27.04.2023

For the first time, astronomers have seen how the big plasma jet shot out by a supermassive black hole connects to the material falling into the black hole.

Last week, I wrote about a new look at the ring of light around M87*, the beefy supermassive black hole that squats at the center of the elliptical galaxy M87, in the Virgo Cluster. Today in Nature, astronomers have unveiled another image of the glow around this black hole — this time made with new observations.

Ru-Sen Lu (Shanghai Astronomical Observatory) and an international team used a global network of radio dishes to peer deep into M87’s heart, following the galaxy’s 5,000-light-year-long plasma jet back to its source. The network is different than the one used by the Event Horizon Telescope project, albeit with some overlap (both in sites and in people).

The EHT observes at 1.3 mm, which enables astronomers to bypass the plasma haze hiding the black hole and detect the glow of material just outside it. The leviathan’s own gravity bends this glow into a ring around where it sits, invisible. But the array did not have enough telescopes to resolve both the ring and the jet at this wavelength.

The new work sacrifices the innermost region to include the jet. The team used the Global Millimetre VLBI Array, ALMA, and the Greenland Telescope to study M87 at a wavelength nearly three times longer, 3.5 mm. The data reveal a thick ring around a central, darker region — which may strike you as familiar. Meanwhile, the jet is a fairly hollow, parabolic cylinder, its edges connecting to bright regions in the ring, which is likely where material is feeding into the jet from the disk of accreting stuff.

This image shows the jet and shadow of the black hole at the center of the galaxy M87. Only barely discernible in the main image is a darkening in the center of the "radio core" the jet emanates from. A spruced-up zoom reveals the underlying ring structure. This image gives scientists the context needed to understand how the powerful jet is formed.
Credit: R.-S. Lu (SHAO), E. Ros (MPIfR), S. Dagnello (NRAO / AUI / NSF)

“This is an amazing result that shows how the jets connect to the accretion disk,” says black hole astrophysicist Sasha Tchekhovskoy (Northwestern University), who wasn’t involved with the work.

The 3.5-mm ring is about 50% larger than that in the EHT data. This is because the central region is foggy at this wavelength, smearing details. Farther from the black hole, the fog thins out, and the ring-like structure emerges. This same size difference shows up in simulations by Koushik Chatterjee (Harvard), Tchekhovskoy, and others of how the shadow of M87* would appear at these wavelengths.

Both the 3.5-mm and 1.3-mm rings likely involve photons coming from the same material, Lu says. It was unclear whether the 1.3-mm emission came from the inner accretion flow or the jet’s base. But given what they see at 3.5 mm and how it compares with simulations, the team thinks the emitting material is the accretion disk. There are also signs of a wind blowing off and outward from the inflow, creating a turbulent mess around the black hole.

Along with the jet’s edges, there’s a third ridge in the radio image: a spine running along the jet’s center. Astronomers have seen such structures on scales 10 times larger before. It’s unclear why the spine exists — it might be because the jet’s core travels faster than the sheath, or due to changes in flow density, energy, or magnetic fields, Lu says.

Simulations don’t include a central ridge, because our current models don’t know what shines inside jets and so remove any emission from the jet interior, Tchekhovskoy says. Incorporating ridge observations into calculations will help us understand how particles accelerate to the whopping energies they do in jets, he adds.

Below, you’ll find a video summarizing the result.

Reference:

Ru-Sen Lu et al. “A ring-like accretion structure in M87 connecting its black hole and jet.” Nature. April 26, 2023.

Quelle: Sky&Telescope