Australian Sky & Telescope - May 2018

20 AUSTRALIAN SKY & TELESCOPE May | June 2018

If we want to hunt for the first galaxies, we need a lot

oflight-collectingarea—thatis,abigtelescope—anda

sensitivespectrograph.Still,therearefartoomanygalaxies

to measure the spectrum of each one. Technological

advances are helping to solve that problem), but astronomers

still commonly save time by narrowing down the search,

preselectingthosefewgalaxiesinanimageofthousandsthat

arelikelytobethemostdistantones.

In the 1990s astronomers developed a deceptively simple

andeconomicmethodtodojustthat,identifyinggalaxies

at very high redshift using images alone. The trick works

because galaxies, and the wider intergalactic medium,

contain a lot of neutral hydrogen gas. This gas is very

efficient at absorbing photons with wavelengths below 91.

nanometres.Thismeansthatthespectrumofatypicalgalaxy

inthedistantuniversehasapronounceddip,or‘break,’at

that wavelength. At wavelengths below this so-calledLyman

break, neutral hydrogen gas, either in the galaxy itself or in

theinterveningmedium,absorbsmostofthephotons.

Inpracticethismeansthatwecanuseafilterthatonly

lets through wavelengths of lightbelowthe Lyman break and

compareittoanimageofthesamepatchofskyfilteredto

only allow wavelengthsabovethe Lyman break. Some galaxies

appearto‘dropout’oftheshorter-wavelengthfilter—they

are simply too faint to detect in the bluest band.

Now, the clever bit is when cosmological redshift comes

intothepicture.Inmoredistantsources,theobserved

MODERN UNIVERSE

EPOCH OF REIONIZATION

First stars and

black holes

DARK

AGES

K

Cosmic

microwave

background

released

Ionized gas

(Plasma)

Ionized gas

Neutral gas (Plasma)

BIG

BANG

Electron

Atomic

nucleus+

SHISTORY OF THE UNIVERSE The period after the Big Bang, when charged particles trapped photons, gave way to the the Epoch of

Recombination, when particles combined into atoms and released the photons we now know as the cosmic microwave background. Soon after

that, in the Epoch of Reionisation, higher-energy emissions from the irst stars began once again stripping electrons off hydrogen atoms.

position of the Lyman break redshifts to longer wavelengths.

That means that a more distant galaxy will disappear at a

longer wavelength than a closer galaxy will. So, as we try

to chase the galaxies to higher and higher redshifts, we can

turn to different combinations of filters. The filter in which

a galaxy vanishes from view gives a pretty strong clue as to

the galaxy’s photometric redshift, even without a spectrum;

spectroscopy can later provide unambiguous confirmation.

But this method for determining redshift, now

more generally referred to as the dropout technique,

has a catch: Distant sources of light are also faint. So,

although the technique is efficient, astronomers still

need an exceptionally sensitive instrument and very deep

observations to find the earliest galaxies. Without question,

the instrument that has allowed us to best exploit the

dropout technique — and given us the clearest glimpse of the

earliest galaxies — is the Hubble Space Telescope. Hubble’s

Deep Fields are tiny ‘windows’ onto the distant universe,

keyholes that the telescope has spent millions of seconds

staring through to reveal thousands of galaxies in small

patches of sky no larger than 1% of the angular area of the

full Moon. Some of those galaxies appear merely as clumps

of a few pixels, nearly lost amid the noise. Nevertheless,

those pixels represent light from some of the earliest

galaxies we know about, and they have allowed us to start

piecing together the picture of galaxy formation within half

a billion years of the Big Bang. LEAH TISCIONE /

S&T

COSMIC DAWN