skyandtelescope.com • JANUARY 2020 23
star-formation rate. Combined with multiwavelength data
from other instruments, these results show that star forma-
tion across the universe peaked between 2.3 and 3.8 billion
years after the Big Bang and has been decreasing ever since.
Astronomers refer to this period of rampant starbirth as
cosmic high noon.
Swathed in dust, many of the distant galaxies we see are
faint at visible wavelengths, even though they blaze in the
infrared. As we look back in time, the concentration of galax-
ies that are oddly bright in the infrared skyrockets. These
systems appear to be predominantly powered by vigorous star
formation, with hundreds to thousands of solar masses of
gas being converted each year into stars. The starbursts are
almost entirely obscured by dust. Thus, the most active period
of star formation in the universe is largely hidden from view
in visible light and accessible only with infrared observations.
For far more distant galaxies, those with a redshift of 6 or
greater (or a lookback time of 12.5 billion years or more), the
galaxy’s light has been stretched so much that the glow from
star-heated dust is undetectable by Spitzer. For a galaxy at red-
shift 6, an observed wavelength of 4.5 microns corresponds to
an emitted wavelength of 0.64 micron, which lies at the red
edge of the visual band. Thus for high redshifts, Spitzer tells
us not about the thermal emission from galaxies but about
the visible light they emit.
This visible light comes from the galaxies’ older stars.
Because these older stars dominate a galaxy’s stellar popula-
tion, we can use their light to measure the total mass of stars
in the galaxy.
Astronomers can also compare Spitzer observations to
those by Hubble or ground-based instruments to extract the
age of the stars producing the ultraviolet and visible light
that’s been redshifted to Spitzer’s domain by cosmic expan-
sion. Observations of one such galaxy, at a redshift of 9.11,
indicate the stars are approximately 300 million years old.Reaching into the Past
The basic tool used to discover galaxies with high red-
shifts is the Lyman dropout technique (S&T: Apr. 2018,
p. 14). This method utilizes the fact that neutral
hydrogen atoms become ionized when they absorb photons with wavelengths shorter than 0.09 micron. So the
universe, which is suffused with neutral hydrogen gas, is effectively opaque to such photons. Thus, if an image
obtained at 0.5 micron shows a galaxy that is not seen at 0.4 micron, we infer that the redshifted wavelength
of hydrogen ionization falls between the two bands, at about 0.45 micron. From there, we can calculate that
the galaxy has a redshift of approximately 4.uGALAXY IN INFRARED The spiral arms of M81 in Ursa Major become
more dramatic in this infrared composite (top). Blue traces the distribu-
tion of stars, whereas green is radiation from hot dust. In visible light, the
bulge is what catches the eye (bottom).INF
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