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Abundance gradient in Local Group galaxies using AGB stars

A study on the distribution of AGB stars in nearby galaxies and how they trace the intermediate age population. AGB stars are classified as O-rich and C-rich and can be observed beyond a distance of 1 Mpc.

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Abundance gradient in Local Group galaxies using AGB stars

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  1. Abundance gradient in Local Group galaxies using AGB stars M.-R. Cioni

  2. Why? • AGB stars trace the intermediate age population of galaxies (1-several Gyr) • are often the brightest and isolated objects in a galaxy • can be observed beyond a Mpc • they are mainly of two kinds: O-rich and C-rich Where? • Mostly in Irregular, Spiral and Elliptical galaxies in the LG (i.e. LMC=30000, SMC=8000, NGC6822=3000) • and in some Spheroidals and Dwarf Spheroidals More C stars in metal poor systems! ESO-Garching

  3. Observationally… Pagel 1981 said that metal abundance in external galaxies exhibits radial gradients. Searches for AGB stars in the solar neighbourhood (Mikami 1978), the MCs (Blanco et al. 1980) and Baade’s Window (Glass et al. 1999) have shown that C/M correlates with metallicity. Theoretically… C/M indicates metallicity because the giant branch shifts to warmer Temp. and less C atoms are needed to form C stars ( Scalo & Miller 1981, Renzini 1983) Studies of AGB stars in the MCs have strongly improved thanks to: DENIS, 2MASS, MACHO, EROS, MCPS, etc… Similar surveys are taking place of other local group galaxies. Especially in the near-IR and monitoring. ESO-Garching

  4. AGB distribution in the LMC (Cioni, Habing & Israel 2000) (DCMC data: Cioni et al. 2000) 2. AGB stars are smoothly and regularly distributed without clear signs of a spiral structure 1. AGB stars are easily distinguished as the plume of objects above the TRGB (Cioni, van der Marel et al. 2000) ESO-Garching

  5. C/M ratio in the LMC (Cioni & Habing 2003) - The combined (I,I-J) + (Ks,J-Ks) selection criteria this is a C/M0+ ratio - M stars are of early type in the outer ring or viceversa if diff. reddening is present Cole et al. 2003: “the bar is more metal rich than the inner disk” To the bridge, or Arm B of Staveley-Smith et al. (2003) Darker regions correspond to a lower ratio. The ratio decreases radially; it implies a variation of 0.75 dex in [Fe/H]. ESO-Garching

  6. The SMC AGB C/M AGB stars have a regular distribution. The C/M ratio corresponds to a variation of 0.75 dex in [Fe/H]; there is no clear radial trend. The ratio is low in the Wing. Cioni et al. 2000 Cioni & Habing 2003 Z=0.008 (red), Z=0.004 (green), Z=0.001 (blue) MCPS – SFH matching theoretical and observed photometry. Star formation activity at a given epoch. The external ring is metal rich because of gas rich mergers or inward propagation of SF. Harris & Zaritsky 2003 ESO-Garching

  7. - Isolated Irr in the LG • - E(B-V)=0.25-0.45 • - Much foreground because of latitude • Large HI envelope • [O/H] intermediate between MCs • First stars 10 Gyr old, increasing rate past 3 Gyr NGC6822 (Cioni & Habing 2004) AGB stars: C/M=551/2124=0.2594 TRGB detection I=19.60, J=18.23, Ks=17.15 All stars detected in three wave bands: I, J and Ks Completeness: about 100% in J&Ks, 95% I=19, 93% I=19.5 80% at I=20, 50% I>20.5 ESO-Garching

  8. NGC6822 All sources detected in IJKs bands from WHT with INGRID and PFIP (Cioni & Habing 2004) AGB RGB Young+Foreg. C/M 2 1 ESO-Garching

  9. NGC6822 All sources detected in J and Ks (Cioni & Habing 2004) Average [Fe/H]=-0.9 but with a spread between -2.0 and -0.3 dex from RGB stars Tolstoy et al. (2001) AGB: bar, tails… RGB: regular [Fe/H] variation of 1.83 dex! [Fe/H]=-1.0(0.3) Davidge (2003) from JKs obs. of 3 small regions. C/M: clumps in a semi-circle. At least some clumps correspond to HI high column density regions (Block & Walter 2000) ESO-Garching

  10. Calibration of C/M versus [Fe/H] NOTE: metallicities are about 0.2 uncertain and C/M could be 50% uncertain! How to improve it? Scheduled observations with FORS2 of about 300 AGB stars in NGC6822 (1/3 C-rich) to derive [Fe/H] via Ca II triplet observations Effect of SFH? Work in progress in collaboration with Girardi and Marigo: interpret the LF of AGB stars, both C and M type as a function of position in these galaxies Data from Gronewegen (2002) ESO-Garching

  11. Other Abundances work in progress… FLAMES Science Verification: LMC 14 AGB stars, 1.5h int., S/N=80, FLAMES+UVES at 580 and 860 nm (Cioni & Wood) (Hill & Primas) + Started and Re-submitted proposal: to measure in both MCs: metallic lines, Ca, Li, ZrO band head CN (5 settings + 2 epochs) GOALs: relations to other AGB properties (colour, L, P,…) (Romaniello) ESO-Garching

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