Pressure Support in Galaxy Disks: Impact on Rotation Curves and Dark Matter Density Profiles

TL;DR

This paper investigates how gas pressure support affects galaxy rotation curves and the inferred dark matter profiles, especially in low-mass galaxies, highlighting potential underestimations of dark matter density due to pressure effects.

Contribution

It quantifies the impact of gas pressure support on rotation curve measurements and discusses its significance across different galaxy masses, emphasizing uncertainties and implications for dark matter studies.

Findings

Pressure support can cause underestimates of dark matter density in galaxies with rotation speeds <75km/s.

Gas turbulence dominates pressure support, which can be sustained over long timescales.

Effects are less significant in higher mass galaxies with faster rotation speeds.

Abstract

Rotation curves constrain a galaxy's underlying mass density profile, under the assumption that the observed rotation produces a centripetal force that exactly balances the inward force of gravity. However, most rotation curves are measured using emission lines from gas, which can experience additional forces due to pressure. In realistic galaxy disks, the gas pressure declines with radius, providing additional radial support to the disk. The measured tangential rotation speed will therefore tend to lag the true circular velocity of a test particle. The gas pressure is dominated by turbulence, and we evaluate its likely amplitude from recent estimates of the gas velocity dispersion and surface density. We show that where the amplitude of the rotation curve is comparable to the characteristic velocities of the interstellar turbulence, pressure support may lead to underestimates of the mass density of the underlying dark matter halo and the inner slope of its density profile. These effects may be significant for galaxies with rotation speeds <75km/s, but are unlikely to be significant in higher mass galaxies. We find that pressure support can be sustained over long timescales, because any reduction in support due to the conversion of gas into stars is compensated for by an inward flow of gas. However, we point to many uncertainties in assessing the importance of pressure support in galaxies. Thus, while pressure support may alleviate possible tensions between rotation curve observations and LambdaCDM on kiloparsec scales, it should not be viewed as a definitive solution at this time.