Oral Presentation

Can the dichotomy scenario explain the observed variety of elliptical galaxies?

Author(s): Rogerio Monteiro-Oliveira (ASIAA), Yen-Ting Lin (ASIAA), Wei-Huai Chen (NTU/ASIAA), Chen-Yu Chuang (NTHU/ASIAA), Abdurro'uf (Johns Hopkins University), Po-Feng Wu (NTU), Eric Emsellem ESO) and Martin Bureau (University of Oxford)

Presenter: Rogerio Monteiro-oliveira (ASIAA)

The identification of patterns in large samples of elliptical galaxies is crucial for understanding their formation, as common features point toward similar assembly histories. Supported by observed correlations (albeit based on small galaxy sample sizes), some researchers have proposed that elliptical galaxies can be split into two distinct groups. On one side lie the most luminous ellipticals, characterized by boxy-shaped isophotes, a high value of alpha-to-iron abundance ratio, and stellar dynamics showing a slow rotator (SR) pattern. On the other hand, the antipode group contains lower luminosity galaxies with disky isophotes, a low alpha-to-iron ratio, and dynamics supported by rotation (fast rotators, FR). This is known as the dichotomy of elliptical galaxies. However, this idea has not yet found full agreement in the literature, as many other researchers have not found the same correlations.

To help shed light on this topic, we used a sample of ~2,000 galaxies identified as ellipticals via deep learning by the MaNGA survey. We extracted from them a diverse set of parameters, including their isophote shapes (boxy or disky), stellar kinematics (SR or FR), chemical composition (alpha-to-iron abundance in terms of [Mg/Fe]), and stellar velocity dispersion. We also added their absolute magnitude and stellar mass from the NASA-Sloan Atlas, totaling seven parameters per galaxy. Then, we conducted an intensive search for correlations among all these parameters. We found that the ellipticals are majority disky-shaped (65%) and FRs (67%), but the abundance of [Mg/Fe] does not show any differences between the different classes. The result of the principal component analysis revealed that our sample of elliptical galaxies shows a single pattern, discarding, therefore, the dichotomy scenario. The different classes (boxy/disky and SR/FR) occupy slightly different loci in the extended parameter space, which may explain the observed trend that led to the misled dichotomy using small sample sizes.