The predictably weird distribution of galaxies

The Local Universe contains a vast structure teeming with only one type of galaxies, apparently violating the standard cosmological model. A new type of cosmological simulation shows that this remarkable configuration is, in fact, precisely what the standard model predicts.
Published in Astronomy
The predictably weird distribution of galaxies
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The "standard model" of cosmology posits that all structures observable in the Universe today originated from random quantum fluctuations that were created during the Big Bang, amplified by inflation, and grown under the force of gravity over 13.8 Gyr until the present day.

On small scales, the physical processes of galaxy and structure formation can be enormously complex, but basic tenet of the standard model is that, on very large scales, the universe appears both homogeneous and isotropic. In apparent contradiction, the Local Universe contains a very large, pancake-like concentration of galaxies measuring around one billion light years (300 Mpc) across. Even more surprisingly, this excess of galaxies in the so-called "supergalactic plane" applies to only one type of galaxy: while the most massive ellipticals dominate in the plane, the most massive spiral or disk galaxies appear to avoid it. This observational fact, known for more than 40 years, features prominently among the list of "cosmological anomalies" recently compiled by Jim Peebles, the 2019 physics Nobel laureate.

The fact that the most massive elliptical galaxies also contain the most massive black holes, whose formation early on in the universe remains an open question, invites an intriguing possibility: perhaps both the alignment of elliptical galaxies and the formation of supermassive black holes are caused by so-called cosmic strings, a proposed extension that would break the isotropy of the standard cosmological model?

Distribution of the brightest galaxies in the Local Universe, as observed in the 2MASS survey (left panel) and as reproduced in the SIBELIUS simulation (right panel)
Distribution of the brightest galaxies in the Local Universe, as observed in the 2MASS survey (left panel) and as reproduced in the SIBELIUS simulation (right panel). Both panels show a hammer projection in supergalactic coordinates, for a redshift range z=0.01 - 0.02 (approximately 85 Mpc). The nearly vertical dark stripe represents the region of the sky masked by our own Milky Way galaxy. The simulation accurately reproduces the structures seen in the Local Universe.

To answer this question, and to see whether the observed galaxy distribution can be explained within the standard model, our work uses a supercomputer simulation of the Local Universe. While most similar simulations consider random patches of the universe which cannot be directly compared to observations, the SIBELIUS DARK simulation uses a special algorithm, BORG, to reproduce the same structures observed in the Local Universe.

This reconstruction, agnostic of the different galaxy types, is combined with a model for galaxy formation which was constructed agnostic of the particular structures in the Local Universe. In this way, we can make a prediction for the distribution of galaxies in the Local Universe that can be directly tested against observations. Our key finding is that the standard model of galaxy formation, applied to the structures in the Local Universe, gives a remarkably good fit to the observations: we find a distribution of bright elliptical galaxies centred on the supergalactic plane, and a distribution of disk galaxies that is far more extended.

Distributions of supergalactic latitude, SGB, for disk galaxies (left), elliptical galaxies (right), and intermediate galaxies (centre). Galaxies directly on the supergalactic plane have sin( SGB) = 0, galaxies above or below the plane have positive or negative values of sin(SGB), respectively. Red lines show the expectations for a uniform distribution on the sky, taking into account the obscuration by the Milky Way.

The top row shows the brightest galaxies in each category, the bottom row shows larger samples. Open black histograms represent the observations, filled coloured histograms show our simulation. The supergalactic plane stands out prominently among the brightest ellipticals but is inconspicuous among the brightest disks in both the simulation and observation. When the sample sizes are increased, the simulations accurately reproduce the observed galaxy distributions.

Our simulation also allows us to examine the origin of these different distributions. We find that the main culprit are the different environments on-and off the supergalactic plane. Within  the plane, galaxies experience frequent mergers and interactions that lead to the transformation of disks into ellipticals, along with the growth of supermassive black holes. By contrast, away from the plane, galaxies can grow in relative isolation, preserving their disk-like structure.

The "supergalactic plane" lies on the equator of the supergalactic projection. As illustrated here, galaxies within this plane experience frequent interactions and mergers, leading to the formation of massive elliptical galaxies. By contrast, galaxies away from the plane evolve in relative isolation, allowing them to preserve their disk-like structure.

It is worth noting that these processes of galactic metamorphosis as a function of environment are not unique to our simulation - they have been known for decades, and extensively studied in both observations and simulations. Our work shows that applying them to the unique environment found in the Local Universe produces the observed distributions of galaxies. We conclude that the observed distributions of elliptical and disk galaxies do not require an overhaul of the standard model - weird as they may appear, they are exactly what the standard model predicts.

Following recent JWST discoveries, some astrophysicists (and many popular voices) have been ready to ring the death knells of the standard model. Instead, our work shows that the solution to a long-standing question can sometimes be found by examining the existing model in more detail.

Our work on this subject was inspired by discussions with Jim Peebles at a workshop in Durham in December 2022. Both the galaxy catalogue and the simulation data we used in this work are public. We are certain that our data contains many more intriguing discoveries waiting to be made and we encourage fellow scientists and interested amateurs to explore it!

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