En massiv galax som utmanar teorier om mörk materia

By | July 30, 2023

Jämförelse mellan galaxer med och utan mörk materia

Jämförelse mellan en konventionell galax (ESO 325-G004) omgiven av en mörk materia halo, som upptar den tyngsta plattan på viktskalan, och galaxen NGC 1277 (vänster), där studien av massfördelningen avslöjar frånvaron av mörk materia . Kredit: Design: Gabriel Pérez Díaz (IAC). Bild av NGC 1277: NASA, ESA och M. Beasley (IAC). Bild av ESO 325-G004: NASA, ESA och Hubble Heritage Team (STScI/AURA); J. Blakeslee (Washington State University)

En internationell undersökning ledd av Canary Islands Institute of Astrophysics (IAC) och University of La Laguna (ULL) har hittat de första bevisen på en massiv galax utan mörk materia. Resultatet är en utmaning för den nuvarande standardmodellen för kosmologi. Studien publicerades i tidskriften Astronomy and Astrophysics.

Ett team av forskare, ledda av IAC och University of La Laguna (ULL) forskare Sebastién Comerón, upptäckte att galaxen NGC 1277 inte innehåller mörk materia. Detta är första gången som en massiv galax (den har en massa som är flera gånger större än den[{” attribute=””>Milky Way) does not show evidence of this invisible component of the universe. “This result does not fit in with the currently accepted cosmological models, which include dark matter,” explains Comerón.

Current Cosmological Models

In the current standard model of cosmology, massive galaxies contain substantial quantities of dark matter, a type of matter which does not interact in the same way as normal matter; the only evidence for its existence is the strong gravitational pull that it exerts on the stars and the gas nearby, and this interaction is observable.

NGC 1277 and Relic Galaxies

NGC 1277 is considered a prototype “relic galaxy” which means a galaxy that has had no interactions with its neighbors. Galaxies of this type are very rare, and they are considered the remnants of giant galaxies which formed in the early days of the universe.

“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,” explains Comerón.

Understanding the Mass Distribution

The team generated kinematic maps from the spectra, which allowed them to ascertain the distribution of mass within the galaxy up to a radius of roughly 20,000 light-years. It was determined that the mass distribution in NGC 1277 corresponded solely to the distribution of the stars. From this, they inferred that there cannot be more than 5% of dark matter within the observed radius, although the observations are consistent with the complete absence of this component.

Discrepancy With Cosmological Models

However, present cosmological models predict that a galaxy with the mass of NGC 1277 should have at least 10 % of their mass in the form of dark matter, with a maximum of 70 % in this form. “This discrepancy between the observations and what we would expect is a puzzle, and maybe even a challenge for the standard model” notes Ignacio Trujillo, a researcher at the IAC and the ULL, who participated in the study.

Explaining the Absence of Dark Matter

The study proposes two possible explanations for the lack of dark matter in NGC 1277. “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” comments Anna Ferré-Mateu, a researcher at the IAC and the ULL who also participated in the study. “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.”

Remaining Questions

Neither of these explanations fully satisfies the authors of the study, “so the puzzle of how a massive galaxy can form without dark matter remains a puzzle” insists Comerón. In order to continue researching the mystery the team plans to make new observations with the WEAVE instrument on the William Herschel Telescope (WHT) at the Roque de los Muchachos Observatory, in the Canary Island of La Palma

Implications on Dark Matter Theories

If this the result, that NGC 1277 does not have dark matter, is confirmed, it would cast strong doubt on alternative models for dark matter, namely theories in which gravity is modified and the major part of the gravitational attraction within galaxies is due to a slight change in the law of gravity on large scales. “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 alternative to dark matter,” notes Trujillo.

Reference: “The massive relic galaxy NGC 1277 is dark matter deficient: From dynamical models of integral-field stellar kinematics out to five effective radii” by Sébastien Comerón, Ignacio Trujillo, Michele Cappellari, Fernando Buitrago, Luis E. Garduño, Javier Zaragoza-Cardiel, Igor A. Zinchenko, Maritza A. Lara-López, Anna Ferré-Mateu and Sami Dib, 11 July 2023, Astronomy and Astrophysics.
DOI: 10.1051/0004-6361/202346291

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