The supermassive black hole and double nucleus of the core elliptical NGC5419

Abstract

We obtained adaptive-optics assisted SINFONI observations of the central regions of the giant elliptical galaxy NGC5419 with a spatial resolution of 0.2 arcsec ($\approx 55$ pc). NGC5419 has a large depleted stellar core with a radius of 1.58 arcsec (430 pc). HST and SINFONI images show a point source located at the galaxy's photocentre, which is likely associated with the low-luminosity AGN previously detected in NGC5419. Both the HST and SINFONI images also show a second nucleus, off-centred by 0.25 arcsec ($\approx 70$ pc). Outside of the central double nucleus, we measure an almost constant velocity dispersion of $\sigma \sim 350$ km/s. In the region where the double nucleus is located, the dispersion rises steeply to a peak value of $\sim 420$ km/s. In addition to the SINFONI data, we also obtained stellar kinematics at larger radii from the South African Large Telescope. While NGC5419 shows low rotation ($v<50$ km/s), the central regions (inside $\sim 4r_b$) clearly rotate in the opposite direction to the galaxy's outer parts. We use orbit-based dynamical models to measure the black hole mass of NGC5419 from the kinematical data outside of the double nuclear structure. The models imply M${}_{\mathrm{BH}}=7.2_{-1.9}^{+2.7}\times 10^9$ M${}_{\odot }$. The enhanced velocity dispersion in the region of the double nucleus suggests that NGC5419 possibly hosts two supermassive black holes at its centre, separated by only $\approx 70$ pc. Yet our measured M${}_{\mathrm{BH}}$ is consistent with the black hole mass expected from the size of the galaxy's depleted stellar core. This suggests, that systematic uncertainties in M${}_{\mathrm{BH}}$ related to the secondary nucleus are small.

Accepted for publication in MNRAS

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