List of Past CTC Theory Lunches : 01-Sep-2003 to 31-Dec-2003

Date:   Thursday 9-Oct-2003
Speaker:   Oleg Gnedin (STScI)
Title:  Formation of Globular Clusters in Hierarchical Cosmology

Numerical simulations of a Milky Way-sized galaxy demonstrate that globular clusters with properties similar to those observed can form naturally at z > 3 in the concordance LCDM cosmology. The clusters in our model form in the strongly baryon-dominated cores of supergiant molecular clouds. The first clusters form at z = 12, while the peak formation appears to be at z = 3-5. The zero-age mass function of globular clusters can be approximated by a power-law with the slope -2, in agreement with observations of young massive star clusters. The total mass of the cluster population is strongly correlated with the mass of its host galaxy, as well as with the local average star formation rate. The first clusters serve as important sources of ionizing radiation within their host galaxies and may lead to gamma-ray bursts and intermediate-mass black holes.

Date:   Thursday 16-Oct-2003
Speaker:   Mario Jiminez-Garate (MIT)
Title:  Accretion Disk Atmospheres

The window on high-resolution X-ray spectroscopy opened by Chandra and XMM-Newton is revealing the nature of the atmosphere and coronae of accretion disks. Observations of X-ray binaries show that dense photoionized plasmas blanket the disk. We compare accretion disk model atmospheres with observed X-ray spectra in order to derive ionization and density structure, opacity, spatial distribution, elemental composition, energetics, thermal stability and kinematics.

We calculate the spectrum for a disk atmosphere surrounding a supermassive Kerr black hole. We show the observable line emission signature in the soft X-rays from the flow near the horizon. This spectrum is subject to line transfer effects. Via a Monte Carlo, we find that the line ratios will be modified after repeated photoionizations and recombinations inside the atmosphere. Lines such as OVII (654eV) and C VI (367eV) are enhanced, while others such as Ne X (1022 eV) and O VIII RRC (871 eV) are suppressed. We predict an anti-correlation between the Fe-K fluorescence and the recombination line intensities, traceable to the atmospheric structure. Our results suggest that the O VIII and N VII lines identified with XMM-Newton in MCG-6-30-15 (Branduardi-Raymont et al., 2001) are overly bright, but may be physically tenable.

Date:   Thursday 30-Oct-2003
Speaker:   Chris O'Dea (STScI)
Title:  Hot News from Cool Cores

The central regions of clusters of galaxies exhibit a wealth of interesting phenomena. After decades of study these objects remain mysterious. We have obtained HST/STIS Far-UV images of Lyman-alpha and the nearby Far-UV continuum of the cooling core clusters A1795 and A2597. We have compared the FUV data with WFPC2 optical data and VLA high resolution radio imaging. We see a wealth of fine detail in the HST images which is not visible from the ground. The Lyman-alpha and FUV continuum exhibits a diffuse component aligned roughly with the radio jet axis, and bright knots which tend to lie along the edges of the radio source. There is a broad positive correlation between the FUV continuum and the Lyman-alpha flux.

The colors of the continuum knots are consistent with those of young stars with ages of about 10 million years. We suggest that the FUV knots are star clusters with ongoing star formation with star formation rates of several solar masses per year. It appears that star formation occurs through out the nebula, though it is strongly enhanced along the edges of the radio source. We suggest that hot young stars are the dominant source of ionization for the nebulae.

Although smaller and less luminous than the Lyman-alpha nebula in high-z radio galaxies, some key properties of the low-z cooling core nebula are similar. This suggests that similar processes occur in the low-z cluster cooling cores and the high-z radio galaxies.

Date:   Thursday 26-Feb-2004
Speaker:   Ed Colbert (C.U.A)

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