arXiv subjects

Kouji Ohta

Publications and source records attributed to Kouji Ohta.

121 records · Page 7Linked to original sources

arXiv2003

Color Gradients in Early-Type Galaxies in ABELL 2199

We performed B and R band surface photometry for E/S0 galaxies in a nearby rich cluster ABELL 2199 to investigate their B-R color gradients (d(B-R)/dlogr). Our aims are to study statistical properties of the color gradients and, by comparing them with those in less dense environments, to examine environmental dependence of color gradients in elliptical galaxies. We studied the distribution of the B-R color gradients in the cluster ellipticals and found that the mean value of the color gradients is -0.09 +- 0.04 mag/dex, which can be converted to a metallicity gradient (dlogZ/dlogr) of ~ -0.3 +- 0.1 assuming an old stellar population. We further studied the relations between the B-R color gradients and global properties of the galaxies. Our data suggest that for the galaxies brighter than L*, more luminous and larger galaxies tend to have steeper color gradients. The typical value of the color gradients seems to be consistent with a recent monolithic collapse model and the correlation could also appear if elliptical galaxies formed through the monolithic collapse. On the contrary, it is found based on data from the literature that any such trend is clearly weaker for ellipticals in less dense environments, while the distribution of the color gradients is quite similar to that found in the rich cluster. Based on the results from our data and the published data, we discuss formation process of elliptical galaxy and its environmental dependence.

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arXiv2002

A Search for Lyman alpha Emitters at Redshift 3.7

We present the results of a survey for emission-line objects based on optical intermediate-band ($λ_{\rm c}$ = 5736 Å~ and $Δλ$ = 280 Å) and broad-band ($B$, $V$, $R$, and $i^\prime$) observations of the Subaru/XMM-Newton Deep Field on the 8.2 m Subaru telescope with the Subaru Prime Focus Camera, Suprime-Cam. All the data were obtained during the guaranteed time observations of the Suprime-Cam instrument. The intermediate-band image covered a sky area with 10\minpoint62 $\times 12\minpoint40 \approx 132$ arcmin$^2$ in the Subaru/XMM-Newton Deep Field (Ouchi et al.). Using this image, we have found 23 emission-line sources whose observed emission-line equivalent widths are greater than 250 Å. Their optical multicolor properties indicate that six emission-line sources are Ly$α$ emitters at $z \approx$ 3.7 ($Δz \approx 0.22$). They are either intense starburst galaxies or active galactic nuclei like quasars at $z \approx$ 3.7. Two more emission-line sources may also be Ly$α$ emitters at $z \approx$ 3.7 although their multicolor properties are marginal. Among the remaining 15 emission-line objects, eight objects appear strong emission-line galaxies at lower redshift; e.g., [O {\sc ii}] $λ$3727 emitters at $z \approx 0.54$, H$β$ at $z \approx 0.18$, or [O {\sc iii}]$λ$5007 emitters at $z \approx 0.15$. The remaining seven objects are unclassified because they are too faint to be detected in broad-band images. We discuss observational properties of these strong emission-line sources. In particular, our data allow us to estimate the star formation density at $z \approx 3.7$ for the first time.

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arXiv2002

Chandra Observations and Optical Identification of Hard X-ray Sources Discovered with ASCA

We present the first results of the Chandra and optical follow-up observations of hard X-ray sources detected in the ASCA Medium Sensitivity Survey (AMSS). Optical identifications are made for five objects. Three of them show either weak or absent optical narrow emission lines and are at low redshift ~0.06. One of them is a broad line object at z=0.910 and one is a z=0.460 object with only narrow lines. All the narrow line objects show strong evidence for absorption in their X-ray spectra. Their line ratios are consistent with a Seyfert II/LINER identification as are the line widths. The three low redshift objects have the colors of normal galaxies and apparently the light is dominated by stars. This could be due to the extinction of the underlying nuclear continuum by the same matter that absorbs X-rays and/or due to the dilution of the central source by starlight. These results suggest that X-ray sources that appear as ``normal'' galaxies in optical and near-IR bands significantly contribute to the hard X-ray background. This population of objects has a high space density and probably dominates the entire population of active galaxies.

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arXiv2002

AGN Populations from Optical Identification of ASCA Surveys

To understand luminous AGNs in the z<1 universe, the ASCA AGN samples are the best at present. Combining the identified sample of AGNs from ASCA Large Sky Survey and Medium Sensitivity Survey, the sample of hard X-ray selected AGNs have been expanded up to 108 AGNs above the flux limit of 10^{-13} erg s^{-1} cm^{-2} in the 2-10 keV hard X-ray band. We discuss the fraction of absorbed AGNs in the hard X-ray selected AGN sample, and nature of absorbed luminous AGNs.

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arXiv2001

A candidate of a type-2 QSO at z=0.9: Large X-ray absorption with a strong broad-H-alpha emission line

Deep hard X-ray and near infrared observations of a type-2 radio-quiet QSO candidate at z=0.9, AXJ08494+4454, are reported. The 0.5-10keV Chandra X-ray Observatory spectrum of AXJ08494+4454 is hard, and is explained well with a power-law continuum absorbed by a hydrogen column density of 2x10^23 cm^-2. The 2-10keV luminosity of the object reaches hard X-ray luminosity of QSOs. The large X-ray absorption and the large intrinsic luminosity support the original identification of AXJ08494+4454 as a type-2 radio-quiet QSO. Nevertheless, the deep Subaru/IRCS J-band spectroscopic observation suggests the presence of a strong broad H-alpha emission line from AXJ08494+4454. If real, the broad H-alpha emission line has a velocity width of 9400+-1000 km/s, which corresponds to a typical broad-Balmer line velocity width of a luminous QSO. The existence of the strong broad H-alpha line means that the object is not a type-2 QSO, but a luminous cousin of a Seyfert 1.9 galaxy in the source-frame optical spectrum. The Balmer decrement of broad lines, and the broad H-alpha emission to the hard X-ray luminosity ratio suggest that the nucleus is affected by dust extinction with A_V of 1-3 mag in the optical wavelength. The estimated amount of dust extinction is much smaller than that expected from the X-ray column density.

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arXiv2000

An Extremely Red Nucleus in an Absorbed QSO at z=0.65

The results of K-band-imaging observations of a candidate of an absorbed QSO at z=0.653, AX J131831+3341, are presented. The B-K color of the object is 4.85 mag, which is much redder than optically-selected QSOs. The K-band image shows nuclear and extended components, the same as in the optical V-, R-, and I-band images. The nuclear component (I-K = 4.29 mag) is much redder than the power-law models with energy indices of 0 to -1.0, which well reproduce the V-R and R-I optical colors of the nuclear component. A heavily absorbed (A_V = 3 mag) nucleus may emerge in the K-band, while optical light may originate from scattered nuclear light. The I-K color of the extended component is 2.2 mag, which is consistent with the post-starburst nature of the host galaxy, which is also suggested from the V-R and R-I colors of the extended component.

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arXiv2000

On the Nature of AX J2049.6+2939 and AX J2050.0+2914

AX J2049.6+2939 is a compact X-ray source in the vicinity of the southern blow-up region of the Cygnus Loop supernova remnant (Miyata et al. 1998a). This source was the brightest X-ray source inside the Cygnus Loop observed during the ASCA survey project. The X-ray spectrum was well fitted by a power-law function with a photon index of $-2.1 \pm 0.1$. Short-term timing analysis was performed and no coherent pulsation was found. Follow-up observations with ASCA have revealed a large variation in X-ray intensity by a factor of $\simeq$ 50, whereas the spectral shape did not change within the statistical uncertainties. In the second ASCA observation, we found another X-ray source, AX J2050.0+2941, at the north east of AX J2049.6+2939. During the three ASCA observations, the X-ray intensity of AX J2050.0+2941 varied by a factor of $\simeq$4. No coherent pulsations could be found for AX J2050.0+2941. We have performed optical photometric and spectroscopic observations in the vicinity of AX J2049.6+2939 at the Kitt Peak National Observatory (KPNO). As a result, all objects brighter than $B$-band magnitude of 22 in the error box can be identified with normal stars. Combined with the X-ray results and the fact that there are no radio counterparts, AX J2049.6+2939 is not likely to be either an ordinary rotation-powered pulsar or an AGN. The nature of AX J2049.6+2939 is still unclear and further observations over a wide energy band are strongly required. As to AX J2050.0+2941, the long-term X-ray variability and the radio counterpart suggests that it is an AGN.

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arXiv2000

Color Gradients in Early-Type Galaxies in Clusters at the Redshift from 0.37 to 0.56

Color gradients in elliptical galaxies in distant clusters ($z=0.37-0.56$) are examined by using the archival deep imaging data of Wide Field Planetary Camera 2 (WFPC2) on-board the Hubble Space Telescope (HST). Obtained color gradients are compared with the two model gradients to examine the origin of the color gradients. In one model, a color gradient is assumed to be caused by a metallicity gradient of stellar populations, while in the other one, it is caused by an age gradient. Both of these model color gradients reproduce the average color gradient seen in nearby ellipticals, but predict significantly different gradients at a redshift larger than $\sim$0.3. Comparison between the observed gradients and the model gradients reveals that the metallicity gradient is much more favorable as the primary origin of color gradients in elliptical galaxies in clusters. The same conclusion has been obtained for field ellipticals by using those at the redshift from 0.1 to 1.0 in the Hubble Deep Field-North by Tamura et al. (2000). Thus, it is also suggested that the primary origin of the color gradients in elliptical galaxies does not depend on galaxy environment.

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arXiv2000

Origin of Color Gradients in Elliptical Galaxies

The origin of color gradients in elliptical galaxies is examined by comparing model gradients with those observed in the Hubble Deep Field. The models are constructed so as to reproduce color gradients in local elliptical galaxies either by a metallicity gradient or by an age gradient. By looking back a sequence of color gradient as a function of redshift, the age - metallicity degeneracy is solved. The observed color gradients in elliptical galaxies at $z=0.1-1.0$ agree excellently with those predicted by the metallicity gradient, while they deviates significantly from those predicted by the age gradient even at $z \sim 0.3$ and the deviation is getting larger with increasing redshift. This result does not depend on cosmological parameters and parameters for an evolutionary model of galaxy within a reasonable range. Thus our results clearly indicate that the origin of color gradients is not the age but the stellar metallicity.

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arXiv1999

A Rich Cluster of Galaxies Near the Quasar B2 1335+28 at z=1.1: Color Distribution and Star-Formation Properties

We previously reported a significant clustering of red galaxies (R-K=3.5--6) around the radio-loud quasar B2 1335+28 at z=1.086. In this paper, we establish the existence of a rich cluster at the quasar redshift, and study the properties of the cluster galaxies through further detailed analysis of the photometric data. The color distribution of the galaxies in the cluster is quite broad and the fraction of blue galaxies (\sim 70%) is much larger than in intermediate-redshift clusters. Using evolutionary synthesis models, we show that this color distribution can be explained by galaxies with various amounts of star-formation activity mixed with the old stellar populations. Notably, there are about a dozen galaxies which show very red optical-NIR colors but also show significant UV excess with respect to passive-evolution models. They can be interpreted as old early-type galaxies with a small amount of star formation. The fact that the UV-excess red galaxies are more abundant than the quiescent red ones suggests that a large fraction of old galaxies in this cluster are still forming stars to some extent. However, a sequence of quiescent red galaxies is clearly identified on the R-K versus K color-magnitude (C-M) diagram. The slope and zero point of their C-M relation appear to be consistent with those expected for the precursors of the C-M relation of present-day cluster ellipticals when observed at z=1.1. We estimate the Abell richness class of the cluster to be R \sim 1. New X-ray data presented here place an upper limit of L_x < 2 10^{44} erg s^{-1} for the cluster luminosity. Inspections of the wider optical images reveal some lumpy structure, suggesting that the whole system is still dynamically young.

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arXiv1999

Expected Sub-mm Emission and Dust Properties of Lyman Break Galaxies at High Redshift

We investigate expected sub-mm emission and dust properties of the Lyman Break Galaxies (LBGs) in the Hubble Deep Field (HDF). The SCUBA Deep Survey (Hughes et al., 1998) provides an upper limit of the 850 micron flux densities of the LBGs. With this constraint, we argue that a typical ultra-violet to far-infrared spectral shape of the high-redshift LBGs is rather close to a template spectrum of low-reddening starburst galaxies in the local universe but different from that of heavily dust-enshrouded ultra-luminous far-infrared galaxies like Arp220. We also evaluate the lower-limit temperature of dust in LBGs assuming single- and two-component modified black-body spectrum. To estimate the total amount of energy re-emitted in FIR wavelength, we take two different approaches; model fitting of the UV spectra of LBGs and an empirical method using the relationship between UV spectral shape and UV/FIR flux ratio observed for local starburst galaxies. Both methods give lower-limit temperature of the LBGs as around 40 K, which is higher than the typical dust temperature of local optical- and infrared-selected galaxies. This result is also supported by the comparison of the expected sub-mm flux of the LBGs with the cosmic FIR background radiation. The high dust temperature may indicate the effective massive-star formation or different dust properties in the high-redshift LBGs.

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arXiv1998

The IRIS Far-Infrared Galaxy Survey : Expected Number Count, Redshift, and Perspective

Infrared Imaging Surveyor (IRIS) is a satellite which will be launched in the beginning of 2003. One of the main purposes of the IRIS mission is an all-sky survey at far-infrared (FIR) with a flux limit much deeper than that of IRAS. In order to examine the performance of the survey, we estimated the FIR galaxy counts in four (50, 70, 120, and 150 $μ$m) bands based on some models. We adopted a multicomponent model which consists of cirrus and starburst components for galaxy spectra, and the nearby FIR luminosity function derived from that of IRAS galaxies. We derived the number counts, redshift distributions, and infrared diffuse background radiation spectra for i) no evolution, ii) pure luminosity evolution, iii) pure density evolution with $q_0 = 0.1$ and 0.5. We found that a large number of galaxies ($\sim {\rm a few} \times 10^6$ in the whole sky) will be detected in this survey. With the aid of a vast number of detection, we will detect the effect of galaxy evolution, and evaluate the amplitude of evolution at least in the nearby universe in the IRIS survey, though it will be still difficult to constrain which type of evolution takes place from the number count alone. We also studied the estimation of redshifts of detected galaxies by their infrared colors alone. Although significant contamination takes place among nearby faint galaxies and high-$z$ ones, we found that rough estimation of galaxy redshift can be practicable by jointly using present and future optical surveys.

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arXiv1998

The hardest X-ray source in the ASCA Large Sky Survey: Discovery of a new type 2 Seyfert

We present results of ASCA deep exposure observations of the hardest X-ray source discovered in the ASCA Large Sky Survey (LSS) project, designated as AX J131501+3141. We extract its accurate X-ray spectrum, taking account of the contamination from a nearby soft source (AX J131502+3142), separated only by 1'. AX J131501+3141 exhibits a large absorption of NH = (6 +4 -2)x 10^22 H/cm^2 with a photon index Γ= 1.5 +0.7 -0.6. The 2--10 keV flux was about 5 x 10^-13 erg/s/cm^2 and was time variable by a factor of 30% in 0.5 year. From the highly absorbed X-ray spectrum and the time variability, as well as the results of the optical follow-up observations (Akiyama et al. 1998, astro-ph/9801173), we conclude that AX J131501+3141 is a type 2 Seyfert galaxy. Discovery of such a low flux and highly absorbed X-ray source could have a significant impact on the origin of the cosmic X-ray background.

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