Cosmology and extragalactic astronomy

Mat Page

Mullard Space Science Lab, UCL

2. Galaxies

1. Galaxies

• This lecture:• Classification of galaxies

– Spirals– Ellipticals– Irregulars

• Characteristics of the different types

Slide 2

Hubble’s tuning fork

• Basic set of galaxy types.

Slide 3

• Ellipticals• Spirals

– barred– normal

• Irregulars

3 groups:Slide 4

• Classified as E0-E7

• En

• n=10(1-b/a)

Ellipticals

M87: Giant elliptical (E1) Classified according to view from Earth!

Slide 5

Most of the galaxies in the local group are dwarf ellipticals!

M 110: dwarf elliptical (E6)

EllipticalsSlide 6

Elliptical characteristics

• Random star motions

• Little dust, little gas– gas is hot, can be X-ray sources

• no star formation

• no hot, young bright stars

• old, red stars

• no spiral structure

• Large range of sizes: 105-1013 Mo.

Slide 7

Spirals

• Like the Milky Way– central bulge– flattened disc– spiral pattern– sometimes a bar

• Classified as • S (for spiral) or • SB (barred spiral)• plus a,b,c,(d)

NGC 1365 SBbc

Slide 8

Credit: Marco Iacobelli (XMM SOC) & ESA

XMM-Newton Optical Monitor

NGC 2997: Sc

This one has no ‘B’ because it has no bar

Slide 9

Spiral classification

• Sa and SBa have:• large central bulges• Tightly-wound spiral

arms

• Sc and SBc have:• small central bulges• Loosely-wound spiral

arms

M81(Sab) NGC4321(Sc)

Slide 10

M81 UV (XMM))

Slide 11

M81 21cm

Slide 12

M100 (AAT))

Slide 13

M51 (HST))

Slide 14

Characteristics of spirals

• Consistent rotational motion in the disc• Lots of gas and dust in the disc• Star formation in disc, especially in the arms• Young, bright blue stars in the arms• Spiral structure

• 109 - 4x1011 Mo

Slide 15

Lenticular galaxies

• Lenticular -- ‘lens-like’• Like a spiral, but without spiral structure• Designated S0 or SB0• I think difficult to tell from ellipticals by eye.

Slide 16

Irregulars

• No definite structure– Everything that

doesn’t fit on the tuning fork

• Mostly small, faint• May have strong star

formation• May have lots of gas

and dust– e.g. LMC and SMC

Slide 17

LMC in the UV (Swift UVOT))

Slide 18

Back to Hubble’s tuning fork

• Hubble thought that galaxies evolved from ellipticals to spirals

• This is very unlikely!

Slide 19

Some key points about galaxies:

• Ellipticals– no overall rotation– old stars, little gas.

• Spirals– rotating disc– old stars in the bulge– young stars in the disc and in bulge

• Lenticulars– bulge and disc without spiral structure

• Irregulars– often lots of gas– sometimes strong star formation

Slide 20