A combined photometric and kinematic recipe for evaluating the nature of bulges using the CALIFA sample

Abstract

Understanding the nature of bulges in disc galaxies can provide important insights into the formation and evolution of galaxies. For instance, the presence of a classical bulge suggests a relatively violent history, in contrast, the presence of simply an inner disc (also referred to as a "pseudobulge") indicates the occurrence of secular evolution processes in the main disc. However, we still lack criteria to effectively categorise bulges, limiting our ability to study their impact on the evolution of the host galaxies. Here we present a recipe to separate inner discs from classical bulges by combining four different parameters from photometric and kinematic analyses: The bulge Sérsic index n b , the concentration index C 20,50 , the Kormendy (1977) relation and the inner slope of the radial velocity dispersion profile ∇σ. With that recipe we provide a detailed bulge classification for a sample of 45 galaxies from the integral-field spectroscopic survey CALIFA. To aid in categorising bulges within these galaxies, we perform 2D image decomposition to determine bulge Sérsic index, bulge-to-total light ratio, surface brightness and effective radius of the bulge and use growth curve analysis to derive a new concentration index, C 20,50 . We further extract the stellar kinematics from CALIFA data cubes and analyse the radial velocity dispersion profile. The results of the different approaches are in good agreement and allow a safe classification for approximately 95% of the galaxies. In particular, we show that our new "inner" concentration index performs considerably better than the traditionally used C 50,90 when yielding the nature of bulges. We also found that a combined use of this index and the Kormendy (1977) relation gives a very robust indication of the physical nature of the bulge.

The Carnegie-Irvine Galaxy Survey. IX. Classification of Bulge Types and Statistical Properties of Pseudo Bulges

“…Combined with the discrepant classification results mentioned above, we suggest that n = 2 is not an appropriate demarcation line for separating classical and pseudo bulges. As with Neumann et al (2017), we find that n = 1.5 is a better criterion; based on the statistics of CGS, this revised criterion would correctly classify 65% of the pseudo bulges and 74% of the classical bulges. Note that even with this modified threshold Sérsic n is still a less effective classifier of bulge type than µ e (Section 3.2).…”

High-resolution MEGARA Integral-field Unit Spectroscopy and Structural Analysis of a Fast-rotating, Disky Bulge in NGC 7025

“…Recently, Neumann et al (2017), Rizzo et al (2018) and Gilhuly & Courteau (2018) fit a 2D Sérsic bulge + exponential disk model to the SDSS images of NGC 7025, without accounting for the intermediate (spiral-arm) component. As warned by Dullo et al (2016Dullo et al ( , 2017Dullo et al ( , 2018, see also Dullo & Graham 2012, 2014Graham et al 2016 Tables 1 and 3).…”

“…Detailed structural and stellar kinematic studies of pseudo-bulges enable us to discriminate between these different formation scenarios, but the lack of robust bulge diagnostic criteria presents a major challenge for the identification pseudo-bulges (e.g., Graham 2013;Neumann et al 2017). Graham (2013, his Section 4.3) provided cautionary remarks about the misidentification of pseudo-bulges and classical bulges when using criteria based e.g., on the bulge's Sérsic index, rotation and stellar age (Kormendy & Kennicutt 2004, their Section 4;Fisher & Drory 2008).…”

The Activation of Galactic Nuclei and Their Accretion Rates Are Linked to the Star Formation Rates and Bulge-types of Their Host Galaxies

“…Gadotti (2009) studied structural properties of a larger sample of about 1000 galaxies from the Sloan Digital Sky Survey (SDSS). He argued that, while n b can be used to distinguish pseudobulges from classical bulges, a more reliable (for low resolution imaging), and physically motivated, separation can be made using the Kormendy relation (see also Neumann et al 2017). He defined pseudobulges as galaxies that lie 3σ below this linear relation of the mean effective bulge surface brightness and the effective radius of the ellipticals.…”

“…This definition was criticized by Fisher & Drory (2016), who showed that a significant population of their bright pseudobulges with n b < 2 lie on the Kormendy relation. Since there is no single ideal way of identifying bulge-types, Fisher & Drory (2016) and Neumann et al (2017) called for a comprehensive approach that combines multiple indicators. Nevertheless, after carefully decomposing the SDSS multiband images into bulge, disc, and bar components, Gadotti (2009) showed that the Petrosian concentration index is a better proxy for the bulge-to-total ratio than the global Sérsic index.…”