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Register a new userMolecular Gas Distribution in Barred and Non-Barred Galaxies along the Hubble Sequence
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We present results from a survey of 12CO(J=1-0) spectra obtained for the central regions of 68 nearby galaxies at an angular resolution of 16 arcseconds using the Nobeyama Radio Observatory 45m telescope, aimed at characterizing the properties of star forming molecular gas. Combined with similar resolution observations in the literature, the compiled sample set of 166 galaxies span a wide range of galactic properties. NGC 4380, which was previously undetected in CO, was detected. This initial paper of a series will focus on the data and the gaseous properties of the samples, and particularly on the degree of central concentration of molecular gas in a range of morphological types, from early (S0/Sa) to late (Sd/Sm) galaxies with and without bars. The degree of molecular central concentration in the central kiloparsec, compared to the central several kiloparsecs of galaxies, is found to vary smoothly with Hubble type, so that early type galaxies show larger central concentration. The comparison of barred and non-barred galaxies within early and late type galaxies suggest that difference in Hubble type, representing the effect of bulges, is the more important factor in concentrating gas into the central regions than bars.
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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.
We investigate possible environmental and morphological trends in the $zsim0$ bar fraction using two carefully selected samples representative of a low-density environment (the isolated galaxies from the AMIGA sample) and of a dense environment (galaxies in the Virgo cluster). Galaxies span a stellar mass range from $10^8$ to $10^{12}$M$_{odot}$ and are visually classified using both high-resolution NIR (H-band) imaging and optical texttt{rgb} images. We find that the bar fraction in disk galaxies is independent of environment suggesting that bar formation may occur prior to the formation of galaxy clusters. The bar fraction in early type spirals ($Sa-Sb$) is $sim$50%, which is twice as high as the late type spirals ($Sbc-Sm$). The higher bar fraction in early type spirals may be due to the fact that a significant fraction of their bulges are pseudo-bulges which form via the buckling instability of a bar. i.e. a large part of the Hubble sequence is due to secular processes which move disc galaxies from late to early types. There is a hint of a higher bar fraction with higher stellar masses which may be due to the susceptibility to bar instabilities as the baryon fractions increase in halos of larger masses. Overall, the $S0$ population has a lower bar fraction than the $Sa-Sb$ galaxies and their barred fraction drops significantly with decreasing stellar mass. This supports the notion that $S0s$ form via the transformation of disk galaxies that enter the cluster environment. The gravitational harassment thickens the stellar disks, wiping out spiral patterns and eventually erasing the bar - a process that is more effective at lower galaxy masses.
We report on the 2.5 arcsec (400 pc) resolution CO (J = 1 -> 0) observations covering the whole length of the bar in the strongly barred late-type spiral galaxy NGC 7479. CO emission is detected only along a dust lane that traverses the whole length of the bar, including the nucleus. The emission is strongest in the nucleus. The distribution of emission is clumpy along the bar outside the nucleus, and consists of gas complexes that are unlikely to be gravitationally bound. The CO kinematics within the bar consist of two separate components. A kinematically distinct circumnuclear disk, < 500 pc in diameter, is undergoing predominantly circular motion with a maximum rotational velocity of 245 km/s at a radius of 1 arcsec (160 pc). The CO-emitting gas in the bar outside the circumnuclear disk has substantial noncircular motions which are consistent with a large radial velocity component, directed inwards. The CO emission has a large velocity gradient across the bar dust lane, ranging from 0.5 to 1.9 km/s/pc after correcting for inclination, and the projected velocity change across the dust lane is as high as 200 km/s. This sharp velocity gradient is consistent with a shock front at the location of the bar dust lane. A comparison of H-alpha and CO kinematics across the dust lane shows that although the H-alpha emission is often observed both upstream and downstream from the dust lane, the CO emission is observed only where the velocity gradient is large. We also compare the observations with hydrodynamic models and discuss star formation along the bar.
`Conspiracy between the dark and the baryonic mater prohibits an unambiguous decomposition of disc galaxy rotation curves into the corresponding components. Several methods have been proposed to counter this difficulty, but their results are widely discrepant. In this paper, I revisit one of these methods, which relies on the relation between the halo density and the decrease of the bar pattern speed. The latter is routinely characterised by the ratio ${cal R}$ of the corotation radius $R_{CR}$ to the bar length $L_b$, ${cal R}=R_{CR}/L_b$. I use a set of $N$-body+SPH simulations, including sub-grid physics, whose initial conditions cover a range of gas fractions and halo shapes. The models, by construction, have roughly the same azimuthally averaged circular velocity curve and halo density and they are all submaximal, i.e. according to previous works they are expected to have all roughly the same ${cal R}$ value, well outside the fast bar range (1.2 $pm$ 0.2). Contrary to these expectations, however, these simulations end up having widely different ${cal R}$ values, either within the fast bar range, or well outside it. This shows that the ${cal R}$ value can not constrain the halo density, nor determine whether galactic discs are maximal or submaximal. I argue that this is true even for early type discs (S0s and Sas).
We use a Cartesian grid to simulate the flow of gas in a barred Galactic potential and investigate the effects of varying the sound speed in the gas and the resolution of the grid. For all sound speeds and resolutions, streamlines closely follow closed orbits at large and small radii. At intermediate radii shocks arise and the streamlines shift between two families of closed orbits. The point at which the shocks appear and the streamlines shift between orbit families depends strongly on sound speed and resolution. For sufficiently large values of these two parameters, the transfer happens at the cusped orbit as hypothesised by Binney et al. over two decades ago. For sufficiently high resolutions the flow downstream of the shocks becomes unsteady. If this unsteadiness is physical, as appears to be the case, it provides a promising explanation for the asymmetry in the observed distribution of CO.
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