Fresh Perspectives on Star
Formation from LEGUS, the Legacy ExtraGalactic Ultraviolet Survey
David Thilker, Johns Hopkins University
The Legacy ExtraGalactic Ultraviolet Survey (LEGUS) was a Cycle 21 Large Treasury HST program which obtained ~parsec resolution NUV- to I-band WFC3 imaging for 50 nearby, representative star-forming Local Volume galaxies, with a primary goal of linking the scales of star formation from the limit of individual stars, to clusters and associations, eventually up through the hierarchy to giant star forming complexes and galaxy-scale morphological features.
I will review the basics of the survey, public data products and science team results pertaining to clusters and the field star hierarchy. I will then describe work to optimize photometric selection methods for massive main sequence O star candidates and LBV candidates, in the former case establishing a means to statistically constrain the fraction of O stars in very isolated locales. I will introduce new ideas on how to quantify the complex spatio-temporal nature of hierarchical star formation using multi-scale clustering methods. The first steps of this work have yielded a landmark OB association database for 36 LEGUS target fields (in 28 of the nearest available galaxies), with tracer stellar populations selected and interpreted uniformly. I will finish with discussion of a pilot HST program to demonstrate remarkably increased survey efficiency of WFC3 UV imaging enabled by use of extra-wide (X) filter bandpasses. Such efficiency is required as we move beyond LEGUS and begin to rigorously explore low surface brightness star-forming environments where canonical results for the IMF and cluster formation efficiency are increasingly called into question.
Galaxy Evolution from Large Surveys of Nearby Resolved Galaxies
Karen Masters (Haverford College, Spokesperson for SDSS-IV)
Abstract: The morphology of a galaxy provides information on the orbits of stars within it. As such, important clues to the formation history of galaxies is revealed by their morphologies, and this information is complimentary, but not identical to, their star formation history and chemical composition as revealed by photometry and spectra. The Galaxy Zoo project (www.galaxyzoo.org) has provided quantitative visual morphologies for over a million galaxies (including the entire Sloan Digital Sky Surveys, or SDSS Main Galaxy Sample), and has been part of a reinvigoration of interest in the morphologies of galaxies and what they reveal about the evolution of galaxies. Mapping Nearby Galaxies at Apache Point Observatory (MaNGA, part of SDSS-IV), is well over halfway through its 6 year plan to obtain spatially resolved spectral maps for 10,000 nearby galaxies (all of which have Galaxy Zoo morphologies). MaNGA is now by far the largest sample of resolved spectroscopy in the world, with over 8000 galaxies observed. In this talk I will review these projects, and show results from them which demonstrate why a resolved view of the internal morphology in large samples of galaxies is interesting and how it provides a unique constraint of our understanding of galaxy evolution.