Detected fluctuations in SDSS LRG magnitudes: Bulk flow signature or systematic?

TL;DR

This study searches for large-scale bulk flow signatures in SDSS LRG magnitudes, finding a fluctuation consistent with other measurements but with an anomalously large velocity, possibly due to data systematics.

Contribution

It introduces a method to detect bulk flow signatures through magnitude fluctuations in SDSS LRGs and constrains bulk flow parameters from the observed data.

Findings

Detected a few percent flux fluctuation over 100 degrees

Bulk flow direction aligns with previous studies

Bulk flow magnitude appears anomalously large (>4000 km/s)

Abstract

In this paper we search for a signature of a large scale bulk flow by looking for fluctuations in the magnitudes of distant LRGs. We take a sample of LRGs from the Sloan Digital Sky Survey with redshifts of z>0.08 over a contiguous area of sky. Neighboring LRG magnitudes are averaged together to find the fluctuation in magnitudes as a function of R.A.. The result is a fluctuation of a few percent in flux across roughly 100 degrees. The source of this fluctuation could be from a large scale bulk flow or a systematic in our treatment of the data set, or the data set itself. A bulk flow model is fitted to the observed fluctuation, and the three bulk flow parameters, its direction and magnitude: alpha_b, delta_b, v_b are constrained. We find that the bulk flow direction is consistent with the direction found by other authors, with alpha_b 180, delta_b -50. The bulk flow magnitude however was found to be anomalously large with v_b>4000km/s. 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 bulk flow signal, or possibly the entire fluctuation. However, the fluctuation indicates a bulk flow direction very close to those found using other data sets and analyses. Further investigation with upcoming data is required to confirm this detection.