3:30–4:45 pm
MCP 201 933 E 56th Street
Dark matter halos provide the gravitational framework within which galaxies form and evolve, and thus underpin the observed large-scale structure of the Universe. Within the cold dark matter (CDM) paradigm, the growth and clustering of halos are well understood in broad terms, making halo clustering a natural starting point for understanding galaxy clustering. When I started my first postdoc at UChicago 20 years ago, halo clustering had long been known to depend strongly on halo mass. During my postdoc, I worked on a number of the early papers which pointed out that halos cluster differentially at fixed mass as a function of additional properties such as formation time, concentration, and accretion history. This phenomenon, commonly referred to as assembly bias, has important implications for models of the association between galaxy properties and the properties of the dark matter halos which host them: the "galaxy–halo connection." If galaxy occupation depends on halo properties beyond mass alone, then mass-only models cannot describe galaxy clustering at high precision, potentially introducing both scatter and systematic biases into inferred galaxy–halo relations and cosmological constraints. I will briefly review the theoretical origin of assembly bias and its impact on halo and galaxy clustering. I will then present two recent observational detections of halo assembly bias in SDSS and DESI data obtained by my collaborators and me, and argue that these signals provide direct, testable evidence for environment-dependent halo and galaxy evolution — a recent result which connects nicely to my earlier time in Chicago. Time permitting, I will then introduce a new aspect of assembly bias related to satellite systems. These findings may have important implications for observational tests of satellite anisotropy and for other large-scale structure observations such as intrinsic alignments in weak gravitational lensing.