Transcript AS2001 - University of St Andrews

Lecture 11:
Age and Metalicity
from Observations
Z
“Closed Box” model with constant yield:

Z(t)    ln(t)
metalicity
Ignores:

gas fraction

Z(t)   as t  

1.
yield
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

t
t


Spirals:
M0
t 
Ý
M
(t)  1 t /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




Irregulars:
1
 

Z(t)   ln1 t /t 
1
t
t
t
t
Z



Age Estimates
Main sequence lifetime:
HR diagram
1
 MS
 MS
M L 
 7 10    yr
M. L . 
L
giants
9
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) 

 log10 
  log10 



 n(H) 
 n(H) .
atoms of Fe
atoms of H
 n(Fe) n(H) 
 
 log10 
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 
 log10 2  0.3
Twice solar abundance: 
 H 


Half solar abundance:
 Fe

 H

 log10 ?  ?


[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) 

 log10 
  log10 



n(H)
n(H)

*

.
n(H)
n(H)  4 n(He)  f n(Mg)
 Z . 
 Z 

 log10 
 log10 


f
X
.
 f X 


Z
n(Mg)

f X n(H)
Z X . 

 log10 
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!