Astronomers have found an explanation for the strange deficiency of dark matter, a critical component of galaxy formation, in the galaxy NGC 1052-DF2 62 million light years that has so far remained a puzzle.
Earlier models have shown that the Ultra-diffuse galaxies (UDGs) NGC 1052-DF2 is deficient in dark matter, which is in conflict with the standard galaxy formation and evolution models.
UDGs are a subclass of low-surface-brightness galaxies, but with an effective radius larger than that expected for their mass content, and hence ultra-diffuse. The UDGs are known to contain so few stars for their size that they appear extremely faint and diffuse compared to the grand majestic spiral galaxies like our Milky Way.
Previous studies of NGC 1052-DF2, predict a total ‘dynamical mass’ within the spatial extent of the galaxy to be less than 340 million solar masses, which is comparable to the estimated mass of the stars of 200 million solar masses. This surprising result implies that within the observed extent of the galaxy, the mass is mostly composed of primarily stars that we are familiar with, and that dark matter is significantly less than expected as compared to spiral galaxies like Milky Way. This contradicts the established paradigm about dark matter which is needed to explain hierarchical structure formation in the Universe.
A recent work from the Indian Institute of Astrophysics (IIA), an autonomous institute of Department of Science and Technology, traces the reason behind this. The research published in the journal, ‘Astronomy & Astrophysics’ shows that the estimation of the dark matter halo is critically dependent on the shape and structure of the dark matter halo, a huge invisible cocoon of dark matter, which gravitationally effects the dynamics of the stars and anisotropy or direction of the stellar velocity dispersion.
Explains K. Aditya, the astronomer who published the recent study, “We find that NGC 1052-DF2 poses interesting questions regarding the formation of galaxies with minimal dark matter content, the astrophysical processes regulating the formation of such galaxies, as well as the potential nature of dark matter itself.”
Fig 1: Image of ultra diffuse galaxy NGC 1052 - DF2, taken by the ACS/WFC instrument on the Hubble Space Telescope.
In order to shed light on this problem and to resolve the discrepancies in the mass estimation of NGC 1052-DF2, Aditya constructed models using the stellar density as an input parameter and constrained the parameters corresponding to the dark matter halo. He then used the observed stellar velocity dispersion profile as a constraint on the model.
“We find that mass models with a “cuspy” (higher density in the central regions) dark matter halo are comparable to models with no dark matter. Moreover, these cuspy dark matter halo fails to consistently account for the observed velocity dispersion in the inner and outer regions of the galaxy. Consequently, we rule out the possibility of a cuspy dark matter halo for describing the mass models of NGC 1052 - DF2,” said Aditya.
In order to reliably determine the total mass of the dark matter, it is essential to use a model called velocity dispersion of the stars in the outer regions of the galaxy, since it is these stars that effectively trace the total gravitational potential of the galaxy. Both dark matter and baryonic (known) matter contribute to this potential. Thus, any mass model of NGC 1052 - DF2 should consistently explain the stellar velocity dispersion in both the inner and outer regions.
Fig 2: The stellar velocity dispersion from the cored dark matter model, from this recent study by K. Aditya.
This new study shows that the “cored” (flat distribution in the central regions) dark matter halo model with a total mass of about 32 billion solar masses explains the observed motion of the stars, but requires an extraordinarily large scale length of 65,000 light years, and an outer cutoff radius of about 85,000 light years.
Aditya explains, “our results suggest that NGC 1052 - DF2 may not only have an ultra-diffuse stellar distribution but that it can, within known uncertainties in the data, potentially host an ultra-diffuse dark matter distribution compatible with the standard galaxy formation and evolution models.”
Publication link: https://doi.org/10.1051/0004-6361/202348078 and
https://arxiv.org/abs/2407.07770
For more details, please contact K. Aditya (aditya[dot]k[at]iiap[dot]res[dot]in).
