CO Maps Research Articles

Molecular Gas in Candidate Double‐barred Galaxies. I. The Diverse Morphology and Dynamics of NGC 2273 and NGC 5728

Double bars have been proposed as a means of transporting molecular gas past inner Lindblad resonances into the nuclear regions, where it can fuel active or starburst nuclei. Thus far, the existence of double bars has been determined predominantly through analysis of near-infrared images, which can tell us little about the dynamics and inflow rates of these systems. We have observed two double-bar galaxy candidates (NGC 2273 and NGC 5728) in 12CO J = 1-0 with the Owens Valley Radio Observatory Millimeter Array. Despite the similarity in the near-infrared images of these galaxies, we see rather different nuclear morphologies in the CO maps. NGC 2273 shows evidence of a nuclear gas bar, aligned with the nuclear stellar bar seen in the near-infrared images. Both the nuclear gaseous and the stellar bars are misaligned from the large-scale bar by ~90°, which also allows the possibility that both are the result of stars and gas populating the x2 orbits of the primary bar. Estimates using dynamical friction arguments and star formation rates suggest significant gas inflow rates along the nuclear bar of NGC 2273. Conversely, NGC 5728 does not show any evidence for a nuclear molecular bar but shows an arc of CO clumps that peaks just to the southwest of the dynamical center and curves to the southeast, where it follows the dust lane to the south. Models of double-barred galaxies suggest that these galaxies should contain large amounts of molecular gas in their nuclei. Our calculations suggest that both galaxies contain sufficient amounts of gas in their nuclei, but only NGC 2273 shows evidence for a nuclear gas bar. This may be the result of past episodes of star formation exhausting and dispersing the nuclear gas of NGC 5728, but it is more likely evidence that NGC 5728 has undergone a minor merger event.

ChandraObservations of Markarian 273: Unveiling the Central Active Galactic Nucleus and the Extended Hot Gas Halo

We report X-ray observations of the field containing the ultraluminous IRAS galaxy Mrk 273. The data were obtained using the Advanced CCD Imaging Spectrometer S3 instrument on board Chandra. The high-resolution X-ray image for the first time reveals a compact hard X-ray nucleus in Mrk 273. Its position is coincident with the northern nucleus identified in the optical, infrared, and radio and in molecular CO maps. Its X-ray energy distribution is well described by a heavily obscured active galactic nucleus (AGN) spectrum with an absorbed power law plus a narrow Fe Kα emission line at 6.4 keV. The neutral hydrogen column density is about 4 × 1023 cm-2, implying an absorption-corrected X-ray luminosity (0.1-10 keV) for the nucleus of LX ≈ 6.5 × 1043 ergs s-1 for H0 = 50 km s-1 Mpc-1. The X-ray properties therefore firmly establish the northern nucleus of Mrk 273 as a Seyfert 2 AGN. There are also bright soft X-ray clumps and diffuse soft X-ray emissions surrounding the central hard X-ray nucleus within the 10'' of the nuclear region. Its spectrum can be fitted by a MEKAL thermal model with a temperature of about 0.8 keV and high metallicity (Z ~ 1.5 Z☉) plus emission lines from α elements and ions. We find that a soft X-ray clump, about 4'' (projected separation of about 4 kpc) southwest of the northern hard X-ray nucleus, is coincident with a nebula with strong [O III] λ5007 emissions. Further outside the central region, the Chandra observations reveal a very extended hot halo in Mrk 273. The X-ray halo encompasses the entire optical tidal tail and plume, with a projected diameter of about 108 × 68 kpc. The total soft X-ray luminosity (0.1-2.4 keV) of the hot halo is LX ≈ 1.9 × 1041 ergs s-1, in the range of the soft X-ray luminosity of bright elliptical galaxies. The temperature of the hot gas is about 0.62 keV with a low metallicity (Z ~ 0.1 Z☉). We discuss the nature of the AGN in Mrk 273 and the implications of our results on the origin of X-ray halos in elliptical galaxies. We also discuss the properties of Mrk 273x, a background AGN at redshift z = 0.46 in the Mrk 273 field. The AGN has an X-ray luminosity of LX ≈ 2.43 × 1044 ergs s-1 in the 0.5-10 keV band. Its X-ray properties resemble those of Seyfert 1 galaxies, while its optical properties are similar to Seyfert 2 galaxies. Such mixed classifications in the optical and X-ray may be a challenge for the unification scheme of AGNs.

Generation of density inhomogeneities by magnetohydrodynamic waves

We show that, in a cold plasma, one of the slow waves of the linear system is a Jordan mode, for which the density grows linearly with time. Although this mode is not present if the temperature is finite, slow waves still generate large density perturbations when the thermal sound speed is small compared with that of the fast and Alfvén waves. Numerical calculations show that non-linear steepening of a fast wave with finite but modest amplitude can readily excite this mode as long as the angle between its direction of propagation and the magnetic field is neither too large nor too small. This produces persistent inhomogeneities with a large density contrast. We suggest that this mechanism is responsible for the clumps identified in CO maps of the Rosette molecular cloud and similar ones in other giant molecular clouds. The same process may also be responsible for the formation of dense cores in the clumps.

A Large-Scale CO Mapping of the Central Region of W 51

Abstract We present a high-resolution (16$^{\prime\prime}$) ${{13\atop} \mathrm{CO}}$ ($J= 1$−$0$) map of the central region of W 51. We observed an area of about ${15\rlap {.}{}^{\mathrm {\prime }}3}\times {16\rlap {.}{}^{\mathrm {\prime }}7}$, which covers the entire region of G49.5–0.4, the brightest source in the W 51 region. Four discrete molecular clouds were identified toward G49.5–0.4, and evidence of cloud–cloud collisions was found. We speculate that a “pileup” of three molecular clouds resulted in a burst of massive star formation in G49.5–0.4.

Mid-Infrared observations of NGC 1068 with the Infrared Space Observatory

We report on Mid-Infrared (MIR) observations of the Seyfert 2 galaxy NGC 1068, obtained with ISOCAM in low-resolution spectro-imaging mode. The spatial resolution (around 5 arcsec) allows us to disentangle the circumnuclear starburst regions from the emission of the active galactic nucleus (AGN). The global spatial distribution of the Unidentified Infrared Bands (UIBs) is similar to the cold dust component, traced by the 450 microns emission and the gaseous component obtained from the CO(1-0) map. However, a shift between the maximum of the UIB and 450 microns emission is clearly seen in our maps. The UIBs in the MIR (5-16 microns) originate almost exclusively from the starburst regions in the galactic disk with an emission peaking at the extremity of the stellar/ gaseous bar at a distance of 1 kpc from the AGN. The spectrum of the nucleus is characterized over the whole 5-16 microns range by a strong continuum which can be fitted with a power law of index -1.7. Moreover, the high [NeIII]/[NeII] ratio (>2.5) in the nuclear region argues for a hard radiation field from the AGN. Observations indicate that the AGN in NGC 1068 contributes less than 5% to the total integrated UIB emission even though its hot dust continuum contributes as much as 75% to the total MIR flux. On the contrary, the nuclear contribution to the cold dust emission decreases considerably at submillimeter wavelengths and does not represent more than 25% of the total integrated emission at 450 microns.

12CO, 13CO and HCN in the Barred Galaxy NGC 1530

Bars probably have a great importance in galactic evolution. The barred potential is able to concentrate large quantities of interstellar gas in the vicinity of the nucleus, feeding any nuclear activity, be it central starbursts or black hole accretion discs. The IRAM millimeter interferometer and 30 m telescope have allowed a precise analysis of the molecular gas in the bar and the nucleus of a typical barred spiral galaxy, NGC 1530. In this galaxy, I have detected CO(1→0) along two lanes that trace shocks in the molecular gas. In these lanes, the gas moves toward the centre of the galaxy, with typical in fall velocities of 100 km s-1. I have shown in these shocks an anticorrelation between shear in the gas and star formation efficiency by comparing Hα and CO maps. I have also studied the centre of this galaxy at higher resolution in12CO(1→0), 12CO(2→1),13CO(1→0) and HCN(1→0). In the central region, the gas distribution is a ring or an unresolved spiral, surrounded by two curved arcs. The nuclear ring contains large amounts of dense gas traced by HCN and 13CO, and shows intense star formation, as indicated by the non-thermal centimetre continuum. The arcs, in contrast, are poor in dense gas and form few stars.

CO Mapping of Spiral Galaxies in the Ursa Major Cluster: An Atlas

We present the first results of the OTF CO (J=1-0) mapping of spiral galaxies in the Ursa Major cluster, obtained with the NRAO 12m radio telescope during June 20-27, 1999 and March 23-30, 2000.

High‐Resolution Imaging of Molecular Gas and Dust in the Antennae (NGC 4038/39): Super Giant Molecular Complexes

We present new aperture synthesis CO maps of the Antennae (NGC 4038/39) obtained with the Caltech Millimeter Array. These sensitive images show molecular emission associated with the two nuclei and a partial ring of star formation to the west of NGC 4038, as well as revealing the large extent of the extranuclear region of star formation (the region), which dominates the CO emission from this system. The largest molecular complexes have masses of × 108 M☉, typically an order of magnitude larger than the largest structures seen to date in more quiescent galaxy disks. The extremely red luminous star clusters identified previously with the Hubble Space Telescope are well correlated with the CO emission, which supports the conclusion that they are highly embedded young objects rather than old globular clusters. There is an excellent correlation between the CO emission and the 15 μm emission seen with the Infrared Space Observatory, particularly for the brightest regions. The most massive complexes in the overlap region have similar [Ne III]/[Ne II] ratios, which implies that all these regions are forming many massive stars. However, only the brightest mid-infrared peak shows strong, rising continuum emission longward of 10 μm, indicative of very small dust grains heated to high temperatures by their proximity to nearby luminous stars. Since these grains are expected to be removed rapidly from the immediate environment of the massive stars, it is possible that this region contains very young (<1 Myr) sites of star formation. Alternatively, fresh dust grains could be driven into the sphere of influence of the massive stars, perhaps by the bulk motions of two giant molecular complexes. The kinematics and morphology of the CO emission in this region provide some support for this second scenario.

Distribution and Kinematics of Molecular Gas in Barred Spiral Galaxies. I. NGC 3504

Abstract We present the results of CO mapping observations of the barred spiral galaxy NGC 3504 with the Nobeyama 45-m telescope. The distribution of the molecular gas shows offset ridges which correspond to the distribution of H II regions along the bar. The velocity perpendicular to the bar decreases abruptly at the ridge. The velocity change implies that the molecular gas changes the direction of its motion to parallel to the bar at the ridge. Since the position angle of the major axis of the bar and the line of nodes are almost the same in NGC 3504, an upper limit to the pattern speed of the bar can be derived directly from the radial velocity. The resultant upper limit is 41 km s−1 kpc−1, which is much smaller than that derived based on an assumption that the corotation radius is located at the end of the bar (77 km s−1 kpc−1). The corotation radius derived from our upper limit is more than two-times larger than the length of the semi-ma jor axis of the bar in NGC 3504.

Formation of Cavities, Filaments, and Clumps by the Nonlinear Development of Thermal and Gravitational Instabilities in the Interstellar Medium under Stellar Feedback

Based on our high-resolution two-dimensional hydrodynamical simulations, we propose that large cavities may be formed by the nonlinear development of the combined thermal and gravitational instabilities, without need for stellar energy injection in a galaxy modeling the Large Magellanic Cloud (LMC). Our numerical model of star formation allows us to follow the evolution of the blast waves due to supernovae in the inhomogenous, multiphase, and turbulent-like media self-consistently. Formation of kiloparsec-scale inhomogeneity, such as cavities as seen in the observed H I map of the LMC, is suppressed by frequent supernovae (the average supernova rate for the whole disk is ~0.001 yr-1). However, the supernova explosions are necessary for the hot component (Tg > 106-107 K). Position-velocity maps show that kiloparsec-scale shells/arcs formed through nonlinear evolution in a model without stellar energy feedback have kinematics similar to explosive phenomena, such as supernovae. We also find that dense clumps and filamentary structure are formed as a natural consequence of the nonlinear evolution of the multiphase interstellar medium (ISM). Although the ISM on a small scale looks turbulent-like and transient, the global structure of the ISM is quasi-stable. In the quasi-stable phase, the volume filling factor of the hot, warm, and cold components are ~0.2, ~0.6, and ~0.2, respectively. We compare observations of H I and molecular gas of the LMC with the numerically obtained H I and CO brightness temperature distributions. The morphology and statistical properties of the numerical H I and CO maps are discussed. We find that the cloud mass spectra of our models represent a power-law shape, but their slopes change between models with and without the stellar energy injection. We also find that the slope depends on the threshold brightness temperature of CO.

Spiral patterns with straight arm segments

The phenomenon of ‘rows’, which are straight geometrical segments in the spiral arms of galaxies, is studied. The Whirlpool nebula, Messier 51 (NGC 5194) in Canes Venatici, is considered to be an example of a giant grand design galaxy. Optical photographs, Hα, ultraviolet and far-ultraviolet images, CO, 21-cm and synchrotron emission maps, and a Ks-band mosaic of M51 are studied. With this observational material, multiple rows can be recognized in the spiral arms of the galaxy. The rows comprise a major part of the arms. The lengths of the rows increase almost linearly with distance from the centre. They intersect one another at an (average) angle ≈2π/3. A possible physical explanation of the phenomenon of rows is discussed on the basis of the assumption that the formation of straight arm segments might be due to the gas-dynamical effect of stability of flat shock fronts, and the tendency of a slightly curved shock front to become flat. A quantitative flattening criterion enables an explanation of the geometrical properties of the arm patterns found in M51 and also in M101. A brief list of spirals with rows is given.

A New High‐Resolution CO Map of the Inner 2.́5 of M51. I. Streaming Motions and Spiral Structure

The Owens Valley millimeter array has been used to map the CO 1-0 emission in the inner 25 of the grand-design spiral galaxy M51 at 2''-3'' resolution. These new images reveal the molecular spiral arms with unprecedented clarity; the emission in the two major arms (northeast and southwest) originates from supermassive cloud complexes, giant molecular associations (GMAs), which are for the first time resolved both along and perpendicular to the arms. The overall morphology of the CO emission is symmetric in reflection about the nucleus, with major complexes occurring opposite each other in the two major arms. On the other hand, the CO line flux in the area of the southwest arm closest to the nucleus is approximately twice as bright as that from the analogous location in the northeast arm. Streaming motions can be studied in detail and appear with great clarity along the major and minor axes of M51. The streaming velocities are very large, 60-150 km s-1. Our maps offer, for the first time, sufficient resolution to resolve the structure in the molecular streaming motions. Both the radial and tangential velocity components show steep gradients, in qualitative accordance with predictions of the density-wave models of Roberts & Stewart. Our data thus support the presence of galactic shocks in the arms of M51. In general, velocity gradients across arms are higher by a factor of 2-10 than previously found. They vary in steepness along the spiral arms, becoming particularly steep in between GMAs. The steep gradients cause conditions of strong reverse shear in several regions in the arms, and thus the notion that shear is generally reduced by streaming motions in spiral arms will have to be modified. Of the three GMAs studied on the southwest arm, only one shows reduced shear. We find an unusual structure, an expansion in the northeast molecular arm at 25'' radius (1.2 kpc) southeast of the center. This broadening occurs right after the end of the northeast arm at the inner Lindblad resonance (ILR). Multiple-peak spectra, velocity twists, and structures with apparently high velocity dispersion are associated with this feature. Bifurcations in the molecular spiral arm structure, at a radius of ≈3.3 kpc (73''), may be evidence of a secondary compression of the gas caused by, and occurring near, the 4/1 ultraharmonic resonance. Several molecular spurs protrude from the main spiral arms, in particular on the western side of the map. Inside the radius of the ILR, we detect narrow (≈5'') molecular spiral arms, possibly related to the K-band arms found in the same region. We find evidence of noncircular motions in the inner 20'' of M51. The magnitude of these deviations is 20-30 km s-1, and they are consistent with gas on elliptical orbits in a bar.

Observations of Middle Atmosphere CO from theUARSISAMS during the Early Northern Winter 1991/92

Structure and kinematics of carbon monoxide in the upper stratosphere and lower mesosphere (10-0.03 hPa) are studied for the early northern winter 1991/92 using the Upper Atmosphere Research Satellite Improved Stratospheric and Mesospheric Sounder (ISAMS) measurements. The study is aided by data from a 6-week parameterized-chemistry run of the Goddard Space Flight Center 3D Chemistry and Transport Model (CTM), initialized on 8 December 1991. Generally, CO mixing ratios increase with height due to the increasing source contribution from CO, photolysis. In the tropical upper stratosphere. however, a local maximum in CO mixing ratio occurs. A simple photochemical model is used to show that this feature results largely from methane oxidation. In the extratropics the photochemical lifetime of CO is long, and therefore its evolution is dictated by large-scale motion of air. evidenced by strong correlation with Ertel potential vorticity. This makes CO one of the few useful observable tracers at the stratopause level and above. Thus CO maps are used to study the synoptic evolution of the polar vortex in early January 1992. Modified Lagrangian mean mixing diagnostics are applied to ISAMS and CTM data to examine the strength of the mixing barrier at the polar vortex edge. It is demonstrated that planetary wave activity weakens the barrier. promoting vortex erosion. The vortex erosion first appears in the lower mesosphere and subsequently descends through the upper stratosphere. and is attributed to effects of planetary wave dissipation. Agreement between ISAMS and CTM is good in the horizontal distribution of CO throughout the examined period, but vertical CO gradients in the CTM weaken with time relative to the ISAMS observations.

Infrared and Millimeter Views of the Helix: the Nearest, Massive, Neutral Remnant of a Circumstellar Envelope

We present new, infrared and millimeter views of the Helix nebula which illustrate the critical role of remnant, neutral AGB envelopes in the formation and evolution of planetary nebulae. Large scale ∼ 1 000″ mapping of the entire nebula in the CO (J = 2 − 1) line with the CSO reveals the global structure of the envelope. The CO emission forms the familiar ring structure seen in optical images of the Helix, and indicates a massive remnant with ≳ 50% the mass of the ionized nebula. High resolution CO mapping with the IRAM 30 m telescope shows that the whole envelope is fragmented into an intricate array of small clumps, closely related to the cometary globules seen in the central, ionized cavity. 5–17 μm spectroscopy of the Helix with ISOCAM reveals a remarkable near infrared spectrum, dominated by the pure (v = 0 − 0) rotational lines of H2. The H2 lines are excited to a temperature of ∼ 900 K, and likely arise in warm, outer layers of the small clumps seen in CO. Imaging of the H2 emission with ISOCAM over the whole nebula provides a striking portrait of the fragmented neutral envelope. 3-dimensional views of the envelope are also presented, based on CO mapping and using 3-dimensional visualization techniques. Point symmetries dominate the toroidal structure, and suggest an origin for the Helix in equatorial mass-loss on the AGB, shaped by the action of bipolar outflows or jets.

Rotating Nuclear Rings and Extreme Starbursts in Ultraluminous Galaxies

New CO interferometer data show that the molecular gas in infrared ultraluminous galaxies is in rotating nuclear disks or rings. The CO maps yield disk radii, kinematic major axes, rotation speeds, enclosed dynamical masses, and gas masses. The CO brightness temperatures, the double-peaked CO line profiles, the limits on thermal continuum flux from dust, and the constraint that the gas mass must be less than the dynamical mass all indicate that the CO lines are subthermally excited and moderately opaque (τ = 4 to 10). We fit kinematic models in which most of the CO flux comes from a moderate-density warm intercloud medium, rather than from self-gravitating clouds. Typical ring radii are 300 to 800 pc. We derive gas masses not from a standard CO-to-mass ratio, but from a model of radiative transfer through subthermally excited CO in the molecular disks. This model yields gas masses of ~5 × 109 M☉, ~5 times lower than the standard method, and a ratio M/L ≈ 0.8 M☉ (K km s-1 pc2)-1. In the nuclear disks, we derive a ratio of gas to dynamical mass of Mgas/Mdyn ≈ 1/6, and a maximum ratio of gas to total mass surface density, μ/μtot, of 1/3. For the galaxies VII Zw 31, Arp 193, and IRAS 10565+2448, the CO position-velocity diagrams provide good evidence for rotating molecular rings with a central gap. In addition to the rotating central rings or disks, a new class of star formation region is identified, which we call an extreme starburst. These have a characteristic sizes of only 100 pc, with about 109 M☉ of gas and an IR luminosity of ≈ 3 × 1011 L☉ from recently formed OB stars. Four extreme starbursts are identified in the 3 closest galaxies in the sample, including Arp 220, Arp 193, and Mrk 273. These are the most prodigious star formation events in the local universe, each representing about 1000 times as many OB stars as 30 Doradus. In Mrk 231, the CO (2-1) velocity diagram along the line of nodes shows a 12 diameter inner disk and a 3'' diameter outer disk. The narrow CO line width, the single-peak line profile, the equality of the major and minor axes, and the observed velocity gradients all imply that the molecular disk is nearly face-on, yielding low optical and UV extinction to the active galactic nucleus (AGN). Such a geometry means that the molecular disk cannot be heated by the AGN; the far-infrared (FIR) luminosity of Mrk 231 is powered by a starburst, not the AGN. In Mrk 273, the CO (1-0) maps show long streamers of radius 5 kpc (7'') with velocity gradients north-south, and a nuclear disk of radius 400 pc (06) with velocity gradients east-west. The nuclear disk contains a bright CO core of radius 120 pc (02). In Arp 220, the CO and 1.3 mm continuum maps show the two nuclei embedded in a central ring or disk at P.A. 50° and a fainter structure extending 7'' (3 kpc) to the east, normal to the nuclear disk. Models of the CO and dust flux indicate that the two K-band sources contain high-density gas, with n(H2) = 2 × 104 cm-3. There is no evidence that these sources really are the premerger nuclei. They are more likely to be compact extreme starburst regions, containing 109 M☉ of dense molecular gas and new stars, but no old stars. Most of the HCN emission arises in the two nuclei. The luminosity-to-mass ratios for the CO sources in Arp 220 are compatible with the early phases of compact starbursts. There is a large mass of molecular gas currently forming stars with plenty of ionizing photons, and no obvious AGN. The entire bolometric luminosity of Arp 220 comes from starbursts, not an AGN. The CO maps show that the gas in ultraluminous IR galaxies is in extended disks that cannot intercept all the power of central AGNs, even if they exist. We conclude that in ultraluminous IR galaxies—even in Mrk 231, which hosts a quasar—the FIR luminosity is powered by extreme starbursts in the molecular rings or disks, not by dust-enshrouded quasars.

The circumstellar envelopes around three protostars in Taurus

We have investigated the distribution of circumstellar material around three Class I protostars in Taurus. TMC1 (IRAS 04381+2540), TMC1A (IRAS 04365+2535) and Haro 6–10 (IRAS 04263+2426) have all been mapped in 790-μm dust continuum emission using the James Clerk Maxwell Telescope, and TMC1A has also been imaged at λ1.1 mm. In addition, we present a 12CO J = 2−1 map of Haro 6–10 obtained using the Caltech Submillimeter Observatory. The dust emission from TMC1 is extended and is oriented perpendicular to its outflow, with little evidence for a compact disc component. The submillimetre emission from TMC1A is centrally peaked, with a weak extension in the direction of the outflow at both 790 μm and 1.1 mm. It is possible that the interaction of the outflow with the surrounding cloud is heating the dust. The emission from Haro 6–10 is oriented roughly perpendicular to the direction of several Herbig—Haro objects in the region. There is no clear bipolar molecular outflow structure in the CO(2−1) map, and the relatively low velocities observed in the line wings suggest that any such system must lie close to the plane of the sky. If the dust emission from the envelopes is optically thin and a temperature distribution can be specified, the broad-band spectral energy distribution (SED) can be used to constrain the density profile. We isolate the envelope SEDs for our sources by subtracting possible disc and wind components from the submillimetre flux densities, and use a simple model to derive the radial power-law dependence of the density in each case. We find that for a density distribution ρ ∝ r−p the values of p are 0.93 ± 0.25, 0.81 ± 0.30 and 1.22 ± 0.11 for TMC1, TMC1A and Haro 6–10 respectively, which for all but Haro 6–10 are considerably shallower than can currently be explained by collapse theory. We suggest that the envelopes around TMC1 and TMC1A may be rotationally supported, but that the Haro 6–10 envelope may still be infalling. The envelope masses are ∼ 0.02–0.03 M⊙ in all cases.

An Atlas of CO (2 → 1) Maps of the Bipolar Outflow NGC 2264G

We describe detailed CO (2 → 1) observations of NGC 2264G. This object is a highly collimated bipolar molecular outflow that has been completely mapped with moderately high resolution as described here. We present the data as an atlas of contoured channel maps and show spectra at numerous positions of importance. We show that at several positions in the low-velocity outflow lobes, the emission from the outflow dominates the emission within the velocity range of the line core.

A Multitransition CO and CS (2–1) Comparison of a Star‐forming and a Non–Star‐forming Giant Molecular Cloud

CO (3-2), CO (1-0), and CS (2-1) observations of clumps in the cold, low-luminosity cloud G216-2.5 discovered by Maddalena & Thaddeus are compared to the star-forming Rosette molecular cloud. The comparisons suggest that the clumps in a cloud may be characterized as being either dormant, incipient star forming, or star forming. In the Rosette molecular cloud, each set accounts for, respectively, 80%, 10%, and 10% of the total mass, but in G216-2.5, nearly 100% of the clumps are dormant. The physical conditions of the clumps in both clouds suggest a mass agglomeration evolutionary sequence from dormant to star-forming clumps. Detailed results for the clumps in both clouds are as follows. Clump excitation conditions are remarkably uniform in G216-2.5 but show wide variation in the Rosette. CO (3-2) integrated intensities and the ratio of (3-2) to (1-0) emission are significantly greater in the star-forming cloud and greatest of all in those clumps with embedded IRAS sources. The ratio of CO (3-2) to (1-0) line widths is also greater in the Rosette cloud. Peak clump CO (1-0) temperatures are greater in the Rosette than G216-2.5, implying higher gas kinetic temperatures, and are highest of all for those clumps associated with IRAS sources. The ratios of peak CO (3-2) to (1-0) temperatures, however, are comparable in the two clouds, which implies that the volume density of emitting gas in the clumps in each cloud is similar, nH2 103 cm-3. The CS observations indicate the presence of denser gas, nH2 ~ 105 cm-3, in the clumps in each cloud. CS integrated intensities are generally an order of magnitude weaker than 13CO emission in each cloud, but the ratio of the two is a factor of 2 less in G216-2.5. CS to 13CO line width ratios are also lower in G216-2.5, which suggests that there is a deficiency of dense gas relative to the Rosette. Again, the star-forming clumps in the Rosette possess the highest ratios. In addition, CO (2-1) emission was mapped over the central region of G216-2.5 and compared to a CO (1-0) map. The ratio of (2-1) to (1-0) integrated intensities increases toward the clump edges, which is opposite to the Sakamoto et al. study of the Orion molecular cloud. High-resolution 13CO (2-1) maps of one clump in each cloud are compared to 13CO (1-0) maps for evidence of further fragmentation. The (2-1) radial profile is steeper than the (1-0) profile in each clump but decreases at the same relative rate in the two clumps despite their different absolute sizes. We conclude that the differences between clouds and clumps that are forming stars are most readily apparent in the warmer, denser gas traced by the CO (3-2) and CS (2-1) observations and note that there are two starless clumps in the Rosette molecular cloud with CO properties that are more characteristic of the star-forming clumps than the other starless clumps: these are the best candidates for the sites of future star formation in the cloud.

Interpreting the HCN/CO Intensity Ratio in the Galactic Center

We study the dense molecular gas in the Galactic center using maps of HCN and CO J = 1 → 0 emission from the central 630 pc of the Milky Way, and images of HCN J = 3 → 2 emission, which requires high densities for excitation (nH2 106 cm-3), from Sgr A and Sgr B. The ratio of integrated HCN and CO J = 1 → 0 intensities is a sensitive measure of molecular gas pressure, and the ratio of integrated HCN J = 3 → 2 and J = 1 → 0 intensities uncovers density enhancements in the maps. However, the HCN/CO ratio is difficult to model without knowing the relative HCN and CO abundances. Furthermore, because of the different filling factors of HCN and CO emission, models that use homogeneous clouds may not be accurate for analyzing the HCN/CO ratio. Most of the mass traced by HCN and CO in the Galactic center is at high densities (nH2 ~ 104 cm-3), roughly an order of magnitude higher than cloud densities in the Galactic disk. Most of the dense gas traced by HCN J = 3 → 2 emission is coincident with star-forming regions and cloud interaction zones, and not necessarily with emission peaks. We smoothed the HCN and CO maps to the typical spatial resolution of extragalactic observations, and repeated the analysis. The single large-scale measurement was sensitive to the mass-weighted average properties of the map. Therefore, if we can extrapolate this result to other spirals, studies such as this are sensitive to the average gas properties in galactic nuclei, despite poor spatial resolution.

CO (J= 2–1) Line Observations of the Galactic Center Molecular Cloud Complex. II Dynamical Structure and Physical Conditions

A large-scale 12C16O (J = 2-1) survey of the inner few hundred parsecs of the Galaxy has been conducted using the University of Tokyo-Nobeyama Radio Observatory 60 cm survey telescope. We have taken about 70012C16O (J = 2-1) spectra in the region -25 ? l ? 25 and |b| ? 1? with 0125 grid spacing, covering the entire region of the huge molecular cloud complex in the Galactic center. We refer to the CO (J = 1-0) data taken with the Columbia 1.2 m telescope and calculate the J = 2-1 to J = 1-0 intensity ratio. Velocity channel maps and longitude-velocity maps of CO (J = 2-1) line are presented, with corresponding maps of J = 2-1/J = 1-0 intensity ratio. Large-scale CO maps enable us to identify several giant molecular cloud complexes and many characteristic features of molecular gas. We identify 15 molecular cloud complexes larger than ~30 pc in our CO (J = 2-1) data. Their virial masses are at least 1 order of magnitude larger than the masses estimated from the CO luminosity. This discrepancy can be removed if we notice that they may not be gravitationally bound but are in pressure equilibrium with the hot gas and/or magnetic field in this region. Using the expressions of virial mass and CO mass for a cloud in the pressure equilibrium case, we get the X-factor for the Galactic center molecular clouds as X = 0.24 ? 1020 cm-2 (K km s-1)-1, which is 1 order of magnitude lower than that in the Galactic disk (X = 3.0 ? 1020 cm-2 [K km s-1]-1). We estimate the total molecular mass in the Galactic center as M(H2) 2 ? 107 M? as a lower limit; the actual total gas mass within the central 400 pc of the Galaxy must be M(H2) = (2-6) ? 107 M?. We diagnose the physical conditions of the molecular gas in the Galactic center using the intensity ratio between the J = 2-1 and J = 1-0 lines. Although the CO J = 2-1/J = 1-0 line intensity ratio is high (~0.74) in the midplane, molecular gas at |b| ? 025 exhibits low J = 2-1/J = 1-0 ratios (~0.6). The overall J = 2-1/J = 1-0 luminosity ratio is R(2-1)/(1-0) = 0.64 ? 0.01 if we include all the emission within |b| ? 1?, -25 ? l ? 25, and |VLSR| ? 150 km s-1. This indicates that low-density gas 50 pc away from the plane dominates the total CO luminosity of the central 400 pc of the Galaxy. The fractional distribution of the molecular gas with R(2-1)/(1-0) for each cloud complex clearly demonstrates the close relationship between the gas with a very high ratio [R(2-1)/(1-0) ? 1.0] and associated UV sources.