We have carried out spectroscopic observations at 3 μm, where extinction is small, of active galaxies to study the obscured emitting regions with LEWIS(Imanishi et al. 1996) mounted on the Steward Observatory 60 and 61 inch telescopes.

Surely the most striking radio feature in the Galactic center may be a sheaf of straight vertical filaments (VFs, e.g., Yusef-Zadeh, Morris, & Chance 1984) of the radio arc. The VFs are believed to be the manifestations of strong magnetic field lines (≥1mG) which have been illuminated by some local relativistic particle source.

Molecular clouds in the Galactic center region are characterised by their large velocity widths and physical conditions which differ from clouds in the Galactic disk (e.g., Morris 1996). These clouds may not be gravitationally bound, but in equilibrium with the high external pressure in the Galactic bulge (Spergel & Blitz 1992, Oka et al. 1997a).

Recent high resolution CO images of the Galactic center (GC) molecular clouds reveal many arc and/or shell structures (Oka et al. 1997). A considerable fraction of them may most probably be formed by an interaction between supernova remnants (SNRs) and molecular clouds. Studies of such cases in less confused areas are needed to clarify this picture. The SNR W44 is a good place for examining the SNR induced shell formation scenario.

We have performed Smoothed Particle Hydrodynamics (SPH) simulations in order to understand the dynamical structures of Galactic Center molecular clouds. In our study it was found that the structures of GC molecular clouds could be induced by the combined effects of a gravitational perturbation by rotating bar potential and the hydrodynamic collisions between the clouds.

We present a model aimed to reproduce the infrared brigtness distribution of the Galaxy, as observed by the COBE-DIRBE experiment, and by the Japanese balloon flights (eg. Hayakawa et al., 1981). Based on the disk brightness profiles of a sample of spiral galaxies, and on the observed brigthness distribution of bars, we adopted new mathematical expressions to represent each component (bulge, bar, disk, spiral arms).

In the last five years, millimeter-wave interferometry has clearly shown the existence of enormous masses (109–1010 M⊙) of molecular gas concentrated in the nuclear regions (R < 500 pc) of many luminous and ultra-luminous infrared galaxies. In these systems, molecular gas is an obvious source of fuel for nuclear starbursts and active galactic nuclei (AGN). For nearer, lower-luminosity systems there exists less systematic characterization of either the properties or the structure of the nuclear gas. Here we review recent results on the molecular gas in the nuclei of two near, lower-luminosity systems (M51 and NGC 1068) and contrast these results with those for the best studied ultra-luminous IRAS galaxy, Arp 220. For all three galaxies, there now exists CO(2–1) interferometry at high resolution which reveals, for the first time, disks of extremely dense, highly excited gas on scales of 50-300 pc. These structures vary in their levels of axisymmetry, thickness, and clumpiness. However, they share the ability to extinguish optical and near-infrared emission from active or stellar nuclei and perhaps to collimate radio jets and ionized outflows. Within the nuclear regions of these three galaxies, the molecular gas constitutes 10-50% of the total mass, with the most luminous systems having the highest gas mass-fractions.

There is now overwhelming evidence that spiral galaxies can experience nuclear outflows, from radio lobes perpendicular to the disks of edge-on galaxies (Hummel et al. 1983), to optical ionization cones (see list in Wilson and Tsvetanov 1994), to spectacular X-ray emission extending many kpc beyond the disk (e.g. Dahlem et al. 1996). These observations indicate that hot and/or energetic components of the ISM can escape away from the plane of the galaxy from their origin in the nuclear vicinity.

We have done a CO(1-0) imaging survey of the central regions of nearby spiral galaxies using the Nobeyama and Owens Valley Millimeter Arrays. The survey aims to reveal the nuclear gas properties in normal galaxies that have been paid relatively little attention compared to Seyferts, mergers, and ultraluminous IRAS galaxies. The sample consists of 20 galaxies that meet the following criteria: (1) i < 70° (2) δ > +5° (3) ICOdV ≥ 10 K(T∗A)km s−1 in the FCRAO Extragalactic CO Survey (4) no evidence of significant perturbation (e.g. merging). No selection was made on the basis of nuclear activity or IRAS data. The average distance of the galaxies is 15 Mpc and the average linear resolution is 300 pc.

We discuss the major results from a study of the molecular gas distribution and dynamics in the nuclear disks (ND) of a limited sample of nearby spirals: NGC891, NGC5907, NGC4565, NGC4013, NGC4321, M82, NGC3626 and NGC3593. The main objective is to search for observational evidences of secular evolution along the Hubble sequence in spirals and to study the specific role of bars in the process.

Bars fuel the prolific star formation rate in many Starburst galaxies. They provide a mechanism for feeding gas into the nuclei as well as a laboratory for the study of molecular gas that is unbound and diffuse due to tidal strain and cloud collisions. A large percentage of galaxies show a stellar bar which is, however, in most cases almost devoid of (molecular) gas, except in the central region. Thus, long gaseous bars are rare and transient phenomena.

Molecular gas in active galactic nuclei (AGN) may play multiple roles: it can fuel a massive black hole, collimate jets, and/or obscure a broad-line region from most viewing angles. In order to understand the physical state and kinematics of molecular gas in AGN, we are using the Owens Valley Millimeter Array to map several nearby active galxies at high resolution in the CO(2–1) line. Our first target is NGC 1068, which hosts an obscured broad-line Seyfert nucleus as well as an extended starburst. We acquired data from 35 baselines out to 242 m, with 10.4 km s−1 resolution. After calibration with MMA (Scoville et al. 1992), we subtracted a 23 mJy nuclear continuum source from the visibility data and made CLEAN maps with AIPS. The resulting sensitivity in each line channel is 32 mJy per 1.8″ × 1.5″ robustly-weighted beam (1″ = 70 pc at 14.4 Mpc).

It is now widely accepted that a combination of torques and dissipation can drive molecular gas into the circumnuclear (inner kpc) region of spirals. However, the fate of the circumnuclear gas is poorly understood and depends on inflow mechanisms, outflow processes, and on star formation. It is unclear why spiral galaxies convert comparable amount of circumnuclear H2 into stars over a timescale (tsf) that varies by nearly two orders of magnitude between galaxies. The ratio M(H2)/L(RC) can be used to show the range in this timescale, where L(RC) is the 1.49 GHz radio continuum (RC) luminosity (Condon et al. 1990), and M(H2) the hydrogen mass traced by CO observations in the central 45″ (4.5 kpc for D=20 Mpc). A similar range is seen if the central 10 micron luminosity is used. Preliminary results from Jogee's thesis suggest that the circumnuclear gas consumption timescale by star formation (tsf) is correlated with the gas morphology (see Fig. 1) and kinematics. In particular, the value of tsf spans a large range in different kinematic regimes within a given galaxy, and together with inflow, this can lead some galaxies to follow evolutionary paths sketched in Fig. 1.

Ultraluminous infrared galaxies (ULIRGs) are major mergers of gas-rich galaxies and are characterized by a molecular interstellar medium that is concentrated in the central kpc, where it forms a dynamically significant, or perhaps dominant, component (Scoville et al., 1991). Sanders et al. (1988) proposed that ULIRGs might harbour, or evolve into, dust-enshrouded quasars, formed in their obscured centers.

Spectra of Active Galactic Nuclei (AGN) indicate that in their central parts a composite motion of emission gas exists. An analysis of broad as well as narrow lines of active galaxies can give information about gas dynamics in central part of these objects (see e.g. Netzer 1990, Osterbrock 1990). Here we present the investigation of the Hβ and O III[4959,5007] line shapes of the two Sy 1 galaxis: Mrk 817 and III Zw2.

NGC4258 and Circinus are both nearby spiral galaxies with active galactic nuclei which show high velocity water maser emission and Seyfert 2 characteristics. Both show radio emission perpendicular to the normal spiral arms which seem to be related to the nuclear activity (van der Kruit et al., 1972; Harnett et al., 1990). In NGC 4258 they coincide with the inner part of the rapidly rotating nuclear disk as expected for nuclear jets. Due to the strong misalignment of the nuclear disk plane with the galaxy's disk plane the jets pass the galactic disk. Molecular gas in NGC 4258 has only been detected in the nucleus and along the Hα jets up to a distance of 2 kpc from the nucleus (Krause et al., 1990).

HCN, one of the most abundant high dipole-moment molecules (traces molecular gas at densities ≳ 3 × 104 cm−3 whereas CO traces at ∼ 500 cm−3), has only been detected in ∼ 25 galaxies, primarily towards the galactic nuclei (Solomon et al. 1992; Nguyen-Q-Rieu et al. 1992; Helfer & Blitz 1993; Aalto et al. 1995). Dense molecular gas properties in “normal” galactic disks, as compared to the centers of “normal” spirals, starbursts, and luminous IR galaxies are not clear. As part of a large HCN survey in 60 galaxies (Gao 1996; Gao & Solomon 1997), we present here HCN observations in central regions of a few nearby galaxies. HCN was mapped at least along the major axes in the inner disks to determine the total HCN luminosity, the distribution with radius of HCN emission, and of the surface brightness ratio of SBR ≡ IHCN/ICO.

It is well known that the radial distributions of atomic and molecular gas differ markedly in spiral galaxies, including our own (e.g. Burton & Gordon 1978, Sofue et al. 1995). We have recently begun a program to obtain multifrequency observations of several nearby galaxies in order to determine whether H I is needed to replenish the H2 consumed by star formation and, if so, how this might be accomplished. Here we present initial results on the nearby RSab galaxy NGC 4736, known for its bright ring of H II regions and recently shown to have a central stellar bar (Möllenhoff et al. 1995). We have mapped the distribution of CO (1-0) emission within r=1′ (1.8 kpc at D=6.2 Mpc) with the BIMA interferometer at ~5″ resolution, and added data from the NRAO 12-m telescope to recover zero-spacing information.

It is very important to know the physical conditions of circumnuclear molecular gas in order to understand the nature of Active Galactic Nuclei (AGN), since the circumnuclear molecular gas in active galaxies might be directly affected by or is affecting the activity of nucleus. To investigate the physical conditions of the molecular clouds in detail, multi-line observations with millimeter arrays are essential.

In order to study the spatial variation of molecular gas properties and their relation to the nuclear activities of galaxies, we have conducted an imaging survey of HCN(1-0) and CO(1-0) emission from nearby active galaxies with the Nobeyama Millimeter Array. Here we present the results of three galaxies, NGC 3504 (Starburst nucleus), NGC 4736 (Post-Starburst nucleus), and NGC 6951 (Seyfert nucleus with a circumnuclear star-forming ring).

In the observations of these three galaxies, we find that (1) the HCN distribution is significantly different from that of CO, and that (2) the HCN emission spatially correlates better with the massive star-forming region than CO emission (Figure 1, 2(a), and 2(b)). For instance, in the Post-Starburst nucleus of NGC 4736, we find a remarkable decrease of the HCN emission, despite of strong CO nuclear concentration. The upper limit (2σ) of the integrated intensity ratio I(HCN)/I(CO) within the central r < 150 pc region is 0.04, which is surprisingly low compared with the value in other galactic nucleus (cf. ratio = 0.08 within r < 300 pc in Milky Way, Jackson et al. 1996, ApJ, 456, 91). Presumably the dense molecular gas must have been consumed by a past starburst event in the nucleus. These results suggest a tight correlation between the gas density measured by the I(HCN)/I(CO) ratio and the star-forming activities in the central region of galaxies.

It seems that there exist a spatial anti-correlation between the I(HCN)/ I(CO) ratio and the gas velocity dispersion in NGC 6951 (Figure 2(c)). The ratio is not enhanced in the CO twin peaks which are interpreted as shock regions due to gas orbit crowding. These imply the importance of gravitational instability on the formation of dense molecular gas and successive massive starformation in this region.