The Link Between Light and Mass in Late-type Spiral Galaxy Disks

TL;DR

This study reveals a tight correlation between central surface brightness and surface mass density in galaxy disks, with corrections for dark matter and gas reducing scatter and enabling precise mass estimates from brightness.

Contribution

It establishes a corrected, low-scatter mu-Sigma relation for galaxy disks, accounting for dark matter and gas effects, improving mass-to-light ratio estimates.

Findings

The mu-Sigma relation has 30% scatter at high surface brightness.

Dark matter causes deviations at low surface brightness, which are correctable.

Corrected relation reduces scatter to 25-50%, enabling 10-20% mass density accuracy.

Abstract

We present the correlation between the extrapolated central disk surface brightness (mu) and extrapolated central surface mass density (Sigma) for galaxies in the DiskMass sample. This mu-Sigma-relation has a small scatter of 30% at the high-surface-brightness (HSB) end. At the low surface brightness (LSB) end, galaxies fall above the mu-Sigma-relation, which we attribute to their higher dark matter content. After correcting for the dark matter, as well as for the contribution of gas and the effects of radial gradients in the disk, the LSB end falls back on the linear mu-Sigma-relation. The resulting scatter about the corrected mu-Sigma-relation is 25% at the HSB end, and about 50% at the LSB end. The intrinsic scatter in the mu-Sigma-relation is estimated to be 10% to 20%. Thus, if the surface brightness is known, the stellar surface mass density is known to within 10-20% (random error). Assuming disks have an exponential vertical distribution of mass, the average (M_L)_K is 0.24 Msun/Lsun, with an intrinsic scatter around the mean of at most 0.05 Msun/Lsun. This value for (M/L)_K is 20% smaller than we found in Martinsson et al., mainly due to the correction for dark matter applied here. This small scatter means that among the galaxies in our sample variations in scale height, vertical density profile shape, and/or the ratio of vertical over radial velocity dispersion must be small.