Chemodynamical analysis of bulge stars for simulated disc galaxies

Abstract

We analyse the kinematics and chemistry of the bulge stars of two simulated disc galaxies using our chemodynamical galaxy evolution code GCD+. First we compare stars that are born inside the galaxy with those that are born outside the galaxy and are accreted into the centre of the galaxy. Stars that originate outside of the bulge are accreted into it early in its formation within 3 Gyrs so that these stars have high [alpha/Fe] as well as having a high total energy reflecting their accretion to the centre of the galaxy. Therefore, higher total energy is a good indicator for finding accreted stars. The bulges of the simulated galaxies formed through multiple mergers separated by about a Gyr. Since [alpha/Fe] is sensitive to the first few Gyrs of star formation history, stars that formed during mergers at different epochs show different [alpha/Fe]. We show that the [Mg/Fe] against star formation time relation can be very useful to identify a multiple merger bulge formation scenario, provided there is sufficiently good age information available. Our simulations also show that stars formed during one of the merger events retain a systematically prograde rotation at the final time. This demonstrates that the orbit of the ancient merger that helped to form the bulge could still remain in the kinematics of bulge stars.Comment: 7 pages, 7 figures, MNRAS in pres

Chemical evolution of the Galactic bulge as traced by microlensed dwarf and subgiant stars

“…In spite of its "proximity" and the many detailed spectroscopic and photometric studies during the last few decades, the origin and evolutionary history of the Galactic bulge is still poorly understood. Its generally very old stellar population, metal-rich nature, and over-abundances of α-elements (e.g., McWilliam & Rich 1994;Zoccali et al 2006;Fulbright et al 2007;Meléndez et al 2008;Bensby et al 2010c) are consistent with a classical bulge formed during the collapse of the proto-galaxy and subsequent mergers, which would have resulted in an intense burst of star formation (e.g., White & Rees 1978;Matteucci & Brocato 1990;Ferreras et al 2003;Rahimi et al 2010). Alternatively, the boxy/peanut-like shape of the bulge suggests an origin through dynamical instabilities in an already established inner disk (e.g., Maihara et al 1978;Combes et al 1990;Shen et al 2010).…”

“…For example, the microlensed bulge dwarf stars will have an important impact on the modelling of the bulge, in particular regarding recent suggestions that the initial mass function (IMF) in the bulge needs to be different from that in the solar neighbourhood in order to explain the MDF based on red giant stars (Cescutti & Matteucci 2011). Additionally, combining the dwarf abundances and kinematics with numerical studies, e.g., Rahimi et al (2010), points to the possibility of disentangling different formation scenarios for the bulge, e.g. secular versus merger origin.…”

Chemical evolution of the Galactic bulge as traced by microlensed dwarf and subgiant stars

“…The similarity between the Bulge and disc results implies that these metallicity-dependent nucleosynthesis yields are a general feature but also argues against substantial differences in the initial mass function (IMF) between the two populations. (Rahimi et al 2010), or early fragmentation of the disc into clumps of stars and gas which then rapidly merge to form the Bulge (Immeli et al 2004). These results may also be consistent with the secular evolution of the disc, depending on the age-metallicity-[α/Fe] relation present in the inner disc.…”

“…Old, low [α/Fe] stars are found in the dwarf spheroid galaxies, but accreting a Sagittarius-like object would not explain the pattern, because the low [α/Fe] stars in Sagittarius have too low metallicities (e.g., Venn et al 2004). In the simulations of Rahimi et al (2010), the bulges experience a series of mergers over a period of ∼5 Gyr, leading to populations of "old", "intermediate" and "young" stars. The distributions of [Mg/Fe] for these populations do show some old stars with low [Mg/Fe], (as well the expected shift to low [Mg/Fe] for the younger stars) but they caution that their code suppresses mixing among gas particles, leading to artificially high abundance ratio dispersions.…”

“…The established concordance model of cosmology provides the framework to understand the formation of the Milky Way by hierarchical merging, but the formation of the components of the Galaxy requires important physics, such as star formation and feedback, that is unresolved and parametrised in current models. However, there are two basic scenarios by which a bulge is built up in simulations (see Rahimi et al 2010, and references therein). The first is by mergers, where subclumps coming together in the early phases of Galactic evolution combine to form the Bulge out of both accreted stars and stars formed in situ.…”