Influence of Barlens on the Bulge Parameters in the 2D Image Decomposition

TL;DR

This work addresses biases in bulge parameter measurements caused by neglecting the barlens component in 2D image decompositions of barred galaxies. By combining mock images from a buckled-bar simulation (Model 1) and a Milky Way–like Auriga galaxy (Model 2) with observations of CGS galaxies NGC 1533 and NGC 7329, the authors develop a systematic, constraint-driven strategy to model the barlens using GALFIT, including barlens geometry and fixed Sérsic index. They demonstrate that including the barlens reduces the bulge-to-total light, bulge Sérsic index, and bulge effective radius by substantial amounts (B/T ~30–50% reduction in many cases; bulge $n$ and $r_{ m e}$ also shift), while improving residuals and model selection criteria (RFF, BIC). The study finds that barlens modeling is robust to moderate inclinations, though uncertainties rise with higher inclination due to projection effects, and that boxy isophotes can better capture B/P features at large inclinations. Overall, accounting for the barlens yields more accurate bulge properties and has implications for bulge scaling relations and galaxy evolution studies; future work will extend to larger samples for statistical validation.

Abstract

Recent observations and simulations have shown that a buckled bar in the face-on view can be considered as a combination of a long flat bar and a short round barlens (corresponding to the boxy/peanut bulge in the edge-on view). However, the barlens component can be misidentified as the bulge, potentially leading to inaccurate bulge parameter measurements in two-dimensional (2D) image decomposition. Our goal is to explore the optimal method for modeling the barlens component and to understand its impact on bulge parameter measurements in 2D image decomposition. We first analyze mock images from two different simulations (with/without bulge) to verify our decomposition method. We then apply the method to two nearby barred galaxies, NGC 1533 and NGC 7329, from the Carnegie-Irvine Galaxy Survey (CGS). Using GALFIT, we conduct 2D image decomposition by gradually increasing the complexity of model configurations. We also explore the effects of inclination by projecting the simulated galaxy to various viewing angles and analyzing the variations in bulge and barlens parameters. From the mock images, we find that the bulge-to-total ratio (B/T) could be overestimated by 50$\%$ without considering the barlens component; the Sérsic index and effective radius of the bulge are also affected to varying degrees. The decomposition results of the two CGS galaxies are consistent with our mock image tests. Uncertainties of the structural parameters of the bulge and barlens are larger at higher inclination angels due to the strong projection effect in the central region. Our findings underscore the necessity of accurately modeling the barlens, revealing that its inclusion in 2D image decomposition can lead to a decrease in B/T by $\sim$30-50$\%$, with other bulge parameters, such as the Sérsic index and effective radius, also affected.

Figures (16)

• Figure 1: Mock images of Model 1 and Model 2 in the face-on view (top), side-on view (middle), and end-on view (bottom). The barlens and the corresponding B/P bulge can be seen clearly in the face-on view and edge-on views, respectively. The blue ellipses in the top panels show the visually identified outer edge of the barlenses.
• Figure 2: $R$-band star-cleaned images of NGC 1533 (left) and NGC 7329 (right) with the barlens component identified in Li2017. The blue ellipses show the visually identified outer edge of the barlenses.
• Figure 3: 2D image decomposition results of the Model 1 mock image in the face-on view. In each column, the top row shows the best-fit 2D model image (left) and the residual image (right). The green and blue ellipses indicate the outlines of the bar and barlens, respectively, with semi-major axis of 2$r_{\rm e}$. The other rows show results of the isophotal analysis, including the radial profiles of ellipticity (second row), the position angle (third row), the surface brightness (forth row), and the residual between data and the best-fit model (bottom row). From left to right, the fitting configuration are bar+D, bar+D1+D2, bl+bar+D1+D2 (free parameters fitting), and bl+bar+D1+D2 (fixing the size and axial ratio of the barlens), respectively (see details in the text). The fifth column (bl+bar+D) shows the result of the fitting with only one exponential disk. The RFF of each configuration is labeled in the model image.
• Figure 4: 2D image decomposition results of the Model 2 mock image in the face-on view. The red, green, and blue ellipses indicate the outlines of the bulge, bar, and barlens, respectively, with semi-major axis of 2$r_{\rm e}$. Other conventions are similar to Figure \ref{['fig:nbody_decomposition']}. The components used in the fitting configurations are shown on the top.
• Figure 5: 2D image decomposition results of NGC 1533. Other conventions are similar to Figure \ref{['fig:nbody_decomposition']}. From left to right, configurations are arranged in the following order: B+D, B+bar+D, B+bar+D1+D2 and B+bar+bl+D1+D2 (fixing the Sérsic index, size and axial ratio of the barlens).