Extremely Red Galaxies at z = 5–9 with MIRI and NIRSpec: Dusty Galaxies or Obscured Active Galactic Nuclei?

Abstract

We study a new population of extremely red objects (EROs) recently discovered by the James Webb Space Telescope (JWST) based on their NIRCam colors F277W − F444W > 1.5 mag. We find 37 EROs in the Cosmic Evolution Early Release Science Survey (CEERS) field with F444W < 28 mag and photometric redshifts between 5 < z < 7, with median z = 6.9 − 1.6 + 1.0 . Surprisingly, despite their red long-wavelength colors, these EROs have blue short-wavelength colors (F150W − F200W ∼ 0 mag) indicative of bimodal spectral energy distributions (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 SEDs of eight of them with MIRI and NIRSpec observations using stellar population models and active galactic nucleus (AGN) templates. We find that dusty galaxies or obscured AGNs provide similarly good SED fits but different stellar properties: massive and dusty, log M ⋆ / M ⊙ ∼ 10 and A V ≳ 3 mag, or low mass and obscured, log M ⋆ / M ⊙ ∼ 7.5 and A V ∼ 0 mag, hosting an obscured quasi-stellar object (QSO). SED modeling does not favor either scenario, but their unresolved sizes are more suggestive of AGNs. If any EROs are confirmed to have log M ⋆ / M ⊙ ≳ 10.5, it would increase the pre-JWST number density at z > 7 by up to a factor ∼60. Similarly, if they are QSOs with luminosities in the L bol > 1045–46 erg s−1 range, their number would exceed that of bright blue QSOs by more than three orders of magnitude. Additional photometry at mid-infrared 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.

Rest-frame UV Colors for Faint Galaxies at z ∼ 9–16 with the JWST NGDEEP Survey

“…We exclude the two galaxies that Leung et al (2023) found had red colors in the longwavelength bands. As discussed in Leung et al (2023), this may indicate that these sources have an AGN component contributing to their emission, similar to other sources identified at brighter magnitudes and lower redshifts (e.g., Kocevski et al 2023;Labbe et al 2023;Matthee et al 2023;Barro et al 2024).…”

Exploring Low-mass Black Holes through Tidal Disruption Events in the Early Universe: Perspectives in the Era of the JWST, Roman Space Telescope, and LSST Surveys

“…The BH masses for those JWST-detected faint AGNs are M • ∼ 10 6−8 M e , which is 1-2 orders of magnitude lower than those of previously known quasars at z > 4. Intriguingly, some of the sources have extremely red colors in the rest-frame optical bands, while the rest-frame UV color seems fairly consistent with those of known Lyman-break galaxies at similar redshifts (e.g., Harikane et al 2023;Kocevski et al 2023;Labbe et al 2023;Barro et al 2024;Greene et al 2024;Matthee et al 2024). The so-called "little red dot" sources are interpreted to be in a transition stage from a dust-obscured starburst to an unobscured luminous quasar due to the expelling of gas and dust (Fujimoto et al 2022).…”

“…2. An intriguing subset consists of the dust-reddened AGNs (referred to as "little red dots"), characterized by moderate obscuration in the SED and compact morphology (Harikane et al 2023;Kocevski et al 2023;Labbe et al 2023;Barro et al 2024;Matthee et al 2024). This class of AGNs shows a distinctive spectral shape, which can be attributed to a dust-reddened AGN continuum coupled with excess emission in the rest-frame UV produced either by the unobscured stellar continuum of the host galaxy or scattered light from the buried AGN (e.g., Zakamska et al 2005;Polletta et al 2006;Noboriguchi et al 2023).…”

Brown Dwarf Candidates in the JADES and CEERS Extragalactic Surveys

“…One source, CEERS-EGS-BD-5, has an EAZY galaxy fit with χ 2 = 18.31, but we included this source in our final sample as the EAZY fit is not consistent with the nondetections in the ACS F606W and F814W filters. We rejected those sources with red LW slopes similar to those described in Endsley et al (2023), Furtak et al (2023, Barro et al (2024), Hainline et al (2023), , and Matthee et al (2023), as these have colors inconsistent with brown dwarf models.…”

“…Multiple authors have looked at similar samples of extremely red sources in extragalactic data sets (e.g., Franco et al 2023;Pérez-González et al 2023, Barro et al 2024, among others), and we have outlined color selection criteria here which may be used to find the brown dwarfs among those samples.…”