Properties of bars and bulges in the Hubble sequence

Abstract

Properties of bars and bulges in the Hubble sequence are discussed, based on an analysis of 216 disc galaxies of S0‐Sm types (S0s from the Near‐Infrared S0 Survey and spirals from the Ohio State University Bright Spiral Galaxy Survey). For this purpose we have collected, and completed when necessary, the various analyses we have previously made separately for early‐ and late‐type galaxies. We find strong evidence of pseudo‐bulges in all Hubble types. Pseudo‐bulges are disc‐like structures formed by secular evolutionary processes in galaxies. Similar to spirals, the early‐type disc galaxies (S0‐S0/a) have on average relatively exponential bulges with Sersic index n < 2, and 56 per cent of them show disc‐like fine structures in the region of the bulge. For some of the galaxies there is also kinematic evidence of pseudo‐bulges. If S0‐S0/a galaxies were once spirals, stripped of their gas, then redistributed gas and star formation in the disc would be a natural explanation for all pseudo‐bulges in the Hubble sequence. However, it is difficult to explain how the bulges of S0 galaxies, which typically include about 30 per cent of the total galaxy mass, were formed by secularly induced central star formation. A more likely explanation is that pseudo‐bulges in barred early‐type galaxies are a combination of secularly induced star formation and the central steepening of the old stellar distribution. Bulges in non‐barred early‐type galaxies could be either classical merger‐built bulges, or pseudo‐bulges formed by similar processes as in barred galaxies, but in response to massive ovals or lenses (70 per cent of S0‐S0/a galaxies have ovals/lenses). Observational support for the outlined picture comes from the fact that bars in early‐type galaxies seem more evolved: their bars are long and massive and frequently (40 per cent) have ansae morphologies. In this scenario it would be possible also to explain why barred early‐type galaxies (preferentially pseudobulges) have slightly smaller B/T flux ratios than the non‐barred early‐type galaxies (mostly classical bulges).

Boxy/Peanut/X-shaped Bulges: Steep Inner Rotation Curve Leads to Barlens Face-on Morphology

“…Recent observational and theoretical studies support the conjecture (Laurikainen et al 2007) that such round inner structures are in fact the face-on counterparts of the vertically extended Boxy/Peanut/X-shaped (B/P/X) structures commonly seen in barred edge-on galaxies (e.g., Lütticke et al 2000;Bureau et al 2006). Namely, the stellar masses and the axial ratio distributions of their host galaxies are consistent with B/P/X and bl galaxies forming a single population (Laurikainen et al 2014;).…”

Early formation of massive, compact, spheroidal galaxies with classical profiles by violent disc instability or mergers

“…A sersic index n = 4 corresponds to the de-Vaucouleurs profile, typical of spheroids and classical bulges. Pseudobulges, on the other hand, have less-concentrated profiles, typically with a low sersic index, n < 2 (Kormendy & Kennicutt 2004;Laurikainen et al 2007;Fisher & Drory 2008;Gadotti 2009). The sersic fitting of the profiles is done using a Monte Carlo, least-squared fitting that samples the parameter space randomly and uniformly, so the probability that the solution converges to wrong secondary minima is low.…”

Measuring the X-shaped structures in edge-on galaxies

“…The Ferrers profile has a central plateau with a steep decrease of the intensity in the outer regions, and it is often used to describe bars (see e.g, Laurikainen et al 2007).…”