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Evolved Stellar Populations as tracers of galaxy properties

Evolved Stellar Populations as tracers of galaxy properties. Maria-Rosa Cioni Institute d’Astrophysique de Paris Workshop: Optical and Infrared Wide-field Astronomy in Antartica Friday 16 th June 2006. Introduction.

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Evolved Stellar Populations as tracers of galaxy properties

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  1. Evolved Stellar Populations as tracers of galaxy properties Maria-Rosa Cioni Institute d’Astrophysique de Paris Workshop: Optical and Infrared Wide-field Astronomy in Antartica Friday 16th June 2006

  2. Introduction • To understand the history of formation and evolution of galaxies we need to understand the distribution of age, chemical abundance and kinematics of both the stars and gas. • The best laboratories for these studies are galaxies of the Local Group and in particular the Magellanic Clouds (nearby, known distance, low extinction, …interacting irregular galaxies). • Distinguish between cluster and field stars. • A global picture of any LGG is lacking.

  3. Star formation historyhow is it measured? • Age • Stars in a specific evolutionary phase. • Fit of the main-sequence turn-off. • Metallicity • Metallic lines from stars and nebula • Ca II triplet • Fit of isochrones/cluster tracks to a CMD • C/M ratio • Kinematics • Atomic and molecular lines

  4. Red giant branch (RGB) stars All stars with 0.4-10 MSun become RGBs Burn H in a shell Cool (bright in IR) Asymptotic giant branch (AGB) stars All stars with 0.8-8 MSun become AGBs Burn H and He alternatively in shells Produce heavy elements (s-process) Pulsate & Loose Mass Cool (brighter in IR) Type of giant stars AGBs are more extreme than RGBs.

  5. The number density The C/M ratio The Ks-band magnitude Low- and high-resolution spectra The structure The metallicity [Fe/H] The star formation history: mean-age and [Fe/H] Dynamics and chemistry of different populations Learning from AGB stars

  6. Selecting AGB stars Cioni, Habing & Israel 2000 Cioni et al. 2006 O-rich: DENIS 2MASS C-rich: 2MASS DENIS • AGB stars are brighter than the tip of the RGB and bluer than younger and foreground objects. • C-rich are redder than O-rich AGB stars • Both criteria exclude upper-RGB stars.

  7. The structure of the LMC Cioni, Israel & Habing 2000 Van der Marel & Cioni 2001 • AGB stars are smoothly distributed across the galaxy. • They trace the orientation of the galaxy in the sky.

  8. …there are also sub-structures • Discovery of an LMC star at 220 from the centre. • Milky Way streams have been discovered using RGB stars (photometry + kinematics). • Sub-structures tracing the dynamical history exists around M31 and other LG galaxies. • Many galaxies have also extended halos. • What is the mechanism that formed giant LG galaxies like the MW and M31? • Accreting satellites? (follow the streams…) • (Chemistry) but not those surviving…

  9. Calibrating C/M vs [Fe/H] Observational correlatation: solar neighbourhood, Magellanic Clouds and Baade’s Window Theoretical correlation: in a metal poor environment - the giant branch shifts to warmer T (less M giants), - less C atoms are needed to form C stars. Battinelli & Demers 2005

  10. LMCMetallicity • C/M ratio map confirms a metallcity gradient across the LMC. • [Fe/H]=0.75 dex. • Metal-weak/metal rich ratio estimated from the location of RGB stars in the near-IR diagram vs. distance. Cioni & Habing 2003 Alves 2004

  11. M33 Cioni, et al., in prep. • C/M ratio map • C/M ratio as a function of distance: - the galaxy is surrounded by metal poor material - at large radii flattening of rotation induces gas outflows (higher Z). Rowe et al. 2005

  12. LMC The dominant stellar population is >2.5 Gyr. The bar is 1-3 Gyr old but stars of 4-8 Gyr are also present. Bursts are associted to the interaction with SMC and MW. SF propagates in the bar from SE to SW. SMC The average stellar population is older and more metal poor. Old stars occupy outer regions and young stars the wing. Half of the stars are >8.4 Gyr; recent bursts; intermediate-age ring (Harris & Zaritsky 2004) Magellanic Clouds SFH status

  13. Magellanic Clouds data • The MCs are an example of interacting Irr galaxies like others in the Universe. • Previous results were obtained from spatially limited regions in the outer/inner disk and in the bar (i.e. using HST). No global picture! • Wide-field observations: • Near-IR DENIS & 2MASS (one step further)

  14. Ks-band method Cioni et al. 2006 • The magnitude distribution of C-rich and O-rich AGB stars as a function of postion in the galaxy is interpreted using stellar evolutionary models. • The selection criteria is homogeneous.

  15. Constructing the theoretical Ks-band AGB distribution • TRILEGAL code: simulates stars according to a SFR, AMR and IMF. • L, Teff, g are interpolated among stellar evolutionary tracks from: • Bertelli et al. 1994 for massive stars • Girardi et al. 2000 for low- and intermediate-mass stars • Marigo et al. 1999 for thermal pulsing AGB stars • Using bolometric tables to derive magnitudes and including photometric errors.

  16. LMC spatial distribution of metallicity, mean-age and chi2 P>80% • Metallicity is high towards the MW and low towards the SMC. Average is Z=0.006. • Average (all stars) mean-age is 5-6 Gyr. • The resolution is 2-5 deg2 (size of sectors).

  17. LMC: how well the C/M ratio indicates metallicity? Z • These maps are corrected for the LMC orientation. • East is younger and metal richer than West. • The C/M ratio is a robust indicator of metallicity. • Regions of low probability: C/M age

  18. SMCMetallicity distribution as a function of mean-age 10.6 Gyr The region of high metallicity moves clockwise along a ring with increasing mean-age of the underlying stellar population….! 6.3 Gyr 8.7 Gyr 3.9 Gyr 2 Gyr

  19. Other Local Group galaxies • M33: bright Spiral • Nucleus, disk, halo and no bulge… • Many giants and abundance gradients. • NGC 6822: isolated magellanic type Irr • Bar embedded in a large HI envelope and in a large decouple C star spheroid. • Present low SFR. Contains RR Lyrae & LPVs. • SagDIG: distant dwarf Irr • Metal poor galaxy with signs of extended SF.

  20. M33metallicity, mean-age & chi2 Cioni, Irwin, Ferguson et al., in prep. P>99% • In the centre the metallicity is low (Z<0.001) compared to a ring around it (Z>0.002). • The stellar population is on average 7-8 Gyr old. • Map resolution of 3-13 arcmin2. • No correction for rotation yet.

  21. NGC 6822metallicity, mean-age & chi2Cioni, Stock, Girardi, Marigo & Habing, in prep. P>90% • The population is on average 8.5 Gyr old. • The metallicity is high SE (Z>0.008), spiraling inwards and low (Z<0.005) in other places. • Map resolution of 19-164 arcmin2. • No correction for orientation yet.

  22. Gullieuszik, Rejkuba, Cioni, Held, in prep. SagDIG Z=0.0005 filled triangles Z=0.001 empty triangles Z=0.004 filled squares Z=0.008 filled squares • C stars are not many but their number trace sufficiently well the Ks-band distribution. • The population is on average young (4 Gyr) and metal poor (Z=0.0005 at least).

  23. SFH: conclusions • Interpreting the Ks-band distribution of AGB stars allows to estimate variations in mean-age and metallicity across stellar populations. • Modest but complete samples produce equally satisfactorily results. • This technique can be applied to more distant systems resolved into stars.

  24. What is missing? • Absolute values of age and Z • Kinematics and detailed chemistry • Effect of interaction (intrinsic versus extrinsic star formation) • 3D picture • VISTA and AAOmega are promising instruments to complete the picture of nearby galaxies and in particular of the Magellanic Cloud system.

  25. A pre-selected VISTA Public Survey proposal - VMC • PI: Cioni • Area:LMC + SMC + Bridge + Stream (a few tiles) • Filters: YJKs • Gross Time: 21h-Ks and 4h-Y&J / tile • Aims: - Spatially resolved SFH - 3D picture 2MASS 10 VMC 10 VMC survey: VISTA Magellanic Clouds survey

  26. Variability issues • Deep Ks-band observations require multiple exposure that if appropriately placed may provide period and amplitude of variable stars: RR Lyrae, Cepheids, LPVs. • Advantages: • Period-magnitude relations involving Ks have a much smaller scatter: can be used for 3D pics. • Combining variations in different bands approaches the study of bolometric variations.

  27. Dome C observations of Magellanic Cloud giants • Favourable RA and DEC. • Large and diverse AGB sample. • Mass-loss vs. metallicity/age/location • Mid-IR (>Ks) monitoring (bolometric variation/evolution) • Polarimetry probing envelope shapes • High spatial resolution (deeper & sharper images) • Wide-area (statistics) • 2.4m telescope is better (Ks saturation)

  28. How far can AGB stars be observed from Antarctica? • Ks=25.8 allow us to well detect AGB stars in the Fornax cluster of galaxies if they are sufficiently isolated. • A resolution of 0.2”/pix allow us to characterise the stellar population of galaxies within 5 Mpc (including Cen A) • Monitoring is clearly an advantage vs. larger telescopes observing plans.

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