Longitude-velocity Diagrams Research Articles

Synthetic 26Al emission from galactic-scale superbubble simulations

ABSTRACT Emission from the radioactive trace element 26Al has been observed throughout the Milky Way with the COMPTEL and INTEGRAL satellites. In particular, the Doppler shifts measured with INTEGRAL connect 26Al with superbubbles, which may guide 26Al flows off spiral arms in the direction of Galactic rotation. In order to test this paradigm, we have performed galaxy-scale simulations of superbubbles with 26Al injection in a Milky Way-type galaxy. We produce all-sky synthetic γ-ray emission maps of the simulated galaxies. We find that the 1809 keV emission from the radioactive decay of 26Al is highly variable with time and the observer’s position. This allows us to estimate an additional systematic variability of 0.2 dex for a star formation rate derived from 26Al for different times and measurement locations in Milky Way-type galaxies. High-latitude morphological features indicate nearby emission with correspondingly high-integrated γ-ray intensities. We demonstrate that the 26Al scale height from our simulated galaxies depends on the assumed halo gas density. We present the first synthetic 1809 keV longitude-velocity diagrams from 3D hydrodynamic simulations. The line-of-sight velocities for 26Al can be significantly different from the line-of-sight velocities associated with the cold gas. Over time, 26Al velocities consistent with the INTEGRAL observations, within uncertainties, appear at any given longitude, broadly supporting previous suggestions that 26Al injected into expanding superbubbles by massive stars may be responsible for the high velocities found in the INTEGRAL observations. We discuss the effect of systematically varying the location of the superbubbles relative to the spiral arms.

Distribution and kinematics of atomic and molecular gas inside the solar circle

The detailed distribution and kinematics of the atomic and the CO-bright molecular hydrogen in the disc of the Milky Way inside the Solar circle are derived under the assumptions of axisymmetry and pure circular motions. We divide the Galactic disc into a series of rings, and assume that the gas in each ring is described by four parameters: its rotation velocity, velocity dispersion, midplane density and its scale height. We fit these parameters to the Galactic HI and CO (J=1-0) data by producing artificial HI and CO line-profiles and comparing them with the observations. Our approach allows us to fit all parameters to the data simultaneously without assuming a-priori a radial profile for one of the parameters. We present the distribution and kinematics of the HI and H2 in both the approaching (QIV) and the receding (QI) regions of the Galaxy. Our best-fit models reproduces remarkably well the observed HI and CO longitude-velocity diagrams up to a few degrees of distance from the midplane. With the exception of the innermost 2.5 kpc, QI and QIV show very similar kinematics. The rotation curves traced by the HI and H2 follow closely each other, flattening beyond R=6.5 kpc. Both the HI and the H2 surface densities show a) a deep depression at 0.5<R<2.5 kpc, analogous to that shown by some nearby barred galaxies, b) local overdensities that can be interpreted in terms of spiral arms or ring-like features in the disk. The HI (H2) properties are fairly constant in the region outside the depression, with typical velocity dispersion of 8.9+/-1.1 (4.4+/-1.2) km/s, density of 0.43+/-0.11 (0.42+/-0.22) cm-3 and HWHM scale height of 202+/-28 (64+/-12) pc. We also show that the HI opacity in the LAB data can be accounted for by using an `effective' spin temperature of about 150 K: assuming an optically thin regime leads to underestimate the HI mass by about 30%.

Wide-field 12CO ( ) and 13CO ( ) Observations toward the Aquila Rift and Serpens Molecular Cloud Complexes. I. Molecular Clouds and Their Physical Properties

Abstract We present the results of wide-field 12CO ( ) and 13CO ( ) observations toward the Aquila Rift and Serpens molecular cloud complexes ( and ) at an angular resolution of 3.′4 ( pc) and at a velocity resolution of 0.079 km s−1 with velocity coverage of . We found that the 13CO emission better traces the structures seen in the extinction map, and derived the -factor of this region. Applying SCIMES to the 13CO data cube, we identified 61 clouds and derived their mass, radii, and line widths. The line width–radius relation of the identified clouds basically follows those of nearby molecular clouds. The majority of the identified clouds are close to virial equilibrium, although the dispersion is large. By inspecting the 12CO channel maps by eye, we found several arcs that are spatially extended to 0.°2–3° in length. In the longitude–velocity diagrams of 12CO, we also found two spatially extended components that appear to converge toward Serpens South and the W40 region. The existence of two components with different velocities and arcs suggests that large-scale expanding bubbles and/or flows play a role in the formation and evolution of the Serpens South and W40 cloud.

Gas flow in barred potentials – III. Effects of varying the quadrupole

We run hydrodynamical simulations of a 2D isothermal non self-gravitating inviscid gas flowing in a rigidly rotating externally imposed potential formed by only two components: a monopole and a quadrupole. We explore systematically the effects of varying the quadrupole while keeping fixed the monopole and discuss the consequences for the interpretation of longitude-velocity diagrams in the Milky Way. We find that the gas flow can constrain the quadrupole of the potential and the characteristics of the bar that generates it. The exponential scale length of the bar must be at least $1.5\rm\, kpc$. The strength of the bar is also constrained. Our global interpretation favours a pattern speed of $\Omega=40\,\rm km s^{-1} {kpc}^{-1}$. We find that for most observational features, there exist a value of the parameters that matches each individual feature well, but is difficult to reproduce all the important features at once. Due to the intractably high number of parameters involved in the general problem, quantitative fitting methods that can run automatic searches in parameter space are necessary.

TRIGONOMETRIC PARALLAXES TO STAR-FORMING REGIONS WITHIN 4 kpc OF THE GALACTIC CENTER

We report four trigonometric parallaxes for high-mass star-forming regions within 4 kpc of the Galactic center. These measurements were made with the VLBA as part of the BeSSeL Survey. By associating these sources kinematically with large-scale features in CO and HI longitude-velocity diagrams, we begin to outline some major features of the inner Milky Way: the Connecting arm, the near and far 3 kpc arms, and the Norma arm. The Connecting arm in the first Galactic quadrant lies closer to the Galactic center than the far 3 kpc arm and is offset by the long-bar's major axis near its leading edge, supporting the presence of an inner Lindblad resonance. Assuming the 3 kpc arms are a continuous physical structure, the relative Galactocentric distance of its near and far sides suggests highly elliptical streamlines of gas around the bar(s) and a bar corotation radius, r_CR > 3.6 kpc. At a Galactic longitude near 10{\deg} and a heliocentric distance of about 5 kpc, the near 3 kpc arm and the Norma arm intersect on a face-on view of our Galaxy, while passing at different Galactic latitudes. We provide an accurate distance measurement to the W31 star-forming complex of 4.95 (+0.5;-0.43) kpc from the Sun, which associates it with a bright CO feature belonging to the near 3 kpc arm.

SiO Maser Surveys of Nearby Miras and their Kinematics in the Galaxy

AbstractI review recent developments of SiO maser observations of miras, and discuss their kinematics in the Galaxy. They are deeply related to problems in other fields, such as the noncircular motion of Galactic spiral arms, streaming motions of stars in the solar neighborhood, and the interface between the interstellar and circumstellar medium revealed by Far-IR imaging of some miras, as well as the bar structure of the Galaxy. Recent automated surveys of variable stars in optical bands have increased the number of red variables enormously. Most of these stars were not observed in the radio before because of their blue colors. We have obtained radial velocities of these long-period variables with SiO masers. The longitude-velocity diagrams have revealed interesting deviations of stellar motions from the circular motion of the Galaxy, some of which show spatial motion similar to that of the Hercules group of stars. Important developments of SiO maser observations have been made for symbiotic stars, one of which showed a nova eruption with concurrent γ ray emission in 2010 (V407 Cyg). SiO masers have been monitored for this star after the nova eruption. I summarize the SiO maser observations of individual stars also.

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.

The Properties of the Galactic Bar Implied by Gas Kinematics in the Inner Milky Way

Longitude-velocity (l-V) diagrams of H I and CO gas in the inner Milky Way have long been known to be inconsistent with circular motion in an axisymmetric potential. Several lines of evidence suggest that the Galaxy is barred, and gas flow in a barred potential could be consistent with the observed velocities and other features in the data. We compare the H I observations to l-V diagrams synthesized from two-dimensional fluid dynamical simulations of gas flows in a family of barred potentials. The gas flow pattern is very sensitive to the parameters of the assumed potential, which allows us to discriminate among models. We present a model that reproduces the outer contour of the H I l-V diagram reasonably well; this model has a strong bar with a semimajor axis of 3.6 kpc, an axis ratio of approximately 3 : 1, an inner Lindblad resonance (ILR), and a pattern speed of 42 km s-1 kpc-1, and it matches the data best when viewed from 34° to the bar major axis. The behavior of the models, combined with the constraint that the shocks in the Milky Way bar should resemble those in external barred galaxies, leads us to conclude that wide ranges of parameter space are incompatible with the observations. In particular we suggest that the bar must be fairly strong, must have an ILR, and cannot be too end-on, with the bar major axis at 35° ± 5° to the line of sight. The H I data exhibit larger forbidden velocities over a wider longitude range than are seen in molecular gas; this important difference is the reason our favored model differs so significantly from other recently proposed models.

Properties of the Galactic Bar from Hydrodynamical Simulations

It is well known that the existence of H I and CO gas at “forbidden” velocities in longitude-velocity (l–V) diagrams of the inner Galaxy (e.g. Liszt &amp; Burton 1980, Liszt 1992) is inconsistent with circular motion in an axisymmetric potential. Gas flow in a barred potential could be consistent with the observations, however. We have compared the observations to 2-D hydrodynamical simulations of gas flows in a family of barred potentials. The gas flow pattern is very sensitive to the assumed potential, and the l–V distribution of cool gas in the inner Galaxy places strong constraints on several parameters of the Milky Way bar.

Ambiguities in the identification of giant molecular cloud complexes from longitude-velocity diagrams

Techniques which use longitude-velocity diagrams to identify molecular cloud complexes in the disk of the Galaxy are investigated by means of model Galactic disks generated from N-body cloud-particle simulations. A procedure similar to the method used to reduce the low-level emission in Galactic l-v diagrams is employed to isolate complexes of emission in the model l-v diagram (LVCs) from the 'background'clouds. The LVCs produced in this manner yield a size-line-width relationship with a slope of 0.58 and a mass spectrum with a slope of 1.55, consistent with Galactic observations. It is demonstrated that associations identified as LVCs are often chance superpositions of clouds spread out along the line of sight in the disk of the model system. This indicates that the l-v diagram cannot be used to unambiguously determine the location of molecular cloud complexes in the model Galactic disk. The modeling results also indicate that the existence of a size-line-width relationship is not a reliable indicator of the physical nature of cloud complexes, in particular, whether the complexes are gravitationally bound objects.

A deep CO survey of molecular clouds in the southern Milky Way

The complete set of observations from the first out-of-plane CO survey of the southern Milky Way is presented. A spatial map, obtained by integrating the CO emission over the entire observed spectral range, 333 km/s, summarizes the survey concisely and can be compared with gamma-ray, IR, and other continuum data. The kinematic information is summarized in two longitude-velocity diagrams, which represent in one case the CO emission integrated over the whole latitude coverage of the survey, and in the other, to improve the contrast for the distant molecular clouds, over a narrower latitude region within 0.75 deg of the Galactic equator. The information is unfolded in a complete set of longitude-velocity contour diagrams, one for each latitude observed. 18 refs.

Can Galactic GMCs be Identified from l-v Diagrams?

Identifying the spiral nature of the distribution of gas in the Galaxy has been a subject of much research in the past thirty years. The position of the sun in the disk of the Galaxy presents us with a problem of perspective: how does one identify the cloud system from within the system? Longitude-velocity (l-v) diagrams have been used to try to determine the distribution of interstellar gas, but problems inherent in the methods have been pointed out previously (Burton 1971). Recent Galactic CO surveys have been used in attempts to map the distribution of molecular cloud complexes in the disk of the Galaxy (Dame, et al. 1986). Here, we use numerical simulations of the molecular cloud system in a spiral galaxy to consider the following question: to what extent can concentrations of emission in the l-v diagram (LVCs) be considered complexes of gas in the disk of the Galaxy (GMCs)?

On the spiral structure in the galactic distribution of co clouds

The CO distribution in the Galaxy is investigated through an analysis of longitude-velocity diagrams of CO emission lines for the two longitude ranges 20°<l<80° and 105°<l<140°. For the kinematics of the Galaxy we adopt the three typical models; the circular rotation, the linear density waves, and the galactic shock waves. It is shown that the distributions and kinematics of CO clouds are consistent with the predictions of the density wave model and the galactic shock model, and that the observed data of CO emissions do not contradict with the claim that the CO clouds form spiral arms.

Interpretation of CO emission from the galactic molecular cloud ensemble

view Abstract Citations (70) References (24) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Interpretation of CO emission from the galactic molecular cloud ensemble Liszt, H. S. ; Burton, W. B. Abstract An analysis is presented of simulated observations of CO emission from a model ensemble of molecular clouds in the inner Galaxy. The model galaxy is constructed in a three-dimensional manner, with each cloud assigned a position and systematic velocity according to specified distribution laws, and the observations are simulated for circular Gaussian telescope beams of different sizes and sampling intervals. Results are presented in the form of longitude-velocity diagrams for models of various cloud numbers, sizes, velocity distributions and kinematic perturbations. The typical cloud diameter in the inner Galaxy is found to be 15-20 pc, with larger clouds containing not more than 50% of the total cloud volume, while many of the large complex features observed in CO emission maps are shown to be the result of the blending of physically unrelated clouds. Results indicate that even though the molecular clouds fill a very small fraction of the galactic layer, they are sufficiently abundant that the gas distribution in the inner Galaxy must be dominated by molecular matter. Finally, the collective emission pattern in the Galactic plane is found to be very susceptible to kinematic perturbations, and kinematic patterns usually ascribed to spiral density enhancements are shown to arise mainly through velocity projection effects. Publication: The Astrophysical Journal Pub Date: February 1981 DOI: 10.1086/158646 Bibcode: 1981ApJ...243..778L Keywords: Carbon Monoxide; Galactic Radiation; Interstellar Gas; Molecular Clouds; Molecular Gases; Antenna Radiation Patterns; Astronomical Models; Galactic Structure; Telescopes; Velocity Distribution; Astrophysics full text sources ADS |