HST Observations of the Earliest Galaxies

HST HST Observations of the Earliest GalaxiesObservations 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

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

yearsbillion 141))((

HodHo

d

Vr

dT

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

due to the relativistic Doppler shift, we may express z as

can be re-written in order to obtain the recessional velocity of what is observed in terms of c

1)1(

1)1(2

2

z

z

c

Vr

1/1

/1

cVr

cVrz

rest

restobsz

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