Ultraviolet spectra of extreme nearby star-forming regions - approaching a local reference sample for JWST
2020-07-27https://academic.oup.com/mnras/article/472/3/2608/4082838
- Speaker : Byeongha Moon
- Date : 2020-07-29 14:00 ~ 15:00
- Place : Lee Won Cheol Hall - Seminar Room 102
- Professor :
- Abstract:
- Nearby dwarf galaxies provide a unique laboratory in which to test stellar population models below Z⊙/2. Such tests are particularly important for interpreting the surprising high-ionization ultraviolet (UV) line emission detected at z > 6 in recent years. We present HST/COS UV spectra of 10 nearby metal-poor star-forming galaxies selected to show He II emission in SDSS optical spectra. The targets span nearly a dex in gas-phase oxygen abundance (7.8 < 12 + log O/H < 8.5) and present uniformly large specific star formation rates (sSFR ∼102 Gyr−1). The UV spectra confirm that metal-poor stellar populations can power extreme nebular emission in high-ionization UV lines, reaching CIII] equivalent widths comparable to those seen in systems at z ∼ 6–7. Our data reveal a marked transition in UV spectral properties with decreasing metallicity, with systems below 12 + log O/H 8.0 (Z/Z⊙ ≤ 1/5) presenting minimal stellar wind features and prominent nebular emission in HeII and CIV. This is consistent with nearly an order of magnitude increase in ionizing photon production beyond the He+-ionizing edge relative to H-ionizing flux as metallicity decreases below a fifth solar, well in excess of standard stellar population synthesis predictions. Our results suggest that often-neglected sources of energetic radiation such as stripped binary products and very massive O-stars produce a sharper change in the ionizing spectrum with decreasing metallicity than expected. Consequently, nebular emission in CIV and HeII powered by these stars may provide useful metallicity constraints in the reionization era.