The Quenching of Star Formation in Massive Galaxies

Kate Whitaker (NASA Postdoctoral Program Fellow, GSFC)

Nearby galaxies exhibit a bimodal color distribution, where actively star-forming galaxies have blue colors and quiescent galaxies have red colors. It is generally thought that red galaxies arise from blue galaxies when star formation is quenched. However, the origin of this color bimodality remains unknown. Furthermore, it is not well understood how actively star-forming galaxies quench and migrate to form the well-defined color-mass relation, known as the "red sequence". In this talk, direct evidence is presented that the massive end of the red sequence is most-rapidly building up when the universe was only 3 billion years old, with an influx of young recently quenched galaxies that are almost non-existent over the past 8 billion years. Presenting recent results from the 3D-HST Survey, I will discuss the properties of these massive galaxies in the context of current galaxy formation and evolution theories.

The Needle in the Hundred-Square-Degree Haystack:
From Fermi GRBs to LIGO Discoveries

Leo Singer (Caltech)

Accurate localizations have driven and enriched our understanding of gamma-ray bursts (GRBs). They could do the same for future gravitational-wave (GW) detections with Advanced LIGO and Virgo, whose sky locations will be uncertain to hundreds of square degrees. I will explain how we will promptly detect GW sources and rapidly estimate their parameters and sky locations within minutes. Having analyzed a comprehensive population of simulated GW sources, we describe the sky localization accuracy that will be achieved in the first two years of Advanced LIGO. Next, in preparation for the optical search with the intermediate Palomar Transient Factory (iPTF), we are following up gamma-ray bursts (GRBs) detected by the Fermi Gamma-ray Burst Monitor. Its comparable error regions offer a close parallel to the Advanced LIGO problem, but Fermi's unique access to MeV--GeV photons and its nearly all-sky coverage may allow us to look at optical afterglows in a relatively unexplored part of GRB parameter space. We present the discovery of iPTF13bxl, the optical afterglow of GRB 130702A. Recovered from a targeted survey of 71 square degrees, it is the first optical afterglow found based solely on a gamma-ray localization. It is also remarkably nearby (z=0.145), and its supernova fills in a gap in the GRB--supernova connection. We also present iPTF13dsw, the optical afterglow of GRB 131011A, recovered from a similarly sized region but much fainter and further away (z=1.874). iPTF13bxl and iPTF13dsw point toward more afterglow discoveries with iPTF and Fermi. Furthermore, they set the stage for finding LIGO optical counterparts with the Zwicky Transient Facility.