Composite of the galaxy image (left) and the spectrum (right)

A colour image (left) of a spiral galaxy in the Hubble Ultra Deep Field, one of the targets included in the Hubble Legacy Archive (HLA) release of slitless spectra from the Advanced Camera for Surveys (ACS). The colour picture was created from ACS images taken through the broadband filters F435W, F606W, F775W and F850LP. The galaxy is a typical spiral with a yellow stellar body and blue-white knots. While the image reveals the colour of the knots, it does not reveal their physical nature. The slitless spectrum (right) provides a one-shot spectrum of the whole galaxy and its constituent parts by spreading all the light in the image into its component colours. The spectrum, formed by combining many exposures with a total exposure time of 22 500 s, shows that the knots shine with very strong emission lines of doubly ionised oxygen (highlighted in green) and hydrogen (highlighted in red), revealing that these are regions of star formation in which the gas is heated by massive stars, as is typical in our own Milky Way and similar spirals. A redshift of 0.42 can be measured from the emission lines in the spectrum, meaning that the galaxy’s light has taken about 4 billion years to reach us.