Department of Astronomy and Astrophysics

In 1964, Refsdal hypothesized that a supernova whose light traversed multiple paths around a strong gravitational lens could be used to measure the rate of cosmic expansion. We report the discovery of such a system. In Hubble Space Telescope imaging, we have found four images of a single supernova forming an Einstein cross configuration around a redshift z = 0.54 elliptical galaxy in the MACS J1149.6+2223 cluster. The cluster's gravitational potential also creates multiple images of the z = 1.49 spiral supernova host galaxy, and a future appearance of the supernova elsewhere in the cluster field is expected. The magnifications and staggered arrivals of the supernova images probe the cosmic expansion rate, as well as the distribution of matter in the galaxy and cluster lenses.

We present a statistical analysis of the first 300 stars observed by the Gemini Planet Imager Exoplanet Survey. This subsample includes six detected planets and three brown dwarfs; from these detections and our contrast curves we infer the underlying distributions of substellar companions with respect to their mass, semimajor axis, and host stellar mass. We uncover a strong correlation between planet occurrence rate and host star mass, with stars M ∗ >1.5 M o more likely to host planets with masses between 2 and 13M Jup and semimajor axes of 3-100 au at 99.92% confidence. We fit a double power-law model in planet mass (m) and semimajor axis (a) for planet populations around high-mass stars (M ∗ >1.5 M o) of the form , finding α = -2.4 +0.8 and β = -2.0 +0.5, and an integrated occurrence rate of % between 5-13M Jup and 10-100 au. A significantly lower occurrence rate is obtained for brown dwarfs around all stars, with % of stars hosting a brown dwarf companion between 13-80M Jup and 10-100 au. Brown dwarfs also appear to be distributed differently in mass and semimajor axis compared to giant planets; whereas giant planets follow a bottom-heavy mass distribution and favor smaller semimajor axes, brown dwarfs exhibit just the opposite behaviors. Comparing to studies of short-period giant planets from the radial velocity method, our results are consistent with a peak in occurrence of giant planets between ∼1 and 10 au. We discuss how these trends, including the preference of giant planets for high-mass host stars, point to formation of giant planets by core/pebble accretion, and formation of brown dwarfs by gravitational instability.

We measure the evolution of the quiescent fraction and quenching efficiency of satellites around star-forming and quiescent central galaxies with stellar mass at . We combine imaging from three deep near-infrared-selected surveys (ZFOURGE/CANDELS, Ultra Deep Survey, and UltraVISTA), which allows us to select a stellar-mass complete sample of satellites with . Satellites for both star-forming and quiescent central galaxies ("centrals") have higher quiescent fractions compared to field galaxies matched in stellar mass at all redshifts. We also observe "galactic conformity": satellites around quiescent centrals are more likely to be quenched compared to the satellites around star-forming centrals. In our sample, this conformity signal is significant at for , whereas it is only weakly significant at and . Therefore, conformity (and thus satellite quenching) has been present for a significant fraction of the age of the universe. The satellite quenching efficiency increases with increasing stellar mass of the central, but does not appear to depend on the stellar mass of the satellite to the mass limit of our sample. When we compare the satellite quenching efficiency of star-forming centrals with stellar masses 0.2 dex higher than quiescent centrals (which should account for any difference in halo mass), the conformity signal decreases, but remains statistically significant at . This is evidence that satellite quenching is connected to the star formation properties of the central galaxy as well as to the mass of the halo. We discuss physical effects that may contribute to galactic conformity, and emphasize that they must allow for continued star formation in the central galaxy even as the satellites are quenched.

A study of the weather records from six stations in the Sierra Nevada at and near Lake Tahoe, plus one station in the Diablo range of central California, reveals the following:

1. Since 1915, the date of the first major autumnal snowstorm has become progressively later at Tahoe City by 0.24 days/year (d/yr).

2. The change in the snowfall date results from an increase in the air temperature (T) at Tahoe City between 1909 and 2004, during the months of Septemberthrough December, of T′max = +0.011 °C/yr, T′min = +0.020 °C/yr, T′mean = +0.016 °C/yr. These values agree substantially with those published by Coats et al. (2006).

3. On average, the other five stations show a much smaller increase, amounting toT′max = +0.001 °C/yr, T′min = +0.012 °C/yr, T′mean = +0.007 °C/yr. Thismean value agrees well with the increase of T′mean = +0.007 – +0.008 °C/yr for the North American continent due to global warming (Jones and Moberg, 2003; Karoly et al., 2003).

4. The rate of temperature increase at Tahoe City is thus greater than would be expected from global warming and reflects a change in the microclimate of the Tahoe Basin, possibly due to human activity.

5. No overall change in precipitation with time is observed at any of the seven stations. However, statistically significant increases in precipitation with time occur, in the month of November, at the three Sierran stations located west of the Sierra crest, as well as at Tahoe City east of the crest. These increases coincide with the onset of the first substantial precipitation at these stations in the fall, and could be the result of “cloud seeding” by atmospheric pollutants, the precipitation becoming greater with time as the amount of pollution increases, and diminishing in December after these pollutants are washed out of the atmosphere by the first rains or snows.

Coats, R., J. Perez-Losada, G. Schladow, R. Richards, and C. Goldman: 2006, ‘The Warming of Lake Tahoe’. Climatic Change 76, 121–148.

Jones, P. D. and A. Moberg: 2003, ‘Hemispheric and Large-Scale Surface Air Temperature Variations: an Extensive Revision and an Update to 2001’. Journal of Climate 16, 206–223.

Karoly, D. J., K. Braganza, P. A. Stott, J. M. Arblaster, G. A. Meehl, A. J. Broccoli, and K. W. Dixon: 2003, ‘Detection of a Human Influence on North American Climate’. Science 302, 1200–1203.