Hosted by the Graduate Student Council.  All are welcome to come and mingle over coffee, tea, and cookies

Title:  Introduction to vector bundles and their classifications

Abstract:  We will introduce the definition of a vector bundle and look at a few examples. Next we will look at how to make new vector bundles from old bundles using familiar algebraic operations like direct sum, tensor product, and the pullback. Finally we will discuss classifying isomorphism classes of bundles over a topological space X, and time permitting, we will show these isomorphism classes are in bijection with homotopy classes of maps from X to Grassmanians on R infinity.

Supermassive black holes (SMBHs) are now known to be ubiquitous, with one present in the center of essentially every galaxy. The energy released by accretion onto an SMBH in the AGN phase is enough to not only outshine its host galaxy, but also to completely unbind the gas and rapidly quench star formation. But do SMBHs actually play an explosive role in galaxy evolution? In most cases, no! I will use a variety of observations - with careful consideration of selection effects - to demonstrate that AGN host galaxies are not particularly special. SMBH growth requires neither massive hosts nor violent angular momentum transport (i.e. mergers, disk instabilities, or large-scale bars). Instead AGNs are generally passengers on the road of galaxy evolution, fueled by the same gas reservoirs which drive star formation.

Small-molecule organic semiconductors form the basis for the emerging field of organic optoelectronics. In order to better understand the intrinsic photo-physical and transport phenomena in this important class of materials, it is necessary to study samples of very high structural order and chemical purity. Such materials exist in the form of molecular single crystals that can be used for fabrication of high-performance prototype devices, such as field-effect transistors, photo-conductors and photo-voltaic cells, in which intrinsic properties of organic semiconductors can be investigated without parasitic effects of disorder (see, e.g., [1,2,3]). This talk will overview some of the main achievements in the area of organic single-crystal devices, present resent progress and discuss novel methods of surface functionalization that result in an extremely low-noise charge transport regime at the surface of molecular crystals, leading to an observation of unprecedentedly clean and quiet (low-noise) Hall effect [4].  In addition, very interesting non-linear effects in photoconductivity originated from long-range exciton diffusion and multi-particle interactions will be discussed [5].     

References:

1. V. Podzorov, MRS Bulletin themed Issue: “Organic Single Crystals: Addressing fundamentals

   of organic electronics” introductory paper, MRS Bulletin 38, 15-24 (Jan. 2013).

2. V. Podzorov et al., "Hall effect in the accumulation layers on the surface of organic

    semiconductors", Phys. Rev. Lett. 95, 226601 (2005).

3. H. Najafov, B. Lee, Q. Zhou, L. C. Feldman, V. Podzorov, "Observation of long-range exciton

    diffusion in highly ordered organic semiconductors", Nature Mater. 9, 938 (2010).

4.  B. Lee, Y. Chen, D. Fu, H. T. Yi, K. Czelen, H. Najafov, V. Podzorov, “Trap healing and

     ultra low-noise Hall effect at the surface of organic semiconductors”, Nature Mater. 12, 1125

     (2013).

5. P. Irkhin, H. Najafov, V. Podzorov, submitted (2014).

  You may think of comets as gossamer, cloudy objects that grace our skies from time-to-time.  All that gas and dust has to come from somewhere. That somewhere is a dusty ball of ice, the comet nucleus. Only when that dirty snowball gets close to the Sun will it begin to grow a visible tail.  The European Space Agency is attempting a space exploration first, to land a probe on a comet while still far enough from the Sun that the snowball is largely quiescent. The landing is scheduled for 11AM EST on November 12. There will be a live feed from ESA, here: #CometLanding webcast.

The MacAdam Student Observatory staff are pleased to welcome the public to our facility. We present a program of public outreach on the second Thursday of every month.  A 40-minute presentation on astronomy will be held  in  the Chemistry-Physics Building, before moving across the street to the observatory, weather permitting. Note that the temperature at the telescope is the same as it is outside. The Observatory is located on Parking Structure #2 on the University of Kentucky campus on this map.)

Parking Note: Guests for the monthly SkyTalk that bring vehicles should plan on leaving them in Parking Structure #2, next to the observatory. Visitors that park elsewhere are subject to citation. Some streets near the observatory will be closed due to construction intermittently over the next few years. The recommended path to Parking Structure #2 is outlined in red, here: 2014-Sept Directions with street closures.pdf.

The imprint of primordial gravitational radiation on the cosmic microwave background polarization, if observed, is considered smoking gun proof of inflation. I will discuss how such an observation can not only provide information about the Universe in the epoch of inflation but also constrain theories of grand unification. In the second part of the talk I will discuss tests of gravity on scales ranging from the tabletop to the cosmological scale. Such tests may shed light on physics beyond the standard model.

Refreshments will be served in CP 179 at 3:15 PM

The average rate at which galaxies are forming stars in the Universe has decreased by more than an order of magnitude over the last 10 billion years. Understanding why certain galaxies shut off their star formation activity, while others do not is one of the key unanswered questions in astrophysics today. Observations in the local Universe suggest that the mechanism responsible for quenching star formation in galaxies may be intimately linked to both their mass assembly and their structural transformation from disks to spheroids. In order to test quenching scenarios, however, it is vital to look beyond the local Universe and identify the first generation of quiescent, "red and dead" galaxies at high redshift. I will discuss my work studying the rest-frame visible morphologies of the first massive systems to appear on the quiescent "red sequence" at redshifts z>1, when the universe was less than half its current age. Interestingly, a significant fraction (~30%) have morphologies dominated by massive exponential disks. The persistence of massive disks, long after star formation has ceased, implies that in at least some cases quenching precedes morphological transformation. I will examine what constraints these observations place on the mechanisms responsible for quenching the first generation of passive galaxies at z~2 and discuss them in context with an emerging picture of massive galaxy formation and evolution. Refreshments will be served in CP 179 at 3:15 PM

Title:  Crossing numbers for tropical curves

Abstract:  In tropical geometry, curves have both an intrinic side, as metric graphs and an embedded representation in terms of so-called balanced polyhedral complexes. I will discuss the relationship between these two representations. Since most curves can't be embedded in the plane, it is often useful to relax the embedding condition by allowing transverse crossings. A tropical crossing number for a metric graph is defined to be the fewest number of crossings in a planar immersion, and I will give some results on this crossing number.