Triaxial Schwarzschild Models of Brightest Cluster Galaxies with Long-Slit LBT Data

Abstract

We present new long-slit stellar kinematics for a sample of 21 Brightest Cluster Galaxies (BCGs) and triaxial Schwarzschild models for 16 of these objects using our orbit modelling code SMART. The new kinematics obtained with the Large Binocular Telescope (LBT) is complemented with high-resolution photometry from HST or new AO-assisted ground-based observations also obtained at LBT and combined with wide-field imaging from the Wendelstein Observatory. These data enable robust modeling from the innermost regions - where the Supermassive Black Hole dominates the potential - to larger radii, where stars and dark matter (DM) are the primary mass contributors. As already discussed in a companion paper, we discovered 8 Ultramassive Black Holes (UMBHs, with mass $> 10^{10}$ M$_\odot$) in this BCG sample, more than doubling the number of galaxies with dynamically detected UMBHs. We show that the DM halos display a wide variety of geometries. Purely kinematical results include low central velocity dispersion with increasing profiles towards the outskirts, and the discovery of one Kinematically Decoupled Core.

Figures (12)

• Figure 1: Left: Histogram of the average ellipticity within and beyond 10", showing that our BCGs become flatter at large radii, although some objects remain round or flat at all radii. Right: Histogram of the twist, computed as max(PA) $-$ min(PA), showing that, in contrast to ordinary ETGs, BCGs often exhibit large twists.
• Figure 2: Surface Brightness profiles for the 21 BCGs of the sample (orange lines) with superimposed Core-Sersic best-fit profiles (blue lines). The red lines mark the postion of the break radius r$_\mathrm{b}$, which we use to estimate the core size. For A292 and A634 we show the cusp radius r$_\gamma$ as vertical green line, while A2107 can only be modeled using a single Sersic profile. Finally, the gray line denotes the radius where we stop the deprojection.
• Figure 3: Isophotes (from top to bottom: Surface Brightness, ellipticity, PA) of the BCG of A2256. The blue points represent the observed photometry, while the orange lines are computed by placing the SB on the grid and performing isophotal fits. The green lines are obtained from the projection of the deprojected Surface Brightness; the orientation is (56, 127, 37)$^\circ$. The black vertical line is placed at $\sigma_\mathrm{PSF} \equiv \mathrm{FWHM}_\mathrm{PSF} / 2.35$, whereas the red line is at r$_\mathrm{match}$: from this radius outwards we take WWFI photometry.
• Figure 4: Example of slit placement on the plane of the sky for A2107. The negative radii are always North for PA = 0$^\circ$. The PA of the individual slits correspond to [MJ, MN, MJ45, MN45] = [290,200,335,245]$^\circ$ (see also Tab. \ref{['Tab.kinematics']}).
• Figure 5: Velocity maps for notable cases in our sample. For A2506 and A1749 (top panels) we observe major-axis rotation. For A1314 (bottom left) we observe minor-axis rotation, which implies triaxiality or at most a prolate geometry. For A2107 (bottom right) we observe a possible KDC with significant rotation inside 2", while the rotation signal is much weaker at larger radii. In all plots, the green ellipse shows the typical orientation of the isophotes — all four galaxies have only weak twists.