The Vulture Survey of MgII and CIV Absorbers: Feasting on the Bones of Spectra Left to Die
Nigel Mathes, NMSU
We present detailed measurements of the absorption properties and redshift evolution of MgII and CIV absorbers as measured in archival spectra from the UVES spectrograph at the Very Large Telescope (VLT/UVES) and the HIRES spectrograph at the Keck Telescope (Keck/HIRES) to equivalent width detection limits below 0.01 angstroms. This survey examines 860 high resolution spectra from various archival data sets representing 700 unique sightlines, allowing for detections of intervening MgII absorbers spanning redshifts 0.1 < z < 2.6 and intervening CIV absorbers spanning redshifts 1 < z < 5. We employ an accurate, automated approach to line detection which consistently detects redshifted absorption doublets. We observe three distinct epochs of evolution in the circumgalactic medium (CGM) as traced by MgII and CIV absorbers. At high redshifts, from 3 < z < 5, galaxies rapidly build up a metal enriched halo where, despite significant evolution in the ionizing background, the production of metals through star formation driven outflows dominates observed trends increasing the number of observed absorbers per redshift path length towards z = 3. At mid redshifts, from 2 < z < 3, a large cosmic increase in the global star formation rate drives large numbers of high column density outflows into the halos of galaxies. At this time, metal line absorption of all species is increased above all other epochs. At low redshifts, for z < 2, the universe becomes more quiescent in both star formation and ionizing background. Weak, low column density MgII absorbers proliferate, while strong MgII absorbers likely fragment or re-accrete onto their host galaxy. Strong CIV absorbers, at this time, still increase in number per absorption path, while their weaker counterparts begin to disappear. MgII and CIV absorbers appear to originate in star formation driven outflows, but their different evolutionary properties imply they represent two physically distinct phases of gas. These two phases comprise the CGM and contribute separately to the cycle of baryons into and out of galaxies.
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.