Yoichi Tamura

The Graduate University for Advanced Studies, Миура, Kanagawa, Japan

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Publications (80)247.08 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: We present < 1 kpc resolution CO imaging study of 37 optically-selected local merger remnants using new and archival interferometric maps obtained with ALMA, CARMA, SMA and PdBI. We supplement a sub-sample with single-dish measurements obtained at the NRO 45 m telescope for estimating the molecular gas mass (10^7 - 10^11 M_sun), and evaluating the missing flux of the interferometric measurements. Among the sources with robust CO detections, we find that 80 % (24/30) of the sample show kinematical signatures of rotating molecular gas disks (including nuclear rings) in their velocity fields, and the sizes of these disks vary significantly from 1.1 kpc to 9.3 kpc. The size of the molecular gas disks in 54 % of the sources is more compact than the K-band effective radius. These small gas disks may have formed from a past gas inflow that was triggered by a dynamical instability during a potential merging event. On the other hand, the rest (46 %) of the sources have gas disks which are extended relative to the stellar component, possibly forming a late-type galaxy with a central stellar bulge. Our new compilation of observational data suggests that nuclear and extended molecular gas disks are common in the final stages of mergers. This finding is consistent with recent major-merger simulations of gas rich progenitor disks. Finally, we suggest that some of the rotation-supported turbulent disks observed at high redshifts may result from galaxies that have experienced a recent major merger.
    The Astrophysical Journal Supplement Series 07/2014; 214(1). · 16.24 Impact Factor
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    ABSTRACT: Long-duration γ-ray bursts are associated with the explosions of massive stars and are accordingly expected to reside in star-forming regions with molecular gas (the fuel for star formation). Previous searches for carbon monoxide (CO), a tracer of molecular gas, in burst host galaxies did not detect any emission. Molecules have been detected as absorption in the spectra of γ-ray burst afterglows, and the molecular gas is similar to the translucent or diffuse molecular clouds of the Milky Way. Absorption lines probe the interstellar medium only along the line of sight, so it is not clear whether the molecular gas represents the general properties of the regions where the bursts occur. Here we report spatially resolved observations of CO line emission and millimetre-wavelength continuum emission in two galaxies hosting γ-ray bursts. The bursts happened in regions rich in dust, but not particularly rich in molecular gas. The ratio of molecular gas to dust (<9-14) is significantly lower than in star-forming regions of the Milky Way and nearby star-forming galaxies, suggesting that much of the dense gas where stars form has been dissipated by other massive stars.
    Nature 06/2014; 510(7504):247-9. · 38.60 Impact Factor
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    ABSTRACT: Sensitive observations with ALMA allow astronomers to observe the detailed distributions of molecules with relatively weak intensity in nearby galaxies. In particular, we report distributions of several molecular transitions including shock and dust related species ($^{13}$CO $J$ = 1--0, C$^{18}$O $J$ = 1--0, $^{13}$CN $N$ = 1--0, CS $J$ = 2--1, SO $J_N$ = 3$_2$--2$_1$, HNCO $J_{Ka,Kc}$ = 5$_{0,5}$--4$_{0,4}$, HC$_3$N $J$ = 11--10, 12--11, CH$_3$OH $J_K$ = 2$_K$--1$_K$, and CH$_3$CN $J_K$ = 6$_K$--5$_K$) in the nearby Seyfert 2 galaxy NGC 1068 observed with the ALMA early science program. The central $\sim$1 arcmin ($\sim$4.3 kpc) of this galaxy was observed in the 100 GHz region covering $\sim$96--100 GHz and $\sim$108--111 GHz with an angular resolution of $\sim4"\times2"$ (290 pc$\times$140 pc) to study the effects of an active galactic nucleus and its surrounding starburst ring on molecular abundances. Here, we present images and report a classification of molecular distributions into three main categories: (1) Molecules concentrated in the circumnuclear disk (CND) (SO $J_N$ = 3$_2$--2$_1$, HC$_3$N $J$ = 11--10, 12--11, and CH$_3$CN $J_K$ = 6$_K$--5$_K$), (2) Molecules distributed both in the CND and the starburst ring (CS $J$ = 2--1 and CH$_3$OH $J_K$ = 2$_K$--1$_K$), (3) Molecules distributed mainly in the starburst ring ($^{13}$CO $J$ = 1--0 and C$^{18}$O $J$ = 1--0). Since most of the molecules such as HC$_3$N observed in the CND are easily dissociated by UV photons and X-rays, our results indicate that these molecules must be effectively shielded. In the starburst ring, the relative intensity of methanol at each clumpy region is not consistent with those of $^{13}$CO, C$^{18}$O, and CS. This difference is probably caused by the unique formation and destruction mechanisms of CH$_3$OH.
    06/2014;
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    ABSTRACT: We present the results from a 1.1 mm imaging survey of the SSA22 field, known for having an overdensity of z=3.1 Lyman-alpha emitting galaxies (LAEs), taken with the AzTEC camera on the Atacama Submillimeter Telescope Experiment (ASTE). We imaged a 950 arcmin$^2$ field down to a 1 sigma sensitivity of 0.7-1.3 mJy/beam to find 125 submillimeter galaxies (SMGs) with a signal to noise ratio >= 3.5. Counterpart identification using radio and near/mid-infrared data was performed and one or more counterpart candidates were found for 59 SMGs. Photometric redshifts based on optical to near-infrared images were evaluated for 45 SMGs of these SMGs with Spitzer/IRAC data, and the median value is found to be z=2.4. By combining these estimation with estimates from the literature we determined that 10 SMGs might lie within the large-scale structure at z=3.1. The two-point angular cross-correlation function between LAEs and SMGs indicates that the positions of the SMGs are correlated with the z=3.1 protocluster. These results suggest that the SMGs were formed and evolved selectively in the high dense environment of the high redshift universe. This picture is consistent with the predictions of the standard model of hierarchical structure formation.
    03/2014; 440(4).
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    ABSTRACT: We present a deep Jansky Very Large Array observation of CO J=1-0 emission line in a proto-cluster at z=2.53, USS1558-003. The target field is an especially dense region, where 20 H-alpha emitters (HAEs) are clustering. We successfully detect CO emission lines from four HAEs and give an upper limit for 16 HAEs. Red, massive star-forming galaxies tend to be gas rich while blue less massive galaxies are faint in the CO emission. Given the close proximity (32 kpc), small velocity offset (130 km/s), and similar stellar masses of the two CO emitters, ID 191 and ID 193, they could be in the phase prior to a gas-rich major merger. ID 193 shows a red color in the rest-frame optical, suggesting the presence of dusty star-formation. Using MIPS 24 um and radio continuum images, we find a total infrared luminosity of L(IR)=5.1e12 Lsun for ID 193, corresponding to SFR~880 Msun/yr. The L(IR)/L'(CO) ratio is significantly enhanced compared to local spirals/high-redshift disks at a fixed CO luminosity, which is indicative of a starburst mode. Our results suggest that ID 193 lies in the late stage of merger where violent star formation is induced by a past interaction with a gas-rich galaxy but the neighboring blue HAE, ID 191. The most intriguing result is that such a system is discovered in an extremely dense region at z>2. The formation processes of slow rotators seen in local massive clusters may involve such multiple gas-rich mergers in proto-clusters at z>2.
    The Astrophysical Journal Letters 02/2014; 788(2). · 6.35 Impact Factor
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    ABSTRACT: We report the detection of a distant star-forming galaxy, ALMA J010748.3─173028, which is identified by a 13σ emission line at 99.75 GHz (SΔv = 3.1 Jy km s─1), behind the nearby merging galaxies VV114 using the Atacama Large Millimeter/submillimeter Array (ALMA) Band 3. We also find an 880 μm counterpart with ALMA Band 7 (S 880μm = 11.2 mJy). A careful comparison of the intensities of the line and the continuum suggests that the line is a redshifted 12CO transition. A photometric redshift analysis using the infrared to radio data favors a CO redshift of z = 2.467, although z = 3.622 is acceptable. We also find a hard X-ray counterpart, suggesting the presence of a luminous (L X ~ 1044 erg s─1) active galactic nucleus obscured by a large hydrogen column (N H ~ 2 × 1023 cm─2 if z = 2.47). A cosmological simulation shows that the chance detection rate of a CO-emitting galaxy at z > 1 with >=1 Jy km s─1 is ~10─3 per single ALMA field of view and 7.5 GHz bandwidth at 99.75 GHz. This demonstrates that ALMA has sufficient sensitivity to find an emission-line galaxy such as ALMA J010748.3─173028 even by chance, although the likelihood of stumbling across such a source is not high.
    01/2014; 781(2).
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    ABSTRACT: We report the detection of a distant star-forming galaxy, ALMA J010748.3-173028, which is identified by a 13-sigma emission line at 99.75 GHz (SdV = 3.1 Jy km/s), behind the nearby merging galaxies VV114 using the Atacama Large Millimeter/submillimeter Array (ALMA) Band 3. We also find an 880-um counterpart with ALMA Band 7 (S_880um = 11.2 mJy). A careful comparison of the intensities of the line and the continuum suggests that the line is a redshifted 12CO transition. A photometric redshift analysis using the infrared to radio data favors a CO redshift of z = 2.467, although z = 3.622 is acceptable. We also find a hard X-ray counterpart, suggesting the presence of a luminous (L_X ~ 10^44 erg/s) active galactic nucleus obscured by a large hydrogen column (N_H ~ 2 x 10^23 cm^-2 if z = 2.47). A cosmological simulation shows that the chance detection rate of a CO-emitting galaxy at z > 1 with \ge 1 Jy km/s is ~ 10^-3 per single ALMA field of view and 7.5-GHz bandwidth at 99.75 GHz. This demonstrates that ALMA has sufficient sensitivity to find an emission-line galaxy such as ALMA J010748.3-173028 even by chance, although the likelihood of stumbling across such a source is not high.
    12/2013;
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    ABSTRACT: We investigate the potential use of nebular emission lines in the rest-frame far-infrared (FIR) for determining spectroscopic redshift of z>8 galaxies with the Atacama Large Millimeter/sub-millimeter Array (ALMA). After making a line emissivity model as a function of metallicity, especially for the [O III] 88 micron line which is likely to be the strongest FIR line from H II regions, we predict the line fluxes from high-z galaxies based on a cosmological hydrodynamics simulation of galaxy formation. Since the metallicity of galaxies reaches at ~0.2 Zsun even at z>8 in our simulation, we expect the [O III] 88 micron line as strong as 1.3 mJy for 27 AB objects, which is detectable at a high significance by <1 hour integration with ALMA. Therefore, the [O III] 88 micron line would be the best tool to confirm the spectroscopic redshifts beyond z=8.
    The Astrophysical Journal 12/2013; 780(2). · 6.73 Impact Factor
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    ABSTRACT: We present preliminary results from our ongoing single-dish and interferometric molecular gas survey toward a sample of optically selected merger remnants. We show structural and kinematical evidence that a significant fraction of merger remnants form rotating molecular disks around their central remnants, and that the size of the resultant disks in some of the galaxies are larger compared to the central bulge size inferred from the Sersic radius. The high occurrence of disks in merger remnants may support the evolutionary scenario where major mergers can evolve into a variety of Hubble types.
    10/2013;
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    ABSTRACT: Since the first light observation of ANIR in June 2009, we have been carrying out a Paα narrow-band imaging survey of nearby luminous infrared galaxies (LIRGs). Because Paα is the strongest hydrogen recombination line in the infrared wavelength ranges, it is a good and direct tracer of dust-enshrouded star forming regions, and enables us to probe the star formation activities in LIRGs. We find that LIRGs have two star-forming modes. The origin of the two modes probably come from differences between merging stage and/or star-forming process.
    10/2013;
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    ABSTRACT: We present observational evidence for forming a molecular gas disk in merging galaxies using new and archival interferometric CO maps of 37 optically selected merger remnants in the local universe. We found that 65% (24/37) of the merger remnants sample shows kinematical signatures of a rotating disk in the interferometer maps. The sources with an extended CO disk have high FIR luminosities (˜ 1011 L⊙), which suggests that active star formation takes place in the extended CO disks and stellar disks will be possibly formed. The molecular gas-to-stellar mass ratio for merger remnants is larger than that of the stellar mass matched early-type galaxies. However, there is no significant evidence suggesting that the high gas mass ratio leads to a more efficient formation of a molecular gas disk.
    10/2013;
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    ABSTRACT: The redshift interval of 1.5-2.5 is the critical era for galaxy formation and early evolution when the star forming and AGN activities are both highest. Also, these activities should be dependent on environment given the clear habitat segregation of different types of galaxies seen today. It is essential therefore to systematically search for star forming galaxies over this critical epoch and across different environments, and investigate their physical properties in detail. For this purpose, we design a fully coordinated program between Subaru and ALMA. With Subaru, we have been conducting the Mahalo-Subaru project which maps out star-forming galaxies (Hɑ and OII emitters) at 1.5 ≤ z ≤ 2.5 across various environments, by employing the unique set of custom-made narrow-band filters and wide-field cameras. With ALMA, we propose to conduct the Gracias-ALMA project which aims to observe CO lines and dust continua of star forming galaxies with great spatial resolution and high sensitivity, based on our Mahalo-Subaru sample. We will scrutinize the physical states and the mode of star formation both globally and locally within galaxies, and identify the physical processes that regulate star-forming activities as a function of time and environment.
    10/2013;
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    ABSTRACT: We present results of our AzTEC/ASTE submillimeter galaxies (SMGs) survey in the Subaru/XMM-Newton Deep Field (SXDF), and VLA (21 cm), Spitzer (3.6-8 μm and 24 μm) and Herschel (100 and 160 μm) counterpart identifications for 1100-μm-selected SMGs. We obtained a 1100-μm map of ˜ 1200 arcmin2 with the rms noise level of ˜ 0.5 mJy (1 σ). Our AzTEC map achieved new detections of 281 sources (S/N ≥ 3.5 σ). For 221 of 281 AzTEC sources, we searched counterparts in order to obtain multi-wavelength data. Deep Herschel PACS 100-μm and 160-μm images cover the whole of the AzTEC map, and 34 of 221 (= 15 %) and 54 of 221 (= 24 %)AzTEC sources have PACS 100-μm and 160-μm counterparts, respectively. We found VLA radio and/or MIPS 24-μm robust (with a significance of ≥ 95 %) counterparts to 102 of 221 the AzTEC SMGs. In addition to this, we have developed an improved identification method using IRAC data (3.6-8 μm) and their colors, in order to cover the AzTEC SMGs located at higher redshift such as z > 3, which is inaccessible with the existing VLA, MIPS and Herschel data alone. With this method, we have achieved another robust detections of 20 IRAC counterparts to AzTEC sources. 131 of 221 AzTEC sources have at least one robust counterparts, and 23 of 221 have more than two robust counterparts. Including tentative identifications (with a significance of ≥ 90 %), 171 of 221 (= 78 %) 1100-μm-selected SMGs have counterparts. We estimated photometric redshifts of the counterparts using deep near-, and med-infrared and optical data, providing zmedian of 2.2. Among them, we found that there were some 1100-μm sources faint in multi-bands data suggesting the extremely high redshift.
    10/2013;
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    ABSTRACT: We present results of 12CO(J = 1-0) observations using NRO 45m radio telescope toward four interacting galaxies in the early and the mid stage of the interaction (Arp 84, VV 219, VV 254 and the Antennae Galaxies). We obtain central concentration of molecular gas and find interacting galaxies in the early and the mid stage have lower gas concentration toward the nuclei compared to isolated galaxies. We find molecular gas fraction in interacting galaxies is higher than isolated galaxies. With model fitting, this high molecular gas fraction is owing to high external pressure induced by the interaction. We also examine the relation between star formation rate and the surface density of interstellar gas (the sum of molecular and atomic hydrogen gas) resolving galactic structures. The relation is same as isolated galaxies, which suggests that high molecular gas fraction does not connect directly to the active star forming activity in interacting galaxies.
    10/2013;
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    ABSTRACT: We investigate the association of multiple mm-bright galaxies to the proto-cluster around 4C 23.56 at z = 2.48 by the combined observation of the AzTEC/ASTE singledish and the Plateau de Bure Interferometer. We detected 1.8 mm emissions from four HAEs associated with this proto-cluster, and detected CO line from two of them. From these observed values, we investigated their star formation rates (SFRs) and variation in mass compositions. Their SFRs are estimated as 60 to 690 M⊙ yr-1 from their submm-mm fluxes. SFRs from mm fluxes for three HAEs are comparable to SFRs from extinction corrected Hα luminosity, but only for HAE 431, large discrepancy in two SFR estimators is seen and large star formation should be dust-obscured.
    10/2013;
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    ABSTRACT: We have been developing a new method for (sub)mm spectroscopy with a frequency-modulating local oscillator (FMLO), which improves observing efficiencies and thus sensitivity of single-dish spectroscopic observations, compared to the conventional position- and frequency-switching methods. The noise removal technique which the FMLO method employes is similar to that used in continuum deep surveys and CMB experiments using multi-pixel bolometer cameras. We have implemented a new FMLO control system on the ASTE 10-m telescope, which demonstrates that the FMLO method does work for (1) minimizing the overheads and maximizing the observing throughput by efficiently removing correlated noises and (2) separating sidebands.
    10/2013;
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    ABSTRACT: We performed 12CO(J = 1-0) mapping observations toward four interacting galaxies in early and mid stages of the interaction using the 45-m telescope. Comparing to isolated galaxies whose data were also obtained with the 45-m, we found that interacting galaxies have (1) lower degree of the central concentration of molecular gas, (2) high molecular gas fraction (the mass of molecular gas to the sum of the mass of molecular gas and atomic gas ratio) and (3) no difference in Kennicutt-Schmidt law.
    10/2013;
  • H. Umehata, Y. Tamura, K. Kohno
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    ABSTRACT: We present the results from the 1.1mm continuum imaging the SSA22 field, which is known as an protocluster field which is traced by Lyα emitting galaxies (LAEs) at z = 3.1, with the AzTEC camera on ASTE telescope. We find 112 submillimeter(1.1mm)-selected galaxies (SMGs) with signal to noise ratio of ≥3.5 within 50% coverage region. Counterpart identification analysis utilizing radio(1.4GHz), MIPS ch1(24 μm) imaging data and IRAC color(3.6 μm, 4.5 μm, 5.8 μm, and 8.0 μm) diagnostics has been carried out to reveal that 48 SMGs have at least one reliable counterpart. We derived photometric redshifts of these sources and consequently seven z = 3.1 candidate SMGs are extracted. Two point angular correlation function between LAEs and these SMGs indicate that high density regions at the high redshift universe are the site of SMG/AGN formation. This picture is consistent with predictions from the standard model of hierarchical structure formation.
    10/2013;
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    ABSTRACT: We performed deep and wide-field surveys at 1.1 mm using AzTEC camera mounted on ASTE. We calculate the two-point angular autocorrelation functions in seven deep survey fields: AKARI Deep Field South (ADF-S), COSMOS, GOODS-N, GOODS-S, Lockman Hole East, SSA 22, and SXDF. We detect evidence for clustering signals in all the fields. By averaging the results on the bright sources measured in the ADF-S, the SSA 22, and the SXDF fields, we derived a more significant correlation function of 1.1 mm sources. A comparison of the correlation length with a bias evolution model of dark halos suggests that dark halos hosting bright 1.1 mm sources evolve into galaxy clusters in the present-day universe.
    10/2013;
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    ABSTRACT: We present a ˜1″ (100 pc) resolution CO(3–2) map of the Antennae galaxies obtained with the Submillimeter Array. We find that only < 30% of the GMAs spatially coincides with the optically detected star clusters, suggesting that the bulk of the CO (3–2) emission traces the regions with very recent or near future star formation activity. A high CO (3–2)/(1–0) ratio is seen in both nuclei and the southern complexes in the overlap region. Higher radiation field associated with intense star formation can account for the nucleus of NGC 4038 and the overlap region, but the nuclear region of NGC 4039 show relatively little star formation or AGN activities and cannot easily explained. We show kinematical evidence that the high line ratio in NGC 4039 is possibly caused by gas inflow into the counter-rotating central disk.
    10/2013;

Publication Stats

183 Citations
247.08 Total Impact Points

Institutions

  • 2014
    • The Graduate University for Advanced Studies
      • Department of Astronomical Science
      Миура, Kanagawa, Japan
  • 2006–2014
    • The University of Tokyo
      • Department of Astronomy
      Edo, Tōkyō, Japan
  • 2007–2013
    • National Astronomical Observatory of Japan
      • Astronomy Data Center
      Edo, Tōkyō, Japan
  • 2010
    • Pontifical Catholic University of Chile
      • Departamento de Astronomía y Astrofísica
      CiudadSantiago, Santiago, Chile