Over the last decade the field of galaxy formation has enjoyed enormous success observing and simulating the broad population of “normal” galaxies in the universe.  However, at the same time, multiple new and unexpected classes of extreme galaxies have been discovered and their origin remains a significant puzzle. For example, the physical origin of the dark matter rich (and perhaps dark matter free?) Ultra-Diffuse Galaxies (UDGs) is still a major debate with numerous possible explanations.  In contrast to these Ultra-Diffuse galaxies, there exists a population of Ultra-Compact Massive Galaxies and Ultra-Compact Dwarf Galaxies whose origins are just as difficult to explain. These galaxies may hold clues as to rare modes of high surface density star formation and its extreme self-regulation. Moreover, surveys probing to lower surface brightness have also demonstrated ubiquity of “Jellyfish” Galaxies with extreme star forming tails that appear to form in dense environments.  These Jellyfish Galaxies may be related to the population of Post-Starburst Galaxies (PSBs) that appear to be nearly an order-of-magnitude larger at high redshift than in the local universe.

Taken together, these extreme galaxies have origins that remain a mystery.  While seemingly very different and disjoint populations of galaxies, one thing that draws them together is that all of them are often preferentially associated with extreme environments such as rich clusters of galaxies and groups.  Indeed, it may be that environment is the key factor in the formation of many of these unusual galaxies.

This 5-day  meeting will focus on 5 key themes under the general topic of extreme objects.The goal is to bring together theoreticians and observational astronomers working at different wavelengths to summarize the state of the field and discuss the recent results on extreme galaxies with the intent of understanding whether some of these objects could simply represent different manifestations of the same physical processes occurring in extreme environments, or if they represent various stages in a larger process.  Likewise we will attempt to determine what new constraints on galaxy evolution processes can be uncovered by examining these populations. The “extreme” landscape and environment of Iceland should provide the ideal backdrop for lively discussion and debate!

The five populations that will be discussed in the meeting  are: Brightest Cluster Galaxies, Post-starburst galaxies, Jellyfish galaxies, Ultra Diffuse galaxies and Ultra Compact galaxies. Some of the topics that will be discussed are

– what are the properties and latest observational and simulation results on UDGs, Jellyfish, PSBs, BCGs and UCGs?

– Can the tails of jellyfish galaxies become dark matter poor (rich?) ultra-diffuse galaxies?

– Are post-starburst galaxies strictly found in high-density environments, and what is the relation between PSBs, UDGs, and Jellyfish?

– What is the evidence that groups and clusters favour the formation of all of these peculiar objects, and what physical processes are involved?

– Brightest Cluster Galaxies (BCGs) are also an extreme population of galaxies, how is their formation entangled with the formation of other extreme galaxies?

– Are the different state-of-the-art simulations able to reproduce all of these extreme populations in clusters?

– What do these galaxies tell us about star formation in the most extreme conditions, e.g. in compact nuclear starbursts or in tails ripped out of galaxies?

We envisage an engaging and lively meeting to discuss the status, challenges and prospects for these specific areas of galaxy evolution. The plan is to focus on connections between the different populations, so we encourage participants to submit abstracts that bridge across fields. The most appealing bridging talks will be granted longer talks. We also envisage intense discussion/brainstorming sessions to summarize where we are, what we do not understand, what is controversial and how can we make progress in the next few years.

This is meant to be a mid-sized conference with ~100-120  participants.