Peanuts at an angle: detecting and measuring the three-dimensional structure of bars in moderately inclined galaxies

Abstract

We show that direct detection and measurement of the vertically thickened parts of bars (so called ‘boxy’ or ‘peanut- shaped’ bulges) are possible not only for edge-on galaxies but also for galaxies with moderate inclinations (i < 70◦), and that examples are relatively common in the nearby Universe. The analysis of a sample of 78 nearby, moderately inclined (i � 65◦) early-type (S0–Sb) barred galaxies shows that the isophotal signature of the box/peanut can usually be detected for inclinations as low as i ∼ 40◦ – and in exceptional cases down to i ∼ 30◦. In agreement with the predictions from N-body simulations, the signature is most easily detectable when the bar’s position angle is within ∼50◦ of the galaxy major axis; in particular, galaxies where the bar lies very close to the minor axis do not show the signature clearly or at all. For galaxies with i = 40◦–65◦ and relative angles <45◦, we find evidence for the signature ≈2/3 of the time; the true frequency of box/peanut structures in bars may be higher. Comparison with N-body models also allows us to link observed photometric morphology with 3D physical structures, and thus estimate the relative sizes of box/peanut structures and bars. For our local sample, we find that box/peanut structures range in radial size (measured along the bar major axis) from 0.4 to 3.8 kpc (mean =1.5 ± 0.9 kpc) and span 0.26–0.58 of the bar length (mean of 0.38 ± 0.08). This is a clear observational confirmation that when bars thicken, it is not the entire bar which does so, but only the inner part. This technique can also be used to identify galaxies with bars which have not vertically thickened. We suggest that NGC 3049 and IC 676 may be particularly good examples, and that the fraction of S0–Sb bars which lack box/peanut structures is at least ∼13 per cent.