News, January 2024
Massive galaxy formation and the reversal of the star formation-density relation from a SCUBA-2 survey of clusters at z=1.6-2.0
Galaxy clusters at high redshifts show accelerated star formation, according to new research by Prof. Ian Smail.
A submillimetre survey of massive clusters at high redshifts found overdensities of galaxies with far-infrared luminosities indicative of star formation rates of at least 100 solar masses per year. The integrated star formation rates in the clusters were found to be two orders of magnitude higher than in local systems, suggesting that star formation in clusters increases more rapidly than in the surrounding low-density field as we look back in time. The results also indicate that the strongly star-forming galaxies responsible for this rapid evolution are likely to be the progenitors of the most massive galaxies in present-day clusters.
Galaxy clusters are the largest collapsed structures in the Universe, consisting of hundreds or thousands of galaxies held together by gravity. In the local Universe, clusters of galaxies are generally devoid of star-forming galaxies, but more distant clusters are known to contains populations of star-forming galaxies. However, it has been unclear how rapidly these star-forming cluster populations change over cosmic time and when (if ever) their activity exceeds that in galaxies in the lower-density field.
To investigate this, Prof. Smail used Durham-funded time on the SCUBA-2 instrument on the James Clerk Maxwell Telescope in Hawaii to survey eight clusters of galaxies at redshifts of 1.6 to 2.0. SCUBA-2 measures emission simultaneously at wavelengths of 450 and 850 micrometers, which corresponds to the submillimeter range. These wavelengths are ideal for detecting emission from cool dust in galaxies which is a tracer of star formation.
The survey uncovered concentrations of dusty galaxies in the centres of the clusters with far-infrared luminosities corresponding to star formation rates greater than 100 solar masses per year (˜ 100 × the current star formation rate of the Milky Way). When combined with earlier studies of systems at lower redshifts (see Figure), these observations demonstrated that the star formation in clusters increases more rapidly with redshift than in the lower-density field, where galaxies tend to be more strongly star forming than those in clusters at low redshifts, such that the activity in the clusters exceeds that in typical field galaxies at redshifts of about 1.8, or 10 billion years ago. This marks the epoch where galaxy formation becomes more active in higher density environments, the reverse of the behaviour seen in the local Universe.
The estimated stellar masses of these submillimeter galaxies in the clusters indicate that their descendants will likely be among the most massive galaxies observed in present-day clusters. This reinforces the idea that the majority of massive early-type galaxies in local clusters likely formed through intense, but dust-obscured, starburst events at redshifts greater than 1.5 to 2.
In summary, Prof. Smail's research demonstrates that galaxy clusters at high redshifts exhibit intense star formation rates, with rates two orders of magnitude higher than in local systems. The results suggest that star formation in clusters increases very rapidly as we look back in time, more rapidly than in the field, and that this accelerated activity led to the formation of most massive galaxies in present-day clusters through intense starburst events in the early Universe.