Y. Taniguchi

Subaru Deep Survey. IV. Discovery of a Large-Scale Structure at Redshift 5

We report the discovery of a large-scale structure of Lyα emitters (LAEs) at z = 4.86 based on wide-field imaging with the prime-focus camera (Suprime-Cam) on the Subaru Telescope. We observed a 25 × 45 area of the Subaru Deep Field in a narrow band (NB711, λc = 7126 A and FWHM = 73 A) together with R and i. We isolate from these data 43 LAE candidates down to NB711 = 25.5 mag using color criteria. Follow-up spectroscopy of five candidates suggests the contamination by low-z objects to be ~20%. We find that the LAE candidates are clustered in an elongated region on the sky of 20 Mpc in width and 50 Mpc in length at z = 4.86, which is comparable in size to present-day large-scale structures (we adopt H0 = 70 km s-1 Mpc-1, Ω0 = 0.3, and λ0 = 0.7). This elongated region includes a circular region of 12 Mpc radius of higher surface overdensity (δΣ = 2), which may be the progenitor of a cluster of galaxies. Assuming this circular region to be a sphere with a spatial overdensity of 2, we compare our observation with predictions by cold dark matter models. We find that an Ω0 = 0.3 flat model with σ8 = 0.9 predicts the number of such spheres consistent with the observed number (one sphere in our survey volume) if the bias parameter of LAEs is b 6. This value suggests that the typical mass of dark halos hosting LAEs at z 5 is of the order of 1012 M☉. Such a large mass poses an interesting question about the nature of LAEs.We report the discovery of a large-scale structure of Lyman alpha emitters (LAEs) at z=4.86 based on wide-field imaging with the prime-focus camera (Suprime-Cam) on the Subaru telescope. We observed a 25 x 45 area of the Subaru Deep Field in a narrow band (NB, lambdaC=7126 A and FWHM=73 A) together with R and i. We isolate from these data 43 LAE candidates down to NB=25.5 mag using color criteria. Follow-up spectroscopy of five candidates suggests the contamination by low-z objects to be ~ 20%. We find that the LAE candidates are clustered in an elongated region on the sky of 20 Mpc in width and 50 Mpc in length at z=4.86, which is comparable in size to present-day large-scale structures (we adopt H0=70 km/s/Mpc, Omega0=0.3, lambda0=0.7). This elongated region includes a circular region of 12 Mpc radius of higher surface overdensity (delta=2), which may be the progenitor of a cluster of galaxies. Assuming this circular region to be a sphere with a spatial overdensity of 2, we compare our observation with predictions by Cold Dark Matter models. We find that an Omega0=0.3 flat model with sigma8=0.9 predicts the number of such spheres consistent with the observed number (one sphere in our survey volume) if the bias parameter of LAEs is b simeq 6. This value suggests that the typical mass of dark haloes hosting LAEs at z simeq 5 is of the order of 10^12 Msolar. Such a large mass poses an interesting question about the nature of LAEs.

A Strong Lyα Emitter at z = 6.33 in the Subaru Deep Field Selected as an i'-Dropout*

We report on the discovery of a star-forming galaxy at z = 6.33 in the Subaru Deep Field. This object is selected as a candidate of an i-dropout high-redshift galaxy around z = 6 because of its red i - z color in our deep optical imaging survey in the Subaru Deep Field. Our follow-up optical spectroscopy reveals that this object is a strong Ly? emitter with only very faint ultraviolet continuum. The rest-frame equivalent width of the detected Ly? emission is as much as 130 ?. Thus the light detected in our z image is largely attributed to the Ly? emission, i.e., ~40% of the z-band flux is the strong Ly? emission, giving a very red i - z color. This is consistent with the photometric property of this object because the narrowband data obtained with the NB921 filter show a significant depression, z - NB921 = -0.54 mag. By using the photometric data, we show that some other objects among the 48 i-dropout high-redshift galaxy candidates found in the Subaru Deep Field also show a significant NB921 depression. We briefly discuss the nature of these NB921-depressed objects.

Cosmic star formation history associated with quasar activity: An approach using the black hole-to-bulge mass correlation

The tight correlation between the masses of central black holes and their host spheroids in nearby galaxies and active galactic nuclei (AGNs) suggests that black hole growth is closely related to their spheroid formation. Based on our previous work regarding such a joint evolutionary scheme and the consequential black hole-to-bulge mass correlation, we use the X-ray luminosity function of AGN and the cosmological evolution rate, which are from ROSAT X-ray surveys, to estimate the cosmic star formation history associated with the black hole growth. By the basic assumption that the major black hole growth occurs during the luminous AGN phase, the luminosity function of AGNs as a function of redshift traces not only the accretion history of the black holes but also the cosmic star formation history of the spheroids. Although the space density of the especially luminous quasars is very low, we show that the total amount of star formation associated with the massive black hole growth is almost the same as that of Lyman break galaxies detected by the current optical deep surveys. We thus argue that the optical deep surveys may miss about half of the net star formation in our universe. This is probably due in part to significant dust extinction, as well as the small field of view of previous optical surveys, which cannot sample such rare events with relatively short timescale. However, the far-infrared emission from the dust heated by ongoing star formation during the black hole growth could sufficiently account for the observed SCUBA number counts and would be the probable dominating energy source of the SCUBA population.

Cryogenic VPH grisms for MOIRCS

We present the development and first astronomical applications of VPH grisms which are now operated at cryogenic temperature in MOIRCS, a Cassegrain near-infrared instrument of the Subaru Telescope. We designed and fabricated the VPH grisms with a resolving power ~3000 for the use in near-infrared bands. The VPH grating, encapsulated in BK7 glass, is glued between two ZnSe prisms with vertex angle of 20 deg. After repeating several thermal cycles down to ~100 K carefully enough not to cause irreparable damage on the grism during cooling, we evaluated the performance at cryogenic temperature in the laboratory and found no deterioration and no large difference in the performance from that measured in room temperature. Based on commissioning observations with MOIRCS, we have confirmed the high efficiency (~0.8) and the resolving power of the original design. Common use of the grisms is due to start in the second semester of 2008.

A 40-cm infrared telescope in Antarctica

In Antarctica the cold and dry air is expected to provide the best observing conditions on the Earth for astronomical observations from infra-red to sub-millimeter. To enjoy the advantages in Antarctica, we have a plan to make astronomical observations at Dome Fuji, which is located at inland Antarctica. However, the harsh environment is very problematic. For example, the temperature comes down to as low as-80 degree Celsius in winter, where instruments designed for temperate environment would not work. In this context, we have developed a 40 cm infra-red telescope, which is dedicated for the use even in winter at Dome Fuji. In designing the telescope, we took account of the difference of the thermal expansion rate among materials, which were used for the telescope. Movable parts like motors were lubricated with grease which would be effective at -80 degrees. Most parts of the telescope are made of aluminum to make the telescope as light as possible, so that it makes the transportation from seacoast to inland and assembling at Dome Fuji easier. We also report the experiment that we have done at Rikubetsu (the coldest city in Japan) in February 2008.

X-raying cosmic star formation history

Based on our previous work about a co-evolution of massive black holes (MBH) and their host spheroids, we estimate the cosmic star formation history associated with AGN accretion by ROSAT X-ray All sky surveys. We show: 1) the total amount of star formation associated with MBH growth is at least half of the net star formation at high redshift, which probably totally missed by the current UV/optical deep surveys; 2) the FIR emission from the dust heated by star formation on-going during the MBH growth could be a significant energy source for SCUBA populations; 3)the peak redshift of the massive spheroid formation in this case is around 1.5, not necessary to be at much higher redshift.

Efficient Star-Formation in the Tidal Arms of the Stephan’s Quintet Group of Galaxies

NGC 7318B in Stephan’s Quintet has two optical arms (toward N and S) emanating from the eastern part of the main body. Since these arms are similar morphologically to the tidal tails of merging galaxies such as NGC 4038/9, it is considered that NGC 7318B itself is a major merger with a retrograde orbit. In order to study the emission-line activity in the tidal arms of NGC 7318B, we took CCD narrow-band (Ha ON and OFF) images and then found a large-scale arc in Ha emission which traces closely the arms. This Ha arc resembles both the radio and the soft X-ray arcs morphologically (van der Hulst & Rots 1981; Pietsch et al. 1997), suggesting that a single physical mechanism is responsible for all of these kinds of emission. Our optical spectroscopic observations of the shell-like feature at the southern tip of the arc reveal both broad Ha emission and stronger-than-normal [NII] and [SII] emission lines, which are typical of supernova remnants (SNRs). The required number of SNRs is estimated to be as much as ∼ 106.

Nuclear Activity in the Hickson Compact Groups of Galaxies

In order to study environmental effects on the nuclear activity in galaxies, we have been conducting a spectroscopic study of Hickson Compact Groups of galaxies (HCGs, Hickson 1982) which are the densest agglomeration of galaxies. We obtained nuclear spectra of 62 galaxies in 29 HCGs in the spectral range 6200–7000A with the 188cm telescope at Okayama Astrophysical Observatory. These spectra were classified into the three types by using the emission line ratio [NII]λ6583/Hα; (1) AGN: [NII]λ6583/Hα >0.6, (2) HII nuclei: [NII]λ6583/Hα <0.6, and (3) Absorption: no emission line. We compared the nuclear activity of galaxies in HCGs with that of nearby galaxies (Ho 1996; Ho, Filippenko & Sargent 1997) which provides a representative sample of field galaxies. In early-type spirals (Sa-Sbc), the fraction of HII nuclei in HCGs is smaller than that in the field galaxies, while the fraction of absorption in HCGs is larger than that in field galaxies. On the other hand, in early-type galaxies (E-SOa) and late-type spirals (Sc-P), we found little difference in the nuclear activity between HCGs and field galaxies.

A K-Band Luminosity Function of Hickson Compact Groups of Galaxies

Figure 1. A K-band luminosity function of Hickson Compact Groups of Galaxies. The data were obtained at the University of Hawaii Planetary Patrol 24 inch telescope with a NICMOS3 camera. The K-band luminosity function of field ellipticals are derived from the data in Faber et al. (1989, ApJS, 69, 763). We adopted 4.19 mag. as (B – K)0 T of ellipticals (as estimated from Govazzi & Boselli 1996, Astro. Lett. & Comm., 35, 1)