Looking for a needle in a haystack : how to measure the length of a stellar bar?

Abstract

One of the challenges related to stellar bars is to determine accurately the length of the bar in a disc galaxy. In past literature, a wide variety of methods has been employed to measure the extent of a bar. However, a systematic study on determining the robustness and accuracy of different bar length estimators is still beyond our grasp. Here, we investigate the accuracy and the correlation (if any) between different bar length measurement methods while using an $N$-body model of a barred galaxy where the bar evolves self-consistently in presence of a live dark matter halo. We investigate the temporal evolution of the bar length, using different estimators (involving isophotal analysis of de-projected surface brightness distribution and Fourier decomposition of surface density), and study their robustness and accuracy. Further attempts have been made towards determining correlation between any two of these bar length estimators used here. In presence of spirals, the bar length estimators which only consider the amplitudes of different Fourier moments (and do not take into account the phase-angle of $m=2$ Fourier moment), systematically overestimate the length of the bar. The strength dark-gaps (produced by bars) correlates strongly with the bar length in early rapid growth phase, and is only weakly anti-correlated during subsequent quiescent phase of bar evolution. However, the location of dark-gaps correlates only weakly with the bar length, hence can not be used as a robust proxy for determining the bar length. In addition, the bar length estimators, obtained using isophotal analysis of de-projected surface brightness distribution, systematically overestimate the bar length. The implications of bar length over(under)estimation in the context of determining fast/slow bars are further discussed.Comment: 12 pages, 10 figures (including appendix), comments are welcome