arXiv subjects

Ichi Tanaka

Publications and source records attributed to Ichi Tanaka.

At least 73 records · Page 4Linked to original sources

arXiv2014

The environments of Ly$α$ blobs I: Wide-field Ly$α$ imaging of TN J1338-1942, a powerful radio galaxy at $z\simeq 4.1$ associated with a giant Ly$α$ nebula

We exploit wide-field Ly$alpha$ imaging with Subaru to probe the environment around TN J1338-1942, a powerful radio galaxy with a >100 kpc Ly$α$ halo at z=4.11. We used a sample of Ly$α$ emitters (LAEs) down to $\log(L_{\rm Lyα} [erg\, s^{-1}])\sim 42.8$ to measure the galaxy density around TNJ1338, compared to a control sample from a blank field taken with the same instrument. We found that TNJ1338 resides in a region with a peak overdensity of $δ_{\rm LAE}=2.8\pm 0.5$ on scales of $8\, h^{-1}\rm Mpc$ (on the sky) and $112\, h^{-1}\rm Mpc$ (line of sight) in comoving coordinates. Adjacent to this overdensity, we found a strong underdensity where virtually no LAEs are detected. We used a semi-analytical model of LAEs derived from the Millennium Simulation to compare our results with theoretical predictions. While the theoretical density distribution is consistent with the blank field, overdense regions such as that around TNJ1338 are very rare, with a number density of $6.4\times 10^{-8}\rm Mpc^{-3}$ (comoving), corresponding to the densest < 0.4 percentile at $z\simeq 4.1$. We also found that the Ly$α$ luminosity function in the TNJ1338 field differs from that in the blank field: the number of bright LAEs ($\log(L_{\rm Lyα}[erg\,s^{-1}]) \gtrsim 43.3$) is enhanced, while the number of fainter LAEs is relatively suppressed. These results suggest that some powerful radio galaxies associated with Ly$α$ nebulae reside in extreme overdensities on $\sim 3$--$6\, \rm Mpc$ scales, where star-formation and AGN activity may be enhanced via frequent galaxy mergers or high rates of gas accretion from the surroundings.

astro-ph.GA↗

arXiv2014

The Redshift and Metallicity of the Host Galaxy of Dark GRB 080325 at z=1.78

We present near-infrared spectroscopy of the host galaxy of dark GRB 080325 using Subaru/MOIRCS. The obtained spectrum provides a clear detection of H$α$ emission and marginal [NII]$λ$6584. The host is a massive (M$_{*}\sim10^{11}$M$_{\odot}$), dusty ($A_{V}\sim 1.2$) star-forming galaxy at z=1.78. The star formation rate calculated from the H$α$ luminosity (35.6-47.0 M$_{\odot}$ yr$^{-1}$) is typical among GRB host galaxies (and star-forming galaxies generally) at z $>$1; however, the specific star formation rate is lower than normal star-forming galaxies at redshift $\sim$ 1.6, in contrast to the high specific star formation rates measured for many of other GRB hosts. The metallicity of the host is estimated to be 12+log(O/H)$_{\rm KK04}$$=$8.88. We emphasize that this is one of the most massive distant host galaxies for which metallcity is measured with emission-line diagnostics. The metallicity is fairly high among GRB hosts. However, this is still lower than the metallicity of normal star-forming galaxies of the same mass at z$\sim$1.6. The metallicity offset from normal star-forming galaxies is close to a typical value of other GRB hosts and indicates that GRB host galaxies are uniformly biased toward low metalicity over a wide range of redshift and stellar mass. The low-metallicity nature of the GRB 080325 host is likely not attributable to the fundamental metallicity relation of star-forming galaxies beacuse it is a metal-poor outlier from the relation and has a low sSFR. Thus we conclude that metallicity is important to the mechanism that produced this GRB.

astro-ph.GA↗

arXiv2014

NIR spectroscopic observation of massive galaxies in the protocluster at z = 3.09

We present the results of near-infrared spectroscopic observations of the $K$-band selected candidate galaxies in the protocluster at $z=3.09$ in the SSA22 field. We observed 67 candidates with $K_{\rm AB}<24$ and confirmed redshifts of the 39 galaxies at $2.0< z_{\rm spec}< 3.4$. Of the 67 candidates, 24 are certainly protocluster members with $3.04\leq z_{\rm spec}\leq 3.12$, which are massive red galaxies those have been unidentified in previous optical observations of the SSA22 protocluster. Many distant red galaxies (DRGs; $J-K_{\rm AB}>1.4$), hyper extremely red objects (HEROs; $J-K_{\rm AB}>2.1$), {\it Spitzer} MIPS 24 $μ$m sources, active galactic nuclei (AGNs) as well as the counterparts of Ly$α$ blobs and the AzTEC/ASTE 1.1-mm sources in the SSA22 field are also found to be the protocluster members. The mass of the SSA22 protocluster is estimated to be $\sim2-5\times10^{14}~M_{\odot}$ and this system is plausibly a progenitor of the most massive clusters of galaxies in the current Universe. The reddest ($J-K_{\rm AB}\geq 2.4$) protocluster galaxies are massive galaxies with $M_{\rm star}\sim10^{11}~M_{\odot}$ showing quiescent star formation activities and plausibly dominated by old stellar populations. Most of these massive quiescent galaxies host moderately luminous AGNs detected by X-ray. There are no significant differences in the [O{\footnotesize III}] $λ$5007/H$β$ emission line ratios, and [O{\footnotesize III}] $λ$5007 line widths and spatial extents of the protocluster galaxies from those of massive galaxies at $z\sim2-3$ in the general field.

astro-ph.GA↗

arXiv2014

High resolution deep imaging of a bright radio quiet QSO at z~3

We have obtained deep J & Ks-band images centered on a bright radio quiet QSO UM402 (z_{em}=2.856) using IRCS camera and AO systems on Subaru Telescope, as well as retrieved WFC3/F140W archive images. A faint galaxy (m_{k}=23.32 +/- 0.05 in the Vega system) that lies ~ 2.4" north of the QSO sightline has been clearly resolved in all three deep high resolution datasets, and appears as an irregular galaxy with two close components in the Ks-band images (separation ~ 0.3"). Given the small impact parameter (b=19.6 kpc, at z_{lls}=2.531), as well as the red color of (J-Ks)_{vega} ~ 1.6, it might be a candidate galaxy giving rise to the Lyman Limit system absorption at z_{abs}=2.531 seen in the QSO spectrum. After carefully subtracting the PSF from the QSO images, the host galaxy of this bright radio quiet QSO at z ~ 3 was marginally revealled. We placed a low limit of the host component of m_{k} ~ 23.3 according to our analyses.

astro-ph.CO↗

arXiv2014

Evidence for a gas-rich major merger in a proto-cluster at z=2.5

Gas-rich major mergers in high-redshift proto-clusters are important events, perhaps leading to the creation of the slowly rotating remnants seen in the cores of clusters in the present day. Here, we present a deep Jansky Very Large Array observation of CO J = 1-0 emission line in a proto-cluster at z = 2.5, USS1558-003. The target field is an extremely dense region, where 20 H-alpha emitters (HAEs) are clustering. We have successfully detected the CO emission line from three HAEs and discovered a close pair of red and blue CO-emitting HAEs. Given their close proximity (~30 kpc), small velocity offset (~300 km/s), and similar stellar masses, they could be in the early phase of a gas-rich major merger. For the red HAE, we derive a total infrared luminosity of L(IR)=5.1e12 Lsun using MIPS 24 um and radio continuum images. The L(IR)/L'(CO) ratio is significantly enhanced compared to local spirals and high-redshift disks with a similar CO luminosity, which is indicative of a starburst mode. We find the gas depletion timescale is shorter than that of normal star-forming galaxies regardless of adopted CO-H2 conversion factors. The identification of such a rare event suggests that gas-rich major mergers frequently take place in proto-clusters at z > 2 and may involve the formation processes of slow rotators seen in local massive clusters.

astro-ph.GA↗

arXiv2014

The environmental impacts on the star formation main sequence: an H-alpha study of the newly discovered rich cluster at z=1.52

We report the discovery of a strong over-density of galaxies in the field of a radio galaxy at z=1.52 (4C65.22) based on our broad-band and narrow-band (H-alpha) photometry with Subaru Telescope. We find that H-alpha emitters are located in the outskirts of the density peak (cluster core) dominated by passive red-sequence galaxies. This resembles the situation in lower-redshift clusters, suggesting that the newly discovered structure is a well-evolved rich galaxy cluster at z=1.5. Our data suggest that the color-density and stellar mass-density relations are already in place at z~1.5, mostly driven by the passive red massive galaxies residing within Rc<200 kpc from the cluster core. These environmental trends almost disappear when we consider only star-forming (SF) galaxies. We do not find SFR-density or SSFR-density relations amongst SF galaxies, and the location of the SF main sequence does not significantly change with environment. Nevertheless, we find a tentative hint that star-bursting galaxies (up-scattered objects from the main sequence) are preferentially located in a small group at ~1-Mpc away from the main body of the cluster. We also argue that the scatter of the SF main sequence could be dependent on the distance to the nearest neighboring galaxy.

astro-ph.GA↗

arXiv2014

Identification of the progenitors of rich clusters and member galaxies in rapid formation at z>2

We present the results of near-infrared spectroscopy of H$α$ emitters (HAEs) associated with two protoclusters around radio galaxies (PKS1138-262 at $z$=2.2 and USS1558-003 at $z$=2.5) with Multi-Object Infrared Camera and Spectrograph (MOIRCS) on the Subaru telescope. Among the HAE candidates constructed from our narrow-band imaging, we have confirmed membership of 27 and 36 HAEs for the respective protoclusters, with a success rate of 70 per cent of our observed targets. The large number of spectroscopically confirmed members per cluster has enabled us for the first time to reveal the detailed kinematical structures of the protoclusters at $z$$>$2. The clusters show prominent substructures such as clumps, filaments and velocity gradients, suggesting that they are still in the midst of rapid construction to grow to rich clusters at later times. We also estimate dynamical masses of the clusters and substructures assuming their local virialization. The inferred masses ($\sim$10$^{14}$M$_\odot$) of the protocluster cores are consistent with being typical progenitors of the present-day most massive class of galaxy clusters ($\sim$10$^{15}$M$_\odot$) if we take into account the typical mass growth history of clusters. We then calculated the integrated star formation rates of the protocluster cores normalized by the dynamical masses, and compare these with lower redshift descendants. We see a marked increase of star-forming activities in the cluster cores, by almost three orders of magnitude, as we go back in time to 11 billion years ago; this scales as (1$+$$z$)$^6$.

astro-ph.GA↗

arXiv2014

Mapping the large scale structure around a z=1.46 galaxy cluster in 3-D using two adjacent narrow-band filters

We present a novel method to estimate accurate redshifts of star-forming galaxies by measuring the flux ratio of the same emission line observed through two adjacent narrow-band filters. We apply this method to our NB912 and new NB921 data taken with Suprime-Cam on the Subaru Telescope of a galaxy cluster, XMMXCS J2215.9-1738, at z=1.46 and its surrounding structures. We obtain redshifts for 170 [OII] emission line galaxies at z~1.46, among which 41 galaxies are spectroscopically confirmed with MOIRCS and FMOS on the Subaru mainly, showing an accuracy of $σ$((z-z_spec})/(1+z_spec))=0.002. This allows us to reveal filamentary structures that penetrate towards the centre of the galaxy cluster and intersect with other structures, consistent with the picture of hierarchical cluster formation. We also find that the projected celestial distribution does not precisely trace the real distribution of galaxies, indicating the importance of the three dimensional view of structures to properly identify and quantify galaxy environments. We investigate the environmental dependence of galaxy properties with local density, confirming that the median colour of galaxies becomes redder in higher density region while the star-formation rate of star-forming galaxies does not depend strongly on local environment in this structure. This implies that the star-forming activity in galaxies is truncated on a relatively short time scale in the cluster centre.

astro-ph.CO↗

arXiv2013

Nature of H-alpha selected galaxies at z>2. I. Main sequence and dusty star-forming galaxies

We present the results from our narrow-band imaging surveys of HAEs at z=2.2 and z=2.5 in the Subaru/XMM-Newton Deep survey Field with near-infrared camera MOIRCS on Subaru Telescope. We have constructed a clean sample of 63 star-forming galaxies at z=2.2 and 46 at z=2.5. For 12 (or ~92%) out of 13 Hαemitters (HAEs) at z=2.2, their Hαemission lines have been successfully detected by the spectroscopy. While about 42% of the red, massive HAEs with M_*>10^{10.8} M_\odot contain AGNs, most of the blue, less massive ones are likely to be star-forming galaxies. This suggests that the AGN may play an important role in galaxy evolution at the late stage of truncation. For the HAEs excluding possible AGNs, we estimate the gas-phase metallicities on the basis of [N~{\sc ii}]/Hαratios, and find that the metallicities of the Hαselected galaxies at z=2.2 are lower than those of local star-forming galaxies at fixed stellar mass, as shown by previous studies. Moreover, we present and discuss the so-called "main sequence" of star-forming galaxies at z>2 based on our unique sample of HAEs. By correlating the level of dust extinction with the location on the main sequence, we find that there are two kinds/modes of dusty star-forming galaxies: star-bursting galaxies and metal-rich normal star-forming galaxies.

astro-ph.GA↗

arXiv2013

Nature of H-alpha selected galaxies at z>2. II. Clumpy galaxies and compact star-forming galaxies

We present the morphological properties of 109 Hα-selected galaxies at z>2 in SXDF-UDS-CANDELS field. With high-resolution optical/near-infrared images obtained by Hubble Space Telescope, we identify giant clumps within the Hαemitters (HAEs). We find that at least 41% of our sample show clumpy structures in the underlying disks. The color gradient of clumps is commonly seen in the sense that the clumps near the galactic center tend to be redder than those in the outer regions. The mid-infrared detection in galaxies with red clumps and the spatial distribution of Hαemission suggest that dusty star-formation activity is probably occurring in the nuclear red clumps. A gas supply to a bulge component through the clump migration is one of the most potent physical processes to produce such dusty star-forming clumps and form massive bulges in local early-type galaxies. They would become large quiescent galaxies at later times just by consumption or blowout of remaining gas. Also, while most of the HAEs have extended disks, we observe two massive, compact HAEs, whose stellar surface densities are significantly higher. They are likely to be the direct progenitors of massive, compact quiescent galaxies at z=1.5-2.0. Two evolutionary paths to massive quiescent galaxies are devised to account for both the size growth of quiescent galaxies and their increased number density from z~2 to z=0.

astro-ph.CO↗

arXiv2013

On the formation time scale of massive cluster ellipticals based on deep near-IR spectroscopy at z~2

We present improved constraints on the formation time scale of massive cluster galaxies based on rest-frame optical spectra of galaxies in a forming cluster located at z=2.16. The spectra are obtained with MOIRCS on the Subaru telescope with an integration time of ~7 hours. We achieve accurate redshift measurements by fitting SEDs using the spectra and broad-band photometry simultaneously, allowing us to identify probable cluster members. Clusters at low redshifts are dominated by quiescent galaxies, but we find that quiescent galaxies and star forming galaxies co-exist in this z=2 system. Interestingly, the quiescent galaxies form a weak red sequence in the process of forming. By stacking the spectra of star forming galaxies, we observe strong emission lines such as [OII] and [OIII] and we obtain a tentative hint of AGN activities in these galaxies. On the other hand, the stacked spectrum of the quiescent galaxies reveals a clear 4000A break with a possible CaII H+K absorption feature and strong emission lines such as [OII] are absent in the spectrum, confirming the quiescent nature of these galaxies. We then perform detailed spectral analyses of the stacked spectrum, which suggest that these massive quiescent galaxies formed at redshifts between 3 and 4 on a time scale of <~0.5 Gyr. This short formation time scale is not reproduced in recent numerical simulations. We discuss possible mechanisms for how these galaxies form 10^11 Msun stellar mass on a short time scale and become red and quiescent by z=2.

astro-ph.CO↗

arXiv2013

Re-evaluating Hot Jupiter WASP-12b: An Update

The hot Jupiter WASP-12b is one of the largest, hottest, and best-studied extrasolar planets. We revisit our recent analysis of WASP-12b's emission spectrum in light of near-infrared spectroscopic measurements which have been claimed to support either a hydride-dominated or carbon-rich atmospheric composition. We show that this new spectrum is still consistent with a featureless blackbody, indicating a nearly isothermal photosphere on the planet's day side. Thus the ensemble of occultation measurements for WASP-12b is still insufficient to constrain the planet's atmospheric composition.

astro-ph.EP↗

arXiv2012

Re-Evaluating WASP-12b: Strong Emission at 2.315 micron, Deeper Occultations, and an Isothermal Atmosphere

We revisit the atmospheric properties of the extremely hot Jupiter WASP-12b in light of several new developments. First, new narrowband (2.315 micron) secondary eclipse photometry that we present here, which exhibits a planet/star flux ratio of 0.45% +/- 0.06 %, corresponding to a brightness temperature of 3640 K +/- 230 K; second, recent Spitzer/IRAC and Hubble/WFC3 observations; and third, a recently observed star only 1" from WASP-12, which has diluted previous observations and which we further characterize here. We correct past WASP-12b eclipse measurements for the presence of this object, and we revisit the interpretation of WASP-12b's dilution-corrected emission spectrum. The resulting planetary emission spectrum is well-approximated by a blackbody, and consequently our primary conclusion is that the planet's infrared photosphere is nearly isothermal. Thus secondary eclipse spectroscopy is relatively ill-suited to constrain WASP-12b's atmospheric abundances, and transmission spectroscopy may be necessary to achieve this goal.

astro-ph.EP↗

arXiv2012

The Massive Distant Clusters of WISE Survey: The First Distant Galaxy Cluster Discovered by WISE

We present spectroscopic confirmation of a z=0.99 galaxy cluster discovered using data from the Wide-field Infrared Survey Explorer (WISE). This is the first z~1 cluster candidate from the Massive Distant Clusters of WISE Survey (MaDCoWS) to be confirmed. It was selected as an overdensity of probable z>~1 sources using a combination of WISE and SDSS-DR8 photometric catalogs. Deeper follow-up imaging data from Subaru and WIYN reveal the cluster to be a rich system of galaxies, and multi-object spectroscopic observations from Keck confirm five cluster members at z=0.99. The detection and confirmation of this cluster represents a first step towards constructing a uniformly-selected sample of distant, high-mass galaxy clusters over the full extragalactic sky using WISE data.

astro-ph.CO↗

arXiv2012

Massive starburst galaxies in a z=2.16 proto-cluster unveiled by panoramic H-alpha mapping

We present a panoramic narrow-band study of H-alpha emitters in the field of the z=2.16 proto-cluster around PKS1138-262 using MOIRCS on the Subaru Telescope. We find 83 H-alpha emitters down to a SFR(Ha)~10Msun/yr across a ~7'x7' region centered on the radio galaxy, and identify ~10-Mpc scale filaments of emitters running across this region. By examining the properties of H-alpha emitters within the large-scale structure, we find that galaxies in the higher-density environments at z=2.16 tend to have redder colours and higher stellar masses compared to galaxies in more underdense regions. We also find a population of H-alpha emitters with red colours ((J-Ks)>1), which are much more frequent in the denser environments and which have apparently very high stellar masses with M*>~10^11Msun, implying that these cluster galaxies have already formed a large part of their stellar mass before z~2. Spitzer Space Telescope 24-micron data suggests that many of these red H-alpha emitters are bright, dusty starbursts (rather than quiescent sources). We also find that the proto-cluster galaxies follow the same correlation between SFR and M* (the "main sequence") of z~2 field star-forming galaxies, but with an excess of massive galaxies. These very massive star-forming galaxies are not seen in our similar, previous study of z~1 clusters, suggesting that their star-formation activity has been shut off at 1<~z<~2. We infer that the massive red (but active) galaxies in this rich proto-cluster are likely to be the products of environmental effects, and they represent the accelerated galaxy formation and evolution in a biased high density region in the early Universe.

astro-ph.CO↗

arXiv2012

Substellar Objects in Nearby Young Clusters (SONYC) VI: The planetary-mass domain of NGC1333

Within the SONYC - Substellar Objects in Nearby Young Clusters - survey, we investigate the frequency of free-floating planetary-mass objects (planemos) in the young cluster NGC1333. Building upon our extensive previous work, we present spectra for 12 of the faintest candidates from our deep multi-band imaging, plus seven random objects in the same fields, using MOIRCS on Subaru. We confirm seven new sources as young very low mass objects (VLMOs), with Teff of 2400-3100K and mid-M to early-L spectral types. These objects add to the growing census of VLMOs in NGC1333, now totaling 58. Three confirmed objects (one found in this study) have masses below 15 MJup, according to evolutionary models, thus are likely planemos. We estimate the total planemo population with 5-15 MJup in NGC1333 is <~8. The mass spectrum in this cluster is well approximated by dN/dM ~ M^-alpha, with a single value of alpha = 0.6+/-0.1 for M<0.6Msol, consistent with other nearby star forming regions, and requires alpha <~ 0.6 in the planemo domain. Our results in NGC1333, as well as findings in several other clusters by ourselves and others, confirm that the star formation process extends into the planetary-mass domain, at least down to 6 MJup. However, given that planemos are 20-50 times less numerous than stars, their contribution to the object number and mass budget in young clusters is negligible. Our findings disagree strongly with the recent claim from a microlensing study that free-floating planetary-mass objects are twice as common as stars - if the microlensing result is confirmed, those isolated Jupiter-mass objects must have a different origin from brown dwarfs and planemos observed in young clusters.

astro-ph.SR↗

arXiv2012

A star-bursting proto-cluster in making associated to a radio galaxy at z=2.53 discovered by H_alpha imaging

We report a discovery of a proto-cluster in vigorous assembly and hosting strong star forming activities, associated to a radio galaxy USS 1558-003 at z=2.53, as traced by a wide-field narrow-band H_alpha imaging with MOIRCS on Subaru Telescope. We find 68 H_alpha emitters with dust-uncorrected SFRs down to 8.6 Msun/yr. Their spatial distribution indicates that there are three prominent clumps of H_alpha emitters, one surrounding the radio galaxy and another located at ~1.5 Mpc away to the south-west, and the other located in between the two. These contiguous three systems are very likely to merge together in the near future and may grow to a single more massive cluster at later times. Whilst most H_alpha emitters reside in the "blue cloud" on the color--magnitude diagram, some emitters have very red colors with J-Ks>1.38(AB). Interestingly, such red H_alpha emitters are located towards the faint end of the red sequence, and they tend to be located in the high density clumps. We do not see any statistically significant difference in the distributions of individual star formation rates or stellar masses of the H_alpha emitters between the dense clumps and the other regions, suggesting that this is one of the notable sites where the progenitors of massive galaxies in the present-day clusters were in their vigorous formation phase. Finally, we find that H_alpha emission of the radio galaxy is fairly extended spatially over ~4.5 arcsec. However it is not as widespread as its Lya halo, meaning that the Lya emission is indeed severely extended by resonant scattering.

astro-ph.CO↗

arXiv2012

Assembly of Massive Galaxies in a High-z Protocluster

We present the results of wide-field deep JHK imaging of the SSA22 field using MOIRCS instrument equipped with Subaru telescope. The observed field is 112 arcmin^2 in area, which covers the z=3.1 protocluster characterized by the overdensities of Ly Alpha emitters (LAEs) and Ly Alpha Blobs (LABs). The 5 sigma limiting magnitude is K_{AB} = 24.3. We extract the potential protocluster members from the K-selected sample by using the multi-band photometric-redshift selection as well as the simple color cut for distant red galaxies (DRGs; J-K_{AB}>1.4). The surface number density of DRGs in our observed fields shows clear excess compared with those in the blank fields, and the location of the densest area whose projected overdensity is twice the average coincides with the large-scale density peak of LAEs. We also found that K-band counterparts with z_{phot} = 3.1 are detected for 75% (15/20) of the LABs within their Ly Alpha halo, and the 40 % (8/20) of LABs have multiple components, which gives a direct evidence of the hierarchical multiple merging in galaxy formation. The stellar mass ofLABs correlates with their luminosity, isophotal area, and the Ly Alpha velocity widths, implying that the physical scale and the dynamical motion of Ly Alpha emission are closely related to their previous star-formation activities. Highly dust-obscured galaxies such as hyper extremely red objects (HEROs; J-K_{AB}>2.1) and plausible K-band counterparts of submillimeter sources are also populated in the high density region.

astro-ph.CO↗