Structural properties of disk galaxies

Abstract

Context. Knowledge of the intrinsic shapes of galaxy components provides crucial information when constraining phenomena driving their formation and evolution. Aims. We analize the structural parameters of a magnitude-limited sample of 148 unbarred S0-Sb galaxies to derive the intrinsic shape of their bulges. Methods. We developed a new method to derive the intrinsic shapes of bulges based on geometrical relationships between the apparent and intrinsic shapes of bulges and disks. Bulges were assumed to be triaxial ellipsoids sharing the same center and polar axis of their surrounding disks. Disks were assumed to be circular, infinitesimally thin, and to lie on the equatorial plane of bulges. The equatorial ellipticity and intrinsic flattening of bulges were obtained from the length of the apparent major and minor semi-axes of the bulge, the twist angle between the apparent major axis of the bulge and the galaxy line of nodes, and the galaxy inclination. Results. We find that the intrinsic shape is well constrained for a subsample of 115 bulges with favorable viewing angles. A large fraction of them are characterized by an elliptical section (B/A < 0.9). This fraction is 33%, 55%, and 43% if using their maximum, mean, or median equatorial ellipticity, respectively. Most are flattened along their polar axis (C < (A + B)/2). Only 18% of the observed bulges have a probability >50% and none has a probability >90% of being elongated along the polar axis. The distribution of triaxiality is strongly bimodal. This bimodality is driven by bulges with Sérsic index n > 2, or equivalently, by the bulges of galaxies with a bulge-to-total ratio B/T > 0.3. Bulges with n ≤ 2 and with B/T ≤ 0.3 follow a similar distribution, which differs from that of bulges with n > 2 and B/T > 0.3. In particular, bulges with n ≤ 2 and B/T ≤ 0.3 exhibit a larger fraction of oblate axisymmetric (or nearly axisymmetric) bulges, a smaller fraction of triaxial bulges, and fewer prolate axisymmetric (or nearly axisymmetric) bulges with respect to bulges with n > 2 and with B/T > 0.3, respectively. No correlation is found between the intrinsic shape and either the luminosity or velocity dispersion of bulges. Conclusions. According to predictions of the numerical simulations of bulge formation, bulges with n ≤ 2, which show a high fraction of oblate axisymmetric (or nearly axisymmetric) shapes and have B/T ≤ 0.3, may be the result of dissipational minor mergers. Both major dissipational and dissipationless mergers seem to be required to explain the variety of shapes found for bulges with n > 2 and B/T > 0.3.

The Intrinsic Shape of Galaxy Bulges

“…2 for definition). This seminal paper promoted the work of Méndez-Abreu et al (2010). They introduced a new method to derive the intrinsic shape of bulges based upon the analytical relations between the observed and intrinsic shapes of bulges and their surrounding discs.…”

“…where φ B , φ C and F θ are functions of the observed quantities a, b, δ , and θ , see equations 12, 13, and 43 of Méndez-Abreu et al (2010). Therefore, Eq.…”

Evolution induced by dry minor mergers onto fast-rotator S0 galaxies

“…11, we compare the structure of our remnant bulges with those of real bulges. In this panel, we show the major and minor axial ratios of our remnant bulges over-plotted on a sample of 115 bulges of S0-Sb galaxies from Méndez-Abreu et al (2010). These authors used a different criterion from that used by Cox et al (2006a) to determine the axial ratios, with semi-axes A and B contained within the equatorial plane of the galaxy (A > B) and the C semi-axis perpendicular to it.…”

“…We estimated the triaxialities of the remnant bulges, defined as (Franx et al 1991;Méndez-Abreu et al 2010)…”

“…Real bulges in ETGs tend to be triaxial (Méndez-Abreu et al 2012). It might be assumed that this initial condition biases the remnants towards low apparent e values in the cases of progenitors with massive (i.e., luminous) bulges, because central mass concentrations tend to make the discs more stable against distortions (see references in EM11).…”