, there are still clear signs of clumps in the simulation. These results are very similar to real observational galaxies as shown in Guo et al. (2018).
Page 2 – Bottom Figure – Left panel shows the star’s metallicity, measured in a logarithmic scale [Fe/H], as a function of the rotational velocity vφ. Each bin is color coded by the mean value of the α-element abundance. In this [Fe/H]-vφ plane, the thin/thick disc corresponds to the α-poor/rich regions marked by the slanted lines as defined in Belokurov et al. (2020). There is a smooth transition from the Splash to the thick disc region which suggests that these two populations are related in the Milky Way. But, an isolated clumpy galaxy also has a thin/thick disc region and a transition towards the Splash region. As this population is formed in the first billion years in the simulation, we suggest that it could already be present during the GES event.
Page 3 – Left Figure – Three snapshots of a clumpy+merger simulation. It shows the progenitor, a disc shaped galaxy, and the satellite's first passage at 1.7 billion years. The merger is completed at about 3 billion years and it completely disturbs the disc-galaxy.
Page 3 – Bottom right Figure – The [Fe/H]- vφ plane for the clumpy+merger simulation, left panel, also shows a clear chemical-kinematic relation for the thin/thick disc and Splash region. Interestingly enough, for a non-clumpy galaxy + merger simulation, the right panel, the merger doesn’t create a clear thin/thick disc dichotomy and the Splash region doesn’t appear as a distinct component. This contradicts the formation scenario via merger of the thick disc (Villalobos & Helmi 2008) and Splash population (Belokurov et al. 2020).
Page 4 - Gaia data and results from state-of-art simulation not only is helping us to understand the Milky Way’s violent past, but it also reopened a few questions which require further investigation. It is undoubted that the GES event happened, but its full effects on the young Milky Way is still under debate.
References:
Amarante J. A. S., Beraldo e Silva L., Debattista V, Smith M. C., 2020, ApJL, 891, L30
Belokurov V., Erkal D., Evans N. W., Koposov S. E., Deason A. J., 2018, MNRAS, 478, 611
Belokurov V., Sanders J. L., Fattahi A., Smith M. C., Deason A. J., Evans N. W., Grand R. J. J.,2020, MNRAS, 494, 388
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Helmi A., Babusiaux C., Koppelman H. H., Massari D., Veljanoski J., Brown A. G. A., 2018,Nature, 563, 85
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