Exploring the structure and evolu4on of the Milky Way disk

Transcription

1 Exploring the structure and evolu4on of the Milky Way disk Results from the Gaia-ESO survey and plans for 4MOST Thomas Bensby Dept. of Astronomy and Theore3cal Physics Lund University Sweden

2 Chemistry of the Solar neighbourhood Bensby et al. (2014, A&A, 562, A71) 712 F and G dwarf stars in the Solar neighbourhood A clear dichotomy: An old and alphaenhanced population Less alphaenhanced young population Similar dichotomy seen in many other Solar neighbourhood studies, e.g., Bensby+2003,2004,2005,2006,2007, Reddy+2003,2006, Adibekyan+2012, Fuhrmann 1998,2001,2004,2008,2011, and others..

3 The Milky Way as a benchmark galaxy Why does the Milky Way have two disk populations? Need to characterize them in terms of velocities abundances ages? Not only in the solar neighbourhood, but throughout the Milky Way galaxy?

4 A bit further away (red giants) Inner disk 4 < R g < 7 kpc Solar neighbourhood Outer disk 9 < R g < 13 kpc Bensby, Alves-Brito, Oey, Yong, & Melendez, 2010, A&A, 516, L13 Alves-Brito et al. (2010) No alpha-enhanced stars in the outer disk Bensby, Alves-Brito, Oey, Yong, & Melendez, 2011, ApJ, 735, L46 => Short scale-length for the thick disk! See also, e.g., Cheng et al. (2012), Bovy et al. (2012)

5 Further away and larger samples - APOGEE No alpha-enhanced stars! Abundance gradient in the thin disk Hayden et al. (2015), based on red giants from APOGEE DR12

6 Scale-lengths in external galaxies Comeron et al. (2012, ApJ, 759, 98) Luminosity profile fitting NGC 891 Thick disk scale-length Thin disk scale-length (kpc) Thick disk scale-lengths are longer than thin disk scale-lengths!

7 Different ways of defining the thick disk Kinematics? Chemistry? Age? Morphology?

8 Kinema4cs Kinematically Thin Kinematically Thick Red colours: >9 Gyr Blue colours: <7 Gyr Fig. 20. Left-hand side plots show the [Fe/Ti] versus [Ti/H] abundance trends when using the kinematical criteria as defined in Bensby et al. (2003, 2005). StarsThin have been colour- and size-coded Thick depending on their ages. Right-hand side plots shows the abundance trends when splitting the sample according to their ages (as indicated). For all plots we have only included stars whose ages have been better determined than 4 Gyr (difference between upper and lower age estimates). 714 nearby dwarfs from Bensby et al, (2014) the stars that are at least ten times more likely to be thick disk Two well-defined, but not perfectly clear trends α-enhanced, as well as kinematically hot stars that are young and

9 Kinema4cs vs Ages Ages seem to better discriminator between thin and thick disk, but Fig. 20. Left-hand side plots show the [Fe/Ti] versus [Ti/H] abundance trends when using the kinematical criteria as defined in Bensby et al. (2003, 2005). Stars have been colour- and size-coded depending on their ages. Right-hand side plots shows the abundance trends when splitting the sample according to ages their ages are (as indicated). rarely For available all plots we have and only included very stars difficult whose agesto have been better determined than 4 Gyr (difference between upper and lower age estimates). 714 nearby dwarfs from Bensby et al, (2014) the stars that are at least ten times more likely to be thick disk α-enhanced, as well as kinematically hot stars that are young and

10 The Gaia-ESO Survey Gilmore et al. (2012, The Messenger, 147, 25) Randich et al. (2013, The Messenger, 154, 47) >400 collaborators First light, New Year s Eve, Dec 31, nights over 5 years with FLAMES on VLT stars, thin/thick disk, halo, bulge, open clusters, globular clusters, and more.

11 Positions First 30 months of observations Stellar parameters for FLAMES- GIRAFFE Milky Way field stars

12 GESiDr4 The known thin and thick disk abundance patterns present

13 Chemistry - GESiDr4, solar cylinder R=1 kpc [Mg/Fe]-rich [Mg/Fe]-poor 3.3 The inner and outer re Toomre diagram: Figure 8 shows the [Mg/Fe]-[F divided into nine di erent R region (R gal < 7 kpc), the sol and the outer disc region (R ga split into three subregions wit from the Galactic plane. To gui over-plotted fiducial lines repr for the thin and thick discs in t that the chemical structures in very di erent, with -rich sta lacking in the outer region th more metal-rich and -poor p seen in Bensby et al. (2011) th disc giants. They concluded th outer disc, a property that has for the thick disc, could be ex shorter scale-length, and the t length, than previously thoug disc was later confirmed by C SEGUE stars, by Bovy & Rix dwarf stars, and recently Bovy Abundance criterion produces kinematical samples that are consistent with what we currently know about the thin and thick disks in the solar neighbourhood: * alpha-rich disk lagging the alpha-poor disk by some ~40 km/s * alpha-rich being kinematically hotter

14 A full three-dimensional mapping of Milky Way stellar disc is currently n of future spectroscopic surveys as for There has been some indications from stars located at constant galactocentri Galactic plane, show a variation with Dashed et al., in line: prep.), making it possible to Fraction of thick-to-thin disk the stars Milky using Way. a Our 10% stars are not loc normalisation some distancesin out the of it. plane, So, for now w and structure 300 pc of and the 1000 Milky pc Way disc is ho scale-heights locations at given for the galactocentric thin radii and thick disks, respectively. Chemistry - GESiDr4, solar cylinder R=1 kpc Thick disk (high alpha): red points Thin disk (low alpha): blue points Green 3.5 line: Thin/thick disc fraction at di Observed fraction of thickto-thin The radial disk extent stars, ofusing the thick disc alpha-enhancement as selection criterion 4 THIN AND THICK DISC SCA Figure 7. The variation of [Mg/Fe] as a function of distance from the plane. 5 DISCUSSION

15 [Mg/Fe] at different Z and R gal >1 kpc above the plane kpc above the plane <0.5 kpc above the plane

16 [Mg/Fe] at different Z and R gal >1 kpc above the plane kpc above the plane <0.5 kpc above the plane

17 [Mg/Fe] at different Z and R gal Lack of alpha-enhanced stars in the outer disk (as in e.g. Bensby+2011, Hayden+2015)

18 Gaia-ESO stellar ages Gaia-ESO targets turnoff region => possible to determine stellar ages However, distances and surface gravities have significant uncertainties Gaia (DR2)!

19 4MOST de Jong, et al. 4MOST: the 4-metre Mul3-Object Spectroscopic Telescope project at preliminary design review, Proc. SPIE 9908 (2016) fibres (1600 LR & 800 HR) First light 2021/ years high-resolution spectra for more than 2 million stars ESO VISTA 4 meter telescope on Paranal

20 4

21

22 Summary Radial abundance gradient in the thin disk Short scale-length for the Galactic thick disk, but. long scale-lengths in external galaxies Gaia, in combination the large spectroscopic surveys, will allow us to explore the thin and thick disks in terms of ages - kinematics - chemistry, throughout the Milky Way.