1. Introduction

2. Data

2.1. The Sample

2.2. Herschel Observations

2.3. The Calibration of Metallicity and the Abundance Gradients

2.4. HI and CO Maps

3. Analysis

3.1. The Mass and Temperature of Dust

3.2. Sources of Uncertainty in Mass and Temperature of Dust

3.3. Radial Profiles

3.4. The $X_{\mathrm{CO}}$ Factor

4. The Method

4.1. I. The Dust-to-Gas Ratio and Metallicity Scales

4.2. II. The Dust-to-Gas Mass Gradient with a Standard $X_{\mathrm{CO}}$

4.3. III. Can We Constrain $X_{\mathrm{CO}}$?

4.4. Other Evidence for a Low $X_{\mathrm{CO}}$

5. Summary and Conclusions