›IAUS 321: Formation and evolution of galaxy outskirts
The evolution of galaxies within their so-called optical radius appears to have been dominated by well-known internal secular processes and star formation recipes at least in the last few Gyr. Their outer parts, however, are more susceptible to other processes such as accretion of satellite stars or halo gas or even (internal) stellar migration, all of which could competitively drive their photometric, chemical, and dynamical evolution. These processes determine how and how much gas and angular momentum are accreted, which are also key input parameters for modeling and understanding internal secular evolution, and allow addressing many fundamental open questions on the evolution of the Universe such as: How relevant is cold accretion as a function of halo mass? How does the (chemical) feedback to the galaxy haloes evolve with redshift and how does relate with the abundances measured in DLAs? How does star formation (triggering mechanisms, IMF) occur at low densities or low metallicities? Does stellar migration and satellite accretion significantly affect the dynamical conditions and effective star formation history of galaxies at any radii?
Major recent discoveries in the study of the outer parts of galaxies, many of which are only the tip of the iceberg of all the information that these regions contain to solve the aforementioned key questions, include (1) the discovery of extended UV (and Hα) emission in disk galaxies, (2) the potential role of stellar radial migration in rearranging material, (3) the presence of positive color profiles in outer disks, (4) recent evidence for cold accretion in dwarf galaxies, (5) the radial variation of the low-mass end of the IMF in elliptical galaxies. All these new results are still only partly understood and disconnected, but represent the beginning of an entire new field of exploration, a new frontier.
The study of the outskirts of galaxies has been traditionally limited either by the low surface brightness of these regions, which restricted their study to broad-band imaging, or to the small volume where single-star photometric and spectroscopic studies (which are not limited by surface brightness) could be carried out. New imaging surveys, ranging from small, dedicated telescopes such as the Dragonfly array to large facilities, in particular LSST, will lead to enormous progress. The availability of 30m-class telescopes in the next decade will allow extending single-star spectroscopic studies by a factor of x100 in volume and, combined with the use of new-generation Integral Field Units, also carrying out low-surface-brightness spectroscopy at several effective radii. Among other things, ELTs will allow using chemical-tagging techniques in the outer parts of the nearest galaxy disks and ellipticals. Besides, JWST will soon allow us to dramatically extend, both in redshift and effective radius, any present-day effort devoted to the study of the growth of galaxies using either imaging (e.g. with WFC3 at HST) or spectroscopy (e.g. SINFONI and KMOS at VLT).
Only 2 years before the planned launch date for JWST and 4 years before the first ELTs plan to see first light, this symposium will provide a perfect timing for (1) getting the clearest and widest view possible on the state-of-the-art of our understanding of outer-galaxy evolution and (2) designing an ambitious plan to best optimize the use of these coming facilities.
This symposium will address the term 'galaxy evolution' from the three points of view originally envisioned by Beatrice Tinsley: (spectro-)photometric, chemical and dynamical. Besides, by devoting a fraction of the symposium sessions to the most recent results coming from single-star imaging and spectroscopic studies, a major workhorse in these sparse, low-background regions, we will set the limits to what we can learn (and what we cannot) when analyzing more distant systems.
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