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The maps of CO cover most of the optical disk of the galaxies. We investigated the influence of bars on the distribution of molecular gas in spiral galaxies using these data. We confirmed that the degree of the central concentration is higher in barred spirals than in nonbarred spirals, as shown by previous studies. Furthermore, we present 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 the molecular gas within the central region in barred spiral galaxies. We found a correlation between the degree of central concentration and the bar strength. Galaxies with stronger bars tend to have a 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.
Since molecular gas is a key material for star formation, which is a fundamental process of galaxy evolution, observations of the molecular gas are essential for an understanding of galaxy evolution. CO observations are the easiest way to observe molecular gas. It is difficult to observe the large-scale distribution of molecular gas in our Galaxy, because we are within the structure.
On the other hand, we can observe the distribution of the molecular gas directly for external galaxies. Therefore, some CO surveys of galaxies have been conducted so far e. These data provide important information about the global properties of galaxies, such as the radial distribution of molecular gas in the galactic disks. On the other hand, surveys observed with interferometers revealed detailed structures of molecular gas, especially in the central region of galaxies e.
Although single-dish telescopes are better at measuring the total flux, mapping CO in the whole disks of external galaxies by large single dishes has so far been limited. This is because much time is needed to map external galaxies with a single-beam receiver with a high angular resolution. Although the angular resolution is poor compared with interferometers, the single dish has an advantage of sampling the total flux, and our maps cover a wider area than those by interferometers.
The data is useful for investigating the relation between the distribution of molecular gas and galactic structures, such as spiral arms and bars. Such a fueling mechanism is needed for starbursts or AGN activities in the central region.
Recently, it has been shown observationally that the degree of the central concentration of molecular gas is higher in barred spirals than in nonbarred spirals Sakamoto et al. Therefore, it is important to clarify how bars work on molecular gas.
We consider the relation between the distribution of molecular gas and the existence of a bar, as well as the correlation between the central concentration and the bar strength. The sample criteria are described in section 2. The details of the observations and data reduction are described in section 3. The data are presented in section 4 and the influence of bars on the distribution of molecular gas is discussed in section 5. We selected our sample of galaxies according to the following criteria: 1 Morphological type ranging from Sa to Scd in the RC3 de Vaucouleurs et al.
We observed 40 galaxies, including galaxies whose data have already been published M Nakai et al. The sample galaxies are listed in table 1. References of the coordinate of the center. Corwin, 1 ; 7: Condon et al. Reference of CO data. Digital spectrometers were used as receiver backends Sorai et al. Since BEARS was operated in the DSB double side band mode, we obtained scaling factors for an intensity calibration by observing a standard source with each of the 25 beams and with a single beam receiver equipped with an SSB single side band filter, and then corrected the DSB intensity into the SSB intensity.
We observed each point with more than 4 different receiver channels to reduce non-uniformity of the noise level due to the variation of the system noise temperature on the receiver channels. The used receivers are listed in table 1. Only M51 was observed with single beam receivers by Nakai et al. We adopted the position angle of the mapping grids so that the grids were parallel or perpendicular to the major axis of the galaxies taken from the literature, except for Maffei 2, whose grids were parallel and perpendicular to the bar.
The observational parameters are summarized in table 1. We checked the consistency of the intensity calibration with other observations.
Figure 1 is a comparison between the total fluxes derived from our maps and those estimated in Young et al. It shows that both data are consistent. Comparison of the total flux in this work with Young et al. Short comments on individual galaxies are also given in the figure captions. NGC Top-right is the CO integrated intensity map. Bottom-left is the velocity field measured in CO.
Bottom-right is the position-velocity diagram of CO along the major axis. The data were taken from Sorai et al. The central peak and the ring of molecular gas surrounding the bar are prominent. Same as figure 2 but for Maffei 2. The contour levels of the position-velocity diagram are 0. Mason and Wilson have shown that there are strong concentrations of molecular gas at the bar ends along with the center.
Our CO map shows a strong peak at the center, offset ridges along the leading side of the bar, condensations at the bar ends, and spiral arms. These are structures often seen in barred spiral galaxies. There are clear gaps between offset ridges and spiral arms.
The gaps are often seen in galaxies that have straight dust lanes along the bar. Large noncircular motion is seen in the bar region. Although the optical and the near-IR images show a disturbed feature, the CO map and velocity field show a symmetric structure. Since Maffei 2 locates behind the Milky Way, large extinction even in the near-IR band seems to be the cause of the asymmetric feature. Same as figure 2 but for NGC The contour levels of the position—velocity diagram are 0.
Same as figure 2 but for IC Since IC is located behind the Milky Way, the optical morphology is not very clear. The southern arm that starts from the southern bar end is much stronger than the northern arm, which starts from the northern bar end in the CO map.
Although IC is classified as a weak bar SAB, the velocity field shows large noncircular motion in the bar region. Same as figure 2 but for UGC The position—velocity diagram along the major axis shows the rigidlike rotation curve in the bar region.
This is because the major axes of the bar and of the galaxy, itself, are almost parallel. For this configuration, the rigid rotation velocity, which corresponds to the pattern speed of the bar, is expected to be observed along the bar Kuno et al. The CO emission distributes along the bar and spiral arms. There is a strong peak at the center and secondary peaks are seen at the bar ends.
The molecular arms continue toward the opposite bar end and make a ringlike structure. This is attributed to a large velocity change due to shock at the ridge.
Since the position angles of the line of nodes and the bar are close, the velocity change can be observed directly. Molecular gas traces the two-arm spiral structure seen in the NIR image. The molecular arms are clumpy. A strong concentration of molecular gas is seen in the center. In addition to the central peak, some clumps of molecular gas distribute along a ring surrounding the bar.
The molecular gas is depleted in the bar region. The data were taken from Kuno et al. There is a strong peak at the center. Offset ridges of the molecular gas along the bar which continue to the bar end smoothly can be seen. On the other hand, the spiral arms are weak in CO as compared with the bar region. Some galaxies that have curved dust lanes along the bar have this trend.
NGC is one of the galaxies that have a kpc-scale central depletion of molecular gas Nishiyama et al. Our map also clearly shows depletion at the center. Molecular gas concentrates at the center and bar ends. Spiral arms are also traced by CO. The asymmetric feature of the arms caused by the interaction with NGC Zhang et al.
The velocity width in the bar region is large, as can be seen in NGC The central peak is prominent. Weak spiral arms are seen in the CO map. Molecular gas concentrates toward the center and the bar ends. Molecular gas concentrates toward the center. The rotation curve rises steeply at the center. Since the inclination angle is large, the distribution of the molecular gas is along the major axis. Kilopc-scale central depletion of molecular gas is seen.
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