Galaxy formation and evolution

Abstract:

We present a summary of the physical and chemical properties we have obtained for the molecular gas found in galaxies, an essential triggering ingredient to any star formation (SF) process. In particular, we focus our studies on the molecular gas phases as traced by CO, HCO+ and CS line emissions, in a sample of galaxies including starbursts-SBs-, irregulars galaxies, low-metallicity sources, normal spirals, mergers, Early Type Galaxies (ETGs), galaxies with an Active Galactic Nucleus (AGN), with Cosmic Ray Dominated Regions (CRDRs) and with Photo-Dominated Regions (PDRs). Our studies mix theoretical developments (i.e. computational models) with the acquisition of large observational datatsets (at both low and high spatial resolution) that we then compare in order to better determine the conditions under which some SF activity is starting and/or is maintained. © 2013 International Astronomical Union.

Abstract:

In the standard picture of disc galaxy formation, baryons and dark matter receive the same tidal torques, and therefore approximately the same initial specific angular momentum. However, observations indicate that disc galaxies typically have only about half as much specific angular momentum as their dark matter haloes. We argue this does not necessarily imply that baryons lose this much specific angular momentum as they form galaxies. It may instead indicate that galaxies are most directly related to the inner regions of their host haloes, as may be expected in a scenario where baryons in the inner parts of haloes collapse first. A limiting case is examined under the idealized assumption of perfect angular momentum conservation. Namely, we determine the density contrast Δ, with respect to the critical density of the Universe, by which dark matter haloes need to be defined in order to have the same average specific angular momentum as the galaxies they host. Under the assumption that galaxies are related to haloes via their characteristic rotation velocities, the necessary Δ is ∼600. This Δ corresponds to an average halo radius and mass which are ∼60per cent and ∼75per cent, respectively, of the virial values (i.e. for Δ= 200). We refer to this radius as the radius of baryonic collapse RBC, since if specific angular momentum is conserved perfectly, baryons would come from within it. It is not likely a simple step function due to the complex gastrophysics involved; therefore, we regard it as an effective radius. In summary, the difference between the predicted initial and the observed final specific angular momentum of galaxies, which is conventionally attributed solely to angular momentum loss, can more naturally be explained by a preference for collapse of baryons within RBC, with possibly some later angular momentum transfer. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.