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18 - Structure of the Universe

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18 - Structure of the Universe. Extragalactic Distance Scale. Cepheids M V =-3.35logΠ-2.13+2.13(B-V) Π=period (days) Novae M V (max)=-9.96-2.31log(Δm/day) first 2 mags Planetary Nebulae Luminosity Function M 5007 (brightest)=-4.48

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extragalactic distance scale
Extragalactic Distance Scale
  • Cepheids MV=-3.35logΠ-2.13+2.13(B-V) Π=period (days)
  • Novae MV(max)=-9.96-2.31log(Δm/day)first 2 mags
  • Planetary Nebulae Luminosity Function M5007(brightest)=-4.48
  • Globular Cluster Luminosity Function MB(turnover)=-6.5
  • Tully-Fisher MH=-10.01log(2vr/sin i)+3.61
  • D-σ logD=1.333logσ + C (for relative distances)
  • Brightest Red Supergiants MV=-8.0
  • SN Ia MB(max)=-19.6 (but correct for decline time and redshift)
  • Brightest Galaxy in Cluster MV=-22.82
  • Surface Brightness Fluctuations
cepheid distance scale
Cepheid Distance Scale
  • L’s for PL relation from Cluster Fitting and a few (~6?) measured parallaxes
  • 1997 (Feast & Catchpole) - Hipparcos parallaxes for 223 Cepheids, of which 26 carry a lot of weight (accurate π’s and spread in P)
  • PLC relation due to width of instability strip (Sandage)
  • Dependent on metallicity
  • Affected by extinction (in near-IR brightnesses are less, too).
  • Blending light with nearby stars
  • Different methods give different results
35 SN Ia’s

1-day averages

Correcting for stretch and time dilation

Original Data

Corrected for Time Dilation (redshift z)

Corrected for Stretch

Slipher (1914-1925) - Radial Velocities of GalaxiesMost (“nearby”) galaxies exhibit spectral shift to longer wavelengths - “redshifts”
1929 - Hubble enters the picture

Note: data originally published in 1929 PNAS, not 1936!

the hubble law
The Hubble Law

actually first introduced by Lamaitre in 1927

These are valid only for small z.

For larger z, need to use true relativitstic formulation:

This gives the Hubble Law (for space with flat geometry - which it seems to be) as:

note implications
NOTE Implications:
  • This sort of law would be derived by any observer in the universe - everyone sees the same law. Everything is (overall) moving away from everything else at the same rate per unit distance.
  • Universe is expanding - space is expanding, carrying the matter with it.
  • The universe need not have a “center” for this to be true.
  • The age of a universe with no acceleration/deceleration is simply 1/H0.
  • If universal, such a law allows one to determine the distance of an object from its value of z.
h 0 from sn ia s
H0 from SN Ia’s

WMAP gives H0=71

large scale structure on many scales
Large-Scale Structure(on many scales)
  • Groups (N<50, D~2 Mpc)
  • Clusters (N~50 (“poor”) - thousands (“rich”), D~8 Mpc)

“regular” - spherical & centrally condensed (Coma)

“irregular” - not (Virgo)

  • Superclusters - clusters of clusters
m 81 group
M 82M 81 Group

NGC 3077

M 81



Chandra (X-rays)

VLA & Merlin (radio)

dark matter
Dark Matter

1933 - Fritz Zwicky uses the virial theorem to deduce the existence of “dunkle Materie” (dark matter) in the Coma cluster.

Helvetica Physica Acta, 6, 110 (1933)

M/L for Coma ~500, compared to ~3 locally.

Considers the possibility there may be “internebular matter” that is giving mass estimates of clusters of galaxies too high a value. Considers independent methods to get masses of individual galaxies...

“Method iv involves the observation of gravitational lens effects. Measurements of deflecting angles combined with data on the absolute distance of the “lens nebula” from the observer suffice to determine the mass of the lens nebula. The chances for the successful application of this method grow rapidly with the size of the available telescopes.” - Zwicky, ApJ, 86, 217 (1937).

hot intergalactic gas
Hot Intergalactic Gas

X-ray emission from Perseus Cluster. Probably >50% of all baryonic matter.

Accounts for a fraction of the “dark matter”. The rest is “non-baryonic”.

even larger scale structure
Even LARGER Scale Structure

CfA - single slice

CfA “hockey puck”

“Great Wall”

Local Supercluster

Pisces-Perseus supercluster