A new study of more than 200,000 galaxies, from the ultraviolet to the far infrared, has provided the most comprehensive assessment of the energy output of the nearby universe. It confirms that the radiation produced by stars in galaxies today is only about half what it was two thousand million years ago. This overall “fading” reflects a decrease in the rate of star formation via the collapse of cool clouds of gas. It seems that the universe is running out of gas – in effect, getting out of breath – and slowly dying.
It is well known to astronomers that the rate of star formation in the universe reached a peak around a redshift z = 2, when the universe was about 3 Gyr old. Over the subsequent 10 Gyr until now, the production of stars in galaxies has steadily decreased in a given co-moving volume of space – that is, a volume expanding at the same rate as the cosmic expansion of the universe, therefore keeping a constant matter content during the history of the universe. Because the most massive stars are also the most luminous ones and have the shortest lifetimes, the energy output of a galaxy is closely related to its star-formation rate. Indeed, some 100 million years after the formation of a star cluster, its brightest stars would have exploded as supernovas leaving only the lower-mass stars, which are much less luminous.
Although the fading trend of the universe has been known since the late 1990s, measuring it accurately has been a challenge. Part of the difficulty is to gather a representative sample of galaxies at different redshifts and to account properly for all biases. Another complication comes from the obscuration by dust in the galaxies, which absorbs ultraviolet and visible radiation and then re-emits this energy in the infrared. A way to overcome these difficulties is to observe the same region of the sky at many different wavelengths to cover fully the energy output. This has now been achieved by a large international collaboration led by Simon Driver from the International Centre for Radio Astronomy Research (ICRAR), University of Western Australia.
The study is part of the Galaxy and Mass Assembly (GAMA) project, the largest multi-wavelength survey ever put together. It used seven of the world’s most powerful telescopes to observe more than 200,000 galaxies, each measured at 21 wavelengths from the ultraviolet at 0.1 μm to the far infrared at 500 μm. Driver and collaborators then used this unique data set to derive the spectral energy distribution of the individual galaxies, and the combined one for three different ranges of redshift up to z = 0.20. For the nearest galaxies, they obtain an average energy output of (1.5±0.3) × 1035 W produced on average by galaxies in a co-moving volume of a cubic megaparsec, which is equivalent to a cube with a side of about 3.3 million light-years. While this is for a redshift range between z = 0.02 and z = 0.08, corresponding to a mean look-back time of 0.75 Gyr, the team finds a significantly higher value of (2.5±0.3) × 10 W35 for a look-back time of 2.25 Gyr (0.14 < z < 0.20). This indicates a decrease by about 1035 W in 1.5 Gyr. This trend occurs across all wavelengths and corresponds roughly to a decrease by a factor two over the past two thousand million years.
The ongoing decay of energy production by stars in galaxies also follows the trend of active galactic nuclei and gamma-ray bursts, which were all more numerous and powerful several gigayears ago. The shining, glorious days of the universe are now long past; instead, it will continue to decline, sliding gently into old age, an age of quiescence.