Early-Type Galaxies: Elliptical and S0 Galaxies, or Fast and Slow Rotators

TL;DR

This work addresses the observed bimodality in early-type galaxies (ETGs) by reframing their classification through kinematics rather than morphology, distinguishing fast and slow rotators and linking them to distinct formation paths. Using integral-field spectroscopy across large surveys, it demonstrates that fast rotators form a continuous sequence with spiral galaxies and that their stellar populations correlate with the central mass concentration, approximated by $\sigma_e \propto \sqrt{M_*/R_e}$. Slow rotators, by contrast, are massive, spheroidal systems that quench early and grow predominantly via dry mergers, and they preferentially inhabit dense environments; their properties align with an early, rapid star-formation epoch followed by hierarchical assembly. The analysis shows that the Fundamental Plane tilt arises mainly from stellar $M/L$ variations, while the Mass Plane adheres to virial expectations, with dark-matter fractions inside $R_e$ remaining modest; environment modulates the mass assembly history and the prevalence of slow rotators. These results provide a coherent framework for ETG evolution, with future high-redshift integral-field studies (via JWST and ELT) poised to test the two-path scenario over cosmic time, elucidating how bulge growth and central density track star-formation quenching and structural transformation.

Abstract

Early-type galaxies (ETGs) show a bimodal distribution in key structural properties like stellar specific angular momentum, kinematic morphology, and nuclear surface brightness profiles. Slow rotator ETGs, mostly found in the densest regions of galaxy clusters, become common when the stellar mass exceeds a critical value of around $M_*^\mathrm{crit}\approx2\times 10^{11}\,M_\odot$, or more precisely when $\lg(R_\mathrm{e}/\mathrm{kpc}) \gtrsim 12.4 - \lg(M_*/M_\odot)$. These galaxies have low specific angular momentum, spheroidal shapes, and stellar populations that are old, metal-rich, and $α$-enhanced. In contrast, fast rotator ETGs form a continuous sequence of properties with spiral galaxies. In these galaxies, the age, metallicity, and $α$-enhancement of the stellar population correlate best with the effective stellar velocity dispersion $σ_\mathrm{e} \propto \sqrt{M_*/R_\mathrm{e}}$ (i.e., properties are similar for $R_\mathrm{e}\propto M_*$), or with proxies approximating their bulge mass fraction. This sequence spans from star-forming spiral disks to quenched, passive, spheroid-dominated fast rotator ETGs. Notably, at a fixed $σ_\mathrm{e}$, younger galaxies show lower metallicity. The structural differences and environmental distributions of ETGs suggest two distinct formation pathways: slow rotators undergo early intense star formation followed by rapid quenching via their dark halos and supermassive black holes, and later evolve through dry mergers during hierarchical cluster assembly; fast rotators, on the other hand, develop more gradually through gas accretion and minor mergers, becoming quenched by internal feedback above a characteristic $\lg(\mathrm{σ_e^{crit}}/\text{ km s}^{-1})\gtrsim2.3$ (in the local Universe) or due to environmental effects.

Figures (20)

• Figure 1: Hubble's tuning-fork diagram for galaxy classification, adapted from Hubble1936. The diagram features elliptical galaxies (E) along the handle, which then splits into two prongs representing spiral galaxies. The top prong depicts normal spirals (S), ranging from tightly wound, bulge-dominated types (Sa) to loosely wound, bulge-less types (Sc). The bottom prong shows barred spirals (SB), characterized by a bar-shaped structure and similar subclasses. Lenticular galaxies (S0), located at the fork's split, bridge the gap between elliptical and spiral galaxies, featuring a central bulge and disk but lacking prominent spiral arms. This version of the diagram was modified by Kormendy1996 to include boxy E(b) and disky E(d) elliptical galaxies, as well as irregular galaxies Im. I added annotations to differentiate early-type galaxies (E and S0), the focus of this chapter, from spiral galaxies.
• Figure 2: Left panel: Sérsic profiles of \ref{['eq:sersic']} for different logarithmically-spaced values of the Sérsic index $n$. All profiles have the same logarithmic slope $\gamma=-2$ at a radius $R\approx1.19R_\mathrm{e}$, indicated by the vertical dotted line with 1% accuracy, where $R_\mathrm{e}$ is the half-light radius. The dashed line shows the asymptotic profile $\Sigma\propto R^{-2}$ for $n\rightarrow\infty$. Right panel: Correlation between Sérsic index $n$ and visual magnitude $M_{VT}$. Red, blue, green, and cyan points represent core ellipticals (Es), non-core ellipticals, spheroidal galaxies, and S0 bulges, respectively. Green triangles, crosses, and open squares indicate spheroidals Kormendy2009.
• Figure 3: HST major-axis surface brightness profiles of elliptical galaxies. The solid lines represent fits using \ref{['eq:core_sersic']}, with dotted lines showing inner and outer extrapolations, and dashed lines indicating inward extrapolations of the Sérsic-like component beyond the break radius. For NGC 5831, the best fit is nearly a pure Sérsic model. In contrast, NGC 3348 (classified as a 'core' galaxy) exhibits a distinct inner break. The root-mean-square (rms) scatter $\Delta \mathrm{mag}$ for each fit is provided below the corresponding panel Graham2003core.
• Figure 4: Morphological classifications based on stellar kinematics. Early-type galaxies are divided into five classes as defined by Krajnovic2011: (a) no detectable rotation, (b) nonregular rotation, (c) kinematically distinct cores (KDCs), (d) counter-rotating disks, and (e) regular disk-like rotation. The kinematic data, obtained from Emsellem2004, were spatially binned using the Voronoi tessellation method of Cappellari2003. Tick marks on the images are spaced at 10 arcsecond intervals. The bottom row displays corresponding SDSS images, with Hubble's morphological classifications indicated in parentheses next to the galaxy names. Classes (d) and (e) are physically related, as both feature stellar disks and are nearly axisymmetric, while the other classes show no evidence of stellar disks. Class (e) is labeled as regular rotators, whereas the remaining classes are classified as nonregular rotators. The symbols below the images represent the different morphological classes and are also used in \ref{['fig:kin_mis']} and \ref{['fig:lam_eps']}.
• Figure 5: Fast-rotator early-type galaxies (ETGs) are axisymmetric out to large radii: For each of the eight galaxies, the left and middle panels show the SDSS surface brightness contours (in black) at two different scales for regular rotator galaxies classified as ellipticals. The red contours represent a Multi-Gaussian Expansion (MGE) fit using https://pypi.org/project/mgefit/Cappellari2002mge, assuming a fixed photometric position angle (PAphot) at all radii. The PAphot, determined using https://pypi.org/project/mgefit/, is indicated by blue arrows. The close match between the observed images and the MGE fit confirms that the photometry is consistent with a constant PAphot across all radii, with no detectable photometric twists. This behavior contrasts with triaxial systems, where radial ellipticity variations typically cause PAphot twists. The right panel displays stellar kinematic maps (from Cappellari2011a) and the best-fitting kinematic position angle (PAkin), measured using https://pypi.org/project/pafit/Krajnovic2006. In all cases, the kinematic misalignment is $|\Psi_\text{mis}| \lesssim 2^\circ$, within measurement uncertainties. The absence of PAphot variation and the negligible $\Psi_\text{mis}$ indicate that these galaxies maintain axisymmetry out to at least $4R_\mathrm{e}$, the extent of the photometric data. These eight galaxies are representative of regular-rotator ETGs as a class, unless barred or otherwise disturbed.