Scientists have studied the closest, largest, brightest galaxies to Earth for centuries.

Located close to the edge of the handle of the Big Dipper, the Whirlpool galaxy, M51, is a classic example of a close, bright, nearby spiral galaxy. (JEAN-DANIEL PAUGET / FLICKR)

Messier 51, the Whirlpool Galaxy, is one of astronomy’s most spectacular objects.

This sketch from the mid-1840s is the first ever one to reveal the spiral structure of any nebula in the night sky. Now known to be a spiral galaxy, Messier 51, the Whirlpool Galaxy, is one of the most well-studied galaxies beyond our Milky Way. (WILLIAM PARSONS, 3RD EARL OF ROSSE (LORD ROSSE))

This enormous, face-on galaxy was the first one ever to reveal its spiral structure.

Modern observations can reveal gas, dust, and stars in the optical, ultraviolet, and near-infrared from most observatories on Earth. Both M51 and its companion display fascinating extended properties. (ADAM BLOCK / MOUNT LEMMON SKYCENTER / UNIVERSITY OF ARIZONA)

The small object alongside it, the galaxy NGC 5195, is interacting and merging with the Whirlpool galaxy.

This ultraviolet image of Messier 51, taken by GALEX, reveals the hottest, youngest, most newly-formed stars found in the merging system of the Whirlpool galaxy and its smaller companion. Note how the gas-rich spiral galaxy forms new stars, but the gas-poor companion does not.(NASA / JPL-CALTECH / GALEX)

Such mergers trigger new waves of star formation, create grand spiral arms, and activate supermassive black holes.

The gas and dust radiates at much cooler temperatures than the stars, and can be imaged by an infrared observatory like NASA’s Spitzer. Note how much rich gas is present in the central regions; that gas should be feeding the central, supermassive black holes. (NASA / JPL-CALTECH / SPITZER SPACE TELESCOPE)

Both galaxies pull on each other, funneling gas onto each central black hole.

When material gets accelerated and funneled into the enormous magnetic field surrounding a supermassive black hole, it can get ‘beamed’ in a particular direction. When those beams arrive at our eyes, we see a tremendous increase in flux. Galaxies undergoing mergers are expected to have their black holes activated. (KIPAC / SLAC / STANFORD)

This matter then accelerates and gets ejected along powerful jets, producing X-ray emissions.

The jet of the active galaxy Pictor A, with X-rays in blue and radio lobes in pink. When galaxies merge together, they’re expected to activate similarly to how this one has. (X-RAY: NASA/CXC/UNIV OF HERTFORDSHIRE/M.HARDCASTLE ET AL., RADIO: CSIRO/ATNF/ATCA)

Prior studies with NASA’s Chandra X-ray telescope showed fewer X-rays than expected.

The galaxy Messier 51, observed in the X-ray, shows point sources and extended emissions that correspond to neutron stars, black holes, and very hot gas. The central regions are far fainter than expected for merging galaxies with supermassive black holes, so it was anticipated that the energy would be found with higher-energy X-ray observatories.(NASA/CXC/WESLEYAN UNIV./R.KILGARD ET AL.)

Exploring higher energies, NuSTAR still showed the same missing X-ray problem.

Bright green sources of high-energy X-ray light captured by NASA’s NuSTAR mission are overlaid on an optical-light image of the Whirlpool galaxy (the spiral in the center of the image) and its companion galaxy, M51b (the bright greenish-white spot above the Whirlpool), taken by the Sloan Digital Sky Survey. The bright green spots at the center of the Whirlpool and NGC 5195 are created by material surrounding supermassive black holes; additional X-ray sources in the vicinity contribute to the emission. The known ultraluminous neutron star is located on the left side of the Whirlpool. (NASA/JPL-CALTECH, IPAC)

Excessive emission isn’t present in these cores; the galactic centers are even outshone by outlying neutron stars.

A composite image of M51, also known as the Whirlpool Galaxy, shows a majestic spiral galaxy. Chandra finds point-like X-ray sources (purple) that are black holes and neutron stars in binary star systems, along with a diffuse glow of hot gas. Data from Hubble (green) and Spitzer (red) both highlight long lanes of stars and gas laced with dust. A view of M51 with GALEX shows hot, young stars that produce lots of ultraviolet energy (blue). (X-RAY: NASA/CXC/WESLEYAN UNIV./R.KILGARD ET AL; UV: NASA/JPL-CALTECH; OPTICAL: NASA/ESA/S. BECKWITH & HUBBLE HERITAGE TEAM (STSCI/AURA); IR: NASA/JPL-CALTECH/ UNIV. OF AZ/R. KENNICUTT)

This is problematic, according to lead author Murray Brightman:

Galactic mergers are supposed to generate black hole growth, and the evidence of that would be strong emission of high-energy X-rays. But we’re not seeing that here.

Earlier observations of the Whirlpool galaxy with observatories like Chandra and XMM-Newton showed fewer soft (i.e., low-energy) X-rays than anticipated. By exploring higher energies with NASA’s NuSTAR mission, scientists were anticipating that the missing energy would appear there, showcasing an active black hole after all. But the black hole’s lack of activity persists. Astronomers have a puzzle to solve. (NASA / JPL-CALTECH / NUSTAR (NUCLEAR SPECTROSCOPIC TELESCOPE ARRAY))

These results imply black holes flicker on and off more rapidly than anticipated.

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. Why some galaxies are active and others are inactive is a deeper puzzle than astronomers realized, and observing the Whirlpool galaxy with NuSTAR is what exposed this hole in our understanding in the first place. (NASA/CXC/CFA/R.KRAFT ET AL.)

Further research is needed; the mystery remains unsolved for now.