An introduction to Galaxies

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An introduction to Galaxies
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The World of Galaxies
Spirals
Ellipticals
Irregulars
barred
unbarred
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Standard CDM Cosmology
Non-baryonic Dark Matter Halos
Baryonic Matter
Gas cooling
Initial Density Field
Clump Mergers
Condensation on to DM halos
Dark Matter Halos

• Initial Mass
• Initial Density
• Angular Momentum
• Dark Matter halo
• Primordial Chemical
• Composition

protogalaxy
Star Formation
First Galaxies
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Galaxy formation through consecutive mergers of
gaseous halos
or from the current picture of the universe
itself.
Super-computer simulation of evolution of young
small galaxies into larger galaxies in pronounced
clusters. See also colliding galaxies Sci. Am.
Aug 90.
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Galaxy Evolution
What changes in a galaxy as a function of time
What causes these changes in a galaxy
Gas fraction
Star Formation
Luminosity Total, L(?)
Stellar Evolution
Total Mass (t)
Galaxy Interactions Tides and mergers
Dust content
Z(t)
Morphology
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Fundamental questions

• Properties of proto-galaxy
• initial mass, initial density, initial
  angular momentum, dark matter
• Initial epoch of astration
• Does a system with similar initial
• characteristics evolve very differently
• in different environments?
• How is Star Formation triggered, propagated and
  regulated in different types of
  galaxies/environments
• How do the various different morphological types
  of galaxies arise?

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Star Formation Defining the problem
SFR (Mo /yr) SFE
(/100Myr) E formation 300
30 Disk formation 10
1 Star Bursts 100-1000 30-50

• Efficiency of star formation (SFE)
• Rate of star formation (SFR)
• Initial mass function (IMF)

• Parameters that define the characteristics of SF

• Log-normal with peak at 0.1 Mo
• Does it change with time/environment?

• Densities of molecular species
• Ionization
• Dust extinction (grain
• composition, size )
• Magnetic field strength
• Dust and gas temperature
• Turbulent velocity field
• Angular momentum
• distribution

• physics and chemistry of the interstellar medium
• galactic dynamics

Fundamental Theory of Star Formation
(??) Determination of the above parameters
from

• Galactic Differential Rotation
• Spiral density waves
• Tidal torques
• Satellite accretion

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The role of galactic interactions increases with
look-back time (or redshift)Colliding Galaxies
at high z HST deep field
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Galactic Interactions in the Local Universe

• Evidence for triggering of Star Formation
• Morphological Disturbances

Antennae
NGC 6745
NGC 1409-1410
Cartwheel
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Study of Galactic Evolution
Observe galaxies at different redshifts
Observe stellar populations of different ages in
nearby galaxies

• snapshots
• (danger of peaking exotic objects
• need to map
• redshift - lookback time
• yield statistical information
• on morphological and
• size evolution

• detailed star formation
• history for individual galaxies
• of different current sizes and
• morphologies
• lookback time known
• yield very little information on
• morphological and size evolution

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Topography of the Local Group of Galaxies
Size 1.2 Mpc
35 galaxies
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Gallery of Local Group Galaxies Spirals
Range of Masses 10 Million to 1 trillion solar
masses
M33
Milky Way
M31
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Sites of Current Star Formation in the Milky Way
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NGC 3603 Massive star formation site in Milky
Way
Trifid Nebula Star Formation site in Milky Way
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Gallery of Local Group Galaxies Irregulars
LMC
SMC
IC 5152
NGC 6822

• Most Irrs are gas rich
• Most show current star
• formation at a significant level
• Most dwarf Irrs are remote
• objects

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Gallery of Local Group Galaxies Dwarf
ellipticals and spheroidals

• The least massive members of the LG (5 orders of
  magnitude less massive than the MW)
• Most of the LG galaxies belong to this category.
• Most are close to the giant spirals
• Most contain little or no gas
• Evidence of dark matter halos (?)

Dwarf Elliptical (nucleated) NGC 205
Dwarf Spheroidal Leo I
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Sagittarius dwarf Irregular galaxy an instance
of galactic cannibalism

• The closest dwarf galaxy to the Milky Way (16
  kpc)
• Tidally distorted merging into the MW
• Its survival up to now is consistent with a flat
• rather than a centrally concentrated dark
  matter halo

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Morphological segregation in Local Group
Most dwarf spheroidals/ ellipticals are located
close to giant spiral Gas-loss ?
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The luminosity function of the Local Group of
Galaxies
The Missing Dwarf Problem too few by a factor
of 10!
(according to hierarchical CDM models)
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For each LG galaxy we want to derive a three
dimensional picture showing the SFR and chemical
enrichment as a function of time
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Approximate age-indicators
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The observed Hertzsprung-Russell Diagram of an
old coeval stellar population
RR-Lyrae variables Distance (Z)
Red Giant Branch Metallicity age
Measure of stellar luminosity
MS turnoff AGE
Measure of surface effective temperature
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How the population synthesis modeling works
Stellar evolution theory isochrones
Initial mass function
Assumed SFR(t)
Synthetic color-magnitude diagram
errors
(Assumed?) Chemical evolution
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How the population synthesis modeling works
Observed diagram areas of number comparisons

• Models for different coeval
• populations

Final Adopted model
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The effect of distance on the c-m diagram
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The simpler systemsStar Formation History of
dwarf spheroidal galaxy Leo I
SFR(t)
Observed Color magnitude diagram
Adopted model color-magnitude diagram
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Carina Dwarf Spheroidal
Burst strength
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3
Age (Gyr)

• What caused this uniquely erratic star
  formation activity in Carina?

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Star Formation History Boxes for 15 Local Group
Dwarf Spheroidal Galaxies
X-axis t(Gyr) Y-Axis SFR Z-Axis Fe/H
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Star Formation History Boxes for 15 Local Group
Irregular Galaxies
X-axis t(Gyr) Y-Axis SFR Z-Axis
Fe/H Distances from LG barycenter
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Direct Evidence of Environmental Influence to
Galactic Evolution in the Local Group

• Morphological type segregation
• Tidal tails and bridges found in
• - Magellanic System
• - Saggitarius
• - Carina dsph
• Possible merger Saggitarius
• Interactions triggered Star Formation
• - Magellanic Bridge
• - Burst of star cluster formation in LMC
• 2-3 Gyr ago

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Dark Matter in Local Group Galaxies

• Galaxy Type DM M/L
  Type of evidence
• 
  (Mo/Lo)
• Spirals - disks yes 1-3
  HI rotation curves
• Spirals - bulges yes 10-20
  stellar kinematics
• Spirals halos yes gt20
  stellar kinematics
• 
  (microlensing)
• Dwarfs yes 1-80
  (?) stellar kinemtics
• 
  survival of Sgr
• 
  ?problem with tides
• Compact HVC yes 10-50(?)
  HI rotation curves

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STAR FORMATION HISTORIES OF LG DWARFS SHOW HUGE
VARIETY

• Stochasticity of SF in low mass galaxies (no
  central control)
• highly variable SFR
• changes in gas content which can result from
• mass (and metal) loss via galactic winds
• gas accretion or gas loss via galactic
  interactions
• gas infall from outer regions of galaxy
• changes in internal dynamics (due to
  interactions, Bar formation) - mergers
• The problem of unknown orbits
• UV radiation from giant spiral can delay cooling
  and SF in close-by dwarf
• role of dark matter halos in modifying SF
  activity (helping retain escaping gas?)

Possible factors
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Galactic Chemical evolutionA function of
galactic mass
Total visible mass seems to be the decisive
parameter that determines the overall chemical
evolution of a galaxy
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Epoch of highest Star Formation Rate A
function of galactic mass?Or of Environment?
Total visible mass is an important parameter that
determines the general characteristics of the
Star Formation History of a galaxy!!
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Closing remarks

• No two LG galaxies have identical SF histories
• The smaller a galaxy the more intermittent its SF
  history
• Gas poor, low mass, old galaxies mostly near
  giant spirals
• Gas rich star forming galaxies mostly isolated
• Minor mergers and tidal interactions currently
  happening
• Triggering of SF in tidal tails observed
• total amount of current visible matter
• seems to be an important parameter for
• the overall chemical evolution of a galaxy and
  
• the epoch when the SFR was highest in a
  particular galaxy
• Mass, interactions, dark matter content