Similarity between Compact Extremely Red Objects Discovered with JWST in Cosmic Dawn and Blue-excess Dust-obscured Galaxies Known in Cosmic Noon

Abstract

Spatially compact objects with extremely red color in the rest-frame optical to near-infrared (0.4–1 μm) and blue color in the rest-frame ultraviolet (UV; 0.2–0.4 μm) have been discovered at 5 < z < 9 using the James Webb Space Telescope (JWST). These extremely red objects (JWST-EROs) exhibit spectral energy distributions (SEDs) that are difficult to explain using a single component of either star-forming galaxies or quasars, leading to two-component models in which the blue UV and extremely red optical are explained using less-dusty and dusty spectra of galaxies or quasars, respectively. Here, we report the remarkable similarity in SEDs between JWST-EROs and blue-excess dust-obscured galaxies (BluDOGs) identified at 2 < z < 3. BluDOGs are a population of active galactic nuclei (AGNs) with black hole masses of ∼108–9 M ⊙, which are 1 order of magnitude larger than those in some JWST-EROs. The Eddington ratios of BluDOGs are 1 or higher, whereas those of JWST-EROs are in the range of 0.1–1. Therefore, JWST-EROs are less massive, less active, and more common counterparts in higher-z of BluDOGs in cosmic noon. Conversely, JWST-EROs have a significantly higher fraction of those with blue excess than DOGs. We present the average UV spectra of BluDOGs as a comparison to JWST-EROs and discuss a coherent evolutionary scenario for dusty AGN populations.

UNCOVER Spectroscopy Confirms the Surprising Ubiquity of Active Galactic Nuclei in Red Sources at z > 5

“…From the available low-resolution spectroscopy and broadband SED data, we cannot rule out that the UV component is due to moderately reddened star formation at the level of a few solar masses per year in the host. AGN with similar shapes (Noboriguchi et al 2023) are known at all redshifts (e.g., Glikman et al 2012;Banerji et al 2015;Veilleux et al 2016;Hamann et al 2017;Assef et al 2018;Pan et al 2021). However, at z < 3, these reddened sources with a UV excess are rare; Noboriguchi et al (2019) estimate that of all the dust-obscured AGN, only 1% have a UV excess.…”

The Galaxies Missed by Hubble and ALMA: The Contribution of Extremely Red Galaxies to the Cosmic Census at 3 < z < 8

“…Recently, similarities have been drawn between JWSTdiscovered sources with blue rest-frame UV colors (∼0 mag) and red rest-frame optical colors (>1.5 mag; e.g., Barro et al 2023;Matthee et al 2023) with so-called blue-excess dustobscured galaxies (bluDOGs) at z ∼ 2.5 (Noboriguchi et al 2023). They exhibit both composite rest-frame UV and optical SEDs, with excess blue emission suggestive of leaked or scattered AGN light, similar to our LRDs.…”

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

“…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). Despite significant extinction levels, typically with A V ∼ a few mag, these reddened sources show broad Balmer line emission, indicating the presence of AGNs.…”

“…The observed spectra of those red sources also show the blue excess at shorter wavelengths of λ rest < 0.3-0.4 μm, requiring additional blue components. Although the origin of the blue excess remains uncertain, in analogy with the SEDs for the blue-excess dust-obscured galaxies, a small fraction f scatt of the radiation flux of the intrinsic AGN spectrum would be scattered to our line of sight and develop the blue component (Zakamska et al 2005;Polletta et al 2006;Alexandroff et al 2018;Noboriguchi et al 2022Noboriguchi et al , 2023. In this paper, we adopt f scatt = 0.03 to reproduce the spectral shapes of the dustreddened AGNs reported in the JWST UNCOVER field (e.g., Greene et al 2024), leading to a luminosity density of ( )…”