Hybridisation among butterflyfishes

[Extract] Hybridisation is defined as the interbreeding of individuals from two distinct populations (sensu stricto species), which are distinguishable on the basis of one or more heritable characters, following Harrison (1993). Hybrids have been recognised, based on intergrading of characteristic features from parent species, since at least the 1700's and have been documented among plants, corals, gastropods, crustaceans, insects, amphibians, reptiles, birds, mammals and fish (Barton and Hewitt, 1985; Schwenk, 1993; Bieme et al., 2003; van Oppen and Gates, 2006). At least 10% of animals and 25% of plant species are known to hybridise, although the true proportion is probably higher because hybridisation often goes unnoticed (Mallet, 2005, 2007). The fact that hybridisation is not a rare phenomenon but occurs in a considerable proportion of species, presents a significant challenge to the fundamental biological definition of a species (Barton and Hewitt, 1985; Mallet, 2005)

The Role of Stellar Feedback in Galaxy Evolution

Aiming at understanding the role of stellar feedback in galaxy evolution, I present a study of the hot interstellar medium in several representative galaxies, based primarily on X-ray observations as well as theoretical modelling. I find that, in the massive disk galaxies NGC2613 and M104, the observed amount of hot gas is much less than that predicted by current galaxy formation models. Such a discrepancy suggests a lack of appropriate treatments of stellar/AGN feedback in these models. I also find that stellar feedback, primarily in the form of mass loss from evolved stars and energy released from supernovae, and presumably consumed by the hot gas, is largely absent from the inner regions of M104, a galaxy of a substantial content of evolved stars but little current star formation. A natural understanding of this phenomenon is that the hot gas is in the form of a galactic-scale outflow, by which the bulk of the stellar feedback is transported to the outer regions and perhaps into the intergalactic space. A comparison between the observed sub-galactic gas structures and model predictions indicate that this outflow is probably subsonic rather than being a classical supersonic galactic wind. Such outflows are likely prevalent in most early-type galaxies of intermediate masses in the present-day universe and thus play a crucial role in the evolution of such galaxies. For the first time I identify the presence of diffuse hot gas in and around the bulge of the Andromeda Galaxy (M31), our well-known neighbor. Both the morphology and energetics of the hot gas suggest that it is also in the form of a large-scale outflow. Assisted with multiwavelength observations toward the circumnuclear regions of M31, I further reveal the relation between the hot gas and other cooler phases of the interstellar medium. I suggest that thermal evaporation, mostly likely energized by Type Ia supernovae, acts to continuously turn cold gas into hot, a process that naturally leads to the inactivity of the central supermassive blackhole as well as the launch of the hot gas outflow. Such a mechanism plays an important role in regulating the multi-phase interstellar medium in the circumnuclear environment and transporting stellar feedback to the outer galactic regions