Bar Evolution in Edge-on Galaxies: A Demographic Study of Boxy/Peanut Bulges

Abstract

Boxy/peanut and X-shaped (BP/X) bulges are prominent features in edge-on disk galaxies and are believed to be vertically thickened bars. Despite their relevance in bar evolution, a statistically robust census of these structures in large surveys has been lacking. We aim to provide the largest catalog of BP/X structures in edge-on galaxies to date, and to investigate their properties and role in shaping galaxy scaling relations. We selected a sample of 6684 edge-on galaxies from SDSS DR8 using Galaxy Zoo classifications, requiring a high edge-on probability ($> 0.9$) and a minimum of 10 independent votes. Two-dimensional image decomposition is performed using GALFIT to obtain structural parameters. Residual images are visually inspected to classify BP/X features into four categories: strong both-sided, both-sided, one-sided, and control (no BP/X). We also estimated stellar mass, distance, and physical size for each galaxy. Out of 6653 classified galaxies, we identified 1673 ($\sim$25%) with both-sided BP/X features-504 ($\sim$8%) strong and 1169 ($\sim$17%) weak-as well as 1112 ($\sim$17%) one-sided structures, making up a total of 2785 BP/X-hosting galaxies ($\sim$42%). One-sided structures, likely signatures of ongoing buckling, are more frequent than strong both-sided bulges across all stellar masses. The fraction of BP/X bulges increases with stellar surface mass density, indicating a connection with bar formation in dense disks. We also find that galaxies with strong BP/X bulges contribute to increased scatter in the stellar mass-size and stellar mass-surface density relations, particularly at higher masses.