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

TL;DR

This work provides the largest census of Boxy/Peanut (BP/X) bulges in edge-on galaxies to date by analyzing 6684 SDSS DR8 edge-on candidates from Galaxy Zoo and performing 2D GALFIT decompositions followed by residual-based BP/X classification. They identify 2785 BP/X-hosting galaxies (∼42%), including 1673 with both-sided features (504 strong, 1169 weak) and 1112 one-sided structures, suggesting ongoing buckling in some systems. The BP/X fraction scales with stellar mass and with the average stellar surface density, indicating a link between bar formation in dense disks and BP/X growth, while strong BP/X bulges increase the scatter in the stellar mass–size and stellar mass–density relations, especially at high masses. The catalog and findings provide a robust foundation for studying bar-driven secular evolution and its imprint on galaxy scaling laws, with prospects for improved mass and dust modeling in future surveys.

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.

Figures (12)

• Figure 1: Figure showing the comparison of fitted position angle, radius and axial ratio between edge-on disk profile fitting by galfit and exponential disk profile fit parameters in SDSS. The red line in left most and middle panel gives the line corresponding to $y=x$. In the right most panel the fitting line is given in red with the fitting parameters. $Z_0$ is the disk height and $R_s$ denotes the scale radius obtained from galfit.
• Figure 2: The figures in the first row are four galaxies in our sample. The second row shows the model fitted using galfit for the corresponding galaxy above it. The third row gives the residual image of the corresponding galaxy above it. Each column of figures gives an example of galaxies classified as strong both-sided, weak both-sided, one-sided, and control, respectively from left to right. The white scale bar marks the scale of 10 arcsec.
• Figure 3: Absolute $r-$band magnitude, redshift, angular scale radius, and mass distribution of galaxies in our base sample. The red color vertical lines correspond to median values, and the dark shaded regions show 16$th$ to 84$th$ percentiles range of the distributions.
• Figure 4: Distribution of different parameters along with the 'log norm' fitting parameters for the distributions of mass, axial ratio, scale height and radius for different samples classified based on the presence of BP/X bulge in them.
• Figure 5: Fraction of galaxies hosting both-sided (green), one-sided (red) and strong both-sided (purple) BP/X structures as a function of their stellar mass. Fraction of BP/X hosting galaxies increases with their stellar mass.