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Register a new userThe supermassive black hole and double nucleus of the core elliptical NGC5419
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X. Mazzalay
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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 galaxys 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 $sim 4 , r_b$) clearly rotate in the opposite direction to the galaxys 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$_{rm BH}=7.2^{+2.7}_{-1.9} times 10^9$ 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$_{rm BH}$ is consistent with the black hole mass expected from the size of the galaxys depleted stellar core. This suggests, that systematic uncertainties in M$_{rm BH}$ related to the secondary nucleus are small.
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The presence of black holes (BHs) at the centers of dwarf elliptical galaxies (dEs) has been argued both theoretically and observationally. Using archival HST/WFPC2 data, we found the Virgo cluster dwarf elliptical galaxy VCC128 to harbor a binary nucleus, a feature that is usually interpreted as the observable signature of a stellar disk orbiting a central massive black hole. Debattista et al. 2006 estimated its mass M sim 6 10^6 - 5 10^7 Msun. One of the most robust means of verifying the existence of a BH is radio continuum and/or X-ray emission, however because of the deficiency of gas in dEs, radio continuum emission forms the best option here. We have tried to detect the X-band radio emission coming from the putative black hole in VCC128 when it accretes gas from the surrounding ISM. While we made a positive 4 sigma detection of a point source 4.63 south-west of the binary nucleus, no statistically significant evidence for emission associated with the nuclei themselves was detected. This implies either that VCC128 has no massive central black hole, which makes the nature of the binary nucleus hard to explain, or, if it has a central black hole, that the physical conditions of the ISM (predominantly its density and temperature) and/or of the surrounding accretion disk do not allow for efficient gas accretion onto the black hole, making the quiescent black hole very hard to detect at radio wavelengths.
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