Extreme nature of four blue-excess dust-obscured galaxies revealed by optical spectroscopy by Akatoki Noboriguchi et al. on Wednesday 30 November

We report optical spectroscopic observations of four blue-excess

dust-obscured galaxies (BluDOGs) identified by Subaru Hyper Suprime-Cam.

BluDOGs are a sub-class of dust-obscured galaxies (DOGs, defined with the

extremely red color $(i-[22])_{\rm AB} \geq 7.0$; Toba et al. 2015), 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. (2019) has

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 CIV line profile. The

redshifts are between 2.2 and 3.3. The averaged rest-frame equivalent widths of

the CIV lines are $160\pm33$ $\mathrm{\mathring{A}}$, $\sim$7 times higher than

the average of a typical type-1 quasar. The FWHMs of their velocity profiles

are between 1990 and 4470 ${\rm km\ s^{-1}}$, and their asymmetric parameters

are 0.05 and 0.25. Such strong CIV lines significantly affect the broad-band

magnitudes, which is partly the origin of the blue excess seen in the spectral

energy distribution of BluDOGs. Their estimated supermassive black hole masses

are $1.1\times10^8 < M_{\rm BH}/M_\odot < 5.5 \times 10^8$. The inferred

Eddington ratios of the BluDOGs are higher than 1 ($1.1< \lambda_{\rm Edd} <

3.8$), suggesting that the BluDOGs are in a rapidly evolving phase of

supermassive black holes.

arXiv: http://arxiv.org/abs/http://arxiv.org/abs/2211.16748v1