The Lyman Continuum escape fraction of emission line-selected $z\sim2.5$ galaxies is less than 15%

TL;DR

This study investigates the Lyman Continuum escape fraction in emission line-selected galaxies at z~2.5, finding it to be less than 15%, suggesting such galaxies may not significantly contribute to cosmic reionization.

Contribution

First to combine archival HST imaging with emission line catalogs to constrain LyC escape fraction in a large, homogeneous galaxy sample at z~2.5.

Findings

No LyC emission detected from the sample.

Upper limits on escape fraction: <5.6% for [OII] emitters, <14% for high [OIII]/[OII] ratio galaxies.

Such galaxies are a small fraction at z~2, likely insufficient for reionization.

Abstract

Recent work suggests that strong emission line, star-forming galaxies may be significant Lyman Continuum leakers. We combine archival HST broadband ultraviolet and optical imaging (F275W and F606W, respectively) with emission line catalogs derived from WFC3 IR G141 grism spectroscopy to search for escaping Lyman Continuum (LyC) emission from homogeneously selected $z\simeq$2.5 SFGs. We detect no escaping Lyman Continuum from SFGs selected on [OII] nebular emission (N=208) and, within a narrow redshift range, on [OIII]/[OII]. We measure 1$\sigma$ upper limits to the LyC escape fraction relative to the non-ionizing UV continuum from [OII] emitters, $f_{esc}<$5.6%, and strong [OIII]/[OII]$>$5 ELGs, $f_{esc}<$14.0%. Our observations are not deep enough to detect $f_{esc}\lesssim$10% typical of the low redshift Lyman continuum emitters. However, we find that this population represents a small fraction of the star-forming galaxy population at $z\simeq$2. Thus, unless the number of extreme emission line galaxies grows substantially to z$>$6, such galaxies may be insufficient for reionization. Deeper survey data in the rest-frame ionizing UV will be necessary to determine whether strong line ratios could be useful for pre-selecting LyC leakers at high redshift.

Tables

Table 1. Table 1: Archival F275W imaging

Field	Survey	Areaa	Survey Depth (3$\sigma$)b	Reference	NOII	N${}_{O_{32}}$	N${}_{O_{32}>5}$
GOODS-North	CANDELS-Deep	120	27.8	Koekemoer et al. (2011)	52	7	4
	HDUV	70	27.9	-‡-	57	15	4
GOODS-South	UVUDF	4.5	28.2	Rafelski et al. (2015)	25	—	—
	ERS	60	26.5	Windhorst et al. (2011)	74	19	5
Notes: a–Approximate area in sq. arcminutes; b–We report published point-source completeness limits [AB mag]
but estimate the depth from the sky variance for the HDUV GOODS-N public mosaic.
c–For HDUV, mosaics at http://www.astro.yale.edu/hduv/DATA/v0.5/
‡–HST Program 13872 (Oesch et al.)

Table 2. Table 2: Measured UV/LyC flux ratios : : : Upper Limits to f e ​ s ​ c subscript 𝑓 𝑒 𝑠 𝑐 f_{esc}

Selection	Nobjs	$\Delta \left(z\right)$	Observed $f_{\nu ,LyC}$a	Observed $f_{\nu ,UV}$	IGM corr. UV/LyC	$f_{esc,rel}^{LyC}$b	$f_{esc}^{LyC}$
[OII]	208		0.45$\pm$0.27 (1.6$\sigma$)	9.12	$>$41.49	7.0%	5.6%
All $O_{32}$	41		0.16$\pm$0.140 (1.1$\sigma$)	4.53	$>$38.44	7.8%	6.7%
$O_{32}$$>5$	13		-0.02$\pm$0.090 (-0.16$\sigma$)	1.15	$>$15.81	18.9%	14.0%
Notes: a–Flux densities reported here in $\mu$Jy; b–We assume ${\left(L_{UV}/L_{LyC}\right)}_{int}$=3; Italicized entries
indicate non-detections and should be interpreted as limits.

Equations

$$f_{esc,rel}=\frac{(L_{UV}/L_{LyC}{)}_{int}}{(L_{UV}/L_{LyC}{)}_{obs}}\cdot \exp [{\tau }_{IGM}],$$

$$f_{esc}=f_{esc,rel}\times \exp [-{\tau }_{UV,dust}]$$

Full text

1 Introduction

Star-forming galaxies (SFGs) likely reionize neutral Hydrogen in the early universe (see review in Loeb & Barkana, 2001), when quasars are not sufficiently numerous to contribute significantly to the ionizing background (Ricci et al. 2016 cf., Giallongo et al. 2015). Verifying this assumption by directly measuring the ionizing output of Lyman Continuum (LyC; $\lambda<912$Å) is impossible—the IGM effectively attenuates all LyC flux emitted along the line of sight to $z>6$ redshift galaxies. Instead, the ionizing output of high redshift SFGs must be constrained by surveys of low-redshift analogs, or indirectly (e.g., Jones et al., 2013). Criteria for pre-selecting LyC emitting candidates from amongst the class of all SFGs that produce LyC are crucial for such studies — large, blind, deep surveys are infeasible with the HST, the only telescope currently capable of obtaining high resolution LyC imaging and spectroscopy.

Previously, pre-selection was made on actively SF galaxies (Schaerer, 2003, young, massive stars emit copious ionizing radiation, Q_{H}>$$10^{47}s*-1*;). Studies with the HUT (Leitherer et al., 1995) and the HST SBC (e.g., Malkan, Webb, & Konopacky, 2003; Siana et al., 2010) do not detect escaping LyC. Large archival studies of LyC emission from SFGs (Cowie, Barger,& Trouille, 2009) and H$\alpha$-selected emission line galaxies (Rutkowski et al., 2016) have generally reported non-detections, likely indicating that the strong star-formation may be conducive to LyC escape, but does not guarantee it. Until recently, few LyC leakers were known; local starbursts Tol 0440-381, Tol 1247-232, Mrk 54, and Haro11 (Leitherer et al., 2016; Puschnig et al., 2016, e.g.) and at z$$\gtrsim2, fewer than $\sim$10, UV-selected star-forming galaxies (e.g., Mostardi et al., 2016; Smith et al., 2016; de Barros et al., 2015).

Recently, five (of five galaxies targeted) z$$\lesssim0.3 compact (r_{e}$$\lesssim1 kpc) SFGs selected for their anomalously high nebular oxygen ratios (O_{32}$$\equiv[OIII$\lambda$5007Å] / [OII$\lambda\lambda$3727,3729Å]$>$5) have been confirmed as LyC leakers (f_{esc}$$\sim5-15%; Izotov et al., 2016a, b), with an additional 2-3 compact $O_{32}$ galaxies predicted to be LyC leakers based upon their Ly$\alpha$ profiles (Verhamme et al., 2017). Furthermore, Naidu et al. (2017) applied a F275W-F336W color selection to identify 3 SFG LyC leakers at $z\lesssim 2$ in the HDUV (PID$:$13779; PI$:$P.Oesch), each with O_{32}$$\gtrsim3. The success rate of LyC detection in $O_{32}$-emitters makes this nebular-line diagnostic appealing for pre-selection. Here, we investigate that potential utility. In Section 2, we discuss the selection of ELGs using HST IR grism spectroscopic catalogs combined with rest-frame UV-optical imaging in the CANDELS fields. In Section 3, we present new measurements to the absolute escape fraction, $f_{esc}$ ,for these ELGs. We assume $\Lambda$CDM cosmology with $\Omega_{m}=0.27$, $\Omega_{\Lambda}$=0.73, and H${}_{0}=70$km s*-1* Mpc*-1* (Komatsu et al., 2011).

2 Emission Line Galaxy Selection in CANDELS

The HST WFC3/IR grism has been remarkably successful for surveying SFGs with strong emission lines at $z\sim 1-3$, as the [OII]$\lambda\lambda$3726,3729Å  doublet, a well-calibrated signature of star-formation (Kewley, Geller, & Jansen, 2004), can be detected with the G141 grism ($\lambda_{c}\simeq 1.4\mu$m) in 2\lesssim$$z$$\lesssim3.5 SFGs. The G141 can not resolve this doublet, thus to avoid potential line mis-identification of a candidate [OII], a photometric redshift is critical. In the HST CANDELS fields, the broadband UV-optical SED for galaxies is well-sampled ensuring the robust identification of z$$\gtrsim2 [OII]-emitters. Unfortunately, rest-frame, broadband LyC imaging is not available across the entire survey footprint. Thus, in our search here for LyC emission from $z\sim 2.5$ SFGs, we are limited to probing $\sim$40% of the area in GOODS-South and North.

Within these regions, we select [OII]-emitters identified by the 3DHST G141 grism survey. We selected ELGs requiring 1) SNR${}_{\mbox{\small OII}}>$3 and 2) within the redshift range $2.38<z<2.9$, where $z\equiv$“z_best” measured by Momcheva et al. (2015) using both grism and broadband photometry. The lower redshift limit of this sample is fixed to ensure the broadband (WFC3/UVIS F275W) imaging is strictly sensitive to rest-frame LyC emission. In total, we select 208 [OII]-emitters (74, 109, and 25 in ERS, GOODS-N, and UVUDF, respectively), with a mean (median) SFR=8.0 (3.9)M*⊙$yr^{-1}$ and stellar mass, M${}_{\star}\simeq$109.9*(109.6), respectively. Included in this sample are 13 ELGs which were also considered in the unpublished LyC survey in the ERS field presented by (Smith et al., 2016).

We note that within a narrow redshift range the G141 grism is simultaneously sensitive to [OII] and [OIII]. Thus, we select a second, independent sample of SFGs requiring$:$ 1) 2.25$<z<$2.31; 2) SNR${}_{[\mbox{\small OIII}]}>$3; and 3) SNR${}_{\mbox{\small H}\beta}>$1.5.

This redshift range implies that the sample’s LyC photometry could include a contribution from non-ionizing emission in the bandpass. As illustrated in Figure 2, the F275W throughput, T, $\gtrsim$1% at $\lambda<3086$Å(910Åat $z=2.39$). Only in the case of zero attenuation by intervening neutral gas and dust (i.e., f_{esc}$$\equiv100%) will the contribution redward of Lyman edge to the F275W photometry be negligible ($<0.5\%$). The contribution by non-ionizing photons to the measured ionizing flux will introduce a systematic uncertainty to $f_{esc}$ strictly less than unity measured using the broadband method we apply in Section 3. Any candidate LyC leakers identified in this sample must be considered tentative pending spectroscopic followup.

We identify 41 $O_{32}$ emitters (22 and 19 in the GOODS-N and ERS respectively). For the measurement of $O_{32}$, we require [OIII]$\lambda$5007, which we derive from [OIII]$\lambda\lambda$4959,5007Åreported in 3DHST catalogs, applying a uniform correction that assumes an intrinsic ratio of $\lambda 5007$/$\lambda 4959$=2.98 (Storey & Zeippen, 2000) to correct for the contribution of [OIII]$\lambda$4959. Of these $O_{32}$ emitters, 13 are identified with O_{32}$$>5. By comparison with the [OII]-emitter sample, these ELGs have similarly high mean (median) SFR=6.8 (4.1) M*⊙$yr^{-1}$ and moderate stellar mass, M${}_{\star}\simeq$1010.0*(109.7).

In the following analysis of both samples, we use publicly-available F275W imaging mosaiced by the individual survey teams (see Table LABEL:tab:thedata). In GOODS-North, the HDUV team has prepared public mosaics ”v0.5” combining 5 of 8 HDUV pointings with CANDELS-Deep data. We note that the WFC3/UVIS is susceptible to significant ($\sim$50% losses) charge transfer inefficiencies. To mitigate this, UVUDF and GOODS-North imaging programs (referenced in Col. 4 of Table LABEL:tab:thedata) included a $\sim$10$e^{-}$ post-flash to minimize charge losses. In the case of the ERS, these data were amongst the first data obtained with the then newly-installed WFC3, and minimally affected by the CTE (see Smith et al., 2016, for details). In preparation for analysis, we extracted 12\farcs$$\times12$\farcs$ postage stamps from the science and associated rms maps centered on each ELG. For uniformity, all stamps were rebinned to a common pixel frame of 0$\farcs$09 pix*-1*, the coarsest scale for which mosaics are available.

3 The LyC escape fraction of $z\simeq 2.3$ ELGs

Broadband imaging surveys readily make differential measurements of the ionizing (LyC) to non-ionizing (UV, measured at $\lambda_{rest}\simeq 1500$Å) luminosity from galaxies. At high redshift, LyC from young stars within galaxies will be attenuated by neutral gas and dust in the ISM, as well as by neutral HI in the IGM along the line of sight. This partly motivates a definition of the relative escape fraction, following Steidel, Pettini, & Adelberger (2001)$:$

[TABLE]

$\tau_{IGM}$ is the (redshift-dependent) IGM attenuation of LyC by neutral HI, typically modeled on measurements from absorption line surveys towards high redshift bright quasars (e.g., Fardal, Giroux, & Shull, 1998). The intrinsic UV-to-LyC ratio must be modeled for each galaxy individually, but typically ranges between 2-10 for star-forming galaxies with ages less than $\sim 10^{8}$yr. If the magnitude of extinction due to dust in the ISM can be estimated from the SED, then the absolute escape fraction can be directly related to $f_{esc,rel}$ as$:$

[TABLE]

We measured ionizing and non-ionizing photometry in the F275W and F606W postage-stamps, respectively, using Source Extractor (Bertins & Arnouts 1996) in dual image mode, with the F606W as the detection image111We use the relevant detection parameters DETECT_MINAREA=6, DETECT_THRESH=3, BACK_SIZE=10, BACK_FILTERSIZE=5 and BACK_FILTTHRESH=1.5, found by extensive testing to determine those parameter that most accurately differentiated source from sky pixels in the segmentation maps..

The median F275W SNR for all ELGs is consistent with a statistical non-detection ($\langle\,SNR\,\rangle=0.12$), as measured within each ELG’s corresponding F606W aperture defined in source extraction. A visual inspection of all F275W stamps confirms that no individual ELGs are LyC leakers, including those [OII]-emitters included in the Smith et al. (2016) sample. Note that for the median F606W (rest-frame UV) continuum $m=$25 AB of this sample, the surveys limits f_{esc,rel}$$\lesssim2% in the deepest (UVUDF) and f_{esc,rel}$$\lesssim12% for the shallowest (ERS) mosaics. Smith et al. report a detection of $f_{esc}$=0.14% for the sample which overlaps with the [OII]-emitter sample. Within the HDUV field no LyC leakers have been previously identified222Naidu et al. (2017) identified 6 candidate LyC leakers, all at $z\simeq 2$..

To measure $f_{esc}$, we apply the stacking procedure defined in Siana et al. (2010), summing over all pixels in the F275W & F606W stamps associated with F606W-defined segmentation map. Furthermore, we sum in quadrature the associated errors from the error maps, applying a (small) correction for correlated noise introduced in the rebinning of the error maps where necessary (Casertano et al., 2000). This stacking yields no statistically significant detections of LyC leakers. A visual inspection of the associated stacked LyC frames (combined using IRAF imcombine; Figure 3) reveals no perceptible LyC flux within an aperture defined by the non-ionizing UV image stack. Here, we have cleaned all LyC stamps before stacking, using the segmentation maps, to replace pixels not associated with the ELG with randomly assigned pixel values consistent with the sky background measured within each stamp. In Table LABEL:tab:fescvals, we report $f_{esc}$ for each stack as upper limits.

In rest-frame UV morphology, these galaxies are compact. For reference, an aperture defined to include 90% of the segmentation pixels common to all galaxies has an area $\sim 0.3$sq. arcsecond (physical radius, r$\lesssim 4$kpc), in good agreement with the measurements of $z\sim$2 galaxy sizes from Shibuya et al. (2015). Many galaxies ($>$70%) do show faint irregular UV features. Though we used the segmentation map (defined by the rest-frame UV morphology) to define pixels to include in the stacking of each galaxy, in principle these asymmetric low surface brightness features could be lost to the sky when stacked.

We measure $f_{esc,rel}$, correcting each galaxy for IGM attenuation using the correction factor from the piecewise parametrization of the redshift distribution and column density of intergalactic absorbers (see Haardt & Madau, 2012). For reference, exp[$\tau_{IGM}$]=1.72 (2.56), at $z=$2.29 (2.56), the median redshift of the $O_{32}$ ([OII])-selected samples. Assuming $(L_{UV}/L_{LyC})_{int}=3$, appropriate for a young ($\sim$107yr), solar metallicity stellar population (Rutkowski et al., 2016), we measure f_{esc,rel}$$\lesssim 7.0, 7.8, and 18.9% (1$\sigma$) for the [OII]-, all $O_{32}$, and high $O_{32}$-selected samples. We measure $f_{esc}$  correcting for dust attenuation for each galaxy individually. We measure $\tau_{UV,dust}$ assuming a Calzetti et al. (2000) reddening law ($R_{V}=4.05$), and calculating stellar E(B-V) from the best-fit $A_{V}$ measured from the broadband SED by Skelton et al. (2014).

We report f_{esc}$$<5.6% for the [OII]-selected sample. Note that the upper limit on $f_{esc}$ measured for random sub-samples of [OII]-emitters drawn exclusively from the individual (unbinned) mosaics scale approximately as $N^{-1/2}$, as expected from purely Poisson statistics. Thus, in future work, using a re-reduction of all available F275W imaging in the CANDELS fields to improve the size of the [OII]-selected sample emitters, we will test for variations in $f_{esc}$ in sub-samples selected on, e.g., UV luminosity or inclination.

For the full $O_{32}$selected sample, we measure f_{esc}$$<6.7%; for the O_{32}$$>5, sample f_{esc}$$<14%, consistent with the expectation for Poissonian statistics if the extinction correction is appropriately re-normalized to reflect the higher average extinction reported for the O_{32}$$>5 sample. The mean IGM transmission for the $O_{32}$- and [OII]-selected samples differs by a factor of $\sim$1.5, the $O_{32}$emitters are intrinsically more luminous ($\sim 3\times$), and the possibility of a non-negligible contribution from non-ionizing flux in the F275 bandpass (see Section 2) makes a direct comparison of $f_{esc}$ upper limits for these samples more difficult.

4 Discussion

If these SFGs are analogs to the high redshift sources of reionization, the measured upper limits can be informative. First, the 1$\sigma$ upper limit to $f_{esc}$ measured for [OII]-emitter sample is inconsistent with the threshold of f_{esc}$$\gtrsim13% required if high redshift SFGs reionize the universe (see Robertson et al., 2015) compatible with the independent constraints on the ionization history of the IGM from the CMB (the electron scattering opacity; $\tau_{es}$, see Planck Collaboration et al. (2016) and QSO absorption line studies (Mesinger & Haiman, 2007). This tension is alleviated considering the $3\sigma$ $f_{esc}$  upper limit and noting that dwarf galaxies less massive than these ELGs (with median M$\simeq 10^{9.5-10}$M*⊙*) are expected to contribute most significantly to reionization (Wise et al., 2014; Robertson et al., 2015).

Note Rutkowski et al. (2016) measured, for (H$\alpha$-selected) $z\sim 1$ SFGs, f_{esc}$$<4% (3$\sigma$). Selecting on more distant SFGs using the same grism spectroscopy here, we are more sensitive to intrinsically brighter line luminosities, $\sim\!4$ brighter at $z\sim 2.5$ than $z\sim 1$, though intrinsically we can expect [OII]/H\alpha$$\lesssim1 ($\simeq 0.5$ at $z\simeq 0.1$; Mouhcine et al., 2005). As such, the average SFR for the [OII]-selected sample is $\sim$2$\times$ that of the H$\alpha$ sample in previous work, though the median SFR is measured for 3DHST sources from the broadband SED in contrast to, e.g., Rutkowski et al. (2016) which used the extinction-corrected H$\alpha$ luminosity. Thus, we caution any strict interpretation of the $f_{esc}$ upper limits derived here for $z\simeq 2.5$ SFGs and previous work at $z\simeq 1$ as evidence for an evolution in $f_{esc}$.

Our observations are not deep enough to detect $f_{esc}\sim 10$% typical of the low redshift LyC emitters (Izotov et al., 2016b), which have comparably high nebular emission line ratios or similar star formation rate surface densities, $\Sigma_{SFR}$333We measure -2 $\lesssim$ log($\Sigma_{SFR}$) $\lesssim$ 1 [M*⊙yr-1* kpc*-2*] for $O_{32}$emitters, using the 3DHST broadband SFR and area from each galaxy’s F606W segmentation map.. Our upper limit on $f_{esc}$ derived for the high $O_{32}$ ELGs is marginally consistent ($f_{esc,rel}(3\sigma)\lesssim 0.57$) with the detection of LyC in a similar galaxy (ion2; $f_{esc,rel}=0.64^{1.1}_{-0.1}$) studied by de Barros et al. (2015).

We call attention to $f_{esc}$  measured for the small number of SFGs identified with high $O_{32}$  emission, and the implication for the contribution of their high redshift analogs to reionization. Generally, reionization proceeds when a sufficient ionizing background can be maintained by either a large number of relatively inefficient LyC leakers or relatively fewer emitters which efficiently source LyC. The number of ionizing background photons in a cosmological volume is proportional to f_{esc}$$\times\,n_{SFG}, where $n_{SFG}$ is the volume density of SF galaxies and f_{esc}$$\simeq10% necessary for reionization. However, not all SF galaxies are LyC leakers. In fact, it is well established that the general population of SFGs have escape fraction $<<10$% (Siana et al., 2010; Grazian et al., 2016), and only the extreme $O_{32}$ galaxies appear to meet the requisite $f_{esc}$(Izotov et al., 2016b). If $f_{leak}$ is the fraction of SF galaxies that are LyC leakers, then the previous relationship for the number of ionizing photons, Nion, can be rewritten as N_{ion}\!\propto$$f_{esc}$$\times\,f_{leak}\times\,n_{SFG}. In the WFC3 spectroscopic parallel survey(WISP, Atek et al., 2010), sensitive to both [OII] and [OIII] emission at 1.4$$\lesssim$$z$$\lesssim$$2.3, 50% of the catalogued galaxies are detected in both oxygen lines (Ross et al., 2016). Only $\sim$4% of these sources are O_{32}$$>5 emitters. With the upper limits presented here, assuming $f_{leak}\sim 4$% and that this fraction does not evolve substantially to $z\sim 6$, such extreme objects would not support reionization. High redshift ($z>3$) SFGs do exhibit, on average, an enhanced ionization state relative to low-redshift SFGs (e.g. Stanway et al., 2014), inferred from the [OIII]/H$\beta$ ratio. Recently, Faisst (2016) modeled this increased ionization state with redshift to predict the evolution of the escape fraction evolution with redshift of $O_{32}$ emitters, and found such galaxies to be nearly sufficient to reionize the universe at $z\sim 6$. Clearly, direct measurement of the median escape fraction for strong emitters (O_{32}$$>5) with HST at $z<3$ is critical. This, in combination with the direct measure of the evolution of the number density of such extreme $O_{32}$  galaxies towards the epoch of reionization ($z>7$), a key result for JWST, will ultimately determine whether such sources may reionize the universe.

5 Conclusion

We have combined archival high resolution HST UV imaging in the rest-frame LyC for $z\sim 2.5$ galaxies in the CANDELS deep fields, selected on the presence of nebular oxygen emission lines in the 3DHST IR grism spectra. We do not detect LyC escaping from [OII]- or $O_{32}$-selected emitters individually.

We stack the individual non-detections, and measure for each stack upper limits to the absolute escape fraction less than 5.6, 6.7, and 14% (1$\sigma$), respectively. Our limits on $f_{esc}$(3$\sigma$) for such relatively massive galaxies do not rule out the possibility that SFGs are able to sustain reionization. However, whether at $z\gtrsim 2$, strong star formation and high $O_{32}$ ratios alone are indicative of significant LyC escape remains uncertain. Furthermore, we note that at $z\sim 2$ the class of galaxies with extreme $O_{32}$ ratios remain exceedingly rare. In order for galaxies to be able to sustain reionization, SFGs must evolve substantially from $z\sim 6$ to present, such that at high redshift most have such highly ionized ISM conditions indicated by the high $O_{32}$ ratio. Such galaxies will be prime targets for JWST at $z>3$ and future grism surveys and further constraints on LyC emission from lower redshift $O_{32}$-selected ELGs will be important for calibrating the evolution of LyC towards the epoch of reionization. Deep HST surveys of large volumes at intermediate redshift will be necessary to obtain the large sample sizes of strong $O_{32}$-emitters necessary to determine whether LyC escape is linked to these observable parameters such that their contribution can be meaningfully extrapolated to the epoch of reionization probed by JWST.

This research was supported by NASA NNX13AI55G, HST–AR Program #12821.01 and GO-#13352 using observations from NASA/ESA HST, operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. STScI is operated by the AURA Inc., under NASA contract NAS5–26555. M.H. acknowledges the support of the Swedish Research Council (Vetenskapsrådet), the Swedish National Space Board (SNSB), and the Knut and Alice Wallenberg Foundation. RAW acknowledges JWST grants NAG5-12460 and NNX14AN10G from NASA GSFC. This research has made use of the NASA ADS.