Galaxies

PHYS390 Astrophysics

Professor Lee Carkner

Lecture 20

The Great Debate In the early 20th century

astronomers could see many strange “nebula”

Were they in our galaxy or independent distant galaxies?

In 1923 Hubble found the distance to M31 using Cepheids

Spirals too distant to be in our Milky Way

Elliptical Galaxies Classified by apparent ellipticity Class = 10E where E = 1 –

Dwarf ellipticals MB ~ Mass = Size ~ 300 pc

Giant ellipticals MB ~ Mass ~ 1013 Msun

Size ~ Largest things in universe

Spiral Galaxies

Less variation in size, but more variation in structure compared to ellipticals MB ~

Mass ~ Size ~ 5 to 100 kpc

Irregular galaxies often look like distorted spirals and have a similar range of properties

Hubble Type

Spirals Sa: Sb: medium bulge, looser arms Sc: If bar present, SBa, SBb, SBc

Ellipticals

Others Irregular: amorphous with no obvious structure

Observing Galaxies

Need to apply K correction to account for red shift of light out of observed band

sky ~ 22 B-mag/arcsec2

Galaxies are diffuse

Have a distribution of magnitude that needs to be parameterized Surface brightness as a function of radius

de Vaucouleurs Profile

Can write in terms of the surface

brightness at some radius ((r))

(r) = e + 8.3268[(r/re)1/4 -1]

radius within which ½ of the light is emitted

e is the surface brightness at re

Rotation Curves

Rotation curve give mass

Sa have higher maximum rotation velocities

Brighter galaxies have higher maximum rotation velocities

Tully-Fisher Relation

Sa: MB = -9.95logVmax+3.15

Sb: MB = -10.2logVmax+2.71

Sc: MB = -11.0logVmax+3.31

Can measure Vmax from Doppler shifts and use to get MB and thus distance

Star Formation

Sc galaxies are bluer than Sb or Sa

21-cm and H II emission increase from Sa to Sc

Since Sc have more young stars, dust and

gas they must have more star formation The more disk (relative to bulge) the more

star formation

Elliptical Classes

cD

Normal (E)

Dwarf (dE) Very small (few kpc)

Dwarf spheroidal (dSph)

Blue compact dwarf (BCD) small ellipticals with star formation

Dust and Gas

dE and dSph have almost no gas and dust

Other ellipticals have some gas

About ½ have dust Ellipticals have very little star formation

Gas and dust may have been acquired after galaxy formation

Faber-Jackson

L proportional to 04

where 0 is the central radial velocity dispersion

Some of this deviation may be due to the galaxy capturing outside material

Luminosity Function

Can examine the relative number of galaxies of a certain brightness and Hubble type

Spirals dominate in mass and luminosity

Strongly dependant on environment

Next Time

Read 26.2 Homework: 25.1, 25.3, 26.1a, 26.3