The Isotropic Center of NGC 5419—A Core in Formation?

Abstract

With its cored surface brightness profile, the elliptical galaxy NGC 5419 appears as a typical high-mass early-type galaxy (ETG). However, the galaxy hosts two distinct nuclei in its center. We use high-signal MUSE (Multi-unit Spectroscopic Explorer (Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO program 099.B-0193(A).)) spectral observations and novel triaxial dynamical orbit models to reveal a surprisingly isotropic central orbit distribution in NGC 5419. Recent collisionless simulations of merging massive ETGs suggest a two-phase core formation model, in which the low-density stellar core forms rapidly by supermassive black holes (SMBHs) sinking into the center due to dynamical friction. Only afterwards do the SMBHs form a hard binary, and the black hole scouring process slowly changes the central orbit distribution from isotropic to tangential. The observed cored density profile, the double nucleus, and the isotropic center of NGC 5419 together thus point to an intermediate evolutionary state where the first phase of core formation has taken place, yet the scouring process is only beginning. This implies that the double nucleus is an SMBH binary. Our triaxial dynamical models indicate a total mass of the two SMBHs in the center of NGC 5419 of M BH = (1.0 ± 0.08) × 1010 M ⊙. Moreover, we find that NGC 5419's complex kinematically distinct core can be explained by a coherent flip of the direction of orbital rotation of stars on tube orbits at ∼3 kpc distance from the galaxy center together with projection effects. This is also in agreement with merger simulations hosting SMBHs in the same mass regime.

Dynamical Stellar Mass-to-light Ratio Gradients: Evidence for Very Centrally Concentrated IMF Variations in ETGs?

“…We have singled out the remaining two galaxies from that previous study, NGC 5419 and NGC 6861, for separate analysis elsewhere. NGC 5419 was modeled using our new triaxial Schwarzschild dynamical modeling code SMART in Neureiter et al (2023b). NGC 6861 will be presented in J.…”

“…As stated above, these models provide worse fits to the kinematics than models with gradients and are here used merely to understand where the differences between the various IMF determinations could arise from. We also add recent Schwarzschild-based constant-ϒ measurements of the ETGs NGC 1600, Holm 15A, and NGC 5419 (Thomas et al 2016;Mehrgan et al 2019;Neureiter et al 2023b). The figure confirms that models with a spatially constant ϒ lead to higher α.…”

“…Modeling larger samples of galaxies with next-generation Schwarzschild models similar to the ones used here and direct comparisons with SSP models galaxy-by-galaxy will be important to constrain the IMF better. We plan to do this in a future paper, also combining gradient models with triaxial symmetry (Neureiter et al 2021;de Nicola et al 2022;Neureiter et al 2023b).…”

“…To prepare the simulation for Schwarzschild dynamical modeling, we set out to generate mock kinematic data in analogy to the data we used in this study (see Section 2.3). We adopt the simulated MUSE and SINFONI binning from Neureiter et al (2023b), assuming a distance of D = 56.2 Mpc (about the largest in our sample). The LOSVDs were generated over v los = ± 1500 km s −1 with N vel = 15 for both mock data sets, in analogy to the sample galaxies.…”

“…Figure9. Same as Figure8, but for comparison models without gradients (left, only for the four galaxies for which we generated constant-ϒ models), to which we add recent constant-ϒ Schwarzschild models ofThomas et al (2016),Mehrgan et al (2019), andNeureiter et al (2023b; here T+16, M+19, and N+23). For NGC 5328 from our new models (at σ ∼ 325 km s −1 ), the error bars are of similar size as the scatter point.…”