• 천문학회보
• /
• 제43권1호
• /
• pp.59.4-60
• /
• 2018

To investigate the impact of the high-redshift quasars on cosmic reionization, the faint end slope of the quasars luminosity function has to be determined precisely. More quasars with low luminosity are needed to constrain the contribution to reionization in the early universe. However, finding these quasars has been regarded as tough process owing to the improper shallow depth of imaging data. In recent days, the release data of Subaru Hyper Suprime-Cam (HSC) Strategic Program survey which provide the deep images reaching ~ 25 mag facilitates searching the faint quasars candidates. To find faint quasar candidates in ELAIS-N1 field, along with the HSC data, two near-infrared (NIR) data sets also be used : The Infrared Medium-deep Survey (IMS) and The UKIRT Infrared Deep Sky Survey (UKIDSS) - Deep Extragalactic Survey (DXS). Quasar candidates selected from the multi-band color cut were observed by the SED camera for QUasars in EArly uNiverse (SQUEAN) instrument. To trace the redshifted Lyman break efficiently, appropriate medium bands comparable to targeted redshift range are chosen. The most reliable quasar candidates are finally determined through SED fitting. Using this less luminous quasars candidates, we can speculate the relation between the quasar growth and the host galaxy unbiasedly and estimate the contribution to the cosmic reionization.

• 천문학회보
• /
• 제46권1호
• /
• pp.31.2-31.2
• /
• 2021

To estimate the contribution of quasars on keeping the IGM ionized, building a quasar luminosity function (LF) is necessary. Quasar LFs derived from multiple quasar surveys, however, are incompatible, especially for the faint regime, emphasizing the need for deep images. In this study, we construct quasar LF reaching M₁₄₅₀~-21.5 AB magnitude at z ~ 5, which is 1.5 mag deeper than previously reported LFs, using deep images from the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP). We trained an artificial neural network (ANN) by inserting the colors as inputs to classify the quasars at z ~ 5 from the late-type stars and low-redshift galaxies. The accuracy of ANN is > 99 %. We also adopted the Bayesian information criterion to elaborate on the quasar-like objects. As a result, we recovered 5/5 confirmed quasars and remarkably minimized the contamination rate of high-redshift galaxies by up to six times compared to the selection using color selection alone. The constructed quasar parametric LF shows a flatter faint-end slope α=-127^+0.16_-0.15 similar to the recent LFs. The number of faint quasars (M₁₄₅₀ < -23.5) is too few to be the main contributor to IGM ionizing photons.

• 천문학회보
• /
• 제35권2호
• /
• pp.36.2-36.2
• /
• 2010

CQUEAN (Camera fo QUasars in EArly uNiverse) is a newly developed camera by CEOU for the 2.1m telescope at the McDonald Observatory, Texas, USA. We report the early science results from the commissioning run of CQUEAN which include the observations of the gamma-ray burst (GRB) afterglows and quasars at z ~ 5.5. Although the data were originally taken to test the instrument performance, the results are already very encouraging. We uncovered GRB afterglows at z = 0.8 - 1.4, with our data being used for the international collaboration research to understand the nature of GRBs. The unique filter sets we employed are providing the data which are effective for selecting quasars at z ~ 5.5. The special aspects of CQUEAN - high sensitivity at 0.8-1.1 ${\mu}m$ and fast readouts - will allow us to produce many interesting through surveys of high redshift quasars and fast follow-up of transient objects such as GRBs and exoplanets in future.

• 천문학회보
• /
• 제39권2호
• /
• pp.59.2-59.2
• /
• 2014

We study the environmental dependences of various quasar properties using the Sloan Digital Sky Survey (SDSS). For an environmental indicator, we construct the galaxy number density field from the latest data (Data Release 12) of Constant MASS (CMASS) galaxies of SDSS in the redshift range 0.46<=z<=0.59. The galaxy number density field is determined by searching the 20 nearest galaxies from each grid point. For quasars, we use the fifth edition of the SDSS Quasar Catalog made by Schneider et al. (2010) and the catalog of properties for the quasars by Shen et al. (2011). We find environmental dependences of quasar properties as a function of the galaxy number density. This will help us to understand the evolution of quasars with their environment, which will be useful to improve modeling Active Galactic Nuclei feedback in cosmological hydrodynamic simulations.

• 천문학회보
• /
• 제36권2호
• /
• pp.55.2-55.2
• /
• 2011

We present the current research activities of the Center for the Exploration of the Origin of the Universe, a center established at Seoul National University with the Creative Research Initiative program. Our activities focus on observational studies of distant objects such as gamma-ray bursts, quasars, and proto-cluster of galaxies, but we also carry out other observational and theoretical studies in related topics. We also developed a new instrument, Camera for Quasars at Early Universe (CQUEAN) in collaboration with Kyunghee University group, and have secured observing facilities such as UKIRT and McDonald 2.1m observatory. Our research highlights include results such as the discovery of high redshift quasars and gamma ray bursts, the discovery of tidal disruption event at z=0.38 and peculiar gamma ray burst events, analysis of proto-clusters of galaxies, the discovery of brown dwarfs, and development of CQUEAN and its usage at the McDonald observatory.

• 천문학회보
• /
• 제42권2호
• /
• pp.74.2-74.2
• /
• 2017

Faint quasars are important to test the possibility that quasars are the main contributor to the cosmic reionization. However, it has been difficult to find faint quasars due to the lack of deep, wide-field imaging data. In this poster, we present our efforts to find faint quasars in the ELAIS-N1 field through the deep data (iAB ~ 25) obtained by the Subaru Hyper Suprime-Cam (HSC) Strategic Program survey. To select reliable quasar candidate, we also use the near-infrared (NIR) data of the Infrared Medium-deep Survey (IMS) and the UKIRT Infrared Deep Sky Survey (UKIDSS) - Deep Extragalactic Survey (DXS). Using multiple-band color cuts, we select high redshift quasar candidates. To confirm them as high redshift quasars, candidates are observed by the SED camera for QUasars in EArly uNiverse (SQUEAN) instrument in several medium band filters that can sample the redshifted Lyman break efficiency. The quasar sample will be used to study the growth of BH and stellar mass, the relation between the quasar activity and the host galaxy, and their contribution to the cosmic re-ionization.

• 천문학회보
• /
• 제41권1호
• /
• pp.37.3-38
• /
• 2016

Recent studies suggest that faint active galactic nuclei may be responsible for the reionization of the universe. Confirmation of this scenario requires spectroscopic identification of faint quasars ($M_{1450}$ > -24 mag) at z > 6, but only a very small number of such quasars have been spectroscopically identified so far. Here, we report the discovery of a faint quasar IMS J220417.92+011144.8 at z ~ 6 in a $12.5deg^2$ region of the SA22 field of the Infrared Medium-deep Survey (IMS). The spectrum of the quasar shows a sharp break at ${\sim}8443{\AA}$, with emission lines redshifted to $z=5.944{\pm}0.002$ and rest-frame ultraviolet continuum magnitude $M_{1450}=-23.59{\pm}0.10$ AB mag. The discovery of IMS J220417.92+011144.8 is consistent with the expected number of quasars at z ~6 estimated from quasar luminosity functions based on previous observations of spectroscopically identified low-luminosity quasars. This suggest that the number of $M_{1450}$ ~ -23 mag quasars at z ~ 6 may not be high enough to fully account for the reionization of the universe. In addition, our study demonstrates that faint quasars in the early universe can be identified effectively with a moderately wide and deep near-infrared survey such as the IMS.

• 천문학회보
• /
• 제35권1호
• /
• pp.64.2-64.2
• /
• 2010

From 2009, CEOU of Seoul National University has been utilizing the United Kingdom Infrared Telescope (UKIRT) at Mauna Kea, Hawaii, as one of its research facilities. UKIRT is a telescope with 3.8m primary mirror, and it is currently the largest telescope specialized for infrared observations. We will summarize our research activities using UKIRT, which include Infrared Medium-deep Survey (IMS) of proto-clusters and high redshift quasars, NIR imaging programs of Gamma Ray Bursts (GRBs), Gamma Ray sources, and SNUQSO quasars. Our research programs include international collaboration with the UK GRB team, the NASA/Swift team, Pomona College, and National Central University of Taiwan. We will also touch on our future plan of using UKIRT.

• 천문학회보
• /
• 제40권1호
• /
• pp.51.4-52
• /
• 2015

From 2010 to 2014, CQUEAN (Camera for QUasars in EArly uNiverse) has been operated for the observation at the 82 inch Otto Struve Telescope of the McDonald Observatory, US. This camera is optimized at wavelength range of 0.7 - 1.1 um with seven (g', r', I', z', Y, Iz and Is) broad-band filters for the survey of high redshift (z > 5) quasars in the early universe. We are upgrading this system to identify more details of SED (Spectral Energy Distribution) of quasar candidates and other astronomical sources. The SQUEAN is comprised of a focal reducer, a CCD camera, a new filter wheel, new auto guiding system and new control software. The new filter wheel consists of interchangeable cartridges for various wavelength and size of filters. 50 nm medium bandwidth filters from 600 - 1050 nm, seven SDSS (Sloan Digital Sky Survey) filters and Johnson-Cousin BVRI filters are installed for now. We also have a plan to use narrow band interference filters to classify high redshift quasars or to obtain SEDs of interesting astronomical sources in details more efficiently. We also developed KAP82 (Kyung Hee University Auto guiding Package for 82 inch telescope) for auto guiding software. CQUEAN and SQUEAN have been developed by CEOU (Center for the Exploration of the Origin of the Universe).

• 천문학논총
• /
• 제32권1호
• /
• pp.163-167
• /
• 2017

Utilizing a unique capability of AKARI that allows deep spectroscopy at $2.5-5.0{\mu}m$, we performed a spectroscopy study of more than 200 quasars through one of the AKARI mission programs, QSONG (Quasar Spectroscopic Observation with NIR Grism). QSONG targeted 155 high redshift (3.3 < z < 6.42) quasars and 90 low redshift active galactic nuclei (0.002 < z < 0.48). In order to provide black hole mass estimates based on the rest-frame optical spectra, the high redshift part of QSONG is designed to detect the $H{\alpha}$ line and the rest-frame optical spectra of quasars at z > 3.3. The low redshift part of QSONG is geared to uncover the rest-frame $2.5-5.0{\mu}m$ spectral features of active galactic nuclei to gain useful information such as the dust-extinction-free black hole mass estimators based on the Brackett lines and the temperatures of the hot dust torus. We outline the program strategy, and present some of the scientific highlights from QSONG, including the detection of the $H{\alpha}$ line from a quasar at z > 4.5 which indicates a rigorous growth of black holes in the early universe, and the $Br{\beta}$-based black hole mass estimators and the hot dust temperatures (~ 1100 K) of low redshift AGNs.