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Starts With A Bang

Astronomers discover how energy escapes the galactic center

The galactic center is home to the most powerful engine in the Milky Way: a supermassive black hole. How does its energy ultimately escape?
The brightest X-ray ridges in this image, appearing in white, seem to trace a path away from the galactic center and perpendicular to the plane of the galaxy. Researchers think these features are the walls of a tunnel, shaped like a cylinder, which helps funnel hot gas as it moves upward along the chimney and away from the Galactic Center. X-ray data is shown in blue, alongside radio data in orange/red.
Credit: X-ray: NASA/CXC/Univ. of Chicago/S.C. Mackey et al.; Radio: NRF/SARAO/MeerKAT; Image Processing: NASA/CXC/SAO/N. Wolk
Key Takeaways
  • The center of the Milky Way, like the centers of most large, massive galaxies, contains a supermassive black hole weighing in at millions of times the mass of our Sun.
  • That strong gravitational pull tears any matter that passes too close to it apart, accelerating it, heating it, and spitting most of it back into the Universe at tremendous energies.
  • But where does that energy go, and how does it ultimately dissipate? A new, X-ray view of a “chimney” perpendicular to the galactic disk may finally reveal the answer.

Unlike the most energetic galaxies, our Milky Way is inactive.

milky way Gaia
Gaia’s all-sky view of our Milky Way Galaxy and neighboring galaxies. The maps show the total brightness and color of stars (top), the total density of stars (middle), and the interstellar dust that fills the galaxy (bottom). Note how, on average, there are approximately ~10 million stars in each square degree, but that some regions, like the galactic plane or the galactic center, have stellar densities well above the overall average.
Credit: ESA/Gaia/DPAC

Although we possess a supermassive black hole, it isn’t actively feeding.

This 20-year time-lapse of stars near the center of our galaxy comes from the ESO, published in 2018. Note how the resolution and sensitivity of the features sharpen and improve toward the end, all orbiting our galaxy’s (invisible) central supermassive black hole. Practically every large galaxy, even at early times, is thought to house a supermassive black hole, but only the one at the center of the Milky Way is close enough to see the motions of individual stars around it, and to thereby accurately determine the black hole’s mass. The actual number density of black holes in the Universe, and their number density as a function of mass, remains only poorly estimated, with large uncertainties remaining.
Credit: ESO/MPE

Actively feeding supermassive black holes represent the most energetic engines in the Universe.

centaurus A x-ray
The galaxy Centaurus A is the closest example of an active galaxy to Earth, with its high-energy jets caused by electromagnetic acceleration around the central black hole. The extent of its jets are far smaller than the jets that Chandra has observed around Pictor A, which themselves are much smaller than the jets of Alcyoneus, which are still smaller than jets found in massive galaxy clusters. This picture, alone, illustrates temperatures ranging from ~10 K to as high as several millions of K, and relativistic jets that are even physically larger than the stellar extent of the galaxy itself.
Credit: X-ray: NASA/CXC/CfA/R.Kraft et al Radio: NSF/VLA/Univ. of Hertfordshire/M.Hardcastle et al. Optical: ESO/VLT/ISAAC/M.Rejkuba et al.

Excepting the Big Bang, no astrophysical events outpower active galactic nuclei (AGNs) and quasars.

ophiuchus x-ray largest explosion cavity
Evidence for the biggest explosion seen in the Universe comes from a combination of X-ray data from Chandra and XMM-Newton. The eruption is generated by a black hole located in the cluster’s central galaxy, which has blasted out jets and carved a large cavity in the surrounding hot gas. Researchers estimate this explosion released five times more energy than the previous record holder and hundreds of thousands of times more than a typical galaxy cluster. The X-ray emitting gas can reach temperatures ranging from millions up to even ~100 million K.
Credit: X-ray: Chandra: NASA/CXC/NRL/S. Giacintucci, et al., XMM-Newton: ESA/XMM-Newton; Radio: NCRA/TIFR/GMRT; Infrared: 2MASS/UMass/IPAC-Caltech/NASA/NSF

From X-rays through radio waves, AGN and quasar engines shine luminously.

To both the left and right of a central, giant elliptical galaxy, multiple images, in X-ray light, of a quasar some ~6 billion light-years away can be seen. By combining data from NASA’s Chandra X-ray Observatory and the ESA’s XMM-Newton Observatory, scientists were able to measure the (rapid) spin of the quasar’s central supermassive black hole. This is just one of many overwhelming lines of evidence supporting the existence of black holes, with no good alternatives remaining.
Credit: X-ray: NASA/CXC/Univ of Michigan/R.C.Reis et al; Optical: NASA/STScI

Emitted bipolar jets of particles and radiation funneling energy outward into the cosmos.

relativistic jet black hole galaxy hercules A
While distant host galaxies for quasars and active galactic nuclei can often be imaged in visible/infrared light, the jets themselves and the surrounding emission is best viewed in both the X-ray and the radio, as illustrated here for the galaxy Hercules A. It takes a black hole to power an engine such as this, but that doesn’t necessarily mean that this is matter/radiation escaping from inside the event horizon.
Credit: NASA, ESA, S. Baum and C. O’Dea (RIT), R. Perley and W. Cotton (NRAO/AUI/NSF), and the Hubble Heritage Team (STScI/AURA)

However, our galaxy’s central black hole isn’t completely quiescent.

Even the Milky Way, a relatively quiet galaxy with a relatively small central supermassive black hole, exhibits giant geysers of charged particles emanating from the galactic center. They can be revealed by radio telescopes, such as this image constructed with data from the Parkes radio telescope, a.k.a. The Dish.
Credit: A. Mellinger (C. Michigan), E. Carretti (CSIRO), S-PASS Team, E. Bressert (CSIRO)

X-ray flares indicate the galactic center occasionally “snacks” on matter.

The supermassive black hole at the center of our galaxy, Sagittarius A*, emits X-rays due to various physical processes. The flares we see in the X-ray indicate that matter flows unevenly and non-continuously onto the black hole, leading to the flares we observe over time.
Credit: NASA/CXC/Amherst College/D.Haggard et al.

Enormous, low-density Fermi bubbles extend for ~25,000 light-years above and below the galactic plane.

energy injection fermi bubble
In the main image, our galaxy’s antimatter jets are illustrated, blowing ‘Fermi bubbles’ in the halo of gas surrounding our galaxy. In the small, inset image, actual Fermi data shows the gamma-ray emissions resulting from this process. These “bubbles” arise from the energy produced by electron-positron annihilation: an example of matter and antimatter interacting and being converted into pure energy via E = mc^2. We are certain that no antimatter signature in our galaxy arises from either antimatter stars or large clumps of antimatter.
Credit: David A. Aguilar (main); NASA/GSFC/Fermi (inset)

Within the galactic center itself, magnetic fields shape the flow of matter and radiation.

Milky Way center
Although the galactic center appears striking at the lower-right of this image, much more puzzling are the “loopy” features seen, which are evidence for filamentary strands of galactic magnetism. These non-thermal filaments have been predicted theoretically, but MeerKAT has identified and imaged them with unexpected and never-before-seen properties.
Credit: I. Heywood et al., 2022, ApJ

Stellar cataclysms — plus young, massive stars — are common in Sagittarius A*’s vicinity.

Milky Way center
This unprecedented view of the galactic center comes from the MeerKAT radio array in South Africa, and highlights never-before seen features, including filaments, previously unseen bubbles, and potentially new supernova remnants and star-forming regions as well.
Credit: I. Heywood et al., 2022, ApJ

How, then, is energy transported outward from the galactic center?

Infrared astronomical image showing various energy signatures in the Milky Way, with annotations indicating distances and coordinates.
An X-ray view of the galactic center in galactic coordinates shows the supermassive black hole at our galaxy’s core (black dot) relative to the location of the exhaust vent (blue dotted box, at left) within the southern part of the chimney (green box, at right) below the galactic center. One parsec (pc) is approximately 3.26 light-years in terms of distance/scale.
Credit: S. C. Mackey et al., ApJ submitted, 2024

The answer is revealed in X-ray data: through a central “exhaust vent” within a chimney-like structure.

Two side-by-side astronomical images of the Milky Way with color spectrums, highlighting differences in spatial data and intensity within a section of the universe.
This linear X-ray emitting feature, located within the southern portion of the galactic center chimney, suggests that a cylinder-shaped plasma outflow channel allows outflowing material to shock/compress/heat the interstellar medium. Serial eruptions may sustain this feature and others like it.
Credit: S. C. Mackey et al., ApJ submitted, 2024

Observed plasma outflow channels depart the galaxy’s center.

Milky Way center
This spectacular composite image, which combines X-ray, infrared, and optical light from NASA’s great observatories, was our best view of what’s going on in the galactic center as of 2009. Over the past ~15 years, however, we’ve taken data that has revealed novel features that, at present, have yet to be fully explained. Perpendicular to the plane of the galaxy, features rise above and below indicating the transport of energy and gas in a chimney-like fashion.
Credit: NASA/JPL-Caltech/ESA/CXC/STScI

Eruptions drive material upward, through the chimney, and outward: through this exhaust vent.

This image, made of composite X-ray (Chandra) and radio (MeerKAT) data, shows evidence for an exhaust vent attached to a previously-identified galactic chimney, highlighting how energy and gas is transported away from the galactic center over time.
Credit: X-ray: NASA/CXC/Univ. of Chicago/S.C. Mackey et al.; Radio: NRF/SARAO/MeerKAT; Image Processing: NASA/CXC/SAO/N. Wolk

Sequential accretion events possibly sustain this structure across long timescales.

Annotated map of the milky way's center in multiple wavelengths with identified regions and sources.
This image shows the magnetized galactic center, with various features highlighted, as imaged by the SOFIA/HAWC+ FIREPLACE survey team. The giant bubble at the left of the image is some 30 light-years wide, several times larger than any other supernova-blown bubble ever discovered.
Credit: D. Paré et al., arXiv:2401.05317v2, 2024

At last, energy transport within the Milky Way’s center finally makes sense.

This updated Radio/X-ray composite of the galactic center, featuring data from both MeerKAT and Chandra, showcases the new information that can be gleaned from stitching together multiple wavelengths of light. In the future, improved observations and superior observatories may help us solve the scientific mysteries of the origin of a variety of features, including lobes, bubbles, and sprites.
Credit: X-ray: NASA/CXC/UMass/Q.D. Wang; Radio: NRF/SARAO/MeerKAT

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