14.02.2022

Dr. Karina Voggel

Relic Nuclear Star Clusters and their hidden Super-Massive Black Holes.

I will summarize the current state of the field of surviving nuclear star clusters and what the future holds for these objects with the advent of large surveys such as Euclid. These former Nuclear Star Clusters (NSCs) end up in the halos of massive galaxies when they are stripped of their surrounding stars by tidal forces. Many of these former nuclei contain "hidden" SMBHs, a signpost of their past in the centers of a large galaxy. I will show how can can identify surviving nuclei and use them to trace galaxy and SMBH formation. I will in particular show NGC7727, a system that has two potential nuclear star clusters, one in the photometric center of the galaxy and one offset by only 500pc. Using high-resolution MUSE data, we detect a SMBH in each Nucleus, confirming that the offset nucleus is the relic of a galaxy that has merged with NGC7727. This is the first dynamically confirmed dual SMBH system at a separation of less than a kpc. The orbital parameters of the SMBHs show that it is in an advanced state of merging and it will constitute a ~1:25 mass ratio SMBH merger and produce a gravitational wave event. The discovery of this offset SMBH is another confirmation that many SMBHs exist outside the centers of galaxies that have not been discovered yet but are a crucial element for our understanding of black holes as well as galaxy assembly.

11.04.2022

Adam Ingram

Measuring black hole mass and the expansion rate of the Universe with X-ray reverberation mapping

Stellar-mass black holes accreting gas from a binary partner (X-ray binaries) and supermassive black holes accreting gas from their host galaxy (active galactic nuclei, AGN) can emit a huge X-ray flux from the vicinity of the black hole event horizon. This can be exploited to probe the strong field regime of General Relativity and measure the properties of the black hole: its mass and angular momentum. For all but two objects in the Universe, the vicinity of the accreting BH is far too small to directly image, necessitating the use of mapping techniques that exploit rapid X-ray variability. I will talk about X-ray reverberation mapping, which utilises the relativistically broadened iron emission line that results from centrally emitted X-rays reflecting from the disk. Modelling the light-crossing delay between reflected and directly observed X-rays returns a black hole mass measurement. I will summarise our efforts to measure the mass of stellar and supermassive black holes with our X-ray reverberation mapping code RELTRANS, including our first proof-of-principle constraint on Cygnus X-1. I will then describe how we can use RELTRANS for an even more ambitious goal: measuring the Hubble constant, H0. This is possible because the shape of the reflection spectrum depends on the intensity of illuminating flux, meaning that modelling with RELTRANS can effectively turn bright nearby AGN into standard candles. New, independent methods to measure H0 are currently highly desirable because modelling of the cosmic microwave background returns an H0 value in >4 sigma tension with the value derived from the traditional distance ladder. I will show that the statistical precision required to prefer one of these two discrepant values is achievable with a sample of ~25 AGN. I will discuss the improvements to our model that are required to achieve such a measurement in reality.