Beyond Hubble's Tuning Fork: is the classification of elliptical galaxies more complex than we thought?

Rogerio Monteiro-Oliveira^1*, Yen-Ting Lin¹, Wei-Huai Chen², Chen-Yu Chuang³, Abdurro'uf⁴, Po-Feng Wu², Eric Emsellem⁵, Martin Bureau⁶

¹IAA, Academia Sinica, Taipei, Taiwan
²Astronomy, NTU, Taipei, Taiwan
³Astronomy, NTHU, Hsinchu, Taiwan
⁴Physics and Astronomy, Johns Hopkins University, Baltimore, MD, USA
⁵European Southern Observatory, Garching, Germany
⁶Astrophysics, University of Oxford, Oxford, UK

* Presenter:Rogerio Monteiro-Oliveira, email:rmonteiro@asiaa.sinica.edu.tw

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, having 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 ishophotes, low alpha-to-iron ratio, and dynamics supported by rotation (hereafter fast rotators, FR). This is known as the dichotomy of the elliptical galaxies. However, this idea has not yet found full agreement in the literature, as many other researchers did not find 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.

Keywords: elliptical galaxies, galaxy formation, galaxy structure, stellar kinematics, chemical abundances