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Register a new userThe Virgo CO Survey: VI. Gas Dynamics and Star Formation Along the Bar in NGC 4303
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J. Koda Caltech
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We present CO interferometer observations of the barred galaxy NGC 4303 (M61). This galaxy has a gas concentration at the central region and offset ridges in the bar. Sharp velocity gradients are apparent across the ridges. Analyses of the CO data and the newborn stellar clusters revealed in HST images indicate the existence of unresolved molecular clouds with masses of 10^4-6Msun. The observed shear velocity gradient across the ridges is too small to break up giant molecular clouds. Therefore, the clouds are likely to survive passage through the ridges. We discuss a cloud orbit model in a bar potential. The model reproduces the narrow offset ridges and sharp velocity gradients across the ridges in NGC 4303. We discuss cloud-cloud collisions (and close interactions) as a possible triggering mechanism for star formation. The newborn stellar clusters in NGC 4303 are located predominantly at the leading sides of the offset ridges, where cloud orbits are densely populated and suggest a high collisional frequency and possibly a high rate of triggered star formation. Cloud-based dynamics is less dissipative than smooth hydrodynamic models, possibly extending the timescales of gas dynamical evolution and gas fueling to central regions in barred galaxies.
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We present new $^{12}$CO(J=1-0) observations of the barred galaxy NGC 4303 using the Nobeyama 45m telescope (NRO45) and the Combined Array for Research in Millimeter-wave Astronomy (CARMA). The H$alpha$ images of barred spiral galaxies often show active star formation in spiral arms, but less so in bars. We quantify the difference by measuring star formation rate and efficiency at a scale where local star formation is spatially resolved. Our CO map covers the central 2$farcm$3 region of the galaxy; the combination of NRO45 and CARMA provides a high fidelity image, enabling accurate measurements of molecular gas surface density. We find that star formation rate and efficiency are twice as high in the spiral arms as in the bar. We discuss this difference in the context of the Kennicutt-Schimidt (KS) law, which indicates a constant star formation rate at a given gas surface density. The KS law breaks down at our native resolution ($sim$ 250 pc), and substantial smoothing (to 500 pc) is necessary to reproduce the KS law, although with greater scatter.
During pilot observations of the Virgo Environmental Survey Tracing Galaxy Evolution (VESTIGE), a blind narrow-band Halpha+[NII] imaging survey of the Virgo cluster carried out with MegaCam at the CFHT, we have observed the spiral galaxy NGC 4254 (M99). Deep Halpha+[NII] narrow-band and GALEX UV images revealed the presence of 60 compact (70-500 pc radius) star forming regions up to ~ 20 kpc outside the optical disc of the galaxy. These regions are located along a tail of HI gas stripped from the disc of the galaxy after a rapid gravitational encounter with another Virgo cluster member that simulations indicate occurred 280-750 Myr ago. We have combined the VESTIGE data with multifrequency data from the UV to the far-infrared to characterise the stellar populations of these regions and study the star formation process in an extreme environment such as the tails of stripped gas embedded in the hot intracluster medium. The colour, spectral energy distribution (SED), and linear size consistently indicate that these regions are coeval and have been formed after a single burst of star formation that occurred ~< 100 Myr ago. These regions might become free floating objects within the cluster potential well, and be the local analogues of compact sources produced after the interaction of gas-rich systems that occurred during the early formation of clusters.
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.
We present new SOFIA [CII] and ALMA CO(J=1-0) observations of the nearby asymmetric barred spiral galaxy NGC 7479. The data, which cover the whole bar of the galaxy and the counter-arms visible in the radio continuum, are analyzed in conjunction with a wealth of existing visible, infrared, radio, and X-ray data. As in most normal galaxies, the [CII] emission is generally consistent with emission from cooling gas excited by photoelectric heating in photo-dissociation regions. However, anomalously high [CII]/CO ratios are seen at the two ends of the counter-arms. Both ends show shell-like structures, possibly bubbles, in H-alpha emission. In addition, the southern end has [CII] to infrared emission ratios inconsistent with normal star formation. Because there is little HI emission at this location, the [CII] emission probably originates in warm shocked molecular gas heated by the interaction of the radio jet forming the counter-arms with the interstellar medium in the galaxy. At two other locations, the high [CII]/CO ratios provide evidence for the existence of patches of CO-dark molecular gas. The [CII] and CO observations also reveal resolved velocity components along the bar. In particular, the CO emission can be separated into two components associated to gas along the leading edge of the bar and gas trailing the bar. The trailing gas component that amounts to approximately 40% of the gas around the bar region may be related to a minor merger.
We present the results of $^{12}$CO($J$=1-0) and $^{13}$CO($J$=1-0) simultaneous mappings toward the nearby barred spiral galaxy NGC 4303 as a part of the CO Multi-line Imaging of Nearby Galaxies (COMING) project. Barred spiral galaxies often show lower star-formation efficiency (SFE) in their bar region compared to the spiral arms. In this paper, we examine the relation between the SFEs and the volume densities of molecular gas $n(rm{H}_2)$ in the eight different regions within the galactic disk with CO data combined with archival far-ultraviolet and 24 $mu$m data. We confirmed that SFE in the bar region is lower by 39% than that in the spiral arms. Moreover, velocity-alignment stacking analysis was performed for the spectra in the individual regions. The integrated intensity ratios of $^{12}$CO to $^{13}$CO ($R_{12/13}$) range from 10 to 17 as the results of stacking. Fixing a kinetic temperature of molecular gas, $n(rm{H}_2)$ was derived from $R_{12/13}$ via non-local thermodynamic equilibrium (non-LTE) analysis. The density $n(rm{H}_2)$ in the bar is lower by 31-37% than that in the arms and there is a rather tight positive correlation between SFEs and $n(rm{H}_2)$, with a correlation coefficient of $sim 0.8$. Furthermore, we found a dependence of $n(rm{H}_2)$ on the velocity dispersion of inter-molecular clouds ($Delta V/ sin i$). Specifically, $n(rm{H}_2)$ increases as $Delta V/ sin i$ increases when $Delta V/ sin i < 100$ km s$^{-1}$. On the other hand, $n(rm{H}_2)$ decreases as $Delta V/ sin i$ increases when $Delta V/ sin i > 100$ km s$^{-1}$. These relations indicate that the variations of SFE could be caused by the volume densities of molecular gas, and the volume densities could be governed by the dynamical influence such as cloud-cloud collisions, shear and enhanced inner-cloud turbulence.
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