Lack of Bulge Alignment in Late-type Galaxies with Large-scale Filaments Plausibly Unveils a Radial Migration Formation Scenario

Abstract

The formation sequence of bulges and disks in late-type galaxies (LTGs) remains debated. While some theories suggest bulges form before disks, others propose the reverse. To address this, we analyze a bulge+disk decomposition catalog from the Sloan Digital Sky Survey, examining the alignment between central bulge major axes and large-scale filaments. For LTGs near filament spines, no significant alignment is found. However, LTGs farther from filaments show a marginal ($\sim 1\sigma$) perpendicular alignment. Notably, central bulges and outer disks exhibit strong alignment in the sky plane, suggesting bulge formation may be driven by material migration from outer disks. Although further simulations are needed, our results provide a novel alignment-based perspective on bulge formation, advancing understanding of galactic structural evolution.