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

1

2

3

4

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