Hubble Finds Evidence of Black Hole's Impact on Stellar Activity

Astronomers using NASA's Hubble Space Telescope have discovered that stars appear to erupt along the path of a supermassive black hole's blowtorch-like jet, located at the center of a massive galaxy. The stars, known as novae, appear in a hazardous area close by rather than inside the jet. The study was published in the journal High Energy Astrophysical Phenomena.

We don't know what's going on, but it's just a very exciting finding. This means there's something missing from our understanding of how black hole jets interact with their surroundings.

Alec Lessing, Study Lead Author, Stanford University

In a double-star system, an aging, swollen normal star releases hydrogen onto a burned-out white dwarf companion star, causing a nova to burst. A mile-deep layer of hydrogen on the surface of the dwarf explodes like a massive nuclear bomb once it has tanked up. The nova eruption, which expels its surface layer and then resumes drawing fuel from its companion, does not destroy the white dwarf; instead, it resets the nova-outburst cycle.

During the survey period, Hubble discovered twice as many novae exploding near the jet as observed elsewhere in the giant galaxy. A central black hole with 6.5 billion solar masses that is encircled by a disk of spinning matter is the source of the jet. Full of material falling into it, the black hole fires a plasma jet that spans 3,000 light-years and travels through space almost as quickly as light. The energetic beam would sizzle anything it caught. According to the latest Hubble findings, however, it also appears to be dangerous to be close to its blistering outflow.

The discovery of twice as many novae close to the jet suggests that either twice as many nova-forming double-star systems are present there or that the frequency of eruptions of these systems is twice that of similar systems in other parts of the galaxy.

There's something that the jet is doing to the star systems that wander into the surrounding neighborhood. Maybe the jet somehow snowplows hydrogen fuel onto the white dwarfs, causing them to erupt more frequently. But it's not clear that it's a physical pushing. It could be the effect of the pressure of the light emanating from the jet. When you deliver hydrogen faster, you get eruptions faster. Something might be doubling the mass transfer rate onto the white dwarfs near the jet.

Alec Lessing, Study Lead Author, Stanford University

Another idea the researchers considered is that the jet is heating the dwarf's companion star, causing it to overflow further and dump more hydrogen onto the dwarf. However, the researchers calculated that this heating is not nearly large enough to have this effect.

We're not the first people who've said that it looks like there's more activity going on around the M87 jet. But Hubble has shown this enhanced activity with far more examples and statistical significance than we ever had before.

Michael Shara, Study Co-Investigator, American Museum of Natural History

Hubble's first-generation Faint Object Camera (FOC) was used by astronomers to look into the center of M87, home to the monster black hole, soon after the spacecraft's 1990 launch. They observed that the black hole was surrounded by strange activity. Hubble's observations nearly always revealed blue "transient events," which may be proof that novae are exploding like paparazzi flashes in the vicinity. However, because of the FOC's extremely limited field of view, Hubble astronomers were unable to compare the near-jet region with the jet. The results remained enigmatic and alluring for more than 20 years.

Hubble observed the erupting novae over nine months using newer, wider-view cameras, providing compelling evidence for the jet's impact on the host galaxy's stars. The telescope's observing schedule was put to the test because a second snapshot of M87 had to be taken exactly every five days. The deepest M87 images ever captured were obtained by summing up all of the M87 images.

Within the third of M87 that Hubble's camera can see, it discovered 94 novae.

“The jet was not the only thing that we were looking at — we were looking at the entire inner galaxy. Once you plotted all known novae on top of M87 you didn't need statistics to convince yourself that there is an excess of novae along the jet. This is not rocket science. We made the discovery simply by looking at the images. And while we were really surprised, our statistical analyses of the data confirmed what we clearly saw,” said Shara.

This achievement is solely due to Hubble's exceptional capabilities. Images obtained with ground-based telescopes are not clear enough to see novae located deep within M87. The surrounding brightness of the black hole prevents them from resolving stars or stellar eruptions near the galactic center. Against the bright M87 background, only Hubble is able to identify novae.

The universe is filled with remarkably frequent novae. Every day, one nova erupts from somewhere in M87. However, since the visible universe contains at least 100 billion galaxies, a million novae explode every second somewhere in the cosmos.

After more than 30 years of operation, the Hubble Space Telescope is still producing revolutionary researchers that influence our basic conception of the cosmos. Hubble is an international collaboration between the European Space Agency (ESA) and NASA.

The telescope and mission operations are overseen by NASA's Goddard Space Flight Center located in Greenbelt, Maryland. Denver also supports Goddard mission operations, Colorado-based Lockheed Martin Space. NASA's Hubble science operations are carried out by the Association of Universities for Research in Astronomy-run Space Telescope Science Institute in Baltimore, Maryland.

Journal Reference:

Lessing, M. A., et al. (2024) A 9-Month Hubble Space Telescope Near-UV Survey of M87. II. A Strongly Enhanced Nova Rate near the Jet of M87. High Energy Astrophysical Phenomena. doi.org/10.48550/arXiv.2309.16856