T.P. Idiart and J.A. de Freitas Pacheco Universidade de São Paulo (Brasil)
description
Transcript of T.P. Idiart and J.A. de Freitas Pacheco Universidade de São Paulo (Brasil)
T.P. Idiart and J.A. de Freitas Pacheco
Universidade de São Paulo (Brasil)Observatoire de la Côte d’Azur (France)
Introduction
Elliptical galaxies are usually think as old, complex stellar systems, whose the stellar population bulk was formed from an
interstellar medium enriched in heavier elements produced mainly by massive stars ( 9M). The chemical clocks used to date
the stellar population are basically the yields of type II and Ia supernovae. Type II supernovae (SNII) ejecta are rich in
elements like Mg, Ca, Ti, O and Si, whereas type Ia supernovae (SNIa) contribute to enrichment of the interstellar medium
mainly in the elements of the iron peak. The high iron content and the mean non-solar [/Fe] ratio in E galaxies indicate that
most of the stars were formed very rapidly.
We developed a multi-population model taking into account stellar evolutionary tracks with non-solar [/Fe] ratios. These
produce a more adequate scaling between -elements and iron abundances, mainly at high metallicities. In our scenario is
included the possibility of a two phase interstellar medium produced by SN explosions. This model is applied to study the
chemical and photometrical evolution of E galaxies. Our picture provides a natural way to stop the star formation after the wind
onset, when the remaining gas is only in the hot and ionized phase.
A two-phase one-zone model with wind outflow Main hypotheses: the interstellar medium contains hot (T 50000 K) and cold gas hot gas mass ≈ ionized gas mass the gas is heated by type II and Ia explosions winds are originated from the SN events stars are not formed in hot gas conditions
The model
Evolution of the Elliptical Interstellar Gas
)()()(
dt
)(df G twinddt
tdf
dt
tdft EJECTSTAR
conservation of total gas mass
Basic equations
)())(1()(
tftfkdt
tdfGHOT
STAR
fraction of formed stars fstar
dmtftfkmMmdt
tdfmGmHOTR
eject )())(1()()()(
fraction of ejected gas mass feject
W
GHOT
t
tftftwind
)()()(
dt
df
f
ftftfC
tf
tCt G
G
HOTHOTG
G
SN
)()()(
)(
dt
)(df 22
1HOT fraction of hot (ionized) gas
fG(t) = total gas mass fractionfHOT(t) = hot gas mass fraction(m) = initial mass functionMR = mass of remanent stark = star formation efficiencytW = time parameter of the wind
C1 = function of ionizing energyC2 = function of physical properties of the gas (hot gas)SN = supernova frequency
Database for stellar population synthesis of E galaxies
• The evolutionary tracks for -enhanced stars
Metallicity References
Z=0.0001 ([Fe/H]=-2.66) Girardi et al.(2000)
Z=0.0004([Fe/H]=-2.01) Girardi et al. (2000)
Z=0.008 ([Fe/H]=-0.75) Salanich et al. (2000)
Z=0.019 ([Fe/H]=-0.34) Salanich et al. (2000)
Z=0.040 ([Fe/H]=+0.07) Salanich et al. (2000)
Z=0.070 ([Fe/H]=+0.36) Salanich et al. (2000)
The conversion from Z to [Fe/H] for an -enhanced mixture of +0.4 can be estimated directly from the expression by Salaris et al. (1993), using the correction by Yong-Cheol et al. (2002) for higher values of metallicity. This relation is scaled to the solar abundances by Anders & Grevesse (1989), which are compatible with the abundance catalog by Thévenin (1998).
• Lick spectroscopic indices for -enhanced stars
As a stellar atmospheric database, we used Thévenin’s (1998) catalog, which has a set of homogeneous parameters: effective temperature, surface gravity and chemical abundances of various nuclear species determined by a detailed spectral analysis. The -elements considered for non-solar [/Fe] classifications were: O, Mg, Si, Ca and Ti. At least three different elements were used to classify stars with an -enhanced abundance pattern ([element/Fe] 0.2). In these conditions a total of 62 stars with observed spectroscopic indices were selected. The figure shows the relationship between atmospheric parameters and spectroscopic indices for the selected -enhanced stars. For comparison, a sample of solar [/Fe] ([element/Fe] < 0.1) stars are overplotted.
Calibration of the free parameters: star formation efficiency and initial mass function using the MV × (U-V) diagram for Virgo-Coma ellipticals
Fiducial models at zero redshift
InitialE
massM
K(Gyrs-
1)
Meanpop age
(Gyrs)
mean[Fe/H]
mean[Mg/Fe]
U-V Mg2
(mag)
H(Å)
2×101
2
0.80 1.89
±0.13
13.45±0.07
+0.24±0.12
+0.53±0.06
1.67
±0.06
0.313
±0.011
1.63
±0.02
7×101
1
0.53 2.00
±0.08
12.91±0.02
+0.19±0.08
+0.47±0.04
1.59
±0.05
0.301
±0.010
1.66
±0.02
2×101
1
0.40 2.11
±0.04
12.41±0.01
+0.12±0.04
+0.41±0.02
1.47
±0.05
0.284
±0.006
1.70
±0.01
5×101
0
0.40 2.32
±0.05
12.48±0.00
-0.10±0.06
+0.27±0.04
1.32
±0.04
0.251
±0.009
1.79
±0.02
2×101
0
0.40 2.42
±0.07
12.53±0.05
-0.21±0.07
+0.18±0.06
1.24
±0.06
0.236
±0.009
1.84
±0.04
K = star formation efficiency = coefficient of IMF
These models show that the most massive galaxies have higher star formation efficiency, flatter IMF, higher mean stellar population ages, more metallic stars and higher non-solar [/Fe] ratio. The predicted observed stellar absorption indices Mg2 and H at z=0 are also shown.
RESULTS
This figure shows the evolution of the hot and
cold gas for galaxies of distinct initial masses
MGAL. The ordinate shows the mass fraction of hot
and cold gas in units of MGAL, and the abscissa the
time evolution in units of age of the galaxy UNIV.
Note that only the cold gas component is
available to form stars, implying that the vast
majority of stars is already formed in early times
of galaxy evolution.
The high efficiency of star formation and the
existence of a hot gas component limits
drastically the number of stars formed more
recently. This is crucial to achieve the MV × U-V
relation for E galaxies, because the U-V color is
very sensible to the existence of younger
population of stars.
As show in histograms, more than 90% of the stars
are formed in the first 4 Gyrs of E galaxy evolution.
The number of younger stars increases as the
initial galaxy mass MGAL decreases. This happens
because a lower star formation efficiency implies
in a lower quantity of hot gas and hence an
extented star formation period.
1 3 5 7 9 11 13 15
1%
19%79%
MGAL = 2x1012
M
Fra
ctio
n o
f fo
rmed
sta
rs
time (Gyrs)
1 3 5 7 9 11 13 15
3%
25%70%
MGAL
= 2x1010
M
time (Gyrs)
Fra
ctio
n o
f fo
rmed
sta
rs
Model predictions for different redshifts
The stellar population at zero redshift
2.0 2.1 2.2 2.3 2.41.6
1.8
2.0
2.2
2.4
2.6
2.8
3.0
3.2
<F
e> A
log (km/s)
The graphs show the behavior of Lick indices
as a function of the central velocity dispersion
(a mass indicator) for different redshifts.
According to the parameters of the fiducial
models, the indices show significant
variations in this redshifts range.
Project supported by IAG/USP, FAPESP and CNPq
2.0 2.1 2.2 2.3 2.41.5
1.6
1.7
1.8
1.9
2.0
2.1
2.2
2.3
2.4
H
(A)
log (km/s)