The Bohdan Paczyński Memorial Colloquium


"On the formation of massive galaxies"

Jeremiah P. Ostriker (Dept. of Astrophysical Sciences, Princeton University )

Looking backwards we have been able to reconstruct from the detailed structure of our own Galaxy and from the fossil evidence derived from the study of nearby galaxies a plausible history of how galaxies formed over the last several billion years. In addition, now that we have a quite definite cosmological model, providing us with a quantitative picture of how perturbations grew from very low amplitude Gaussian fluctuations, we can perform the forward modeling of representative pieces of the universe using standard physical processes to see how well we match our local knowledge and the time-reversed modeling based on the fossil evidence. Finally, we can employ large ground and space based telescopes to use the universe as a time-machine – directly observing the past history of our light-cone. While none of these approaches can give us at the present time results accurate to more than roughly the 5% -> 10% level, a coherent and plausible picture is emerging. Massive galaxies form in two phases. In the first phase, which peaks at redshift z = 6 and ends by redshift z = 2, cold gas streams in, making stars in a small (<1kpc) region, but as the stellar mass approaches 1011 Msolar, a hot bubble forms which suppresses further inflow of cold gas. But from redshift z = 3 to the present time, small stellar satellite systems are accreted at typically 10kpc from the center and the size of the total system grows by about a factor of three as the mass doubles. Energy release from gravitational infall in various forms will greatly reduce star-formation even in the absence of feedback from SN or MBHs. And, output from massive black holes at the centers of these systems significantly helps to terminate or “quench” star formation. This physical picture seems naturally to lead to the mass, size, scale and epoch of galaxy formation and, increasingly, to a first understanding of the detailed internal structure of these systems.

"Accretion Disks, Jets, and Black Hole Spin"

Ramesh Narayan (Center for Astrophysics, Harvard University)

An astrophysical black hole is fully described with only two parameters: mass and spin. This simplicity makes the black hole a perfect laboratory for exploring both the physics of accretion disks and the mechanisms by which accreting black holes eject relativistic jets. Using a combination of numerical simulations and observational data, we have studied the properties of accretion disks near and inside the innermost stable circular orbit around a spinning black hole. These studies have enabled us to measure spin parameters of ten stellar-mass black holes. We find an intriguing correlation between the radio power of relativistic jets and black hole spin parameter. This is the first direct evidence that jets may be powered by black hole spin energy.

  Ramesh Narayan's home page

"Relativistic Transient Explosions"

Tsvi Piran (The Hebrew University, Jerusalem)

Among the most interesting fireworks observed on the sky are the brightest - gamma ray bursts, GRBs, the least known - neutron star mergers, and the recently observed puzzling tidal disruption events. I present new results on GRBs progenitors, demonstrating on one hand the existence of a new group of objects: low-luminosity GRBs and providing on the other hand the first direct observational evidence for the Collapsar mechanism. I examine the links between these conclusions and short GRBs that are expected to arise from neutron star mergers and I predict the existence of long lasting flares from merger events. These could help identify gravitational radiation emission from mergers events, increasing the effective sensitivity of gravitational radiation detectors by a large factors. I examine the puzzling Swift events: J1644 and J2058 and explain why they were observed in non-thermal X-ray and not in the expected thermal UV. I also demonstrate surprising (theoretical) links between these three unrelated objects.

  Tsvi Piran's home page.

"Hot Intergalactic Gas in Clusters of Galaxies, Cosmic Microwave Background Radiation and Cosmology"

Rashid Sunyaev (Max-Planck Institute for Astrophysics, Space Research Institute, Moscow, Institute for Advanced Study, Princeton)

Clusters of galaxies are the most massive objects in our Universe. Each of them contains dark matter, thousands of galaxies and is filled with hot intergalactic gas radiating in X-rays. Unusual method to detect clusters of galaxies is possible due to presence of extremely isotropic Cosmic Microwave Background Radiation (CMB) filling our Universe. Interaction of hot electrons with CMB photons changes the CMB spectrum in the directions toward clusters of galaxies. As a result clusters become 'negative sources' of radiation in cm and mm spectral bands. The brightness and spectrum of these sources does not depend on distance or redshift. This property opens the way to detect all the clusters of galaxies (more than 100 000 !!!) in the observable Universe. Planck Surveyor spacecraft, ground based South Pole Telescope and Atacama Cosmology Telescope discovered recently more than thousand of extremely massive clusters of galaxies at different redshifts looking for such 'negative sources'on microwave sky. This new discoveries opened new doors for observational cosmology. They are providing us with unique data on properties of our Universe as a whole, about its past and even future. It gives us clues about the physics working under the conditions and scales which we cannot test in the ground based laboratories.

  Rashid Sunyaev's home page.