Did Dragonfly 44 fail as a galaxy?
In Phys.Org there was an article telling about the discovery of a dark galaxy - Dragonfly 44 - with mass, which is of the same order of magnitude as that of Milky Way from the estimate based on standard model of galactic dark matter, for which the region within half-light radius is deduced to be 98 per cent dark. The dark galaxies found earlier have been much lighter. Dragonfly 44 posesses 94 globular clusters and in this respects remembles ordinary galaxies in this mass range.
The abstract of the article telling about the discovery gives a more quantitative summary about the finding.
Recently a population of large, very low surface brightness, spheroidal galaxies was identified in the Coma cluster. The apparent survival of these Ultra Diffuse Galaxies (UDGs) in a rich cluster suggests that they have very high masses. Here we present the stellar kinematics of Dragonfly 44, one of the largest Coma UDGs, using a 33.5 hr integration with DEIMOS on the Keck II telescope. We find a velocity dispersion of 47 km/s, which implies a dynamical mass of Mdyn=0.7× 1010 Msun within its deprojected half-light radius of r1/2=4.6 kpc. The mass-to-light ratio is M/L=48 Msun/Lsun, and the dark matter fraction is 98 percent within the half-light radius. The high mass of Dragonfly 44 is accompanied by a large globular cluster population. From deep Gemini imaging taken in 0.4" seeing we infer that Dragonfly 44 has 94 globular clusters, similar to the counts for other galaxies in this mass range. Our results add to other recent evidence that many UDGs are "failed" galaxies, with the sizes, dark matter content, and globular cluster systems of much more luminous objects. We estimate the total dark halo mass of Dragonfly 44 by comparing the amount of dark matter within r=4.6 kpc to enclosed mass profiles of NFW halos. The enclosed mass suggests a total mass of ∼ 1012 Msun, similar to the mass of the Milky Way. The existence of nearly-dark objects with this mass is unexpected, as galaxy formation is thought to be maximally-efficient in this regime.
To get some order of manitude perspective it is good to start by noticing that r1/2=4.6 kpc is about 15,000 ly - the distance of Sun from galactic center is about 3 kpc. The diameter of Milky Way is 31-55 kpc and the radius of the blackhole in the center of Milky Way, which is smaller than 17 light hours.
The proposed interpretation is as a failed galaxy. What could this failure mean? Did Dragonfly 44 try become an ordinary galaxy but dark matter remained almost dark inside the region defined by half radius? It is very difficult to imagine what the failure of dark matter to become ordinary matter could mean. In TGD framework this would correspond to phase transition transforming dark identified as heff=n×h phases to ordinary matter and could be imagined but this is not done in the following. Could the unexpected finding challenge the standard assumption that dark matter forms a halo around galactic center?
The mass of Dragonfly 44 is deduce from the velocities of stars. The faster they move, the larger the mass. The model for dark matter assumes dark matter halo and this in turn gives estimate for the total mass of the galaxy. Here a profound difference from TGD picture emerges.
- In TGD most of dark matter and energy are concentrated at long cosmic strings transformed to magnetic flux tubes like pearls along string. Galaxies are indeed known to be organized to form filaments. Galactic dark energy could correspond to the magnetic energy. The twistorial lift of TGD predicts also cosmological constant (see this). Both forms of dark energy could be involved. The linear distribution of dark matter along cosmic strings implies a effectively 2-D gravitational logarithmic potential giving in Newtonian approximation and neglecting the effect of the ordinary matter constant velocity spectrum serving as a good approximation to the observed velocity spectrum. A prediction distinguishing TGD from halo model is that the motion along the cosmic string is free. The self-gravitation of pearls however prevents them from decaying.
- Dark matter and energy at galactic cosmic string (or flux tube) could explain most of the mass of Dragonfly 44 and the velocity spectrum for the stars of Dragonfly 44. No halo of dark stars would be needed and there would be no dark stars within r1/2. Things would be exactly what they look like apart from the flux tube!
The "failure" of Dragonfly 44 to become ordinary galaxy would be that stars have not been gathered to the region within r1/2. Could the density of the interstellar gas been low in this region? This would not have prevented the formation of stars in the outer regions and feeling the gravitational pull of cosmic string.
See the article Some astrophysical and cosmological findings from TGD point of view. For background see the chapter TGD and Astrophysics.
For a summary of earlier postings see Latest progress in TGD.