According to our predominant cosmological models, Dark Matter accounts for roughly 85% of the mass in the Universe. While ongoing efforts to study this mysterious, invisible mass have yielded no direct evidence, astrophysicists have been able to measure its influence by observing Dark Matter Haloes, gravitational lenses, and the effect of General Relativity on large-scale cosmic structures. And with the help of next-generation missions like the ESA’s Euclid and NASA’s Nancy Grace Roman space telescopes, Dark Matter may not be a mystery for much longer!
And then something like this comes along: a massive galaxy that appears to have little or no Dark Matter! This is precisely what a team of astronomers led by members of the Instituto Astrofisica de Canarias (IAC) noticed when observing NGC 1277. This lenticular galaxy, located 240 million light-years away in the constellation Perseus, is several times more massive than the Milky Way. This is the first time a massive galaxy has been found that doesn’t show signs of Dark Matter, which is a serious challenge to our current cosmological models.
The research was led by Sébastien Comerón, an extragalactic astronomer at the Universidad de La Laguna (ULL), the IAC, and the leader of the Archæology of Thick discs (ArcThick) collaboration. He was joined by researchers from the Instituto Nacional de Astrofísica, Óptica y Electrónica (INAOE), the Consejo Nacional de Ciencia y Tecnología, the National Academy of Sciences of Ukraine, Instituto de Física de Partículas y del Cosmos (IPARCOS), the Max Planck Institute for Astronomy (MPA), and multiple universities. The paper that describes their findings recently appeared in the journal Astronomy & Astrophysics.
Hubble Space Telescope image of the galactic group in the Perseus Cluster that NGC 1277 is a member of. It is the bright galaxy just to the left of the center of the image. Credit: ESA/Hubble
According to the Standard Model of Cosmology – aka. the Lambda Cold Dark Matter (?CDM) model – Dark Matter played an intrinsic role in the formation and evolution of the cosmos (and still does). In theory, this invisible mass existed shortly after the Big Bang and formed halos that attracted neutral hydrogen gas into swirling disks. This gas was pulled into denser and denser clouds, triggering the formation of the first stars and galaxies. Today, DM is a major component of all massive galaxies and is evident by their rotational curves, the lenses they create, and their interactions with surrounding stars and the intergalactic medium (IGM).
However, when the team measured the mass distribution of NGC 1277, they observed just the distribution of the star. From this, they inferred that DM could not account for more than 5% of the galaxy’s mass within the observed radius – although their observations indicated that there might be no DM at all. As Comerón explained in a recent IAC press release:
“This result does not fit in with the currently accepted cosmological models, which include dark matter. The importance of relic galaxies in helping us to understand how the first galaxies formed was the reason we decided to observe NGC 1277 with an integral field spectrograph. From the spectra, we made kinematic maps which enabled us to work out the distribution of mass within the galaxy out to a radius of some 20,000 light years.”
In their paper, the team describes NGC 1277 as a prototype “relic galaxy,” a very rare class that does not interact with neighboring galaxies. These galaxies are believed to be the remnants of giant galaxies that formed shortly after the Big Bang. However, the ?CDM model predicts that DM should account for at least 10% of galaxies as massive as NGC 1277, with a maximum of 70% for this particular type. Said co-author Anna Ferré-Mateu, a researcher at the IAC and the ULL, there are two possible explanations for this discrepancy:
“One is that the gravitational interaction with the surrounding medium within the galaxy cluster in which this galaxy is situated has stripped out the dark matter. The other is that the dark matter was driven out of the system when the galaxy formed by the merging of protogalactic fragments, which gave rise to the relic galaxy”.
This diagram shows how the diameter of the 17-billion-solar-mass black hole in the heart of galaxy NGC 1277 compares with the orbit of Neptune around the Sun. Credit: D. Benningfield/K. Gebhardt/StarDate
However, neither of these explanations is fully satisfactory as far as the team is concerned. In the near future, the team plans to investigate the mystery further by conducting observations with the WHT Enhanced Area Velocity Explorer (WEAVE) instrument on the William Herschel Telescope (WHT), located at the Roque de los Muchachos Observatory on the island of La Palma. If WEAVE’s velocity measurements should confirm that NGC 1277 has no DM, it could cast serious doubt on alternative theories – such as Modified Newtonian Dynamics (MOND). Said Trujillo:
“This discrepancy between the observations and what we would expect is a puzzle, and maybe even a challenge for the standard model. Although the dark matter in a specific galaxy can be lost, a modified law of gravity must be universal, it cannot have exceptions, so that a galaxy without dark matter is a refutation of this type of alternatives to dark matter.”
These observations could also shed some light on the galaxy’s especially huge Supermassive Black Hole (SMBH), which is roughly 17 billion solar masses, or 4,250 times that of Sagittarius A* (the SMBH at the center of the Milky Way)! According to some astronomers, black holes might be the source of DM, which formed from the collapse of DM Halos during the early Universe. There’s also the mystery of Dark Matter Galaxies, like the curious case of FAST J0139+4328, which are almost entirely composed of DM.
Next-generation missions like Euclid and the Nancy Grace Roman space telescopes will also provide fresh insight by examining the expansion of the cosmos since the Big Bang. These observations aim to measure the influence of Dark Matter (and Dark Energy) on the largest of cosmic scales. The results of these and other studies will settle the ongoing debate by revealing that either a mysterious invisible mass exists or that our understanding of gravity (as described by General Relativity) needs to be revised.
Further Reading: IAC, Astronomy & Astrophysics
