The Extragalactic Proper Motion of the Acoustic Scale

Once considered stationary objects, galaxies are increasingly becoming an interest in proper motion studies. As the precision of astrometry increases, extragalactic proper motion can be used to trace the real-time evolution of cosmological phenomena, such as the growth of large-scale structure. Large-scale structure density fluctuations cause correlated peculiar galaxy motions on various angular scales. Characterizing the overall proper motion field as a function of angular scale and/or redshift requires precisely modeling and distinguishing between a variety of competing cosmological and observer-induced proper motion signals. The acoustic scale in the clustering of matter offers an opportunity to predict a correlated proper motion signal at a precise angular scale. Caused by the baryon acoustic oscillations in the early universe, the acoustic scale serves as a robust cosmological standard ruler. However, at low redshift, gravity among large-scale structure creates flows on the ~10 Mpc scale, causing galaxy pairs originally separated by the acoustic scale to shift to smaller scales. As these pairs of galaxies shift closer together, a redshift-dependent pattern of converging galaxies should appear at the angular acoustic scale. We present a theoretical framework for the proper motion signal caused by the non-linear shift of the acoustic scale. Our prediction is redshift dependent and reaches a minimum of -0.025 μas yr^-1 (negative for converging motion) at a redshift of z = 0.018.

The authors acknowledge support from NSF grant AST-1411605 and NASA grant 14-ATP14-0086.