Concept illustration of a grism image
This image represents the data that comes from using the NASA/ESA Hubble Space Telescope's highly-sensitive Wide Field Camera 3 in its grism spectroscopy mode.
A grism is a combination of a grating and a prism, and it splits up the light from a galaxy into its constituent colours, producing a spectrum. In this image the continuum of each galaxy is shown as a "rainbow".
Astronomers can look at a galaxy's spectrum and identify light emitted by the hydrogen gas in the galaxy. If there are stars being formed in the galaxy then the intense radiation from the newborn stars heats up the hydrogen gas and makes it glow.
All of the light from the hydrogen gas is emitted in a small number of very narrow and bright emission lines. For dwarf galaxies in the early Universe the emission lines are much easier to detect than the faint, almost invisible, continuum.