Download AS2001 - University of St Andrews

Lecture 11:
Age and Metalicity
from Observations
Z
“Closed Box” model with constant yield:

Z(t)    ln (t)
metalicity
yield
gas fraction
Z(t)   as t  

Ignores:

1

1.
IGM--ISM exchanges: IGM falls into galaxy
ISM blown out of galaxy
2.
SN Ia, stellar winds, PNe, novae, etc.
3.
Initial enrichment by e.g. Pop.III stars prior to galaxy formation?
4.
Faster enrichment (more SNe) in high-density regions of galaxy.

 

Z
Z(t)    ln (t)



1
(t)  e
Z
Z(t)   t /t


1
t / t
t


Ellipticals: 

t

M0
t 
Ý
M
(t) 1 t /t



t 

Spirals:
 

t
1
t t
Z
Z(t)    ln 1 t /t 


t
t

Z
Z(t)    ln (t)



(t)  1 t /t

M0
t  f
Ýburst
M
f 
1

1

1
Irregulars:
 

Z
Z(t)   ln 1 t /t 


t
t
t
t



Age Estimates
Main sequence lifetime:
HR diagram
1
 MS
 MS
M L 
 7 10    yr
M. L . 
L
9
giants
Lmax
 43
L 
 7 10  
L .
9
yr
(since L  M 4  M  L1/ 4 )
Main sequence
B-V
Lmax (top of main sequence)
gives age t.
Abundance Measurements
•
•
•
•
Star spectra: absorption lines
Gas spectra: emission lines
Galaxy spectra: both
Metal-rich/poor stars: stronger/weaker metal lines
.
relative to H.
HII region spectra
Stellar spectra
• Lab measurements: Unique line signature for each element.
High-Resolution Spectra
Abundance Measurements

Spectra
Line strengths (equivalent widths)
+
Astrophysics

Stellar atmosphere models
+
Physics

Laboratory calibrations

 Fe

 H

, etc.


(Full details of this process are part of other courses)

Bracket Notation
Bracket notation for Fe abundance of a star relative to the Sun:
 Fe

 H
n(Fe) 
n(Fe) 

 log 10
  log 10



 n(H) 
 n(H) .
atoms of Fe
atoms of H
 n(Fe) n(H) 
 
 log 10
n(Fe) n(H) . 


And similarly for other metals, e.g. relative to Fe:

 O

 Fe
  C 
,
, ...

 
 Fe 



Star with solar Fe abundance:  Fe  0.0
 H 

 Fe 
 log 10 2  0.3
Twice solar abundance: 
 H 

 Fe 

Half solar abundance:
 log 10 ?  ?

 H 

[Xi/Fe] vs [Fe/H]
Metallicity
Metalicity (by mass):
f n(Mg)
Z
n(H)  4 n(He)  f n(Mg)
X
vs Abundance
Abundance (by number):
 Mg

 H
n(Mg) 
n(Mg) 

 log 10
  log 10



 n(H) *
 n(H) .
n(H)
n(H)  4 n(He)  f n(Mg)
 Z . 
 Z 

 log 10
 log 10


f
X
.
 f X 


Z
n(Mg)

f X n(H)
Z X . 

 log 10
Z . X 

 

(where f = some factor)
Z X

10 Mg
X.
 Z .
H

10 Mg
H

Primordial:
Xp = 0.75,
Yp = 0.25,
Zp = 0.00
Solar:
X . = 0.70,
Y . = 0.28,
Z . = 0.02
.
Key Observational Results
Z log()
1.
More gas used --> higher metallicity.

Z 1/distance from galaxy centre
2.
More metals near centres of galaxies.
Infall of IGM on outskirts + gas migrates to centre.
More star generations -->  lower at centre.

Z  L0B.3
3.
Small (low luminosity) galaxies have lower metallicities.
•
Dwarf irregulars: form later (young galaxies),  high
•
Dwarf ellipticals: SN ejecta leave the galaxy,  low
Less Metals in Small Galaxies
faint
---->
bright
More metals near Galaxy Centres
Ellipticals
(NGC 3115)
Spirals
(M100)
Review of Course
• Main events in the evolution of the Universe:
–
–
–
–
–
–
The Big Bang (caused by ???)
Symmetry breaking  matter/anti-matter ratio
Quark + antiquark annihilation  photon/baryon ratio
The quark soup  heavy quark decay
Quark-Hadron phase transition and neutron decay  n/p ratio
Big Bang nucleosynthesis  primordial abundances
Xp = 0.75
Yp = 0.25
Zp = 0.0
– Matter-Radiation equality
– Recombination/decoupling  the Cosmic Microwave
Background
– Galaxy formation and chemical evolution of galaxies
• Main events in the chemical evolution of galaxies:
– Galaxy formation  Jeans Mass
• Ellipticals
• Spirals
• Irregulars
 SFRs, gas fraction (t)
– Star formation  = efficiency of star formation
• The IMF (e.g., Salpeter IMF)
– Stellar nucleosynthesis  metals up to Fe  yield 
• Black holes, white dwarfs, neutron stars
– Supernova (i.e., SN 1987A)  metals beyond Fe
• r, p, and s process
• Abundances  X = 0.70 Y = 0.28 Z = 0.02
(solar abundances)
– Planet formation  Life!