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

2. are often the brightest and isolated objects in a galaxy
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!
20th May 2004
ESO-Garching

3. Pagel 1981 said that metal abundance in external
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.
We have seen by Gronenewegen the number trend with luminosity and other parameters.
The only elliptical galaxy in the Local Group is M32.
20th May 2004
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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
Extent of the LMC “disk”? Center about 0.6 deg above the bar, same as HI.
The LMC is elliptical with major axis towards the galactic center (tidally shaped)
HI indicates distinct spiral features, AGB stars are much more uniform.
Tidal distorsion must apply to star and gas. Evolved stars are not tracing the same HI disk?
Orbit-crossing and dissipation made HI density to evolve non linearly? (Hypothesis by Staveley-Smith 2003)
1. AGB stars are easily distinguished as
the plume of objects above the TRGB (Cioni, van der Marel et al. 2000)
20th May 2004
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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)
If we assume that the reddening decreases radially with a step of (I-J)=0.025 in a range between 0.1 and 0.
In low metallicity environment O-rich stars move out of the M star temperature region
Darker regions correspond to a lower ratio.
The ratio decreases radially; it implies
a variation of 0.75 dex in [Fe/H].
20th May 2004
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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.
The metal abundance in external galaxies exibits radial gradients with larger metallicities in the center (Pagel 1981 and references therein).
Harris & Zaritsky 2003
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7. NGC6822 All stars detected in three wave bands: I, J and Ks
- Isolated Irr in the LG
- E(B-V)=
- 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/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= % at I=20, 50% I>20.5
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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
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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.
Thought this is an irregular galaxy it is quite isolated and might have been less perturbated than the MCs.
This might explain the correspondence between HI and C/M ratio.
Compare some findings by Davidge and others.
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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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
Add selected sample for FORS2 observations.
Show Ca II triplet from proposal and list calibrations between Ca II triplet and [Fe/H].
Mention that metallicity can be measured directly in the Magellanic Clouds. This one of the objective of an already started program: Ca II triplet observations for one FLAMES field taken!
Data from Gronewegen (2002)
20th May 2004
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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)
Low mass stars have heavier s-process elements.
+ 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)
20th May 2004
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