Transcript HST Observations of the

HST
Observations of the Earliest Galaxies
Expansion of the Universe
 1912 Edwin Hubble discovered that a galaxy’s recessional velocity Vr is
proportional to its distance d
 known today as Hubble’s law and the constant of proportionality Ho is called
Hubble constant
 current best estimate of Ho is given by 70 km/s/Mpc
 assuming that the radial velocity of a galaxy remains constant throughout time;
the time it took for the a given galaxy to be at its present distance from us can
be calculated by the following formula
T
d
d

1
 14 billion years
Ho
Vr ( Ho )( d )
 Hubble’s law implies that all the galaxies in the universe started moving from
a single point and is the foundation for the Big Bang Theory
Big Bang Theory
 the Universe consisted entirely of radiation at a single point that erupted into a
rapidly expanding mass of hot gas called plasma
 300,000 years later temperatures within the expanding plasma cooled to about
3,000 K which is a temperature low enough for protons to bond with electrons
to form neutral hydrogen
 light from the initial big bang began a 500-million year fade to black, thus the
beginning of the dark ages
 between 100 million and 300 million years later, dark matter began to
gravitationally form into dark matter halos
Big Bang
 dark matter halos collapsed thus allowing the formation of stars in isolation
within the halo
 stars eventually burned/ionized their surrounding shell of neutral hydrogen
allowing their photons to escape and begin re-ionizing the universe.
 eventually the fog of neutral hydrogen lifted, and the universe became
transparent once again
Redshift
 as a photon moves through space, its wavelength is increased by the expansion
of the universe, shifting them towards a redder spectrum
 astronomers may use redshift as a means of expressing time
 astronomers observe a star or galaxy moving away from or toward us by using
spectral lines; most common line used is the Lyman-alpha line of hydrogen
 Lyman-alpha line is found to be at 121.5 nanometers under laboratory
conditions. This is inside the ultraviolet spectrum normally too short to be
observed visually. However due to redshift this line is shifted into the red and
infrared wavelengths
 Redshift Desert: A difficult but hard to observe point in time when galaxies
began assembling themselves and started to assume more “normal” forms.
Parameters for Redshift
 redshift parameter z is used to describe the change in wavelength which is
defined as
z
obs  rest
rest
 due to the relativistic Doppler shift, we may express z as
z
1  Vr / c
1
1  Vr / c
 can be re-written in order to obtain the recessional velocity of what is observed
in terms of c
Vr ( z  1) 2  1

c ( z  1) 2  1
Hubble Space Telescope (HST)
 named after Edwin Hubble, the man credited for the foundation of the big
bang theory
 launched on April 25, 1990 with an expected life of 20 years and at a cost at
launch of $1.5 billion
 currently there has only been 4 servicing missions to the HST to date; the last
one was Servicing Mission 3B on February 2002
 Operational costs to run and maintain the largest telescopes in the world is
approximately fifty thousand dollars a day or about one dollar a second.
Hubble Space Telescope estimated operational cost is twice as much.
HST Specifications
 Hubble Space Telescope consists of two mirrors. The primary mirror is at a
diameter of 2.4m while the second is a minor 0.3m
 a combined weight of mirrors is 840.3 kg
 HST detectors rage from visible, near-infrared and near ultra-violet (100nm –
2200 nm)
Hubble Deep Field (HDF)
 Ten consecutive days of image collection between December 10 and 28, 1995
with 342 separate exposures of the same region each ranging between 15 and
40 minutes
 images taken of a small region of space near the handle of the big dipper
because it did not become obstructed during orbit by the Earth or Moon and it
is also relatively free of foreground stars and other galaxy clusters
 images of the region were taken using ultraviolet, optical and infrared
wavelengths and then later combined and their distance, ages, and composition
of many galaxies were able to be calculated
 faintest galaxies are less than a 30th magnitude and have never been seen by
telescopes before
HDF
 Individual stars known as Cepheids were observed in the distant galaxies, and
knowing how bright they appeared and relating them to other Cepheid stars
which were used as standard candles, we can estimate the distance to the
galaxies.
 calculated estimates have the nearest galaxies within the HDF approximately
2.5 billion light years from us and the most distant are 10.5 billion light years
 galaxies in the HDF are smaller than those in the present universe and stay
small even at faint magnitudes; this agrees with the theory of the universe
being open rather than closed
 In October 1998 HST made a second deep field exposure, nearly equivalent to
the HDF except the region selected was in the southern sky. The resulting
exposure was very similar to the HDF. Both images contained roughly about
the same number of odd-looking galaxies
Hubble Ultra Deep Field (HUDF)
 observations began Sept. 24, 2003 and continued through Jan. 16, 2004
 million-second-long exposure reveals the first galaxies to emerge from the
time shortly after the big bang when the first stars reheated the cold
 two different instruments are used : Hubble's Advanced Camera for Surveys
(ACS) and the Near Infrared Camera and Multi-object Spectrometer
(NICMOS)
 both reveal images not resolvable by ground-based telescopes and the quality
also surpasses the images taken by HDFs back in 1995 and 1998
 search for galaxies that existed between 400 and 800 million years
(corresponding to a redshift range of 7 to 12) after the big bang
HUDF
 estimated 10,000 galaxies in HUDF which is located in the constellation
Fornax, the region below the constellation Orion
 ACS uncovered galaxies that existed 800 million years after the big bang (at a
redshift of 7)
 NICMOS may have spotted galaxies that lived just 400 million years after the
birth of the cosmos (at a redshift of 12)
 NICMOS: the longer infrared wavelengths are sensitive to galaxies that are
intrinsically red, such as elliptical galaxies and galaxies that have red colors
due to a high degree of dust absorption
 ACS picture required a series of 800 exposures taken over the course of 400
Hubble orbits around Earth