http://www.skyandtelescope.com.au 23the universe around them. Some of this
radiation was energetic enough to again
start stripping electrons from the
hydrogen atoms in the surrounding
intergalactic medium. As galaxy
formation took hold, bubbles
of ionised gas grew around new
galaxies, spreading outward like
some sort of disease. After another
500 million years or so, a time period
known as the Epoch of Reionisation, the
universe’s neutral hydrogen was almost
totally turned into plasma.
What did these reionising galaxies look like?
Observations show that when they were bursting into life,
themajorityofthesegalaxiesweresmall,justafewhundred
light-yearsacross—comparabletothesizeofindividual
star-formingregionsintheMilkyWayanditscompanions,
such as 30 Doradus. These young galaxies also had quite
lowmasses,perhapslessthan1%oftheMilkyWay’smass.
Simplynotenoughtimehadyetpassedforlargenumbersof
starstoformoutofthegasreservoirs,orforlotsofgalaxy
mergerstotakeplace.Nevertheless,observationsofdropout
galaxies show that the average rate of star formation ramped
upquicklyinthesesmall,low-massgalaxies,startinghalfa
billionyearsaftertheBigBang,ifnotearlier.
We also have some evidence that galaxies were settling
intorotatingdisksatquiteearlytimes.Butratherthan
sedately spinning, Milky Way–like pinwheels, which became
prevalentmorethanabillionyearslater,earlydiskgalaxies
were probably quite clumpy and turbulent.
Oneofthemostimportantdifferencesbetweenvery
high-redshiftgalaxiesandthoseweseeintoday’suniverse
is in the chemistry of their interstellar medium. Our Sun
formedfromagascloudcomposedmostlyofhydrogenanda
bitofheliumbut‘polluted’withheavierelements,knownto
astronomers asmetals,thatformedinpreviousgenerationsof
stars.Withoutmetalenrichmentoftheinterstellarmedium,
neither our planet nor the people on it could have formed.
Butthesemetalstaketimetobuildup:Theyaremadeinside
starsduringnuclearfusion,insupernovaexplosions,andin
otherextremecosmicevents.Soweexpectthefirstgalaxies
to be ‘metal poor’ compared to the Milky Way.
Alackofmetalswouldmeanthatthesegalaxiesshould
alsocontainrelativelysmallamountsofinterstellardust,
which tends to absorb starlight. The ionising radiation from
massivestarswouldescapethegalaxiesmoreeasily,making
thereionisationprocessquiteefficient.SEPOCH OF RECOMBINATION As the universe cooled enough
to allow electrons and positively charged nuclei to come together
into neutral atoms, the space between stars was freed of the ionised
plasma that trapped light. Photons escaped, free to roam the universe
as what is known as the cosmic microwave background. The Planck
telescope’s view of this radiation shows us the universe at 370,000
years of age.Moreover, the stars that formed from the metal-poor gas
would be metal-poor themselves. Metals absorb ultraviolet
photons, which is one reason why metal-poor stars tend to
emit lots more of this high-energy light compared to stars
today. Observations of some of the earliest star-forming
galaxies reveal gas containing oxygen and carbon atoms that
are missing more than one electron. Ultraviolet light must
have stripped these electrons, which means that much of the
starlight escaping from early galaxies was in the form of high-
energy photons. These observations again point to an efficient
reionisation process and a quick cosmic dawn.Pulling back the veil
Hubble’s observations have taken us far, but astronomers are
gearing up for two new telescopes that will help us see back
even further in time. One is the James Webb Space Telescope
(JWST), due to be launched in 2019.
“We have squeezed out every last drop of what the
Hubble Space Telescope has to offer,” explains Renske Smit
(University of Cambridge, UK), a hunter of distant galaxies.
“But the fact is, Hubble is limited in how far it can look back
in time due to the wavelength range that it can see. The light
from the very first stars is redshifted too far into the infrared,
out of Hubble’s view.”
“JWST’s uniquely designed infrared capabilities will allow
us to look back beyond where Hubble has seen,” Smit says.
On the ground, we can look forward to the next
generation of 30- to 40-metre megatelescopes. One of these
— the European Extremely Large Telescope (ELT) — is now
being built some 3,000 metres up in the Atacama Desert in
Chile. The ELT’s huge segmented primary mirror will provideWFARTHEST GALAXY In 2016 Hubble revealed the infant galaxy GN-
z11, a galaxy that existed just 400 million years after the Big Bang and
is the most distant galaxy known to date. The background image shows
the GOODS-N project, which combines images of a small square of sky
in Ursa Major from the Hubble, Spitzer and Chandra space telescopes,
FARTHEST GALAXY: NASA, ESA, AND G. BACON (STSCI); PLANCK MAP: ESA / PLANCK COLalong with data from ground-based telescopes.
LABORATION