arXiv subjects

S. Matsushita

Publications and source records attributed to S. Matsushita.

At least 37 records · Page 2Linked to original sources

arXiv2012

Giant Molecular Clouds and Star Formation in the Tidal Molecular Arm of NGC 4039

The properties of tidally induced arms provide a means to study molecular cloud formation and the subsequent star formation under environmental conditions which in principle are different from quasi stationary spiral arms. We report the properties of a newly discovered molecular gas arm of likely tidal origin at the south of NGC 4039 and the overlap region in the Antennae galaxies, with a resolution of 1"68 x 0"85, using the Atacama Large Millimeter/submillimeter Array science verification CO(2-1) data. The arm extends 3.4 kpc (34") and is characterized by widths of ~ 200 pc (2") and velocity widths of typically \DeltaV ~ 10-20 km/s . About 10 clumps are strung out along this structure, most of them unresolved, with average surface densities of Σ_gas ~ 10-100 Msun pc^{-2}, and masses of (1-8) x 10^6 Msun. These structures resemble the morphology of beads on a string, with an almost equidistant separation between the beads of about 350 pc, which may represent a characteristic separation scale for giant molecular associations. We find that the star formation efficiency at a resolution of 6" (600 pc) is in general a factor of 10 higher than in disk galaxies and other tidal arms and bridges. This arm is linked, based on the distribution and kinematics, to the base of the western spiral arm of NGC 4039, but its morphology is different to that predicted by high-resolution simulations of the Antennae galaxies.

astro-ph.CO↗

arXiv2012

225 GHz Atmospheric Opacity Measurements from Two Arctic Sites

We report the latest results of 225 GHz atmospheric opacity measurements from two arctic sites; one on high coastal terrain near the Eureka weather station, on Ellesmere Island, Canada, and the other at the Summit Station near the peak of the Greenland icecap. This is a campaign to search for a site to deploy a new telescope for submillimeter Very Long Baseline Interferometry and THz astronomy in the northern hemisphere. Since 2011, we have obtained 3 months of winter data near Eureka, and about one year of data at the Summit Station. The results indicate that these sites offer a highly transparent atmosphere for observations in submillimeter wavelengths. The Summit Station is particularly excellent, and its zenith opacity at 225 GHz is statistically similar to the Atacama Large Milllimeter/submillimeter Array in Chile. In winter, the opacity at the Summit Station is even comparable to that observed at the South Pole.

astro-ph.IM↗

arXiv2012

ALMA Temporal Phase Stability and the Effectiveness of Water Vapor Radiometer

Atacama Large Millimeter/submillimeter Array (ALMA) will be the world largest mm/submm interferometer, and currently the Early Science is ongoing, together with the commissioning and science verification (CSV). Here we present a study of the temporal phase stability of the entire ALMA system from antennas to the correlator. We verified the temporal phase stability of ALMA using data, taken during the last two years of CSV activities. The data consist of integrations on strong point sources (i.e., bright quasars) at various frequency bands, and at various baseline lengths (up to 600 m). From the observations of strong quasars for a long time (from a few tens of minutes, up to an hour), we derived the 2-point Allan Standard Deviation after the atmospheric phase correction using the 183 GHz Water Vapor Radiometer (WVR) installed in each 12 m antenna, and confirmed that the phase stability of all the baselines reached the ALMA specification. Since we applied the WVR phase correction to all the data mentioned above, we also studied the effectiveness of the WVR phase correction at various frequencies, baseline lengths, and weather conditions. The phase stability often improves a factor of 2 - 3 after the correction, and sometimes a factor of 7 improvement can be obtained. However, the corrected data still displays an increasing phase fluctuation as a function of baseline length, suggesting that the dry component (e.g., N2 and O2) in the atmosphere also contributes the phase fluctuation in the data, although the imperfection of the WVR phase correction cannot be ruled out at this moment.

astro-ph.IM↗

arXiv2012

Disentangling the circumnuclear environs of Centaurus A: Gaseous Spiral Arms in a Giant Elliptical Galaxy

We report the existence of spiral arms in the recently formed gaseous and dusty disk of the closest giant elliptical, NGC 5128 (Centaurus A), using high resolution 12CO(2-1) observations of the central 3 arcmin (3 kpc) obtained with the Submillimeter Array (SMA). This provides evidence that spiral-like features can develop within ellipticals if enough cold gas exists. We elucidate the distribution and kinematics of the molecular gas in this region with a resolution of 4.4 x 1.9 (80 pc x 40 pc). The spiral arms extend from the circumnuclear gas at a radius of 200 pc to at least 1 kiloparsec. The general properties of the arms are similar to those in spiral galaxies: they are trailing, the width is \sim 500 \pm 200 pc, and the pitch angle is 20 degrees. From independent estimates of the time when the HI-rich galaxy merger occurred, we infer that the formation of spiral arms happened on a time scale of less than \sim10^8 yr. The formation of spiral arms increases the gas density and thus the star formation efficiency in the early stages of the formation of a disk.

astro-ph.GA↗

arXiv2011

SMA/PdBI multiple line observations of the nearby Seyfert2 galaxy NGC 1068: Shock related gas kinematics and heating in the central 100pc?

We present high angular resolution (0.5-2.0") observations of the mm continuum and the 12CO(J=3-2), 13CO(J=3-2), 13CO(J=2-1), C18O(J=2-1), HCN(J=3-2), HCO+(J=4-3) and HCO+(J=3-2) line emission in the circumnuclear disk (r=100pc) of the proto-typical Seyfert type-2 galaxy NGC1068, carried out with the Submillimeter Array. We further include in our analysis new 13CO(J=1-0) and improved 12CO(J=2-1) observations of NGC1068 at high angular resolution (1.0-2.0") and sensitivity, conducted with the IRAM Plateau de Bure Interferometer. Based on the complex dynamics of the molecular gas emission indicating non-circular motions in the central ~100pc, we propose a scenario in which part of the molecular gas in the circumnuclear disk of NGC1068 is radially blown outwards as a result of shocks. This shock scenario is further supported by quite warm (Tkin>=200K) and dense (nH2=10^4cm^-3) gas constrained from the observed molecular line ratios. The HCN abundance in the circumnuclear disk is found to be [HCN]/[12CO]=10^-3.5. This is slightly higher than the abundances derived for galactic and extragalactic starforming/starbursting regions. This results lends further support to X-ray enhanced HCN formation in the circumnuclear disk of NGC1068, as suggested by earlier studies. The HCO+ abundance ([HCO+]/[12CO]=10^-5) appears to be somewhat lower than that of galactic and extragalactic starforming/starbursting regions. When trying to fit the cm to mm continuum emission by different thermal and non-thermal processes, it appears that electron-scattered synchrotron emission yields the best results while thermal free-free emission seems to over-predict the mm continuum emission.

astro-ph.CO↗

arXiv2010

Molecular gas properties of galaxies: The SMA CO(2-1) B0DEGA legacy project

In the last two decades high resolution (< 5 arcsec) CO observations for ~ 150 galaxies have provided a wealth of information about the molecular gas morphologies in the circumnuclear regions. While in samples of 'normal' galaxies the molecular gas does not seem to peak toward the nuclear regions for about 50% of the galaxies, barred galaxies and mergers show larger concentrations. However, we do not exactly know from an observational point of view how the molecular gas properties of a galaxy evolve as a result of an interaction. Here we present the SMA CO(2-1) B0DEGA (Below 0 DEgree GAlaxies) legacy project in which we are imaging the CO(2-1) line of the circumnuclear regions (1 arcmin) of a large (~ 70) sample of nearby IR-bright spiral galaxies, likely interacting, and that still remained unexplored due to its location in the southern hemisphere. We find different molecular gas morphologies, such as rings, nuclear arms, nuclear bars and asymmetries. We find a centrally peaked concentration in about 85% of the galaxies with typical size scales of about 0.5 - 1 kpc. This might be related to perturbations produced by recent interactions.

astro-ph.CO↗

arXiv2010

High-resolution mapping of the physical conditions in two nearby active galaxies based on 12CO(1-0), (2-1) and (3-2) lines

We present a detailed analysis of high resolution observations of the three lowest CO transitions in two nearby active galaxies, NGC4569 and NGC4826. The CO(1-0) and (2-1) lines were observed with the Plateau de Bure Interferometer and the CO(3-2) line with the Submillimeter Array. Combining these data allows us to compare the emission in the three lines and to map the line ratios, R21=I_{CO(2-1)}/I_{CO(1-0)} and R32=I_{CO(3-2)}/I_{CO(1-0)} at a resolution of ~2", i.e., a linear resolution of 160 pc for NGC4569 and 40 pc for NGC4826. In both galaxies the emission in the three lines is similarly distributed spatially and in velocity, and CO is less excited (R32<0.6) than in the Galactic Center or the centers of other active galaxies studied so far. According to a pseudo-LTE model the molecular gas in NGC4569 is cold and mainly optically thick in the CO(1-0) and (2-1) lines; less than 50% of the gas is optically thin in the CO(3-2) line. LVG modeling suggests the presence of an elongated ring of cold and dense gas coinciding with the ILR of the stellar bar. More excited gas is resolved in the circumnuclear disk of NGC4826. According to our pseudo-LTE model this corresponds to warmer gas with a ~50% of the CO(3-2) emission being optically thin. LVG modeling indicates the presence of a semicircular arc of dense and cold gas centered on the dynamical center and ~70 pc in radius. The gas temperature increases and its density decreases toward the center. A near side/far side asymmetry noticeable in the CO, R32 and Pa-alpha maps suggests that opacity effects play a role. Examining published CO maps of nearby active galaxies we find similar asymmetries suggesting that this could be a common phenomenon in active galaxies. These mainly qualitative results open new perspectives for the study of active galaxies with the future Atacama Large Millimeter/submillimeter Array.

astro-ph.GA↗

arXiv2010

Disentangling the circumnuclear environs of Centaurus A: II. On the nature of the broad absorption line

We report on atomic gas (HI) and molecular gas (as traced by CO(2-1)) redshifted absorption features toward the nuclear regions of the closest powerful radio galaxy, Centaurus A (NGC 5128). Our HI observations using the Very Long Baseline Array allow us to discern with unprecedented sub-parsec resolution HI absorption profiles toward different positions along the 21 cm continuum jet emission in the inner 0."3 (or 5.4 pc). In addition, our CO(2-1) data obtained with the Submillimeter Array probe the bulk of the absorbing molecular gas with little contamination by emission, not possible with previous CO single-dish observations. We shed light with these data on the physical properties of the gas in the line of sight, emphasizing the still open debate about the nature of the gas that produces the broad absorption line (~55 km/s). First, the broad H I line is more prominent toward the central and brightest 21 cm continuum component than toward a region along the jet at a distance ~ 20 mas (or 0.4 pc) further from it. This suggests that the broad absorption line arises from gas located close to the nucleus, rather than from diffuse and more distant gas. Second, the different velocity components detected in the CO(2-1) absorption spectrum match well other molecular lines, such as those of HCO+(1-0), except the broad absorption line that is detected in HCO+(1-0) (and most likely related to that of the H I). Dissociation of molecular hydrogen due to the AGN seems to be efficient at distances <= 10 pc, which might contribute to the depth of the broad H I and molecular lines.

astro-ph.CO↗

arXiv2010

Unveiling the Nature of Submillimeter Galaxy SXDF850.6

We present an 880 micron Submillimeter Array (SMA) detection of the submillimeter galaxy SXDF850.6. SXDF850.6 is a bright source (S(850 micron) = 8 mJy) detected in the SCUBA Half Degree Extragalactic Survey (SHADES), and has multiple possible radio counterparts in its deep radio image obtained at the VLA. Our new SMA detection finds that the submm emission coincides with the brightest radio emission that is found ~8" north of the coordinates determined from SCUBA. Despite the lack of detectable counterparts in deep UV/optical images, we find a source at the SMA position in near-infrared and longer wavelength images. We perform SED model fits to UV-optical-IR photometry (u, B, V, R, i', z', J, H, K, 3.6 micron, 4.5 micron, 5.8 micron, and 8.0 micron) and to submm-radio photometry (850 micron, 880 micron, 1100 micron, and 21 cm) independently, and we find both are well described by starburst templates at a redshift of z ~= 2.2 (+/- 0.3). The best-fit parameters from the UV-optical-IR SED fit are a redshift of z = 1.87 (+0.15/-0.07), a stellar mass of M_star = 2.5 +2.2/-0.3 x 10^11 M_sun, an extinction of A_V = 3.0 (+0.3/-1.0) mag, and an age of 720 (+1880/-210) Myr. The submm-radio SED fit provides a consistent redshift of z ~ 1.8-2.5, an IR luminosity of L_IR = (7-26) x 10^12 L_sun, and a star formation rate of 1300-4500 M_sun/yr. These results suggest that SXDF850.6 is a mature system already having a massive amount of old stellar population constructed before its submm bright phase and is experiencing a dusty starburst, possibly induced by major mergers.

astro-ph.CO↗

arXiv2009

Cosmic Ray Tests of the Prototype TPC for the ILC Experiment

A time projection chamber (TPC) is a strong candidate for the central tracker of the international linear collider (ILC) experiment and we have been conducting a series of cosmic ray experiments under a magnetic field up to 4 T, using a small prototype TPC with a replaceable readout device: multi-wire proportional chamber (MWPC) or gas electron multiplier (GEM). We first confirmed that the MWPC readout could not be a fall-back option of the ILC-TPC under a strong axial magnetic field of 4 T since its spatial resolution suffered severely from the so called E x B effect in the vicinity of the wire planes. The GEM readout, on the other hand, was found to be virtually free from the E x B effect as had been expected and gave the resolution determined by the transverse diffusion of the drift electrons (diffusion limited). Furthermore, GEMs allow a wider choice of gas mixtures than MWPCs. Among the gases we tried so far a mixture of Ar-CF4-isobutane, in which MWPCs could be prone to discharges, seems promising as the operating gas of the ILC-TPC because of its small diffusion constant especially under a strong magnetic field. We report the measured drift properties of this mixture including the diffusion constant as a function of the electric field and compare them with the predictions of Magboltz. Also presented is the spatial resolution of a GEM-based ILC-TPC estimated from the measurement with the prototype.

physics.ins-det↗

arXiv2009

Disentangling the circumnuclear environs of Centaurus A: I. High resolution molecular gas imaging

We present high resolution images of the 12CO(2-1) emission in the central 1' (1 kpc) of NGC 5128 (Centaurus A), observed using the SMA. We elucidate for the first time the distribution and kinematics of the molecular gas in this region with a resolution of 6'.0 x 2'.4 (100 pc x 40 pc). We spatially resolve the circumnuclear molecular gas in the inner 24'' x 12'' (400 pc x 200 pc), which is elongated along a position angle P.A. = 155 deg and perpendicular to the radio/X-ray jet. The SE and NW components of the circumnuclear gas are connected to molecular gas found at larger radii. This gas appears as two parallel filaments at P.A. = 120 deg, which are coextensive with the long sides of the 3 kiloparsec parallelogram shape of the previously observed dust continuum, as well as ionized and pure rotational H2 lines. Spatial and kinematical asymmetries are apparent in both the circumnuclear and outer gas, suggesting non-coplanar and/or non-circular motions. We extend to inner radii (r < 200 pc) previously studied warped disk models built to reproduce the central parallelogram-shaped structure. Adopting the warped disk model we would confirm a gap in emission between the radii r = 200 - 800 pc (12'' - 50''), as has been suggested previously. Although this model explains this prominent feature, however, our 12CO(2-1) observations show relevant deviations from this model. Namely, the physical connection between the circumnuclear gas and that at larger radii, brighter SE and NW sides on the parallelogram-shaped feature, and an outer curvature of its long sides. Overall it resembles more closely an S-shaped morphology, a trend that is also found in other molecular species. Hence, we explore qualitatively the possible contribution of a weak bi-symmetric potential which would naturally explain these peculiarities.

astro-ph.GA↗

arXiv2008

Tracing star formation in galaxies with molecular line and continuum observations

We report our recent progress on extragalactic spectroscopic and continuum observations, including HCN(J=1-0), HCO$^+$(J=1-0), and CN(N=1-0) imaging surveys of local Seyfert and starburst galaxies using the Nobeyama Millimeter Array, high-J CO observations (J=3-2 observations using the Atacama Submillimeter Telescope Experiment (ASTE) and J=2-1 observations with the Submillimeter Array) of galaxies, and $λ$ 1.1 mm continuum observations of high-z violent starburst galaxies using the bolometer camera AzTEC mounted on ASTE.

astro-ph↗

arXiv2007

SMA High Angular Resolution Imaging of the Lensed Quasar APM08279+5255

We present Submillimeter Array observations of the z=3.91 gravitationally lensed broad absorption line quasar APM08279+5255 which spatially resolve the 1.0mm (0.2mm rest-frame) dust continuum emission. At 0.4" resolution, the emission is separated into two components, a stronger, extended one to the northeast (46+/-5mJy) and a weaker, compact one to the southwest (15+/-2mJy). We have carried out simulations of the gravitational lensing effect responsible for the two submm components in order to constrain the intrinsic size of the submm continuum emission. Using an elliptical lens potential, the best fit lensing model yields an intrinsic (projected) diameter of ~80pc, which is not as compact as the optical/near-infrared (NIR) emission and agrees with previous size estimates of the gas and dust emission in APM08279+5255. Based on our estimate, we favor a scenario in which the 0.2mm (rest-frame) emission originates from a warm dust component (T_d=150-220K) that is mainly heated by the AGN rather than by a starburst (SB). The flux is boosted by a factor of ~90 in our model, consistent with recent estimates for APM08279+5255.

astro-ph↗

arXiv2007

The Circumnuclear Molecular Gas in the Seyfert Galaxy NGC4945

We have mapped the central region of NGC 4945 in the $J=2\to1$ transition of $^{12}$CO, $^{13}$CO, and C$^{18}$O, as well as the continuum at 1.3 mm, at an angular resolution of $5\farc \times 3\farc$ with the Submillimeter Array. The relative proximity of NGC 4945 (distance of only 3.8 Mpc) permits a detailed study of the circumnuclear molecular gas and dust in a galaxy exhibiting both an AGN (classified as a Seyfert 2) and a circumnuclear starburst in an inclined ring with radius $\sim$2\farcs5 ($\sim$50 pc). We find that all three molecular lines trace an inclined rotating disk with major axis aligned with that of the starburst ring and large-scale galactic disk, and which exhibits solid-body rotation within a radius of $\sim$5\farc ($\sim$95 pc). We infer an inclination for the nuclear disk of $62^{\circ} \pm 2^{\circ}$, somewhat smaller than the inclination of the large-scale galactic disk of $\sim$$78^{\circ}$. The continuum emission at 1.3 mm also extends beyond the starburst ring, and is dominated by thermal emission from dust. If it traces the same dust emitting in the far-infrared, then the bulk of this dust must be heated by star-formation activity rather than the AGN. We discover a kinematically-decoupled component at the center of the disk with a radius smaller than $1\farcs4$ (27 pc), but which spans approximately the same range of velocities as the surrounding disk. This component has a higher density than its surroundings, and is a promising candidate for the circumnuclear molecular torus invoked by AGN unification models.

astro-ph↗

arXiv2007

High Resolution Molecular Gas Maps of M33

New observations of CO (J=1->0) line emission from M33, using the 25 element BEARS focal plane array at the Nobeyama Radio Observatory 45-m telescope, in conjunction with existing maps from the BIMA interferometer and the FCRAO 14-m telescope, give the highest resolution (13'') and most sensitive (RMS ~ 60 mK) maps to date of the distribution of molecular gas in the central 5.5 kpc of the galaxy. A new catalog of giant molecular clouds (GMCs) has a completeness limit of 1.3 X 10^5 M_sun. The fraction of molecular gas found in GMCs is a strong function of radius in the galaxy, declining from 60% in the center to 20% at galactocentric radius R_gal ~ 4 kpc. Beyond that radius, GMCs are nearly absent, although molecular gas exists. Most (90%) of the emission from low mass clouds is found within 100 pc projected separation of a GMC. In an annulus 2.1< R_gal <4.1 kpc, GMC masses follow a power law distribution with index -2.1. Inside that radius, the mass distribution is truncated, and clouds more massive than 8 X 10^5 M_sun are absent. The cloud mass distribution shows no significant difference in the grand design spiral arms versus the interarm region. The CO surface brightness ratio for the arm to interarm regions is 1.5, typical of other flocculent galaxies.

astro-ph↗

arXiv2007

High Resolution Imaging of Warm and Dense Molecular Gas in the Nuclear Region of the Luminous Infrared Galaxy NGC6240

We present ~2'' resolution CO(3-2), HCO+(4-3) and 880micron continuum images of the luminous infrared galaxy NGC6240 obtained at the Submillimeter Array. We find that the spatially resolved CO(3-2), HCO+(4-3) and the 880micron emission peaks between the two nuclear components that are both known to harbor AGNs. Our Large Velocity Gradient (LVG) analysis performed on each velocity channel suggests that the peak of the molecular gas emission traced in our observations is warm (T = 20 - 100K), dense (nH2 = 10^(5.0 - 5.4) cm^-3) and moderately optically thin (tau = 0.2 - 2) in the central 1 kpc. We also find large column densities of ~10^(23) cm^(-2). Such extreme conditions are observed over ~300 km s^(-1) centered around the CO derived systemic velocity. The derived molecular gas mass from the CO(3-2) emission and a CO-to-H2 conversion factor commonly used for ULIRGs is (6.9 +/- 1.7) x 10^9 Msun, and this is consistent with the mass derived from previous CO(2-1) observations. The gas is highly turbulent in the central kpc (Delta v_(FWZI) ~ 1175 km s^(-1)). Furthermore, possible inflow or outflow activity is suggested from the CO(3-2) velocity distribution. We tentatively state that 3.5 x 10^8 Msun of isolated CO(3-2) emission seen west of the northern disk may be associated with outflows from starburst superwinds, but the gas outflow scenario from one of the central AGN is not completely ruled out. Piecing all of the information together, the central region of NGC 6240 harbors 2 AGNs, ~10^(10) Msun of molecular gas mass, 5 x 10^7 Msun of dust mass, and has possible evidence of inflow and outflow activity.

astro-ph↗

arXiv2006

Structure and Kinematics of CO (J=2-1) Emission in the Central Region of NGC 4258

We present 12CO (J=2-1) observations towards the central region of the Seyfert 2 galaxy NGC 4258 with the Submillimeter Array (SMA). Our interferometric maps show two arm-like elongated components along the major axis of the galaxy, with no strong nuclear concentration. The CO (2-1) morphology and kinematics are similar to previous CO (1-0) results. The velocity field of the components agrees with the general galactic rotation, except for the east elongated component, which shows a significant velocity gradient along the east-west direction. In order to account for the velocity field, we propose the kinematical model where the warped rotating disk is also expanding. The line ratio of CO(2-1)/CO(1-0) reveals that the eastern component with the anomalous velocity gradient appears to be warmer and denser. This is consistent with the gas in this component being closer to the center, being heated by the central activities, and possibly interacted by expanding motions from the nuclear region.

astro-ph↗

arXiv2006

A Detection of [CII] line emission in the z=4.7 QSO BR1202-0725

We present ~3'' resolution imaging of the z=4.7 QSO BR1202-0725 at 900 micron from the Submillimeter Array. The two submillimeter continuum components are clearly resolved from each other, and the positions are consistent with previous lower frequency images. In addition, we detect [CII] line emission from the northern component. The ratio of [CII] to far-infrared luminosity is 0.04% for the northern component, and an upper limit of < 0.03% is obtained for the southern component. These ratios are similar to the low values found in local ultraluminous galaxies, indicating that the excitation conditions are different from those found in local field galaxies. X-ray emission is detected by Chandra from the southern component at L$_{0.5-2keV}=3\times10^{45}$~erg~s$^{-1}$, and detected at 99.6% confidence from the northern component at L$_{0.5-2keV}\sim3\times10^{44}$erg~s$^{-1}$, supporting the idea that BR1202-0725 is a pair of interacting galaxies at z=4.7 that each harbor an active nucleus.

astro-ph↗