The two phases of core formation – orbital evolution in the centres of ellipticals with supermassive black hole binaries

Saglia

et al. 2023

ApJ

We present the first systematic study of the detailed shapes of the line-of-sight velocity distributions (LOSVDs) in nine massive early-type galaxies (ETGs) using the novel nonparametric modeling code WINGFIT. High-signal spectral observations with the Multi-Unit Spectroscopic Explorer (MUSE) at the Very Large Telescope allow us to measure between 40 and 400 individual LOSVDs in each galaxy at a signal-to-noise ratio level better than 100 per spectral bin and to trace the LOSVDs all the way out to the highest stellar velocities. We extensively discuss potential LOSVD distortions due to template mismatch and strategies to avoid them. Our analysis uncovers a plethora of complex, large-scale kinematic structures for the shapes of the LOSVDs. Most notably, in the centers of all ETGs in our sample, we detect faint, broad LOSVD “wings” extending the line-of-sight velocities, v los, well beyond 3σ to v los ∼ ± 1000–1500 km s−1 on both sides of the peak of the LOSVDs. These wings likely originate from point-spread function effects and contain velocity information about the very central unresolved regions of the galaxies. In several galaxies, we detect wings of similar shape also toward the outer parts of the MUSE field of view. We propose that these wings originate from faint halos of loosely bound stars around the ETGs, similar to the cluster-bound stellar envelopes found around many brightest cluster galaxies.

supporting

confidence: 72%

mentioning

confidence: 72%

Detailed Shapes of the Line-of-sight Velocity Distributions in Massive Early-type Galaxies from Nonparametric Spectral Models

Mehrgan

Saglia

et al. 2023

ApJ

“…Several observations suggest that the stellar populations of KDCs in some slowly rotating ETGs show little or no difference to the stellar populations of the surrounding host galaxies (Davies et al 2001;McDermid et al 2006;Nedelchev et al 2019;Kuntschner et al 2010). Recent high-resolution numerical simulations of gas-free mergers hosting SMBHs (Rantala et al 2019;Frigo et al 2021) provide another possible explanation for the origin of kinematically distinct velocity structures in such ETGs.…”

Section: Origin Of the Kinematically Distinct Corementioning

confidence: 99%

“…Such cores naturally form in gas-poor mergers with supermassive black holes (SMBHs) (e.g., Begelman et al 1980;Hills & Fullerton 1980;Ebisuzaki et al 1991;Milosavljević & Merritt 2001;Merritt 2006;Rantala et al 2018;Nasim et al 2021). By using high-resolution simulations of galaxy mergers with SMBHs Rantala et al (2018) and Frigo et al (2021) recently revealed that the core formation actually happens in two phases: First, dynamical friction causes the two SMBHs of the progenitor galaxies to sink to the center of the merger remnant. This causes the surrounding stars to move to larger radii, happens rapidly (some tens of millions of years), and is the main driver of the formation of the shallow central stellar density core.…”

Section: Introductionmentioning

confidence: 99%

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

Neureiter

Rantala

et al. 2023

ApJ

Self Cite

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.

“…This tangential bias is observed in several massive ellipticals (Thomas et al 2014(Thomas et al , 2016Mehrgan et al 2019). It is likely due to the core scouring mechanism, which leads to an ejection of stars with radial orbits in the central regions (Rantala et al 2018(Rantala et al , 2019Frigo et al 2021). We tested four different projections of the simulation at four different viewing angles (see Fig.…”

Section: Data and Codementioning

confidence: 99%

Accuracy and precision of triaxial orbit models – II. Viewing angles, shape, and orbital structure

Nicola

Neureiter

Monthly Notices of the Royal Astronomical Society

et al. 2022

Self Cite

We explore the potential of our novel triaxial modeling machinery in recovering the viewing angles, the shape and the orbit distribution of galaxies by using a high-resolution N-body merger simulation. Our modelling technique includes several recent advancements. (i) Our new triaxial deprojection algorithm SHAPE3D is able to significantly shrink the range of possible orientations of a triaxial galaxy and therefore to constrain its shape relying only on photometric information. It also allows to probe degeneracies, i.e. to recover different deprojections at the same assumed orientation. With this method we can constrain the intrinsic shape of the N-body simulation, i.e. the axis ratios p = b/a and q = c/a, with Δp and Δq ≲ 0.1 using only photometric information. The typical accuracy of the viewing angles reconstruction is 15-20○. (ii) Our new triaxial Schwarzschild code SMART exploits the full kinematic information contained in the entire non-parametric line-of-sight velocity distributions (LOSVDs) along with a 5D orbital sampling in phase space. (iii) We use a new generalised information criterion AICp to optimise the smoothing and to select the best-fit model, avoiding potential biases in purely χ2-based approaches. With our deprojected densities, we recover the correct orbital structure and anisotropy parameter β with Δβ≲ 0.1. These results are valid regardless of the tested orientation of the simulation and suggest that even despite the known intrinsic photometric and kinematic degeneracies the above described advanced methods make it possible to recover the shape and the orbital structure of triaxial bodies with unprecedented accuracy.