How Large Are the Bars in Barred Galaxies?

Abstract

I present a study of the sizes (semimajor axes) of bars in disc galaxies, combining a detailed study of 65 S0-Sb galaxies with measurements of 70 Sb-Sd galaxies from Martin (1995). As has been noted before with smaller samples, bars in early-type (S0-Sb) galaxies are clearly larger than bars in late-type (Sc-Sd) galaxies; this is true both for relative sizes (bar length as fraction of isophotal radius R_25 or exponential disc scale length h) and absolute sizes (kpc). S0-Sab bars extend to ~1-10 kpc (mean ~3.3 kpc), ~0.2-0.8 R_25 (mean \~0.38 R_25) and ~0.5-2.5 h (mean ~1.4 h). Late-type bars extend to only \~0.5-3.5 kpc, 0.05-0.35 R_25 and 0.2-1.5 h; mean sizes are ~1.5 kpc, 0.14 R_25 and 0.6 h. Sb galaxies resemble earlier-type galaxies in terms of bar size relative to h; their smaller R_25-relative sizes may be a side effect of higher star formation, which increases R_25 but not h. For S0-Sbc galaxies, bar size correlates well with disc size (both R_25 and h); these correlations are stronger than the known correlation with M_B. All correlations appear to be weaker or absent for late-type galaxies; in particular, there seems to be no correlation between bar size and either h or M_B for Sc-Sd galaxies. I show that the bars detected in HST near-IR images at z ~ 1 by Sheth et al. (2003) have absolute sizes consistent with those in bright, nearby S0-Sb galaxies. I also compare the sizes of real bars with those produced in simulations, and discuss some possible implications for scenarios of secular evolution along the Hubble sequence. Simulations often produce bars as large as -- or larger than -- those seen in S0-Sb galaxies, but rarely any as small as those in Sc-Sd galaxies. (Abridged.)