Since the 1970s, astronomers have observed that supermassive black holes (SMBHs) reside at the centers of most massive galaxies. In some cases, these black holes accelerate gas and dust from their poles, forming relativistic jets that can extend for thousands of light-years. Using the NASA/ESA Hubble Space Telescope, a team of astronomers observed the jet emanating from the center of M87, the supermassive galaxy located 53.5 million light-years away. To their surprise, the team observed nova erupting along the jet’s trajectory, twice as many as they observed in M87 itself.
The team was led by Alec M. Lessing, a Stanford University astronomer, and included researchers from the American Museum of Natural History, the University of Maryland Baltimore, Columbia University, Yale University, the SETI Institute, and NASA’s Goddard Space Flight Center. The paper detailing their findings recently appeared in The Astrophysical Journal. Their research was part of a 9-month survey of the M87 galaxy using Hubble’s near-UV Cosmic Origins Spectrograph (COS).
To date, all novae have been observed in double-star systems consisting of a red giant star and a white dwarf companion. In these systems, the outer layers of the red giant are siphoned away by the white dwarf and accreted onto its surface. When the white dwarf has accumulated enough hydrogen, the layer explodes in a “nova eruption,” and the cycle begins again. When the team observed M87 using Hubble’s COS, they found twice as many novae eruptions near the 3000-light-year-long jet than in the galaxy itself during the surveyed period.
A Hubble image of M87 shows a 3,000-light-year-long jet of plasma blasting from the galaxy’s 6.5-billion-solar-mass central black hole. Credit: NASA/ESA/STScI/A. Lessing et al. (2004).
These findings imply that there are twice as many nova-forming double-star systems near the jet or that these systems erupt twice as often as similar systems elsewhere in the galaxy. Another possibility is that the jet is heating the red giant stars in these binary systems, causing them to overflow further and dump more hydrogen onto the dwarf companion. However, the researchers determined that this heating is not significant enough to have this effect. As Lessing explained in an ESA press release:
“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… 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.”
This is not the first time astronomers have noticed increased levels of activity around the M87 jet. Shortly after Hubble launched in 1990, astronomers observed the galaxy’s SMBH using its first-generation Faint Object Camera (FOC). These observations revealed “transient events” that could be evidence of novae, but the FOC’s view was too narrow to compare what was happening between the jet and in the near-jet region. Thanks to the nine-month campaign that relied on Hubble’s upgraded cameras (which have a wider view) and viewed the jet’s environment every five days, the team was able to count the novae along the jet’s trajectory.
Sag A* compared to M87* and the orbit of Mercury. Credit: EHT collaboration
The observations, which were the deepest images of M87 ever taken, revealed 94 novae within the M87 galaxy’s inner region. Said co-author Michael Shara, the Curator of Astrophysics at the American Museum of Natural History:
“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.”
These observations confirm that the venerable Hubble still has the capability to reveal new and interesting things about the Universe. In addition, these findings provide an opportunity for follow-up studies to learn more about how relativistic jets could influence star systems extending well beyond their galaxies.
Further Reading: ESA Hubble, The Astrophysical Journal
