December 2013

December 13Possible evidence for the disappearance of the AGN torus at low luminosities - Leonard Burtscher

Theoretical studies suggest that low-luminosity AGNs are not able to sustain an obscuring torus. This holds both for tori that originate in an outflowing (wind) structure and for ones that are part of the accretion flow. So far, however, there is little evidence for a dependance of torus properties on AGN luminosity. Instead it seems that relations such as the mid-IR X-Ray correlation are unchanged down to about 10^41 erg/s. We compiled a large (> 50 sources) sample of IFU data for AGNs that span a wide range in luminosities around the expected threshold. The sources are nearby so that we are sensitive to the dilution of the stellar features on small spatial scales. With this sample, we study the dilution of the stellar light, traced by the near-infrared CO absorption features, by the non-stellar continuum as a function of radius and find a change in the properties of the diluting continuum that happens very close to the expected threshold luminosity. We will discuss whether this supports predictions that the torus disappears at low luminosities.

December 6Dynamics and structures of the ISM around AGNs - Keiichi Wada

I am going to talk about recent progress in our on-going radiation-hydrodynamic simulations of the ISM on pc-10 pc scales around AGNs. We found that non-steady, non-uniform outflows are often formed, and as a result the nucleus is covered over wide solid angles.

November 2013

November 29Not another narrow-angle astrometric interferometer? - Yitping Kok

Ever since the successful high-precision narrow-angle astrometric prototype observations of binary stars with optical long baseline interferometry (OLBI) more than two decades ago, the OLBI community has been putting significant effort to replicate the success by building similar astrometric instruments at stellar interferometers for similar and different scientific goals. The ASTRA facility at the Keck Interferometer, the PRIMA facility at the VLTI and the GRAVITY instrument we are currently assembling in the next building are examples of such effort. This talk will describe similar effort at the Sydney University Stellar Interferometer (SUSI). The main scientific goal of the recently installed astrometric instrument at SUSI is to carry out Jupiter-mass exoplanet search around bright binary stars. This talk will describe the interferometric methodology adopted and the observational strategy carried out at SUSI. Initial results and challenges going forward will also be discussed.

November 20Accretion and stellar mass growth in low mass galaxies - Katharina Lutz - Master thesis defense

We combine deep long-slit optical spectra, atomic and molecular gas measurements, as well as public SDSS and GALEX imaging for a sample of 30 galaxies in the mass range 9.0 < log M⋆/M⊙ < 10.0. These galaxies are the first observed as part of a new survey whose goal is to study the formation histories of low mass galaxies, and the role of gas accretion and star formation efficiency in regulating their growth. This new survey, COLD GASS 2, builds upon the results of the GASS and COLD GASS surveys which targeted massive galaxies (log M⋆/M⊙ > 10.0). In this thesis, we use the long-slit optical spectra for 27 low mass galaxies to study the radial variation of key quantities such as metallicity, star formation surface density and stellar age, and correlate these variations with global galaxy properties. The results are combined with the COLD GASS sample to probe the full stellar mass range between 10^9 and 10^11.5 M⊙ . Our analysis reveals a mass threshold of log M⋆ ≈ 10.5, above and below which the stellar mass growth of galaxies appears to proceed differently. While high mass galaxies show clear signs of growing inside-out, the low mass galaxies appear to be forming stars throughout their disk, despite displaying significant radial metallicity variations. A simple closed-box chemical model is unable to reproduce all our observations, indicating that significant exchange of material with the intergalactic medium must be taking place, likely through both inflows and outflows.

November 8The ALMA view of one of the nearest starburst galaxies - Alberto D. Bolatto

Department of Astronomy and Joint Space Institute, University of Maryland, Visiting Humboldt Fellow at MPIA-Heidelberg

In the context of galaxy evolution, it is particularly interesting to understand better the mechanisms that regulate starburst activity in galaxies. In this talk I will present an analysis of the molecular ISM properties in the prototypical circumnuclear starburst galaxy, NGC 253, derived from ALMA observations. I will discuss the evidence for a molecular superwind, our measurements of the mass loss rate, and the possible gas entraining mechanisms. I will show our measurement of the properties of giant molecular clouds in the starburst, in an effort to better understand the conditions in the starburst. Finally, I will present and discuss some of the spectroscopic complexity we see in the data. This extremely rich spectroscopy, a common feature in many ALMA datasets, opens new windows for the study of physical conditions in extragalactic systems.

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