Stellar populations in the Milky Way halo

Transcription

1 Stellar populations in the Milky Way halo Paula Jofre Pfeil Max Planck Institute for Astrophysics (Achim Weiss, Ben Panter, Camilla Hansen)

2 Introduction Todayʼs Universe Scenario: Dark matter haloes connected by filaments contain the baryonic matter interact, merge, collapse to form the galaxies How much contribute the collapse, how much the mergers?

3 Introduction The Milky Way Galactic Halo Old component: reveals epoch of the MW formation How old are the field halo stars?

4 Introduction Stellar Ages Age using isochrones Globular Cluster A GC is a group of stars with same age, metallicity and distance One of the best ageestimates for old stellar systems. Still many uncertainties, i.e. 1) Color - Teff relations 2) Physics in the stellar evolution models 3) Initial conditions Chaboyer 2001 Meissner and Weiss 2006

5 Introduction Stellar Ages Isochrone ages 2Gyr 1Gyr 3Gyr 5Gyr Field Stars 10Gyr 15Gyr Field stars are alone and can have any age, metallicity or distance Ages for field distant stars are one of the most inaccurate quantities in astrophysics Still many uncertainties, i.e. 1) Atmosphere parameters 2) Physics in the stellar evolution models 3) Distance estimation Pont & Eyer 2004 Especially the halo!!!

6 The aim An age for the Galactic halo Sloan Digital Sky Survey provides large amounts of halo field stars We can use the turn-off stars to estimate ages (explain technique later!) 1) SDSS spectra allow to estimate atmosphere parameters independently from photometry 3) MPA stellar evolutionary tracks allow to study dependency of input physics in the final results 2) SDSS globular clusters allow to compare with field star properties to of the Galactic halo 4) Develop tools to deal with the next generation of big surveys of the Milky Way, such as Gaia or LAMOST

7 Ages without distance If the absolute magnitude is not known, the turn-off can be found by looking at the hottest stars The common TO between isochrone and field stars suggests an age of 11 Gyr for the youngest stars

8 Methodology Estimate atmosphere parameters from SDSS spectra (MAX) Find the turn-off temperature as a function of metallicity Find the turn-off isochrones and the ages calibrate results with globular clusters Conclude

9 MAX - Motivation Big Surveys Era For our purpose Tools to analyze them! Temperature and metallicity of ~140,000 SDSS spectra MAX : MAssive Compression of chi-square for stellar spectra (Jofre et al. 2010) Derivative of MOPED (Heavens et al. 2000, Panter et al. 2003) Makes fast and automatic estimates of stellar atmosphere parameters

10 MAX - Method Compression algorithm b vectors concentrate all the information about the parameters in a number with this numbers we generate a compress likelihood which behaves the same as the full likelihood at the peak Compression allows to scan the whole likelihood without time-consuming.

11 MAX - Method Compression algorithm b vectors concentrate all the information about the parameters in a number with these numbers we generate a compressed likelihood which behaves the same as the full likelihood at the peak Compression allows to scan the whole likelihood without time-consuming. Time needed for the calculation of likelihoods: compressed: ms full: hours

12 MAX - Application SDSS SDSS data: R = 2000, wave=[3850,9000] total = 10,000 TO stars Grid of models: 761,280 elements reproducing metal-poor F-G dwarfs ~Agreements with other independent techniques in the parameter estimation from SDSS spectra

13 MAX - Application UVES UVES data: R = 40, TO stars ~Agreements with other classical techniques in the parameter estimation from Hi-Res spectra

14 Yale Isochrones (Green, Demarque & King 1987) Paula Jofre - MPA Galactic halo turn-off Previous Results ʻ96 Unavane et al 1996 Isochrones: Bergbusch and Vandenberg (1992) 5Gyr 10Gyr 15Gyr 17Gyr Schuster et al 1996 Age of Universe from WMAP: 13.7 Gyr (Bennett et al 2003)

15 Galactic halo turn-off Previous Results ʻ06 Allende Prieto et al 2006 Isochrones: Girardi et al 2004 Schuster et al What means ʻbetter isochronesʼ?

16 Galactic halo turn-off Stellar Evolution: Atomic Diffusion Gravitational settling of heavy elements in stellar interiors accelerates the evolution during the main sequence The TO temperature of diffusive isochrones is lower than that of canonical models with the same age For a given TO temperature, the diffusive isochrone yields younger ages than the canonical one

17 Age determination Stellar Evolution: Atomic Diffusion Agreements with the 96ʼs 4 Gyr of relative difference in the ages!!! Agreements with the 06ʼs Garstec Isochrones: Weiss and Schattl 2008

18 Age-metallicity relation Galactic Evolution: One dominating population Because the halo stars cannot be older than the Universe, they have to be determined using diffusive isochrones Age of halo stars: Gyr No real trend of age as a function of [Fe/H] Rapid star formation scenario: collapse? YES: this is the inner halo

19 Globular clusters CHECK 1 - TO color as a function of metallicity SDSS photometry from 10 galactic GC (An et al. 2008, 2009) Field TO colors agree well with those of Globular Clusters

20 Globular clusters CHECK 2 - color distribution NGC5466 / [Fe/H] = SDSS photometry from 10 galactic GC (An et al. 2008, 2009)

21 Globular clusters CHECK 2 - color distribution NGC5466 / [Fe/H] = SDSS photometry from 10 galactic GC (An et al. 2008, 2009) In the distribution, same features seen for Field and GC

22 Globular clusters CHECK 2 - color distribution SDSS photometry from 10 galactic GL (An et al. 2008, 2009)

23 Globular clusters CHECK 2 - color distribution SDSS photometry from 10 galactic GL (An et al. 2008, 2009) 1.- We are finding the turn-off for the halo 2.- Cluster and field stars are dominated by one coeval population 3.- Ages of GC v/s Field?

24 Age determination Age metallicity relation - GC Our ages agree well with GC, considering different methods An et al Salaris & Weiss 2002 Dotter et al. 2010

25 Summary - Implications Galactic formation using [Fe/h]-Teff diagram

26 Summary - Implications Galactic formation using [Fe/h]-Teff diagram 1.- SDSS low resolution spectra provides metallicties and temperatures which can be used to understand the formation of the inner halo

27 Summary - Implications Galactic formation using [Fe/h]-Teff diagram 2.- the new and independent MAX method provides realistic atmosphere parameters

28 Summary - Implications Galactic formation using [Fe/h]-Teff diagram 3.- We ARE finding the turnoff the Galactic halo, independently of data set used

29 Summary - Implications Galactic formation using [Fe/h]-Teff diagram 5.- In the inner halo, field and clusters are equally old

30 Summary - Implications Galactic formation using [Fe/h]-Teff diagram 5.- Signatures of recent mergers in the inner halo as well, supporting current hierarchical formation scenarios Schuster et al sample