Search

We have exploited the new, deep, near-infrared UltraVISTA imaging of the COSMOS field, in tandem with deep optical and mid-infrared imaging, to conduct a new search for luminous galaxies at redshifts z ~ 7. The unique multi-wavelength dataset provided by VISTA, CFHT, Subaru, HST and Spitzer over a common area of 1 deg^2 has allowed us to select galaxy candidates at z > 6.5 by searching first for Y+J-detected ( 6.5 which we present in this paper. The first four of these appear to be robust galaxies at z > 6.5, and fitting to their stacked SED yields z = 6.98+-0.05 with a stellar mass M* = 5x10^9 Msun, and rest-frame UV spectral slope beta = -2.0+-0.2. The next three are also good candidates for z > 6.5 galaxies, but the possibility that they are low-redshift galaxies or dwarf stars cannot be excluded. Our final subset of three additional candidates is afflicted not only by potential dwarf-star contamination, but also contains objects likely to lie at redshifts just below z = 6.5. We show that the three even-brighter z > 7 galaxy candidates reported in the COSMOS field by Capak et al. (2011) in fact all lie at z ~ 1.5-3.5. Consequently the new z ~ 7 galaxies reported here are the first credible z ~ 7 Lyman-break galaxies discovered in the COSMOS field and, as the most UV-luminous discovered to date at these redshifts, are prime targets for deep follow-up spectroscopy. We explore their physical properties, and briefly consider the implications of their inferred number density for the form of the galaxy luminosity function at z = 7.

We investigate the effects of the environment on star-formation in a sample of massive luminous and ultra-luminous infrared galaxies (LIRGs and ULIRGs) with S(24 micron)>80 uJy and i+, Accepted for publication in ApJ. 8 pages, 8 figures

To derive a new H$\alpha$ luminosity function and to understand the clustering properties of star-forming galaxies at $z \approx 0.24$, we have made a narrow-band imaging survey for H$\alpha$ emitting galaxies in the HST COSMOS 2 square degree field. We used the narrow-band filter NB816 ($\lambda_c = 8150$ \AA, $\Delta \lambda = 120$ \AA) and sampled H$\alpha$ emitters with $EW_{\rm obs}(\rm H\alpha + [N\textsc{ii}]) > 12$ \AA in a redshift range between $z=0.233$ and $z=0.251$ corresponding to a depth of 70 Mpc. We obtained 980 H$\alpha$ emitting galaxies in a sky area of 5540 arcmin$^2$, corresponding to a survey volume of $3.1 \times 10^4 {\rm Mpc^3}$. We derive a H$\alpha$ luminosity function with a best-fit Schechter function parameter set of $\alpha = -1.35^{+0.11}_{-0.13}$, $\log\phi_* = -2.65^{+0.27}_{-0.38}$, and $\log L_* ({\rm erg s^{-1}}) = 41.94^{+0.38}_{-0.23}$. The H$\alpha$ luminosity density is $2.7^{+0.7}_{-0.6} \times 10^{39}$ ergs s$^{-1}$ Mpc$^{-3}$. After subtracting the AGN contribution (15 %) to the H$\alpha$ luminosity density, the star formation rate density is evaluated as $1.8^{+0.7}_{-0.4} \times 10^{-2}$ $M_{\sun}$ yr$^{-1}$ Mpc$^{-3}$. The angular two-point correlation function of H$\alpha$ emitting galaxies of $\log L({\rm H\alpha}) > 39.8$ is well fit by a power law form of $w(\theta) = 0.013^{+0.002}_{-0.001} \theta^{-0.88 \pm 0.03}$, corresponding to the correlation function of $\xi(r) = (r/1.9{\rm Mpc})^{-1.88}$. We also find that the H$\alpha$ emitters with higher H$\alpha$ luminosity are more strongly clustered than those with lower luminosity.

We investigated the fraction of [OII] emitters in galaxies at z~0.9 as a function of the local galaxy density in the Hubble Space Telescope (HST) COSMOS 2 square degree field. [OII] emitters are selected by the narrow-band excess technique with the NB711-band imaging data taken with Suprime-Cam on the Subaru telescope. We carefully selected 614 photo-z selected galaxies with M_U3500 < -19.31 at z=0.901-0.920, which includes 195 [OII] emitters, to directly compare results with our previous study at z~1.2. We found that the fraction is almost constant at 0.3 Mpc^-2 < Sigma_10th < 10 Mpc^-2. We also checked the fraction of galaxies with blue rest-frame colors of NUV-R < 2 in our photo-z selected sample, and found that the fraction of blue galaxies does not significantly depend on the local density. On the other hand, the semi-analytic model of galaxy formation predicted that the fraction of star-forming galaxies at z~0.9 decreases with increasing the projected galaxy density even if the effects of the projection and the photo-z error in our analysis were taken into account. The fraction of [OII] emitters decreases from ~60% at z~1.2 to ~30% at z~0.9 independent of the galaxy environment. The decrease of the [OII] emitter fraction could be explained mainly by the rapid decrease of the star formation activity in the universe from z~1.2 to z~0.9., 14 pages, 13 figures, 2 tables, Accepted for publication in ApJ

With this paper, we release accurate photometric redshifts for 1692 counterparts to Chandra sources in the central square degree of the COSMOS field. The availability of a large training set of spectroscopic redshifts that extends to faint magnitudes enabled photometric redshifts comparable to the highest quality results presently available for normal galaxies. We demonstrate that morphologically extended, faint X-ray sources without optical variability are more accurately described by a library of normal galaxies (corrected for emission lines) than by AGN-dominated templates, even if these sources have AGN-like X-ray luminosities. Preselecting the library on the bases of the source properties allowed us to reach an accuracy sigma_(Delta z/(1+z_spec)) \sim0.015 with a fraction of outliers of 5.8% for the entire Chandra-COSMOS sample. In addition, we release revised photometric redshifts for the 1735 optical counterparts of the XMM-detected sources over the entire 2 sq. deg.of COSMOS. For 248 sources, our updated photometric redshift differs from the previous release by Delta z>0.2. These changes are predominantly due to the inclusion of newly available deep H-band photometry H_AB=24 mag. We illustrate once again the importance of a spectroscopic training sample and how an assumption about the nature of a source together with the number and the depth of the available bands influence the accuracy of the photometric redshifts determined for AGN. These considerations should be kept in mind when defining the observational strategies of upcoming large surveys targeting AGN, such as eROSITA at X-ray energies and ASKAP/EMU in the radio band., ApJ, accepted for publication. Data also available at http://www.ipp.mpg.de/~msalv/PHOTOZ_XCOSMOS

We searched for quasars that are ~ 3 mag fainter than the SDSS quasars in the redshift range 3.7 < z < 4.7 in the COSMOS field to constrain the faint end of the quasar luminosity function. Using optical photometric data, we selected 31 quasar candidates with 22 < i' < 24 at z ~ 4. We obtained optical spectra for most of these candidates using FOCAS on the Subaru telescope, and identified 8 low-luminosity quasars at z ~ 4. In order to derive the quasar luminosity function (QLF) based on our spectroscopic follow-up campaign, we estimated the photometric completeness of our quasar survey through detailed Monte Carlo simulations. Our QLF at z ~ 4 has a much shallower faint-end slope beta = -1.67^{+0.11}_{-0.17} than that obtained by other recent surveys in the same redshift. Our result is consistent with the scenario of downsizing evolution of active galactic nuclei inferred by recent optical and X-ray quasar surveys at lower redshifts., 13 pages, 4 figure, Accepted for publication in ApJL

This is an erratum for the article 2009 ApJ 696 546.

We present results of a survey for Ly alpha emitters at z=4.86 based on optical narrowband (lambda_c=7126 angstrom, Delta lambda=73 angstrom) and broadband (B, V, r', i', and z') observations of the Cosmic Evolution Survey (COSMOS) field using Suprime-Cam on the Subaru Telescope. We find 79 LAE candidates at z=4.86 over a contiguous survey area of 1.83 deg^2, down to the Ly alpha line flux of 1.47 x 10^-17 ergs s^-1 cm^-2. We obtain the Ly alpha luminosity function with a best-fit Schechter parameters of log L^*=42.9^+0.5_-0.3 ergs s^-1 and phi^* = 1.2^+8.0_-1.1 x 10^-4 Mpc^-3 for alpha=-1.5 (fixed). The two-point correlation function for our LAE sample is xi(r) = (r/4.4^+5.7_-2.9 Mpc)^-1.90+/-0.22. In order to investigate the field-to-field variations of the properties of Ly alpha emitters, we divide the survey area into nine tiles of 0.5^circ x 0.5^circ each. We find that the number density varies with a factor of ~ 2 from field to field with high statistical significance. However, we find no significant field-to-field variance when we divide the field into four tiles with 0.7^circ x 0.7^circ each. We conclude that at least 0.5 deg^2 survey area is required to derive averaged properties of LAEs at z~5, and our survey field is wide enough to overcome the cosmic variance., Comment: 24 pages, 13 figures, 4 tables, uses emulateapj sty, accepted for publication in ApJ

International audience; We present detailed morphological properties of Lyα emitters (LAEs) at z ≈ 5.7 in the COSMOS field based on Hubble Space Telescope Advanced Camera for Surveys (ACS) data. The ACS imaging in the F814W filter covered 85 LAEs of the 119 LAEs identified in the full two square degree field, and 47 LAEs of them are detected in the ACS images. Nearly half of them are spatially extended with a size larger than 0.15 arcsec (~0.88 kpc at z = 5.7) and up to 0.4 arcsec (~2.5 kpc at z = 5.7). The others are nearly unresolved compact objects. Two LAEs show double-component structures indicating interaction or merging of building components to form more massive galaxies. By stacking the ACS images of all the detected sources, we obtain a Sersic parameter of n ~ 0.7 with a half-light radius of 0.13 arcsec (0.76 kpc), suggesting that the majority of ACS detected LAEs have not spheroidal-like but disk-like or irregular light profiles. Comparing ACS F814W magnitudes (I 814) with Subaru/Suprime-Cam magnitudes in the NB816, i', and z' bands, we find that the ACS imaging in the F814W band mainly probes UV continuum rather than Lyα line emission. UV continuum sizes tend to be larger for LAEs with larger Lyα emission regions as traced by the NB816 imaging. The nondetection of 38 LAEs in the ACS images is likely due to the fact that their surface brightness is too low both in the UV continuum and Lyα emission. Estimating I 814 for the ACS-undetected LAEs from the z' and NB816 magnitudes, we find that 16 of these are probably LAEs with a size larger than 0.15 arcsec in UV continuum. All these results suggest that our LAE sample contains systematically larger LAEs in UV continuum size than those previously studied at z ~ 6. Based on observations with NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555; and also based on data collected at Subaru Telescope, which is operated by the National Astronomical Observatory of Japan.

We have carried out a wide-field imaging survey for [OII]3727 emitting galaxies at z~1.2 in the HST COSMOS 2 square degree field using the Suprime-Cam on the Subaru Telescope. The survey covers a sky area of 6700 arcmin^2 in the COSMOS field, and a redshift range between 1.17 and 1.20 (Delta_z = 0.03), corresponding to a survey volume of 5.56*10^5 Mpc^3. We obtain a sample of 3176 [OII] emitting galaxies with observed emission-line equivalent widths greater than 26 AA. Since our survey tends to sample brighter [OII]3727 emitting galaxies, we also analyze a sample of fainter [OII]3727 emitting galaxies found in the Subaru Deep Field (SDF). We find an extinction-corrected [OII] luminosity density of 10^{40.35^+0.08_-0.06} ergs s^-1 Mpc-3, corresponding to star formation rate density of 0.32^+0.06_-0.04 M_sun yr-1 Mpc^-3 in the COSMOS field at z~1.2. This is the largest survey for [OII]3727 emitters beyond z=1 currently available., Comment: 33 pages, 9 figures. to appear in the ApJ Supplement COSMOS Special Issue

We present our new deep optical imaging and long-slit spectroscopy for Arp 220 that is the archetypical ULIRG in the local universe. Our sensitive Ha imaging has newly revealed large-scale, Ha absorption, i.e., post-starburst regions in this merger; one is found in the eastern superbubble and the other is in the two tidal tails that are clearly reveled in our deep optical imaging. The size of Ha absorption region in the eastern bubble is 5 kpc x 7.5 kpc and the observed Ha equivalent widths are ~2 A +- 0.2 A. The sizes of the northern and southern Ha-absorption tidal tails are ~5 kpc x 10 kpc and ~6 kpc x 20 kpc, respectively. The observed Ha equivalent widths range from 4 A to 7 A. In order to explain the presence of the two post-starburst tails, we suggest a possible multiple-merger scenario for Arp 220 in which two post-starburst disk-like structures merged into one, and then caused the two tails. This favors that Arp 220 is a multiple merging system composed of four or more galaxies, arising from a compact group of galaxies. Taking our new results into account, we discuss a star formation history in the last 1 Gyr in Arp 220., 6 pages, 7 figures, Accepted for publication in the Astrophysical Journal