Breaking Up Star Clusters: The Source of Blue Light in NGC1275
The galaxy NGC1275, sitting at the center of the Perseus Cluster, has long been one of astronomy’s great puzzles. While it is classified as a giant elliptical galaxy, its central regions contain unusual features, bright star clusters, a spiral-like disk, and mysterious arcs of bluish light. In this work, Levitskiy and collaborators investigate the origin of this extended blue light and propose that it comes from the tidal disruption of super star clusters (SSCs).
A Complex Galaxy at the Cluster Center
NGC1275 has fascinated astronomers for decades because of its tangled history. It is associated with the bright radio source 3C 84, and it hosts both a filamentary nebula of glowing gas and a second galaxy passing in front of it, called the High Velocity System. Observations have revealed a mix of old stars typical of massive galaxies, but also many young and massive star clusters, some resembling globular clusters in size but far younger. These were first called “super star clusters” (SSCs) to distinguish them from the more common “blue star clusters” found farther out in the galaxy.
The Extended Blue Light
One of the most striking features of NGC1275 is the diffuse blue glow stretching from about 5,000 to 14,000 light-years from its center. Earlier studies suggested that this light came from ongoing star formation caused by gas cooling out of the cluster environment. However, the arcs and trails within this blue glow hinted at a more complicated story. Levitskiy and colleagues argue that these arcs are the remnants of SSCs that have been pulled apart by the galaxy’s strong gravitational tides.
Observations and Analysis
To study the extended blue light, the authors used data from the William Herschel Telescope and the Hubble Space Telescope. Spectra and colors of the stars in this region show evidence of two distinct populations: an old population with high metallicity (rich in heavy elements), and a younger population with sub-solar metallicity. The young stars are about 160 million years old on average, with a total mass of several billion Suns. The light curves and absorption features suggest that the blue glow could not come from a single burst of star formation, but rather from disrupted clusters spread over time.
Linking to Super Star Clusters
The SSCs in the center of NGC1275 are slightly older, around 500 million years old, and have an unusual distribution of masses, shallower than expected if none had been destroyed. This supports the idea that many lower-mass clusters were torn apart, leaving behind stellar streams that now make up the arcs of blue light. In this view, the surviving SSCs are simply the most massive clusters that escaped total disruption.
A Trigger from the Active Galaxy
The authors connect this star cluster formation episode to an outburst of activity from the central black hole about 500 million years ago. Evidence for this comes from bubbles seen in X-rays, thought to be blown by jets from the galaxy’s active nucleus. These jets may have cooled the hot gas in the cluster’s core, fueling the birth of numerous star clusters. Over hundreds of millions of years, many of those clusters were disrupted, while the largest survived as SSCs in the central disk.
A Unified Picture
Putting it all together, Levitskiy and collaborators propose a scenario in which an AGN outburst triggered cluster formation in NGC1275. Some of these clusters survived as SSCs, others were disrupted to form the extended blue light, and the spiral disk of stars in the galaxy formed slightly later. This model provides a consistent explanation for the central spiral, the arcs of blue light, and the unusual mass distribution of the surviving SSCs. It also highlights how the activity of a galaxy’s black hole can shape not just its gas, but also its stars.
Source: Levitskiy
