Galaxy Morphology The Tuning Fork that Blossomed into a Lemon
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Transcript of Galaxy Morphology The Tuning Fork that Blossomed into a Lemon
Galaxy MorphologyThe Tuning Fork that Blossomed into
a Lemon
Lance SimmsMASS Talk
9/8/08
Hubble’s Tuning Fork
Tuning Fork Diagram used by Hubble from 1925-1935 Irregular class was later added to right hand side Hubble originally thought evolution was from left to right
LenticularsS0 galaxies with large central bulgeNo spiral arms, gas, or dustFlattened disc of stars
Ellipticals – En n=10(1-b/a)
b: semi-minor axisa: semi-major axis
Ellipticals – En n=10(1-b/a)
b: semi-minor axisa: semi-major axis Bulge/Disc Ratio
Loose Arms
Gas and Dust
Irregulars would fall over here
Lemon Classification of Vaucouleurs
Image: Mod. Phys Rev, G. De Vucouleurs, Large-Scale Structure and Direction of Rotation in Galaxies
B=‘Barred’
A=‘Normal’
Rotational Velocity Curves
Hα-656.28 nm (in rest frame)N II-658.53 nm (in rest frame)Note: Galaxy should be edge-on
Image for illustrative purposes
Towards us
Away from us
• Differential rotation can be observed through spectra
• Useful for Spiral Galaxies that are viewed edge-on
• Difficult to use for Ellipticals• Overall shift in spectral lines gives velocity and
with Hubble Law, approximate distance away
Velocity Dispersions• Profile width gives velocity dispersion σ
– Spectral fitting methods vary
• Mass is obtained via the virial theorem
• Very useful for elliptical galaxies
Increasing dispersion
Virial Theorem
K – Kinetic EnergyU – Potential Energy
α – Constant that depends on distribution of mass within galaxy
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2K +U = 0
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M ≅Rv 2
αG
Irregular Galaxies
IC 1613 – Cetus
IC 10 - Cassiopea
• Small percentage of known galaxies are irregulars (~3%)
• Galaxies that do not show spiral or elliptical structure• No nuclear bulge• No spiral arms
• Divided into two main types• Irr-I : some structure• Irr-II : chaotic mess
• Some are Starburst Galaxies • Very high rate of star formation
Mass range: 108 −1010 solar massesSize range: 1 − >10 kiloparsecsMagnitudes: −13 to −20 in B bandpass Composition: Varied Young Stars HII regionsColor: Varied, toward blue
Spiral Galaxies
We think about 66% of galaxies are spiralsMost have active Star Formation (SF) occurring in spiral arms Appearance depends on angle relative to our line of sight Consist of 4 Distinct Components
4 MAIN COMPONENTS OF
SPIRAL1) Flattened, rotating disc of stars
and gas − Arms are in plane of disc2) Central bulge with mainly
old stars − Brightest component of
galaxy3) Nearly spherical halo of
stars − Globular Clusters − Dark Matter 4) Supermassive black hole at
center
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2
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Spiral Galaxies: A Slice of the Lemon
r – internal ring around nucleus -- spiral arms begin on ring
s – no internal ring -- spiral arms begin directly at nucleus
A – Normal spiral -- no bar
B – Barred Spiral
Spiral Galaxies
Mass range: 109 −1012 solar massesSize range: 5 − >100 kiloparsecsMagnitudes: −16 to −23 in B bandpass Composition: Young and Old Stars
Active Star Formation (SF) occurring in spiral arms is very bright in UV Young stars emit towards
UV Several types shown below
Spiral Galaxies
Our Spiral – The Milky Way
Our Sun
10,000 ly
Mapping the Milky Way In past, mostly done with 2 methods:
1)Mapping HI regions with radio observations
- 21 cm line measurements2)Mapping HII regions via Hα emission lines
- HII regions trace active star formation
Old data showed that there were 4 arms New data from Spitzer indicates that there are only 2 major spiral arms:
-Scutum and Perseus Arms
Elliptical Galaxies Ellipticals appear to have very little gas or dust Approximately 10% of known galaxies are elliptical Stars orbit the galaxy center in all different planes
Circular orbital velocity measurements do not work very well Sometimes a preferred direction of very slow rotation
Luminosity decreases quickly from center so measurements are always made within 10 kpc. Detailed kinematic observations ( σ(r) and Vsys(r) ) only exist for some 10s of galaxies
Usually limited to σo and Vsys at center M32
http://www.astr.ua.edu/
Before 1977Theorists thought they understood ellipticals well in 1970s = axially symmetric isothermal ensembles = increasingly flattened the more rapidly they rotate about center
After 1977Observations proved them wrong = Spectroscopic data (stellar absorption lines) showed that ellipticals do not rotate globally = Not isothermal = Velocity dispersion is anisotropic = Now strong evidence that they are triaxial ellipsoids
Elliptical Galaxies
Mass range: 107 −1013 solar massesSize range: 0.1 − >100 kiloparsecsSmallest: Dwarf Ellipticals Composition: Mostly old, red starsColor: Towards the red end
M87 –Largest Galaxy in Virgo Cluster
Luminosity Profiles:Hubble’s Law (1930)
I is intensity emitted per unit area at r from center a is core radius; Io is intensity per unit area at center
De Vaucouleurs’s Law (1948)
re is radius containing half of total luminosity Ie is intensity at a distance re from center
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I /Io = [(r /a) +1]−2
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log(I /Ie ) = −3.33[(r /re )1/ 4 −1]
Dwarf Spheroidal Galaxies
• Low luminosity galaxies• More spherical than elliptical• Companions to Milky Way or
other galaxies such as M31• Little or no gas or dust• No recent star formation• Approximately spheroidal in
shape NGC 147 – Dwarf Spheroidal in Local Group
Spheroids:A spheroid is basically an ellipsoid with to of its axes equalSaturn is an oblate spheroid, flattened near equatorEquation in 3-d:
Oblate Spheroid
Globular Clusters Large, gravitationally bound groups of stars
10,000 – 1,000,000 stars Not galaxies; considered a part of our galaxy Orbit center of our galaxy in elliptical orbits
Some orbits are highly extended Some contain “Tidal Tails”
Highly concentrated in Galactic Longitude (337°) Tidal Tails
When globulars pass by bulge of Milky Way, gravity is strong enough to rip stars away Trail of stars left behind is called a Tidal Tail
NGC 5466
Dwarf Spheroidal or Globular Cluster?
• Distinction between globulars (GCs) and Dwarf Spheroidal Galaxies (dSphs) is ambiguous – Globular clusters are
generally more compact, but some dwarf galaxies are also
– Small galaxies have about same mass as globulars
– Galaxies are more “isolated”, but there are intergalactic ‘tramp’ globulars
– Color Magnitude Diagrams (CMD) look similar
• As of 2003, there were – ~150 GCs – ~9 dSphs
• Now, there are ~20 dSphs
Globulars and DSphs
• There is significant overlap in i) Mass iii) Luminosity iii) Sizeii) Mass-to-light ratio iv) Spread in Metallicity
• Apparently, ellipticity may be a distinguishing factor • only 20 galaxies in plot, 1.4 data points per plot point
Taken from van den Bergh
Dwarf Spheroidal or Globular?• Carina Low Surface
Brightness (LSB) dSph
Dwarf Spheroidal or Globular?
NGC 288 Globular Cluster