Extreme Nature of Four Blue-excess Dust-obscured Galaxies Revealed by Optical Spectroscopy
Journal article, 2022
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.
Author
Akatoki Noboriguchi
Ehime University
Shinshu University
Tohoku University
Tohru Nagao
Ehime University
Y. Toba
Academia Sinica
Kyoto University
Ehime University
National Astronomical Observatory of Japan
Kohei Ichikawa
Tohoku University
Masaru Kajisawa
Ehime University
Nanako Kato
Ehime University
Toshihiro Kawaguchi
Onomichi City University
Hideo Matsuhara
The Graduate University for Advanced Studies (SOKENDAI)
JAXA Institute of Space and Astronautical Science
Yoshiki Matsuoka
Ehime University
Kyoko Onishi
Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics
Masafusa Onoue
Beijing University of Technology
Max Planck Society
University of Tokyo
Nozomu Tamada
Ehime University
Koki Terao
National Astronomical Observatory of Japan
Tohoku University
Yuichi Terashima
Ehime University
Yoshihiro Ueda
Kyoto University
Takuji Yamashita
National Astronomical Observatory of Japan
Ehime University
Astrophysical Journal
0004-637X (ISSN) 1538-4357 (eISSN)
Vol. 941 2 195Subject Categories
Astronomy, Astrophysics and Cosmology
Atom and Molecular Physics and Optics
Condensed Matter Physics
DOI
10.3847/1538-4357/aca403