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Register a new userHigh Resolution Imaging of Warm and Dense Molecular Gas in the Nuclear Region of the Luminous Infrared Galaxy NGC6240
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D. Iono
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We present ~2 resolution CO(3-2), HCO+(4-3) and 880micron continuum images of the luminous infrared galaxy NGC6240 obtained at the Submillimeter Array. We find that the spatially resolved CO(3-2), HCO+(4-3) and the 880micron emission peaks between the two nuclear components that are both known to harbor AGNs. Our Large Velocity Gradient (LVG) analysis performed on each velocity channel suggests that the peak of the molecular gas emission traced in our observations is warm (T = 20 - 100K), dense (nH2 = 10^(5.0 - 5.4) cm^-3) and moderately optically thin (tau = 0.2 - 2) in the central 1 kpc. We also find large column densities of ~10^(23) cm^(-2). Such extreme conditions are observed over ~300 km s^(-1) centered around the CO derived systemic velocity. The derived molecular gas mass from the CO(3-2) emission and a CO-to-H2 conversion factor commonly used for ULIRGs is (6.9 +/- 1.7) x 10^9 Msun, and this is consistent with the mass derived from previous CO(2-1) observations. The gas is highly turbulent in the central kpc (Delta v_(FWZI) ~ 1175 km s^(-1)). Furthermore, possible inflow or outflow activity is suggested from the CO(3-2) velocity distribution. We tentatively state that 3.5 x 10^8 Msun of isolated CO(3-2) emission seen west of the northern disk may be associated with outflows from starburst superwinds, but the gas outflow scenario from one of the central AGN is not completely ruled out. Piecing all of the information together, the central region of NGC 6240 harbors 2 AGNs, ~10^(10) Msun of molecular gas mass, 5 x 10^7 Msun of dust mass, and has possible evidence of inflow and outflow activity.
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High spatial resolution observations of the 12CO(1-0), HCN(1-0), HCO+(1-0), and 13CO(1-0) molecular lines toward the luminous infrared merger NGC 6240 have been performed using the Nobeyama Millimeter Array and the RAINBOW Interferometer. All of the observed molecular emission lines are concentrated in the region between the double nuclei of the galaxy. However, the distributions of both HCN and HCO+ emissions are more compact compared with that of 12CO, and they are not coincident with the star-forming regions. The HCN/12CO line intensity ratio is 0.25; this suggests that most of the molecular gas between the double nuclei is dense. A comparison of the observed high HCN/13CO intensity ratio, 5.9, with large velocity gradient calculations suggests that the molecular gas is dense [n(H_2)=10^{4-6} cm^-3] and warm (T_kin>50 K). The observed structure in NGC 6240 may be explained by time evolution of the molecular gas and star formation, which was induced by an almost head-on collision or very close encounter of the two galactic nuclei accompanied with the dense gas and star-forming regions.
We present our initial results on the CO rotational spectral line energy distribution (SLED) of the $J$ to $J$$-$1 transitions from $J=4$ up to $13$ from Herschel SPIRE spectroscopic observations of 65 luminous infrared galaxies (LIRGs) in the Great Observatories All-Sky LIRG Survey (GOALS). The observed SLEDs change on average from one peaking at $J le 4$ to a broad distribution peaking around $J sim,$6$-$7 as the IRAS 60-to-100 um color, $C(60/100)$, increases. However, the ratios of a CO line luminosity to the total infrared luminosity, $L_{rm IR}$, show the smallest variation for $J$ around 6 or 7. This suggests that, for most LIRGs, ongoing star formation (SF) is also responsible for a warm gas component that emits CO lines primarily in the mid-$J$ regime ($5 lesssim J lesssim 10$). As a result, the logarithmic ratios of the CO line luminosity summed over CO (5$-$4), (6$-$5), (7$-$6), (8$-$7) and (10$-$9) transitions to $L_{rm IR}$, $log R_{rm midCO}$, remain largely independent of $C(60/100)$, and show a mean value of $-4.13$ ($equiv log R^{rm SF}_{rm midCO}$) and a sample standard deviation of only 0.10 for the SF-dominated galaxies. Including additional galaxies from the literature, we show, albeit with small number of cases, the possibility that galaxies, which bear powerful interstellar shocks unrelated to the current SF, and galaxies, in which an energetic active galactic nucleus contributes significantly to the bolometric luminosity, have their $R_{rm midCO}$ higher and lower than $R^{rm SF}_{rm midCO}$, respectively.
We performed 12CO(1-0), 13CO(1-0), and HCN(1-0) single-dish observations (beam size ~14-18) toward nearby starburst and non-starburst galaxies using the Nobeyama 45 m telescope. The 13CO(1-0) and HCN(1-0) emissions were detected from all the seven starburst galaxies, with the intensities of both lines being similar (i.e., the ratios are around unity). On the other hand, for case of the non-starburst galaxies, the 13CO(1-0) emission was detected from all three galaxies, while the HCN(1-0) emission was weakly or not detected in past observations. This result indicates that the HCN/13CO intensity ratios are significantly larger (~1.15+-0.32) in the starburst galaxy samples than the non-starburst galaxy samples (<0.31+-0.14). The large-velocity-gradient model suggests that the molecular gas in the starburst galaxies have warmer and denser conditions than that in the non-starburst galaxies, and the photon-dominated-region model suggests that the denser molecular gas is irradiated by stronger interstellar radiation field in the starburst galaxies than that in the non-starburst galaxies. In addition, HCN/13CO in our sample galaxies exhibit strong correlations with the IRAS 25 micron flux ratios. It is a well established fact that there exists a strong correlation between dense molecular gas and star formation activities, but our results suggest that molecular gas temperature is also an important parameter.
The role of star formation in luminous and ultraluminous infrared galaxies is a hotly debated issue: while it is clear that starbursts play a large role in powering the IR luminosity in these galaxies, the relative importance of possible enshrouded AGNs is unknown. It is therefore important to better understand the role of star forming gas in contributing to the infrared luminosity in IR-bright galaxies. The J=3 level of 12CO lies 33K above ground and has a critical density of ~1.5 X 10^4 cm^-3. The 12CO(J=3-2) line serves as an effective tracer for warm-dense molecular gas heated by active star formation. Here we report on 12CO (J=3-2) observations of 17 starburst spirals, LIRGs and ULIRGs which we obtained with the Heinrich Hertz Submillimeter Telescope on Mt. Graham, Arizona. Our main results are the following: 1. We find a nearly linear relation between the infrared luminosity and warm-dense molecular gas such that the infrared luminosity increases as the warm-dense molecular gas to the power 0.92; We interpret this to be roughly consistent with the recent results of Gao & Solomon (2004a,b). 2. We find L_IR/M_H2 ratios ranging from ~10 to ~128 L_sun/M_sun using a standard CO-H2 conversion factor of 3 X 10^20 cm^-2 (K km s^-1)^-1. If this conversion factor is ~an order of magnitude less, as suggested in a recent statistical survey (Yao et al. 2003), then 2-3 of our objects may have significant contributions to the L_IR by dust-enshrouded AGNs.
The mechanism behind the shaping of bipolar planetary nebulae is still poorly understood. Accurately tracing the molecule-rich equatorial regions of post-AGB stars can give valuable insight into the ejection mechanisms at work. We investigate the physical conditions, structure and velocity field of the dense molecular region of the planetary nebula NGC 6302 by means of ALMA band 7 interferometric maps. The high spatial resolution of the $^{12}$CO and $^{13}$CO J=3-2 ALMA data allows for an analysis of the geometry of the ejecta in unprecedented detail. We built a spatio-kinematical model of the molecular region with the software SHAPE and performed detailed non-LTE calculations of excitation and radiative transfer with the SHAPEMOL plug-in. We find that the molecular region consists of a massive ring out of which a system of fragments of lobe walls emerge and enclose the base of the lobes visible in the optical. The general properties of this region are in agreement with previous works, although the much greater spatial resolution of the data allows for a very detailed description. We confirm that the mass of the molecular region is 0.1 M$_{odot}$. Additionally, we report a previously undetected component at the nebular equator, an inner, younger ring inclined $sim$60$^circ$ with respect to the main ring, showing a characteristic radius of 7.5$times$10$^{16}$ cm, a mass of 2.7$times$10$^{-3}$ M$_{odot}$, and a counterpart in optical images of the nebula. This inner ring has the same kinematical age as the northwest optical lobes, implying it was ejected approximately at the same time, hundreds of years after the ejection of the bulk of the molecular ring-like region. We discuss a sequence of events leading to the formation of the molecular and optical nebulae, and briefly speculate on the origin of this intriguing inner ring.
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