Abundance gradient in Local Group galaxies using AGB stars

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Abundance gradient in Local Group galaxies using
AGB stars

• M.-R. Cioni

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• 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. LMC30000, SMC8000, NGC68223000)
• and in some Spheroidals and Dwarf Spheroidals
• More C stars in metal poor systems!

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• 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 Baades
  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.

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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)
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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.
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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
Z0.008 (red), Z0.004 (green), Z0.001 (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
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NGC6822

• - 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

(Cioni Habing 2004)
AGB stars C/M551/21240.2594
TRGB detection
I19.60, J18.23, Ks17.15
All stars detected in three wave bands I, J and
Ks
Completeness about 100 in JKs, 95 I19, 93
I19.5 80 at I20, 50 Igt20.5
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NGC6822
All sources detected in IJKs bands from WHT with
INGRID and PFIP
(Cioni Habing 2004)
AGB
RGB
YoungForeg.
C/M
2
1
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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)
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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)
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Other Abundances
work in progress
FLAMES Science Verification LMC 14 AGB stars,
1.5h int., S/N80, FLAMESUVES 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)