Detailed Astrophysical Properties of Lyman Break Galaxies

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Astro 105 Our Place in the Universe
Lecture 10

• Lecturers
• J.P.Ostriker
• A.E.Shapley
• J.E. Gunn
• P. Steinhardt

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Reading

• Chapters 5-7 in Rees, Just Six Numbers
• Science article, The Cosmic Triangle
• Scientific American article, Surveying
  Spacetime with Supernovae
• Link to John Huchras Hubble Constant Webpage
  http//cfa-www.harvard.edu/huchra/hubble/
• Problem Set 4 is due on Thursday, November 10th

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Overview

• Expansion rate of the Universe
• -- Expansion rate today, H0
• -- Expansion history, H(t)
• What do you need to measure expansion rate?
• -- VH0 x d
• -- Velocities (redshifts)
• -- Distances

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Redshifts (the easy part)
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Redshift/Doppler Shift
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Redshift/Doppler Shift

• We measure the spectra of galaxies (light is
  dispersed into different wavelengths)
• The spectra contain features from stars and gas
• These features have known wavelengths (e.g. from
  Hydrogen, Oxygen, and Nitrogen in gas clouds
  where stars are forming, or from Calcium, Iron,
  and Magnesium from stars that have already
  formed)

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Emission line spectra HII regions

• NGC604 (right) is a star-forming region,
  so-called HII region
• Galaxies forming stars contain HII regions, as
  seen on the left
• Radiation from young, massive stars ionizes
  Hydrogen, gas is heated to 10,000 K, emission
  from H, O (other metals) is highlighted
• What is the spectrum?

(HST/WFPC2 image from Yang Hester, NASA)
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Emission line spectra galaxy

• Strong features from H, O, N, S
• Example, Ha emission line from Hydrogen. Rest
  wavelength is 6563 Å. Observed wavelength in this
  spectrum is 7153 Å
• z l/l0 - 1 0.09

SDSS galaxy at z0.09
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Absorption line spectra galaxy

• Strong stellar absorption features, no gas
• 4000 A break, which appears at 5520 Å, 5175 Å Mg
  line which appears at 7141 Å A.
• zl/l0-10.38

SDSS galaxy at z0.38
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Distances (the hard part)
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Distance Measures

• Distances are very tricky to measure, compared
  with redshifts, which you can read directly off
  of the spectrum.
• At the same time, distances were crucial for
  establishing the scale of the Milky Way and the
  Universe
• Distances are also crucial for measuring the
  expansion rate of the Universe

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Distance Measures

• Standard ruler (compare angular size with
  physical size, get distance)
• Standard candle (you know the intrinsic
  luminosity, and measuring the apparent brightness
  tells you how far away something is)
• An important idea is that of The Distance
  Ladder, which well talk about later
• Certain types of distance indicators work best
  close by, and can be used to calibrate distance
  indicators that work at very large distances.

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Distance Measures

• What are some ways of measuring distances?

• Parallax, Main Sequence fitting (close by)

• Cepheids (out to 20 Mpc)

• Tully Fisher Relation, Type Ia Supernovae
  (really far out, to zgt1, which is gt1000 Mpc),
  Sunyaev-Zeldovich Effect, gravitational lensing,
  surface-brightness fluctuations

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What are Cepheids?

• Pulsating stars, pulsation period ranges from
  one to a few hundred days (NB Polaris is a
  Cepheid)
• Very bright, can be observed out to 20 Mpc
  (100-104 times more luminous than the sun)
• Two types of Cepheids (Type I classical, found
  in open clusters, and Type II, found in globular
  clusters)

LR2T4
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What are Cepheids?
Light curve of d Cephei, eponymous Cepheid,
period is 5-6 days.
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Cepheids History

• First Cepheid variable found in 1784 by amateur
  English astronomer, John Goodricke
• Henrietta Swann Leavitt (1868-1921), a computer
  at Harvard College Observatory
• Hired to measure positions and brightnesses of
  stars in astronomical photographs, including
  those of the Small and Large Magellanic Clouds

LMC
SMC
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Cepheids History
Women working at the Harvard College observatory
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Cepheids History

• Leavitt found that the period of pulsation is
  related to the brightness A remarkable relation
  between the brightness of these variables and the
  length of their periods will be noticed.

Data for Cepheids in the SMC
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Cepheids History

• This is a big deal!
• However, it only tells you the relative
  brightnesses of different Cepheids. In order to
  know how far away they are, you need to know the
  absolute brightness of a Cepheid of a given
  period.
• The P-L relationship needs to be calibrated.
  Another way of saying it is that we need to know
  the zeropoint of the Cepheid P-L relationship.
• Instead of LPa, LConstant x Pa

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Cepheids History

• Ejnar Hertzprung, Danish astronomer (1873-1967),
  found that most stars make a roughly
  one-dimensional sequence in temperature and
  luminosity, plotted as the Hertzprung-Russell (or
  H-R) diagram
• 1914 Used motion of the sun to obtain
  statistical parallax to Cepheids
  in our MW (estimate of their distances)
• With distances, and apparent
  brightnesses, enabled a calculation of
  luminosities -- the first attempt to calibrate
  the P-L relation

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Cepheids History

• Shapley used Cepheids to map out the Milky Way
  (used Type II when he thought they were Type I,
  but thats another story)
• Hubble used Cepheids in M31/Andromeda and M33 to
  find their distances
• Now Cepheids are the cornerstone of our estimate
  of H0, the expansion rate of the Universe,
  because they provide accurate distance
  measurements to galaxies out to 20 Mpc. We now
  know H0 with only a 10 uncertainty.

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History of H0, and Current Value

• Why is H0 important?
• Expansion of the Universe is one of the main
  pieces of evidence for the Big Bang
• H0 indicates the age of the universe (1/H0)
  (compare with ages of the oldest stars)
• H0 indicates the scale of the universe (c/H0)

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History of H0, and Current Value
Hubbles value, H0500 km/s/Mpc
Today, H072 km/s/Mpc
Why was Hubble so far off?
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History of H0, and Current Value
Freedman
de Vaucouleurs
Sandage
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History of H0, and Current Value
Best value of H072 km/s/Mpc, from the HST Key
Project. The Universe is 13.4 billion years old.
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History of H0, and Current Value

• HST Key Project, led by Wendy Freedman
• Use Cepheids to measure distances to galaxies out
  to 20 Mpc
• Then use Cepheid distances to calibrate other
  distance indicators that go further out to 1000
  Mpc and beyond
• Secondary distance indicators calibrated by
  Cepheids include Type Ia SNe (1000 times further
  out than Cepheids) and the Tully Fisher relation
• Example find Cepheid distance to a galaxy in
  which a Type Ia supernova explosion occurred

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Other Distance Indicators

• The Distance Ladder
• Use nearby methods to calibrate Cepheids
• Find Cepheid distances to galaxies within 20 Mpc
• Use Cepheid distances to calibrate Type Ia
  Supernovae luminosities
• Look out to zgt1

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Other Distance Indicators

• The Distance Ladder
• Tully-Fisher relation
• Relation between rotational speed of a galaxy and
  its luminosity
• Measure rotational speed and predict luminosity
• Measure flux
• Comparison of flux and luminosity yields distance

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Other Distance Indicators
The Distance Ladder Type Ia Supernovae -- very
very bright explosions (billion times Lsun) What
are the progenitors? Model binary star, mass
gets dumped onto White Dwarf until it gets pushed
over the Chandrasekhar limit --gt Explosion!
White Dwarf
It turns out that Type Ia Supernovae are not
exactly standard candles
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Expansion History of the Universe

• So far, weve talked about measuring H0, the rate
  at which the Universe is expanding today
• It is also fundamental to measure how the
  expansion rate has evolved with time.
• To do this, we take advantage of the finite speed
  of light.
• If a galaxy is at a distance of d Mpc, it will
  take light a time td/c to reach us. The further
  away galaxies are, the further back in time we
  see.
• The redshifts of galaxies at great distances
  indicate the expansion rate when the Universe was
  younger

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Next time

• More on the expansion history of the Universe
• What are the other parameters of our Universe,
  in terms of the global matter and energy density,
  and the geometry?
• What is cosmological redshift?