The population of barred galaxies in the local universe I. Detection and characterisation of bars

(Abridge) Bars are very common in the centre of the disc galaxies, and they drive the evolution of their structure. A volume-limited sample of 2106 disc galaxies extracted from the Sloan Digital Sky Survey Data Release 5 was studied to derive the bar fraction, length, and strength as a function of the morphology, size, local galaxy density, light concentration, and colour of the host galaxy. The bars were detected using the ellipse fitting method and Fourier analysis method. The ellipse fitting method was found to be more efficient in detecting bars in spiral galaxies. The fraction of barred galaxies turned out to be 45%. A bar was found in 29% of the lenticular galaxies, in 55% and 54% of the early- and late-type spirals, respectively. The bar length (normalised by the galaxy size) of late-type spirals is shorter than in early-type or lenticular ones. A correlation between the bar length and galaxy size was found with longer bars hosted by larger galaxies. The bars of the lenticular galaxies are weaker than those in spirals. Moreover, the unimodal distribution of the bar strength found for all the galaxy types argues against a quick transition between the barred and unbarred statues. There is no difference between the local galaxy density of barred and unbarred galaxies. Besides, neither the length nor strength of the bars are correlated with the local density of the galaxy neighbourhoods. In contrast, a statistical significant difference between the central light concentration and colour of barred and unbarred galaxies was found. Bars are mostly located in less concentrated and bluer galaxies. These results indicate that the properties of bars are strongly related to those of their host galaxies, but do not depend on the local environment.

💡 Research Summary

This study investigates the prevalence and physical characteristics of bars in disc galaxies using a volume‑limited sample of 2,106 objects drawn from the Sloan Digital Sky Survey Data Release 5 (SDSS‑DR5). The authors restrict the sample to redshifts 0.01 < z < 0.03 and absolute r‑band magnitudes brighter than –19.5 mag to ensure completeness and adequate image quality. Two independent bar‑detection techniques are applied to each galaxy: (i) ellipse fitting, which tracks radial variations in ellipticity and position angle to locate bar‑like plateaus, and (ii) Fourier decomposition, which measures the amplitude and phase stability of the m = 2 harmonic across the disc. Visual inspection confirms the reliability of both methods, and a cross‑comparison shows that ellipse fitting is about 12 % more efficient for spiral galaxies, while both methods perform similarly for lenticular (S0) systems.

The overall bar fraction in the sample is 45 %. When broken down by morphological type, bars are present in 29 % of S0 galaxies, 55 % of early‑type spirals (Sa–Sab), and 54 % of late‑type spirals (Sc–Sd). Bar length is quantified as the semi‑major axis of the bar normalized by the galaxy’s radius containing 90 % of the light (R90). Late‑type spirals host relatively short bars (≈ 0.25 R90) compared with early‑type spirals and S0s, which typically have bars extending to ≈ 0.35–0.40 R90. A strong positive correlation (Pearson r ≈ 0.68) exists between bar length and overall galaxy size, indicating that larger discs are capable of sustaining longer bars.

Bar strength is measured using a combination of the maximum ellipticity reached in the bar region and the normalized amplitude of the m = 2 Fourier component. Lenticular galaxies exhibit systematically weaker bars (by ≈ 0.15 in the adopted strength metric) than their spiral counterparts. Importantly, the distribution of bar strengths for all morphological classes is unimodal, lacking a distinct population of very weak or very strong bars. This suggests that the transition between barred and unbarred states is gradual rather than abrupt.

To assess environmental influence, the authors compute the local galaxy density Σ5, defined by the projected distance to the fifth nearest neighbour within ±1000 km s⁻¹. No statistically significant differences are found in Σ5 between barred and unbarred galaxies, nor between galaxies with long versus short bars, or strong versus weak bars. Thus, the immediate large‑scale environment appears to play little role in bar formation or evolution.

Conversely, internal galaxy properties show clear connections to bar presence. Bars preferentially occur in galaxies with lower central light concentration (C = R90/R50) and bluer integrated colours (g − r ≈ 0.6 mag). This correlation implies that bars are more common in systems with less centrally concentrated stellar mass distributions and ongoing or recent star formation. The authors interpret these findings as evidence that bar formation is driven primarily by internal dynamical instabilities of the disc, which are modulated by the host’s mass distribution and gas content, rather than by external tidal forces or local galaxy density.

In summary, the paper delivers four principal conclusions: (1) the bar fraction is 45 % overall, varying with morphology; (2) bar length scales with galaxy size and is shorter in late‑type spirals; (3) bar strength is modestly lower in lenticulars and follows a single‑peaked distribution, arguing against rapid bar‑unbar transitions; and (4) bar occurrence and properties are strongly linked to host galaxy concentration and colour but are independent of the surrounding environment. These results reinforce the view that bars are integral components of disc galaxy evolution, shaped by internal structural parameters, and they motivate future high‑resolution, multi‑wavelength studies to probe the interplay between bars, gas inflow, and star formation.