MAY 20 - 24, 2013

WEDNESDAY, MAY 22

11:00 am: Optical and Infrared Astronomy Division Seminar. "VHE Blazars: Observations and Modeling," Dr. Matteo Cerruti, CfA. Pratt Conference Room.

Abstract: The current generation of atmospheric Cherenkov telescopes has significantly improved our knowledge of active galactic nuclei emitting very-high-energy (VHE; E > 100 GeV) gamma-ray photons. The VHE extra-galactic sky is dominated, in particular, by the class of blazars, even though some radio-galaxies and starburst galaxies have been detected as well. I will present a review of VHE blazar observations, in particular from the VERITAS telescope array. In the second part of the talk I will focus on theoretical modeling of blazar emission. The models developed to explain the blazar spectral energy distributions (SEDs) can be divided in two big families, leptonic and hadronic models. While in the first scenario the emission is dominated by electrons and positrons, in the latter the gamma-ray flux is associated with protons in the emitting region. I will discuss both families of models, with applications to some interesting sources.

Noon: High Energy Astrophysics Division Lunch Talk. "Probing the Nature of kpc-scale Relativistic Jets with Hubble and Fermi," Eileen Meyer, Space Telescope Science Institute. Pratt Conference Room.

Abstract: I will first present recent work utilizing Hubble archival imaging data of the famous optical jet in the nearby radio galaxy M87. Using state-of-the-art astrometry techniques and over 12 years of archival data, we reach unprecedented accuracies in measuring the velocity field of the optical jet to less than 0.1c accuracy. We find large-scale deceleration over the length of the jet, as well as an surprising array of behavior in individual knots, visible to the naked eye. We find clear evidence of apparently relativistic material moving through standing shocks, as well as non-radial motions and decelerations. In the second half of the talk I will present a recent study on another famous large-scale jet, 3C 273, conducted with the Fermi gamma-ray telescope. With 4 years of data from Fermi, we put new limits on the gamma-ray emission from inverse-Compton upscattering of CMB photons by the relativistic plasma in the large-scale jet. This limit is well below that expected from a continuation of the UV-Xray spectrum, all but ruling out IC/CMB as the source of the X-rays in this source, a long-standing source of debate. This result has strong implications for our understanding of the energetics of jets and the scale of their impact on their environments.

THURSDAY, MAY 23

4:00 pm: Colloquium. Sackler Lecture: "Observing the Evolution of Galaxies," Prof. Marijn Franx, Leiden Observatory. Preceded by tea at 3:30 pm. Phillips Auditorium.

Abstract: Modern telescopes allow us to observe galaxies out to high redshift. These observations, while still far from complete, allow us to give first answers to questions like: at what redshift was a typical galaxy assembled? At what redshift were half the stars formed? What processes played a role in galaxy formation? Is the observed galaxy formation and evolution consistent with our standard cosmological model?

FRIDAY, MAY 24

12:30 pm: Radio and Geoastronomy Division Lunch Talk. "The Case for Global Gravitational Contraction in Star-Forming Molecular Clouds," Enrique Vazquez, CRYA. Room M-340, 160 Concord Avenue.

Abstract: I discuss the question of the relative contribution of turbulence and gravitational contraction to the nonthermal motions observed in molecular gas, from giant molecular clouds (GMCs) to dense cores. I first discuss both observational and numerical evidence suggesting that the motions constitute gravitational contraction. From energetics alone, observations of apparent virialization may be just as well interpreted as infall. This includes the recent generalization of Larson's relations by Heyer et al. In numerical simulations of cloud formation and evolution, cold clouds are born transonically turbulent, and develop highly supersonic motions as the collapse progresses. The role of the initial turbulence is to provide nonlinear density fluctuations that are later "captured" by gravity during the global collapse. The global contraction starts in the cold atomic gas. Star formation as well as physical conditions leading to molecule formation do not start until several Myr after global gravitational contraction has started, although long before it ends. Thus, the feedback from the newly formed stars must act against the ongoing contraction. The observed effect is that the feedback does reduce the star formation efficiency (SFE), but does so by eroding the infalling gas, rather than by maintaining the clouds in approximate dynamical equilibrium. The topology of the velocity field in this regime is highly chaotic, rather than monolithic, and gravitational contraction occurs at all scales. An evolutionary semi-analytical model based on this phenomenology correctly reproduces the locus of clouds in the Kenniccutt-Schmidt diagram, the evolutionary stages of GMCs, and the observed stellar age distribution in individual clouds, while predicting that present-day low-mass star forming regions may evolve into high-mass regions within a few Myr.

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