Outer Galactic Disks and a Quantitative Test of Gravity at Low Accelerations

TL;DR

This study tests gravity at low accelerations using star-forming knots in the outer disk of galaxy NGC 628, finding marginal discrepancies with modified gravity theories and potential for more definitive future tests.

Contribution

It introduces a new observational test of gravity at low accelerations using outer-disk star-forming knots and demonstrates its potential for high-confidence discrimination.

Findings

Marginal (>97%) discrepancy with modified gravity given current data.

Potential to reach >4 sigma discrimination with additional observations.

Method links rotation curves to vertical scaleheight of star-forming knots.

Abstract

We use the recent measurement of the velocity dispersion of star-forming, outer-disk knots by Herbert-Fort et al. in the nearly face-on galaxy NGC 628, in combination with other data from the literature, to execute a straightforward test of gravity at low accelerations. Specifically, the rotation curve at large radius sets the degree of non-standard acceleration and then the predicted scaleheight of the knots at that radius provides the test of the scenario. For our demonstration, we presume that the H alpha knots, which are young (age < 10 Myr), are distributed like the gas from which they have recently formed and find a marginal (> 97% confidence) discrepancy with a modified gravity scenario given the current data. More interestingly, we demonstrate that there is no inherent limitation that prevents such a test from reaching possible discrimination at the > 4 sigma level with a reasonable investment of observational resources.