Stellar Properties of z ~ 8 Galaxies in the Reionization Lensing Cluster Survey

TL;DR

This study analyzes high-redshift galaxies at z~8 using HST and Spitzer data, revealing diverse stellar properties and evidence of early star formation, with some galaxies showing evolved stellar populations just 100 million years after the Big Bang.

Contribution

It incorporates Spitzer/IRAC imaging into spectral energy distribution fitting for z~8 galaxy candidates, providing new insights into their stellar masses, ages, and star formation histories.

Findings

Most z~8 candidates are confirmed at high redshift after Spitzer data inclusion.

Detected evolved stellar populations in some galaxies imply early star formation within 100 Myr of the Big Bang.

Deep Spitzer data supports the high-redshift nature of a z~10 candidate.

Abstract

Measurements of stellar properties of galaxies when the universe was less than one billion years old yield some of the only observational constraints of the onset of star formation. We present here the inclusion of \textit{Spitzer}/IRAC imaging in the spectral energy distribution fitting of the seven highest-redshift galaxy candidates selected from the \emph{Hubble Space Telescope} imaging of the Reionization Lensing Cluster Survey (RELICS). We find that for 6/8 \textit{HST}-selected $z\gtrsim8$ sources, the $z\gtrsim8$ solutions are still strongly preferred over $z\sim$1-2 solutions after the inclusion of \textit{Spitzer} fluxes, and two prefer a $z\sim 7$ solution, which we defer to a later analysis. We find a wide range of intrinsic stellar masses ($5\times10^6 M_{\odot}$ -- $4\times10^9$ $M_{\odot}$), star formation rates (0.2-14 $M_{\odot}\rm yr^{-1}$), and ages (30-600 Myr) among our sample. Of particular interest is Abell1763-1434, which shows evidence of an evolved stellar population at $z\sim8$, implying its first generation of star formation occurred just $< 100$ Myr after the Big Bang. SPT0615-JD, a spatially resolved $z\sim10$ candidate, remains at its high redshift, supported by deep \textit{Spitzer}/IRAC data, and also shows some evidence for an evolved stellar population. Even with the lensed, bright apparent magnitudes of these $z \gtrsim 8$ candidates (H = 26.1-27.8 AB mag), only the \textit{James Webb Space Telescope} will be able further confirm the presence of evolved stellar populations early in the universe.

Tables

Table 1. Table 1 z ≳ 8 greater-than-or-equivalent-to 𝑧 8 z\gtrsim 8 galaxy candidates and selected photometry

Object ID	R.A.	Dec.	${\mathrm{F160W}}^{\text{<span><sup>1</sup><span><span><sup>1</sup><span>footnotemark: </span></span></span></span>}}$	Ks	${\left[3.6\right]}^{\text{<span><sup>2</sup><span><span><sup>2</sup><span>footnotemark: </span></span></span></span>}}$	$R_{3.6}^{\text{<span><sup>3</sup><span><span><sup>3</sup><span>footnotemark: </span></span></span></span>}}$	${\left[4.5\right]}^{\text{<span><sup>2</sup><span><span><sup>2</sup><span>footnotemark: </span></span></span></span>}}$	$R_{4.5}^{\text{<span><sup>3</sup><span><span><sup>3</sup><span>footnotemark: </span></span></span></span>}}$
	(deg.)	(deg.)	(mag)	(mag)	(mag)		(mag)
Abell1763-1434	203.8333744	+40.9901793	$26.1\pm 0.1$		$25.5\pm 0.4$	0.29	$24.5\pm 0.2$	0.28
Abell1763-0460	203.8249758	+41.0091170	$27.8\pm 0.2$		$>$25.9	0.37	$24.5\pm 0.2$	0.39
MACS0553-33-0219	88.3540349	-33.6979484	$27.2\pm 0.2$		$25.0\pm 0.3$	0.34	$25.5\pm 0.6$	0.34
PLCKG287+32-2032	177.7225936	-28.0850703	$26.7\pm 0.2$	$26.6\pm 0.3$	$>$26.6	0.53	$>$26.4	0.59
SPT0615-JD	93.9792550	-57.7721477	$25.8\pm 0.1$		$26.0\pm 0.6$	1.43	$25.4\pm 0.4$	1.13
RXC0911+17-0143	137.7939712	+17.7897516	$26.5\pm 0.1$		$>$26.4	0.05	$>$26.1	0.04
AbellS295-0568	41.4010242	-53.0405184	$26.3\pm 0.1$		$>$26.2	0.16	$>$26.3	0.14

Table 2. Table 2 Photometric Redshift and Stellar Population Modeling Results

Object ID	$z_{\mathrm{peak}}^{\text{<span><sup>1</sup><span><span><sup>1</sup><span>footnotemark: </span></span></span></span>}}$	${\mu }_{\mathrm{med}}^{\text{<span><sup>2</sup><span><span><sup>2</sup><span>footnotemark: </span></span></span></span>}}$	$M_{\mathrm{stellar}}\times f_{\mu }^{\text{<span><sup>3</sup><span><span><sup>3</sup><span>footnotemark: </span></span></span></span>}}$	$SFR\times f_{\mu }^{\text{<span><sup>3</sup><span><span><sup>3</sup><span>footnotemark: </span></span></span></span>}}$	${\mathrm{Age}}^{\text{<span><sup>4</sup><span><span><sup>4</sup><span>footnotemark: </span></span></span></span>}}$	${\mathrm{sSFR}}^{\text{<span><sup>5</sup><span><span><sup>5</sup><span>footnotemark: </span></span></span></span>}}$	$E{\left(B-V\right)}^{\text{<span><sup>6</sup><span><span><sup>6</sup><span>footnotemark: </span></span></span></span>}}$
			(${10}^9{M}_{\odot }$)	($M_{\odot }y{r}^{-1}$)	(Myr)	(${\mathrm{Gyr}}^{-1}$)	(mag)
Abell1763-1434	${8.2}_{-0.2}^{+0.6}$	${2.0}_{-0.4}^{+0.5}$	${4.3}_{-2.1}^{+2.4}$	${13.7}_{-4.0}^{+9.1}$	${510}_{-190}^{+60}$	${2.9}_{-0.3}^{+1.5}$	$0.10{\pm }_{0.00}^{0.05}$
$\mathrm{Abell1763}-{0460}^{\text{<span><sup>*</sup><span><span><sup>*</sup><span>footnotemark: </span></span></span></span>}}$	${8.9}_{-0.8}^{+0.2}$	${2.9}_{-1.0}^{+0.9}$	${0.3}_{-0.2}^{+4.3}$	${1.2}_{-0.6}^{+10.4}$	${510}_{-500}^{+60}$	${2.9}_{-0.3}^{+102.1}$	${0.00}_{-0.00}^{+0.25}$
MACS0553-33-219	${8.2}_{-2.4}^{+0.2}$	${6.5}_{-0.6}^{+0.7}$	${0.6}_{-0.5}^{+0.9}$	${1.8}_{-1.3}^{+2.5}$	${570}_{-320}^{+240}$	${2.6}_{-0.7}^{+2.9}$	${0.20}_{-0.20}^{+0.10}$
PLCKG287+32-2032	${7.9}_{-0.9}^{+0.5}$	${36}_{-6}^{+12}$	${0.005}_{-0.002}^{+0.003}$	${0.2}_{-0.1}^{+2.5}$	${31}_{-11}^{+9}$	${36.9}_{-7.7}^{+18.4}$	${0.05}_{-0.05}^{+0.10}$
SPT0615-JD	${10.2}_{-0.5}^{+1.1}$	${6.0}_{-1.7}^{+2.7}$	${0.4}_{-0.3}^{+0.7}$	${6.2}_{-3.2}^{+6.9}$	${80}_{-70}^{+320}$	${15.4}_{-11.9}^{+73.0}$	${0.05}_{-0.05}^{+0.05}$
RXC0911+17-0143	${8.1}_{-0.6}^{+0.4}$	${1.5}_{-0.2}^{+0.2}$	${0.2}_{-0.1}^{+0.8}$	${5.6}_{-1.4}^{+10.6}$	${34}_{-20}^{+200}$	${33.9}_{-28.0}^{+71.0}$	${0.00}_{-0.00}^{+0.05}$
$\mathrm{AbellS295}-{0568}^{\text{<span><sup>*</sup><span><span><sup>*</sup><span>footnotemark: </span></span></span></span>}}$	${8.1}_{-0.7}^{+0.3}$	${4.0}_{-0.4}^{+1.5}$	${0.07}_{-0.02}^{+0.03}$	${2.4}_{-0.4}^{+2.2}$	${28}_{-16}^{+13}$	${41.2}_{-11.9}^{+47.3}$	${0.00}_{-0.00}^{+0.50}$

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1. Introduction

High-$z$ galaxies are key sources in the epoch of reionization, and to understand the contributions of the faint $z\sim 8-10$ population by way of ionizing photon production, we need measurements of star formation rate (SFR) and stellar mass. However in practice, robust constraints on physical properties of $z\sim 8-10$ galaxies are difficult to place. Surveys using lensing and blank fields to target high-$z$ galaxies in recent years have rapidly grown the sample. In particular, measurements of ages of galaxies in the high-$z$ universe have provided one of the few observational probes of the onset of star formation (e.g., Egami et al., 2005; Richard et al., 2011; Huang et al., 2016). The most recent spectroscopically confirmed example by Hashimoto et al. (2018) (see also Zheng et al., 2012; Bradač et al., 2014; Hoag et al., 2018) implies first star formation at $\sim$ 250 Myr after the Big Bang as evidenced by an old stellar population in the galaxy MACS1149-JD.

There are also a number of galaxies that are not yet spectroscopically confirmed and show signs of a possible evolved stellar population at high-$z$. At $z\sim 8$, spectral energy distribution (SED) results are heavily influenced by near-IR fluxes, since the Balmer/$D_{n}$(4000) break (hereafter Balmer break) falls into Spitzer channel 1 (3.6$\mu$m, [3.6] or ch1 hereafter) from $z\sim 7-10$, requiring Spitzer fluxes for robust measurements of stellar mass, SFR, and age. Complicating the problem, strengths of nebular emission lines and dust content at these redshifts are unknown, creating a degeneracy between emission lines and the Balmer break that is difficult to disentangle with the currently available near-IR broadband observations. When a spectroscopic redshift is available, it is sometimes possible to disentangle the degeneracy if the emission lines fall outside of a broadband, as in Hashimoto et al. (2018). While the James Webb Space Telescope (JWST) will ultimately be able to break most of these degeneracies, identifying candidates with broadband photometry for follow-up and an initial investigation of their stellar properties are important scientific goals.

So far, there have been 100-200 $z\gtrsim 8$ candidates identified in Hubble Space Telescope (HST) surveys that utilize gravitational lensing by massive galaxy clusters and in blank field surveys (e.g., Bradley et al., 2014; Bouwens et al., 2015; Finkelstein et al., 2015; Oesch et al., 2015; Ishigaki et al., 2018; Morishita et al., 2018; Bouwens et al., 2019; De Barros et al., 2019). Photometric redshifts of this sample are largely based on rest-frame UV + optical photometry (HST + Spitzer/IRAC), and only a small subset are spectroscopically confirmed. Without a spectroscopic confirmation, Spitzer fluxes can aid in removing low-redshift interlopers from these samples. Even with a spectroscopic confirmation, Spitzer/IRAC (rest-frame optical) fluxes are essential for robust measurements of stellar properties (González et al., 2011; Ryan et al., 2014; Salmon et al., 2015).

Here we use HST and Spitzer/IRAC imaging data from the Reionization Lensing Cluster Survey (RELICS, PI Coe) and companion survey, Spitzer-RELICS (S-RELICS, PI Bradač) to probe rest frame optical wavelengths of seven $z\gtrsim 8$ candidates originally selected with HST. Details of the HST-selected high-$z$ candidates can be found in Salmon et al. (2017, 2018) (hereafter S17, S18). We present measurements of stellar mass, SFR, and age inferred from HST and Spitzer broadband fluxes.

In §2 we describe HST and Spitzer imaging data and photometry. In §3 we discuss the lens models used in our analysis. In §4 we describe our photometric redshift procedure, SED modeling procedure and calculation of stellar properties. We present our SED fitting and stellar properties results in §5 and we conclude in §6. Throughout the paper, we give magnitudes in the AB system (Oke, 1974), and we assume a $\Lambda$CDM cosmology with $h=0.7$, $\Omega_{m}=0.3$, and $\Omega_{\rm\Lambda}=0.7$.

2. Observations and Photometry

HST reduced images and catalogs are publicly available on Mikulski Archive for Space Telescopes (MAST111https://archive.stsci.edu/prepds/relics/) and Spitzer reduced images on NASA/IPAC Infrared Science Archive (IRSA222https://irsa.ipac.caltech.edu/data/SPITZER/SRELICS/). Details of the survey can be found in Coe et al. (2019). Here we focus on the six clusters with $z\gtrsim 8$ candidates (Abell 1763, MACSJ0553-33, PLCKG287+32, Abell S295, RXC0911+17, and SPT0615-57, Figure 1).

2.1. HST

Each cluster was observed with two orbits of WFC3/IR imaging in F105W, F125W, F140W, and F160W and with three orbits in ACS (F435W, F606W, F814W), with the exception of Abell1763 which received seven additional WFC3/IR orbits. In this work, we use the catalogs based on a detection image comprised of the $0.06\arcsec$/pix weighted stack of all WFC3/IR imaging, optimized for detecting small high-$z$ galaxies, described in Coe et al. (2019).