Sustained formation of progenitor globular clusters in a giant
  elliptical galaxy
  Globular clusters (GCs) are thought to be ancient relics from the early
formative phase of galaxies, although their physical origin remains uncertain.
GCs are most numerous around massive elliptical galaxies, where they can
exhibit a broad colour dispersion, suggesting a wide metallicity spread. Here,
we show that many thousands of compact and massive (~5$\times$10$^{\rm
3}-$3$\times$ 10$^{\rm 6} M_{\odot}$) star clusters have formed at an
approximately steady rate over, at least, the past ~1Gyr around NGC 1275, the
central giant elliptical galaxy of the Perseus cluster. Beyond ~1Gyr, these
star clusters are indistinguishable in broadband optical colours from the more
numerous GCs. Their number distribution exhibits a similar dependence with
luminosity and mass as the GCs, whereas their spatial distribution resembles a
filamentary network of multiphase gas associated with cooling of the
intracluster gas. The sustained formation of these star clusters demonstrates
that progenitor GCs can form over cosmic history from cooled intracluster gas,
thus contributing to both the large number and broad colour dispersion$-$owing
to an age spread, in addition to a spread in metallicity$-$of GCs in massive
elliptical galaxies. The progenitor GCs have minimal masses well below the
maximal masses of Galactic open star clusters, affirming a common formation
mechanism for star clusters over all mass scales irrespective of their
formative pathways.
