The extended Planetary Nebula Spectrograph (ePN.S) early-type galaxy survey: The specific angular momentum of ETGs

¹ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany
e-mail: cpulsoni@mpe.mpg.de
² Department of Physics and Astronomy, Johns Hopkins University, 3400 N. Charles Street, Baltimore, MD 21218, USA
³ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
⁴ Facultad de Ciencias Astronómicas y Geofísicas de la Universidad Nacional de La Plata, and Instituto de Astrofísica de La Plata (CCT La Plata – CONICET, UNLP), Paseo del Bosque s/n, B1900FWA La Plata, Argentina
⁵ Consejo Nacional de Investigaciones Científicas y Técnicas, Godoy Cruz 2290, C1425FQB Ciudad Autónoma de Buenos Aires, Argentina
⁶ Waterloo Centre for Astrophysics, University of Waterloo, 200 University Ave W, Waterloo, Ontario N2L 3G1, Canada
⁷ Perimeter Institute for Theoretical Physics, Waterloo, Ontario N2L 2Y5, Canada
⁸ European Southern Observatory, Alonso de Córdova 3107, Santiago de Chile, Chile
⁹ Sub-Department of Astrophysics, Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
¹⁰ Finnish Centre for Astronomy with ESO (FINCA), University of Turku, 20014 Turku, Finland
¹¹ School of Physics and Astronomy, Sun Yat-sen University, Zhuhai Campus, 2 Daxue Road, Xiangzhou District, Zhuhai, PR China
¹² CSST Science Center for Guangdong-Hong Kong-Macau Great Bay Area, Zhuhai 519082, PR China
¹³ INAF – Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy

Received: 24 February 2023
Accepted: 24 April 2023

Abstract

Context. Mass and angular momentum are key parameters of galaxies. Their coevolution establishes an empirical relation between the specific stellar angular momentum j_* and the stellar mass M_* that depends on morphology.

Aims. For this work, we measured j_* in a sample of 32 early-type galaxies (ETGs) from the ePN.S survey, using the full two-dimensional kinematic information. We present local λ profiles and projected j_* profiles in apertures. We derived the distribution of these galaxies on the total j_* − M_* plane and determined the ratio between the stellar j_* and the specific angular momentum of the host dark matter halo.

Methods. We used integral-field-spectroscopic data in the central regions (one to two effective radii, R_e) and planetary nebula (PN) kinematics in the outskirts (out to a mean 6R_e). In the j_* determination, we accounted for misaligned rotation and for the differences between light-weighted j_* and mass-weighted j_*, estimating also the effects of gradients in the mass-to-light ratio driven by variations in the initial mass function. We used simulated ETGs from the IllustrisTNG simulation TNG100 to correct for the limited radial coverage of the PN data and to account for projection effects on j_*.

Results. The radially extended, two-dimensional kinematic data show that the stellar halos of ETGs do not contain large stellar mass fractions of high j_*. The j_* profiles of fast-rotator ETGs are largely converged within the range of the data. For slow rotators, j_* is still rising and is estimated to increase beyond 6R_e by up to 40%, using simulated galaxies from TNG100. More than 60% of their stellar halo angular momentum is in misaligned rotation. We find that the ePN.S ETG sample displays the well-known correlation between j_*, M_*, and morphology: elliptical galaxies have systematically lower j_* than similar mass S0 galaxies. However, fast and slow rotators lie on the same relation within errors with the slow rotators falling at the high M_* end. A power-law fit to the mass-weighted j_* − M_* relation gives a slope of 0.55 ± 0.17 for the S0s and 0.76 ± 0.23 for the ellipticals, with normalization about four and nine times lower than spirals, respectively. The estimated retained fraction of angular momentum at 10¹⁰ ≤ M_* ≤ 10^10.5 M_⊙ is ∼25% for S0s and > 10% for ellipticals, and decreases by ∼1.5 orders of magnitude at M_* ∼ 10¹² M_⊙.

Conclusions. Our results show that ETGs have substantially lower j_* than spiral galaxies with similar M_*. Their j_* must be lost during their evolution, and/or retained in the hot gas component and the satellite galaxies that have not yet merged with the central galaxy.