Evolved Stellar Populations as
tracers of galaxy properties
Maria-Rosa Cioni
Institute d’Astrophysique de Paris
Workshop: Optical and Infrared Widefield Astronomy in Antartica
Friday 16th June 2006
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.
Star formation history
how 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
Type of giant stars
 Red giant branch
(RGB) stars
 All stars with 0.4-10
MSun become RGBs
 Burn H in a shell
 Cool (bright in IR)
AGBs are more
extreme than RGBs.
 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)
Learning from AGB stars
 The number density
 The C/M ratio
 The Ks-band
magnitude
 Low- and highresolution spectra
 The structure
 The metallicity
[Fe/H]
 The star formation
history: mean-age
and [Fe/H]
 Dynamics and
chemistry of
different populations
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.
The structure of the LMC
Cioni,
Israel &
Habing
2000
 AGB stars are smoothly
distributed across the galaxy.
 They trace the orientation of
the galaxy in the sky.
Van der Marel & Cioni 2001
…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…
Calibrating C/M vs [Fe/H]
Observational correlatation:
solar neighbourhood,
Magellanic Clouds and
Baade’s Window
Battinelli & Demers 2005
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.
LMC
Metallicity
 C/M ratio map confirms
a metallcity gradient
across the LMC.
 [Fe/H]=0.75 dex.
Cioni & Habing 2003
 Metal-weak/metal rich
ratio estimated from the
location of RGB stars in
the near-IR diagram vs.
distance.
Alves 2004
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
Magellanic Clouds SFH status
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 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)
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.
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.
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).
LMC: how well the C/M ratio
indicates metallicity?
Z
C/M




age
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:
SMC
Metallicity 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….!
2 Gyr
3.9 Gyr
6.3 Gyr
8.7 Gyr
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.
M33
metallicity, 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.
NGC 6822
metallicity, mean-age & chi2
Cioni, 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.
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).
SFH: conclusions
 Interpreting the Ks-band distribution of AGB
stars allows to estimate variations in meanage 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.
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.
A pre-selected VISTA Public
Survey proposal - VMC
2MASS
10
VMC
10
 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
VMC survey: VISTA Magellanic Clouds survey
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.
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)
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.