Searching for z > 6.5 Analogs Near the Peak of Cosmic Star Formation

Abstract

Strong [O III]lambda lambda 4959,5007 + H beta 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 alpha 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 similar to 2 galaxies selected as analogs of those in the reionization era based on [O III]lambda lambda 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 1.36 <= z <= 2.49. We find that the median Ly alpha and C.III]lambda lambda 1907,1909 EWs of our sample are significantly greater than those of z similar to 2 UV-continuum-selected starforming 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 alpha and [O III] at EW[O III] less than or similar to 1000 angstrom. At higher EW[O III], extreme Ly alpha emission starts to emerge. Using stacked spectra, we find that Ly alpha 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 lambda lambda 1548,1550, He II lambda 1640, and O III]lambda lambda 1661,1665 emission in addition to intense Ly alpha 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.