LERMA UMR8112

While the effective radius is a robust parameter, its use to
characterise galaxy sizes has provided a counter-intuitive definition of
what the actual extent of a galaxy is. Current deep imaging therefore
offers a unique opportunity to critically review the convention that the
size of a galaxy is its effective radius and rethink how one best
measures the extent of galaxies using a physically motivated parameter.
We introduce a new definition of galaxy size based on the gas density
threshold for star formation in galaxies. Remarkably, our new size
definition not only captures what the human visual system identifies as
the edge of a galaxy, but also dramatically decreases the scatter in the
stellar mass - size plane by a factor of three. Our size parameter
unifies galaxies spanning five orders of magnitude in stellar mass on a
single mass-size relationship. To demonstrate the implications of our
results, we show that ultra-diffuse galaxies have the same sizes as
regular dwarfs when a size indicator that describes the global structure
of galaxies is used. This work may be extended for larger samples of
galaxies using upcoming wide, deep imaging surveys.

The supermassive black holes that lurk in galaxy centers should form binaries as galaxies merge over cosmic time, emanating gravitational waves in the nanohertz sensitivity band of networks of Milky Way millisecond pulsars. Pulsar-timing arrays (PTAs) are poised to chart this new frontier of gravitational wave discovery within the next several years. With this new window onto the warped Universe, PTAs will bring a dramatic change in our understanding of supermassive binary black-hole demographics and dynamical interactions. The International Pulsar Timing Array (IPTA) combines precision timing data from regional collaborations in Europe, North America, Australia (and new partners in South Africa, India, and China) to accelerate these goals. I will review ongoing work by my IPTA colleagues to place stringent gravitational-wave limits and increase near-future detection prospects using the recently published "IPTA Data Release 2".

Stephen Taylor a fait ses études à Cambridge et est aujourd'hui membre du consortium nord-américain NanoGrav et de son équivalent mondial IPTA, pour la recherche d'ondes gravitationnelles avec un réseau de pulsars stables (PTA -- Pulsar Timing Array). Il est actuellement en poste dans le Department of Physics & Astronomy de l'Université de Vanderbilt à Nashville, USA. Il collabore depuis de nombreuses années avec les équipes d'Orléans/Nançay et de l'APC, sur les méthodes d'analyse pour les programmes PTA.