Nishant Mishra, Department of Astronomy, University of Michigan
Light from a background quasar at redshift z = 0.8 crosses the gas around twenty foreground
galaxies, and many faint intergalactic clouds, on its way to the Hubble Space Telescope. Astronomers use
redshift as a measure of distance. The universe expands while light travels, so light from a source at
redshift z arrives with its wavelength stretched by a factor of 1 + z, and this quasar's
light set out about 7 billion years ago. The spectrum's top axis gives the wavelengths at which the
quasar emitted the light, and its bottom axis follows them as they stretch, until they match the
wavelengths Hubble observes. Each crossing
imprints absorption lines at that gas's own redshift, so the same atomic transitions from different
galaxies land at different observed wavelengths. The spectrum grows from a clean quasar continuum into
a dense forest of overlapping lines. Each galaxy leaves many lines, and lines from galaxies at very
different distances can fall on top of each other, so every line must be assigned to the right galaxy
and redshift before the gas can be measured.