Day 1402

Thursday.

1804.06412
Circular polarization in a spherical basis
Kamionkowski

Circular polarization of the CMB arises in the standard cosmological model from Faraday conversion of the linear polarization generated at the surface of last scatter by various sources of birefringence along the line of sight.  If the sources of birefringence are generated at linear order in primordial density perturbations the principal axes of the index-of-refraction tensor are determined by gradients of the primordial density field.  Since linear polarization at the surface of last scatter is generated at linear order in density perturbations, the circular polarization thus arises at second order in primordial perturbations.  Revisit the calculation of the circular polarization using the total-angular-momentum formalism, which allows for some simplifications in the calculation of the angular power spectrum of the circular polarization --- especially for the dominant photon-photon scattering contribution --- and also provides some new intuition.


1804.06421
Tidal stripping as a possible origin of the ultra diffuse galaxy lacking dark matter
Ogiya

Recent observations revealed a mysterious UDG, NGC1052-DF2, in the group of a large elliptical galaxy, NGC1052.  Compared to expectations from abundance matching models, the DM mass contained in NGC1052-DF2 is smaller by a a factor of ~400.  Utilize controlled N-body sims of the tidal interaction between NGC1052 and a smaller satellite galaxy, that is supposedly the progenitor of NGC1052-DF2, to test if tidal stripping can explain DM deficiency at such levels.  Find that when assuming a tightly bound orbit as well as a relatively low concentration and cored density profile for the dark halo of the satellite, the simulations reproduce well both the mass profile and the effective radius inferred form the observations of NGC1052-DF2.  Orbital parameters and halo concentration are in the tail, but still consistent with measurements of their distributions from cosmological simulations.  Such strongly DM deficient galaxies, in the scenario, are thus expected to be relatively rare in groups and clusters and not present in the field.