Extremely red galaxies at $z=5-9$ with MIRI and NIRSpec: dusty galaxies or obscured AGNs?

TL;DR

This paper leverages JWST CEERS data to identify 37 extremely red objects with F277W−F444W>1.5 at z>5, all of which are unresolved and exhibit blue rest-frame UV colors alongside red rest-frame optical continua. Through MIRI photometry and NIRSpec spectroscopy for a subset, the authors perform multi-code SED modeling (including galaxy-only and hybrid galaxy+AGN templates) to test whether these objects are dusty massive galaxies or obscured AGNs. The 8 EROs with MIRI/NIRSpec data can be fit by either scenario, though MIRI-detected continua tend to favor non-stellar red continua in some models, and the inferred stellar masses—and thus number densities—are highly model-dependent. The results imply potentially large implications for the high-redshift stellar mass function and AGN demographics, emphasizing the need for deeper mid-IR data and spectroscopy to break degeneracies and place these objects in the proper context of early galaxy evolution.

Abstract

We study a new population of extremely red objects (EROs) recently discovered by JWST based on their NIRCam colors F277W$-$F444W $>1.5$ mag. We find 37 EROs in the CEERS field with F444W $<28$ mag and photometric redshifts between $5<z<7$, with median $z=6.9^{+1.0}_{-1.6}$. Surprisingly, despite their red long-wavelength colors, these EROs have blue short-wavelength colors (F150W$-$F200W$\sim$0 mag) indicative of bimodal SEDs with a red, steep slope in the rest-frame optical, and a blue, flat slope in the rest-frame UV. Moreover, all these EROs are unresolved, point-like sources in all NIRCam bands. We analyze the spectral energy distributions of 8 of them with MIRI and NIRSpec observations using stellar population models and AGN templates. We find that a dusty galaxy or an obscured AGN provide similarly good SED fits but different stellar properties: massive and dusty, log M/M_sun$\sim$10 and A$_{\rm V}\gtrsim3$ mag, or low mass and obscuration, log M/M_sun$\sim$7.5 and A$_{\rm V}\sim0$ mag, hosting an obscured QSO. SED modeling does not favor either scenario, but their unresolved sizes are more suggestive of an AGN. If any EROs are confirmed to have log M/M_sun$\gtrsim10.5$, it would increase pre-JWST number densities at $z>7$ by up to a factor $\sim$60. Similarly, if they are OSOs with luminosities in the L$_{\rm bol}>10^{46-47}$ erg s$^{-1}$ range, their number would exceed that of bright blue QSOs by more than two orders of magnitude. Additional photometry at mid-IR wavelengths will reveal the true nature of the red continuum emission in these EROs and will place this puzzling population in the right context of galaxy evolution.

Figures (10)

• Figure 1: Color-magnitude and color-color diagrams showing the selection threshold for F277W EROs (circles; F277W$-$F444W$>$1.5 mag), relative to the bulk of the CEERS galaxy catalog, color-coded by stellar mass and A$_{\rm V}$, and a subset of F150W EROs (F150W$-$F444W$>$2 mag). The blue and purple markers indicate the EROs observed with MIRI and NIRSpec. The black squares show the EROs from labbe23. The left and central panels show the general trends toward redder colors with increasing mass and dust attenuation (arrows), which suggest that F277W EROs are massive and dusty galaxies. However, the central panel reveals that F277W EROs have surprisingly blue colors at shorter wavelengths, F150W$-$F200W$\sim0$ mag, very different from those of F150W EROs and massive dusty galaxies in general. The red square shows a massive, dusty, sub-mm galaxy at $z=5.1$ from zavala23 which is also red in all bands. This implies that F277W EROs have bimodal SEDs with blue SW colors and red LW colors. The right panel shows the correlation between photometric redshift and F150W$-$F277W color for the F277W EROs. As the F277W filter shifts from the steep, rest-frame optical range to the flat rest-UV range with increasing redshift the color declines to F150W$-$F277W$\sim$0 mag.
• Figure 2: Left: Photometric redshift vs. stellar mass diagram for the F277W EROs (circles), the CEERS galaxy sample (green density map), and the F150W EROs (red). The F277W EROs are relatively massive, $\log\mathrm{M\!_\star/M}_{\odot}$$\sim$10, and dusty, A$_{V}\sim$3 mag and they span a redshift range from $5<z<9$. Overall, F277W EROs are among the most massive galaxies at their redshift, but less massive than the F150W EROs at lower redshift, following the expected decline in the number of very massive galaxies with redshift. However, a few of them are much more massive ($\log\mathrm{M\!_\star/M}_{\odot}$$\gtrsim$10.5)m suggesting that there might be limitations in the fitting of their bimodal SEDs or perhaps that the continuum is not stellar, but AGN-dominated (see discussion in § \ref{['s:stellarpop']}). Right: Stellar mass vs. F356W effective radius for the same galaxies. The blue and red lines show the mass-size relations for star-forming and quiescent galaxies from vdw14. The dashed lines indicate the approximate resolution limit from the HWHM of the PSF in F356W (FHWM=0.15$^{\prime\prime}$) at $z=5$ and $z=9$. Remarkably, all the F277W EROs appear to be unresolved point-like sources in contrast with the typical spread of F150W EROs and other massive galaxies. We find similar results in the other NIRCam bands suggesting that the EROs are unresolved at all wavelengths. The panels on the right show the best fit to a PSF in F444W for the 4 galaxies with MIRI detections showing negligible residuals.
• Figure 3: Left: Stacked spectral energy distribution of the 37 EROs (grey squares) divided into 2 groups below and above z=7, shown in purple and red, respectively. The MIRI photometry is shown in green. All galaxies exhibit a characteristic bimodal SED. Representative best-fit SEDs with EAZYPy at z$=$5.5 and z$=$7.5 (purple and red solid lines) show that this peculiar SED shape is typically reproduced by a composite SED with a blue, flat continuum in the rest-frame UV and red, steep continuum in the optical. Indeed, the best-fit power-law to the fluxes redward of F277W is quite large ($\alpha_{\nu}\sim3-4$) indicative of a heavily reddened continuum. The stacked SEDs also highlight the difference in F277W as the bands shift from the steep to the flat slope with increasing redshift. Right-top: 2.5$^{\prime\prime}$$\times$2.5$^{\prime\prime}$ cutouts of EROs in the two redshift bins showing their similar compact and featureless visual appearances. Right-bottom: List of some of the strongest emission lines that can potentially cause emission-line-driven excesses in the NIRCAM and MIRI photometry at different redshifts. The locations of the strongest H$\alpha$ and [ O iii] lines are also indicated in the left panel.
• Figure 4: Photometric redshift distributions (PDFz) for the 4 MIRI detected EROs computed using EAZY, EAZYpy, and Prospector. The PDFz's derived with the default and blue version of the EAZYpy templates agree well with one another and with the Prospector estimates for all the galaxies. For the 3 galaxies at $z<7$, the PDFz's based on the templates with very high EW lines (blue) suggest a secondary peak at higher redshift that is not supported by the detections in F150W. The key difference between the low and high-z peaks is typically an emission line-driven excess in F444W which could be attributed to H$\alpha$ or [ O iii] respectively (see also Figure \ref{['fig:stack_lines']}).
• Figure 5: Multi-band 2.5$^{\prime\prime}$$\times$2.5$^{\prime\prime}$ cutouts of the MIRI detected EROs and best-fit SED models computed with EAZYpy, Prospector, Synthesizer and a hybrid of galaxy plus a red QSO2 template from polletta06. The left panels illustrate that fits based on a single stellar population component provide a good fit to the overall LW NIRCam and MIRI photometry (black and green squares) but they systematically fail to reproduce the rest-frame UV probed by the SW NIRCam bands. The middle panels show that a composite model consisting of two (or more) stellar populations provides an excellent fit to all the bands by combining a red, massive, and dusty galaxy that fits the LW bands and a blue, low-mass galaxy that fits the SW bands but has little impact on the stellar mass. The right panels show that the hybrid of galaxy + QSO2 models provide an equally good (or better) fit to the SED than the other models. Here, a dust-obscured QSO dominates the LW photometry but does not contribute to the stellar mass of a blue unobscured host, and consequently leads to total stellar masses $\sim2$ orders of magnitude smaller than the other scenarios.