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

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

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

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

•  The distance is calculated from the comparison with the apparent magnitude, assuming no extinction

M⟨V ⟩ = –3.53 log10 Pd – 2.13 + 2.13 ( B – V )

Classical Cepheids have B – V ≈ 0.4 to 1.1

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

with R(t) the radius of the expanding gas shell •  Assuming the velocity to not change, we can write

•  Once the luminosity is known, the absolute magnitude can be calculated

L = 4 π R 2 ( t ) σ Te 4

L = 4 π v 2 t 2 σ Te 4

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 time-stretched 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 Doppler-broadened 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 MB and MB + d MB as a function of the blue magnitude MB

•  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 = H0 d

•  H0 is the Hubble constant for the time of observation

•  Red-shift z =

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

Supercluster BAS11

Sun

λobserved - λemitted

λemitted

H0 = 71 ± 4 km/s/Mpc 1 Mpc = 3.26 million light years

•  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

(z + 1) 2 – 1

Cosmic Expansion

v = H0 d = c

1 MPc = 3.26 million light years

(z + 1) 2 + 1