galaxies( - astronomy group – university of st andrewsstar-spd3/teaching/phys2220/phys... ·...
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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
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
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
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
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