We present new, more precise measurements of the mass and distance of ourGalaxy's central supermassive black hole, Sgr A*. These results stem from a newanalysis that more than doubles the time baseline for astrometry of faint starsorbiting Sgr A*, combining two decades of speckle imaging and adaptive opticsdata. Specifically, we improve our analysis of the speckle images by usinginformation about a star's orbit from the deep adaptive optics data (2005 -2013) to inform the search for the star in the speckle years (1995 - 2005).When this new analysis technique is combined with the first completere-reduction of Keck Galactic Center speckle images using speckle holography,we are able to track the short-period star S0-38 (K-band magnitude = 17,orbital period = 19 years) through the speckle years. We use the kinematicmeasurements from speckle holography and adaptive optics to estimate the orbitsof S0-38 and S0-2 and thereby improve our constraints of the mass ($M_{bh}$)and distance ($R_o$) of Sgr A*: $M_{bh} =4.02\pm0.16\pm0.04\times10^6~M_{\odot}$ and $7.86\pm0.14\pm0.04$ kpc. Theuncertainties in $M_{bh}$ and $R_o$ as determined by the combined orbital fitof S0-2 and S0-38 are improved by a factor of 2 and 2.5, respectively, comparedto an orbital fit of S0-2 alone and a factor of $\sim$2.5 compared to previousresults from stellar orbits. This analysis also limits the extended dark masswithin 0.01 pc to less than $0.13\times10^{6}~M_{\odot}$ at 99.7% confidence, afactor of 3 lower compared to prior work.

We obtained adaptive-optics assisted SINFONI observations of the centralregions of the giant elliptical galaxy NGC5419 with a spatial resolution of 0.2arcsec ($\approx 55$ pc). NGC5419 has a large depleted stellar core with aradius of 1.58 arcsec (430 pc). HST and SINFONI images show a point sourcelocated at the galaxy's photocentre, which is likely associated with thelow-luminosity AGN previously detected in NGC5419. Both the HST and SINFONIimages also show a second nucleus, off-centred by 0.25 arcsec ($\approx 70$pc). Outside of the central double nucleus, we measure an almost constantvelocity dispersion of $\sigma \sim 350$ km/s. In the region where the doublenucleus 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 atlarger radii from the South African Large Telescope. While NGC5419 shows lowrotation ($v < 50$ km/s), the central regions (inside $\sim 4 \, r_b$) clearlyrotate in the opposite direction to the galaxy's outer parts. We useorbit-based dynamical models to measure the black hole mass of NGC5419 from thekinematical data outside of the double nuclear structure. The models implyM$_{\rm BH}=7.2^{+2.7}_{-1.9} \times 10^9$ M$_{\odot}$. The enhanced velocitydispersion in the region of the double nucleus suggests that NGC5419 possiblyhosts two supermassive black holes at its centre, separated by only $\approx70$ pc. Yet our measured M$_{\rm BH}$ is consistent with the black hole massexpected from the size of the galaxy's depleted stellar core. This suggests,that systematic uncertainties in M$_{\rm BH}$ related to the secondary nucleusare small.

Stellar scattering off irregularities in a galaxy disk has been shown to makean exponential radial profile, but no fundamental reason for this has beensuggested. Here we show that exponentials are mathematically expected fromrandom scattering in a disk when there is a slight inward bias in thescattering probability. Such a bias was present in our previous scatteringexperiments that formed exponential profiles. Double exponentials can arisewhen the bias varies with radius. This is a fundamental property of scatteringand may explain why piece-wise exponential profiles are ubiquitous in galaxies,even after minor mergers and other disruptive events.