Nuclear star clusters are dense and compact stellar systems, with sizes of a few parsecs, found at the centers of many galaxies. Their formation is thought to be closely connected to the assembly history of their host galaxies, and astronomers think that these clusters contain important clues about how galaxies formed and evolved over cosmic time. Recent studies suggest that different formation pathways may operate in late- and early-type galaxies, but the dominant mechanisms and their dependence on galaxy morphology remain unclear.
While most observational studies have focused on early-type galaxies, here we investigate the nuclear star cluster of M74 (NGC 628), a massive, gas-rich spiral galaxy with active star formation, observed as part of the Physics at High Angular resolution in Nearby GalaxieS (PHANGS) survey. In this work, PHANGS data from the Multi Unit Spectroscopic Explorer (MUSE) integral-field spectrograph, mounted at the Very Large Telescope (VLT) in Paranal (Chile) were used. In these observations, the very small nuclear star cluster (few parsecs in size) is not spatially resolved and its light is spread on a larger region by the point spread function (PSF). Furthermore, within a PSF aperture, the light of the nuclear star cluster is blended with that of the host galaxy, coming from other components overlapped in the line of sight. Therefore, a special technique was used to isolate the nuclear-star-cluster light from that of the host galaxy. This technique consists of a two-dimensional spectro-photometric decomposition of the MUSE cube, employing a modified version of a previous code (C2D, Méndez-Abreu et al. 2019), which was now adapted for nuclear star clusters. This method provided separate data cubes for the nuclear star clusters and the host galaxy, allowing for their comparison in a PSF aperture, as well as a spatially resolved analysis of the host.
The stellar-population analysis from these cubes reveals a very old and metal-poor nuclear star cluster, in contrast to the surrounding regions, which are much younger and richer in metals. While similar properties have been found in nuclear star clusters hosted by galaxies with masses and morphologies different from those of M74, such a cluster is somewhat unexpected in a relatively massive star-forming spiral galaxy. The spatially resolved stellar populations of the host galaxy display much younger ages and higher metallicities, especially in the central region (∼500 pc) surrounding the nuclear star cluster.
These results suggest that the nuclear star cluster formed at very early epochs, evolved passively until today, and experienced little subsequent growth, despite the continued evolution of its host galaxy. The absence of younger stellar populations in the nuclear star cluster suggests that little gas reached the central few parsecs over the last eight billion years. This makes the nuclear star cluster of M74 a valuable fossil record of the earliest stages of galaxy assembly and highlights how central star clusters can preserve information about the formation history of their host galaxies.
