Download Lecture 12: Galaxy Evolution

Lecture 12: Galaxy Evolution
•  An empirically driven subject:
–  The Mass versus Age plot of all surveys
•  Completing the local census:
–  New dwarf galaxies in the local group
–  The dwarf galaxy problem
•  Comparative evolution:
–  Luminosity function evolution
•  Luminosity evolution
•  Number evolution
•  Practicalities
–  K-correction
–  Dust
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MA
V. SMOOTH
NO METALS
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V. LUMPY,
METAL RICH
SS
AS
SE
MB
LY
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The Mass-Age plot
1.  Completing the local census
2.  Comparative studies
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Dwarf galaxies
•  Dwarf galaxies are a crucial part of the galaxy evolution
puzzle but we know very little about them.
•  Main theory (see later) proposes that galaxies built-up from
smaller units through repeated merging.
•  Numerical simulations typically predict several thousand
dark matter haloes in the local group.
•  ~ 55 Local Group galaxies known.
•  ~ 1 new Local Group dwarf galaxy discovered every 18
months.
•  Very wide range of properties = a combination of latestarters, relics, debris and stunted systems.
•  Space-density extremely poorly constrained, I.e., important
to appreciate that our current backyard census is woefully
incomplete.
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2 New Local Group
galaxies discovered
recently…
• 
• 
• 
• 
Bootes
Mv=-5.7 mag
µo=28.1 mag/sq arcsec
Belokurov et al (2006)
• 
• 
• 
• 
Canes Venatici
MV=-7.9 mag
µo=27.8 mag/sq arcsec
Zuker et al (2006)
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The Luminosity-Surface Brightness Plane
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Comparative studies
•  Many comparisons are possible, e.g.,
–  Profile shapes
–  Gas, dust, plasma and stellar content
–  Fundamental plane and Faber-Jackson relation
–  Tully-Fischer relation
–  Star-formation rates
–  Line indices, metallicity and colours
–  Morphologies and luminosity-size relations
–  Overall and component luminosity functions
•  Main issues are sample selection bias and
demonstrating that a comparison of the high and
low z samples is valid, comprehensive and
complete.
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The evolution of the galaxy luminosity
function ?
•  Typically allow luminosity and number-density to
evolve according too:
Lz = Lo (1+ z) β ,
φ z = φ o (1+ z)γ
⇒ J z = J o (1+ z) β +γ
•  By constructing two luminosity functions at two
redshifts one can constrain β and γ
Luminosity
Evolution
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logφ
Abs. magnitude
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Number
Evolution
logφ
Abs. magnitude
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Results from the
VLT VIRMOS
Deep Survey
(Ilbert et al 2006)
•  Basically no
or little
change seen
out to z=1
except
possible in the
blue
spheroids !
•  More data
needed
subject rapidly
advancing
due to new
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technologies.
Redshift
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Evolutionary forms
•  Pure luminosity evolution (brighter in past)
•  Pure number evolution (more or less in past)
•  Hierarchical merging (more but fainter in past)
•  Downsizing (big things form first then fade)
•  Upsizing (little things form first then fade)
•  Early monolithic collapse (everything formed at
high-z, no change over low z)
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E.g., if galaxies are evolving such that β=0.5 and
γ=-0.3 what kind of evolution is this and what is
the relative luminosity density at z=1 ?
•  L(z=1)=1.41L(z=0), I.e., galaxies brighter in past
•  φ(z=1)=0.81φ(z=1), I.e., fewer galaxies in past
•  20% of galaxies have formed below z=1, the
galaxies have either faded or the new formers
been of lower luminosity (downsizing).
•  J(z=1)=1.15J(z=0)
•  The luminosity density of the Universe has
decreased since z=1.
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Practicalities
•  2 effects which can dramatically change a
galaxies magnitude !
–  Extinction
•  Caused by attenuation of flux by intervening medium,
e.g., dust
–  K-correction
•  Important over cosmological distances where
observed wavelength is substantially different to
emmitted wavelength
m − M = 5log d + 25 + A + K
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Extinction
correction
K-correction
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Practicalities: Attenuation
•  E.g., Dust (either in our galaxy, the target galaxy or some
inbetween IGM).
•  Flux received less than it should be by some factor F (0<F<1).
FLABS
F
⇒ 10−0.4 m ∝ 2 10−0.4 M
2
d
d
M = m − 5log f − 25 + 2.5log F
M = m − 5log d − 25 − A
i.e.,LApp ∝
•  Av=-2.5log(F)=Attenuation in units of V mag (λ dependent)
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Practicalities: Attenuation
•  Extinction, if not corrected for can give low values
for H0 (I.e., distance are overestimated, H0=v/d)
•  Where can extinction occur ?
HERE
HERE
HERE
HERE
HERE
•  Big problem, typically dust is more transparent at
longer wavelength as dust grains are very small.
•  Major advantage of the near-IR now opening up.
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NO DUST
i = 0o
(face-on)
i = 60o
i = 88o
(edge-on)
WITH DUST
(B band)
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NGC891
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Impact of dust on global B LF
Luminosity density doubles, I.e., only 50% of photons escape
~0.8mag
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Practicalities: K-corrections
•  Normally we observe through filters, I.e., over some dλ:
Typical Galaxy
Spectrum at z=0
f(λ)
λ1
dλ
λ2
T(λ)=Transfer or filter function
λ2
Lλ =
∫
λ1
∞
f ( λ)dλ =
∫ T(λ) f (λ)dλ
0
•  As we look towards galaxies at higher redshift through the
same filter (or (T(λ)) we are actually receiving flux from shorter
wavelengths at a slower
€ rate. If we know the spectral shape
we can correct for this using the K-correction.
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Practicalities: K-correction
f(λ)
Typical Galaxy
Spectrum at z=0
f1
λ1
λ2
f2
f(λ)
f3
Typical Galaxy
Spectrum at z=1
λ2 (1+z)λ1 (1+z)λ2
λ1
•  We measure f2 but we want f3 to compare to f1
∞
∫ T(λ (1+ z)) f (λ (1+ z))dλ
K(z) =
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∞
(1+ z) ∫ T( λ ) f ( λ )dλ
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0
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Practicalities: K-correction
E/S0
dM
Sabc
Sd/Irr
Redshift
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Practicalities: K-correction
•  Typically K(z) is given as: K(z)=az+bz2
•  Where a and b are derived for different types
Type a
b
E/S0 3.13 0.24
Sabc 2.63 -0.107
(V-band values)
Sd/Irr 0.62 0.14
•  E.g., an elliptical galaxy at z=0.3 has an apparent
magnitude of mV=23 mag, assuming Av=0 mag
what is its absolute magnitude ?
cz
M = m − 5log( ) + 5log h0.5 − 25 − AV − K(z)
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M = −18.3 + 5log h0.5 − (3.13z + 0.24z 2 )
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M = −19.3 + 5log h0.5
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