47 research outputs found
X-Ray bright optically faint active galactic nuclei in the Subaru Hyper Suprime-Cam wide survey
We construct a sample of X-ray bright optically faint active galactic nuclei
by combining Subaru Hyper Suprime-Cam, XMM-Newton, and infrared source
catalogs. 53 X-ray sources satisfying i band magnitude fainter than 23.5 mag
and X-ray counts with EPIC-PN detector larger than 70 are selected from 9.1
deg^2, and their spectral energy distributions (SEDs) and X-ray spectra are
analyzed. 44 objects with an X-ray to i-band flux ratio F_X/F_i>10 are
classified as extreme X-ray-to-optical flux sources. SEDs of 48 among 53 are
represented by templates of type 2 AGNs or starforming galaxies and show
signature of stellar emission from host galaxies in the optical in the source
rest frame. Infrared/optical SEDs indicate significant contribution of emission
from dust to infrared fluxes and that the central AGN is dust obscured.
Photometric redshifts determined from the SEDs are in the range of 0.6-2.5.
X-ray spectra are fitted by an absorbed power law model, and the intrinsic
absorption column densities are modest (best-fit log N_H = 20.5-23.5 cm^-2 in
most cases). The absorption corrected X-ray luminosities are in the range of
6x10^42 - 2x10^45 erg s^-1. 20 objects are classified as type 2 quasars based
on X-ray luminsosity and N_H. The optical faintness is explained by a
combination of redshifts (mostly z>1.0), strong dust extinction, and in part a
large ratio of dust/gas.Comment: 25 pages, 14 figures, 5 tables, accepted for publication in PAS
Ratio of black hole to galaxy mass of an extremely red dust-obscured galaxy at z = 2.52
We present a near-infrared (NIR) spectrum of WISE J1042+1641, an extremely
red dust-obscured galaxy (DOG), which has been observed with the LIRIS on the
4.2m William Hershel Telescope. This object was selected as a hyper-luminous
DOG candidate at z ~ 2 by combining the optical and IR photometric data based
on the SDSS and WISE, although its redshift had not yet been confirmed. Based
on the LIRIS observation, we confirmed its redshift of 2.521 and total IR
luminosity of log(L_IR/L_sun) = 14.57, which satisfies the criterion for an
extremely luminous IR galaxy (ELIRG). Moreover, we indicate that this object
seems to have an extremely massive black hole with M_BH = 10^10.92 M_sun based
on the broad Halpha line: the host stellar mass is derived as M_star = 10^13.55
M_sun by a fit of the spectral energy distribution. Very recently, it has been
reported that this object is an anomalous gravitationally lensed quasar based
on near-IR high-resolution imaging data obtained with the Hubble Space
Telescope. Its magnification factor has also been estimated with some
uncertainty (i.e., mu = 53-122). We investigate the ratio of the black hole to
galaxy mass, which is less strongly affected by a lensing magnification factor,
instead of the absolute values of the luminosities and masses. We find that the
M_BH/M_star ratio (i.e., 0.0140-0.0204) is significantly higher than the local
relation, following a sequence of unobscured quasars instead of obscured
objects (e.g., submillimeter galaxies) at the same redshift. Moreover, the
LIRIS spectrum shows strongly blueshifted oxygen lines with an outflowing
velocity of ~ 1100 km/s, and our Swift X-ray observation also supports that
this source is an absorbed AGN with an intrinsic column density of N_H = 4.9 x
10^23 cm^-2. These results imply that WISE J1042+1641 is in a blow-out phase at
the end of the buried rapid black hole growth.Comment: 6 pages, 4 figures, accepted for publication in A&
Hyper-luminous Dust Obscured Galaxies discovered by the Hyper Suprime-Cam on Subaru and WISE
We present the photometric properties of a sample of infrared (IR) bright
dust obscured galaxies (DOGs). Combining wide and deep optical images obtained
with the Hyper Suprime-Cam (HSC) on the Subaru Telescope and all-sky mid-IR
(MIR) images taken with Wide-Field Infrared Survey Explorer (WISE), we
discovered 48 DOGs with and , where
, , and [22] represent AB magnitude in the -band,
-band, and 22 m, respectively, in the GAMA 14hr field
( 9 deg). Among these objects, 31 ( 65 %) show power-law
spectral energy distributions (SEDs) in the near-IR (NIR) and MIR regime, while
the remainder show a NIR bump in their SEDs. Assuming that the redshift
distribution for our DOGs sample is Gaussian, with mean and sigma = 1.99
0.45, we calculated their total IR luminosity using an empirical relation
between 22 m luminosity and total IR luminosity. The average value of the
total IR luminosity is (3.5 1.1) L, which
classifies them as hyper-luminous infrared galaxies (HyLIRGs). We also derived
the total IR luminosity function (LF) and IR luminosity density (LD) for a
flux-limited subsample of 18 DOGs with 22 m flux greater than 3.0 mJy and
with -band magnitude brighter than 24 AB magnitude. The derived space
density for this subsample is log = -6.59 0.11 [Mpc]. The
IR LF for DOGs including data obtained from the literature is well fitted by a
double-power law. The derived lower limit for the IR LD for our sample is
3.8 10 [L Mpc] and
its contributions to the total IR LD, IR LD of all ultra-luminous infrared
galaxies (ULIRGs), and that of all DOGs are 3 %, 9 %, and 15 %,
respectively.Comment: 15 pages, 15 figures, and 3 tables, accepted for publication in PASJ
(Subaru special issue
Extreme Nature of Four Blue-excess Dust-obscured Galaxies Revealed by Optical Spectroscopy
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 \ub1 33 \uc5, ∼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
7 108 < M BH/M ⊙ <5.5
7 108. 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