Download Evolution of galaxy morphology - Lecture 1 - NCRA-TIFR

Evolution of galaxy morphology
Lecture 1
Yogesh Wadadekar
July 2014
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Galaxies are a very active field of research today!
About 39% of papers published on Arxiv:astro-ph in 2013 were on
Cosmology and Extragalactic astrophysics - 5510 papers in 2013
alone.
Most of the content in my lectures reached its present form in the last
15 years.
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My lectures will cover
1
A brief History of extragalactic research
2
Morphological Classification of galaxies
3
Quantitative morphology
4
Evolution of galaxy morphology primarily with time and
environment
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Study material
I am putting in significant effort to make good slides for my talks. Use
these as your primary reference material and remember to stop me
and ask questions if you don’t understand something. Some of the
slides will cite papers which you are encouraged to read to know more.
A number of excellent online courses are available. I will use material
from these as appropriate. I particularly like the Galaxies and
Cosmology course by S. G. Djorgovski on Coursera. A similar course
is also available on edX. For textbooks, I recommend Schneider’s
Extragalactic Astronomy and Cosmology and Galactic Astronomy by
Binney and Merrifield.
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The big picture
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Three ubiqitous basic structures in the Universe
Atoms
Stars
Galaxies
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Kant’s “Island Universes”
In 1755 he wrote a book called General Natural History & Theory of
the Heavens in which he said:
1
MW like a huge solar system, rotating; origin from rotating cloud.
2
stars far from disk plane on different orbits
3
disks (like MW) project to ellipses
4
oval nebulae (seen by de Maupertius) = “Island Universe”
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Major milestones since Kant
1755: Kant’s Island Universes
1912: Slipher discovered that nebulae are rotating
1920: Shapley-Curtis debate “Are Spiral Nebulae Island
Universes” - 30 minute presentations by each - Shapley “won” the
debate, although Curtis was right.
1925: Hubble discovered Cepheids in the Andromeda galaxy
Extragalactic astrophysics < 100 years old!
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What are galaxies?
gravitationally bound agglomerations of stars, dust, gas, dark
matter.
Mass ratio Gas:Stars:Dark Matter - 1:10:100
they are the basic building blocks of the Universe on large scales
they show a broad range in their physical properties
Understanding of galaxy formation and evolution is one of the
main outstanding problems in modern cosmology
there are ∼ 1011 galaxies in the observable universe
typical total mass of 108 − 1012 M
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Counting and cataloging galaxies
In late 1700s,Messier made a catalog of 109 nebulae so that
comet hunters wouldn’t mistake them for comets! About 40 of
these were galaxies.
NGC New General Catalogue (Dreyer 1888) had 7840 objects, of
which about 50% were galaxies.
In the 20th century, many catalogs were produced UGC, RC3 etc.
Nowadays we have automated surveys, e.g. Sloan Digital Sky
Survey, with tens to hundreds of millions of galaxies
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First step census, second step classification
third step understanding are common steps in any empirical science.
Hubble proposed a scheme for classifying galaxies (the ”tuning fork”
diagram) in his 1936 book, The Realm of the Nebulae. This scheme
survives in its essence to the present day.
ellipticals, lenticulars, spirals and irregulars are the main types.
A better approach may be to look at the properties of subsystems
within galaxies (e.g., disks, spheroids, halos, etc.), and deduce their
origins and evolution
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Why is galaxy classification useful?
Classes bring order to diversity of galaxy forms
Span/include majority of galaxies
Unambiguous & easily identified criteria
Relate to important physical properties → provide insight into
internal processes, formation, & evolution
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Why classify galaxies in the optical band?
Historically, optical photometry was the method used to observe
galaxies. Hence Hubble’s classification is most widely used.
Today, many other criteria such as color indices, spectroscopic
parameters (based on emission or absorption lines), the broad-band
spectral energy distribution (galaxies with/without radio- and/or X-ray
emission) are also used to group similar galaxies together.
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Hubble tuning fork diagram
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Elliptical Galaxies
About 20% of field galaxies are Ellipticals, but most E’s are in
clusters
There are a number of different subtypes:E’s (normal ellipticals),
cD’s (massive bright ellipticals at the centers of galaxy clusters),
dE’s (dwarf ellipticals) dSph’s (dwarf spheroidals)
Smooth and almost featureless. no spiral arms or dust lanes.
Generally lacking in cool gas, and hence few young blue stars
Elliptical galaxies are denoted En where:b/a = 1 − n/10 i.e. an
E4 galaxy has an axis ratio of b/a = 0.6, and E0’s have circular
isophotes.
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Elliptical
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Lenticular
are characterised by the presence of a central bulge and disk and
the absence of spiral arms i.e. little or no ongoing star formation
intermediate in many of their properties between ellipticals and
spirals.
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Lenticular
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Spiral
Named for their bright spiral arms, which are prominent due either to
bright O and B stars (evidence for recent star formation), or due to dust
lanes.
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Grand Design Spiral
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What is the diameter of the Milky Way Galaxy?
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M/L ratio for stars
For main sequence stars, we have L ∝ M 3.5 , giving :
(M/L) ∝ M −2.5 ∝ L−0.71 showing, as one expects, later spectral types
have higher M/L. eg K stars : M ∼ 0.5M → M/L ∼ 10; A stars :
M ∼ 2.0M → M/L ∼ 0.1
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Mass to light ratio M/L of a galaxy
M/L defined in units of M /L
For galaxies, M/L reflects the average M/L over the population
Pop I (young) : massive stars dominate light; low mass stars dominate
mass Pop II (old) : giants dominate light; M.S. stars dominate mass
Typical galaxy (& solar neighborhood) has M/LV 6, M/LB ∼ 10 In
general : M/L increases with age and metallicity Maximum range :
2 < M/LB < 20.
What are Pop III stars?
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Initial Mass Function IMF
The IMF ξ(M) specifies the distribution in mass of a newly formed
stellar population and it is frequently assumed to be a simple power
law: ξ(M) = c M −(1+x) . In general, ξ(M) is assumed to extend from a
lower (M1 ) to an upper cutoff (M2 ).
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Initial Mass Function
IMF
Salpeter
Scalo
Miller & Scalo
M1
0.10
0.10
0.18
0.42
0.62
1.18
3.50
0.10
1.00
2.00
10.0
M2
125.
0.18
0.42
0.62
1.18
3.50
125.
1.00
2.00
10.0
125.
x
1.35
-2.60
0.01
1.75
1.08
2.50
1.63
0.25
1.00
1.30
2.30
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Question
For a Salpeter IMF, what is the relative number of K and A stars in a
cluster that has just formed?
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M100 nucleus
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Barred galaxies
Half of all disk galaxies - Milky Way included - show a central bar
which contains up to 1/3 of the total light Bars are a form of
dynamical instability in differentially rotating stellar disks
S0 galaxies also have bars – a bar can persist in the absence of
gas
Bar patterns are not static, they rotate with a pattern speed, but
unlike spiral arms they are not density waves. Stars in the bar stay
in the bar
The asymmetric gravitational forces of a disk allow gas to lose
angular momentum (via shocks) compressing the gas along the
edge of the bar. The gas loses energy (dissipation) and moves
closer to the center of the galaxy
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Barred Spiral
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Lopsided galaxy - unstable disks
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Irregular
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Large Megallanic Cloud
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Nomenclature: Early and late type
Objects along the sequence are often referred to as being either an
early-type or a late-type. Ellipticals and S0 galaxies are collectively
called and early-type and spirals are called late-type. Within spirals, an
Sa galaxy is called an early-type spiral, and an Sc galaxy a late-type
spiral. This nomenclature is not a statement of the evolutionary stage
of the objects but is merely a nomenclature of purely historical origin.
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Galaxy classification affected by projection
Morphological classification is at least partially affected by projection
effects. If, for instance, the spatial shape of an elliptical galaxy is a
triaxial ellipsoid, then the observed ellipticity will depend on its
orientation with respect to the line-of-sight. Also, it will be difficult to
identify a bar in a spiral that is observed from its side (“edge-on”).
Similarly a weak disk in an “face-on S0” is hard to spot.
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∼30% of galaxies at z ∼ 1 are peculiar
First, distant galaxies.
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Dwarf galaxies are also not included.
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Dwarf galaxies
Low-luminosity: 106 − 1010 L , low-mass: 107 − 1010 M , small in
size, ∼few kpc, dark matter dominated
Often low surface brightness, so they are hard to find!
More than one family of objects:
Gas-poor, passive (dE and dSph)
Gas-rich, star forming
Why are dwarf galaxies important?
Majority of galaxies are dwarfs!
Dwarf galaxies may be remnants of galaxy formation process:
“proto-dwarf” gas clouds came together to form larger galaxies
(hierarchical formation)
Dwarf galaxies are currently being cannibalized by larger galaxies
Dwarf galaxies are relatively simple systems, not merger products:
in some sense, “pristine” low metallicity galaxies
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I Zwicky 18
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Questions
If you go look at the night sky most of the stars look white or blue with
a few red ones which are all red giants. But the IMF tells us that most
stars should be red looking M-dwarfs or G and K type dwarfs? Why
are these common stars extremely uncommon in the night sky?
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Mergers can alter morphology
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The milky way - Andromeda galaxy merger
Galaxies in the process of transformation, generally from disks to
ellipticals
In late stages of a merger, the 2 galaxies are no longer distinguishable.
What does the merger product look like?
Show movie
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Other limitations of the Hubble classification
Based on photographic images in the blue emphasises star
formation (not mass distribution) High z galaxies sample the rest
frame UV.
Requires reasonably good spatial resolution across the galaxy
( 20 elements) progressively more difficult for cz > 8000 km/s
from ground.
To summarise, three kinds of galaxies don’t fit into the Hubble scheme:
(1) Disturbed or interacting galaxies (2) Galaxies at high-z and (3) Low
Surface Brightness (LSB) galaxies - almost always dwarf galaxies.
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Modifications to the Hubble Sequence e.g. by van den
Bergh 1976
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Kormendy’s version of the Hubble sequence
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Why do we need quantitative morphological
classification?
Modern CCD imaging surveys generate vast numbers of galaxy
images
There is a need for fast, objective, robust classification.
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Quantitative morphology: bulge-disk decomposition
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Wadadekar et al. (1999)
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Connection between star formation history and
morphology
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Galaxy morphology, star formation history, stellar mass Galaxies
and projected
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Quantitative morphology: bulge-disk decomposition
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Wadadekar et al. (1999)
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χ2 minimisation
χ2 =
X (Ii − I0 )2
σi2
where Ii , I0 and σi are the observed flux, model flux and the error in Ii
respectively. The error depends on the signal to noise ratio of the
data. Exactly how this is to be computed should be covered in your
Astronomical Techniques I course.
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Analytics model for the disk
Exponential Disk:
Idisk (x, y ) = Is e−rdisk /rs ,
q
rdisk =
x 2 + y 2 /(1 − ed )2 .
ed
= 1 − cos(i),
rdisk is the galactocentric radius [putting it more accurately, the length
of the semi-major axis of the (elliptical!) isophote], rd the length scale
of the disk and IS refers to intensity at the centre.
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Inner and outer disk - double exponentials
It is possible to fit two different disks (inner and outer disks) that share
the same position angle and ellipticity, but have different central
brightness and length scales. This ability tends to become very useful
since galaxies with double exponential disks are being now frequently
found with ever deeper and finer images.
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Analytic model for the bulge
Sérsic Bulge:
1/n
Ibulge (x, y ) = Ie e−bn [(rbulge /re ) −1] ,
q
rbulge =
x 2 + y 2 /(1 − eb )2 ,
Where e refers to effective values and n is the Sérsic index. For n=4 it
becomes the de Vaucouleurs function; for n=1 an exponential, and
when n=0.5, a Gaussian! For values in the range 1-4, approximately, it
describes bulges in late-type spiral galaxies (or pseudo-bulges) to
bulges in early-type spirals (or classical bulges) and elliptical galaxies.
It is easy to realize that the larger the value of n the more concentrated
is the light (and mass!) of the bulge (or elliptical) in the center.
The effective radius of a galaxy is the one that contains half of the total
light emitted by the galaxy. The numerical constants bn is chosen so
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The Sérsic profile
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Other equivalent forms of the Sérsic function
Ibulge (x, y ) = I0 10−cn [(rbulge /re )
1/n ]
1/n −1]
Ibulge (x, y ) = Ie 10−dn [(rbulge /re )
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