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Register a new userProperties of giant molecular clouds in the strongly barred galaxy NGC1300
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Star formation activity depends on galactic-scale environments. To understand the variations in star formation activity, comparing the properties of giant molecular clouds (GMCs) among environments with different star formation efficiency (SFE) is necessary. We thus focus on a strongly barred galaxy to investigate the impact of the galactic environment on the GMCs properties, because the SFE is clearly lower in bar regions than in arm regions. In this paper, we present the $^{12}$CO($1-0$) observations toward the western bar, arm and bar-end regions of the strongly barred galaxy NGC1300 with ALMA 12-m array at a high angular resolution of $sim$40 pc. We detected GMCs associated with the dark lanes not only in the arm and bar-end regions but also in the bar region, where massive star formation is not seen. Using the CPROPS algorithm, we identified and characterized 233 GMCs across the observed regions. Based on the Kolmogorov-Smirnov test, we find that there is virtually no significant variations in GMC properties (e.g., radius, velocity dispersion, molecular gas mass, and virial parameter) among the bar, arm and bar-end region. These results suggest that systematic differences in the physical properties of the GMCs are not the cause for SFE differences with environments, and that there should be other mechanisms which control the SFE of the GMCs such as fast cloud-cloud collisions in NGC1300.
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In many barred galaxies, star formation efficiency (SFE) in the bar is lower than those in the arm and bar-end, and its cause has still not been clear. Focusing on the strongly barred galaxy NGC 1300, we investigate the possibility that the presence of a large amount of diffuse molecular gas, which would not contribute to the SF, makes the SFE low in appearance. We examine the relation between the SFE and the diffuse molecular gas fraction ($f_{rm dif}$), which is derived using the $^{12}$CO($1-0$) flux obtained from the interferometer of ALMA 12-m array, which has no sensitivity on diffuse (extended; FWHM $gtrapprox 700$ pc) molecular gases due to the lack of ACA, and the total $^{12}$CO($1-0$) flux obtained from Nobeyama 45-m single-dish telescope. We find that the SFE decreases with increasing $f_{rm dif}$. The $f_{rm dif}$ and ${rm SFE}$ are $0.74 - 0.91$ and $(0.06 - 0.16) ~rm Gyr^{-1}$ in the bar regions, and $0.28 - 0.65$ and $(0.23 - 0.96) ~rm Gyr^{-1}$ in the arm and bar-end regions. This result supports the idea that the presence of a large amount of diffuse molecular gas makes the SFE low. The suppression of the SFE in the bar has also been seen even when we exclude the diffuse molecular gas components. This suggests that the low SFE appears to be caused not only by a large amount of diffuse molecular gases but also by other mechanisms such as fast cloud-cloud collisions.
We present a high spatial resolution ($approx 20$ pc) of $^{12}$CO($2-1$) observations of the lenticular galaxy NGC4526. We identify 103 resolved Giant Molecular Clouds (GMCs) and measure their properties: size $R$, velocity dispersion $sigma_v$, and luminosity $L$. This is the first GMC catalog of an early-type galaxy. We find that the GMC population in NGC4526 is gravitationally bound, with a virial parameter $alpha sim 1$. The mass distribution, $dN/dM propto M^{-2.39 pm 0.03}$, is steeper than that for GMCs in the inner Milky Way, but comparable to that found in some late-type galaxies. We find no size-linewidth correlation for the NGC4526 clouds, in contradiction to the expectation from Larsons relation. In general, the GMCs in NGC4526 are more luminous, denser, and have a higher velocity dispersion than equal size GMCs in the Milky Way and other galaxies in the Local Group. These may be due to higher interstellar radiation field than in the Milky Way disk and weaker external pressure than in the Galactic center. In addition, a kinematic measurement of cloud rotation shows that the rotation is driven by the galactic shear. For the vast majority of the clouds, the rotational energy is less than the turbulent and gravitational energy, while the four innermost clouds are unbound and will likely be torn apart by the strong shear at the galactic center. We combine our data with the archival data of other galaxies to show that the surface density $Sigma$ of GMCs is not approximately constant as previously believed, but varies by $sim 3$ orders of magnitude. We also show that the size and velocity dispersion of GMC population across galaxies are related to the surface density, as expected from the gravitational and pressure equilibrium, i.e. $sigma_v R^{-1/2} propto Sigma^{1/2}$.
Tidal dwarf galaxies (TDGs) are gravitationally bound condensations of gas and stars formed during galaxy interactions. Here we present multi-configuration ALMA observations of J1023+1952, a TDG in the interacting system Arp 94, where we resolve CO(2-1) emission down to giant molecular clouds (GMCs) at 0.64 ~ 45pc resolution. We find a remarkably high fraction of extended molecular emission (~80-90%), which is filtered out by the interferometer and likely traces diffuse gas. We detect 111 GMCs that give a similar mass spectrum as those in the Milky Way and other nearby galaxies (a truncated power law with slope of -1.76+/-0.13). We also study Larsons laws over the available dynamic range of GMC properties (~2 dex in mass and ~1 dex in size): GMCs follow the size-mass relation of the Milky Way, but their velocity dispersion is higher such that the size-linewidth and virial relations appear super-linear, deviating from the canonical values. The global molecular-to-atomic gas ratio is very high (~1) while the CO(2-1)/CO(1-0) ratio is quite low (~0.5), and both quantities vary from north to south. Star formation is predominantly taking place in the south of the TDG, where we observe projected offsets between GMCs and young stellar clusters ranging from ~50pc to ~200pc; the largest offsets correspond to the oldest knots, as seen in other galaxies. In the quiescent north, we find more molecular clouds and a higher molecular-to-atomic gas ratio (~1.5); atomic and diffuse molecular gas also have a higher velocity dispersion there. Overall, the organisation of the molecular ISM in this TDG is quite different from other types of galaxies on large scales, but the properties of GMCs seem fairly similar, pointing to near universality of the star-formation process on small scales.
We present the initial results of a census of 684 barred galaxies in the MaNGA galaxy survey. This large sample contains galaxies with a wide range of physical properties, and we attempt to link bar properties to key observables for the whole galaxy. We find the length of the bar, when normalised for galaxy size, is correlated with the distance of the galaxy from the star formation main sequence, with more passive galaxies hosting larger-scale bars. Ionised gas is observed along the bars of low-mass galaxies only, and these galaxies are generally star-forming and host short bars. Higher-mass galaxies do not contain H{alpha} emission along their bars, however, but are more likely to host rings or H{alpha} at the centre and ends of the bar. Our results suggest that different physical processes are at play in the formation and evolution of bars in low- and high-mass galaxies.
We present CO (1-0) and (2-1) observations of the dwarf starburst galaxy NGC 1569 with the IRAM interferometer on Plateau de Bure. We find the CO emission is not spatially associated with the two super star clusters in the galaxy, but rather is found in the vicinity of an HII region. With the resolution of our data, we can resolve the CO emission into five distinct giant molecular clouds, four are detected at both transitions. In the (1-0) transition the sizes and linewidths are similar to those of GMCs in the Milky Way Galaxy and other nearby systems, with diameters ranging from about 40 to 50 pc and linewidths from 4 to 9 kms. The (2-1)/(1-0) line ratios range from 0.64 +- 0.30 to 1.31 +- 0.60 in the different clouds. The lower line ratios are similar to those seen in typical Galactic GMCs, while values higher than unity are often seen in interacting or starburst galaxies. We use the virial theorem to derive the CO-H(2) conversion factor for three of the clouds, and we adopt an average value of 6.6 +-1.5 times the Galactic conversion factor for NGC 1569 in general. We discuss the role of the molecular gas in NGC 1569, and its relationship to the hot component of the ISM. Finally, we compare our observations with blue compact dwarf galaxies which have been mapped in CO.
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