Transcript Marcolini et al 2006, MNRAS 371, 643 Draco

The chemical evolution of the
peculiar “Globular Cluster”
Omega Centauri
Andrea Marcolini (Uclan, Central Lancashire)
Antonio Sollima
(Bologna University)
Annibale D’Ercole
(Bologna Observatory)
Brad Gibson
(Uclan)
Francesco Ferraro (Bologna University)
Fabrizio Brighenti
(Bologna University)
Simone Recchi
(Trieste Observatory)
Dwarf spheroidals galaxies of the local group were
originally thought to be very similar in their
metallicity and star formation histories to the galactic
globular clusters, but their star formation history is
now known to be much more complex.
Relative SFR
Draco
Relative SFR
Star Formation History in Dwarf Spheroidal Galaxies:
Time (Gyr)
Relative SFR
Relative SFR
Time (Gyr)
Sculptor
Time (Gyr)
Time (Gyr)
Mateo 1998 (Review)
“The small dynamical mass of dSphs means that their
binding energy is small compared to the energy released
by several SNe, which leads the high metallicity spread
and relatively high mean metallicity derived for these
galaxies puzzling: how did the gas stay bound enough to
have an extended star formation and gas enrichment?”
Babusiaux, Gilmore & Irwin 2005
“It is not currently understood how low-mass dSphs
managed to hold on to enoigh gas to form stars over an
extended period of time.”
Simon & Geha 2007
Goals of the simulations
 Find a galaxy model and a SFH able to reproduce
the observed stellar content in a consistent way
(e.g. without ejecting the whole ISM too soon).
 Reproduce the range of the observed metallicity
and chemical properties of dSphs.
Marcolini et al 2006, MNRAS 371, 643
Assumptions for the dSph Model:
Star
formation
history
Stellar
Gas
Component
:
Type
Type
Ia
IIComponent:
Supernovae:
Dark
Matter
Halo:
King
Mgas
Model
=Msol
0.18*Mdark,
following
Peterson
in stars,
hydrostatic
& uniformally
Caldwell
equilibrium
1992
and
with
Mateo
T=Tvir
1 SN
IIThe
every
same
100
as
before
of formed
but
following
Matteucci
&distributed
Recchi
2001
in time
in1998
for 30 Myr
We
choose
several
sequences
of
instantaneous
bursts
differing
intime.
number
Modified isothermal Halo obtained following Burkert 1995 so that M/Lv
agrees
afterand
eachintensity
istantaneous
burst.
Stochastically
distributed
in
space
proportionally
such a way that
stellar mass
formed
after 3 Gyr is
withinobservations
(e.g.the
Peterson
& Caldwell
1992)
toalways
the stellar
density.
the same.
Draco:
M
 80
Lv
M tot
 220
Lv
N burst  50
N SNII  112
Ebind  831051 erg
t sleep  60 Myr
Marcolini et al 2006, MNRAS 371, 643
If the explosion of a
SNIa occurres during
the re-collapse phase
its ejecta results to be
much more localized.
At the beginning of the
simulation there is a high
spread in metallicity, while
at later times the SNII
ejecta becomes more
uniform.
ISM density
SNII ejecta density
SNIa ejecta density
Inside dark matter halo
Inside stellar region
SNII
60 % inside the
galaxy region
15 % inside the star
forming region (~600
pc)
SNIa
70 % inside the
galaxy region
17 % inside the star
forming region (~600
pc)
{
{
SNIa
SNII
SNII
Shetrone et al. 2003
M ej  1.4 M 
M ej , Fe  0.74 M 
M ej ,O  0.15M 
M Z  1 .4 M 
M ej  10 M 
M ej , Fe  0.07 M 
M ej ,O  1.0 M 
M Z  3 .0 M 
SNIa
Inhomogeneous!!
homogeneous
Draco, Fornax, Leo I, Sextans, Sculptor and Ursa Minor
Marcolini et al in preparation (Maybe)
ISM
SNII
ejecta
SNIa
ejecta
How is the metal
enrichment in the central
region of the dSph?
Time~1Gyr
=Star with
[alpha/Fe]<0.2
7%
30%
W Cen!!!
Strong inhomogeneous
pollution by SNIa
Time~1Gyr
=Star with
[alpha/Fe] <0.2
SNe Ia produce a lot of Fe
but expel relatively few
metals (few percent)
compared to the total
number of SNe II.
log( Z / Z  )  0.42  [ Fe / H ]
The Z is not proportional to the
[Fe/H] content of the star
Z  0.015 10
[ Fe / H ]
[ / Fe]
10
[1  0.08(2.82 10
[ / Fe]
 1)]
Marcolini et al 2007 astroph-07083445
MNRAS accepted
Did you miss it?
Consistent with a model in which the ancient dwarf galaxy lost
most but not all of its gas with the first interaction with the
Milky Way and its Halo (e.g. Mayer et al 2006).
Simulated and observed [Fe/H] and [Ca/H]
distributions.
Sollima et al 2005
Norris et al. 1996
[Fe/H]=-1.3
[Fe/H]=-0.6
Ref: Francois et al 1988; Brown & Wallerstein 1993; Norris & Da Costa 1995; Smith
et al 1995 and 2000; Pancino et al 2002, Vanture et al. 2002; Origlia et al. 2003;
Villanova et al. 2007
Age-metallicity relation
Isochrones:
Cassisi et al 2004
Pietrinferni et al. 2006
Extra Helium??
AGB pollution is
fundamental!!!
Main sequence
[Fe/H]=-1.3
[Fe/H]=-1.7
The need for Helium
enrichment is
reduced of 40%.
Z
Conclusion:
dSph
nucleus
Conclusion: Open problem and
critical points:
dSph:
• At the end of the simulation (3 Gyr) quite all the gas is still inside the dark
matter potential well, we need a mechanism to remove it: ram pressure
stripping (e.g. Marcolini at al 2003) plus tidal interaction (Mayer et al 2006).
We need more models to take such an interaction into account.
Omega Cen:
• One of the strong constrain of the model is the presence of a non
negligible amount of alpha-depleted stars. Even if this is consistent with
preliminary results (which I did NOT show you), these results must be
confirmed.
• Even if the spread of the SGB-a is quite satisfactory its morphology is not.
• We fail in reproducing the double main sequence even if due to the
inhomogeneous pollution the enigmatic extra Helium required is halved.
• We need more models taking the AGB pollution into account (plus MW
interaction .... again)
Preliminary results form
Pancino et al. (in preparation).
Conclusions:
• Under the hypothesis that Omega Cen is the survivor nucleus of a dSph we
are able to reproduce the main features of this peculiar Globular Cluster.
• To fit better the observation we must assume that the SFH is constant till
1Gyr after that it drops to 0. after further 600Myr due to the interaction with
the Milky Way. The total SF lasted ~1.6 Gyr.
• With this SFH, we are able to reproduce the [Fe/H] and [Ca/H] distribution as
well as the general trend observed in the [alpha/Fe]-[Fe/H] diagram. In both
these diagram the peculiar features are due to the inhomogeneous pollution
of SNe Ia. Due to this inhomogeneous pollution the metal mass fraction
content of the stars is not proportional to the Iron.
• The general properties of CMD diagram are reproduce quite satisfactory as
well as the peculiar SGB-a and RGB-a, even if the morphology of the SGB-a
is not satisfactory.
• We fail in reproducing the double main sequence but in our model the He
content to reproduce it is halved.
Brown et al.
Small Iron Gradient