Detected fluctuations in Sloan Digital Sky Survey luminous red galaxy magnitudes: bulk flow signature or systematic?


E-mail: (AA); (HAF)


In this paper, we search for a signature of a large-scale bulk flow by looking for fluctuations in the magnitudes of distant luminous red galaxies (LRGs). We take a sample of LRGs from the Sloan Digital Sky Survey with redshifts of z > 0.08 over a contiguous area of the sky. Neighbouring LRG magnitudes are averaged together to find the fluctuation in magnitudes as a function of right ascension. The result is a fluctuation of a few per cent in flux across roughly 100°. The source of this fluctuation could be from a large dipole motion with respect to the LRG sample or a systematic in our treatment of the data set, or the data set itself. A dipole model is fitted to the observed fluctuation, and the three flow parameters, its direction (αb, δb) and magnitude (vb), are constrained . We find that the flow direction is consistent with the direction found by other authors, with αb∼ 180 and δb∼−50. The flow magnitude, however, was found to be anomalously large, with vb > 4000 km s−1. The LRG angular selection function cannot be sufficiently taken into account in our analysis with the available data, and may be the source of either the anomalous magnitude of the flow signal or possibly the entire fluctuation. However, the fluctuation indicates a flow direction very close to those found using other data sets and analyses. Further investigation with upcoming data is required to confirm this detection.