The Surface Mass Density and Structure of the Outer Disk of NGC 628

TL;DR

This study measures the surface mass density and vertical structure of NGC 628's outer disk using H_alpha knots, finding it to be dynamically cold and explainable by visible matter without dark matter.

Contribution

It provides the first detailed kinematic analysis of the outer disk of NGC 628, estimating its mass density and demonstrating the disk's low velocity dispersion and minimal dark matter influence.

Findings

Vertical velocity dispersion < 11 km/s in outer disk

Surface mass density of 7.5 solar masses/pc^2

Outer disk is dynamically cold and consistent with visible matter

Abstract

We study the kinematics of GALEX-selected H_alpha knots in the outer disk (beyond R25) of NGC 628 (M74), a galaxy representative of large, undisturbed, extended UV (Type 1 XUV) disks. Our spectroscopic target sample of 235 of the bluest UV knots surrounding NGC 628 yielded 15 H_alpha detections (6%), roughly the number expected given the different mean ages of the two populations. The measured vertical velocity dispersion of the H_alpha knots between 1 - 1.8 R25 (13.5 - 23.2 kpc) is < 11 km/s. We assume that the H_alpha knots trace an 'intermediate' vertical mass density distribution (between the isothermal sech(z)^2 and exponential distributions) with a constant scaleheight across the outer disk (h_z = 700 pc) and estimate a total surface mass density of 7.5 solar masses/pc^2. This surface mass density can be accounted for by the observed gas and stars in the outer disk (little or no dark matter in the disk is required). The vertical velocity dispersion of the outer disk H_alpha knots nearly matches that measured from older planetary nebulae near the outskirts of the optical disk by Herrmann et al., suggesting a low level of scattering in the outer disk. A dynamically cold stellar component extending nearly twice as far as the traditional optical disk poses interesting constraints on the accretion history of the galaxy.