Download Lecture 12: Age, Metalicity, and Observations Abundance

Lecture 12:
Age, Metalicity, and Observations
Main sequence lifetime:
HR diagram
L
*1
!
" MS
$ M '$ L '
= 7 #10 9 & )& )
% M. (% L . (
" MS
$L '
= 7 #10 & )
% L .(
Abundance Measurements
• 
• 
• 
• 
Star spectra: absorption lines
Gas spectra: emission lines
Galaxy spectra: both
Metal-rich/poor stars: stronger/weaker metal lines
relative to H.
giants
yr
HII region spectra
Lmax
Stellar spectra
* 43
9
4
yr
Main sequence
1/ 4
(since L + M , M + L )
B-V
Lmax (top of main sequence)
gives age t.
!
•  Lab measurements: Unique signature (pattern of
wavelengths and strengths of lines) for each element.
High-Resolution Spectra
Measure line strengths (equivalent widths) for individual elements.
Equivalent
Width
measures the
strength
(not the width)
of a line.
EW is width of
a 100% deep
line with same
area.
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Abundance Measurements
Bracket Notation
Bracket notation for Fe abundance of a star relative to the Sun:
!
Spectra
Line strengths (equivalent widths)
) n(Fe) ,
) n(Fe) ,
" Fe %
$# H '& ( log10 + n(H) . 0 log10 + n(H) .
*
-/
*
-.
+
!
Astrophysics
Stellar atmosphere models
!
Laboratory calibrations
"
"
atoms of H
) ( n(Fe) n(H)) ,
/ .
= log10 ++
.
* ( n(Fe) n(H)) . And similarly for other metals, e.g. relative to Fe:
+
Physics
atoms of Fe
" O % " C %
$# Fe '&, $# Fe '&, ...
!
%
Abundances: $ Fe ', etc.
# H &
"
%
Star with solar Fe abundance: $ Fe ' = 0.0
# H &
!
" Fe %
= log10 (2) = +0.3
Twice solar abundance: $# H '&
(Full details of this process are part of other courses)
!!
!
Half solar abundance:
" Fe %
= log10 (1/2) = (0.3
#$ H '&
!
!
Metallicity
Metallicity (by mass):
"A
i
Z=
ni
metals
n(H) + 4 n(He) +
"A
i
ni
Abundance (by number):
• 
) n(Mg) ,
) n(Mg) ,
" Mg %
$# H '& ( log10 +* n(H) .- / log10 +* n(H) .- .
*
• 
metals
X=
n(H)
n(H) + 4 n(He) +
!
"A n
i
Z
n(Mg)
= f
X
n(H)
#A n
i
f "
metals
n(Mg)
!
!
) Z ,
) Z ,
= log10 +
. / log10 +
.
* f X -*
* f X- .
)Z f . X . ,
..
= log10 ++ 0
* Z . f0 X0 -
i
metals
!
i
Z"
X f
= " * 10[ Mg
Z. X.f.
H
]
# 10
Primordial:
Xp = 0.75,
Yp = 0.25,
Zp = 0.00
Solar:
X . = 0.70,
Y . = 0.28,
Z . = 0.02
!
!
Some Key Observational Results
vs Abundance
• 
Gas consumption: Z = "# log(µ) for Z < #
More gas used --> higher metallicity.
Z
"
Radius: Z "1/distance from galaxy centre
!
Near centre!of galaxy: Shorter orbit period-->
More passes1thruµ
spiral shocks --> More star generations!--> µ lower --> Z
higher. (Also, more infall of IGM on outskirts.)
!
Luminosity:
• 
Z ! L0B.3
! !
Small (low luminosity) galaxies have lower metallicities.
• 
Dwarf irregulars: form late (young galaxies), have low Z
because µ is still high.
• 
Dwarf ellipticals: SN ejecta expel gas from the galaxy,
making µ low without increasing Z.
[ Mg H ]
2
Metallicity gradient in MW
Young
Old
•  Emission lines from
H II regions in low-metalicity
galaxies.
•  Measure abundance ratios: He/
H, O/H, N/H, …
•  Stellar nucleosynthesis
increases He along with metal
abundances.
•  Find Yp by extrapolating to
zero metal abundance.
A spectroscopic sequence
Primordial He/H measurement
α-element enhancement v radius in MW
MW metallicity
gradient implies an
inside-out formation
process for the MW
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α-element enhancement
Less Metals in Small Galaxies
α-element = easy multiples
of He, byproducts of
normal SN (Type II)
Z ! L0B.3
Galaxies with high α must
Have formed fast before
Many SnIa’s injected the
non-α elements into the
ISM
MW bulge formed fast and
early, before the disc.
Zoccali et al (2006)
faint
Mass-metallicity relation
• Low mass galaxies
have lower metalicity,
why?
Low mass
galaxies
Baldwin, Philips & Terlevich
Known as BPT plot.
Seyfert’s
Normal
Star-forming
galaxies
n
Transitio
region
WHY: AGN are a more
energetic process than
SF and are able to
sustain high energy
states longer leading to
a distinct line ratio mix
High mass
galaxies
bright
The BPT diagnostic diagram
Used to separate star-forming
galaxies from weak AGN
(Seyferts/LINERS)
- Younger
- More gas loss
- Higher infall
- IMF variation (IGIMF)
---->
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Example:
Star-formation rate
Hα
•  Star-formation can be measured in many ways:
– 
– 
– 
– 
FUV flux
Optical colours e.g., (g-r)
Hα, [OII], [OIII]
FIR flux
OI
•  See review by Kennicutt (1998), ARA&A
•  Here we look at one: The Hα recombination line.
To measure L(Hα) we need to measure the equivalent width of Hα and
multiple the mean intensity of the continuum at the mid-point of the line.
•  FUV rad from recent star-form ionises cloud which
recombined and cascades
•  Only stars with M>10Mo with lifetimes <20Myr produce FUV
SFR(M ! yr "1 ) = 7.9 #10 "42 L(H ! )(ergs "1 )
SFR(M " yr #1 ) = 7.9 $10#42 L(H% )(ergs#1 ) = 1.08 $10#53 Q(H o )(s#1 )
Q(H o ) is the ionising photon luminosity
constants are derived from evol. synthesis models (e.g., Kennicutt 1982)
!
Cosmic star-formation history
Star-formation has declined 10-fold since
a redshift of 1 (~half age of Universe).
SFR
Most stellar mass formed early
Stellar Mass
•  The fundamental parameters
that seem to determine observed
metallicity are mass and SFR.
•  This forms the fundamental
metallicity relation (FMR).
•  Despite extremely complex
underlying physics, the relation
exists out to z=2.5 and in a huge
range of galaxies/ environments.
Stellar Mass
5
M31: Andromeda in Ultraviolet Light
UV light traces
hot young stars,
current star
formation.
More metals near Galaxy Centres
Ellipticals
(NGC 3115)
Gas depleted,
hence no current
star formation in
the inner disk.
Spirals
(M100)
Brief Review
•  Main events in the evolution of the Universe:
–  The Big Bang (inflation of a bubble of false vacuum)
–  Symmetry breaking à matter/anti-matter ratio
–  Quark + antiquark annihilation à photon/baryon ratio
–  The quark soup à heavy quark decay
–  Quark-Hadron phase transition and neutron decay à n/p ratio
–  Big Bang nucleosynthesis à primordial abundances
Xp = 0.75
Yp = 0.25
Zp = 0.0
–  Matter-Radiation equality R ~ t1/2 à R ~ t2/3
–  Recombination/decoupling à the Cosmic Microwave
Background
–  CMB ripples (ΔT/T~10-5 at z=1100) seed galaxy formation
–  Galaxy formation and chemical evolution of galaxies
•  Main events in the chemical evolution of galaxies:
–  Galaxy formation à Jeans Mass
•  Ellipticals
•  Spirals
•  Irregulars
Initial mass and angular momentum, plus mergers.
à Star formation history S(t), gas fraction µ(t)
–  Star formation à α = efficiency of star formation
•  The IMF (e.g., Salpeter IMF power-law with slope -7/3)
•  First stars (Population III) from gas with no metals.
–  Stellar nucleosynthesis à metals up to Fe
–  Supernovae (e.g. SN 1987A) à metals beyond Fe
•  r, p, and s processes
•  white dwarfs (M < 8 Msun) or black holes, neutron stars (M > 8 Msun).
–  Galaxy enrichment models: (e.g. Z = - ρ ln(µ) , yield ρ )
•  Metal abundances rise à
X = 0.70 Y = 0.28 Z = 0.02
(solar abundances)
–  Gas with metals à Stars with Planets à Life!
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[Xi/Fe] vs [Fe/H]
All metals enrich at
approx same rate as
Fe (e.g. to a factor
of 2 over a factor of
30 enrichment).
Fini
Some elements
(Mg,O,Si,Ca,Ti,Al)
formed early, up to
~2 times faster than
Fe in metal-poor
stars.
Lowest metal
abundance seen in
stars: [Fe/H] ~ -4
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