32 research outputs found
Spectroscopy of i-Dropout Galaxies with an NB921-Band Depression in the Subaru Deep Field
We report new spectroscopy of two star-forming galaxies with strong Ly_alpha
emission at z=6.03 and z=6.04 in the Subaru Deep Field. These two objects are
originally selected as i'-dropouts (i'-z' > 1.5) showing an interesting
photometric property, the ``NB921 depression''. The NB921-band (centered at
9196A) magnitude is significantly depressed with respect to the z'-band
magnitude. The optical spectra of these two objects exhibit asymmetric
emission-lines at lambda_obs ~ 8540A and ~ 8560A, suggesting that these objects
are Ly_alpha emitters at z~6. The rest-frame equivalent widths of the Ly_alpha
emission of the two objects are 94A and 236A; the latter one is the Ly_alpha
emitter with the largest Ly_alpha equivalent width at z > 6 ever
spectroscopically confirmed. The spectroscopically measured Ly_alpha fluxes of
these two objects are consistent with the interpretation that the NB921
depression is caused by the contribution of the strong Ly_alpha emission to the
z'-band flux. Most of the NB921-depressed i'-dropout objects are thought to be
strong Ly_alpha emitters at 6.0 < z < 6.5; Galactic L and T dwarfs and
NB921-dropout galaxies at z > 6.6 do not dominate the NB921-depressed
i'-dropout sample. Thus the NB921-depression method is very useful for finding
high-z Ly_alpha emitters with a large Ly_alpha equivalent width over a large
redshift range, 6.0 < z < 6.5. Although the broadband-selected sample at z ~ 3
contains only a small fraction of objects with a Ly_alpha equivalent width
larger than 100A, the i'-dropout sample of the Subaru Deep Field contains a
much larger fraction of such strong Ly_alpha emitters. This may imply a strong
evolution of the Ly_alpha equivalent width from z > 6 to z ~ 3.Comment: 21 pages, 6 figures, to appear in The Astrophysical Journa
An Observational Pursuit for Population III Stars in a Ly_alpha Emitter at z=6.33 through HeII Emission
We present a very deep near-infrared spectroscopic observation of a strong
Ly_alpha emitter at z=6.33, SDF J132440.6+273607, which we used to search for
HeII 1640. This emission line is expected if the target hosts a significant
number of population III stars. Even after 42 ksec of integration with the
Subaru/OHS spectrograph, no emission-line features are detected in the JH band,
which confirms that SDF J132440.6+273607 is neither an active galactic nucleus
nor a low- emission-line object. We obtained a 2sigma upper-limit of
9.06e-18 ergs/s/cm^2 on the HeII 1640 emission line flux, which corresponds to
a luminosity of 4.11e42 ergs/s. This upper-limit on the HeII 1640 luminosity
implies that the upper limit on population III star-formation rate is in the
range 4.9--41.2 M_sun/yr if population III stars suffer no mass loss, and in
the range 1.8--13.2 M_sun/yr if strong mass loss is present. The non-detection
of HeII in SDF J132440.6+273607 at z=6.33 may thus disfavor weak feedback
models for population III stars.Comment: 13 pages, 3 figures, to appear in The Astrophysical Journal Letter
The Quasar-LBG Two-point Angular Cross-correlation Function at z ~ 4 in the COSMOS Field
In order to investigate the origin of quasars, we estimate the bias factor
for low-luminosity quasars at high redshift for the first time. In this study,
we use the two-point angular cross-correlation function (CCF) for both
low-luminosity quasars at and Lyman-break galaxies
(LBGs). Our sample consists of both 25 low-luminosity quasars (16 objects are
spectroscopically confirmed low-luminosity quasars) in the redshift range
and 835 color-selected LBGs with at
in the COSMOS field. We have made our analysis for the following two
quasar samples; (1) the spectroscopic sample (the 16 quasars confirmed by
spectroscopy), and (2) the total sample (the 25 quasars including 9 quasars
with photometric redshifts). The bias factor for low-luminosity quasars at
is derived by utilizing the quasar-LBG CCF and the LBG
auto-correlation function. We then obtain the upper limits of the bias
factors for low-luminosity quasars, that are 5.63 and 10.50 for the total and
the spectroscopic samples, respectively. These bias factors correspond to the
typical dark matter halo masses, log
and , respectively. This result is not inconsistent with the predicted
bias for quasars which is estimated by the major merger models.Comment: 13 pages, 9 figures, Accepted for publication in Ap
Chemical properties in the most distant radio galaxy
We present a deep optical spectrum of TN J0924-2201, the most distant radio
galaxy at z = 5.19, obtained with FOCAS on the Subaru Telescope. We
successfully detect, for the first time, the CIV1549 emission line from the
narrow-line region (NLR). In addition to the emission-line fluxes of Ly alpha
and CIV, we set upper limits on the NV and HeII emissions. We use these line
detections and upper limits to constrain the chemical properties of TN
J0924-2201. By comparing the observed emission-line flux ratios with
photoionization models, we infer that the carbon-to-oxygen relative abundance
is already [C/O] > -0.5 at a cosmic age of ~ 1.1 Gyr. This lower limit on [C/O]
is higher than the ratio expected at the earliest phases of the galaxy chemical
evolution, indicating that TN J0924-2201 has already experienced significant
chemical evolution at z = 5.19.Comment: Accepted for publication in A&A, 5 pages, 3 figure
The End of the Reionization Epoch Probed by Ly-alpha Emitters at z=6.5 in the Subaru Deep Field
We report an extensive search for Lyman-alpha emitters (LAEs) at z=6.5 in the
Subaru Deep Field. Subsequent spectroscopy with Subaru and Keck identified
eight more LAEs, giving a total of 17 spectroscopically confirmed LAEs at
z=6.5. Based on this spectroscopic sample of 17, complemented by a photometric
sample of 58 LAEs, we have derived a more accurate Lyman-alpha luminosity
function of LAEs at z=6.5, which reveals an apparent deficit at the bright end
of ~0.75 mag fainter L*, compared with that observed at z=5.7. The difference
in the LAE luminosity functions between z=5.7 and 6.5 is significant at the
3-sigma level, which is reduced to 2-sigma when cosmic variance is taken into
account. This result may imply that the reionization of the universe has not
been completed at z=6.5. We found that the spatial distribution of LAEs at
z=6.5 was homogeneous over the field. We discuss the implications of these
results for the reionization of the universe.Comment: To appear in APJ vol.648. Only minor corrections have been made.
Black&White version is available at
http://zone.mtk.nao.ac.jp/~kashik/sdf/z6p5lae/paper/sdf_z6p5lae_bw.pd
Subaru Studies of the Cosmic Dawn
An overview on the current status of the census of the early universe
population is given. Observational surveys of high redshift objects provide
direct opportunities to study the early epoch of the Universe. The target
population included are Lyman Alpha Emitters (LAE), Lyman Break Galaxies (LBG),
gravitationally lensed galaxies, quasars and gamma-ray bursts (GRB). The basic
properties of these objects and the methods used to study them are reviewed.
The present paper highlights the fact that the Subaru Telescope group made
significant contributions in this field of science to elucidate the epoch of
the cosmic dawn and to improve the understanding of how and when infant
galaxies evolve into mature ones.Comment: 14 pages, 11 figures, accepted for publication in the Proceedings of
the Japan Academy, Series
Constraints on the faint end of the quasar luminosity function at z~5 in the COSMOS field
We present the result of our low-luminosity quasar survey in the redshift
range of 4.5 < z < 5.5 in the COSMOS field. Using the COSMOS photometric
catalog, we selected 15 quasar candidates with 22 < i' < 24 at z~5, that are ~
3 mag fainter than the SDSS quasars in the same redshift range. We obtained
optical spectra for 14 of the 15 candidates using FOCAS on the Subaru Telescope
and did not identify any low-luminosity type-1 quasars at z~5 while a
low-luminosity type-2 quasar at z~5.07 was discovered. In order to constrain
the faint end of the quasar luminosity function at z~5, we calculated the
1sigma confidence upper limits of the space density of type-1 quasars. As a
result, the 1sigma confidence upper limits on the quasar space density are Phi<
1.33*10^{-7} Mpc^{-3} mag^{-1} for -24.52 < M_{1450} < -23.52 and Phi<
2.88*10^{-7} Mpc^{-3} mag^{-1} for -23.52 < M_{1450} < -22.52. The inferred
1sigma confidence upper limits of the space density are then used to provide
constrains on the faint-end slope and the break absolute magnitude of the
quasar luminosity function at z~5. We find that the quasar space density
decreases gradually as a function of redshift at low luminosity (M_{1450} ~
-23), being similar to the trend found for quasars with high luminosity
(M_{1450}<-26). This result is consistent with the so-called downsizing
evolution of quasars seen at lower redshifts.Comment: 8 pages, 9 figures, 1 table, accepted for publication in Ap
The [OII]3727 Luminosity function and Star Formation Rate at z~1.2 in the COSMOS 2 Square-degree Field and the Subaru Deep Field
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
Issu