Galaxies  Our  working  model  

•  Schema6c  •  Example  •  Components  

Galaxy  morphology  •  Ellip6cals  •  Spirals  •  Iregulars  •  HTF  v  bulges  &  discs  

Galaxy  fundamentals  •  number  of  stars  •  space  density  •  mean  separa6on  •  mass-­‐to-­‐light  ra6o  •  mean  mass  density  

Galaxies  –  AS  3011   2  

Our  Working  Galaxy  Model  

BULGE HALO

STELLAR DISK

HI GAS DISK GLOBULAR CLUSTER

COMPANION

NUCLEUS SMBH/AGN

Andromeda  (M31)  

BULGE  

DISK  

DUST  LANE  

COMPANION  

HII  REGION  

Galaxy  Components  •  Main  Ingredients  (%M):  –  Dark  MaRer  (90%)  

•  Baryonic,  e.g.  Dust,  neutrinos  •  Exo6c,  e.g.  WIMPS  

–  Stars  (9%)  –  Gas  Disk  (0.9%)  –  Planets,  Asteroids,  Comets  

•  Principle  Features:  –  Bulge  –  Halo  –  Disk  (Thin,  Thick,  Gas)  –  Spiral  Arms  

•  Other  (Interior)  –  Open  Clusters  –  Giant  Molecular  Clouds  –  HII  regions  –  Dust  lane  

•  Other  (Exterior)  –  Globular  Clusters  –  Tidal  tails  –  Polar  ring  –  Companion  

Galaxy  Morphology  

•  Hubble  Tuning  Fork:  1929  •  Originally  covered  Ellip6cals  to  Spirals  

Three  Generic  Galaxy  Types  

•  Ellip6cals:  E0-­‐E7  –  En  where  n  =  10(a-­‐b)  /  a  (a=major  and  b=minor  axis)  –  S0  or  Len6cular.  A  transi6on  class  where  a  very  faint  disk  is  just  seen  

•  Spirals:  Sa,  Sb,  Sc,  Sd  –  Sa  =  Dominant  Bulge,  6ghtly  wound  arms  –  Sb=  Obvious  Bulge,  spiral  arms  –  Sc=  Faint  bulge,  spiral  arms  –  Sd=  No  bulge,  diffuse  spiral  arms  

•  Irregulars:  Im,  Irr  –  m  =  Magellanic,  no  bulge,  asymmetrical  

1822 !<<! VM

Ellip6cal  •  Red  ,  i.e.,  (B-­‐V)  >1  •  Smooth  profile  •  High  Surface  Brightness  •  Egg  shaped  •  LiRle  or  no  dust  lane  •  Absorp6on  lines  only  •  Many  Globular  Clusters  •  No  rota6on  •  Found  in  Clusters  •  Typically:  

!22 <MV < !18

Ellip6cal  •  Red  ,  i.e.,  (B-­‐V)  >1  •  Smooth  profile  •  High  Surface  Brightness  •  Egg  shaped  •  LiRle  or  no  dust  lane  •  Absorp6on  lines  only  •  Many  Globular  Clusters  •  No  rota6on  •  Found  in  Clusters  •  Typically:  

=  Old  stellar  popula6on  =  Relaxed  old  system  =  Densely  packed  =  Massive/Old  =  Gas  reservoir  exhausted  =  No  star-­‐forma6on  =  Formed  via  mergers  =  Formed  via  mergers  =  Formed  via  mergers  =  Massive  

Why  are  Ellip6cals  red  ?  •  A  galaxy’s  light  is  dominated  by  the  stars    •  A  spectrum  of  a  galaxy  =  Sum  of  stellar  spectra  •  Stellar  spectra  ~  Black  body  ,  i.e.,     4TL!

I  

BLUE            λ            RED  

I  

BLUE            λ            RED  

Hot  &  Short  Lived  Star  

Cold  &  Long  Lived  Star  

An  Ellip6cal  Galaxy  Spectra  •  A  galaxy  spectrum  is  the  sum  of  many  stellar  spectra.      •  If  the  galaxy  is  no  longer  forming  stars  there  are  fewer  (short-­‐

lived)  blue  stars  the  overall  shape  will  look  red:  

I  

Blue                                        λ                                        Red    

FEW  *s   MANY  *s  

COMPOSITE  GALAXY    SPECTRUM  

Spirals  •  Red  bulge  (B-­‐V)  >1  •  Bluish  Arms/Disk,  (B-­‐V)  ~1  •  Moderate  Surface  Brightness  •  Dusty  •  Emission+absorp6on  lines  •  Rota6ng  disk  •  Numerous  Globular  Clusters  •  Seen  in  high  and  low  density  

environments  •  Typically:  

!21<MV < !17

Spirals  •  Red  bulge  (B-­‐V)  >1  •  Bluish  Arms/Disk,  (B-­‐V)  ~1  •  Moderate  Surface  Brightness  •  Dusty  •  Emission+absorp6on  lines  •  Rota6ng  disk  •  Numerous  Globular  Clusters  •  Seen  in  high  and  low  density  

environments  •  Typically:  

1721 !<<! VM

=  Central  bulge  is  old  =  Disk  is  s6ll  star-­‐forming  =  Relaxing  =  SF  will  con6nue  =  SF  ongoing,  old  &  young  pop  =  Formed  via  gas  collapse  =  plus  some  merging  =  Collapse+merging  

Irregulars  •  Blue  (B-­‐V)  <0.8  •  Strong  Emission  lines  •  Very  dusty  •  Low  surface  brightness  •  Highly  Asymmetrical  •  Rota6ng  •  Few  Globular  clusters  •  Typically:  

1018 !<<! VM

Irregulars  •  Blue  (B-­‐V)  <0.8  •  Strong  emission  lines  •  Very  dusty  •  Low  surface  brightness  •  Highly  Asymmetrical  •  Rota6ng  •  Few  Globular  clusters  •  Typically:  

1018 !<<! VM

•  Young  stellar  popula6on  •  Lots  of  Star-­‐forma6on  •  SF  will  con6nue  •  Forming  •  Forming/low  mass  •  Formed  via  collapse  •  Formed  via  collapse  

Other  Galaxy  Types  

•  Globular  Clusters(?)    •  Dwarfs  –  Dwarf  Ellip6cals  –  Dwarf  Irregulars  –  Dwarf  Spheroidals  

•  Crouching  Giants  –  LSBGs  or  Low  Surface  Brightness  Galaxies  

 

The  Hubble  Tuning  Fork  

•  Objects  classified  from  Early-­‐to-­‐Late  –  Ellip6cals  =  Early,  Spirals=  Late  

•  Not  an  evolu6onary  sequence  –   isolated  systems  will  not  spontaneously  start  to  rotate  

•  Spirals  subdivided  according  to  whether  they  exhibit  a  bar  or  not  –  i.e.,  Sa  or  SBa  

•  Lateness  is  given  by  the  bulge-­‐to-­‐disk  ra6o  and  6ghtness  of  the  spiral  arms  –  i.e.,  Sa,  Sb,  Sc  or  Sd  

•  Discovery  of  new  dwarf  types  is  now  making  HTF  unwieldy  –  dE,  dEn,  dSp,  dIrr,  dS0,  BCD,  UCD,  cE,  LSBG,  HSBG,  GC  

•  New  systems  recognises  three  types  or  two  primary  components  –  Ellip6cals,  Spirals,  Irregulars  –  Spheroids  (dynamically  hot),  discs  (dynamically  cool)  

SdSb Sc

SBa SBb SBc

HSBG

LSBG

MALINs

dE

dE(N)BCD

dISm

dS

ceP

cD E0 E7 S0 Im

CE

Sa

The  modern  day  tuning  fork  is  not  looking  so  elegant  as  we    discover  more  galaxy  types,  par6cularly  low  mass  systems.  Dwarf  popula6ons  represent  a  great  unknown…  

Back  to  square  one….  

3  types  or  2  components?  

Modern  view  is  that  galaxies  consist  of  two    primary  components,  a  spheroid  and  a  disc  

•  The  disc  may  also  contain  a  bar  •  The  spheroid  may  contain  a  nucleus  

Galaxy Fundamentals •  How many stars are in a galaxy ? •  How did galaxies form ? •  How many galaxies are there ? •  How far apart are they ? •  How are they clustered ? •  What is the mass of a typical galaxy ? •  What is the mass density of the Universe ?

How many stars in a Galaxy ?   Andromeda is at 0.9Mpc and has an apparent magnitude

mB=3.5 mag   1)   2) Adopt M*,B=+5.48 mag (i.e., Solar)   3)

  4)

M =m! 5log10 (d)! 25= !21.3 mag

10)(4.0*

*

**10

*10*

**

10510

)(log5.2)(log5.2

* !"=

#=#=#

=

## MM

GALGAL

GAL

GALnffn

ffMM

fnf

n*=50 billion stars

How many galaxies are there ?   STEP1: Take deep all sky images

  STEP2: Count galaxies brighter than some magnitude

  STEP3: Assume most galaxies are like the MW*

  STEP4: Calculate depth and volume of sky sampled

  STEP5: Calculate the SPACE-DENSITY of galaxies

  [* This is a bit of a fudge but works because the most easily detected galaxies are like the Milky Way, i.e., big bright spirals. This does not mean they’re the most numerous just the most visible!]

The Space Density of Galaxies •  For  example  the  MW  has  MB  =  -­‐20  mag  and  there  are  ~10,000    MW-­‐

like  galaxies  known,  brighter  than  14th  mag  over  the  whole  sky.  How  many  galaxies  are  there  per  Mpc  cubed  ?  

•  i.e.,  There  is  ~1  MW-­‐like  galaxy  every  100Mpc  cubed  

m =M + 5log10 (d)+ 25d =100.2[m!M!25] =100.2[14!(!20)!25]

d = 63Mpc

V =43!d3 = 4

3! (63)3

V =106Mpc3

n = NV=104

106=10!2gals /Mpc3

Use magnitude equation to get the distance =>

Use geometry to get the volume =>

n = number density

•  The mean separation of galaxies is therefore ~(100) = 4.6 Mpc

•  In reality though we know that galaxies are strongly clustered

How far apart are they ? 1/3

100Mpc 4.6 Mpc

3

Large Scale Structure in the Universe

Mass-to-Light Ratios •  Let  us  assume  that  the  amount  of  light  a  galaxy  emits  

relates  to  its  mass  •  i.e.,    there  exists  a  mass-­‐to-­‐light  ra6o  [we  will  explore  the  

validity  of  this  later]  •  Typically  this  is  expressed  in  solar  units:  

     X=1  For  our  sun      X~10  For  a  galaxy  

!

!=L

XL

MM Solar Mass

Solar Luminosity

Mass-to-light ratio

The Mass of M31   Given that the average mass-to-light ratio is about 10 what mass

does this imply for M31 which has MV = -20.5 mag ?

!

M =MLL =10M"

L"L =10M"

LL"

M =10M"10#0.4(MV #MV" )

M =10 $ 2 $1030 $10#0.4(#20.5#4.6)

!"="= Mkg 1141 101.1102.2M

M for mass M for Absolute magnitude

The Density of the Universe •  By multiplying together the space density of galaxies and the

mass of the typical galaxy we can get an approximate value for the density of the visible Universe:

•  This is only the matter that is inside galaxies. •  However…

32839 /10~/101.1 mkgMpcn !"#== MM$

•  More precise calculations incorporate the dynamics of galaxies and clusters, yield the total density (including luminous and dark matter in galaxies and clusters):

•  Most of matter in Universe we cannot see! •  Mass of hydrogen atom: mH=1.7 x 10-27 kg •  If the Universe was smoothly spread out there would be a

couple of hydrogen atoms per cubic metre. But the air we breathe contains about 1025 atoms per cubic metre.

327 /1042~ mkg!"!#

The Density of the Universe