Extreme Nature of Four Blue-excess Dust-obscured Galaxies Revealed by Optical Spectroscopy

Abstract

We report optical spectroscopic observations of four blue-excess dust-obscured galaxies (BluDOGs) identified by the Subaru Hyper Suprime-Cam. BluDOGs are a subclass of dust-obscured galaxies (DOGs; defined with the extremely red color (i − [22])AB ≥ 7.0; Toba et al., showing a significant flux excess in the optical g and r bands over the power-law fits to the fluxes at the longer wavelengths. Noboriguchi et al. have suggested that BluDOGs may correspond to the blowing-out phase involved in a gas-rich major-merger scenario. However, the detailed properties of BluDOGs are not understood because of the lack of spectroscopic information. In this work, we carry out deep optical spectroscopic observations of four BluDOGs using Subaru/FOCAS and VLT/FORS2. The obtained spectra show broad emission lines with extremely large equivalent widths, and a blue wing in the C iv line profile. The redshifts are between 2.2 and 3.3. The averaged rest-frame equivalent widths of the C iv lines are 160 ± 33 Å, ∼7 times higher than the average of a typical type 1 quasar. The FWHMs of their velocity profiles are between 1990 and 4470 km s−1, and their asymmetric parameters are 0.05 and 0.25. Such strong C iv lines significantly affect the broadband magnitudes, which are partly the origin of the blue excess seen in the spectral energy distribution of BluDOGs. Their estimated supermassive black hole masses are 1.1 × 108 < M BH/M ⊙ <5.5 × 108. The inferred Eddington ratios of the BluDOGs are higher than 1 (1.1 < λ Edd < 3.8), suggesting that the BluDOGs are in a rapidly evolving phase of supermassive black holes.

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

“…In this Letter, we use the JWST-ERO samples taken from Barro et al (2023) and Matthee et al (2023) and the BluDOG sample taken from Noboriguchi et al (2022). Barro et al (2023) selected 37 JWST-EROs in CEERS fields (Finkelstein et al 2023) based on the single color criterion (F277W − F444W) AB >1.5.…”

“…However, Noboriguchi et al (2019) found eight DOGs with blue excess in optical bands (blueexcess DOGs; BluDOGs) using an observed-frame optical slope (α opt < 0.4, where α opt denotes the observed-frame optical spectral index of the power law fitted to the HSC g-, r-, i-, z-, and y-band fluxes, f opt l µ n a ). After spectroscopic follow-up observations, Noboriguchi et al (2022) reported that BluDOGs are broad-line AGNs at 2 < z < 3 and the origin of the blue-excess are blue continuum and large equivalent widths (EWs) of the broad emission lines. In addition, C IV lines exhibit a blue tail, suggesting that BluDOGs have nuclear outflows (Noboriguchi et al 2022).…”

“…After spectroscopic follow-up observations, Noboriguchi et al (2022) reported that BluDOGs are broad-line AGNs at 2 < z < 3 and the origin of the blue-excess are blue continuum and large equivalent widths (EWs) of the broad emission lines. In addition, C IV lines exhibit a blue tail, suggesting that BluDOGs have nuclear outflows (Noboriguchi et al 2022). The Eddington ratio (λ Edd = L bol /L Edd , where L bol and L Edd denote the bolometric luminosity and the Eddington luminosity, respectively) of BluDOGs is greater than 1, i.e., it is in a super Eddington phase.…”

“…The Eddington ratio (λ Edd = L bol /L Edd , where L bol and L Edd denote the bolometric luminosity and the Eddington luminosity, respectively) of BluDOGs is greater than 1, i.e., it is in a super Eddington phase. Therefore, BluDOGs are likely to be in a transition phase between dusty AGN and optically thin quasar phases (Noboriguchi et al 2022).…”

“…The BluDOG sample consists of eight objects selected by Noboriguchi et al (2019). Noboriguchi et al (2022) conducted spectroscopic observations for the four brightest BluDOGs (r AB < 23) among them. The spectroscopic redshifts are between 2.2 and 3.3.…”