Physical Conditions Of Molecular Gas Research Articles

The Correlation between WISE 12 μm Emission and Molecular Gas Tracers on Subkiloparsec Scales in Nearby Star-forming Galaxies

We complement the MALATANG sample of dense gas in nearby galaxies with archival observations of 12CO and its isotopologues to determine scaling relations between Wide-field Infrared Survey Explorer (WISE) 12 μm emission and molecular gas tracers at subkiloparsec scales. We find that 12 μm luminosity is more tightly correlated with 12CO than it is with 13CO or dense gas tracers. Residuals between predicted and observed 12CO are only weakly correlated with molecular gas mass surface density (Σmol) in regions where Σmol is very low (∼10 M ⊙ pc−2). Above this limit, the 12CO residuals show no correlations with physical conditions of molecular gas, while 13CO residuals depend on the gas optical depth and temperature. By analyzing differences from galaxy to galaxy, we confirm that the 12CO−12 μm relation is strong and statistically robust with respect to star-forming galaxies and active galactic nucleus hosts. These results suggest that WISE 12 μm emission can be used to trace total molecular gas instead of dense molecular gas, likely because polycyclic aromatic hydrocarbons (PAHs, a major contributor to WISE 12 μm emission) may be well mixed with the gas that is traced by 12CO. We propose that WISE 12 μm luminosity can be used to estimate molecular gas surface density for statistical analyses of the star formation process in galaxies.

CO Multi-line Imaging of Nearby Galaxies (COMING). X. Physical conditions of molecular gas and the local SFR–mass relation

Abstract We investigate the molecular gas properties of galaxies across the main sequence of star-forming (SF) galaxies in the local Universe using 12CO(J = 1–0), hereafter 12CO, and 13CO(J = 1–0), hereafter 13CO, mapping data of 147 nearby galaxies obtained in the COMING project, a legacy project of the Nobeyama Radio Observatory. In order to improve the signal-to-noise ratios of both lines, we stack all the pixels where 12CO emission is detected after aligning the line center expected from the first-moment map of 12CO. As a result, 13CO emission is successfully detected in 80 galaxies with a signal-to-noise ratio larger than three. The error-weighted mean of the integrated-intensity ratio of 12CO to 13CO lines (R1213) of the 80 galaxies is 10.9, with a standard deviation of 7.0. We find that (1) R1213 positively correlates to specific star-formation rate (sSFR) with a correlation coefficient of 0.46, and (2) both the flux ratio of IRAS 60 μm to 100 μm (f60/f100) and the inclination-corrected linewidth of 12CO stacked spectra ($\sigma _{{\rm ^{12}CO},i}$) also correlate with sSFR for galaxies with the R1213 measurement. Our results support the scenario where R1213 variation is mainly caused by changes in molecular gas properties such as temperature and turbulence. The consequent variation of the CO-to-H2 conversion factor across the SF main sequence is not large enough to completely extinguish the known correlations between sSFR and Mmol/Mstar (μmol) or star-formation efficiency (SFE) reported in previous studies, while this variation would strengthen (weaken) the sSFR–SFE (sSFR–μmol) correlation.

ALMA images the many faces of the NGC 1068 torus and its surroundings

Aims. We investigate the fueling and the feedback of nuclear activity in the nearby (D = 14 Mpc) Seyfert 2 barred galaxy NGC 1068 by studying the distribution and kinematics of molecular gas in the torus and its connections to the host galaxy disk. Methods. We used the Atacama Large Millimeter Array (ALMA ) to image the emission of a set of molecular gas tracers in the circumnuclear disk (CND) and the torus of the galaxy using the CO(2–1), CO(3–2), and HCO+(4–3) lines and their underlying continuum emission with high spatial resolutions (0.03″ − 0.09″ ≃ 2 − 6 pc). These transitions, which span a wide range of physical conditions of molecular gas (n(H2)⊂103 − 107 cm−3), are instrumental in revealing the density radial stratification and the complex kinematics of the gas in the torus and its surroundings. Results. The ALMA images resolve the CND as an asymmetric ringed disk of D ≃ 400 pc in size and ≃1.4 × 108 M⊙ in mass. The CND shows a marked deficit of molecular gas in its central ≃130 pc region. The inner edge of the ring is associated with the presence of edge-brightened arcs of NIR polarized emission, which are identified with the current working surface of the ionized wind of the active galactic nucleus (AGN). ALMA proves the existence of an elongated molecular disk/torus in NGC 1068 of Mtorusgas ≃ 3 × 105 M⊙, which extends over a large range of spatial scales D ≃ 10 − 30 pc around the central engine. The new observations evidence the density radial stratification of the torus: the HCO+(4–3) torus, with a full size DHCO+(4 − 3) = 11 ± 0.6 pc, is a factor of between two and three smaller than its CO(2–1) and CO(3–2) counterparts, which have full sizes of DCO(3 − 2) = 26 ± 0.6 pc and DCO(2 − 1) = 28 ± 0.6 pc, respectively. This result brings into light the many faces of the molecular torus. The torus is connected to the CND through a network of molecular gas streamers detected inside the CND ring. The kinematics of molecular gas show strong departures from circular motions in the torus, the gas streamers, and the CND ring. These velocity field distortions are interconnected and are part of a 3D outflow that reflects the effects of AGN feedback on the kinematics of molecular gas across a wide range of spatial scales around the central engine. In particular, we estimate through modeling that a significant fraction of the gas inside the torus (≃ 0.4 − 0.6 × Mtorusgas) and a comparable amount of mass along the gas streamers are outflowing. However, the bulk of the mass, momentum, and energy of the molecular outflow of NGC 1068 is contained at larger radii in the CND region, where the AGN wind and the radio jet are currently pushing the gas assembled at the Inner Lindblad Resonance (ILR) ring of the nuclear stellar bar. Conclusions. In our favored scenario a wide-angle AGN wind launched from the accretion disk of NGC1068 is currently impacting a sizable fraction of the gas inside the torus. However, a large gas reservoir (≃1.2 − 1.8 × 105 M⊙), which lies close to the equatorial plane of the torus, remains unaffected by the feedback of the AGN wind and can therefore continue fueling the AGN for at least ≃1 − 4 Myr. Nevertheless, AGN fueling currently seems thwarted on intermediate scales (15 pc ≤r ≤ 50 pc).

Spatially resolved physical conditions of molecular gas and potential star formation tracers in M 83, revealed by theHerschelSPIRE FTS

Since the launch of the Herschel Space Observatory, our understanding about the photo-dissociation regions (PDR) has taken a step forward. In the bandwidth of the Fourier Transform Spectrometer (FTS) of the Spectral and Photometric Imaging REceiver (SPIRE) on board Herschel, ten CO rotational transitions, including J=4-3 to J=13-12, and three fine structure lines, including [CI] 609, [CI] 370, and [NII] 250 micron, are covered. In this paper, we present our findings from the FTS observations at the nuclear region of M83, based on the spatially resolved physical parameters derived from the CO spectral line energy distribution (SLED) map and the comparisons with the dust properties and star-formation tracers. We discuss (1) the potential of using [NII] 250 and [CI] 370 micron as star-formation tracers; (2) the reliability of tracing molecular gas with CO; (3) the excitation mechanisms of warm CO; (4) the possibility of studying stellar feedback by tracing the thermal pressure of molecular gas in the nuclear region of M83.

Physical conditions of molecular gas in the Circinus galaxy Multi-JCO and Ci3PP0observations

We report mapping observations of the $^{12}$CO $J=3-2$, $4-3$, $6-5$, and $7-6$ transitions and the CI 492 GHz transition toward the central 40$''\times$40$''$ region of the Circinus galaxy, using the Atacama Pathfinder EXperiment (APEX) telescope. We also detected $^{13}$CO $J=3-2$ at the central position of Circinus. These observations are to date the highest CO transitions reported in Circinus. With large velocity gradient (LVG) modeling and likelihood analysis we try to obtain density, temperature, and column density of the molecular gas in three regions: the nuclear region ($D< 18''\sim$ 360 pc), the entire central 45$''$ ($D<45''\sim$ 900pc) region, and the star-forming (S-F) ring (18$''<D<45''$). In the nuclear region, we can fit the CO excitation with a single excitation component, yielding an average condition of $n_{\rm H_2} \sim 10^{3.2}$cm$^{-3}$, $T_{\rm kin}$ $\sim$ 200 K, and d$v$/d$r \sim$ 3 km s$^{-1}$pc$^{-1}$. In the entire 45$''$ region, two excitation components are needed with $n_{\rm H_2}$ $\sim$ 10$^{4.2}$ and 10$^{3.0}$ cm$^{-3}$, $T_{\rm kin}\sim$ 60 K and 30 K, and $M_{\rm H_2}\sim$ $2.3\times 10^7$ M$_\odot$ and $6.6 \times 10^7$ M$_\odot$, respectively. The gas excitation in the S-F ring can also be fitted with two LVG components, after subtracting the CO fluxes in the 18$''$ region. The S-F ring region contributes 80\% of the molecular mass in the 45$''$ region. For the 45$''$ region, we find a conversion factor of $N({\rm H_2})/I_{\rm CO\ 1-0}$ = $0.37\times 10^{20}$ cm$^{-2} ({\rm K\ km\ s}^{-1})^{-1}$, about 1/5 of the Galactic disk value. The luminosity ratios of CI and $^{12}$CO $J=3-2$ ($R_{\rm CI/CO\ J=3-2}$) in Circinus basically follow a linear trend. The average $R_{\rm CI/CO\ J=3-2}$ in Circinus is $\sim$ 0.2, lying at an intermediate value between non-AGN nuclear region and high-redshift galaxies.

Physical Conditions of Molecular Gas in the Galactic Center

Abstract We estimated the physical conditions of molecular gas in the central molecular zone (CMZ) of the Galaxy, using our CO $J = 3-2$ data obtained with the Atacama Submillimeter Telescope Experiment (ASTE) in conjunction with J = 1–0 $^{12}$CO and $^{13}$CO data previously observed with the Nobeyama Radio Observatory (NRO) 45 m telescope. The large velocity gradient (LVG) approximation was employed. The distributions of gas density, kinetic temperature, and CO column density were derived as functions of the position and the velocity for the entire coverage of the CO $J = 3-2$ data. We fairly determined the physical conditions for 69% of data points in the CMZ with $\geq 1 \hskip2pt\sigma$ CO detections. The kinetic temperature was found to be roughly uniform in the CMZ, while the gas density was higher in the 120 pc star-forming ring than in the outer dust lanes. The physical conditions of high $J = 3-2/J = 1-0$ features are also discussed.

Mass and Physical Conditions of Molecular Gas at Cosmological Distances Probed with CO Lines

A new way of estimating molecular gas mass at cosmological distances is proposed. This method utilizes the CO-to-H2 conversion factors for higher J transitions calculated in various combinations of physical conditions and redshifts on the basis of the large-velocity gradient (LVG) approximation. This method (LVG mass) is an extension of the LTE mass, and its application includes not only high-z objects but also Galactic objects such as gravitationally unbound gas. The calculated CO-to-H2 conversion factor of the J = 1-0 transition at z = 0 naturally explains both the absolute value and the narrow range of variation of that measured in Galactic molecular clouds. By contrast, the CO-to-H2 conversion factors for higher J transitions are very sensitive to the physical conditions of molecular gas. Although only lower limits to molecular gas mass may be reliably obtained from the luminosity of the single higher J transition of CO, we may still be able to estimate the plausible value of the conversion factor by specifying the physical conditions of the molecular gas or by multitransition analyses. Dependence of CO line ratios on physical conditions of molecular gas is examined for various pairs of transitions at different redshifts. At high redshifts, ratios of low-J transitions exhibit narrower ranges of variation because of warmer cosmic background radiation and thus are not good probes of the physical conditions of molecular gas. Ratios of higher J transitions may be less affected by the warmer background and may remain good probes of the physical conditions of molecular gas. They may be analyzed in the same framework established in the present-day universe as long as the CO abundance is similar to that in the present day.

An Out‐of‐Plane CO (J= 2–1) Survey of the Milky Way. II. Physical Conditions of Molecular Gas

Physical conditions of molecular gas in the first quadrant of the Galaxy are examined through comparison of the CO J = 2-1 data of the Tokyo-Nobeyama Radio Observatory survey with the CO J = 1-0 data of the Columbia survey. A gradient of the CO J = 2-1/J = 1-0 intensity ratio (≡ R2-1/1-0) with Galactocentric distance is reported. The ratio varies from 0.75 at 4 kpc to 0.6 at 8 kpc in Galactocentric distance. This confirms the early in-plane results reported by Handa et al. We classify molecular gas into three categories in terms of R2-1/1-0 on the basis of a large velocity gradient model calculation. Very high ratio gas (VHRG; R2-1/1-0 > 1.0) is either dense, warm, and optically thin gas or externally heated, dense gas. High ratio gas (HRG; R2-1/1-0 = 0.7-1.0) is warm and dense gas with high-excitation temperature of the J = 2-1 transition (Tex 10 K), and it is often observed in central regions of giant molecular clouds. Low ratio gas (LRG; R2-1/1-0 < 0.7) has low-excitation temperature of the J = 2-1 transition (Tex 10 K) because of low density or low kinetic temperature, or both, and is often observed in dark clouds and outer envelopes of giant molecular clouds. It is shown that the CO J = 2-1 emission is better characterized as a tracer of dense gas rather than a tracer of warm gas for molecular gas with kinetic temperature higher than 10 K. The observed large-scale decrease in R2-1/1-0 as a function of Galactocentric distance is ascribed to the fractional decrease of HRG and VHRG from 40% near 5 kpc to 20% near the solar circle. The HRG and VHRG are found predominantly along the Sagittarius and Scutum arms, probably in their downstream. This fact and the deficiency of atomic gas compared with molecular gas in the inner Galaxy indicate that physical conditions of interstellar gas are affected by grand-design, nonlinear processes, such as compression by spiral density waves followed by gravitational collapse, and not by dissociation of low-density molecular gas by young stars.

Extensive High‐Resolution12CO Imaging of L1641 in Orion

We obtained extensive 12CO J = 1-0 images along the minor axis of the L1641 cloud in Orion with a 15'' (0.03 pc) beam spaced by 34''. Besides bright ridgelike emission in the middle of the cloud, we detected CO emission almost everywhere within the outermost boundary of the cloud. There are at least two distinct populations of molecular gas: well-defined with high brightness temperature (~25 K) and small line width (1.5 km s-1), and components with low brightness temperature (~2.5 K) and broad line width (2.5 km s-1). The 12CO J = 1-0 emission shows clumpy features even in the periphery of the cloud. The clumps dominate the CO luminosity from the central ridge region and are severely overlapped, while they are isolated in the periphery of the cloud. The extended component is widely distributed, especially in the eastern side of the cloud, and seems to be connected to the atomic gas layer in the Galactic plane. It lacks notable substructures at our spatial resolution and exhibits a surface filling factor very close to unity. The extended component does not fill the interclump space, at least in the velocity domain. The ridge component may be separating from the extended molecular material. To deduce physical conditions of molecular gas, we conducted simultaneous 12CO J = 2-1/J = 1-0 strip-scan observations along the minor axis of the cloud with higher sensitivity and denser sampling. Molecular gas density estimated from excitation analysis of the 12CO emission was 3 × 103 cm-3 in the clumps and 2 × 102 cm-3 in the extended component. The peripheral region of the cloud is fainter in CO brightness not because the surface filling factor of emitting gas is smaller, but because the emission is dominated by the tenuous extended component with low brightness temperature.

Physical Conditions of Molecular Gas in the Galaxy

Physical conditions of molecular gas are key parameters to the formation rate and initial mass function of stars formed in molecular clouds. The ongoing Tokyo-NRO survey has been observing the Galactic CO (J=2–1) emission with a beamsize matched to the Columbia CO (J=1–0) survey. Intensities of the two lines should reflect physical conditions of the CO-emitting gas. An out-of-plane survey of the inner Galaxy which covers from 20° to 60° in galactic longitude and from −1° to +1° in galactic latitude with grid spacings of 0.25° has already been made (Sakamoto et al. 1994). Its coverage is large enough to draw conclusions on global properties of molecular gas in the inner Galaxy.

PHYSICAL CONDITIONS OF MOLECULAR GAS IN THE GALAXY

ABSTRACT Physical conditions of molecular clouds in the Milky Way are examined through extensive observations of CO (J = 2-1) emission with the University of Tokyo-NRO 60 cm radio telescope. We take two complementary approaches-- large-scale mapping observations of nearby molecular clouds and an out-of-plane survey of the Milky Way. The observed CO (J = 2-1) intensity is compared with the CO (J = 1-0) data taken with the same angular resolution. Through a numerical calculation based on a large velocity gradient approximation, the CO (J = 2-1)/CO (J = 1-0) intensity ratio (equiv R2-1/1-0) is found to be a useful tool for researches of large-scale structure of molecular clouds. Molecular gas is classified in terms of the R}2-1/1-0 value; Very High Ratio Gas (VHRG; R2-1/1-0 >>1), High Ratio Gas (HRG; R_{2-1/1-0 ~0.8), Low Ratio Gas (LRG; R2-1/1-0 ~0.5) and Very Low Ratio Gas (VLRG; R2-1/1-0 <0.4). The VHRG is optically thin, dense and warm gas. The HRG is opaque and dense enough for low-J transitions of CO to be excited to the local thermodynamical equilibrium through collisions, while the LRG is less dense and the excitation of these low-J transitions is inefficient. For the HRG and the LRG, the R2-1/1-0 value is sensitive to the variation of gas density rather than gas kinetic temperature. The VLRG is faint and usually not detected in quick surveys. The apparent discrepancy of the observed line intensity and that predicted by the ratio of these intensities can be used to estimate the degree of beam dilution. Giant molecular clouds show large-scale systematic variation of gas density according to the location in the molecular clouds. Contrary to the widely-accepted model of molecular clouds, peripheral regions of GMCs are faint not because surface filling factor of line-emitting gas clumps is small but because even the low-J transitions of 12 CO are not excited enough due to low gas density in clumps in these regions. Heating and compression by associated young stars are efficient within several parsecs from the stars, and they do not enhance the R2-1/1-0 value in a cloud-scale because of this small range of influence. A systematic variation of the R2-1/1-0 value across the Galactic plane was observed. The ratio varies from ~0.75 at 4 kpc to ~0.6 at 8 kpc in galactocentric distance. The corresponding nT values derived by the one-zone model analysis are 8 X 103 and 4 X 103 cm-3 K, respectively. This trend can be understood by large-scale variation of mixing ratio of HRG and LRG as a function of galactocentric distance. On the other hand, there is little, if any, evidence for the difference of physical conditions between the in-plane gas and the out-of-plane gas. There is concentration of HRG along 'the Sagittarius arm' and 'the Scutum arm' found in l- V diagrams. This supports an idea that strong arms exist in the inner Galaxy and they compress molecular gas to contain much of HRG. Analysis of tangential components indicates that the scale height of molecular gas in interarm regions is larger by the factor of 1.5 than in arm regions. Molecular gas with higher R2-1/1-0 value are concentrated to the central ridge-like condensation of massive cloud, the inner Galaxy and the arms, rather than to the cloud envelopes, the outer Galaxy and the interarm regions, respectively. These tendency indicate that virtually all of the regions with high R2-1/1-0 ratio have one feature in common-- they concentrate to the regions with higher gas pressure. This fact as well as the central deficiency of atomic gas compared to molecular gas indicate that the large-scale properties of molecular gas are dominated by the compression processes such as shock compression by density-waves and gravitational collapse, and not by dissociative stripping of low-density gas due to UV photons. Since the Milky Way is the only galaxy in which we can learn about the effects of heating and compression in various scales on physical conditions of molecular gas, the above knowledge about the dominant mechanism for the compression of molecular gas will be used as a guideline for extragalactic works.

NH3 Observations of the Sgr A Complex Region with the Nobeyama Millimeter Array

We report NH3 observations of the Sgr A complex region including Sgr A West and the 20 km/s and 50 km/s molecular clouds (M–0.13–0.08 and M–0.02–0.07) using the Nobeyama Millimeter Array and the 45m telescope. NH3(1,1) and (2,2) lines were simultaneously observed to estimate the kinetic temperature. Our results suggest strong interaction between the molecular clouds and the continuum sources in the Sgr A complex. The interaction with continuum sources might be an important factor in determining the physical conditions of molecular gas in the galactic center region.