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Kentaro Aoki

Publications and source records attributed to Kentaro Aoki.

At least 55 records · Page 3Linked to original sources
arXiv2008

No Evidence for Variability of Intervening Absorption Lines toward GRB 060206: Implications for the Mg II Incidence Problem

We examine variability of absorption line strength of intervening systems along the line of sight to GRB 060206 at $z = 4.05$, by the low-resolution optical spectra obtained by the Subaru telescope from six to ten hours after the burst. Strong variabilities of Fe\emissiontype{II} and Mg\emissiontype{II} lines at $z=1.48$ during $t=$5--8 hours have been reported for this GRB \citep{Hao07}, and this has been used to support the idea of clumpy Mg\emissiontype{II} cloudlets that was originally proposed to explain the anomalously high incidence of Mg\emissiontype{II} absorbers in GRB spectra compared with quasars . However, our spectra with higher signal-to-noise ratio do not show any evidence for variability in $t=$6-10 hours. There is a clear discrepancy between our data and Hao et al. data in the overlapping time interval. Furthermore, the line strengths in our data are in good agreement with those observed at $t \sim$ 2 hours by \citet{Tho08}. We also detected Fe\emissiontype{II} and Mg\emissiontype{II} absorption lines for a system at $z = 2.26$, and these lines do not show evidence for variability either. Therefore we conclude that there is no strong evidence for variability of intervening absorption lines toward GRB 060206, significantly weakening the support to the Mg\emissiontype{II} cloudlet hypothe sis by the GRB 060206 data.

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arXiv2008

The Stellar Populations of Lyman Break Galaxies at z~5

We present the results of Spectral Energy Distribution(SED) fitting analysis for Lyman Break Galaxies(LBGs) at z~5 in the GOODS-N and its flanking fields (the GOODS-FF). With the publicly available IRAC images in the GOODS-N and IRAC data in the GOODS-FF, we constructed the rest-frame UV to optical SEDs for a large sample (~100) of UV-selected galaxies at z~5. Comparing the observed SEDs with model SEDs generated with a population synthesis code, we derived a best-fit set of parameters (stellar mass, age, color excess, and star formation rate) for each of sample LBGs. The derived stellar masses range from 10^8 to 10^11M_sun with a median value of 4.1x10^9M_sun. The comparison with z=2-3 LBGs shows that the stellar masses of z~5 LBGs are systematically smaller by a factor of 3-4 than those of z=2-3 LBGs in a similar rest-frame UV luminosity range. The star formation ages are relatively younger than those of the z=2-3 LBGs. We also compared the results for our sample with other studies for the z=5-6 galaxies. Although there seem to be similarities and differences in the properties, we could not conclude its significance. We also derived a stellar mass function of our sample by correcting for incompletenesses. Although the number densities in the massive end are comparable to the theoretical predictions from semi-analytic models, the number densities in the low-mass part are smaller than the model predictions. By integrating the stellar mass function down to 10^8 M_sun, the stellar mass density at z~5 is calculated to be (0.7-2.4)x10^7M_sun Mpc^-3. The stellar mass density at z~5 is dominated by massive part of the stellar mass function. Compared with other observational studies and the model predictions, the mass density of our sample is consistent with general trend of the increase of the stellar mass density with time.

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arXiv2008

The Stellar Populations of Lyman Break Galaxies at $z\sim5$

We present the results of Spectral Energy Distribution (SED) fitting analysis for Lyman Break Galaxies (LBGs) at $z\sim5$ in the GOODS-N and its flanking fields. From the SED fitting for $\sim100$ objects, we found that the stellar masses range from $10^{8}$ to $10^{11}M_{\odot}$ with a median value of $4\times10^{9}M_{\odot}$. By using the large sample of galaxies at $z\sim5$, we construct the stellar mass function (SMF) with incompleteness corrections. By integrating down to $10^{8}M_{\odot}$, the cosmic stellar mass density at $z\sim5$ is calculated to be $7\times10^{6}M_{\odot}\textrm{Mpc}^{-3}$.

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arXiv2008

Asphericity in Supernova Explosions from Late-Time Spectroscopy

Core-collapse supernovae (CC-SNe) are the explosions that announce the death of massive stars. Some CC-SNe are linked to long-duration gamma-ray bursts (GRBs) and are highly aspherical. One important question is to what extent asphericity is common to all CC-SNe. Here we present late-time spectra for a number of CC-SNe from stripped-envelope stars, and use them to explore any asphericity generated in the inner part of the exploding star, near the site of collapse. A range of oxygen emission-line profiles is observed, including a high incidence of double-peaked profiles, a distinct signature of an aspherical explosion. Our results suggest that all CC-SNe from stripped-envelope stars are aspherical explosions and that SNe accompanied by GRBs exhibit the highest degree of asphericity.

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arXiv2007

Lyman Break Galaxies at z~5: Rest-frame UV Spectra II

We present the results of spectroscopy of Lyman Break Galaxies (LBGs) at z~5 in the J0053+1234 field with the Faint Object Camera and Spectrograph on the Subaru telescope. Among 5 bright candidates with z' < 25.0 mag, 2 objects are confirmed to be at z~5 from their Ly alpha emission and the continuum depression shortward of Ly alpha. The EWs of Ly alpha emission of the 2 LBGs are not so strong to be detected as Ly alpha emitters, and one of them shows strong low-ionized interstellar (LIS) metal absorption lines. Two faint objects with z' \geq 25.0 mag are also confirmed to be at z~5, and their spectra show strong Ly alpha emission in contrast to the bright ones. These results suggest a deficiency of strong Ly alpha emission in bright LBGs at z~5, which has been discussed in our previous paper. Combined with our previous spectra of LBGs at z~5 obtained around the Hubble Deep Field-North (HDF-N), we made a composite spectrum of UV luminous (M_1400 \leq -21.5 mag) LBGs at z~5. The resultant spectrum shows a weak Ly alpha emission and strong LIS absorptions which suggests that the bright LBGs at z~5 have chemically evolved at least to ~0.1 solar metallicity. For a part of our sample in the HDF-N region, we obtained near-to-mid infrared data, which constraint stellar masses of these objects. With the stellar mass and the metallicity estimated from LIS absorptions, the metallicities of the LBGs at z~5 tend to be lower than those of the galaxies with the same stellar mass at z \lesssim 2, although the uncertainty is very large.

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arXiv2007

CO Observations of a FeLoBAL Quasar with an H alpha Absorption Line at z=2.3

SDSS J083942.11+380526.3 is an Iron Low-ionization Broad Absorption Line (FeLoBAL) quasar at z = 2.3, and Aoki et al. (2006) recently found the presence of an H alpha absorption line in the broad H alpha emission line. Motivated by an idea that this quasar may be a huge molecular gas reservoir in the early phase of quasar evolution, we made CO(J=3-2) observations of it using the Nobeyama Millimeter Array. No significant CO emission was detected; although an emission-like feature (2.5 sigma) was seen close (~ 2") to the quasar, we regard it as a noise. The obtained 3 sigma upper limit on the CO luminosity is L'_{{\rm CO}(J=3-2)} = 4.5 \times 10^{10} K km/s pc^2, which corresponds to M({\rm H}_2) = 3.6 times 10^10 M_{\odot} if we adopt the CO-to-H_2 conversion factor of 0.8 M_{\odot} (K km/s pc^2)^{-1}. This upper limit is comparable to L'_{{\rm CO}(J=3-2)} (and thus the molecular gas mass) detected in quasars and BAL quasars at z=1-3, and no sign of the presence of the huge amount of molecular gas in this FeLoBAL quasar was obtained.

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arXiv2006

The Long Faint Tail of the High-Redshift Galaxy Population

We study the properties of very faint, sub-L* Lyman break galaxies at z~2-5 - thus far a largely neglected but numerically and energetically very important population. We find that the LBG luminosity function undergoes luminosity-dependent evolution: the number of luminous galaxies remains constant while the number of faint ones grows with time. The total UV luminosity density increases with cosmic time from at least z~5 until reaching a peak or a plateau around z~2 - behaviour that is governed by the sub-L* galaxies in the LF's "faint tail". Using broadband SED fitting we find a nearly-linear relationship between SFR and galaxy stellar mass at z~2. A typical L* LBG at z~2 shows a stellar mass of ~10^10M_sun, remarkably similar to the bimodality mass at low redshift. This similarity suggests that the mechanisms responsible for the galaxy bimodality at low-z may have also been at play at z~2.

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arXiv2006

Implications for the Cosmic Reionization from the Optical Afterglow Spectrum of the Gamma-Ray Burst 050904 at z = 6.3

The gamma-ray burst (GRB) 050904 at z = 6.3 provides the first opportunity of probing the intergalactic medium (IGM) by GRBs at the epoch of the reionization. Here we present a spectral modeling analysis of the optical afterglow spectrum taken by the Subaru Telescope, aiming to constrain the reionization history. The spectrum shows a clear damping wing at wavelengths redward of the Lyman break, and the wing shape can be fit either by a damped Ly alpha system with a column density of log (N_HI/cm^{-2}) ~ 21.6 at a redshift close to the detected metal absorption lines (z_metal = 6.295), or by almost neutral IGM extending to a slightly higher redshift of z_{IGM,u} ~ 6.36. In the latter case, the difference from z_metal may be explained by acceleration of metal absorbing shells by the activities of the GRB or its progenitor. However, we exclude this possibility by using the light transmission feature around the Ly beta resonance, leading to a firm upper limit of z_{IGM,u} < 6.314. We then show an evidence that the IGM was largely ionized already at z=6.3, with the best-fit neutral fraction of IGM, x_HI = N_HI/N_H = 0.00, and upper limits of x_HI < 0.17 and 0.60 at 68 and 95% C.L., respectively. This is the first direct and quantitative upper limit on x_HI at z > 6. Various systematic uncertainties are examined, but none of them appears large enough to change this conclusion. To get further information on the reionization, it is important to increase the sample size of z >~ 6 GRBs, in order to find GRBs with low column densities (log N_HI <~ 20) within their host galaxies, and for statistical studies of Ly alpha line emission from host galaxies.

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arXiv2006

A Bar Fuels a Super-Massive Black Hole?: Host Galaxies of Narrow-Line Seyfert 1 Galaxies

We present optical images of nearby 50 narrow-line Seyfert 1 galaxies (NLS1s) which cover all the NLS1s at z<0.0666 and $δ\ge -25^{\circ}$ known at the time of 2001. Among the 50 NLS1s, 40 images are newly obtained by our observations and 10 images are taken from archive data. Motivated by the hypothesis that NLS1s are in an early phase of a super-massive black hole (BH) evolution, we present a study of NLS1 host galaxy morphology to examine trigger mechanism(s) of active galactic nuclei (AGNs) by seeing the early phase of AGN. With these images, we made morphological classification by eye inspection and by quantitative method, and found a high bar frequency of the NLS1s in the optical band; the bar frequency is $85 \pm 7% among disk galaxies ($64- 71$% in total sample) which is more frequent than that (40-70%) of broad-line Seyfert 1 galaxies (BLS1s) and normal disk galaxies, though the significance is marginal. Our results confirm the claim by Crenshaw et al. (2003) with a similar analysis for 19 NLS1s. The frequency is comparable to that of HII/starburst galaxies. We also examined the bar frequency against width of the broad H$β$ emission line, Eddington ratio, and black hole mass, but no clear trend is seen. Possible implications such as an evolutionary sequence from NLS1s to BLS1 are discussed briefly.

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arXiv2006

Discovery of H alpha absorption in the unusual broad absorption line quasar SDSS J083942.11+380526.3

We discovered an H alpha absorption in a broad H alpha emission line of an unusual broad absorption line quasar, SDSS J083942.11+380526.3 at z=2.318, by near-infrared spectroscopy with the Cooled Infrared Spectrograph and Camera for OHS (CISCO) on the Subaru telescope. The Presence of non-stellar H alpha absorption is known only in the Seyfert galaxy NGC 4151 to date, thus our discovery is the first case for quasars. The H alpha absorption line is blueshifted by 520 km/s relative to the H alpha emission line, and its redshift almost coincides with those of UV low-ionization metal absorption lines. The width of the H alpha absorption (~ 340 km/s) is similar to those of the UV low-ionization absorption lines. These facts suggest that the H alpha and the low-ionization metal absorption lines are produced by the same low-ionization gas which has a substantial amount of neutral gas. The column density of the neutral hydrogen is estimated to be ~ 10^18 cm^-2 by assuming a gas temperature of 10,000 K from the analysis of the curve of growth. The continuum spectrum is reproduced by a reddened (E(B-V) ~ 0.15 mag for the SMC-like reddening law) composite quasar spectrum. Furthermore, the UV spectrum of SDSS J083942.11+380526.3 shows a remarkable similarity to that of NGC 4151 in its low state, suggesting the physical condition of the absorber in SDSS J083942.11+380526.3 is similar to that of NGC 4151 in the low state. As proposed for NGC 4151, SDSS J083942.11+380526.3 may be also seen through the close direction of the surface of the obscuring torus.

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arXiv2006

Deficiency of large equivalent width Lyman-alpha emission in luminous Lyman break galaxies at z~5-6?

We report a deficiency of luminous Lyman break galaxies (LBGs) with a large rest-frame equivalent width (EW_rest) of Lyman-alpha emission at z~5-6. Combining our spectroscopic sample of LBGs at z~5 and those from the literature, we found that luminous LBGs at z~5-6 generally show weak Lyman-alpha emissions, while faint LBGs show a wide range of Lyman-alpha EW_rest and tend to have strong (EW_rest >20A) Lyman-alpha emissions; i.e., there is a deficiency of strong Lyman-alpha emission in luminous LBGs. There seems to be a threshold UV luminosity for the deficiency; it is M_1400 = -21.5 \~ -21.0 mag, which is close to or somewhat brighter than the M* of the UV luminosity function at z~5 and 6. Since the large EW_rest of Lyman-alpha emission can be seen among the faint LBGs, the fraction of Lyman-alpha emitters in LBGs may change rather abruptly with the UV luminosity. If the weakness of Lyman-alpha emission is due to dust absorption, the deficiency suggests that luminous LBGs at z=5-6 tend to be in dusty and more chemically evolved environments and started star formation earlier than faint ones, though other causes cannot be ruled out.

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arXiv2006

Host Galaxies of Hard X-ray Selected Type-2 Active Galactic Nuclei at Intermediate Redshifts

We study properties of the host galaxies of 15 hard X-ray selected type-2 active galactic nuclei (AGNs) at intermediate redshifts (0.05$<z<$0.6) detected in $ASCA$ surveys. The absorption corrected hard X-ray luminosities $L_{\rm 2-10 keV}$ range from 10$^{42}$ erg s$^{-1}$ to $10^{45}$ erg s$^{-1}$. We took the $R$-band image of these AGNs with the University of Hawaii 2.2 m telescope. Thanks to the intrinsic obscuration of nuclear light, we can decompose the galaxies with a spheroid component and a disk component. The resulting spheroid luminosities correlate with $L_{\rm 2-10 keV}$; higher (lower) X-ray luminosity AGNs tend to reside in luminous (less luminous) spheroids. It is also found that the hosts of luminous AGNs show a large spheroid-to-disk luminosity ratio ($\sim$1), while those of less luminous AGNs spread between 0 and 1. The correlation between $L_{\rm 2-10keV}$ and spheroid luminosity indicates that the relation between mass of a supermassive black hole (SMBH) and spheroid luminosity (BS-relation) at the intermediate redshifts. The BS-relation agrees with that in the local universe if the Eddington ratio of 0.24 is adopted, which is a mean value determined from our $ASCA$ type-1 AGN sample at similar redshifts through the broad-line width and continuum luminosity. The present study demonstrates the effectiveness of using type-2 AGNs at high redshifts to study their host properties.

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arXiv2005

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-$z$ 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.

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arXiv2004

The Largest Blueshifts of [O III] emission line in Two Narrow-Line Quasars

We have obtained optical intermediate resolution spectra (R = 3000) of the narrow-line quasars DMS 0059-0055 and PG 1543+489. The [O III] emission line in DMS 0059-0055 is blueshifted by 880 km/s relative to Hbeta. We also confirm that the [O III] emission line in PG 1543+489 has a relative blueshift of 1150 km/s. These two narrow-line quasars show the largest [O III] blueshifts known to date among type 1 active galactic nuclei (AGNs). The [O III] emission lines in both objects are broad (1000 - 2000 km/s) and those in DMS 0059-0055 show strong blue asymmetry. We interpret the large blueshift and the profile of the [O III] lines as the result of an outflow interacting with circumnuclear gas. Among type 1 AGNs with large blueshifted [O III], there is no correlation between the Eddington ratios and the amount of [O III] blueshifts. Combining our new data with published results, we confirm that the Eddington ratios of the such AGNs are the highest among AGNs with the same black hole masses. These facts suggest that the Eddington ratio is a necessary condition or the [O III] blueshifts weakly depend on the Eddington ratio. Our new sample suggests that there are possible necessary conditions to produce an outflow besides a high Eddington ratio: large black hole mass (> 10^7 M_solar) or high mass accretion rate (> 2 M_solar/yr) or large luminosity (lambda L_{lambda} (5100A) > 10^44.6 erg/s).

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arXiv2004

Lyman Break Galaxies at $z\sim5$: Rest-Frame UV Spectra

We report initial results for spectroscopic observations of candidates of Lyman Break Galaxies (LBGs) at $z\sim5$ in a region centered on the Hubble Deep Field-North by using the Faint Object Camera and Spectrograph attached to the Subaru Telescope. Eight objects with $I_C\leq25.0$ mag, including one AGN, are confirmed to be at $4.5<z<5.2$. The rest-frame UV spectra of seven LBGs commonly show no or weak Lyalpha emission line (rest-frame equivalent width of 0-10Å) and relatively strong low-ionization interstellar metal absorption lines of SiII $λ$1260, OI+SiII $λ$1303, and CII $λ$1334 (mean rest-frame equivalent widths of them are $-1.2 \sim -5.1 $Å). These properties are significantly different from those of the mean rest-frame UV spectrum of LBGs at $z\sim3$, but are quite similar to those of subgroups of LBGs at $z\sim3$ with no or weak Lyalpha emission. The weakness of Lyalpha emission and strong low-ionization interstellar metal absorption lines may indicate that these LBGs at $z\sim5$ are chemically evolved to some degree and have a dusty environment. Since the fraction of such LBGs at $z\sim5$ in our sample is larger than that at $z\sim3$, we may witness some sign of evolution of LBGs from $z\sim5$ to $z\sim3$, though the present sample size is very small. It is also possible, however, that the brighter LBGs tend to show no or weak Lyalpha emission, because our spectroscopic sample is bright (brighter than $L^{\ast}$) among LBGs at $z\sim5$. More observations are required to establish spectroscopic nature of LBGs at $z\sim5$.

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arXiv2003

Dusty ERO Search behind Two Massive Clusters

We performed deep K'-band imaging observations of 2 massive clusters, MS 0451.6-0305 at z = 0.55 and MS 0440.5+0204 at z = 0.19, for searching counterparts of the faint sub-mm sources behind these clusters, which would provide one of the deepest extremely red object(ERO) samples. Comparing our near-infrared images with optical images taken by the Hubble Space Telescope and by the Subaru Telescope, we identified 13 EROs in these fields. The sky distributions of EROs are consistent with the previous results, that there is a sign of strong clustering among detected EROs. Also, the surface density with corrected lensing amplification factors in both clusters are in good agreement with that derived from previous surveys. We found 7 EROs and 3 additional very red objects in a small area (\sim 0.6 arcmin^2) of the MS 0451.6-0305 field around an extended SCUBA source. Many of their optical and near-infrared colors are consistent with dusty star-forming galaxies at high redshifts(z \sim 1.0-4.0), and they may be constituting a cluster of dusty starburst galaxies and/or lensed star-forming galaxies at high redshift. Their red J-K' colors and faint optical magnitudes suggest they are relatively old massive stellar systems with ages(>300 Mega years) suffering from dust obscuration. We also found a surface-density enhancement of EROs around the SCUBA source in the MS 0440.5+0204 field.

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arXiv2002

Subaru Deep Survey III. Evolution of Rest-Frame Luminosity Functions Based on the Photometric Redshifts for A K'-band Selected Galaxy Sample

We have constructed a very deep K'-selected multicolor BVRIz'JK' sample of 439 field galaxies. Photometric redshifts for sample galaxies were estimated. The overall redshift distribution N(z) for the K'<21.0 sample is consistent with previous observations, and for the first time we derive the N(z) down to K'=24.0. After taking account of the dust extinction and selection effects of the sample, the observed N(z) distribution is well described with the PLE model, while the hierarchical galaxy formation model shows an apparent deficiency of galaxies especially at z>2. The photometric redshift and the best-fit SED model evaluations allow the derivation of the rest-frame K', B, and UV(2000A) luminosity functions (LFs) and their evolutions. The rest-K' LF shows almost no evolution up to z=3, while the rest B LF shows mild luminosity evolution, and the rest UV LF shows strong luminosity evolution. This trend corresponds to the evolution of the rest UV-K' color, which gets bluer with increasing redshift. We also found that more massive galaxies are redder in this rest-frame color in every epoch. The rest-UV LF of our K'-selected galaxies shows a much shallower faint end slope at z=3 than that of previous estimations for rest-UV selected Lyman break galaxies. As a consequence, the contribution to the global star formation rate of our K'-selected galaxies is about 42% of that derived from the integration of LF of Lyman break galaxies at z=3. This result suggests that a large fraction of the star formation rate density at z>1.5 comes from the contribution from the faint (M_2000A>-20) blue galaxy population at high redshift universe that have not yet obviously been identified. (abridged)

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