Sachiko K. Okumura

Bar-driven Transport of Molecular Gas to Galactic Centers and Its Consequences

We study the characteristics of molecular gas in the central regions of spiral galaxies on the basis of our CO(J = 1-0) imaging survey of 20 nearby spiral galaxies using the NRO and OVRO millimeter arrays. Condensations of molecular gas at galactic centers with size scales 1 kpc and CO-derived masses Mgas(R < 500 pc) ~ 108-109 M☉ are found to be prevalent in the gas-rich ~ L* galaxies. Moreover, the degree of gas concentration to the central kiloparsec is found to be higher in barred systems than in unbarred galaxies. This is the first statistical evidence for the higher central concentration of molecular gas in barred galaxies, and it strongly supports the theory of bar-driven gas transport. It is most likely that more than half of molecular gas within the central kiloparsec of a barred galaxy was transported there from outside by the bar. The supply of gas has exceeded the consumption of gas by star formation in the central kiloparsec, resulting in the excess gas in the centers of barred systems. The mean rate of gas inflow is statistically estimated to be larger than 0.1-1 M☉ yr-1. There is no clear correlation between gas mass in the central kiloparsec and the type of nuclear spectrum (H II, LINER, or Seyfert), suggesting that the amount of gas at this scale does not determine the nature of the nuclear activity. There is, however, a clear correlation for galaxies with larger gas-to-dynamical mass ratios to have H II nuclear spectra, while galaxies with smaller ratios show spectra indicating active galactic nuclei (AGNs). This trend may well be related to the gravitational stability of the nuclear gas disk, which is generally lower for larger gas mass fractions. It is therefore possible that all galaxies have active nuclei, but that dwarf AGNs are overwhelmed by the surrounding star formation when the nuclear molecular gas disk is massive and unstable. The theoretical prediction of bar dissolution by condensation of gas to galactic centers is observationally tested by comparing gas concentration in barred and unbarred galaxies. If a bar is to be destroyed so abruptly that the gas condensation at the nucleus does not have enough time to be consumed, then there would be currently unbarred but previously barred galaxies with high gas concentrations. The lack of such galaxies in our sample, together with the current rates of gas consumption at the galactic centers, suggests that the timescale for bar dissolution is larger than 108-1010 yr or a bar in a L* galaxy is not destroyed by a condensation of 108-109 M☉ gas in the central kiloparsec.

Digital spectrometers for the Nobeyama 45-m telescope

New digital spectrometers for the 25-BEam Array Receiver System (BEARS) of the Nobeyama 45-m telescope are described. A commercial digital oscilloscope is used as a digitizer. The digitizer samples analogue data with 2 bits (4 levels). Data of 512 MHz bandwidth are processed for four beams at the same time. The data-formatting unit demultiplexes 2 bits 8 data in parallel to 32 parallels and sends outputs to LSIs. General purpose LSIs for autocorrelation read the digital data with a clock rate of 32 MHz. Thirty-two LSIs of 32 lags connected in cascades calculate 1024-lag autocorrelation, and output a 1024-channel power spectrum of 512 MHz bandwidth. The bandwidth of 32 MHz is achieved by picking up the data in a rate of 1/16 in the front part of the autocorrelator module. The total performances have been demonstrated by long-term integration of noise signals from receivers and observations of the Galactic star-forming region W51 in CO line.

BEARS - SIS 25-BEam Array Receiver system for the NRO 45-m telescope

A large focal plane array receiver system for the NRO 45 m telescope (SIS 25-BEam Array Receiver System, or BEARS) is described. This new array receiver uses SIS junctions and has 25 elements. It can operate at the frequency range of 82 - 116 GHz. The development of this new system is almost complete. We describe about the whole system in detail, which includes the receiver, the IF systems, the new spectrometers and the remote control systems. We also describe about the performances and the uniformity of the system and show the astronomical result.

Formation of a Massive Black Hole at the Center of the Superbubble in M82

We performed 12CO (1-0), 13CO (1-0), and HCN (1-0) interferometric observations of the central region (about 450 pc in radius) of M82 with the Nobeyama Millimeter Array and have successfully imaged a molecular superbubble and spurs. The center of the superbubble is clearly shifted from the nucleus by 140 pc. This position is close to that of the massive black hole (BH) of 460 M☉ and the 2.2 μm secondary peak (a luminous supergiant-dominated cluster), which strongly suggests that these objects may be related to the formation of the superbubble. Consideration of star formation in the cluster based on the infrared data indicates that (1) the energy release from supernovae can account for the kinetic energy of the superbubble, (2) the total mass of stellar-mass BHs available for building up the massive BH may be much higher than 460 M☉, and (3) it is possible to form the middle-mass BH of 102-103 M☉ within the timescale of the superbubble. We suggest that the massive BH was produced and is growing in the intense starburst region.

The Virgo High-Resolution CO Survey: I. CO Atlas

We present the results of the Virgo high-resolution CO survey (ViCS) obtained with the Nobeyama Millimeter-wave Array (NMA). This survey was made in the course of a long-term project at Nobeyama from 1999 December through 2002 April. The objects were selected from Virgo cluster members, considering CO richness from single dish flux, mild inclination, and lack of strong tidal perturbations. The central 1 arc min regions (4.7 kpc) of 15 spiral galaxies were observed with resolutions of 2 to 5 arcsec and 10 to 20 km/s, and sensitivities of 20 mJy/ beam for a 10 km/s channel. The objects lie at the same distance of the Virgo cluster (16.1 Mpc), which is advantageous for comparisons among individual galaxies. We describe the details of observations and data reduction, and present an atlas of integrated CO intensity maps, velocity fields and position-velocity diagrams along the major axes. The molecular gas morphology in the Virgo galaxies shows a wealth of variety, not specifically depending on the Hubble types.

Aperture synthesis observations of the molecular environment of the Sgr A complex. I. The M-0. 13-0. 08 molecular cloud

NH{sub 3} and H{sub 2}O maser observations of the 20 km/s molecular cloud M-0.13-0.08 in the Sgr A complex region are reported. NH{sub 3} (1,1) and (2,2) lines were observed simultaneously, and the molecular gas temperature and density are estimated. The NH{sub 3} emission is elongated in the same direction of the entire M-0.13-0.08 cloud and has a large velocity gradient along its major axis. Strong NH{sub 3} emission is located in the northern part of the cloud, where a perturbed velocity field and broad line widths are observed. In addition, a new H{sub 2}O maser spot was detected near one of the nonthermal continuum sources. These observational results suggest the physical association between a part of the M-0.13-0.08 molecular cloud and the nonthermal continuum sources in the Sgr A complex. 39 refs.

Ultra-High-Density Molecular Core and Warped Nuclear Disk in the Deep Potential of Radio Lobe Galaxy NGC 3079

We have performed high-resolution synthesis observations of the 12CO (J = 1-0) line emission from the radio lobe edge-on spiral galaxy NGC 3079 using a seven-element millimeter-wave interferometer at the Nobeyama Radio Observatory, which consisted of the 45 m telescope and six-element array. The nuclear molecular disk (NMD) of 750 pc radius is found to be inclined by 20° from the optical disk, and the NMD has spiral arms. An ultra-high-density core (UHC) of molecular gas was found at the nucleus. The gaseous mass of the UHC within 125 pc radius is as large as ~3 × 108 M☉, an order of magnitude more massive than that in the same area of the Galactic center, and the mean density is as high as ~3 × 103H2 cm-3. A position-velocity diagram along the major axis indicates that the rotation curve already starts at a finite velocity exceeding 300 km s-1 from the nucleus. The surface mass density in the central region is estimated to be as high as ~105 M☉ pc-2, producing a very deep gravitational potential. We argue that the very large differential rotation in such a deep potential will keep the UHC gravitationally stable during the current star formation.

A finger-like extension of the 20 kilometer per second cloud toward the Galactic center

High-resolution ammonia-line images of the M − 0.13 − 0.08 («20 km s −1 ») cloud in the Galactic center region have been obtained with the Nobeyama Millimeter Array. The 20 km s −1 cloud has a finger-like structure extending toward the Galactic center. It almost reaches to the circumnuclear ring (disk) surrounding Sgr A West in projection. A large systematic velocity gradient (5-10 km s −1 arcmin −1 ) along the elongation is found over the whole length of the cloud. We suggest that these characteristics are caused by tidal force in the gravitational field of the Galactic center.

Molecular gas bar and expanding molecular ring in the nucleus of the spiral galaxy Maffei 2

Aperture synthesis observations of the central 1.5 kpc region of the spiral galaxy Maffei 2 have been made in CO(J = 1-0) line. A central narrow ridge of molecular gas with a size 1000 pc x 200 pc and a ringlike feature with a large noncircular motion (60 km/s) with a size 500 pc x 240 pc have been found. The narrow ridge is interpreted as a bar of molecular gas in shocks generated at the leading edges in the bar potential. The observational results suggest that the ring is an expanding and rotating ring of molecular gas in the plane of the galaxy. The ring may have been formed by a starburst induced by the efficient gas supply in the bar potential. 38 refs.

Digital Spectro-Correlator System for the Atacama Compact Array of the Atacama Large Millimeter/Submillimeter Array

We have developed an FX-architecture digital spectro-correlator for the Atacama Compact Array (ACA) of the Atacama Large Millimeter/submillimeter Array. The correlator is able to simultaneously process four pairs of dual polarization signals with the bandwidth of 2 GHz, which are received by up to sixteen antennas. It can calculate auto- and cross-correlation spectra including cross-polarization in all combinations of all the antennas, and output correlation spectra with flexible spectral configuration such as multiple frequency ranges and multiple frequency resolutions. Its spectral dynamic range is estimated to be higher than 10^4 relative to Tsys from processing results of thermal noise for eight hours with a typical correlator configuration. The sensitivity loss is also confirmed to be 0.9 % with the same configuration. In this paper, we report the detailed design of the correlator and the verification results of the developed hardware.