Shunji S. Sasaki

COSMOS PHOTOMETRIC REDSHIFTS WITH 30-BANDS FOR 2-deg2

We present accurate photometric redshifts in the 2-deg2 COSMOS field. The redshifts are computed with 30 broad, intermediate, and narrow bands covering the UV (GALEX), Visible-NIR (Subaru, CFHT, UKIRT and NOAO) and mid-IR (Spitzer/IRAC). A chi2 template-fitting method (Le Phare) was used and calibrated with large spectroscopic samples from VLT-VIMOS and Keck-DEIMOS. We develop and implement a new method which accounts for the contributions from emission lines (OII, Hbeta, Halpha and Ly) to the spectral energy distributions (SEDs). The treatment of emission lines improves the photo-z accuracy by a factor of 2.5. Comparison of the derived photo-z with 4148 spectroscopic redshifts (i.e. Delta z = zs - zp) indicates a dispersion of sigma_{Delta z/(1+zs)}=0.007 at i<22.5, a factor of 2-6 times more accurate than earlier photo-z in the COSMOS, CFHTLS and COMBO-17 survey fields. At fainter magnitudes i<24 and z<1.25, the accuracy is sigma_{Delta z/(1+zs)}=0.012. The deep NIR and IRAC coverage enables the photo-z to be extended to z~2 albeit with a lower accuracy (sigma_{Delta z/(1+zs)}=0.06 at i~24). The redshift distribution of large magnitude-selected samples is derived and the median redshift is found to range from z=0.66 at 22

Stellar and Total Baryon Mass Fractions in Groups and Clusters Since Redshift 1

We investigate if the discrepancy between estimates of the total baryon mass fraction obtained from observations of the cosmic microwave background (CMB) and of galaxy groups/clusters persists when a large sample of groups is considered. To this purpose, 91 candidate X-ray groups/poor clusters at redshift 0.1 ≤ z ≤ 1 are selected from the COSMOS 2 deg^2 survey, based only on their X-ray luminosity and extent. This sample is complemented by 27 nearby clusters with a robust, analogous determination of the total and stellar mass inside R_(500). The total sample of 118 groups and clusters with z ≤ 1 spans a range in M_(500) of ~10^(13)-10^(15) M_☉. We find that the stellar mass fraction associated with galaxies at R_(500) decreases with increasing total mass as M^(–0.37 ± 0.04)_(500), independent of redshift. Estimating the total gas mass fraction from a recently derived, high-quality scaling relation, the total baryon mass fraction (f^(stars+gas)_(500) = f^(stars)_(500) + f^(gas)_(500)) is found to increase by ~25%, when M_(500) increases from = 5 × 10^(13) M_☉ to = 7 × 10^(14) M_☉. After consideration of a plausible contribution due to intracluster light (11%-22% of the total stellar mass) and gas depletion through the hierarchical assembly process (10% of the gas mass), the estimated values of the total baryon mass fraction are still lower than the latest CMB measure of the same quantity (WMAP5), at a significance level of 3.3σ for groups of = 5 × 10^(13) M_☉. The discrepancy decreases toward higher total masses, such that it is 1σ at = 7 × 10^(14) M_☉. We discuss this result in terms of nongravitational processes such as feedback and filamentary heating.

COSMOS Morphological Classification with the Zurich Estimator of Structural Types (ZEST) and the Evolution Since z = 1 of the Luminosity Function of Early, Disk, and Irregular Galaxies

Motivated by the desire to reliably and automatically classify structure of thousands of COSMOS galaxies, we present ZEST, the Zurich Estimator of Structural Types. To classify galaxy structure, ZEST uses (1) five nonparametric diagnostics: asymmetry, concentration, Gini coefficient, second-order moment of the brightest 20% of galaxy pixels, and ellipticity; and (2) the exponent n of single-Sersic fits to the two-dimensional surface brightness distributions. To fully exploit the wealth of information while reducing the redundancy present in these diagnostics, ZEST performs a principal component (PC) analysis. We use a sample of ~56,000 I_(AB) ≤ 24 COSMOS galaxies to show that the first three PCs fully describe the key aspects of the galaxy structure, i.e., to calibrate a three-dimensional classification grid of axes PC_1, PC_2, and PC_3. We demonstrate the robustness of the ZEST grid on the z = 0 sample of Frei et al. The ZEST classification breaks most of the degeneracy between different galaxy populations that affects morphological classifications based on only some of the diagnostics included in ZEST. As a first application, we present the evolution since z ~ 1 of the luminosity functions (LFs) of COSMOS galaxies of early, disk, and irregular galaxies and, for disk galaxies, of different bulge-to-disk ratios. Overall, we find that the LF up to a redshift z = 1 is consistent with a pure luminosity evolution (of about 0.95 mag at z ~ 0.7). We highlight, however, two trends that are in general agreement with a downsizing scenario for galaxy formation, i.e., (1) a deficit of a factor of about 2 at z ~ 0.7 of M_B > -20.5 structurally classified early-type galaxies and (2) an excess of a factor of about 3, at a similar redshift, of irregular galaxies.

Large structures and galaxy evolution in COSMOS at z < 1.1

We present the first identification of large-scale structures (LSSs) at z 2), compared to the lower mass galaxies there. Over the range z < 1.1, we do not see evolution in the mass of galaxies by more than a factor of ~2 separating active and inactive star-forming galaxy populations.

Evolution of the Frequency of Luminous (?LV) Close Galaxy Pairs at z < 1.2 in the COSMOS Field

We measure the fraction of luminous galaxies in pairs at projected separations of 5-20 kpc out to z = 1.2 in the Cosmic Evolution Survey (COSMOS) field using ACS images and photometric redshifts derived from an extensive multiwavelength data set. Analysis of a complete sample of 106,188 galaxies more luminous than MV = -19.8 (~ LV) in the redshift range 0.1 < z < 1.2 yields 1749 galaxy pairs. These data are supplemented by a local (z = 0-0.1) value for the galaxy pair fraction derived from the Sloan Digital Sky Survey. After statistically correcting the COSMOS pair sample for chance line-of-sight superpositions, the evolution in the pair fraction is fit by a power law (1 + z)n=3.1±0.1. If this strongly evolving pair fraction continues out to higher redshift, ~50% of all luminous galaxies at z ~ 2 are in close pairs. This clearly signifies that galaxy mergers are a very significant and possibly dominant mechanism for galaxy evolution during the epoch of galaxy formation at z = 1-3.

A Photometric Survey for Lyα-He II Dual Emitters: Searching for Population III Stars in High-Redshift Galaxies

We present a new photometric search for high-z galaxies hosting Population III (Pop III) stars based on deep intermediate-band imaging observations obtained in the Subaru Deep Field (SDF), using Suprime-Cam on the Subaru Telescope. By combining our new data with the existing broadband and narrowband data, we searched for galaxies that emit strongly in both Lyα and He II λ1640 (dual emitters) and are promising candidates for Pop III-hosting galaxies at 3.93 z 4.01 and 4.57 z 4.65. Although we found 10 dual emitters, most of them turn out to be [O II]-[O III] dual emitters or Hβ-(Hα+[N II]) dual emitters at z < 1, as inferred from their broadband colors and from the ratio of the equivalent widths. No convincing candidate Lyα-He II dual emitter of SFRPopIII 2 M☉ yr−1 was found by our photometric search in 4.03 × 105 Mpc3 in the SDF. This result disfavors low-feedback models for Pop III star clusters and implies an upper limit on the Pop III SFR density of SFRDPopIII < 5 × 10−6 M☉ yr−1 Mpc−3. This new selection method to search for Pop III-hosting galaxies should be useful in future narrowband surveys to achieve the first observational detection of Pop III-hosting galaxies at high redshifts.

Narrowband Survey of the GOODS Fields: Search for Lyα Emitters at z = 5.7

We present results from optical narrowband (λc = 8150 A and Δλ = 120 A) observations of the Great Observatories Origins Deep Survey (GOODS) fields, using Suprime-Cam on the Subaru Telescope. Using these narrowband data, we then perform a survey of Lyα emitters (LAEs) at z ~ 5.7. The LAE survey covers an area of ≈320 arcmin2 and a comoving volume of 8.0 × 104 Mpc3. We found a total of 10 (GOODS-N) and 4 (GOODS-S) LAE candidates at z ~ 5.7. We perform a study of the spatial distribution, space density, and star formation properties of the LAEs at z ~ 5.7.

ENVIRONMENTAL EFFECTS ON THE STAR FORMATION ACTIVITY IN GALAXIES AT z ≃ 1.2 IN THE COSMOS FIELD*

We investigate the relation between the star formation activity in galaxies and environment at z ≃ 1.2 in the Cosmic Evolution Survey field, using the fraction of [O II] emitters and the local galaxy density. The fraction of [O II] emitters appears to be almost constant over the surface density of galaxies between 0.2 and 10 Mpc^(–2). This trend is different from that seen in the local universe where the star formation activity is weaker in higher density regions. To understand this difference between z ~ 1 and z ~ 0, we study the fraction of non-isolated galaxies as a function of local galaxy density. We find that the fraction of non-isolated galaxies increases with increasing density. Our results suggest that the star formation in galaxies at z ~ 1 is triggered by galaxy interaction and/or mergers.

High-redshift Ly

We report new follow-up spectroscopy of i′-dropout galaxies with an NB921-band depression found in the Subaru Deep Field. The NB921-depressed 1′-dropout selection method is expected to select galaxies with large equivalent-width Lyα emission over a wide redshift range, 6.0 ≲ z ≲ 6.5. Two of four observed targets show a strong emission line with a clear asymmetric profile, identified as Lyα emitters at z = 6.11 and 6.00. Their rest-frame equivalent widths are 153 A and 114 A, which are lower limits on the intrinsic equivalent widths. Through our spectroscopic observations (including previous ones) of NB921 -depressed i′-dropout galaxies, we identified 5 galaxies in total with a rest-frame equivalent width larger than 100 A at 6.0 ≲ z ≲ 6.5 out of 8 photometric candidates, which suggests that the NB921 -depressed i′-dropout selection method is possibly an efficient way to search for Lyα emitters with a large Lyα equivalent width, in a wider redshift range than with usual narrow-band excess techniques. By combining these findings with our previous observational results, we infer that the fraction of broad-band selected galaxies having a rest-frame equivalent width larger than 100 A is significantly higher at z ∼ 6 (the cosmic age of ∼1 Gyr) than that at z ∼ 3 (∼2 Gyr), being consistent with the idea that the typical stellar population of galaxies is significantly younger at z ∼ 6 than that at z ∼ 3. The NB921-depressed i′-dropout galaxies may be interesting candidates for hosts of massive, zero-metallicity Population III stars.

\mathsf{\alpha}

We have identified a complex galaxy cluster system in the COSMOS field via a wide-angle tail (WAT) radio galaxy, consistent with the idea that WAT galaxies can be used as tracers of clusters. The WAT galaxy, CWAT-01, is coincident with an elliptical galaxy resolved in the HST ACS image. Using the COSMOS multiwavelength data set, we derive the radio properties of CWAT-01 and use the optical and X-ray data to investigate its host environment. The cluster hosting CWAT-01 is part of a larger assembly consisting of a minimum of four X-ray luminous clusters within ~2 Mpc distance. We apply hydrodynamic models that combine ram pressure and buoyancy forces on CWAT-01. These models explain the shape of the radio jets only if the galaxys velocity relative to the intracluster medium (ICM) is in the range of about 300-550 km s^(-1), which is higher than expected for brightest cluster galaxies (BCGs) in relaxed systems. This indicates that the CWAT-01 host cluster is not relaxed but is possibly dynamically young. We argue that such a velocity could have been induced through subcluster merger within the CWAT-01 parent cluster and/or cluster-cluster interactions. Our results strongly indicate that we are witnessing the formation of a large cluster from an assembly of multiple clusters, consistent with the hierarchical scenario of structure formation. We estimate the total mass of the final cluster to be approximately 20% of the mass of the Coma Cluster.