arXiv subjects

Takuji Yamashita

Publications and source records attributed to Takuji Yamashita.

47 records · Page 3Linked to original sources

arXiv2018

ALMA observations of HCN and HCO+ outflows in the merging galaxy NGC 3256

We report ~2" resolution Atacama Large Millimeter/submillimeter Array observations of the HCN(1-0), HCO+(1-0), CO(1-0), CO(2-1), and CO(3-2) lines towards the nearby merging double-nucleus galaxy NGC 3256. We find that the high density gas outflow traced in HCN(1-0) and HCO+(1-0) emission is co-located with the diffuse molecular outflow emanating from the southern nucleus, where a low-luminosity active galactic nucleus (AGN) is believed to be the dominant source of the far-infrared luminosity. On the other hand, the same lines were undetected in the outflow region associated with the northern nucleus, whose primary heating source is likely related to starburst activity without obvious signs of AGN. Both HCO+(1-0)/CO(1-0) line ratio (i.e. dense gas fraction) and the CO(3-2)/CO(1-0) line ratio are larger in the southern outflow (0.20$\pm$0.04 and 1.3$\pm$0.2, respectively) than in the southern nucleus (0.08$\pm$0.01, 0.7$\pm$0.1, respectively). By investigating these line ratios for each velocity component in the southern outflow, we find that the dense gas fraction increases and the CO(3-2)/CO(1-0) line ratio decreases towards the largest velocity offset. This suggests the existence of a two-phase (diffuse and clumpy) outflow. One possible scenario to produce such a two-phase outflow is an interaction between the jet and the interstellar medium, which possibly triggers shocks and/or star formation associated with the outflow.

astro-ph.GA↗

arXiv2018

Spatially-resolved Dense Molecular Gas Excitation in the Nearby LIRG VV 114

We present high-resolution observations (0".2-1".5) of multiple dense gas tracers, HCN and HCO$^+$ ($J$ = 1-0, 3-2, and 4-3), HNC ($J$ = 1-0), and CS ($J$ = 7-6) lines, toward the nearby luminous infrared galaxy VV 114 with the Atacama Large Millimeter/submillimeter Array. All lines are robustly detected at the central gaseous filamentary structure including the eastern nucleus and the Overlap region, the collision interface of the progenitors. We found that there is no correlation between star formation efficiency and dense gas fraction, indicating that the amount of dense gas does not simply control star formation in VV 114. We predict the presence of more turbulent and diffuse molecular gas clouds around the Overlap region compared to those at the nuclear region assuming a turbulence-regulated star formation model. The intracloud turbulence at the Overlap region might be excited by galaxy-merger-induced shocks, which also explains the enhancement of gas-phase CH$_3$OH abundance previously found there. We also present spatially resolved spectral line energy distributions of HCN and HCO$^+$ for the first time, and derive excitation parameters by assuming optically-thin and local thermodynamic equilibrium (LTE) conditions. The LTE model revealed that warmer, HCO$^+$-poorer molecular gas medium is dominated around the eastern nucleus, harboring an AGN. The HCN abundance is remarkably flat ($\sim$3.5 $\times$ 10$^{-9}$) independently of the various environments within the filament of VV 114 (i.e., AGN, star formation, and shock).

astro-ph.GA↗

arXiv2017

No sign of strong molecular gas outflow in an infrared-bright dust-obscured galaxy with strong ionized-gas outflow

We report the discovery of an infrared (IR)-bright dust-obscured galaxy (DOG) that shows a strong ionized-gas outflow but no significant molecular gas outflow. Based on detail analysis of their optical spectra, we found some peculiar IR-bright DOGs that show strong ionized-gas outflow ([OIII]$λ$5007) from the central active galactic nucleus (AGN). For one of these DOGs (WISE J102905.90+050132.4) at $z_{\rm spec} = 0.493$, we performed follow-up observations using ALMA to investigate their CO molecular gas properties. As a result, we successfully detected $^{12}$CO($J$=2-1) and $^{12}$CO($J$=4-3) lines, and the continuum of this DOG. The intensity-weighted velocity map of both lines shows a gradient, and the line profile of those CO lines is well-fitted by a single narrow Gaussian, meaning that this DOG has no sign of strong molecular gas outflow. The IR luminosity of this object is $\log\,(L_{\rm IR}/L_{\odot})$ = 12.40 that is classified as ultraluminous IR galaxy (ULIRG). We found that (i) the stellar mass and star-formation rate relation and (ii) the CO luminosity and far-IR luminosity relation are consistent with those of typical ULIRGs at similar redshifts. These results indicate that the molecular gas properties of this DOG are normal despite that its optical spectrum showing a powerful AGN outflow. We conclude that a powerful ionized-gas outflow caused by the AGN does not necessarily affect the cold interstellar medium in the host galaxy at least for this DOG.

astro-ph.GA↗

arXiv2017

X-Ray bright optically faint active galactic nuclei in the Subaru Hyper Suprime-Cam wide survey

We construct a sample of X-ray bright optically faint active galactic nuclei by combining Subaru Hyper Suprime-Cam, XMM-Newton, and infrared source catalogs. 53 X-ray sources satisfying i band magnitude fainter than 23.5 mag and X-ray counts with EPIC-PN detector larger than 70 are selected from 9.1 deg^2, and their spectral energy distributions (SEDs) and X-ray spectra are analyzed. 44 objects with an X-ray to i-band flux ratio F_X/F_i>10 are classified as extreme X-ray-to-optical flux sources. SEDs of 48 among 53 are represented by templates of type 2 AGNs or starforming galaxies and show signature of stellar emission from host galaxies in the optical in the source rest frame. Infrared/optical SEDs indicate significant contribution of emission from dust to infrared fluxes and that the central AGN is dust obscured. Photometric redshifts determined from the SEDs are in the range of 0.6-2.5. X-ray spectra are fitted by an absorbed power law model, and the intrinsic absorption column densities are modest (best-fit log N_H = 20.5-23.5 cm^-2 in most cases). The absorption corrected X-ray luminosities are in the range of 6x10^42 - 2x10^45 erg s^-1. 20 objects are classified as type 2 quasars based on X-ray luminsosity and N_H. The optical faintness is explained by a combination of redshifts (mostly z>1.0), strong dust extinction, and in part a large ratio of dust/gas.

astro-ph.GA↗

arXiv2017

A universal correlation between star-formation activity and molecular gas properties across environments

We present the molecular gas mass fraction ($f_\mathrm{H_2}$) and star-formation efficiency (SFE) of local galaxies on the basis of our new CO($J=1-0$) observations with the Nobeyama 45m radio telescope, combined with the COLDGASS galaxy catalog, as a function of galaxy environment defined as the local number density of galaxies measured with SDSS DR7 spectroscopic data. Our sample covers a wide range in the stellar mass and SFR, and covers wide environmental range over two orders of magnitude. This allows us to conduct the first, systematic study of environmental dependence of molecular gas properties in galaxies from the lowest- to the highest-density environments in the local universe. We confirm that both $f_\mathrm{H_2}$ and SFE have strong positive correlations with the SFR offset from the star-forming main sequence ($Δ$MS), and most importantly, we find that these correlations are universal across all environments. Our result demonstrates that star-formation activity within individual galaxies is primarily controlled by their molecular gas content, regardless of their global environment. Therefore, we claim that one always needs to be careful about the $Δ$MS distribution of the sample when investigating the environmental effects on the H$_2$ gas content in galaxies.

astro-ph.GA↗

arXiv2017

Cold Molecular Gas Along the Merger Sequence in Local Luminous Infrared Galaxies

We present an initial result from the 12CO (J=1-0) survey of 79 galaxies in 62 local luminous and ultra-luminous infrared galaxy (LIRG and ULIRG) systems obtained using the 45 m telescope at the Nobeyama Radio Observatory. This is the systematic 12CO (J=1-0) survey of the Great Observatories All-sky LIRGs Survey (GOALS) sample. The molecular gas mass of the sample ranges 2.2 x 10^8 - 7.0 x 10^9 Msun within the central several kiloparsecs subtending 15" beam. A method to estimate a size of a CO gas distribution is introduced, which is combined with the total CO flux in the literature. The method is applied to a part of our sample and we find that the median CO radius is 1-4 kpc. From the early stage to the late stage of mergers, we find that the CO size decreases while the median value of the molecular gas mass in the central several kpc region is constant. Our results statistically support a scenario where molecular gas inflows towards the central region from the outer disk, to replenish gas consumed by starburst, and that such a process is common in merging LIRGs.

astro-ph.GA↗

arXiv2017

Clustering of quasars in a wide luminosity range at redshift 4 with Subaru Hyper Suprime-Cam wide field imaging

We examine the clustering of quasars over a wide luminosity range, by utilizing 901 quasars at $\overline{z}_{\rm phot}\sim3.8$ with $-24.73<M_{\rm 1450}<-22.23$ photometrically selected from the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) S16A Wide2 date release and 342 more luminous quasars at $3.4<z_{\rm spec}<4.6$ having $-28.0<M_{\rm 1450}<-23.95$ from the Sloan Digital Sky Survey (SDSS) that fall in the HSC survey fields. We measure the bias factors of two quasar samples by evaluating the cross-correlation functions (CCFs) between the quasar samples and 25790 bright $z\sim4$ Lyman Break Galaxies (LBGs) in $M_{\rm 1450}<-21.25$ photometrically selected from the HSC dataset. Over an angular scale of \timeform{10.0"} to \timeform{1000.0"}, the bias factors are $5.93^{+1.34}_{-1.43}$ and $2.73^{+2.44}_{-2.55}$ for the low and high luminosity quasars, respectively, indicating no luminosity dependence of quasar clustering at $z\sim4$. It is noted that the bias factor of the luminous quasars estimated by the CCF is smaller than that estimated by the auto-correlation function (ACF) over a similar redshift range, especially on scales below \timeform{40.0"}. Moreover, the bias factor of the less-luminous quasars implies the minimal mass of their host dark matter halos (DMHs) is $0.3$-$2\times10^{12}h^{-1}M_{\odot}$, corresponding to a quasar duty cycle of $0.001$-$0.06$.

astro-ph.GA↗

arXiv2016

Spatially resolved CO SLED of the Luminous Merger Remnant NGC 1614 with ALMA

We present high-resolution (1".0) Atacama Large Millimeter/submillimeter Array (ALMA) observations of CO (1-0) and CO (2- 1) rotational transitions toward the nearby IR-luminous merger NGC 1614 supplemented with ALMA archival data of CO (3-2), and CO (6-5) transitions. The CO (6-5) emission arises from the starburst ring (central 590 pc in radius), while the lower-$J$ CO lines are distributed over the outer disk ($\sim$ 3.3 kpc in radius). Radiative transfer and photon dominated region (PDR) modeling reveal that the starburst ring has a single warmer gas component with more intense far-ultraviolet radiation field ($n_{\rm{H_2}}$ $\sim$ 10$^{4.6}$ cm$^{-3}$, $T_{\rm{kin}}$ $\sim$ 42 K, and $G_{\rm{0}}$ $\sim$ 10$^{2.7}$) relative to the outer disk ($n_{\rm{H_2}}$ $\sim$ 10$^{5.1}$ cm$^{-3}$, $T_{\rm{kin}}$ $\sim$ 22 K, and $G_{\rm{0}}$ $\sim$ 10$^{0.9}$). A two-phase molecular interstellar medium with a warm and cold ($>$ 70 K and $\sim$ 19 K) component is also an applicable model for the starburst ring. A possible source for heating the warm gas component is mechanical heating due to stellar feedback rather than PDR. Furthermore, we find evidence for non-circular motions along the north-south optical bar in the lower-$J$ CO images, suggesting a cold gas inflow. We suggest that star formation in the starburst ring is sustained by the bar-driven cold gas inflow, and starburst activities radiatively and mechanically power the CO excitation. The absence of a bright active galactic nucleus can be explained by a scenario that cold gas accumulating on the starburst ring is exhausted as the fuel for star formation, or is launched as an outflow before being able to feed to the nucleus.

astro-ph.GA↗

arXiv2016

Investigating the Relation between CO (3-2) and Far Infrared Luminosities for Nearby Merging Galaxies Using ASTE

We present the new single dish CO (3-2) emission data obtained toward 19 early stage and 7 late stage nearby merging galaxies using the Atacama Submillimeter Telescope Experiment (ASTE). Combining with the single dish and interferometric data of galaxies observed in previous studies, we investigate the relation between the CO (3-2) luminosity (L'CO(3-2)) and the far Infrared luminosity (LFIR) in a sample of 29 early stage and 31 late stage merging galaxies, and 28 nearby isolated spiral galaxies. We find that normal isolated spiral galaxies and merging galaxies have different slopes (alpha) in the log L'CO(3-2) - log LFIR plane (alpha ~ 0.79 for spirals and ~ 1.12 for mergers). The large slope (alpha > 1) for merging galaxies can be interpreted as an evidence for increasing Star Formation Efficiency (SFE=LFIR/L'CO(3-2)) as a function of LFIR. Comparing our results with sub-kpc scale local star formation and global star-burst activity in the high-z Universe, we find deviations from the linear relationship in the log L'CO(3-2) - log LFIR plane for the late stage mergers and high-z star forming galaxies. Finally, we find that the average SFE gradually increases from isolated galaxies, merging galaxies, and to high-z submillimeter galaxies / quasi-stellar objects (SMGs/QSOs). By comparing our findings with the results from numerical simulations, we suggest; (1) inefficient star-bursts triggered by disk-wide dense clumps occur in the early stage of interaction and (2) efficient star-bursts triggered by central concentration of gas occur in the final stage. A systematic high spatial resolution survey of diffuse and dense gas tracers is a key to confirm this scenario.

astro-ph.GA↗

arXiv2015

Revised calibration for near- and mid-infrared images from ~4000 pointed observations with AKARI/IRC

The Japanese infrared astronomical satellite AKARI performed ~4000 pointed observations for 16 months until the end of 2007 August, when the telescope and instruments were cooled by liquid Helium. Observation targets include solar system objects, Galactic objects, local galaxies, and galaxies at cosmological distances. We describe recent updates on calibration processes of near- and mid-infrared images taken by the Infrared Camera (IRC), which has nine photometric filters covering 2-27 um continuously. Using the latest data reduction toolkit, we created calibrated and stacked images from each pointed observation. About 90% of the stacked images have a position accuracy better than 1.5". Uncertainties in aperture photometry estimated from a typical standard sky deviation of stacked images are a factor of ~2-4 smaller than those of AllWISE at similar wavelengths. The processed images together with documents such as process logs as well as the latest toolkit are available online.

astro-ph.IM↗

arXiv2015

Spatially-resolved Radio-to-Far-infrared SED of the Luminous Merger Remnant NGC 1614 with ALMA and VLA

We present the results of Atacama Large Millimeter/Submillimeter Array (ALMA) 108, 233, 352, and 691 GHz continuum observations and Very Large Array (VLA) 4.81 and 8.36 GHz observations of the nearby luminous merger remnant NGC 1614. By analyzing the beam (1".0 * 1".0) and uv (> 45 kλ) matched ALMA and VLA maps, we find that the deconvolved source size of lower frequency emission (< 108 GHz) is more compact (420 pc * 380 pc) compared to the higher frequency emission (> 233 GHz) (560 pc * 390 pc), suggesting different physical origins for the continuum emission. Based on an SED model for a dusty starburst galaxy, it is found that the SED can be explained by three components, (1) non-thermal synchrotron emission (traced in the 4.81 and 8.36 GHz continuum), (2) thermal free-free emission (traced in the 108 GHz continuum), and (3) thermal dust emission (traced in the 352 and 691 GHz continuum). We also present the spatially-resolved (sub-kpc scale) Kennicutt-Schmidt relation of NGC 1614. The result suggests a systematically shorter molecular gas depletion time in NGC 1614 (average τ_gas of 49 - 77 Myr and 70 - 226 Myr at the starburst ring and the outer region, respectively) than that of normal disk galaxies (~ 2 Gyr) and a mid-stage merger VV 114 (= 0.1 - 1 Gyr). This implies that the star formation activities in U/LIRGs are efficiently enhanced as the merger stage proceeds, which is consistent with the results from high-resolution numerical merger simulations.

astro-ph.GA↗