A Spectroscopic Study of the H-alpha Surface Brightness Profiles in the Outer Disks of Galaxies

TL;DR

This study investigates the H-alpha surface brightness profiles in the outer disks of 15 galaxies, revealing a broken-exponential shape that extends beyond the optical radius, challenging the idea of a sharp truncation.

Contribution

It provides the first detailed spectroscopic analysis showing that H-alpha profiles are generally broken-exponentials, not sharply truncated, and links these profiles to the underlying stellar populations.

Findings

H-alpha profiles extend beyond R_25 up to 50%.

Profiles are best described by broken-exponential laws.

Outer disk H-alpha drops more steeply than stellar continuum.

Abstract

The surface brightness profile of H-alpha emission in galaxies is generally thought to be confined by a sharp truncation, sometimes speculated to coincide with a star formation threshold. Over the past years, observational evidence for both old and young stellar populations, as well as individual H II regions, has demonstrated that the outer disk is an actively evolving part of a galaxy. To provide constraints on the origin of the aforementioned H-alpha truncation and the relation of H-alpha emission in the outer disk to the underlying stellar population, we measure the shape of the outer H-alpha surface brightness profile of 15 isolated, edge-on late-type disk galaxies using deep, long-slit spectroscopy. Tracing H-alpha emission up to 50% beyond the optical radius, R_25, we find a composite H-alpha surface brightness profile, well described by a broken-exponential law, that drops more steeply in the outer disk, but which is not truncated. The stellar continuum and H-alpha surface brightness both exhibit a break at ~0.7 R_25, but the H-alpha emission drops more steeply than the stellar continuum beyond that break. Although profiles with truncations or single exponential laws correctly describe the H-alpha surface brightness profiles of some individual galaxies, flexible broken-exponentials are required in most cases and are therefore the more appropriate generic description. The common existence of a significant second surface brightness component beyond the H-alpha break radius disfavors the hypothesis that this break is a purely stochastic effect.