arXiv subjects

Yasuhiro Shioya

Publications and source records attributed to Yasuhiro Shioya.

At least 19 recordsLinked to original sources

arXiv2016

Morphological Properties of Lyman Alpha Emitters at Redshift 4.86 in the COSMOS Field: Clumpy Star Formation or Merger?

We investigate morphological properties of 61 Lyman-alpha emitters (LAEs) at z = 4.86 identified in the COSMOS field, based on Hubble Space Telescope Advanced Camera for Surveys (ACS) imaging data in the F814W-band. Out of the 61 LAEs, we find the ACS counterparts for the 54 LAEs. Eight LAEs show double-component structures with a mean projected separation of 0."63 (~ 4.0 kpc at z = 4.86). Considering the faintness of these ACS sources, we carefully evaluate their morphological properties, that is, size and ellipticity. While some of them are compact and indistinguishable from the PSF half-light radius of 0."07 (~ 0.45 kpc), the others are clearly larger than the PSF size and spatially extended up to 0."3 (~ 1.9 kpc). We find that the ACS sources show a positive correlation between ellipticity and size and that the ACS sources with large size and round shape are absent. Our Monte Carlo simulation suggests that the correlation can be explained by (1) the deformation effects via PSF broadening and shot noise or (2) the source blending in which two or more sources with small separation are blended in our ACS image and detected as a single elongated source. Therefore, the 46 single-component LAEs could contain the sources which consist of double (or multiple) components with small spatial separation (i.e., < 0."3 or 1.9 kpc). Further observation with high angular resolution at longer wavelengths (e.g., rest-frame wavelengths of > 4000 A) is inevitable to decipher which interpretation is adequate for our LAE sample.

astro-ph.GA↗

arXiv2015

Discovery of Massive, Mostly Star-formation Quenched Galaxies with Extremely Large Lyman-alpha Equivalent Widths at z ~ 3

We report a discovery of 6 massive galaxies with both extremely large Lya equivalent width and evolved stellar population at z ~ 3. These MAssive Extremely STrong Lya emitting Objects (MAESTLOs) have been discovered in our large-volume systematic survey for strong Lya emitters (LAEs) with twelve optical intermediate-band data taken with Subaru/Suprime-Cam in the COSMOS field. Based on the SED fitting analysis for these LAEs, it is found that these MAESTLOs have (1) large rest-frame equivalent width of EW_0(Lya) ~ 100--300 A, (2) M_star ~ 10^10.5--10^11.1 M_sun, and (3) relatively low specific star formation rates of SFR/M_star ~ 0.03--1 Gyr^-1. Three of the 6 MAESTLOs have extended Ly$α$ emission with a radius of several kpc although they show very compact morphology in the HST/ACS images, which correspond to the rest-frame UV continuum. Since the MAESTLOs do not show any evidence for AGNs, the observed extended Lya emission is likely to be caused by star formation process including the superwind activity. We suggest that this new class of LAEs, MAESTLOs, provides a missing link from star-forming to passively evolving galaxies at the peak era of the cosmic star-formation history.

astro-ph.GA↗

arXiv2012

A Ly-alpha Emitter with an Extremely Large Rest-frame Equivalent Width of ~900A at z=6.5: A Candidate of Population III-dominated Galaxy?

We have identified a very interesting Ly-alpha emitter, whose Ly-alpha emission line has an extremely large observed equivalent width of EW_0=436^{+422}_{-149}A, which corresponds to an extraordinarily large intrinsic rest-frame equivalent width of EW_0^{int}=872^{+844}_{-298}A after the average intergalactic absorption correction. The object was spectroscopically confirmed to be a real Ly-alpha emitter by its apparent asymmetric Ly-alpha line profile detected at z=6.538. The continuum emission of the object was definitely detected in our deep z'-band image; thus, its EW_0 was reliably determined. Follow-up deep near-infrared spectroscopy revealed emission lines of neither He II lambda1640 as an apparent signature of Population III, nor C IV lambda1549 as a proof of active nucleus. No detection of short-lived He II lambda1640 line is not necessarily inconsistent with the interpretation that the underlying stellar population of the object is dominated by Population III. We found that the observed extremely large EW_0 of the Ly-alpha emission and the upper limit on the EW_0 of the He II lambda1640 emission can be explained by population synthesis models favoring a very young age less than 2-4Myr and massive metal-poor (Z<10^{-5}) or even metal-free stars. The observed large EW_0 of Ly-alpha is hardly explained by Population I/II synthesis models with Z>10^{-3}. However, we cannot conclusively rule out the possibility that this object is composed of a normal stellar population with a clumpy dust distribution, which could enhance the Ly-alpha EW_0, though its significance is still unclear.

astro-ph.CO↗

arXiv2011

The Role of Galaxy Interaction in Environmental Dependence of the Star Formation Activity at z~1.2

In order to understand environmental effects on star formation in high-redshift galaxies, we investigate the physical relationships between the star formation activity, stellar mass, and environment for z ~1.2 galaxies in the 2 deg^2 COSMOS field. We estimate star formation using the [OII] emission line and environment from the local galaxy density. Our analysis shows that for massive galaxies M_*>10^10 M_sun, the fraction of [OII] emitters in high-density environments is 1.7 times higher than in low-density environments, while the [OII] emitter fraction does not depend on environment for low-mass M_* < 10^10 M_sun galaxies. In order to understand what drives these trends, we investigate the role of companion galaxies in our sample. We find that the fraction of [OII] emitters in galaxies with companions is 2.4 times as high as that in galaxies without companions at M_* > 10^10 M_sun. In addition, massive galaxies are more likely to have companions in high-density environments. However, although the "number" of star forming galaxies increases for massive galaxies with close companions and in dense environments, the "average" star formation rate of star forming galaxies at a given mass is independent of environment and the presence/absence of a close companion. These results suggest that interactions and/or mergers in high-density environment could induce star formation in massive galaxies at z~1.2, increasing the fraction of star-forming galaxies with M_* > 10^10 M_sun.

astro-ph.CO↗

arXiv2011

Completing the Census of Ly-alpha Emitters at the Reionization Epoch

We carried out extended spectroscopic confirmations of Ly-alpha emitters (LAEs) at z=6.5 and 5.7 in the Subaru Deep Field. Now, the total number of spectroscopically confirmed LAEs is 45 and 54 at z=6.5 and 5.7, respectively, and at least 81% (70%) of our photometric candidates at z=6.5 (5.7) have been spectroscopically identified as real LAEs. We made careful measurements of the Ly-alpha luminosity, both photometrically and spectroscopically, to accurately determine the Ly-alpha and rest-UV luminosity functions (LFs). The substantially improved evaluation of the Ly-alpha LF at z=6.5 shows an apparent deficit from z=5.7 at least at the bright end, and a possible decline even at the faint end, though small uncertainties remain. The rest-UV LFs at z=6.5 and 5.7 are in good agreement, at least at the bright end, in clear contrast to the differences seen in the Ly-alpha LF. These results imply an increase in the neutral fraction of the intergalactic medium from z=5.7 to 6.5. The rest-frame equivalent width (EW_0) distribution at z=6.5 seems to be systematically smaller than z=5.7, and it shows an extended tail toward larger EW_0. The bright end of the rest-UV LF can be reproduced from the observed Ly-alpha LF and a reasonable EW_0-UV luminosity relation. Integrating this rest-UV LF provides the first measurement of the contribution of LAEs to the photon budget required for reionization. The derived UV LF suggests that the fractional contribution of LAEs to the photon budget among Lyman break galaxies significantly increases towards faint magnitudes. Low-luminosity LAEs could dominate the ionizing photon budget, though this inference depends strongly on the uncertain faint-end slope of the Ly-alpha LF.

astro-ph.CO↗

arXiv2009

The Nature of Optically Dull Active Galactic Nuclei in COSMOS

We present infrared, optical, and X-ray data of 48 X-ray bright, optically dull AGNs in the COSMOS field. These objects exhibit the X-ray luminosity of an active galactic nucleus (AGN) but lack broad and narrow emission lines in their optical spectrum. We show that despite the lack of optical emission lines, most of these optically dull AGNs are not well-described by a typical passive red galaxy spectrum: instead they exhibit weak but significant blue emission like an unobscured AGN. Photometric observations over several years additionally show significant variability in the blue emission of four optically dull AGNs. The nature of the blue and infrared emission suggest that the optically inactive appearance of these AGNs cannot be caused by obscuration intrinsic to the AGNs. Instead, up to ~70% of optically dull AGNs are diluted by their hosts, with bright or simply edge-on hosts lying preferentially within the spectroscopic aperture. The remaining ~30% of optically dull AGNs have anomalously high f_x/f_o ratios and are intrinsically weak, not obscured, in the optical. These optically dull AGNs are best described as a weakly accreting AGN with a truncated accretion disk from a radiatively inefficient accretion flow.

astro-ph.CO↗

arXiv2008

Clustering Properties of Low-Luminosity Star-Forming galaxies at z = 0.24 and 0.40 in the Subaru Deep Field

We present our analysis on the clustering properties of star-forming galaxies selected by narrow-band excesses in the Subaru Deep Field. Specifically we focus on Halpha emitting galaxies at z = 0.24 and z = 0.40 in the same field, to investigate possible evolutionary signatures of clustering properties of star-forming galaxies. Based on the analysis on 228 Halpha emitting galaxies with 39.8 < log L(Halpha) < 40.8 at z = 0.40, we find that their two-point correlation function is estimated as xi = (r/1.62^{+0.64}_{-0.50} Mpc)^{-1.84 +/- 0.08}. This is similar to that of Halpha emitting galaxies in the same Halpha luminosity range at z = 0.24, xi = (r/1.88^{+0.60}_{-0.49} Mpc)^{-1.89 +/- 0.07}. These correlation lengths are smaller than those for the brighter galaxy sample studied by Meneux et al. (2006) in the same redshift range. The evolution of correlation length between z = 0.24 and z = 0.40 is interpreted by the gravitational growth of the dark matter halos.

astro-ph↗

arXiv2008

Star-Forming Galaxies at z=0.24 in the Subaru Deep Field and the Sloan Digital Sky Survey

We make a search for Halpha emitting galaxies at z=0.24 in the Subaru Deep Field (SDF) using the archival data set obtained with the Subaru Telescope. We carefully select Halpha emitters in the narrowband filter NB816, using B, V, Rc, i', and z' broad-band colors. We obtain a sample of 258 emitting galaxies with observed equivalent widths of (Halpha+[NII]6548,6584) greater than 12 angstrom. We also analyze a sample of Halpha emitters taken from the Sloan Digital Sky Survey (SDSS) to constrain the luminous end of Halpha luminosity function. Using the same selection criteria as for the SDF, and after excluding AGNs, we obtain 317 Halpha emitting star-forming galaxies. Combining these two samples of Halpha emitters found in both SDF and SDSS, we derive a Halpha luminosity function with best-fit Schechter function parameters of alpha = -1.31^+0.17_-0.17, log phi^* = -2.46^+0.34_-0.40 Mpc^-3, log L^* = 41.99^+0.08_-0.07 ergs s^-1. An extinction-corrected Halpha luminosity density is 4.45^+2.96_-1.75 x 10^39 ergs s^-1 Mpc^-3. Using the Kennicutt relation between the Halpha luminosity and star formation rate, the star formation rate density in the survey volume is estimated as 0.035^+0.024_-0.014 M_sun yr^-1 Mpc^-3. The angular two-point correlation function of Halpha emitters over 875 arcmin^2 at z = 0.24 is well fitted by a power-law form with w(theta) = 0.047^+0.017_-0.013 theta^-0.66 +- 0.08, corresponding to the correlation function of xi(r) = (r/2.6^+1.0_-0.8 Mpc)^(-1.66 +- 0.08). The small correlation length of Halpha emitters may imply the weak clustering of active star-forming galaxies.

astro-ph↗

arXiv2008

A photometric survey for Lyalpha-HeII dual emitters: Searching for Population III stars in high-redshift galaxies

We present a new photometric search for high-z galaxies hosting Population III (PopIII) stars based on deep intermediate-band imaging observations obtained in the Subaru Deep Field (SDF), by using Suprime-Cam on the Subaru Telescope. By combining our new data with the existing broad-band and narrow-band data, we searched for galaxies which emit strongly both in Ly_alpha and in HeII 1640 (``dual emitters'') that are promising candidates for PopIII-hosting galaxies, at 3.93 2 Msun/yr was found by our photometric search in 4.03 x 10^5 Mpc^3 in the SDF. This result disfavors low feedback models for PopIII star clusters, and implies an upper-limit of the PopIII SFR density of SFRD_PopIII < 5 x 10^-6 Msun/yr/Mpc^3. This new selection method to search for PopIII-hosting galaxies should be useful in future narrow-band surveys to achieve the first observational detection of PopIII-hosting galaxies at high redshifts.

astro-ph↗

arXiv2007

High-redshift Ly alpha emitters with a large equivalent width: Properties of i-dropout galaxies with an NB921-band depression in the Subaru Deep Field

We report new follow-up spectroscopy of i-dropout galaxies with an NB921-band depression found in the Subaru Deep Field. The NB921-depressed i-dropout selection method is expected to select galaxies with large equivalent width Ly alpha 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 Ly alpha emitters at z=6.11 and 6.00. Their rest-frame equivalent widths are 153A and 114A, 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 100A 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 Ly alpha emitters with a large Ly alpha equivalent width, in a wider redshift range than 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 100A 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.

astro-ph↗

arXiv2007

The HI content of star-forming galaxies at z = 0.24

We use observations from the Giant Metrewave Radio Telescope (GMRT) to measure the atomic hydrogen gas content of star-forming galaxies at z = 0.24 (i.e. a look-backtime of ~3 Gyr). The sample of galaxies studied were selected from Halpha-emitting field galaxies detected in a narrow-band imaging survey with the Subaru Telescope. The Anglo-Australian Telescope was used to obtain precise optical redshifts for these galaxies. We then coadded the HI 21 cm emission signal for all the galaxies within the GMRT spectral line data cube. From the coadded signal of 121 galaxies, we measure an average atomic hydrogen gas mass of (2.26 +- 0.90)*10^9 solar masses. We translate this HI signal into a cosmic density of neutral gas at z = 0.24 of Omega_gas = (0.91 +- 0.42)*10^-3. This is the current highest redshift at which Omega_gas has been constrained from 21 cm emission and our value is consistent with that estimated from damped Lyman-alpha systems around this redshift. We also find that the correlations between the Halpha luminosity and the radio continuum luminosity and between the star formation rate and the HI gas content in star-forming galaxies at z = 0.24 are consistent with the correlations found at z = 0. These two results suggest that the star formation mechanisms in field galaxies ~3 Gyr ago were not substantially different from the present, even though the star formation rate is 3 times higher.

astro-ph↗

arXiv2006

Environmental Effects on Gaseous Disks of the Virgo Spiral Galaxies

We found high molecular fractions ($f_{\rm mol}$; ratio of the molecular to total gas surface densities) in three of five Virgo spiral galaxies in spite of their low total gas column density, based on $^{12}$CO$(J=1-0)$ observations with the Nobeyama 45 m telescope equipped with a multi-beam receiver, BEARS. We interpret this as a result of environmental effects. Combining the CO data with HI data, the relationship between the surface density of the total gas (HI plus H$_2$) and $f_{\rm mol}$ indicates that the three galaxies near the cluster center have larger $f_{\rm mol}$ values than expected for field galaxies, while the others show normal $f_{\rm mol}$. The large $f_{\rm mol}$ is interpreted as being due either to effective HI gas stripping, even in the inner disks, or to large ISM pressure induced by the high ICM pressure and/or ram pressure, although the possibility of an unusually high metallicity cannot be ruled out.

astro-ph↗

arXiv2006

Dissipative transformation of non-nucleated dwarf galaxies into nucleated systems

Recent photometric observations by the {\it Hubble Space Telescope (HST)} have revealed the physical properties of stellar galactic nuclei in nucleated dwarf galaxies in the Virgo cluster of galaxies. In order to elucidate the formation processes of nucleated dwarfs, we numerically investigate gas dynamics, star formation, and chemical evolution within the central 1 kpc of gas disks embedded within the galactic stellar components of non-nucleated dwarfs. We find that high density, compact stellar systems can be formed in the central regions of dwarfs as a result of dissipative, repeated merging of massive stellar and gaseous clumps developed from nuclear gaseous spiral arms as a result of local gravitational instability. The central stellar components are found to have stellar masses which are typically $~$5% of their host dwarfs and show very flattened shapes, rotational kinematics, and central velocity dispersions significantly smaller than those of their host dwarfs. We also find that more massive dwarfs can develop more massive, more metal-rich, and higher density stellar systems in their central regions, because star formation and chemical enrichment proceed more efficiently owing to the less dramatic suppression of star formation by supernovae feedback effects in more massive dwarfs. Based on these results, we suggest that gas-rich, non-nucleated dwarfs can be transformed into nucleated ones as a result of dissipative gas dynamics in their central regions. We discuss the origin of the observed correlations between physical properties of stellar galactic nuclei and those of their host galaxies.

astro-ph↗

arXiv2006

New Supporting Evidence for the Overdensity of Galaxies around the Radio-Loud Quasar SDSS J0836+0054 at z =5.8

Recently, Zheng et al. (2005) found evidence for an overdensity of galaxies around a radio-loud quasar, SDSS J0836+0054, at z=5.8 (a five arcmin$^2$ region). We have examined our deep optical imaging data (B, V, r', i', z', and NB816) taken with the Suprime-Cam on the Subaru Telescope. The NB816 narrow-band filter (lambda_c = 815 nm and $Δλ= 12$ nm) is suitable for searching for Ly$α$ emitters at $z\approx 5.7$. We have found a new strong Ly$α$ emitter at $z \approx 5.7$ close to object B identified by Zheng et al. Further, the non detection of the nine objects selected by Zheng et al. (2005) in our B, V, and r' images provides supporting evidence that they are high-z objects.

astro-ph↗

arXiv2005

Strong Emission-Line Galaxies at Low Redshift in the Field around the Quasar SDSSp J104433.04-012502.2

We discuss observational properties of strong emission-line galaxies at low redshift found by our deep imaging survey for high-redshift Ly alpha emitters. In our surveys, we used the narrowband filter, NB816 (lambda_center=8150A with FWHM = 120A), and the intermediate-band filter, IA827 (lambda_center = 8270A with FWHM = 340A). In this survey, 62 NB816-excess (> 0.9 mag) and 21 IA827-excess (> 0.8 mag) objects were found. Among them, we found 20 NB816-excess and 4 IA827-excess Ly alpha emitter candidates. Therefore, it turns out that 42 NB816-excess and 17 IA827-excess objects are strong emission-line objects at lower redshift. Since 4 objects in the two low-z samples are common, the total number of strong low-z emitters is 55. Applying our photometric redshift technique, we identify 7 H alpha emitters at z~0.24, 20 H beta-[OIII] ones at z~0.65, and 11 [OII] ones at z~1.19. However, we cannot determine reliable photometric redshifts of the remaining 17 emitters. The distributions of their rest frame equivalent widths are consistently understood with recent studies of galaxy evolution from z~1 to z~0.

astro-ph↗

arXiv2005

An Intermediate-band imaging survey for high-redshift Lyman Alpha Emitters: The Mahoroba-11

We present results of our intermediate-band optical imaging survey for high-$z$ Ly$α$ emitters (LAEs) using the prime focus camera, Suprime-Cam, on the 8.2m Subaru Telescope. In our survey, we use eleven filters; four broad-band filters ($B$, $R_{\rm c}$, $i^\prime$, and $z^\prime$) and seven intermediate-band filters covering from 500 nm to 720 nm; we call this imaging program as the Mahoroba-11. The seven intermediate-band filters are selected from the IA filter series that is the Suprime-Cam intermediate-band filter system whose spectral resolution is $R = 23$. Our survey has been made in a $34^\prime \times 27^\prime$ sky area in the Subaru XMM Newton Deep Survey field. We have found 409 IA-excess objects that provide us a large photometric sample of strong emission-line objects. Applying the photometric redshift method to this sample, we obtained a new sample of 198 LAE candidates at $3 < z < 5$. We found that there is no evidence for evolution of the number density and the star formation rate density for LAEs with $\log L({\rm Ly}α) ({\rm erg s^{-1}}) > 42.67$ between $z \sim 3$ and 5.

astro-ph↗

arXiv2005

Narrow-Band Survey of the GOODS Fields: Search for Lyman-Alpha Emitters at z = 5.7

We present results from optical narrow-band lambda_c = 8150A ~ and Delta lambda = 120A) observations of the Great Observatories Origins Deep Survey (GOODS) fields, using Suprime-Cam on the Subaru Telescope. Using these narrow-band data, we then perform a survey of Lyman alpha Emitters (LAEs) at z~5.7. The LAE survey covers an area of approx 320 arcmin^2 and a co-moving volume of ~8.0 x 10^4 Mpc^3. 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.

astro-ph↗

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.

astro-ph↗