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Do Galactic Haloes Have A Constant Density?

Article by Dr. Amira Val Baker, Astrophysicist, Resonance Science Foundation Research Scientist

Image ©Pearson Education

Galactic halos – comprising of the theorized dark matter halo– show an interesting characteristic in the constant nature of their volume density.

Galaxies come in all shapes and sizes with the most common and well-studied being similar to our Milky way galaxy and known as disc or spiral galaxies. A well-known problem in physics is the observed rotation velocities of stars with respect to the distance from their galactic centre. These rotation curves, as they are known, intriguingly do not appear as expected – that is they appear flat instead of falling off and decreasing with distance. Read more here.

In the cold dark matter model of the universe these flat rotation curves are attributed to dark matter, hence the name ‘dark matter’ haloes.

In an effort to better understand this, scientists have been comparing the density profiles of these so-called dark matter haloes – those found from observations with those predicted by cosmological N-body simulations. Now in a recent study, led by American scientist Pengfei Li, the scaling relationship between these dark matter haloes and their galactic disks is explored. Utilizing data from the Spitzer Photometry and Accurate Rotation Curves (SPARC) database, 175 galaxies were analyzed and compared with cosmological N-body simulations. The results showed, that not only was there a strong correlation between the properties of dark matter haloes and their galactic discs, but also that the characteristic volume density of the dark matter halo is constant. So, although the different galaxies show quite different rotation curves, they all have a constant characteristic density.

This implies that the luminosity is independent of the volume density, which is instead found to correlate with the characteristic surface density. This has huge implications as not only does the constant volume density and scaling relations provide new insights into galaxy formation, they could also provide an empirical benchmark to cosmological simulations of galaxy formation.

In the unified physics approach the vacuum density along with the matter density is governed by a scale factor – the surface entropy. The galactic halo would thus be driven by a specific ratio maintaining a constant density, independent of its size. On the other hand, the luminosity – which is the amount of energy emitted per unit time – would depend on the surface entropy and would therefore scale accordingly.


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