Galactic Center Molecular Clouds Research Articles

The Origin of Diffuse X‐Ray and γ‐Ray Emission from the Galactic Center Region: Cosmic‐Ray Particles

The inner couple of hundred parsecs of our Galaxy are characterized by a significant amount of synchrotron-emitting gas. Many of best studied sources in this region exhibit a mixture of 6.4 keV Fe Kα emission, molecular line emission, and nonthermal radio continuum radiation. The spatial correlation between fluorescent Fe Kα line emission at 6.4 keV and molecular line emission from Galactic center molecular clouds has been explained as reflected X-rays from a past outburst of Sgr A*. Here we present a multiwavelength study of this region and find a correlation between nonthermal radio filaments and X-ray features. This correlation, when combined with distribution of molecular gas, suggests against irradiation model. Instead, we account for this distribution in terms of impact of relativistic particles from local (nonthermal filaments) and extended sources with diffuse neutral gas producing both nonthermal bremsstrahlung X-ray continuum emission and diffuse 6.4 keV line emission. The production rate of Fe Kα photons associated with injection of electrons into a cloud as a function of column density is calculated. The required energy density of low-energy cosmic rays associated with synchrotron-emitting radio filaments or extended features is estimated to be in range between 20 and ~103 eV cm-3 for Sgr C, Sgr B1, Sgr B2, and the 45 and -30 km s-1 clouds. We also generalize this idea to explain cosmic-ray heating of molecular gas, interstellar cosmic-ray ionization, pervasive production of diffuse Kα line, and TeV emission from Galactic center molecular clouds. In particular, we suggest that inverse Compton scattering of submillimeter radiation from dust by relativistic electrons may contribute substantially to large-scale diffuse TeV emission observed toward central regions of Galaxy.

A 2 Millimeter Spectral Line Survey of the Starburst Galaxy NGC 253

We present the first unbiased molecular line survey toward an extragalactic source, namely the nuclear region of the starburst galaxy NGC 253. The scan covers the frequency band from 129.1 to 175.2 GHz, i.e., most of the 2 mm atmospheric window. We identify 111 spectral features as transitions from 25 different molecular species. Eight of which (three tentatively) are detected for the first time in the extragalactic interstellar medium. Among these newly detected species, we detected the rare isotopomers 34SO and HC18O+. Tentative detections of two deuterated species, DNC and N2D+, are reported for the first time from a target beyond the Magellanic Clouds. In addition, three hydrogen recombination lines are identified, while no organic molecules larger than methanol are detected. Column densities and rotation temperatures are calculated for all the species, including an upper limit to the ethanol abundance. A comparison of the chemical composition of the nuclear environment of NGC 253 with those of selected nearby galaxies demonstrates the chemical resemblance of IC 342 and NGC 4945 to that of NGC 253. On the other hand, the chemistries characterizing NGC 253 and M82 are clearly different. We also present a comparison of the chemical composition of NGC 253 with those observed in Galactic prototypical sources. The chemistry of NGC 253 shows a striking similarity with the chemistry observed toward the Galactic center molecular clouds, which are thought to be dominated by low-velocity shocks. This resemblance strongly suggests that the heating in the nuclear environment of NGC 253 is dominated by the same mechanism as that in the central region of the Milky Way.

The solar boron, stellar lithium and deuterium, interstellar deuterium, and extragalactic deuterium abundances

D, Li, Be, and B are not by formed by ordinary stellar nucleosynthesis. D is primarily formed in the big bang. Because each generation of stars replenished the ISM with material depleted in D, the D abundance decreases with time and is larger in low-metallicity regions. Because Li, Be and B are primarily formed via cosmic-ray spallation reactions, their abundances will increase with time. Some Li and B is produced in supernovae via ν-spallation reactions and some Li is produced via mass loss from Li-rich AGB stars. To determine if the B abundance has increased during past 4.5 Gyr, an accurate Solar B abundance will be determined from ongoing observations of the 1.6 μ lines of B. Initial negative results yield B/H < 3.5 × 10 −10. The Balmer D α line was not detected in the high-metallicity star HD 82943; the low-metallicity Pop II halo star HD 140283 (both with detected 6Li); or in the slowly rotating B stars τ Her and γ Peg with D/H < 1.0 × 10 −5. Observations of Li in super-Li-rich AGB C and S stars with strong Li lines confirms that these stars have the largest Li abundance in the Galaxy (Li/H = 10 −7 and that mass loss from these stars may contribute to the ISM Li abundance. Observations of the DCN/HCN ratio in the Galaxy yield D/H < 1.4 × 10 −6 in the Galactic Center molecular clouds (10 pc from the center) and a positive D/H gradient in the Galaxy implying that there are no Galactic sources of D and D is cosmological. Extragalactic D (in DCN) in currently being searched for in two gravitational lenses against quasars at z = 0.7 and 0.9 and in the Seyfert Galaxy NGC 1068.

A Hyperenergetic CO Shell in the Galactic Center Molecular Cloud Complex

Abstract We report on the detection of a high-velocity feature, CO 1.27$+$0.01, in the Galactic-center molecular-cloud complex. In our 12CO images taken with the NRO 45 m telescope, CO 1.27$+$0.01 appears as the second prominent high-velocity feature, next to CO 0.02–0.02, having a velocity width of $\Delta V \simeq 100 \,\mathrm{km} \,\mathrm{s}^{-1}$ and a size of $9 \,\mathrm{pc} \times 12 \,\mathrm{pc}$. It seems to consist of two expanding shells with sizes of $\sim 10 \,\mathrm{pc}$. The major shell has a kinetic energy of about an order of magnitude larger than the gravitational energy, indicating that the shell is apparently gravitationally unbound. We propose that CO 1.27$+$0.01 may have been accelerated by a series of supernova explosions which occurred within the last $\sim 6 \times 10^4 \,\mathrm{yr}$. No association of a nonthermal radio continuum source suggests a strong magnetic field ($B \gtrsim 0.4 \,\mathrm{mG}$) in the vicinity of CO 1.27$+$0.01.

The Tokyo‐Onsala‐ESO‐Calan Galactic CO J = 2–1 Survey. I. The Galactic Center Region

As a part of the Tokyo-Onsala-ESO-Calán Galactic CO survey, we have made large-scale mapping observations of the Galactic center in the CO J = 2-1 line using the 60 cm survey telescope in Chile. The data were taken with the same beam size (9') and sampling grid (75) as the CO J = 1-0 Columbia survey, so that the two data sets can be compared directly. Velocity channel maps and longitude-velocity diagrams of the 12CO J = 2-1 line, covering -6° ≤ l ≤ 6°, -2° ≤ b ≤ 2°, -300 ≤ vLSR ≤ 300 km s-1, are presented, along with the corresponding maps of the 12CO J = 2-1/J = 1-0 intensity ratio [R2-1/1-0(12CO)]. A longitude-velocity diagram of the 13CO J = 2-1 line intensity and corresponding 13CO J = 2-1/12CO J = 2-1 intensity ratio [R13/12(J = 2-1)] is also presented at b = 0°. The overall R2-1/1-0(12CO) in the central 900 pc of the Galaxy is 0.96 ± 0.01, which is higher than the typical value in the Galactic disk, 0.6-0.7. The isotopic intensity ratio R13/12(J = 2-1) is 0.10 ± 0.01. The two observed intensity ratios [R2-1/1-0(12CO) and R13/12(J = 2-1)] indicate that the optical depth of the 12CO J = 1-0 line is ~1 or smaller in the Galactic center molecular clouds, much smaller than those of the giant molecular clouds (GMCs) in the Galactic disk. Longitude-velocity distributions of physical properties of molecular gas are derived using a large velocity gradient analysis. Molecular gas in the Galactic center generally shows a high pressure. In particular, there is a high-pressure region in the central ∼100 pc where the pressure is an order of magnitude higher than that in GMCs in the Galactic disk. This region is dominated by high-density gas and contains star-forming regions.

Hot Expanding Shells in the Envelope of the Sagittarius B2 Molecular Cloud

We present high-resolution (3'') maps of the (3, 3) and (4, 4) lines of NH3 toward the southern part of the molecular envelope of the Sagittarius B2 star-forming region. These maps reveal, for the first time, that the morphology of the hot gas in the Sgr B2 envelope is dominated by at least six rings, two arcs, and a filament. The sizes of the rings are between 1 and 2.6 pc and their thicknesses between 0.2 and 0.4 pc. Most of the gas in the rings is warm, with kinetic temperatures, Tk, of 40-70 K, although some parts of the low-velocity rings reach temperatures larger than 100 K. These hot rings and arcs represent regions of enhanced H2 density and/or enhanced NH3 abundance in the Sgr B2 envelope. Some of the hot rings show radial velocity gradients, which suggests that the rings and arcs correspond to three-dimensional shells expanding at velocities of ~6-10 km s-1. The walls of the hot shells are highly clumped and contain unresolved condensations (1'') in the line maps. There are two kinds of unresolved condensations: those appearing in the (3, 3) line maps with only weak emission in the (4, 4) line and those with rather strong emission in the (4, 4) line compared with that of the (3, 3) line. For the first kind, we identify six condensations in the NH3 (3, 3) line maps that have brightness temperatures larger than the kinetic temperatures. It is likely that these are newly found NH3 masers. We also find that other masers in the region such as class II CH3OH and H2CO masers are very well correlated in velocity and position with the hot shells. The large number of masers observed in the hot shells can be explained as a result of the combination of high abundance of volatile molecules like NH3, CH3OH, and H2CO (produced by hot temperature and/or shock chemistry) and sufficient velocity coherence at the edges of the expanding shells. For the second kind of condensations, we have identified three compact sources in the walls of the shells that are hot (Tk 300 K) and have high H2 densities (106-107 cm-3). These exhibit characteristics similar to those of the hot cores associated with newly formed massive stars. The inferred dust luminosity for the hot cores is at least 105 L☉, similar to the Orion A hot core. The high luminosity of the hot cores and the lack of associated radio continuum emission indicates that these are internally heated and very likely associated with the dense circumstellar material surrounding newly formed stars. The detection of hot cores suggests that massive star formation is not restricted to only Sgr B2N and Sgr B2M but is also taking place in the envelope of Sgr B2. Very likely, this star formation has been triggered by the expanding bubbles that produce the shells seen in the NH3 inversion lines. We discuss the possible origin of these hot shells. We find that a wind-blown bubble driven by typical Galactic Wolf-Rayet stars could account for the kinetic energies and the momenta observed in the hot NH3 shells. A cluster containing massive evolved stars such as the Quintuplet could easily explain the large concentration of hot shells, the heating of the warm envelope, and the peculiar chemistry observed in the envelope of Sgr B2. The implication of these findings for the heating of Galactic center molecular clouds is briefly discussed.

Density structure of the Sgr B2 molecular cloud probed by low-J lines of CS

We have observed the CS (2-1) and (3-2), and 13CS (2-1) transitions toward the Galactic Center molecular cloud Sgr B2 which consists of several clumps with different chemical properties. We have newly identified a cloud at 30 km s −1 from a CS (2-1) optical depth map. This cloud lies 1.5′ South from the Sgr B2 (M) position and has a diameter of ∼2.5 pc and a total column density of 7 × 10 23 cm −2 assuming optically thin emission of the 13CS (2-1) line. Towards the 2′N Cloud no evidence for a density enhancement is found, which suggests that the strong emission from HNCO and HCO 2 + is due to chemical effects. The main isotopic CS lines show broad wing components similar to previous studies, but we find rotational temperatures T rot(CS) < 10 K at this region.

G359.87+0.18, An FR II Radio Galaxy 15′ from Sagittarius A*: Implications for the Scattering Region in the Galactic Center

G359.87+0.18 is an enigmatic object located 15' from Sgr A*. It has been variously classified as an extragalactic source, a Galactic jet source, and a young supernova remnant. We present new observations of G359.87+0.18 between 0.33 and 15 GHz and an H I absorption spectrum and use these to argue that this source is an FR II radio galaxy. We are able to place a crude limit on its redshift of z0.1. The source has a spectral index of α<-1 (S∝να), suggestive of a radio galaxy with a redshift z2. The scattering diameters of Sgr A* and several nearby OH masers (≈1'' at 1 GHz) indicate that a region of enhanced scattering is along the line of sight to the Galactic center. If the region covers the Galactic center uniformly, the implied diameter for a background source is at least 600'' at 0.33 GHz, which is in contrast with the observed 20'' diameter of G359.87+0.18. Using the scattering diameter of a nearby OH maser OH 359.762+0.120 and the widths of two nearby nonthermal threads, G0.08+0.15 and G359.79+0.17, we show that a uniform scattering region should cover G359.87+0.18. We therefore conclude that the Galactic center scattering region is inhomogeneous on a scale of 5' (≈10 pc at a distance of 8.5 kpc). This scale is comparable with the size scale of molecular clouds in the Galactic center. The close agreement between these two length scales is an indication that the scattering region is linked intimately to the Galactic center molecular clouds.

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.

4.14. Molecular shell formation by supernova remnants in the Galactic center—what we learn from the case of W44

Recent high resolution CO images of the Galactic center (GC) molecular clouds reveal many arc and/or shell structures (Oka et al. 1997). A considerable fraction of them may most probably be formed by an interaction between supernova remnants (SNRs) and molecular clouds. Studies of such cases in less confused areas are needed to clarify this picture. The SNR W44 is a good place for examining the SNR induced shell formation scenario.

4.15. Dynamics of the Galactic center molecular clouds

We have performed Smoothed Particle Hydrodynamics (SPH) simulations in order to understand the dynamical structures of Galactic Center molecular clouds. In our study it was found that the structures of GC molecular clouds could be induced by the combined effects of a gravitational perturbation by rotating bar potential and the hydrodynamic collisions between the clouds.

Infrared Identification of SiO Maser Sources toward the SGR B2 Molecular Cloud

J, H, and K imaging observations have been made in order to identify infrared counterparts for 6 new SiO maser sources toward the Sgr B2 molecular cloud. Only two of these 6 new sources were previously identified with known objects in other catalogues: one with an OH/IR object, another with an IRAS point source. With the imaging observations, we find infrared counterparts for four of the objects: #1, #2, #4, and #7. These are late-type stars either nearby or near to the galactic center. The remaining two sources of which we could not find counterparts can be candidates of young stellar objects with SiO masers. However, it is highly possible that these two are late-type stars located behind the galactic-center molecular cloud, suffering from very high dust extinction.

S[CLC]i[/CLC]O Emission from the Galactic Center Molecular Clouds

We have mapped the J=1-0 line of SiO in a 1 degree x 12 arcmin. (lxb) region around the Galactic center (GC) with an angular resolution of 2 arcmin. (approx. 4 pc). In contrast to the spatial distribution of other high dipole moment molecules like CS, whose emission is nearly uniform, the SiO emission is very fragmented and it is only associated with some molecular clouds. In particular, it is remarkable that the SiO emission closely follows the non-thermal radio arc in the GC. The SiO clouds are more extended than the beam with typical sizes between 4 and 20 pc. High angular resolution (26 arcsec.) mapping in the J=1-0 line of SiO toward the molecular clouds in Sgr B2 and Sgr A shows that the SiO emission is relatively smooth with structures of typically 2 pc. From the line intensities of the J=1-0, J=3-2 and J=5-4 transitions of SiO we derive H2 densities for these clouds of a few 1e4 cm-3. The SiO fractional abundances are approx. 1e-9 for the SiO clouds and 1e-10 for the other molecular clouds in the GC. The characteristics (size and H2 densities) of the SiO emission in the GC are completely different from those observed in the Galactic disk, where the SiO emission arises from much smaller regions with larger H2 densities. We briefly discuss the implications of the SiO emission in the molecular clouds of the GC. We conclude that the particular chemistry in these clouds is probably related to large scale fast shocks occurring in the Galactic center region.

Probing Galactic Center Black Hole Candidates with Far-Infrared Spectroscopy

The narrow positron annihilation line source in the Galactic center has been identified with the X-ray source 1E 1740.7-2942, which is coincident in projection with a Galactic center molecular cloud and exhibits synchrotron jets in the radio spectrum. It has been suggested that the source is powered by Bondi-Hoyle accretion from the cloud onto a compact object. Here we show that, regardless of the source of the X-ray luminosity, the irradiation of the interstellar medium (ISM) in its vicinity by ~1 keV X-rays can generate substantial luminosities in a number of far-infrared atomic fine-structure lines, such as [O I] 63 μm, [C II] 158 μm, and [Si II] 35 μm. Similar predictions hold for the nearly identical (in luminosity, spectrum, and presence of radio jets) Galactic center source GRS 1758-258. The corresponding line fluxes will be easily detectable using the Infrared Space Observatory; if the sources are embedded in dense ISM, the observations will allow estimates of the gas density, column density, and volume filling factor in the vicinity of the X-ray sources, the X-ray luminosity of the source averaged over the last t ~ 100 yr, and will constrain models for accretion from the ISM.

The Nature of the Faraday Screen Toward the Galactic Center Nonthermal Filament G359.54+0.18

We present multifrequency VLA observations of the intensity and polarized emission from G359.54+0.18, a system of nonthermal filaments near the Galactic center. The intrinsic magnetic field lines run primarily along the filaments. The rotation measure (RM) varies between -4200 and -370 rad m-2 on scales of several arcseconds, implying that the ionized, magnetized medium responsible for the Faraday rotation is less than 0.1 pc thick. In turn, this implies that the magnetic field in the Faraday screen is large, suggesting that it is located close to the Galactic center. Further evidence is provided by the anisotropy of the RM fluctuations. The structure of the eastern portion of G359.54+0.18 suggests that the magnetic field in the filaments is highly distorted as a result of an interaction with an adjacent Galactic center molecular cloud.

A large-scale cloud collision in the galactic center molecular cloud near Sagittarius B21

A large-scale cloud collision in the galactic center molecular cloud near Sagittarius B21

Infrared observations of the center of the Galaxy

We report velocity resolved maps of the 158 μm [CII] fine structure line, of the 63 μm [OI] fine structure line, and of the 88 μm and 52 μm [OIII] fine structure lines toward the center region of the Galaxy. Observations were carried out on board the Kuiper Airborne Observatory (KAO) using the MPE/UCB Far-infrared Imaging Fabry-Perot interferometer (FIFI). The angular resolution was 55″ for the [CII] line and 22″ for the oxygen lines. The [CII] emission, tracing the distribution of dense gas dissociated and partially ionized by far-UV photons, is strongest toward the rotating circumnuclear disk surrounding Sgr A West. A continuous bridge of [CII] line emission connects the Sgr A complex to the thermal radio filaments in the Radio Arc 10' north of the center, thus strongly suggesting a direct physical connection between the two. Outside the nuclear region, there is an anticorrelation between the [CII] intensity and the distribution of dense molecular material. The brightest emission occurs near the edges of the massive Galactic Center molecular clouds. We conclude that these clouds are predominantly ionized by external UV photons, partly from the Sgr A West region and partly from OB stars near the center. Our data indicate a physical connection between the center and the massive interstellar clouds in the surrounding 50 pc. Our 88 μm [OIII] maps of the “sickle” region within the Radio Arc and of the thermal arches approximately follow the thermal radio continuum. The line width varies substantially over the mapped area; we find a narrow ( Δv ≈ 60 km/ s) and a broad ( Δv ≈ 150 km/ s) component, with the broad component being more prominent in the sickle. From the 52 μm 88 μm line ratio we derive a density ≈ 300 cm −3 and a hydrogen column density ≈3 × 10 21 cm −1 in the [OIII] emitting medium. This rules out shocks or fast winds from stars as excitation mechaniμs. UV excitation, however, could produce the observed amount of O 2+. About 10 to 100 O-stars would be required to ionize the gas in the “pistol/sickle” region. The broad linewidth could be due to interaction between the ionized gas and magnetic fields. Our maps of both [OIII] lines toward the circum-nuclear disk show the brightest emission from inside the central, “ionized cavity” in the molecular ring. The high 52 μm/88μm ratio requires a density ⩾ 10 4 cm −3. The derived hydrogen column density, however, is low, which could be understood by a small ionization ratio N o2+ N o0 or a small beam filling factor of the ionized gas. We infer about 10 m⊙ of ionized gas inside the cavity, with no indication of an additional low-density component. The [OI] data, on the other hand, reveal the presence of a large amount of neutral atomic gas inside the central cavity, and associated with the “northern” and “eastern” arms of ionized gas. We estimate about 300 M⊙ of dense neutral hydrogen gas within the 1.5 pc radius of the circumnuclear ring, about a factor 10 more than the amount of ionized gas. The northern arm appears to be a bright, ionized rim at the surface of this neutral gas streamer, which is probably in the plane of the circumnuelear disk and falling from a gap in the 1.5pc ring into the central 0.5pc. We infer a mass inflow rate into the central parsec of 0.03 M⊙/year.

Deuterated Ammonia in Sagittarius B2

The 85.927 GHz J = 1<SUB>11</SUB> → 1<SUB>01</SUB> transition of deuterated ammonia (NH<SUB>2</SUB>D) has been detected in the northern "hot-core" in Sagittarius B2 with the BIMA millimeter-wave interferometer. Emission and absorption in the (3, 3) inversion transition of the <SUP>15</SUP>NH<SUB>3</SUB> isotope has also been detected from the same region with the VLA. The <SUP>15</SUP>NH_{3 }observations confirm the large NH<SUB>3</SUB> abundance enhancements previously inferred. The abundance of NH<SUB>2</SUB>D relative to NH<SUB>3</SUB> is enhanced by more than an order of magnitude compared to the local elemental (D/H) ratio, but is an order of magnitude lower than in hot-core regions outside the Galactic center. The NH<SUB>2</SUB>D and NH<SUB>3</SUB> enhancements are consistent with models in which the molecules are formed on dust grains in an earlier, cooler epoch; the comparatively small NH<SUB>2</SUB>D enhancement can be explained by the relatively high temperatures in Galactic center giant molecular clouds. The difference in physical conditions between Galactic center and local molecular clouds must be considered in deriving the elemental (D/H) ratio from observations of deuterated molecules.

Millimeter continuum observations of Galactic center giant molecular cloud cores

Results are presented of observations of 1.3- and 0.8-mm continuum emission toward the cores of three Galactic center molecular clouds with ongoing massive star formation, Sagittarius B2, C, and D, which were made in order to study possible variations in the high-mass star formation rate per unit mass between the Galactic center and the disk. The luminosity-to-mass ratio, based on the mass estimates derived from the millimeter continuum emission, is used as a tracer of the high-mass star formation rate in GMC cores. The magnitude of errors involved in using millimeter continuum emission for determining the core mass is estimated through radiative transfer modeling. It is inferred from the present millimeter data, along with previously published far-infrared data, that the Sgr C and D cores are very similar in terms of mean dust optical depth and temperature. The luminosity-to-mass ratios derived for the Sgr C and D cores are found to be consistent with those of typical disk GMC cores with comparable far-infrared luminosities.

Understanding the kinematics of Galactic Centre gas

A coherent picture is constructed of the Galaxy’s H I, CO and CS emissions in the region |$|l|\le10^\circ,\enspace |b|\le0.5^\circ$|⁠. The flow of gas at the Galactic Centre is dominated by a bar that has corotation at r= 2.4 ± 0.5 kpc, which we view at an angle of |$\theta_\text {incl}=16\pm2^\circ$| from its major axis. The first CO emission arises where gas is obliged to switch from x1 orbits to x2 orbits in the notation of Contopoulos. This gives rise to a shock and a clear signature in the (l, v) diagram. The great Galactic Centre molecular clouds such as Sgr B are on x2 orbits. From the structure of the H I terminal velocity envelope we deduce that the central mass density scales as |$\rho \propto r^{-1.75}$| out to at least ∼ 1.2 kpc along the bar’s major axis. Consequently the circular velocity curve is rising significantly through the radius range where naive analysis of the tangent velocity leads to a falling rotation curve. The great ring of molecular material at |$r \simeq 3.5$| kpc is probably associated with the bar’s outer Lindblad resonance, and the region of low gas densities inwards from there with corotation.