Understanding how star formation ceases is one of the most significant challenges faced by astrophysicists today. As the Universe evolves, galaxies typically transition from vibrant hubs of activity, glowing with the brilliant blue light of massive, newly-formed stars, to much quieter and less active states known as quiescence.

This phenomenon is commonly referred to as quenching, but recent research utilizing data from the Hubble Space Telescope has unveiled that quenched galaxies may not be as lifeless as once believed.

Led by astrophysicist Michael Rutkowski from Minnesota, the research team has discovered what they have termed 'zombie galaxies'. Despite being classified as quenched, these galaxies still exhibit sporadic, significant bursts of star formation that challenge the conventional understanding of galactic evolution.

The galaxies under investigation are some of the most massive in the Universe, which logically should have ceased star formation long ago. The study focused on objects from the UVCANDELS survey (Ultraviolet Imaging of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey Fields), one of several deep imaging surveys conducted by Hubble in recent years. The researchers specifically selected galaxies that possess a stellar mass exceeding 10 billion times that of the Sun and that showed no indications of active black holes to ensure the integrity of their final sample.

Active black holes, found at the centers of galaxies, radiate bright light, thus complicating the observations of surrounding structures. The redshifts of the galaxies studied ranged from 0.5 to 1.5, allowing researchers to look back in time approximately 5 to 9 billion years, a period shortly after the cosmic rate of star formation had peaked.

Out of the 1,067 galaxies that remained in the study, many appeared to be the large, unremarkable elliptical types one might expect to have halted star formation earlier in their development. In contrast, it is typically spiral galaxies that showcase the majority of stellar nurseries in our present-day Universe.

What sets this study apart is the availability of ultraviolet observations for these galaxies. Ultraviolet light is crucial for tracing hidden star formation, especially since the farther away a galaxy is, the more its light is redshifted to longer wavelengths. The luminous, massive young stars, which have relatively short lifespans, emit brightly within this ultraviolet spectrum.

Using this data, the research team determined that around 15% of the galaxies in their sample were sufficiently bright to indicate that these seemingly dormant galaxies had experienced a recent uptick in star formation, contributing no more than 10% of their stellar mass over the last billion years. For a galaxy of this size, even a modest star formation event represents a substantial resurgence of activity.

The question arises: how do these galaxies come back to life? One plausible explanation for their revival could be the phenomenon of galaxy mergers. By merging with a younger, more active galaxy, a quenched galaxy could gain a fresh influx of gas, the raw material required for star formation.

However, this theory is complicated by the lack of correlation between the density of the environment in which a galaxy resides and the likelihood of it exhibiting recent star formation. Galaxy mergers tend to occur more frequently in denser environments, so a strong connection should be evident if this were the cause of the observed resurgence in star formation. Therefore, it suggests that there may be underlying processes at play that are yet to be understood.

It is possible that the black holes residing at the centers of these galaxies, operating in a dormant state that renders their growth hard to detect, might be influencing the star formation process somehow. Looking ahead, plans are already underway to obtain better images and spectra of these fascinating systems to further explore their characteristics.

After all, who doesn’t enjoy a good zombie movie?

Chris Lintott was reviewing 'Recent Star Formation in 0.5