Missing the Point - a Brief Reply to Foreman & Scott and Gnedin

TL;DR

This paper defends the baryonic Tully-Fisher relation's significance in testing galaxy dynamics theories, critiques recent criticisms, and emphasizes the need to understand why MOND predictions are so effective, questioning LCDM's completeness.

Contribution

It clarifies misconceptions about the BTFR analysis and highlights the importance of understanding the fundamental reasons behind MOND's success.

Findings

The apparent difference in acceleration scales is due to disk rotation effects.

The intrinsic scatter in the BTFR is consistent with zero within uncertainties.

The critique of the baryonic mass explanation for missing baryons is addressed.

Abstract

In recent postings, Foreman & Scott and Gnedin criticize my work on the baryonic Tully-Fisher relation (BTFR) of gas rich galaxies as tests of MOND and LCDM (McGaugh 2011a,b). These criticisms are rather redundant, as they mostly rehash material I have already discussed. Gnedin is concerned with explaining away the problem of the apparently missing baryons with ionized gas. This is one hypothetical possibility that I have previously discussed (McGaugh et al. 2010; McGaugh & Wolf 2010; McGaugh 2011b). Foreman & Scott claim to find a difference between the acceleration scale that they measure and that I measure from the same data, but the apparent difference results simply from their failure to account for the well known fact (Binney & Tremaine 1987) that flattened disks rotate faster than the equivalent spherical mass distribution (as spelled out for this application in McGaugh 2011b). They further argue that the the intrinsic scatter in the BTFR is merely tiny rather than zero. This difference is within the uncertainty of the uncertainties, so confirms rather than refutes my result. Worse, these papers miss the basic point. Why does MOND have any predictions come true? Why does it provide such an effective and economical description of galaxy dynamics? They do nothing to address these deeper questions. One can imagine that the apparently MONDian behavior of galaxies is merely an approximation to some phenomenology that emerges in the context of LCDM, but we have yet to demonstrate how this occurs. Until we confront this issue seriously, and confirm the existence of non-baryonic cold dark matter in the laboratory, it is reasonable to remain skeptical of LCDM as well as of MOND.