MIDIS: Strong (Hb + [OIII]) and Ha emitters at redshift $z\simeq 7-8$ unveiled with JWST/NIRCam and MIRI imaging in the Hubble eXtreme Deep Field (XDF)

Abstract

We make use of \textit{JWST} medium and broad-band NIRCam imaging, along with ultra-deep MIRI $5.6\mu \mathrm{m}$ imaging, in the Hubble eXtreme Deep Field (XDF) to identify prominent line emitters at $z\simeq 7-8$. Out of a total of 58 galaxies at $z\simeq 7-8$, we find 18 robust candidates ($\simeq$31\%) for (H$\beta$ + [OIII]) emitters, based on their enhanced fluxes in the F430M and F444W filters, with EW${}_0$(H$\beta$ +[OIII]) $\simeq 87-2100$ Å. Among these emitters, 16 lie in the MIRI coverage area and 12 exhibit a clear flux excess at $5.6\mu \mathrm{m}$, indicating the simultaneous presence of a prominent H$\alpha$ emission line with EW${}_0$(H$\alpha$) $\simeq 200-3000$ Å. This is the first time that H$\alpha$ emission can be detected in individual galaxies at $z>7$. The H$\alpha$ line, when present, allows us to separate the contributions of H$\beta$ and [OIII] to the (H$\beta$ +[OIII]) complex, and derive H$\alpha$-based star formation rates (SFRs). We find that in most cases [OIII]/H$\beta >1$. Instead, two galaxies have [OIII]/H$\beta <1$, indicating that the NIRCam flux excess is mainly driven by H$\beta$. This could potentially imply extremely low metallicities. Most prominent line emitters are very young starbursts or galaxies on their way to/from the starburst cloud. They make for a cosmic SFR density $\mathrm{lo}{\mathrm{g}}_{10}({\rho }_{\mathrm{SF}{\mathrm{R}}_{\mathrm{H}\alpha }})\simeq -2.35$, which is about a quarter of the total value ($\mathrm{lo}{\mathrm{g}}_{10}({\rho }_{\mathrm{SF}{\mathrm{R}}_{\mathrm{tot}}})\simeq -1.76$) at $z\simeq 7-8$. Therefore, the strong H$\alpha$ emitters likely had a significant role in reionization.

arXiv comment18 pages, 11 figures. The paper has been accepted for publication in ApJ

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