Barlenses and X-shaped features compared: two manifestations of boxy/peanut bulges

Abstract

Aims. We study the morphological characteristics of boxy/peanutshaped bulges. In particular, we are interested to determine whether most of the flux associated with bulges in galaxies with masses similar to those of the Milky Way at redshift z ~ 0 might belong to the vertically thick inner part of the bar, in a similar manner as in the Milky Way itself. At high galaxy inclinations, these structures are observed as boxy/peanut/Xshaped features, and when the view is near to faceon, they are observed as barlenses. We also study the possibility that bulges in some fraction of unbarred galaxies might form in a similar manner as the bulges in barred galaxies. Methods. We used the Spitzer Survey of Stellar Structure in Galaxies (S G) and the NearIR S0 galaxy Survey (NIRS0S) to compile complete samples of galaxies with barlenses (N = 85) and Xshaped features (N = 88). A sample of unbarred galaxies (N = 41) is also selected. For all 214 galaxies unsharp mask images were created, used to recognize the Xshaped features and to measure their linear sizes. To detect possible boxy isophotes (using the B parameter), we also performed an isophotal analysis for the barlens galaxies. We use recently published Nbody simulations: the models that exhibit boxy/peanut/X/barlens morphologies are viewed from isotropically chosen directions that cover the full range of galaxy inclinations in the sky. The synthetic images were analyzed in a similar manner as the observations. Results. This is the first time that the observed properties of barlenses and Xshaped features are directly compared across a wide range of galaxy inclinations. A comparison with the simulation models shows that the differences in their apparent sizes, a/r ? 0.5 for barlenses and a/r ? 0.5 for Xshapes, can be explained by projection effects. Observations at various inclinations are consistent with intrinsic a a 0.5r : here intrinsic size means the faceon semimajor axis length for bars and barlenses, and the semilength of the Xshape when the bar is viewed exactly edgeon. While Xshapes are quite common at intermediate galaxy inclinations (for i = 40-60 their frequency is about half that of barlenses), they are seldom observed at smaller inclinations. This is consistent with our simulation models, which have a small compact classical bulge that produces a steep inner rotation slope, whereas bulgeless shallow rotation curve models predict that Xshapes should be visible even in a faceon geometry. The steep rotation curve models are also consistent with the observed trend that B is positive at low inclination and with negative values for i ? 40-60; this implies boxy isophotes. In total, only about one quarter of the barlenses (with i = 60) show boxy isophotes. Conclusions. Our analyses are consistent with the idea that barlenses and Xshaped features are physically the same phenomenon. However, the observed nearly round faceon barlens morphology is expected only when at least a few percent of the disk mass is located in a central component, within a region much smaller than the size of the barlens itself. Barlenses contribute to secular evolution of By using this website, you agree that EDP Sciences may store cookies on your browser to enhance your experience.