Abstract: The first Journal Club of the quarter will be an All-Hands CASS Meeting this Friday, October 9, from 12:00 to 1:00 pm in SERF 383. This meeting will be an opportunity for new members of CASS to introduce themselves and present a quick overview of their research. It will also give established members of CASS a chance to summarize their recent work. New graduate students are especially encouraged to attend. Pizza and drinks will be provided.Journal Club is an informal environment in which graduate students can give short talks on any subject of interest to them, be it a recent paper, their own research, or any other topic. Talks will resume next week. All are welcome to attend.

Christopher's Abstract: Abstract: In recent years there have been a handful of stars observed to have large amounts of excess infrared flux, indicative of copious amounts of orbiting dust. It is believed that these large dust populations are the aftermath of planetary collisions, similar to the Moon-forming event in our own Solar System. One candidate star, BD +20 307, is older ($\sim$1 Gyr) than the expected timescale for such an event ($\lesssim$100 Myr), and may indicate that planetary collisions can occur over a longer timescale. With the ubiquity of low-mass stars in the Galaxy, and their penchant for building numerous, close-in terrestrial planets, I investigate the occurrence of giant impacts in the local Galaxy. Using initial results from SDSS and \textit{WISE}, I present 175 low-mass field stars exhibiting large infrared excesses. These results suggest the occurrence of large amounts of warm dust is more common around low-mass stars than higher mass stars ($\sim$0.1\% versus $\sim$0.001\%). This high frequency of giant impacts may also provide an explanation for the so called "\textit{Kepler} Dichotomy." With an increasing interest to search for Earth-like planets around low-mass stars, giant impacts have serious implications on the habitability of these planetary systems.

Prof. Hughes's Abstract: Abstract: Motivated by the problem of the scale of the motions driving the geodynamo, I shall revisit the classical problem of rapidly rotating Boussinesq convection. For the purely hydrodynamic case, convection at onset takes the form of narrow cells. However, on a long time scale there can be the emergence of large-scale, cyclonic vortices. Although similar findings have resulted from studies of decaying rotating turbulence, the problem has not been addressed in depth for convectively unstable layers. I shall then go on to look at the nature of the magnetic field that can be generated by such convective flows, highlighting the possibility of small- and large-scale dynamo action.

Jiacong's Abstract: Abstract: Plasma rotation is important in reducing turbulent transport, suppressing macroscopic instabilities, and is beneficial to confinement. Intrinsic rotation without an external momentum input is of interest for its plausible application on ITER. Spectral asymmetry is required for residual Reynolds stress that drives the intrinsic rotation. Conventional wisdom for symmetry breaking tied to magnetic field configuration does not apply to linear cylindrical devices with zero magnetic shear where intrinsic axial flows are suggested by experiments. A dynamical symmetry breaking mechanism is proposed. Rather than an intrinsic torque, the new mechanism introduces a negative viscosity increment. The modified total viscosity will then set the flow profile. Implications for tokamaks are also proposed.

Prof. Vilenkin's Abstract: Abstract: Spherical domain walls and vacuum bubbles can spontaneously nucleate and expand during the inflationary epoch in the early universe. After inflation ends, the walls and/or bubbles form black holes with a wide spectrum of masses. For some parameter values, the black holes can serve as dark matter or as seeds for supermassive black holes at galactic centers. This mechanism of black hole formation is very generic and has important implications for the global structure of the universe. Black holes with mass greater than certain critical value contain inflating universes inside. The resulting multiverse has a very nontrivial spacetime structure, with a multitude of eternally inflating regions connected by wormholes.

 Abstract: Research conducted in the Research Experience with Diversity (R.E.D.) Laboratory at Morehouse College is focused on three broad categories: Atomic, Molecular, & Optical (AMO) Physics – both computational and experimental; Materials Science Physics; and Nuclear Physics. AMO research projects are centered on studying the dynamics governing electronic processes associated with low kinetic energy photo-ionized charged particles. Research projects include an experimental component that is married with a computational calculation for verification of results. Projects in Materials Science Physics are focused on the growth and characterization of pure- and doped- metal oxide nanotube arrays, which have applications in solar cell technology, batteries, and sensors. Lastly, projects pursued in the area of Nuclear Physics are geared towards developing techniques & materials that will aid in nuclear nonproliferation and remote detection. In addition, projects also include the characterization of materials suitable for use in Light Water Reactors (LWR).

 Abstract: World-renowned astrophysicist Michel Mayor, Ph.D., will also speak at UCSD on March 16 at 3:30 p.m., as part of the annual Kyoto Prize Symposium. To attend that free talk, which is open to the public, please click here. Read UCSD News story here.

Angela's Abstract: Abstract: Galactic conformity is a correlation between the colors and star formation rates of massive central galaxies and their satellites. The effect was first observed in SDSS, and has since been found to extend beyond the scale of a single dark matter halo (so-called "2-halo" conformity). Explanations of the cause of especially 2-halo conformity were initially illusive, but theories have emerged within the past few years. I will discuss our measurement of galactic conformity from z=0.2 to 1.0 to a projected distance of 5 Mpc using spectroscopic redshifts from the PRism MUlti-object Survey (PRIMUS), as well as recent ideas about the origin of galactic conformity, and how the magnitude of the effect is predicted to scale with redshift and halo mass. We test these predictions with a sample of ~60,000 galaxies in five separate fields covering 5.5 square degrees, which allows us to account for the effects of cosmic variance. We detect a significant conformity signal (>3 sigma) of ~5% on scales of 0-1 Mpc and a 2.5-sigma signal of ~1% on scales of 1-3 Mpc.

Angela's Abstract: Abstract: The Zooniverse has been facilitating scientific research since 2009 by connecting scientists whose research requires large-scale human pattern detection with members of the public who are keen to contribute to real research. With a community of over 1.3 million and growing, Zooniverse projects such as Galaxy Zoo, Planet Hunters, Penguin Watch and Snapshot Serengeti are leading a new generation of online citizen science; collectively the projects have over 100 publications with over 2,500 citations. The newest feature of the Zooniverse is an online Project Builder that allows anyone to create a citizen research project without needing to know how (or have funding) to build a website or maintain a database. Paired with a scalable, open-source back-end software platform that allows those with some coding knowledge to create fully custom projects, the Project Builder is a tool designed both for researchers who need millions of clicks and for those who just need an easy, reliable interface for an internal team project.

The purpose of this informal talk is to give a brief overview of the Zooniverse followed by a project creation tutorial. If you already have a project in mind and a bit of sample data in hand, bring it along: you can create your own working classification project in under an hour. Really.

 Abstract: I shall present the results of a new analysis of the green valley galaxy population, which shows that - contrary to previous work - a diverse range of morphologically dependent star formation histories are required to create this population. Using classifications from Galaxy Zoo - an online citizen science project which asks the public to classify images of galaxies - I shall show that a range of quenching rates are possible for all morphological types. I will discuss the nature of the mechanisms that could cause such quenching rates, including secular evolution, galaxy interactions and mergers with and without AGN.

I will particularly explore the relationship between these quenched star formation histories and the presence of a type 2 AGN with evidence showing that a population of these AGN host galaxies have recently undergone a rapid drop in their star formation rate. This result provides strong observational support for AGN feedback in these systems, but this work also shows that feedback cannot be responsible for all of the quenching across the AGN host population. I shall discuss these results in the context of the co-evolution of galaxies and supermassive black holes.

 Abstract: The existence of bright QSOs at redshifts as high as 6-7 poses stringest constraints on the time available to grow the first supermassive black holes (SMBHs). Models based on rapid accretion from ~ 100 Mo black holes resulting from the collapse of an early generation of Pop III stars require very low radiative effiencies and sustained growth at Eddington rate for more than half a billion year, which seems unlikely. Alternative, much bigger seed BHs, weighing 10^5 Mo or more, could form from direct gas collapse in massive halos at z ~ 10-20. There could be more than one flavour of direct collapse, requiring, or not, an intermediate stage with a supermassive and/or quasi star. The mechanism of angular momentum loss triggered at galactic scales is also subject to debate. While the dominant scenario of direct collapse studied in the literature requires metal-free gas to avoid fragmentation into normal stars in gas-rich protogalactic disks, we present an alternative scenario in which a supermassive nuclear disky cloud of less than a pc in size can form in massive protogalaxies at z ~ 10 as a result of their first major merger with another similarly massive galaxy. Here gas is metal-enriched at solar values, consistent with the behaviour of a high-sigma peak associated with high-z QSOs. This scenario provides naturally an efficient way to lose angular momentum and produce a SMBH precursor in <~ 10^5 yr. The optically thick cloud produced by the accumulation of gas infalling at near free-fall velocity due to the merger dynamics naturally avoids fragmentation leading to a stage similar to first core formation in protostars. During infall fragmentation is also limited due to heating by high Mach number shocks. The disky cloud reaches formidable densities at our resolution limit of 0.1 pc. It is unstable to bar-like modes, which would continue to transport angular momentum and bring mass inwards, possibly reaching the critical limit for the general relativistic radial instability. If this happens , a SMBH would form in a dynamical time, with no stellar precuorsor. This "dark collapse" could produce a gravitational wave signal detectable with the future eLISA interferometer. Prospects to study this with general relativistic codes are also briefly discussed.

 Abstract: Hydrogen column density maps of molecular clouds are one of the most important observables in the context of molecular cloud- and star-formation studies. They reflect the structure of the ISM and constitute the gas reservoir out of which stars form. The Herschel photometric maps (70-500 micron) allow now to determine column density maps over a very large dynamic range.

Our group of observers and theorists specialized in the last years on the interpretation of probability distribution functions (N-PDFs) of column density. N-PDFs are used to evaluate the relative importance of gravity, turbulence, magnetic fields, geometry, and radiative feedback governing the cloud's density structure and star-formation activity. I will present a selection of our most important results:

1. For a proper interpretation of N-PDFs, line-of-sight contamination, completeness limits, and resolution effects need to be considered.

2. We found clear indications that the power-law tail commonly found in N-PDFs is caused by self-gravity of filaments and clumps, and free-fall collapse of cores.

3. Infrared dark clouds reveal a power-law tail and are thus dominated by gravity, and not turbulence, even at an early evolutionary state.

4. The most massive giant molecular clouds show an additional power-law tail with flatter slope at the highest column densities, indicating a slowed-down collapse.

5. N-PDFs combined with PDFs from HI observations allow to trace precisely the HI/H2 transition in the ISM.

6. The N-PDFs of high-density molecular line tracers such as N2H+ and CS show a power-law tail that corresponds to the one from dust. However, abundance variations and different regimes of excitation limit the functionality of molecular line PDFs.

 Abstract: The cosmic microwave background contains a wealth of information about cosmology as well as high energy physics. It tells us about the composition and geometry of the universe, the properties of neutrinos, dark matter, and even the conditions in our universe long before the cosmic microwave background was emitted. After a brief introduction, I will discuss various aspects of the recently released Planck full mission data before turning to a discussion of string inspired models and the search for their signatures. Finally, I will turn to the search for primordial B-modes.