Using gravitational lensing to probe for bright quintessential galaxies in the Epoch of Reionization

TL;DR

This work addresses the challenge of obtaining high-S/N spectroscopy for bright high-redshift galaxies in the Epoch of Reionization by harnessing strong gravitational lensing from clusters to magnify intrinsically faint sources. The authors combine wide-field ground-based lens searches with space-based HST data to identify five bright dropout candidates, followed by multi-wavelength follow-up and rigorous SED modeling with Prospector to classify two targets at $z\sim6.5$–$7$ and three at $z\sim2$. Their analysis demonstrates that, with limited photometry, high-$z$ galaxies can be distinguished from red dusty interlopers when the dropout is bright and the continuum colors in redder bands are flat, and highlights two highly magnified $z\sim7$ galaxies as prime targets. Overall, the study increases the sample of bright ($m_{AB}<24$) EoR galaxies and provides a practical path for leveraging lensing to probe early galaxy formation with future facilities like JWST and ALMA.

Abstract

Understanding the properties of the first generation of galaxies is an ongoing challenge in observational astrophysics. While advances in deep field observation have led to the identification of large numbers of galaxies within the Epoch of Reionization, there are very few observed galaxies at this range that are sufficiently bright for high signal-to-noise spectroscopy. To this end, we analyse HST and ground-based photometry of five candidate strongly lensed galaxies, all projected behind the cores of massive clusters and with similarly red optical-NIR colors. All are characterized by a drop-off in their spectra between the near-infrared and optical wavelengths, corresponding to a Lyman-break that sets a lower bound on their redshifts. Using the open-source SED modeling software Prospector, we characterize two of these galaxies as high-z (z $\sim$ 6.5-7) while the other three are low-z (z $\sim$ 2) despite all five having similar apparent magnitudes at the observed wavelengths. We demonstrate that for the brightest dropout candidates we can distinguish high-z galaxies from red or dusty low-z galaxies using limited photometric data. The bright sources enable deep constraints on the dropout color which, in combination with flat continua measured in redder bands, require high-z solutions when searching the parameter space. At the time of writing this work significantly increases the number of m_AB < 24 galaxies at or above a redshift of 6, and provides a path forward for future analysis on the early era of galaxy formation

Figures (7)

• Figure 1: Multiband thumbnail imaging of SPT0216-DO. The source exhibits strong dropout behaviour, with rest-frame emission (indicated by the red circles) being well-detected in the F110W (rightmost) and z-band (center) bandpasses but not in the F200LP (leftmost).
• Figure 2: As Figure \ref{['fig:0216_cutout']} for SPT0304-DO. The source likewise exhibits strong dropout behaviour, being well detected in the F110W (rightmost) and z-band (center) bandpasses, and not detected in the F200LP (leftmost).
• Figure 3: As Figure \ref{['fig:0216_cutout']} for the three dropouts in the SDSS field. The positions of DO-1, DO-2, and DO-3 are indicated by the label next to each circle.
• Figure 4: GALFIT models of the SPT targets showing the spatially resolved clump structures in each dropout galaxy.
• Figure 5: Prospector fits for the SDSS dropout candidates, with the probability density of the fitted redshift plotted as insets. Photometric error bars are included in every plot but are too small to be visible for some sources. The 'SSP' and 'Exp.' terms within the legend refer to different fitted star-formation histories as detailed in Section \ref{['fitting']}. Residuals for the photometric data are plotted in green (SSP) and magenta (Exp.).