Abstract
We use simulations to study the growth of a pseudobulge in an isolated thin exponential stellar disc embedded in a static spherical halo. We observe a transition from later to earlier morphological types and an increase in bar prominence for higher disc-to-halo mass ratios, for lower disc-to-halo size ratios, and for lower halo concentrations. We compute bulge-to-total stellar mass ratios B/T by fitting a two-component Sérsic-exponential surface-density distribution. The final B/T is strongly related to the disc’s fractional contribution fd to the total gravitational acceleration at the optical radius. The formula B/T = 0.5 fd1.8 fits the simulations to an accuracy of 30%, is consistent with observational measurements of B/T and fd as a function of luminosity, and reproduces the observed relation between B/T and stellar mass when incorporated into the GALICS 2.0 semi-analytic model of galaxy formation.
Highlights
According to the standard theory of galaxy formation, the dissipative infall of gas in the gravitational potential wells of dark-matter (DM) haloes forms discs (Fall & Efstathiou 1980); elliptical galaxies are formed by mergers (Toomre & Toomre 1972)
Semianalytic models (SAMs) of galaxy formation build on this theory and describe a galaxy as the sum of two components: a disc and a bulge
If the observations that we aim to explain cannot distinguish between different types of bulges, it makes sense to compute the bulge-to-total mass ratio B/T in such a way that any stellar surface-density excess above an exponential fit is assigned to the bulge component, independently of its origin, structure, and kinematics
Summary
According to the standard theory of galaxy formation, the dissipative infall of gas in the gravitational potential wells of dark-matter (DM) haloes forms discs (Fall & Efstathiou 1980); elliptical galaxies are formed by mergers (Toomre & Toomre 1972). Semianalytic models (SAMs) of galaxy formation build on this theory and describe a galaxy as the sum of two components: a disc and a bulge. Observers perform a similar decomposition when they fit galaxies with the sum of an exponential and a Sérsic (1963) profile to compute quantitative morphologies (Simard et al 2011; Meert et al 2015, 2016; Dimauro et al 2018). This simplification brushes over the complexity and diversity of galactic morphologies, for example, the distinction between normal and barred spirals (Hubble 1926). It would be much more difficult to perform the same analysis on large samples, such as the Sloan Digital Sky Survey (SDSS), or on high-redshift data with even poorer spatial resolution
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