The relationship between nuclear rings and the overall galaxy morphology

NGC 7742 is well known for its prominent blue nuclear ring around an EO-like core, and so appears as a Hoag-type galaxy, an elliptical galaxy with an outer ring (Schweizer et al. 1987). The galaxy is classified as Sa(r!) in the Revised Shapley-Ames Catalog (Sandage and Tammann 1987) with an exclamation mark to emphasize the prominence of the ring. Its photographs are published in Laustsen et al. (1987), Wray (1988), and Sandage & Bedke (1994).The ring has a diameter of 19″ = 1.6 kpc at a distance of 17.1 Mpc (Buta & Crocker 1993), and so should be a nuclear ring of the galaxy. Nuclear rings and pseudorings are often detected in strongly barred (SB) galaxies, and interpreted to be linked to the inner Lindblad resonance (Buta & Crocker 1993). These nuclear features are, however, also found in some weakly-barred (SAB) and non-barred (SA) galaxies. NGC 7742 is a galaxy of the highest circular symmetry in its core, ring, and main body, and so the best object for a detailed study of formation mechanisms of nuclear rings in non-barred galaxies.

We present high spatial resolution (≈24 pc) Atacama Large Millimeter/sub-millimeter Array 12CO(2-1) observations of the central region of the nearby barred spiral galaxy NGC 5806. NGC 5806 has a highly structured molecular gas distribution with a clear nucleus, a nuclear ring, and offset dust lanes. We identify 170 spatially and spectrally resolved giant molecular clouds (GMCs). These clouds have comparable sizes (Rc) and larger gas masses, observed linewidths (σobs, los), and gas mass surface densities than those of clouds in the Milky Way disc. The size–linewidth relation of the clouds is one of the steepest reported so far ($\sigma _{\mathrm{obs,los}}\propto R_{\mathrm{c}}^{1.20}$ ), the clouds are on average only marginally bound (with a mean virial parameter 〈αvir〉 ≈ 2), and high velocity dispersions are observed in the nuclear ring. These behaviours are likely due to bar-driven gas shocks and inflows along the offset dust lanes, and we infer an inflow velocity of ≈120 km s−1 and a total molecular gas mass inflow rate of ≈5 M⊙ yr−1 into the nuclear ring. The observed internal velocity gradients of the clouds are consistent with internal turbulence. The number of clouds in the nuclear ring decreases with azimuthal angle downstream from the dust lanes without clear variation of cloud properties. This is likely due to the estimated short lifetime of the clouds (≈6 Myr), which appears to be mainly regulated by cloud–cloud collision and/or shear processes. Overall, it thus seems that the presence of the large-scale bar and gas inflows to the centre of NGC 5806 affect cloud properties.

If active galactic nuclei (AGN) are powered by the accretion of matter onto massive black holes, how does the gas in the host galaxy lose the required angular momentum to approach the black hole? Gas easily transfers angular momentum to stars in strong bars, making them likely candidates. Although ground-based searches for bars in active galaxies using both optical and near-infrared surface brightness have not found any excess of bars relative to quiescent galaxies, the searches have not been able to rule out small-scale nuclear bars. To look for these nuclear bars we use Hubble Space Telescope Wide Field and Planetary Camera 2 and Near Infrared Camera and Multi-Object Spectrometer color maps to search for the straight dust lane signature of strong bars. Of the twelve Seyfert galaxies in our sample, only three have dust lanes consistent with a strong nuclear bar. Therefore, strong nuclear bars cannot be the primary fueling mechanism for Seyfert nuclei. We do find that a majority of the galaxies show an spiral morphology in their dust lanes. These spiral arms may be a possible fueling mechanism.

Nuclear rings are excellent laboratories for probing diverse phenomena such as the formation and evolution of young massive star clusters and nuclear starbursts, as well as the secular evolution and dynamics of their host galaxies. We have compiled a sample of 17 galaxies with nuclear rings, which are well resolved by high-resolution Hubble and Spitzer Space Telescope imaging. For each nuclear ring, we identified the ring star cluster population, along with their physical properties (ages, masses, and extinction values). We also determined the integrated ring properties, including the average age, total stellar mass, and current star formation rate (SFR). We find that Sb-type galaxies tend to have the highest ring stellar mass fraction with respect to the host galaxy, and this parameter is correlated with the ring’s SFR surface density. The ring SFRs are correlated with their stellar masses, which is reminiscent of the main sequence of star-forming galaxies. There are striking correlations between star-forming properties (i.e., SFR and SFR surface density) and nonaxisymmetric bar parameters, appearing to confirm previous inferences that strongly barred galaxies tend to have lower ring SFRs, although the ring star formation histories turn out to be significantly more complicated. Nuclear rings with higher stellar masses tend to be associated with lower cluster mass fractions, but there is no such relation for the ages of the rings. The two youngest nuclear rings in our sample, NGC 1512 and NGC 4314, which have the most extreme physical properties, represent the young extremity of the nuclear ring age distribution.

We present hydrodynamic simulations of gas clouds inflowing from the disk to a few hundred parsec region of the Milky Way. A gravitational potential is generated to include realistic Galactic structures by using thousands of multipole expansions (MEs) that describe 6.4 million stellar particles of a self-consistent Galaxy simulation. We find that a hybrid ME model, with two different basis sets and a thick-disk correction, accurately reproduces the overall structures of the Milky Way. Through non-axisymmetric Galactic structures of an elongated bar and spiral arms, gas clouds in the disk inflow to the nuclear region and form a central molecular zone-like nuclear ring. We find that the size of the nuclear ring evolves into ∼ 240 pc at T ∼ 1500 Myr , regardless of the initial size. For most simulation runs, the rate of gas inflow to the nuclear region is equilibrated to ∼ 0.02 M ⊙ yr − 1 . The nuclear ring is off-centered, relative to the Galactic center, by the lopsided central mass distribution of the Galaxy model, and thus an asymmetric mass distribution of the nuclear ring arises accordingly. The vertical asymmetry of the Galaxy model also causes the nuclear ring to be tilted along the Galactic plane. During the first ∼100 Myr, the vertical frequency of the gas motion is twice that of the orbital frequency, thus the projected nuclear ring shows a twisted, ∞ -like shape.

We present results obtained from ALMA CO (2–1) data of the double-barred galaxy NGC 3504. With three times higher angular resolution (∼0${_{.}^{\prime\prime}}$8) than previous studies, our observations reveal an inner molecular gas bar, a nuclear ring, and four inner spiral arm-like structures in the central 1-kpc region. Furthermore, the CO emission is clearly aligned with the two dust lanes in the outer bar region, with differences in shape and intensity between them. The total molecular gas mass in the observed region (50 arcsec × 57 arcsec) is estimated to be ${\sim}3. 1\times 10^9 \, {\rm M}_{\odot }$, which is 17 per cent of the stellar mass. We used the Kinemetry package to fit the velocity field and found that circular motion strongly dominates at R = 0.3–0.8 kpc, but radial motion becomes important at R < 0.3 kpc and R = 1.0–2.5 kpc, which is expected due to the presence of the inner and outer bars. Finally, assuming that the gas moves along the dust lanes in the bar rotating frame, we derived the pattern speed of the outer bar to be 18 ± 5 km s−1 kpc−1, the average streaming velocities on each of the two dust lanes to be 165 and 221 km s−1, and the total mass inflow rate along the dust lanes to be $12\, {\rm M}_{\odot }$ yr−1. Our results give a new example of an inner gas bar within a gas-rich double-barred galaxy and suggest that the formation of double-barred galaxies could be associated with the existence of such gas structures.

We present Ks-band surface photometry of NGC 7690 (Hubble type Sab) and NGC 4593 (SBb). We find that, in both galaxies, a major part of the bulge is as flat as the disk and has approximately the same color as the inner disk. In other words, the bulges of these galaxies have disklike properties. We conclude that these are examples of pseudobulges, that is, products of secular dynamical evolution. Nonaxisymmetries such as bars and oval disks transport disk gas toward the center. There star formation builds dense stellar components that look like?and often are mistaken for?merger-built bulges, but that were constructed slowly out of disk material. These pseudobulges can most easily be recognized when, as in the present galaxies, they retain disklike properties. NGC 7690 and NGC 4593 therefore contribute to the growing evidence that secular processes help to shape galaxies. NGC 4593 contains a nuclear ring of dust that is morphologically similar to nuclear rings of star formation that are seen in many barred and oval galaxies. The nuclear dust ring is connected to nearly radial dust lanes in the galaxy's bar. Such dust lanes are a signature of gas inflow. We suggest that gas is currently accumulating in the dust ring and hypothesize that the gas ring will starburst in the future. The observations of NGC 4593 therefore suggest that major starburst events that contribute to pseudobulge growth can be episodic.

NGC 4314 (Benedict et al. 1996) has a complex nuclear morphology with recent star formation confined to a nuclear ring. HST observations resolve the nuclear ring into multiple sites of new star formation and resolve the associated dust lanes into discrete clouds. We construct dynamical models of this galaxy in order to provide plausible identification of the dynamical processes that led to the formation of the observed structure. We assume that the center of this galaxy contains a very massive double black-hole, surrounded by relatively low-mass particles — the star clusters as well as gas and dust complexes. Our previous work (Anosova & Anandarao 1994, Anosova & Tanikawa 1995) showed that the dynamical evolution of such a model produces many structures similar to those observed in galaxies of diverse types: spiral and elliptical galaxies, interacting galaxies, and various types of flows and jets. In the present work, we consider a number of such models with different initial parameters. We study their dynamical evolution of the gravitational N-body problem, taking into account strong interactions of bodies. Comparison of the evolution of our models with the observed structure, distributions and motions of stars, gas, and dust complexes in NGC 4314 shows good agreement. The model predicts the velocity fields observed in this galaxy.

Aims. We wish to determine the influence of bars on the building of galaxy bulges through analysis of ages and metallicities derived from stellar absorption line-strength indices.

The nuclear region of the barred galaxy NGC 4314 was observed and modeled by VIJHK photometry, long-slit spectroscopy, and SPH simulations. Young stellar populations is supposed to be made from the gas which are accumulated in the nuclear ring and/or inner spiral arms by the strong bar potential.

Using the methods of spectrophotometric imagery and multislit spectroscopy, we have derived the radial abundance profiles from O/H measurements in 549 H II regions of eight early-type spiral galaxies. We then compared the characteristic abundance levels and radial distributions of this group of spirals with those of late-type galaxies. The early-type galaxies of our sample display gradients that are flatter and overall levels of O/H abundances that are higher than those of normal late-type galaxies. Early-type galaxies show an identical trend in the behavior of extrapolated central abundance versus morphological type to that shown by late-type galaxies with strong bars, even in the absence of a bar. On a diagram showing extrapolated central abundances versus morphological types, two clearly separated sequences appear: late-type barred galaxies and early-type (barred or unbarred) galaxies clearly fall on a sequence ~0.5 dex in abundance below that of normal late-type galaxies. This behavior is consistent with a scenario of morphological evolution of disk galaxies by the formation and dissolution of a bar over a period of a few 109 yr, where later type galaxies (Sd, Sc, Sbc) evolve into earlier-type disk galaxies (Sb, Sa) through transitory SBc and SBb phases.

We investigate photometric properties of spiral arms and stellar complexes/associations inside these arms in the grand design NGC 628 (M74) galaxy.We analyze GALEX ultraviolet, optical UBVRI, and H-alpha? surface photometry data, including those obtained with 1.5 m telescope at the Maidanak Observatory. In the longer arm, the large and bright stellar complexes are located at regular intervals along the arm, but only farther from the galaxy center. They are joined with the narrow lane of dust, visible only in the infrared bands. The usual dust lane along the stellar arm inner side is seen there only at distances closer to the galaxy center. It is well expressed in CO (H_2) image. We have found, that the second, short arm hosts two dust lanes, the strong and wide at the inner side, and narrow and irregular along its outer edge. This outer dust lane is well seen in IR images only. The shorter arm contains no star complexes at all. Gradients of age and luminosity of stars across both arms are missing (again excepting the parts of arms located closer to the center), which is confirmed by our photometric cuts across both arms. The drastic difference in the morphology of the two symmetric arms (grand design type) of a galaxy has now been confirmed by objective measurements in the case of M74. It is unclear why about two third of galaxies with beaded arms host these "beads" (star complexes) in one arm only.

We are carrying out a study on the stellar content (age and metallicity) along the bars in a small sample of barred spirals of different morphological types (SB0~SBb). The goal is to search for observational evidence of the evolution of stellar populations in barred spirals, and to compare with the predictions of numerical models found in the literature for the evolution of bars (e.g. Friedli and Benz: 1995, A&A 301, 649). We measure a large set of spectral indices (mainly the Lick system of indices, plus the CaII NIR triplet), complemented with optical-NIR color maps. Comparing them with stellar population synthesis models, we try to determine the mean luminosity-weighted age and metallicity of the (spatially averaged) stellar populations along the bars. We focus on the relative ages/metallicities, as absolute values are quite uncertain and more model dependent.We have found evidence of a difference in the mean stellar age between the nuclear and circumnuclear ring (CNR) of the SBa galaxy NGC 4314, greater than (at least) 3–4 Gyr. Also, there is a hint that a relatively small amount of gas has been supplied to the nuclear region by the bar, as shown by the spectra, broad-band images and line strength diagrams. We have found Mg overabundant relative to Fe in the nuclear region. Some of the uncertainties in the comparison of model predictions and observations are investigated (Lourenso et al.: 2001b, Ap&SS, in press). Recent results show a tight linear distribution in the color-magnitude and color-color diagrams along the nuclear spiral arms of NGC 4314, getting redder and fainter from the CNR outwards and suggesting, in principle, an age sequence (Lourenso et al.: 2001c, in prep.). A discussion on the stellar populations of the prototype double-barred galaxy NGC 5850 (SBb) will be presented in Lourenso et al. (200la, ApJ, submitted).

This thesis explores the molecular gas (the raw material for star formation), especially the dense molecular gas content of luminous infrared galaxies (LIGs) and ''normal'' galaxies using millimeter line observations. Most LIGs are closely interacting/merging galaxies and ''normal'' spiral galaxies are believed to be the building blocks of LIGs. We here study and compare the distributions and masses of the total molecular gas and the dense molecular gas, traced by CO and HCN emission respectively, in LIGs, starburst galaxies and ''normal'' spiral galaxies. The molecular gas properties are then compared with the stages of galaxy- galaxy interaction and the far-IR luminosity to understand the star formation process and evolution of LIGs. We first study the dense molecular gas extent and distribution in ~ 10 nearby ''normal'' galaxies by mapping HCN emission (complementary with CO) at least along the major axes. We present the first detailed observational evidence that HCN emission in galaxies, \\ie, the dense molecular gas, is not confined to the inner ~ 1 kpc nuclear region, although the highest concentrations of dense molecular gas are in the center. A significant fraction of dense molecular gas is distributed in the inner disks of galaxies outside the nuclear or inner ring starburst regions, and can be detected to radii as large as a few kpc, perhaps to diameters of ~ D25/4. Then we have further surveyed HCN emission in more than ~ 40 relatively distant IR/CO bright ''normal'' galaxies and LIGs. Therefore, a statistically significant sample of HCN data in galaxies is established. We find that LIGs, especially ultraluminous ones, contain tremendous amount of dense molecular gas fueling the starbursts. Although LIGs are rich in molecular gas, some ''normal'' galaxies could have as much molecular gas as LIGs. However, the dense molecular gas content of even gas-rich ''normal'' galaxies is much less than that of LIGs of comparable molecular gas content. We show that HCN emission is better correlated with IR emission than that of CO. The total dense molecular content, the ratio of HCN/CO luminosity and the distribution of the surface brightness ratio IHCN/ICO are important high mass star formation indicators. We also confirm that the star formation efficiency indicated by LIR/LCO depends on the fraction of dense molecular gas (LHCN/LCO) and that the LIR/LHCN ratio is similar in all galaxies, ultraluminous or not, hot or cold in dust temperature, illustrating the starburst nature of ultraluminous IR galaxies. A second goal of this thesis is to study LIGs in the intermediate merging process, to determine the relationship between the various IR/CO properties and galaxy-galaxy interactions. We find a correlation between the CO luminosity and the projected separation of merger nuclei in a sample of more than ~ 50 LIG mergers, which suggests that the molecular content is decreasing as merging advances. Although the correlation is weak for the truly ultraluminous (LIR>1012 ls ) IR mergers, which could simply be due to the incompleteness of the sample since ultraluminous mergers are at great distances, the correlation is better established with less luminous LIG mergers, more close to a volume-limited statistically complete sample. The correlation slope for this nearby almost complete sample is the same as that of the large, heterogeneous sample of 50 mergers. We conducted new CO observation in ~ 20 LIG mergers to provide the CO data for this statistically complete sample of nearby LIG mergers. This correlation seems to have important constraints on both the merger- induced star formation models and the evolution scenario of LIGs. We have also conducted high resolution interferometry CO imaging in two spectacular LIGs, Arp 118 and Arp 119. We detected strong CO emission from rings/tails more than 10 kpc away from the nuclei, and most molecular gas is extranuclear. Detailed study of the distribution and kinematics of the molecular gas can provide the dynamical clues to the origin and evolution of these LIGs. Finally, we study the environment and spatial distribution of LIGs. We found that LIGs have stronger clustering than that of \\IRAS galaxies, yet most of them are not inside the groups of galaxies. Using various previous studies along with our own present investigations, we suggest that some LIGs are remnants of the merged groups of gas-rich galaxies.

We present a CO(3–2) survey of selected regions in the M31 disc as part of the JCMT large programme, HARP and SCUBA-2 High-Resolution Terahertz Andromeda Galaxy Survey (HASHTAG). The 12 CO(3–2) fields in this survey cover a total area of 60 arcmin2, spanning a deprojected radial range of 2–14 kpc across the M31 disc. Combining these observations with existing IRAM 30 m CO(1–0) observations and JCMT CO(3–2) maps of the nuclear region of M31, as well as dust temperature and star formation rate surface density maps, we are able to explore the radial distribution of the CO(3–2)/CO(1–0) integrated intensity ratio (R31) and its relationship with dust temperature and star formation. We find that the value of R31 between 2 and 9 kpc galactocentric radius is 0.14, significantly lower than what is seen in the nuclear ring at 1 kpc (R31 ∼ 0.8), only to rise again to 0.27 for the fields centred on the 10 kpc star forming ring. We also found that R31 is positively correlated with dust temperature, with Spearman’s rank correlation coefficient ρ = 0.55. The correlation between star formation rate surface density and CO(3–2) intensity is much stronger than with CO(1–0), with ρ = 0.54 compared to –0.05, suggesting that the CO(3–2) line traces warmer and denser star forming gas better. We also find that R31 correlates well with star formation rate surface density, with ρ = 0.69.