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Register a new userDynamically Driven Inflow onto the Galactic Center and its Effect upon Molecular Clouds
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The Galactic bar plays a critical role in the evolution of the Milky Ways Central Molecular Zone (CMZ), as its potential drives mass inward toward the Galactic Center via gas flows known as dust lanes. To explore the interaction between the CMZ and the dust lanes, we run hydrodynamic simulations in Arepo, modeling the potential of the Milky Ways bar in the absence of gas self-gravity and star formation physics, and we study the flows of mass using Monte Carlo tracer particles. We estimate the efficiency of the inflow via the dust lanes, finding that only about a third (30 +/- 12%) of the dust lanes mass initially accretes onto the CMZ, while the rest overshoots and accretes later. Given observational estimates of the amount of gas within the Milky Ways dust lanes, this suggests that the true total inflow rate onto the CMZ is 0.8 +/- 0.6 Msun yr$^{-1}$. Clouds in this simulated CMZ have sudden peaks in their average density near apocenter, where they undergo violent collisions with inflowing material. While these clouds tend to counter-rotate due to shear, co-rotating clouds occasionally occur (~52% are strongly counter-rotating, and ~7% are strongly co-rotating of the 44 cloud sample), likely due to the injection of momentum from collisions with inflowing material. We investigate the formation and evolution of these clouds, finding that they are fed by many discrete inflow events, providing a consistent source of gas to CMZ clouds even as they collapse and form stars.
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We demonstrate that the accretion disk model for the Galactic Center region by Linden et al (1993a) is applicable for at least one order of magnitude in radius from the Galactic Center (10 ... 100 pc). The viscosity $ u$ is shown to be weakly dependent on the radius $s$: $ u sim s^{0.4}$. Finally, we discuss the influence of the inner boundary on the structure of the inner disk regions.
This letter presents a Nyquist-sampled, high-resolution [CI] 3P1-3P0 map of the -0.2 deg < l < 1.2 deg x -0.1 deg < b < 0 deg region in the Central Molecular Zone (CMZ) taken with the Atacama Submillimeter Telescope Experiment (ASTE) 10 m telescope. We have found that molecular clouds in the CMZ can be classified into two groups according to their [CI]/13CO intensity ratios: a bulk component consisting with clouds with a low, uniform [CI]/13CO ratio (0.45) and another component consisting of clouds with high [CI]/13CO ratios (> 0.8). The [CI]-enhanced regions appear in M-0.02-0.07, the circumnuclear disk, the 180-pc ring and the high velocity compact cloud CO+0.02-0.02. We have carried out a large velocity gradient (LVG) analysis and have derived the C^0/CO column density ratio for M-0.02-0.07 as 0.47, which is approximately twice that of the bulk component of the CMZ (0.26). We propose several hypotheses on the origin of high C^0 abundance in M-0.02-0.07, including cosmic-ray/X-ray dissociation and mechanical dissociation of CO in the pre-existing molecular clouds. We also suggest the possibility that M-0.02-0.07 is a cloud at an early stage of chemical evolution from diffuse gas, which was possibly formed by the bar-induced mass inflow in the Galactic Center region.
We present a survey of molecules in a sample of Galactic center molecular clouds using the Karl G. Jansky Very large Array, which includes M0.25+0.01, the clouds near Sgr A, and Sgr B2. The molecules detected are primarily NH3 and HC3N; in Sgr B2-N we also detect nonmetastable NH3, vibrationally-excited HC3N, torsionally-excited CH3OH, and numerous isotopologues of these species. 36 GHz Class I CH3OH masers are ubiquitous in these fields, and in several cases are associated with new NH3 (3,3) maser candidates. We also find that NH3 and HC3N are depleted or absent toward several of the highest dust column density peaks identified in submillimeter observations, which are associated with water masers and are thus likely in the early stages of star formation.
Thermal images of cold dust in the Central Molecular Zone of the Milky Way, obtained with the far-infrared cameras on-board the Herschel satellite, reveal a 3x10^7 solar masses ring of dense and cold clouds orbiting the Galactic Center. Using a simple toy-model, an elliptical shape having semi-major axes of 100 and 60 parsecs is deduced. The major axis of this 100-pc ring is inclined by about 40 degrees with respect to the plane-of-the-sky and is oriented perpendicular to the major axes of the Galactic Bar. The 100-pc ring appears to trace the system of stable x_2 orbits predicted for the barred Galactic potential. Sgr A* is displaced with respect to the geometrical center of symmetry of the ring. The ring is twisted and its morphology suggests a flattening-ratio of 2 for the Galactic potential, which is in good agreement with the bulge flattening ratio derived from the 2MASS data.
We present 74 MHz radio continuum observations of the Galactic center region. These measurements show nonthermal radio emission arising from molecular clouds that is unaffected by free-free absorption along the line of sight. We focus on one cloud, G0.13--0.13, representative of the population of molecular clouds that are spatially correlated with steep spectrum (alpha^{74MHz}_{327MHz}=1.3pm0.3) nonthermal emission from the Galactic center region. This cloud lies adjacent to the nonthermal radio filaments of the Arc near l~0.2^0 and is a strong source of 74 MHz continuum, SiO (2-1) and FeI Kalpha 6.4 keV line emission. This three-way correlation provides the most compelling evidence yet that relativistic electrons, here traced by 74 MHz emission, are physically associated with the G0.13--0.13 molecular cloud and that low energy cosmic ray electrons are responsible for the FeI Kalpha line emission. The high cosmic ray ionization rate ~10-13 s-1 H-1 is responsible for heating the molecular gas to high temperatures and allows the disturbed gas to maintain a high velocity dispersion. LVG modeling of multi-transition SiO observations of this cloud implies H2 densities ~104-5 cm-3 and high temperatures. The lower limit to the temperature of G0.13-0.13 is ~100K, whereas the upper limit is as high as 1000K. Lastly, we used a time-dependent chemical model in which cosmic rays drive the chemistry of the gas to investigate for molecular line diagnostics of cosmic ray heating. When the cloud reaches chemical equilibrium, the abundance ratios of HCN/HNC and N2H+/HCO+ are consistent with measured values. In addition, significant abundance of SiO is predicted in the cosmic ray dominated region of the Galactic center. We discuss different possibilities to account for the origin of widespread SiO emission detected from Galactic center molecular clouds.
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