Constraining the thick disc formation scenario of the Milky Way

By Annie Robin, Céline Reylé, J. Fliri, M. Czekaj, Christian P. Robert, Ana M. Martins. 2014

134 citations recorded by the source corpus.

More than 30 years after its discovery, the thick disc of the Milky Way is not fully explored. We examine the shape of the thick disc in order to gain insight into the process of its formation. Methods. The shape of the thick disc is studied in detail using photometric data at high and intermediate latitudes from SDSS and 2MASS surveys. We adopted the population synthesis approach using an ABC-MCMC method to determine the potential degeneracies in the parameters, that can be caused by the mixing with the halo and with the thin disc. We characterised the thick disc shape, scale height, scale length, local density, and flare, and we investigated the extent of the thick disc formation period by simulating several formation episodes. Results. We find that the vertical variation in density is not exponential, but much closer to a hyperbolic secant squared. Assuming a single formation epoch, the thick disc is better fitted with a s ech 2 scale height of 470 pc and a scale length of 2.3 kpc. However, if one simulates two successive formation episodes, which mimicks an extended formation period, the older episode has a higher scale height and a longer scale length than the younger episode, which indicates a contraction during the collapse phase. The scale height decreases from 800 pc to 340 pc, the scale length from 3.2 kpc to 2 kpc. The likelihood is much higher when the thick disc formation extends over a longer period. We also show that star formation increases from the old episode to the young and that there is no flare in the outskirt of the thick disc during the main episode. We compare our results with formation scenarios of the thick disc. During the fitting process, the halo parameters are determined as well. If a power-law density is assumed, it has an exponent of 3.3 and an axis ratio of 0.7. Alternatively, a Hernquist shape would have an exponent of 2.76, an axis ratio of 0.77, and a core radius of 2.1 kpc. The constraint on the halo shows that a transition between an inner and outer halo, if it exists, cannot be at a distance shorter than about 30 kpc, which is the limit of our investigation using turno halo stars. Finally, we show that extrapolating the thick disc towards the bulge region explains well the stellar populations observed there, that there is no longer need to invoke a classical bulge. Conclusions. The facts that the thick-disc episode lasted for several billion years, that a contraction is observed during the collapse phase, and that the main thick disc has a constant scale height with no flare argue against the formation of the thick disc through radial migration. The most probable scenario for the thick disc is that it formed while the Galaxy was gravitationally collapsing from well-mixed gas-rich giant clumps that were sustained by high turbulence, which prevented a thin disc from forming for a time, as proposed previously. This scenario explains well the observations in the thick-disc region and in the bulge region.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/9da103913dfd27ec72ab93bf8f097864f39f8259

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Citations in the corpus

Massive optimal data compression and density estimation for scalable, likelihood-free inference in cosmology (2018, methodology): “…in the astronomy literature). abc methods have been applied successfully to a number of problems in cosmological data analysis (Schafer & Freeman 2012; Cameron & Pettitt 2012; Weyant et al. 2013; Robin et al. 2014; Lin & Kilbinger 2015; Hahn et al. 2017; Kacprzak et al. 2017; Davies et al. 2017).”
Chemical tagging with APOGEE: Discovery of a large population of N-rich stars in the inner Galaxy (2016, methodology): “An alternative mass estimate can be obtained from consideration of a detailed model for the inner Galaxy, such as the one by Robin et al. (2014), which matches stellar counts in the 2MASS and SDSS catalogs.”
BANYAN. XI. The BANYAN Σ Multivariate Bayesian Algorithm to Identify Members of Young Associations with 150 pc (2018, methodology): “The thick disk and halo populations are simulated with the best-fitting parameters obtained in the analysis of Robin et al. (2014) based on SDSS (Alam et al. 2015) and 2MASS data, and the isochrones of Bergbusch & Vandenberg (1992).”
The Galaxy in Context: Structural, Kinematic, and Integrated Properties (2016, background): “Rather than cross-matching sources common to both catalogs (e.g., Finlator et al. 2000), future studies will need to combine both surveys in order to provide better input catalogs (Robin et al. 2014).”
PLATO as it is: a legacy mission for Galactic archaeology (2017, unknown): “…et al. 2013, 2014; Snaith et al. 2015), where current data suggest that the thick disk formation started at z ∼ 3.5 (12 Gyear ago) while the thin disk began to form at z ∼ 1.5 (8 Gyear ago) (e.g., Bensby et al. 2014; Bergemann et al. 2014; Fuhrmann 2011; Haywood et al. 2013; Robin et al. 2014).”
StarHorse : a Bayesian tool for determining stellar masses, ages, distances, and extinctions for field stars (2017, methodology): “The E component was removed based on the revision of the thick-disk structure and its extrapolation towards the inner Galactic regions made by Robin et al. (2014), which effectively rendered the classical ellipsoid bulge unnecessary.”
Generalized massive optimal data compression (2017, background): “…as a viable approach to analyzing large and complex astronomical datasets (Schafer & Freeman 2012; Cameron & Pettitt 2012; Weyant et al. 2013; Robin et al. 2014; Lin & Kilbinger 2015; Akeret et al. 2015; Ishida et al. ⋆ E-mail: jalsing@flatironinstitute.org 2015; Jennings et al. 2016; Hahn…”
Measuring the vertical age structure of the Galactic disc using asteroseismology and SAGA (2015, background): “…with [Fe/H] > −1, implying that this overdensity is associated with disc stars, rather than the halo, and it could be the signature e.g. associated with the formation of the thick disc or enhanced star formation in the early Galaxy (cf. Haywood et al. 2013; Robin et al. 2014; Snaith et al. 2014).”
Nuisance hardened data compression for fast likelihood-free inference (2019, unknown): “…known effects that can be successfully simulated without relying on approximate likelihoods (Schafer & Freeman 2012; Cameron & Pettitt 2012; Weyant et al. 2013; Robin et al. 2014; Lin & Kilbinger 2015; Ishida et al. 2015; Akeret et al. 2015; Jennings et al. 2016; Hahn et al. 2017; Kacprzak et al. ?”