How to find a distant galaxy in the early Universe ?

Using the VLT + the gravitational telescope

To search for the most distant galaxies in the early Universe we combine two powerful tools:

1) The larges European optical and near infra-red telescope (the ESO VLT - Very Large Telescope) -- see here

2) The "gravitational telescope", or more precisely the natural phenomenon of "gravitational lensing" provided by clusters of galaxies, whose mass focusses and magnifies the light of distant galaxies lying behind them -- see here for more explanations.

The gravitational telescope "coupled" to the VLT allows us thereby to see much deeper in the Universe. In other words this corresponds to exploring the history of the Universe further in the past.

In the case of the record-breaking galaxy just discovered, its light is amplified at least 25. Without the gravitational lensing effect this faint galaxy would only be observable out to a distance of approximately 11.2 billion light-years, which is ~2.5 billion years after the Big Bang (redshift ~2.5). The "gravitational telescope" has allowed us to see this galaxy much further, at an age where the Universe was barely ~470 million years old!!

Selection trough spectral features

Given the high redshift (i.e. large distance) of such galaxies, they are observable only in the near infra-red. An appropriate instrument - here ISAAC/VLT - must hence be used.

To distinguish these distant galaxies from other objects (stars or galaxies) peculiar spectral signatures ("colors") are used. These criteria have partly been established from our previous simulations.

In this manner serveral "candidate" distant galaxies have been found in the observed field of the galaxy cluster Abell 1835. Among these we have been able to measure the distance (hence age etc.) for one of them, Abell 1835 IR 1916, the record-breaking galaxy.