MEOW: The increase in the obscured AGN fraction in mid-infrared from 0 < z < 6 with JWST MIRI

Abstract

Obscured active galactic nuclei (AGN) are often invoked to explain the rapid emergence of young quasars at high redshift and are crucial for building a complete census of AGN activity and black hole growth. The advent of the James Webb Space Telescope (JWST) extends the discovery space for obscured AGN into the mid-infrared (mid-IR) with unprecedented precision through reprocessed dust emission. In this work, we use deep JWST Mid-Infrared Instrument (MIRI) imaging from the MIRI Early Obscured AGN Wide Survey (MEOW), together with existing JWST Near Infrared Camera (NIRCam), spectroscopic, and Hubble Space Telescope imaging data, to identify a previously unrecognized population of obscured AGN out to z ~ 6. Using spectral energy distribution (SED) modeling of the MIRI-detected sources, we identify 883 AGN over an area of ~ 131 arcmin2 and construct the AGN bolometric luminosity function, including both obscured and unobscured sources, across five redshift bins. We find an excess in AGN abundance relative to UV-selected AGN luminosity functions, indicating a substantial obscured population missed by optical/UV surveys, with the inferred obscured fraction increasing with redshift and reaching ~ 98-99% in our highest-redshift bin, 4.5 < z < 6. We also find higher AGN abundances and obscured fractions than X-ray-based studies, consistent with a previously unrecognized population of heavily obscured, Compton-thick AGN revealed by mid-IR selection. These results suggest that a large fraction of supermassive black hole growth at early times occurs during heavily obscured phases largely inaccessible at other wavelengths.

Figures (11)

• Figure 1: Framework for quantifying the AGN obscured fraction. We illustrate the layout in characterizing obscuration, from the MEOW source detections (§ \ref{['sect:sources_ident']}), followed by the identification of AGN, and corresponding bolometric luminosities and uncertainties $L_\mathrm{bol},\Delta L_\mathrm{bol}$ (§ \ref{['sect:AGN_identification']}), as well as photometric redshift determinations (§ \ref{['sec:z-det']}) through SED fitting. These measurements enable the construction of AGN luminosity functions across five redshift bins, incorporating both obscured and unobscured populations (§ \ref{['Sec:LF']}). Finally, we quantify the obscured fraction as a function of luminosity and redshift by evaluating the excess of MEOW AGN relative to type-1, unobscured quasars (§ \ref{['sect:f_obsc']}).
• Figure 2: CIGALE redshift estimates compared to eazy (blue) and spectroscopic measurements (green). We show only the sources included in the analysis presented in this paper (§ \ref{['Sec:LF']}), namely those with $f_\mathrm{AGN} > 0.1$ and $L_\mathrm{bol} > 10^{44}\,\mathrm{erg\,s^{-1}}$. Overall, the CIGALE redshifts are in good agreement with both the eazy and spectroscopic measurements; however, we identify 41 outliers, as well as a bias toward lower redshift estimates from CIGALE (see text). Of the 254 sources, 83 have secure spectroscopic redshifts, which are adopted for the analysis in this paper. For the remaining 171, we rely on the CIGALE redshift estimates, noting that the choice between CIGALE and eazy does not affect the population-level results of this work (Appendix \ref{['sec:app_LF']}).
• Figure 3: Examples of SED fits, and corresponding cut-outs in the F444W, F1000W, F2100W filters, for a SF object at $z = 0.92$ (top-left), and three AGN, with increasing $f_\mathrm{AGN}$ (quoted on the plot) at $z = 1.36$ (top-right), $z = 3.17$ (bottom-left) and $z=4.13$ (bottom-right). Photometric measurements are plotted as open black squares; uncertainties are included but not visible due to the high signal to noise. In the first two cases, the negligible AGN contribution in the star-forming system and the modest enhancement above the dust component in the top-right object support their classifications. The last two objects are clearly AGN-dominated, highlighted by the mid-IR bump that can only be explained through the hot AGN circumnuclear dust, which exceeds the galactic dust responsible for attenuating the blue stellar continuum.
• Figure 4: Stacked histogram of the redshift distribution for sources in GOODS-N (top) and GOODS-S (bottom). Star-forming systems dominate both fields, except at $z \gtrsim 4$, where the MIRI selection becomes increasingly sensitive to AGN and mixed sources. Despite the difference in overall number counts (see text), the relative fractions of star-forming, composite, and AGN sources remain broadly consistent across the two fields.
• Figure 5: Bolometric luminosity distribution across redshift, color-coded by $f_\mathrm{AGN}$. We show only sources with $f_\mathrm{AGN} > 0.1$ (mixed and AGN-dominated), and include typical error bars in grey, corresponding to $|\Delta z| \approx 0.1$ and $|\Delta \mathrm{log} L_\mathrm{bol}| \approx 0.4$ dex. Overall, the MEOW depth allows us to detect down to $L_\mathrm{bol} \approx 10^{44} \,\mathrm{erg\,s^{-1}}$ across cosmic time, and for the identification of high redshift AGN up to $z\lesssim 7$, with 18 AGN at $z \gtrsim 4.5$.