8 research outputs found
FAST reveals new evidence for M94 as a merger
We report the first high-sensitivity HI observation toward the spiral galaxy
M94 with the Five-hundred-meter Aperture Spherical radio Telescope (FAST). From
these observations, we discovered that M94 has a very extended HI disk, twice
larger than that observed by THINGS, which is accompanied by an HI filament and
seven HVCs (high velocity clouds) at different distances. The projected
distances of these clouds and filament are less than 50 kpc from the galactic
center. We measured a total integrated flux (including all clouds/filament) of
127.3 (1) Jy km s, corresponding to a H I mass of
(6.510.06)10M, which is 63.0% more than that
observed by THINGS. By comparing numerical simulations with the HI maps and the
optical morphology of M94, we suggest that M94 is likely a remnant of a major
merger of two galaxies, and the HVCs and HI filament could be the tidal
features originated from the first collision of the merger happened about 5 Gyr
ago. Furthermore, we found a seemingly isolated HI cloud at a projection
distance of 109 kpc without any optical counterpart detected. We discussed the
possibilities of the origin of this cloud, such as dark dwarf galaxy and RELHIC
(REionization-Limited HI Cloud). Our results demonstrate that high-sensitivity
and wide-field HI imaging is important in revealing the diffuse cold gas
structures and tidal debris which is crucial to understanding the dynamical
evolution of galaxies.Comment: 14 pages, 8 figure
The Influence of Oscillation Parameters on the Formation of Overhead Welding Seams in the Narrow-Gap GMAW Process
Thick-walled X80 pipelines for oil and gas transportation are difficult to relocate due to their large size. In the process of narrow-gap overhead welding, welding defects, such as bulges and lack of sidewall fusion, can appear easily. To avoid these defects and to improve the welding quality of thick-walled pipelines, the GMAW welding method is adopted in this paper. The formation characteristics of the weld and the influence of arc oscillation parameters, such as the oscillation width and sidewall dwell time, on the formation process of narrow-gap overhead welding seams are studied. In this research, it was found that, in the NG-GMAW overhead welding position, there was a downward trend in the middle of the formed surface of the weld pool. Defects, such as finger-shaped penetrations and lack of sidewall fusion, were prone to occur due to gravity. The increased oscillation width was beneficial for reducing the protrusion in the middle of the weld seam, but an excessive oscillation width can easily cause undercut defects. The sidewall dwell time has little effect on the protrusion in the middle of the weld seam, but it can increase sidewall penetration, thereby avoiding the occurrence of incomplete sidewall penetration
FAST Reveals New Evidence for M94 as a Merger
We report the first high-sensitivity H i observation toward the spiral galaxy M94 with the Five-hundred-meter Aperture Spherical radio Telescope. From these observations, we discovered that M94 has a very extended H i disk, twice larger than that observed by THINGS, which is accompanied by a H i filament and seven high velocity clouds (HVCs) at different distances. The projected distances of these clouds and filaments are less than 50 kpc from the galactic center. We measured a total integrated flux (including all clouds/filament) of 127.3 ( ±1) Jy km s ^−1 , corresponding to a H i mass of (6.51 ± 0.06)×10 ^8 M _⊙ , which is 63.0% more than that observed by THINGS. By comparing numerical simulations with the H i maps and the optical morphology of M94, we suggest that M94 is likely a remnant of a major merger of two galaxies, and the HVCs and H i filament could be the tidal features that originated from the first collision of the merger, which happened about 5 Gyr ago. Furthermore, we found a seemingly isolated H i cloud at a projection distance of 109 kpc without any optical counterpart detected. We discuss the possibilities of the origin of this cloud, such as dark dwarf galaxy and RELHIC (REionization-Limited H i Cloud). Our results demonstrate that high-sensitivity and wide-field H i imaging is important in revealing diffuse cold gas structures and tidal debris, which is crucial to understand the dynamical evolution of galaxies