Addition to the Lecture on Galactic Evolution

Rapid Encounters In case the encounter of two galaxies is quite fast, there will be not much dynamical friction due to lack of the density enhancement The potential energy of the galaxies remain unchanged The internal kinetic energy increases in the collision When regaining equilibrium after the collision, the gained energy is dissipated by Increasing the gravitational potential leading to a slight expansion of the galaxies After regaining equilibrium a ring may have formed Tidal forces removing mass from the galaxies (tidal stripping)

Polar-ring Galaxy NGC 4650A

Simulation of NGC 4038/9 Alar Toomre simulated in the 1970s the collision between two spiral galaxies Shown is the simulation of NGC 4038 and NGC 4039, referred to as the Antennae Galaxies Clearly visible are the two tidal tails left behind after the encounter

Peculiar Galaxies Halton Arp cataloged 338 peculiar galaxies into an atlas in 1966

Galaxies: The Structure of the Universe

Extragalactic Distance Scale The first astronomical distance measurements based on measuring the parallax of a nearby object, like a star, taken at different times during the year The Cepheid Distance Scale was discovered by Henrietta Leavitt and later deployed by Harlow Shapley and Edwin Hubble Nowadays, this period-luminosity relation is used in the V band involves a color correction M V = 3.53 log 10 P d 2.13 + 2.13 ( B V ) Classical Cepheids have B V 0.4 to 1.1 The distance is calculated from the comparison with the apparent magnitude, assuming no extinction

Extragalactic Distance Scale In 1928, Edwin Hubble detected Cepheids in the Andromeda galaxy and estimated the distance to be 285 kpc, compared to the modern value of 770 kpc This established the Andromeda galaxy as the first object outside our Milky Way galaxy

Supernovae as Distance Indicators Assume that a supernova s expanding shell of hot gas radiates like a blackbody The luminosity is then given by L = 4 π R 2 ( t ) σ T e 4 with R(t) the radius of the expanding gas shell Assuming the velocity to not change, we can write L = 4 π v 2 t 2 σ T e 4 Once the luminosity is known, the absolute magnitude can be calculated

Supernovae as Distance Indicators A different approach is using supernova type Ia light curves involves SN Ia light curves are calculable with the maximum luminosity very reproducible Example by S. Perlmutter of timestretched SN Ia light curves The peak magnitude is then determined by the stretch factor S. Perlmutter, Physics Today 56, No. 4, 53 (2003)

Tully-Fisher Relation We saw that there exists a relation between the luminosity of a spiral galaxy and its maximum rotation velocity The relation was determined first in 1977 by Tully and Fisher by measuring the Dopplerbroadened 21 cm radio emission line from neutral hydrogen in a sample of spiral galaxies

Globular Cluster Luminosity Function The brightest globular clusters around a galaxy are measured The luminosity function ϕ is the number of clusters with magnitude between M B and M B + d M B as a function of the blue magnitude M B The distribution can be described with a Gaussian function The peak value can be used to compare the apparent to the absolute magnitude

Galactic Distance Indicators

Hubble s Law Edwin Hubble measured in 1929 distances d and red-shifts z of galaxies and noticed a linear relation between their recessional velocity v and d v = H 0 d H 0 is the Hubble constant for the time of observation Red-shift z = λ observed - λ emitted λ emitted measures the increase in wavelength due to a receding light source (compare to Doppler effect) This was the first measurement showing that the universe is expanding H 0 = 71 ± 4 km/s/mpc 1 Mpc = 3.26 million light years Sun Supercluster BAS11

Cosmic Expansion Comparison of red-shifts for different astronomical objects The more the spectral lines are shifted towards the red end of the spectrum, the more far away the object is and the larger its recessional velocity v = H 0 d = c (z + 1) 2 1 (z + 1) 2 + 1 1 MPc = 3.26 million light years