Galaxy formation and evolution

Abstract:

The quest to discover the most distant galaxies has developed rapidly in the last decade. We are now exploring redshifts of 6 and beyond, when the Universe was less than a billion years old, an epoch when the previously neutral intergalactic medium was reionized. The continuing discovery of galaxies at progressively higher and higher redshifts has been driven by the availability of large telescopes on the ground and in space, improvements in detector technology, and new search strategies. Over the past 4 years, the Lyman break technique has been shown to be effective in isolating z ≈ 6 star-forming i′-drop galaxies through spectroscopic confirmation with large ground-based telescopes (Keck, Gemini and the ESO VLTs). Analysis of the Hubble Ultra Deep Field (HUDF-the deepest images obtained so far, and likely to remain so until the James Webb Space Telescope, JWST), has enabled us to explore the faint end of the luminosity function, which may contribute the bulk of the total star formation. The discovery of this i′-drop galaxy population has been used to infer the global star formation rate density at this epoch (z «6), and we are now beginning to constrain the contribution to reionization of the UV flux from these galaxies. Infrared data from the Spitzer Space Telescope has been used to determine the Spectral Energy Distributions (SEDs) from the rest-frame UV to the optical of some i′-drops, and constrain the previous star formation histories, masses and age of these sources. The indications are that much of the stellar mass of these galaxies might have formed in vigorous bursts at z > 6. The next big advances would be to test the population synthesis modelling of these z ∼ 6 galaxies through spectroscopy of the rest-frame optical (rather than crude broad-band SEDs), and also to push the observational horizon for galaxies further to directly explore star formation during the reionization epoch. JWST is likely to have a profound impact on realising these goals. © Società Italiana di Fisica.

Abstract:

The analysis of Lyman-break galaxies lying at z > 5 has greatly enhanced our understanding of star formation at this cosmologically important epoch. However the vast majority of candidate galaxies at these redshifts either lack spectroscopic confirmation or were selected for their extreme youth and strong Lyman-α emission. Spectroscopy is essential both to constrain the redshifts and properties of the candidate galaxies, and to eliminate lower-redshift interlopers. We discuss results from our recent spectroscopy of high-redshift, rest-UV selected sources using 8 and 10 m telescopes and the contribution of such observations to our overall understanding of the cosmological evolution of the galaxy population. © Società Italiana di Fisica.

Abstract:

We revisit the analysis of the nonlinear thin shell instability (NTSI) numerically, including magnetic fields. The magnetic tension force is expected to work against the main driver of the NTSI - namely, transverse momentum transport. However, depending on the field strength and orientation, the instability may grow. For fields aligned with the inflow, we find that the NTSI is suppressed only when the Alfvén speed surpasses the (supersonic) velocities generated along the collision interface. Even for fields perpendicular to the inflow, which are the most effective at preventing the NTSI from developing, internal structures form within the expanding slab interface, probably leading to fragmentation in the presence of self-gravity or thermal instabilities. High Reynolds numbers result in local turbulence within the perturbed slab, which in turn triggers reconnection and dissipation of the excess magnetic flux. We find that when the magnetic field is initially aligned with the flow, there exists a (weak) correlation between field strength and gas density. However, for transverse fields, this correlation essentially vanishes. In light of these results, our general conclusion is that instabilities are unlikely to be erased unless the magnetic energy in clouds is much larger than the turbulent energy. Finally, while our study is motivated by the scenario of molecular cloud formation in colliding flows, our results span a larger range of applicability, from supernova shells to colliding stellar winds. © 2007. The American Astronomical Society. All rights reserved.