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Register a new userSED Analysis of 47 Spectroscopically Confirmed Galaxies at $zsimeq6$ to Constrain Possible Relationships between UV Slope, Dust attenuation, and Escape Fraction
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Junehyoung Jeon
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The reionization of the Universe is thought to have been completed by redshifts $zsimeq5.5$. Observations of galaxies in the Subaru Deep Field (SDF) found more than 100 galaxies at $zsimeq6$, many spectroscopically confirmed through follow-up observations. Among a sample of such 67 galaxies, seven galaxies were examined previously, and six showed steep UV-Continuum slopes ($beta$) between $-3.4<beta<-2.6$, indicating young stellar populations. Using the CIGALE package, we model the SEDs of 47 SDF galaxies with Subaru WFCAM K-band data of the seven galaxies studied previously, using a large model parameter space. The seven blue-$beta$ galaxies are compared to the other 40 SDF galaxies to trace any differences or possible relationships in e.g. the model-implied SED age, dust extinction, and the Lyman continuum (LyC) escape fraction $f_{esc}$. We found that the average implied $f_{esc}$ value is $sim42%$, exceeding the minimum for galaxies to finish reionization at $zsimeq6$. Furthermore, we found slight trends between the CIGALE UV-slope, $f_{esc}$, and E(B-V) values: for a given CIGALE E(B-V) value, the implied $beta$ values are steeper than at $z=0$. Lower implied CIGALE $f_{esc}$ values appear to be associated with bluer $beta$ and lower E(B-V), but the trends between the parameters are weak. This suggests that LyC could have escaped through holes in the surrounding the ISM produced by outflows from supernovae and/or weak AGN, with sufficiently wide opening angles for on average $42%$ of the LyC to escape. Further IR spectroscopic observations with the emph{James Webb Space Telescope} will be needed to constrain this possibility.
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We use a newly assembled large sample of 3,545 star-forming galaxies with secure spectroscopic, grism, and photometric redshifts at z=1.5-2.5 to constrain the relationship between UV slope (beta) and dust attenuation (L(IR)/L(UV)=IRX). Our sample benefits from the combination of deep Hubble WFC3/UVIS photometry from the Hubble Deep UV (HDUV) Legacy survey and existing photometric data compiled in the 3D-HST survey, and extends the range of UV luminosity and beta probed in previous UV-selected samples. IRX is measured using stacks of deep Herschel/PACS 100 and 160 micron data, and the results are compared with predictions of the IRX-beta relation for different assumptions of the stellar population model and obscuration curve. We find that z=1.5-2.5 galaxies have an IRX-beta relation that is consistent with the predictions for an SMC extinction curve if we invoke sub-solar metallicity models that are currently favored for high-redshift galaxies, while the commonly assumed starburst attenuation curve over-predicts the IRX at a given beta by a factor of ~3. The IRX of high-mass (M*>10^9.75 Msun) galaxies is a factor of >4 larger than that of low-mass galaxies, lending support for the use of stellar mass as a proxy for attenuation. The commonly observed trend of fainter galaxies having bluer beta may simply reflect bluer intrinsic UV slopes for such galaxies, rather than lower obscurations. The IRX-beta for young/low-mass galaxies implies a dust curve that is steeper than the SMC, suggesting a lower attenuation at a given beta relative to older/more massive galaxies. The lower attenuations and higher ionizing photon output implied by low metallicity stellar population models point to Lyman continuum production efficiencies, xi_ion, that may be elevated by a factor of ~2 relative to the canonical value for L* galaxies, aiding in their ability to keep the universe ionized at z~2. [Abridged]
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