Searching for z > 6.5 Analogs Near the Peak of Cosmic Star Formation
Xinnan 楠 Du 杜辛, Alice E. Shapley, Mengtao Tang, Daniel P. Stark, Crystal L. Martin, Bahram Mobasher, Michael W. Topping, Jacopo Chevallard
Abstract
Abstract Strong [O iii ] λλ 4959,5007 + H β emission appears to be typical in star-forming galaxies at z > 6.5. As likely contributors to cosmic reionization, these galaxies and the physical conditions within them are of great interest. At z > 6.5, where Ly α is greatly attenuated by the intergalactic medium, rest-UV metal emission lines provide an alternative measure of redshift and constraints on the physical properties of star-forming regions and massive stars. We present the first statistical sample of rest-UV line measurements in z ∼ 2 galaxies selected as analogs of those in the reionization era based on [O iii ] λλ 4959,5007 equivalent width (EW) or rest-frame U − B color. Our sample is drawn from the 3D- HST Survey and spans the redshift range . We find that the median Ly α and C iii ] λλ 1907,1909 EWs of our sample are significantly greater than those of z ∼ 2 UV-continuum-selected star-forming galaxies. Measurements from both individual and composite spectra indicate a monotonic, positive correlation between C iii ] and [O iii ], while a lack of trend is observed between Ly α and [O iii ] at EW [O iii ] ≲ 1000 Å. At higher EW [O iii ] , extreme Ly α emission starts to emerge. Using stacked spectra, we find that Ly α and C iii ] are significantly enhanced in galaxies with lower metallicity. Two objects in our sample appear comparable to z > 6.5 galaxies with exceptionally strong rest-UV metal line emission. These objects have significant C iv λλ 1548,1550, He ii λ 1640, and O iii ] λλ 1661,1665 emission in addition to intense Ly α or C iii ]. Detailed characterization of these lower-redshift analogs provides unique insights into the physical conditions in z > 6.5 star-forming regions, motivating future observations of reionization-era analogs at lower redshifts.