# BHI Colloquium

## Date:

Tuesday, September 17, 2019, 1:30pm to 2:30pm

## Location:

BHI Meeting Room

Dr. Vivienne Baldassare (Yale University)

Title: Searching for active galactic nuclei in low-mass galaxies with optical photometric variability

Abstract: The population of massive black holes (BHs) at the centers of nearby low-mass galaxies provides some of the best observational constraints on the masses of “black hole seeds” at high redshift. Furthermore, while BHs are ubiquitous and well-studied in Milky Way-sized and larger galaxies, relatively little is known about the population and properties of BHs in low-mass galaxies. However, BHs in these systems can be difficult to identify due to star formation dilution of the active galactic nucleus (AGN) signal and metallicity effects. In Baldassare et al. 2018, we showed that long-term optical variability in low-mass galaxies can identify AGNs missed by other selection techniques. I will present a new analysis of the nuclear variability of more than 50,000 nearby galaxies using data from the Palomar Transient Factory. The fraction of galaxies with AGN-like variability increases from 0.3% for galaxies with stellar masses less than 1e9 Msun to 2% for galaxies with stellar masses greater than 1e11 Msun. I will present our sample of low-mass, variability-selected AGNs and discuss implications for the active fraction and occupation fraction at low galaxy masses. I also study the variability properties of AGNs as a function of BH mass down to BH masses of 1e5 Msun, and find no dependence of variability properties on BH mass.

Prof. Jeremy Butterfeld (Cambridge University)

Title: Deriving time evolution in general relativity: a philosopher's viewpoint. (Joint work with Henrique Gomes, Cambridge)

Abstract: This talk will advertise three results that give, in a broadly
general-relativistic framework, sufficient conditions for time-evolution being generated by the ADM Hamiltonian. The philosophical background is
functionalism. Roughly speaking, it is the idea that one specifies an
entity or concept by its web of relations to other entities and concepts.
Recently philosophers have debated 'spacetime functionalism': "spacetime is as spacetime does". Thus their focus is on how the physics of matter and radiation contributes to determining, or perhaps even determines or explains, chrono-geometry: a Machian theme. The results are as follows:---
(i) The recovery of geometrodynamics, i.e. general relativity's usual ADM Hamiltonian, from requirements on deformations of hypersurfaces in a Lorentzian spacetime; due to Hojman, Kuchar and Teitelboim (1976: Annals of Physics); (ii) The deduction from judicious assumptions about matter and radiation in a 4-dimensional manifold that there is an (appropriately related) Lorentzian metric; due to Schuller, Duell, Giesel et al. (2012, 2018: Physical Review D); (iii) The deduction of a Lorentzian metric and of general relativity's usual ADM Hamiltonian, assuming (not a 4-manifold: but merely) that how a 3-geometry changes over time should be locally definable (Gomes and Shyam 2016: Journal of Mathematical Physics).