**TITLE** ASP Conference Series, Vol. **VOLUME**, **PUBLICATION YEAR** **EDITORS** AGN Population at 10−13 erg s−1 cm−2 : Results from Optical Identification of ASCA Surveys Masayuki Akiyama Subaru Telescope, NAOJ, 650 North A’ohoku Place, Hilo, HI, 96720, 1 U.S.A. 0 0 Abstract. In this paper, results of optical identification of ASCA sur- 2 veys are summarized. To understand luminous AGNs in the z ≤ 1 uni- n verse, the ASCA AGN sample is still better than samples of AGNs from a deepChandra orXMM-Newtonsurveys. Combiningtheidentifiedsample J of AGNs from ASCA Large Sky Survey and Medium Sensitivity Survey, 4 the sample of hard X-ray selected AGNs have been expanded up to ∼80 −13 −1 −2 1 AGNs above the flux limit of 10 erg s cm in the 2–10 keV band. v One fifth of the ASCA AGNs are absorbed narrow-line AGNs. The lumi- 1 45 −1 nosity distribution of the absorbed AGNs is limited below 10 erg s , 5 though there are 14 non-absorbed broad-line AGNs are detected above 0 1 the luminosity. The result suggests lack of absorbed luminous AGNs 0 (deficiency of type 2 QSO). 1 0 / h p 1. Introduction - o r A hard X-ray selection is one of good selection methods that can sample AGNs t s without bias against heavy absorption to the nucleus. ASCA is the firstsatellite a that is sensitive enough to detect AGNs beyond nearby universe in the hard : v X-ray band. Many optical spectroscopic-follow-up observations have been and Xi being done for X-ray sources detected in ASCA surveys that range from wide- area medium sensitivity surveys (e.g., Della Ceca et al. 2000) to deep surveys r a with a few pointings (e.g., Georgantopoulos et al. 1997). To understand lumi- nous AGNs in the z ≤ 1 universe, the ASCA AGN sample is still better than samples of AGNs from deep Chandra or XMM-Newton surveys (Figure 1). Here, the results of optical identification of the ASCA Large Sky Survey (hereafterLSS;Uedaetal. 1999a; Akiyamaetal. 2000a)andtheASCAMedium Sensitivity Survey (hereafter MSS; Ueda et al. 1999b) are summarized. An advantage of the two surveys is high completeness of the optical identification (33/34 for ASCA LSS and 51/55 for ASCA MSS, so far). The ASCA LSS covers 5 degree2 area near the north galactic pole down to 2×10−13 erg−1 cm−2 −1 s . The optical identification is done for 34 X-ray sources detected by SIS detector and 30 AGNs are identified. On the other hand, the ASCA MSS is a serendipitous survey of GIS pointing observations from 1993 to 1996. The original sample consists of 55 X-ray sources with flux more than 3×10−13 erg−1 −2 −1 cm s in the northern sky selected from 15 arcmin radius areas of 176 GIS field of views. The sample contains 48 AGNs so far. The optical identification of the ASCAMSSis now beingextended to X-ray sources detected in outer area 1 2 Masayuki Akiyama Figure 1. Redshift-luminosity distributions of hard X-ray selected AGNs. Smallcircles,largesquares,largecircles,andtrianglesrepresent sample of AGNs from the HEAO1 A2, ASCA LSS, ASCA MSS, and Chandra Surveys (in detail see text). Open symbols represent AGNs 22 −2 with X-ray absorption column densities less than NH of 10 cm (for ASCA LSS and HEAO1 A2 samples) and AGNs with a strong broad- line(s) (for ASCA MSS and Chandra samples). of GIS and in southern sky observations. Finally the ASCA MSS sample will contain more than 100 identified sources. 2. Deficiency of Type-2 QSOs A sample of hard X-ray selected AGNs enables us to estimate the fraction of absorbedAGNs. Theredshiftvs. luminosity distributionoftheidentified AGNs is shown in Figure 1. Squares and large circles represent ASCA LSS and MSS AGNs respectively. One fifth of identified AGNs are narrow-line (broad-line- 44 −1 weak)andabsorbedAGNs. Intheluminosityrangebelow10 ergs ,onethird ofAGNsareabsorbednarrow-lineAGNs. Consideringthefactthatobjectswith 22.5 −2 absorption column densities of up to NH of 10 cm can bedetected without bias using 2–10 keV emission, the fraction of absorbed narrow-line AGNs is consistent with the absorption column density distribution of nearby Seyfert galaxies (Risaliti et al. 1999). On the other hand, in the luminosity range above 45 −1 10 erg s , 14 broad-line AGNs are detected in total, but no narrow-line AGN is found. The result suggests lack of absorbed luminous AGNs (deficiency of type 2 QSOs). Similar tendencies also appear in the HEAO1 A2 sample (small circles in Figure 1; Piccinotti et al. 1982) and recent results of Chandra optical Optical Identification of ASCA Surveys 3 Figure 2. Fitted column density versus 2–10 keV luminosity dia- gram of the ASCA LSS AGNs. The X-ray luminosities are not cor- rected for the absorption. Narrow-line AGNs with X-ray absorption 22 −2 with hydrogen column density larger than 10 cm are marked with dots. Upper limits for column densities are indicated with downward arrows. identification (smalltriangles inFigure1; Hornschemeieretal. 2000; Mushotzky et al. 2000; Brandt et al. 2000; Fiore et al. 2000). Chandra AGNs with a strong broad-line(s) in their optical spectra or listed as a QSO are plotted with open symbols, and filled symbols represent other AGNs or galaxies. It should be noted that the optical identificaiton of Chandra sources is not complete at this moment and unidentified X-ray sources with red optical couterparts may be a missing population of type 2 QSOs. 3. X-ray Spectra of Hard X-ray Selected QSOs Some of broad-line z∼1 QSOs have harder X-ray spectra than typical X-ray spectra of QSOs (power-law with photon index of 1.7), although their optical colors, optical spectra, and X-ray to optical flux ratios are same as normal QSOs. In the ASCA LSS AGNs, four high-redshift broad-line QSOs show hard X-ray spectra with apparent photon indices of 1.3±0.3. The hard X-ray spectra may also be explained by absorption with log NH(cm−2)=22∼23 at the object’s redshift, if we assume an intrinsic photon index of 1.7 (Figure 2, it should be noted that redshifts and hard X-ray luminosities correlate each other in the ASCA LSS sample. Therefore, the hard X-ray luminosity of L2−−10 keV ∼ 1045 ergs−1 correspondstotheredshiftofz ∼ 1). SimilarhardeningsofX-rayspectra of high-redshiftbroad-lineQSOsarealso reportedinotherASCAsurveys(Della Ceca et al. 2000) and Beppo-SAX surveys (La Franca et al. 2000). Theorigin of the hardness is not clear yet. The hardness of the X-ray spectra of high-redshift 4 Masayuki Akiyama QSOsmaybeexplainedbytheexistenceofthereflectioncomponent. Becauseof a bump of the reflection component, observed photon indices of type 1 Seyferts in the 0.7–10 keV band is expected to get harder to z ∼ 2. 4. A Case of an Absorbed QSO at z=0.65 A few candidates of narrow-line-strong absorbed QSOs were found in the ASCA surveys, though the fraction of absorbed AGNs in the high luminosity range is not as high as that in the low luminosity range. AX J131831+3341 is a moderately absorbed (NH of 1022 cm−2) QSO at z=0.65 found in the ASCA LSS (Akiyama et al. 2000b, Akiyama et al. 2001). Its optical spectrum shows strong emission lines, such as broad Mg II 2800 ˚A, narrow [O II] 3727 ˚A, and narrow [O III] 5007 ˚A, but no broad Hβ emission line. Its small Hβ-to-[O III] 5007 ˚A equivalent width ratio [log(Hβ/[O III])= −0.54] is comparable to those of Seyfert 1.8–2 galaxies (Winkler 1992). The X-ray luminosity is estimated to 45 −1 be 10 erg s , which is as large as the luminosity of the knee of the AGN luminosity function in the 2–10 keV band at z ∼ 0.6 (e.g., Boyle et al. 1998), and corresponds to the luminosities of QSOs. The optical and near-infrared images show a nucleus and an extended host galaxy around the QSO (left panel in Figure 3). In the right panel of Figure 3, the R−I and I −K colors of nuclear and host galaxy components are shown. The nuclear component have blue R−I colors but red I −K color. The I−K color of the nuclear component is much redder than optically-selected QSOs at similar redshifts (open squares; Elvis et al. 1994). A possible explanation of these colors is that, in the K band, heavily absorbed (AV ∼ 3 mag) nucleus emerges, while optical light originates from scattered blue nuclear light. The estimated fraction of scattered light is 2%, which is similar to that observed in narrow-line radio galaxies (Alighieri et al. 1994). Such a red optical to near- infrared color and a blue optical color may be common characteristics of X-ray selected absorbed QSOs at intermediate redshifts. Recent observations reveal that another absorbed QSO at a redshift of 0.9, AX J08494+4454, also has a blueoptical color and a red optical to near-infrared color (R−I = 0.67 mag and I −K = 3.4 mag; Nakanishi et al. 2000; Akiyama et al. in preparation). Also, a significant fraction of the optical and near-infrared counterparts of Chandra hardX-ray sources show similar red near-infrared colors (I−HK′ =4 ∼ 5 mag) and blue optical colors (B −I = 1 ∼ 2 mag) (Mushotzky et al. 2000). Thehostgalaxyextends62kpcawayfromthenucleus. Theasymmetricand extended structureof thecomponentsuggests agalaxy interaction ormergingin thehostgalaxy. TheV −R,R−I, andI−K colors ofthehostgalaxy areconsis- tent with a stellar population with age of 1 Gyr. Thus, AX J131831+3341 may have a post-starburst galaxy as the host galaxy like a ”post-starburst quasar”, UN J1025-0040 (Brotherton et al. 1999). Acknowledgments. I would like to thank members of ASCA Large Sky Survey, and ASCA Medium Sensitivity Surveys, especially Yoshihiro Ueda, Ta- dayuki Takahashi, Kouji Ohta, and Toru Yamada, and Suprime-Cam team. Optical Identification of ASCA Surveys 5 10" = 78kpc Figure 3. Right) Optical R band image of AX J131831+3341 taken with the 8.2m Subaru telescope with integration time of 1800 s. Left) The R−I and I−K color-color diagram of the nuclear (filled square) and the host galaxy (filled circle) components of AX J131831+3341. The open squares represents colors of optically-selected QSOs at red- shifts between 0.2 and 1.5 from Elvis et al. (1994). Pentagons show colors of power-law model with indices (fν = να) of −1.0 (top) and 0.0 (bottom). 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