A Radio Burst from a Giant “Dead” Galaxy

For the first time ever, astronomers have detected a fast radio burst (FRB) in a large elliptical galaxy. The discovery, announced at the FRB2024 conference in Khao Lak, Thailand, supports earlier indications that there are various ways to form the extreme objects that produce these ultra-brief flashes of radio waves.

Astronomers first discovered fast radio bursts in 2007, and more than 800 have been observed to date (see S&T’s September 2022 issue). In just one-thousandth of a second, they radiate as much energy as the Sun does in a week. However, their origin is still a mystery.

Most FRBs for which accurate sky positions are known can be traced to small dwarf or spiral galaxies with a high rate of star formation. That’s one of the reasons why scientists think that the bursts occur on magnetars — young and strongly magnetized neutron stars left behind when massive stars end their short lives in catastrophic supernova explosions.

However, FRB 20240209A doesn’t fit this picture. The Canadian Hydrogen Intensity Mapping Experiment (CHIME) radio observatory in Canada detected it on February 9th, and between then and July, the same source produced another 21 bursts. In fact, FRB 20240209A was the first repeater whose sky position CHIME could precisely pinpoint, says Vishwangi Shah (McGill University, Montreal). That feat was possible thanks to the recent addition of a smaller outrigger telescope 66 kilometers (41 miles) away that dramatically increases the observatory’s spatial resolution.

Surprisingly, the bursts emanate from the outskirts of an old, massive, and luminous elliptical galaxy almost 2 billion light-years away that doesn’t show any signs of recent star formation. “It’s the most massive and luminous FRB host galaxy known to date,” says Shah. According to Tarraneh Eftekhari (Northwestern University), who presented observations of the galaxy by the Keck and Gemini North telescopes at the conference,  “it’s a very old and dead galaxy. Compared to other FRB hosts, it’s definitely an outlier.”

Given the galaxy’s properties, it’s extremely unlikely that FRB 20240209A occurred on a young magnetar resulting from the recent death of a massive star (a so-called core-collapse supernova) — especially since the burst location lies at least 130,000 light-years away from the galaxy’s center, the biggest offset for any FRB found to date, according to Shah. Massive stars are unlikely to form so far out.

In two as-yet unpublished papers (https://arxiv.org/pdf/2410.23374 and https://arxiv.org/pdf/2410.23336), Shah, Eftekhari, and their colleagues suggest that the repeating FRB could be located in one of the many old globular clusters expected to swarm around the large elliptical galaxy. If so, it might resemble FRB 20200120E, also discovered by CHIME, which sits in a globular cluster belonging to the nearby spiral galaxy M81.

With two fast radio bursts originating from very old stellar populations, it becomes evident that a core-collapse supernova is not the only way to produce the objects that generate FRBs. Alternative scenarios include the collision and merger of two old white dwarf stars or the non-explosive collapse of a particular type of white dwarf into a neutron star as a result of mass transfer from a companion (so-called accretion-induced collapse). Both of these events could potentially also result in magnetars, even billions of years after the birth of new stars died down.

However, coauthor Mohit Bhardwaj (McGill University) notes that “in the case of FRB 20240209A, you can’t completely rule out a regular core-collapse supernova.” To really know how often alternative scenarios play out, you’d need a bigger sample, he says.

That may not take too long. Sometime next year, CHIME will work with three outrigger telescopes (in Canada, California and West-Virginia), enabling extremely precise localizations for many hundreds or even thousands of fast radio bursts per year.