Day 1390

Thursday.

1803.10237
A galaxy lacking dark matter
van Dokkum, et al

Studies of galaxy surveys in the context of the CDM paradigm have shown that the mass of the DM halo and the total stellar mass are coupled through a function that varies smoothly with mass.  Their average ratio M_halo/M_stars has a minimum of about 30 for galaxies with stellar masses near that of the Milky Way (approximately 5e10 Msun) and increases both towards lower masses and towards higher masses.  The scatter in this relation is not well known; it is generally thought to be less than a factor of two for massive galaxies but much larger for dwarf galaxies.  Here, report the radial velocities of ten luminous globular-cluster-like objects in the ultra-diffuse galaxy NGC1052-DF2, which has a stellar mass of approximately 2e8 Msun.  Infer that its velocity dispersion is less than 10.5 km/s with 90 % confidence, and determine from this that its total mass within a radius of 7.6 kpc is less than 3.4e8 Msun.  This implies that the ratio M_halo/M_stars is of order unity (and consistent with zero), a factor of at least 400 lower than expected.  NGC1052-DF2 demonstrates that DM is not always coupled with baryonic matter on galactic scales.


1803.10240
An enigmatic population of luminous globular clusters in a galaxy lacking dark matter
van Dokkum, et al

Recently found an ultra diffuse galaxy (UDG) with half-light radius of R_e=2.2 kpc and little or no DM.  The total mass of NGC1052-DF2 was measured from the radial velocities of bright compact objects that are associated with the galaxy.  Here, analyze these objects using a combination of HST imaging and Keck spectroscopy.  Their average size is <r_h>=6.2±0.5 pc and their average ellipticity is <epsilon>=0.18±0.02.  From a stacked Keck spectrum, derive an age >9 Gyr and a metallicity of [Fe/H]=-1.35±0.12.  Their properties are similar to omega Centauri, the brightest and largest globular cluster in the MW, and the results demonstrate that the luminosity function of metal-poor globular clusters is not universal.  The fraction of the total stellar mass that is in the globular cluster system is similar to that in other UDGs, and consistent with "failed galaxy" scenarios where star formation terminated shortly after the clusters were formed.  However, the galaxy is a factor of ~1000 removed from the relation between globular cluster mass and total galaxy mass that has been found for other galaxies, including other UDGs.  Infer that a DM halo is not a prerequisite for the formation of metal-poor globular cluster-like objects in high redshift galaxies.


1803.10741
The existence of the cosmic neutrino background is predicted from the hot big bang model.  These neutrinos were a dominant component of the energy density in the early universe and, therefore, played an important role in the evolution of cosmological perturbations.  In particular, fluctuations in the neutrino density produced a distinct shift in the temporal phase of sound waves in the primordial plasma, which has recently been detected in the CMB.  In this paper, report the first measurement of this neutrino-induced phase shift in the spectrum of BAO of BOSS DR12 data.  Constraining the acoustic scale using Planck data, and marginalizing over the effects of neutrinos in the CMB, find evidence for a non-zero phase shift at greater than 95% CL.  Also demonstrate the robustness of this result in simulations and forecasts.  Besides being a new measurement of the cosmic neutrino background, this work is the first application of the BAO signal to early universe physics and a non-trivial confirmation of the standard cosmological history.