This is the loneliest galaxy in the known Universe

All throughout the cosmos, stars and galaxies appear everywhere.

The main galaxies of Stephan’s Quintet, as revealed by JWST on July 12, 2022. The galaxy on the left is only about ~15% as distant as the other galaxies, and the background galaxies are many scores of times farther away. And yet, they’re all equally sharp to JWST’s eyes, demonstrating that the Universe is full of stars and galaxies practically everywhere we look.

In our own backyard, we inhabit the Local Group of galaxies.

This three-dimensional view of the Local Group showcases the three largest galaxies and their relative positions in space: Andromeda (M31), the Milky Way, and Triangulum (M33). Both Andromeda and Triangulum are visible with the naked human eye, as are the Large and Small Magellanic clouds. Over the next few billion years, these galaxies will interact and merge, as our entire Local Group is gravitationally bound.

We’re one of many groups on the outskirts of a large galaxy cluster.

The various galaxies of our local supercluster, dominated by the dense Virgo Cluster of galaxies. Here, all galaxies appear grouped and clustered together. On the largest scales, the Universe is uniform, but as you look to galaxy or cluster scales, overdense and underdense regions dominate. Each galaxy group and cluster within this larger structure will remain gravitationally bound, individually, but the groups making up the apparent, larger structure are not bound together. As the Universe expands, they will disappear from one another’s reach.

On grander cosmic scales, our Universe clusters into a great filamentary network.

This snippet from a medium-resolution structure-formation simulation, with the expansion of the Universe scaled out, represents billions of years of gravitational growth in a dark matter-rich Universe. Note that filaments and rich clusters, which form at the intersection of filaments, arise primarily due to dark matter; normal matter plays only a minor role. The larger-scale your simulation is, however, the more that smaller-scale structure is intrinsically underestimated and “smoothed-out.”

Galaxies inhabit these filaments, with clusters forming at their nexuses.

The Coma Cluster of galaxies, as seen with a composite of modern space and ground-based telescopes. The Coma Cluster, the largest galaxy cluster in our nearby Universe, is dominated by two giant elliptical galaxies, with over 1000 other spirals and ellipticals inside. The speed of the individual galaxies within the Coma Cluster is too great for the cluster to remain a bound entity based on its normal matter content alone. It can only remain a bound object under Einstein’s laws of General Relativity if a substantial amount of additional matter, i.e., a source of dark matter, exists throughout this cluster. The total mass of the cluster comes in at a few quadrillion solar masses.

But the matter-poor space between these filaments creates great cosmic voids.

A region of space devoid of matter in our galaxy reveals the Universe beyond, where every point is a distant galaxy. The cluster/void structure can be seen very clearly, demonstrating that our Universe is not of exactly uniform density on all scales. While there are many galaxy-rich regions, galaxy-poor or even galaxy-free regions are also abundant, like holes within a cosmic Swiss cheese.

These underdense regions lose their matter to the surrounding denser areas.

Over time, gravitational interactions will turn a mostly uniform, equal-density Universe into one with large concentrations of matter and huge voids separating them. Because simulations are limited in the number of particles they can handle at once, the largest-scale cosmic simulations are inherently limited in their ability to resolve individual, early galaxies. However, the great underdense regions, the cosmic voids within our Universe, are well-understood.

They typically have fewer, smaller, and fainter galaxies than the richer clustered regions.

The relative attractive and repulsive effects of overdense and underdense regions on the Milky Way are mapped out here on distance scales of hundreds of millions of light-years. Overdense and underdense regions both pull and push on matter, giving it speeds of hundreds or even thousands of kilometers in excess of what we’d expect from redshift measurements and the Hubble flow alone. These giant collections of galaxies can be divided up into superclusters, but their underdense counterparts, the great cosmic voids, play an equally important role on cosmic scales.

However, one relatively deep void possesses the remarkable galaxy MCG+01-02-015.

The galaxy MCG+01–02–015, shown here, lies in the center of a void in the direction of the constellation Pisces. The galaxy, located some 293 million light-years away, is the only known galaxy within 100 million light-years of its position.

This impressive, Milky-Way like spiral has many classic features.

The spiral arms of galaxy NGC 6384 are where new stars primarily form in this galaxy. Under normal circumstances, spiral arms in the disk of a spiral galaxy are where the majority of new stars form. With many features in common with our own Milky Way, NGC 6384 is one of the best candidates for a near-twin of the Milky Way. Its similarities to MCG+01-02-015 are also striking.

It has a gas-rich disk, dusty arms, central bar and bulge, and abundant heavy elements.

In between the great clusters and filaments of the Universe are great cosmic voids, some of which can span hundreds of millions of light-years in diameter. While some voids are larger in extent than others, spanning a billion light-years or more, they all contain matter at some level. Even the void that houses MCG+01–02–015, the loneliest galaxy in the Universe, likely contains small, low surface brightness galaxies that are below the present detection limit of telescopes like Hubble.

But it’s centrally located within a particularly sparse, underdense “void” region.

Despite the visual appearance of other galaxies near MCG+01-02-015, none of these galaxies are actually located in the same region of space. With only one or two notable exceptions of nearby, small, faint galaxies, almost all of the galaxies shown in this image lie behind MCG+01-02-015; they are background galaxies simply found along the same line-of-sight.

Our best telescopes identify no other substantial galaxies within ~100 million light-years.

The Large (top right) and Small (lower left) Magellanic Clouds are visible in the southern skies, and helped guide Magellan on his famous voyage some 500 years ago. In reality, the LMC is located some 160-165,000 light-years away, with the SMC slightly farther at 198,000 light-years. Both galaxies, along with Triangulum and Andromeda, are visible to the naked human eye.

Contrariwise, four galaxies beyond the Milky Way are visible to naked human eyes.

Hubble’s discovery of a Cepheid variable in the Andromeda galaxy, M31, opened up the Universe to us, giving us the observational evidence we needed for galaxies beyond the Milky Way and leading us, in short order, to the discovery of the expanding Universe.

Their extragalactic nature was confirmed with 1920s technology.

Edwin Hubble’s original plot of galaxy distances versus redshift (left), establishing the expanding Universe, versus a more modern counterpart from approximately 70 years later (right). In agreement with both observation and theory, the Universe is expanding, and the slope of the line relating distance to recession speed is a constant.

This cosmic companionship first revealed the expanding Universe.

Italian astronomer Paolo Maffei’s promising work on infrared astronomy culminated in the discovery of galaxies — like Maffei 1 and 2, shown here — in the plane of the Milky Way itself. Maffei 1, the giant elliptical galaxy at the lower left, is the closest giant elliptical to the Milky Way, yet went undiscovered until 1967. For more than 40 years after the Great Debate, no spirals in the plane of the Milky Way were known, due to light-blocking dust that’s very effective at visible wavelengths.

But uncovering another galaxy, from within MCG+01-02-015, requires 1960s-level telescopes.

Galaxy 3C 295, at the center of the galaxy cluster ClG J1411+5211, is shown with a composite X-ray/optical view in purple, with the X-rays blown up to reveal the central radio and X-ray loud core. At 5.6 billion light-years away, this was the most distant object known in the Universe from 1960-1964. Only with X-ray or radio telescopes, or with an enormous optical telescope, would the first extragalactic objects from MCG+01-02-015’s perspective be detectable.

From this isolated perspective, discovering our cosmic origins would’ve been far more challenging.

According to the original observations of Penzias and Wilson, the galactic plane emitted some astrophysical sources of radiation (center), but above and below, all that remained was a near-perfect, uniform background of radiation. The temperature and spectrum of this radiation has now been measured, and the agreement with the Big Bang’s predictions are extraordinary. If we could see microwave light with our eyes, the entire night sky would look like the green oval shown.

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words.