Nario Kuno

Nobeyama CO Atlas of Nearby Spiral Galaxies: Distribution of Molecular Gas in Barred and Nonbarred Spiral Galaxies

The data from a CO(1 - 0) mapping survey of 40 nearby spiral galaxies performed with the Nobeyama 45-m telescope are presented. The criteria of the sample selection were (1) RC3 morphological type in the range Sa to Scd, (2) distance less than 25 Mpc, (3) inclination angle less than 79deg (RC3), (4) flux at 100 um higher than ~ 10 Jy, (5) spiral structure is not destroyed by interaction. The maps of CO cover most of the optical disk of the galaxies. We investigated the influence of bar on the distribution of molecular gas in spiral galaxies using these data. We confirmed that the degree of central concentration is higher in barred spirals than in non-barred spirals as shown by the previous works. Furthermore, we present an observational evidence that bars are efficient in driving molecular gas that lies within the bar length toward the center, while the role in bringing gas in from the outer parts of the disks is small. The transported gas accounts for about half of molecular gas within the central region in barred spiral galaxies. We found a correlation between the degree of central concentration and bar strength. Galaxies with stronger bars tend to have higher central concentration. The correlation implies that stronger bars accumulate molecular gas toward the center more efficiently. These results are consistent with long-lived bars.

On-The-Fly Observing System of the Nobeyama 45-m and ASTE 10-m Telescopes

We have developed a spectral line On-The-Fly (OTF) observing mode for the Nobeyama Radio Observatory 45- ma nd the Atacama Submillimeter Telescope Experiment 10-m telescopes. Sets of digital autocorrelation spectrometers are available for OTF with heterodyne receivers mounted on the telescopes, including the focal-plane 5 � 5a rray receiver, BEARS, on the 45-m. During OTF observations, the antenna is continuously driven to cover the mapped region rapidly, resulting in a high observing efficiency and accuracy. Pointing of the antenna and readouts from the spectrometer are recorded as fast as 0.1 s. In this paper we report on improvements made to the software and instruments, requirements and optimization of observing parameters, the data-reduction process, and verification of the system. It is confirmed that, using optimal parameters ,t he OTF is about twice as efficient as the conventional position-switch observing method.

The AMIGA sample of isolated galaxies - IX. Molecular gas properties

Aims. We characterize the molecular gas content (ISM cold phase) using CO emission of a redshift-limited subsample of isolated galaxies from the AMIGA (Analysis of the interstellar Medium of Isolated GAlaxies) project in order to provide a comparison sample for studies of galaxies in different environments. Methods. We present the 12 CO(1–0) data for 273 AMIGA galaxies, most of them (n = 186) from our own observations with the IRAM 30 m and the FCRAO 14 m telescopes and the rest from the literature. We constructed a redshift-limited sample containing galaxies with 1500 km s −1 <v< 5000 km s −1 and excluded objects with morphological evidence of possible interaction. This sample (n = 173) is the basis for our statistical analysis. It contains galaxies with molecular gas masses, MH2 , in the range of ∼10 8 −10 10 M� .I t is dominated, both in absolute number and in detection rate, by spiral galaxies of type T = 3–5 (Sb-Sc). Most galaxies were observed with a single pointing towards their centers. Therefore, we performed an extrapolation to the total molecular gas mass expected in the entire disk based on the assumption of an exponential distribution. We then studied the relationships between MH2 and other galactic properties (LB, D 2 , LK, LFIR ,a ndMHI). Results. We find correlations between MH2 and LB, D 2 , LK ,a ndLFIR. The tightest correlation of MH2 holds with LFIR and, for T = 3– 5, with LK, and the poorest with D 2 . The correlations with LFIR and LK are very close to linearity. The correlation with LB is nonlinear so that MH2 /LB increases with LB. The molecular and the atomic gas masses of our sample show no strong correlation. We find a low mean value, log(MH2 /MHI) = −0.7 (for T = 3–5), and a strong decrease in this ratio with morphological type. The molecular gas column density and the surface density of the star formation rate (the Kennicutt-Schmidt law) show a tight correlation with a rough unity slope. We compare the relations of MH2 with LB and LK found for AMIGA galaxies to samples of interacting galaxies from the literature and find an indication for an enhancement of the molecular gas in interacting galaxies of up to 0.2–0.3 dex.

Near-Infrared and Millimeter Constraints on the Nuclear Energy Source of the Infrared-luminous Galaxy NGC 4418

We present near-infrared and millimeter investigations of the nucleus of the infrared-luminous galaxy NGC 4418, which previous observations suggest possesses a powerful buried active galactic nucleus (AGN). We find the following main results: (1) The infrared K-band spectrum shows CO absorption features at 2.3–2.4 μm from stars and very strong H2 emission lines. The luminosity ratios of H2 emission lines are suggestive of a thermal origin, and the equivalent width of the H2 1–0 S(1) line is the second largest observed to date in an external galaxy, after the well-studied strong H2-emitting galaxy NGC 6240. (2) The infrared L-band spectrum shows a clear polycyclic aromatic hydrocarbon (PAH) emission feature at 3.3 μm, which is usually found in star-forming galaxies. The estimated star formation luminosity from the observed PAH emission can account for only a small fraction of the infrared luminosity. (3) Millimeter interferometric observations of the nucleus reveal a high HCN (1–0) to HCO+ (1–0) luminosity ratio of ~1.8, as has been previously found in pure AGNs. (4) The measurements of HCN (1–0) luminosity using a single-dish millimeter telescope show that the HCN (1–0) to infrared luminosity ratio is slightly larger than the average, but within the scattered range, for other infrared-luminous galaxies. All of these results can be explained by the scenario in which, in addition to energetically insignificant, weakly obscured star formation at the surface of the nucleus, a powerful X-ray–emitting AGN deeply buried in dust and high-density molecular gas is present.

Resolved Giant Molecular Clouds in Nearby Spiral Galaxies: Insights from the CANON CO (1-0) Survey

We resolve 182 individual giant molecular clouds (GMCs) larger than 2.5 × 10^5 M ☉ in the inner disks of 5 large nearby spiral galaxies (NGC 2403, NGC 3031, NGC 4736, NGC 4826, and NGC 6946) to create the largest such sample of extragalactic GMCs within galaxies analogous to the Milky Way. Using a conservatively chosen sample of GMCs most likely to adhere to the virial assumption, we measure cloud sizes, velocity dispersions, and ^(12)CO (J = 1-0) luminosities and calculate cloud virial masses. The average conversion factor from CO flux to H_2 mass (or X_(CO)) for each galaxy is 1-2 × 10^(20) cm^(–2) (K km s^(–1))^(–1), all within a factor of two of the Milky Way disk value (~2 × 10^(20) cm^(–2) (K km s^(–1))^(–1)). We find GMCs to be generally consistent within our errors between the galaxies and with Milky Way disk GMCs; the intrinsic scatter between clouds is of order a factor of two. Consistent with previous studies in the Local Group, we find a linear relationship between cloud virial mass and CO luminosity, supporting the assumption that the clouds in this GMC sample are gravitationally bound. We do not detect a significant population of GMCs with elevated velocity dispersions for their sizes, as has been detected in the Galactic center. Though the range of metallicities probed in this study is narrow, the average conversion factors of these galaxies will serve to anchor the high metallicity end of metallicity-X_(CO) trends measured using conversion factors in resolved clouds; this has been previously possible primarily with Milky Way measurements.

TRACING MOLECULAR GAS MASS IN EXTREME EXTRAGALACTIC ENVIRONMENTS: AN OBSERVATIONAL STUDY

U.L. acknowledges financial support from the research project AYA2007-67625-C02-02 from the Spanish Ministerio de Ciencia y Educacion and from the Junta de Anaducia.

A New 100-GHz Band Front-End System with a Waveguide-Type Dual-Polarization Sideband-Separating SIS Receiver for the NRO 45-m Radio Telescope

We developed a waveguide-type dual-polarization sideband-separating SIS receiver system of the 100GHz band for the 45-m radio telescope at the Nobeyama Radio Observatory, Japan. This receiver is composed of an ortho-mode transducer and two sideband-separating SIS mixers, which are both based on the waveguide technique. The receiver has four intermediate frequency bands of 4.0–8.0 GHz. Over the radio frequency range of 80–120 GHz, the single-sideband receiver noise temperatures are 50–100 K and the image rejection ratios are greater than 10 dB. We developed new matching optics for the telescope beam as well as new IF chains for the four IF signals. The new receiver system was installed in the telescope, and we successfully observed the 12 CO, 13 CO and C 18 O emission lines simultaneously toward the Sagittarius B2 region to confirm the performance of the receiver system. The SSB noise temperature of the system, including the atmosphere, became approximately half of that of the previous receiver system. The Image Rejection Ratios (IRRs) of the two 2SB mixers were calculated from the 12 CO and HCO + spectra from the W51 giant molecular cloud, resulting in > 20 dB for one polarization and > 12 dB for the other polarization.

PHYSICAL CONDITIONS IN MOLECULAR CLOUDS IN THE ARM AND INTERARM REGIONS OF M51

We report systematic variations in the emission line ratio of the CO J = 2–1 and J = 1–0 transitions (R2–1/1–0) in the grand-design spiral galaxy M51. The R2–1/1–0 ratio shows clear evidence for the evolution of molecular gas from the upstream interarm regions into the spiral arms and back into the downstream interarm regions. In the interarm regions, R2–1/1–0 is typically 0.7 (often 0.8–1.0) in the spiral arms, particularly at the leading (downstream) edge of the molecular arms. These trends are similar to those seen in Galactic GMCs with OB star formation (presumably in the Galactic spiral arms). R2–1/1–0 is also high, ∼0.8–1.0, in the central region of M51. Analysis of the molecular excitation using a Large Velocity Gradient radiative transfer calculation provides insight into the changes in the physical conditions of molecular gas between the arm and interarm regions: cold and low-density gas (10 K, 300 cm −3 ) is required for the interarm GMCs, but this gas must become warmer and/or denser in the more active star-forming spiral arms. The ratio R2–1/1–0 is higher in areas of high 24 μm dust surface brightness (which is an approximate tracer of star formation rate surface density) and high CO(1–0) integrated intensity (i.e., a well-calibrated tracer of total molecular gas surface density). The systematic enhancement of the CO(2–1) line relative to CO(1–0) in luminous star-forming regions suggests that some caution is needed when using CO(2–1) as a tracer of bulk molecular gas mass, especially when galactic structures are resolved.

Observations of Cold Dust in Nearby Elliptical Galaxies

Spectral energy distribution (SED) analyses that include new millimeter to far-infrared (FIR) observations obtained with continuum instruments on the Nobeyama and James Clerk Maxwell Telescopes and the Infrared Space Observatory are presented for seven nearby (<45 Mpc) FIR-bright elliptical galaxies. These are analyzed together with archival FIR and shortwave radio data obtained from the NASA/IPAC Extragalactic Database (NED). The radio to infrared SEDs are best-fitted by power law plus graybody models of dust residing in the central galactic regions within a 2.4 kpc diameter and with temperatures between ~21 and 28 K, emissivity index 2, and masses from ~1.6 to 19 × 105 M☉. The emissivity index is consistent with dust constituting amorphous silicate and carbonaceous grains previously modeled for stellar-heated dust observed in the Galaxy and other nearby extragalactic sources. Using updated dust absorption coefficients for this type of dust, dust masses are estimated that are similar to those determined from earlier FIR data alone, even though the latter results implied hotter dust temperatures. Fluxes and masses that are consistent with the new FIR and submillimeter data are estimated for dust cooler than 20 K within the central galactic regions. Tighter physical constraints for such cold, diffuse dust (if it exists) with low surface brightness will need sensitive FIR to submillimeter observations with the Spitzer Space Telescope, SCUBA2, or ALMA.

Resolved Measurements of X_(CO) in NGC 6946

We present the largest sample to date of giant molecular clouds (GMCs) in a substantial spiral galaxy other than the Milky Way. We map the distribution of molecular gas with high resolution and image fidelity within the central 5 kpc of the spiral galaxy NGC 6946 in the ^(12)CO (J = 1-0) transition. By combining observations from the Nobeyama Radio Observatory 45 m single dish telescope and the Combined Array for Research in Millimeter Astronomy interferometer, we are able to obtain high image fidelity and accurate measurements of L_CO compared with previous purely interferometric studies. We resolve individual GMCs, measure their luminosities and virial masses, and derive X CO—the conversion factor from CO measurements to H2 masses—within individual clouds. On average, we find that X_CO = 1.2 × 10^(20) cm^(–2) (K km s^(–1))^(–1), which is consistent within our uncertainties with previously derived Galactic values as well as the value we derive for Galactic GMCs above our mass sensitivity limit. The properties of our GMCs are largely consistent with the trends observed for molecular clouds detected in the Milky Way disk, with the exception of six clouds detected within ~400 pc of the center of NGC 6946, which exhibit larger velocity dispersions for a given size and luminosity, as has also been observed at the Galactic center.