1. Introduction

2. Data
2.1. Ancillary photometric data and rest-frame galaxy properties

3. The spectroscopic overdensity
3.1. The galaxy density contrast
3.2. The overdensity total mass

4. Searching for diffuse cold gas

5. Galaxy properties
5.1. Stellar mass, star formation, and colors
5.2. Stacked spectra

6. Discussion
6.1. The overdensity
6.2. The diffuse gas

7. Summary and conclusions