The Carnegie-Irvine Galaxy Survey. V. Statistical study of bars and buckled bars

Abstract

Simulations have shown that bars are subject to a vertical buckling instability that transforms thin bars into boxy or peanut-shaped structures, but the physical conditions necessary for buckling to occur are not fully understood. We use the large sample of local disk galaxies in the Carnegie-Irvine Galaxy Survey to examine the incidence of bars and buckled bars across the Hubble sequence. Depending on the disk inclination angle ($i$), a buckled bar reveals itself as either a boxy/peanut-shaped bulge (at high $i$) or as a barlens structure (at low $i$). We visually identify bars, boxy/peanut-shaped bulges, and barlenses, and examine the dependence of bar and buckled bar fractions on host galaxy properties, including Hubble type, stellar mass, color, and gas mass fraction. We find that the barred and unbarred disks show similar distributions in these physical parameters. The bar fraction is higher (70\%--80\%) in late-type disks with low stellar mass ($M_{*} < 10^{10.5}\, M_{\odot}$) and high gas mass ratio. In contrast, the buckled bar fraction increases to 80\% toward massive and early-type disks ($M_{*} > 10^{10.5}\, M_{\odot}$), and decreases with higher gas mass ratio. These results suggest that bars are more difficult to grow in massive disks that are dynamically hotter than low-mass disks. However, once a bar forms, it can easily buckle in the massive disks, where a deeper potential can sustain the vertical resonant orbits. We also find a probable buckling bar candidate (ESO 506$-$G004) that could provide further clues to understand the timescale of the buckling process.