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

Abstract

Strong [OIII]$\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 also 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 $\sim$ 2 galaxies selected as analogs of those in the reionization era based on [OIII]$\lambda \lambda$4959,5007 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 CIII]$\lambda \lambda$1907,1909 EWs of our sample are significantly greater than those of z $\sim$ 2 UV-continuum-selected star-forming galaxies. Measurements from both individual and composite spectra indicate a monotonic, positive correlation between CIII] and [OIII], while a lack of trend is observed between Ly$\alpha$ and [OIII] at [OIII] EW < 1000$\unicode{x212B}$. At higher [OIII] EW, extreme Ly$\alpha$ emission starts to emerge. Using stacked spectra, we find that Ly$\alpha$ and CIII] 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 CIV$\lambda \lambda$1548,1550, HeII$\lambda$1640, and OIII]$\lambda \lambda$1661,1665 emission in addition to intense Ly$\alpha$ or CIII]. 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.

22 pages, 9 figures, accepted for publication in ApJ

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• JWST/NIRCam Observations of Stars and HII Regions in $z\simeq 6-8$ Galaxies: Properties of Star Forming Complexes on 150 pc Scales
• Lyman-alpha spectroscopy of extreme [OIII] emitting galaxies at $z\simeq 2-3$: Implications for Ly$\alpha$ visibility and LyC leakage at $z>6$
• The MUSE Hubble Ultra Deep Field Survey. XV. The mean rest-UV spectra of Ly-alpha emitters at z>3
• Early results from GLASS-JWST. VI: Extreme rest-optical equivalent widths detected in NIRISS Wide Field Slitless Spectroscopy
• Recovery and analysis of rest-frame UV emission lines in 2052 galaxies observed with MUSE at $1.5<z<6.4$
• The Detection of Ionized Carbon Emission at z~8
• Spectroscopy of CASSOWARY gravitationally-lensed galaxies in SDSS: characterisation of an extremely bright reionization-era analog at $z=1.42$
• The high-redshift Universe with Spitzer