Astronomy Colloquium

I describe what can be learned about galaxy formation theories from studying how active galactic nuclei (AGN) occupy galaxies. In simulations, AGN play the role of quenching and preventing further star formation in the most massive galaxies, preventing them from becoming even higher stellar mass than they are. The cosmological simulations require subgrid physics to describe both star formation processes as well as black hole growth and feedback, both of which are complicated, nonlinear processes occurring at AU scales, within simulations with at best about 10 to 100 pc resolution. Among the major cosmological simulation codes, there are many different implementations of this subgrid physics that all lead to predictions of the stellar mass function and star formation rates of galaxies that agree with observations, and this agreement is taken very seriously as validation of the basic picture. These different implementations nevertheless lead to quite different predictions for how AGN populate galaxies as a function of mass and star formation rate, predictions which, in contrast to those regarding stellar properties of galaxies, are almost never compared against observations. I describe a research program to perform these tests, which requires a major revision of the statistics of AGN demographics in local galaxies, measurements which in many cases have been performed previously without regard to the selection effects of AGN in galaxies, which have dramatic effects. Although there are many methodological difficulties in interpreting the AGN predictions from galaxy formation simulations, we are finding that these simulations basically never show agreement with observations in this respect.

Cosmic ray propagation describes how cosmic rays travel through space, exchanging energy and momentum with the ambient medium as they go. Understanding how cosmic rays propagate is fundamental to using them for astrophysical probes, and for elucidating their role in star formation and AGN feedback, and in galaxy evolution. Advances in observation, computation, and theory are creating opportunities to develop and test our understanding of cosmic ray propagation theory as never before, and revealing that the success of existing theory is mixed. I will describe some recent work on resetting and testing cosmic ray propagation theory, show why it matters, and indicate where new development may be needed.

TBD

TBD

TBD

TBD

TBD

In over 40 years at JPL and Caltech I have been fortunate to participate in two great scientific revolutions, the shift of IR astronomy into space and the discovery and characterization of exoplanets. The first of these owed much to Gerry's development of ground-based astronomy at Mt. Wilson and Palomar which set the stage for the dramatic leap to space with IRAS, followed by Spitzer, WISE and ultimately JWST. The second is the ongoing explosion in exoplanet science which happened mostly after Gerry retired, but as I pursue my own research, I continually hear him urging me to think about the key questions and to make sure I get the data right.

My scientific arc has been defined by increasing capabilities, from searching for massive protostars in giant molecular clouds with a 24" telescope on Mauna Kea, to identifying solar-type protostars with IRAS, to finding signposts of planet formation with IRAS and Spitzer, to detecting Jovian-mass brown dwarfs with 2MASS and WISE, and now searching for and characterizing planets from gas giants to super-Earths. Gerry shared a vision that interferometry would be important for finding other Earths using both astrometry and direct detection. But apart from getting a second Keck telescope built with NASA's partnership, that proved to be a road not taken—at least in the US. But many valuable capabilities came from those initial efforts which are enabling new instruments including single mode fiber spectrometers like Parvi and HISPEC and even the picometer control of optical structures for HWO.

I will describe some waypoints along this voyage of discovery and describe ways, direct and indirect, where I, along with the astronomical community, owe Gerry a great debt.