Galaxy Morphology The Tuning Fork that Blossomed into a Lemon Lance Simms MASS Talk 9/8/08

Hubble’s Tuning Fork  Tuning Fork Diagram used by Hubble from  Irregular class was later added to right hand side  Hubble originally thought evolution was from left to right Lenticulars S0 galaxies with large central bulge No spiral arms, gas, or dust Flattened disc of stars Ellipticals – En n=10(1-b/a) b: semi-minor axis a: semi-major axis Ellipticals – En n=10(1-b/a) b: semi-minor axis a: 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 nm (in rest frame) N II 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 Energy U – Potential Energy α – Constant that depends on distribution of mass within galaxy

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: 10 8 − solar masses Size range: 1 − >10 kiloparsecs Magnitudes: − 13 to − 20 in B bandpass Composition: Varied Young Stars HII regions Color: Varied, toward blue

Spiral Galaxies  We think about 66% of galaxies are spirals  Most 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 SPIRAL 1) Flattened, rotating disc of stars and gas − Arms are in plane of disc 2) Central bulge with mainly old stars − Brightest component of galaxy 3) Nearly spherical halo of stars − Globular Clusters − Dark Matter 4) Supermassive black hole at center

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: 10 9 − solar masses Size range: 5 − >100 kiloparsecs Magnitudes: − 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 measurements 2)Mapping HII regions via H α 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 V sys (r) ) only exist for some 10s of galaxies  Usually limited to σ o and V sys at center M32 Before 1977 Theorists thought they understood ellipticals well in 1970s = axially symmetric isothermal ensembles = increasingly flattened the more rapidly they rotate about center After 1977 Observations 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: 10 7 − solar masses Size range: 0.1 − >100 kiloparsecs Smallest: Dwarf Ellipticals Composition: Mostly old, red stars Color: 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; I o is intensity per unit area at center De Vaucouleurs’s Law (1948) r e is radius containing half of total luminosity I e is intensity at a distance r e from center

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 equal Saturn is an oblate spheroid, flattened near equator Equation 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)Massiii) Luminosityiii) Size ii)Mass-to-light ratioiv) 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