Browsing by Subject "GAS"

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    HALOGAS observations of NGC 4414: fountains, interaction, and ram pressure
    (2014) de Blok, W J G; Józsa, G I G; Patterson, M; Gentile, G; Heald, G H; JYtte, E; Kamphuis, P; Rand, R J; Serra, P; Walterbos, R A M
    We present deep H I imaging of the nearby spiral galaxy NGC 4414, taken as part of the Westerbork HALOGAS (Hydrogen Accretion in LOcal GAlaxieS) survey. The observations show that NGC 4414 can be characterized by a regularly rotating inner H I disk, and a more disturbed outer disk. Modeling of the kinematics shows that the outer disk is best described by a U-shaped warp. Deep optical imaging also reveals the presence of a low surface brightness stellar shell, indicating a minor interaction with a dwarf galaxy at some stage in the past. Modeling of the inner disk suggests that about 4 percent of the inner H I is in the form of extra-planar gas. Because of the disturbed nature of the outer disk, this number is difficult to constrain for the galaxy as a whole. These new, deep observations of NGC 4414 presented here show that even apparently undisturbed galaxies are interacting with their environment.
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    Is GBT 1355+5439 a dark galaxy?
    (2013) Oosterloo, T A; Heald, G H; de Blok, W J G
    We present H i imaging of GBT 1355+5439 performed with the Westerbork Synthesis Radio Telescope. This is a dark H i object recently discovered close to the nearby galaxy M101. We find GBT 1355+5439 to be an H i cloud 5 × 3 arcmin in size. The total H i image and the kinematics show that the cloud consists of condensations that have small (~10 km s-1) motions with respect to each other. The column densities of the H i are low; the observed peak value is 7.1 × 1019 cm-2. The velocity field shows a mild velocity gradient over the body of GBT 1355+5439, possibly due to rotation, but it may also indicate large-scale radial motions. Although our data are limited in sensitivity, at all positions the H i velocity dispersion is higher than 5 km s-1 and no narrow, cold, H i component is seen. Because its distance is not known, we considered various possibilities for the nature of GBT 1355+5439. Both the scenarios that it is a tidal remnant near M101 and that it is a dark dwarf companion of M101 meet difficulties. Neither do the data fit the properties of known compact high-velocity clouds in the Galactic halo exactly, but we cannot entirely exclude this option and deeper observations are required. We also considered the possibility that GBT 1355+5439 is a gas-rich dark minihalo in the outer regions of the Local Group. Interestingly, it would then have similar properties as the clouds of a proposed Local Group population recently found in the ALFALFA survey. In this case, the H i mass of GBT 1355+5439 would be about a few times 105M⊙, its size about 1 kpc, and the dynamical mass Mdyn > 5 × 107M⊙. However, if GBT 1355+5439 is a dark Local Group object, the internal kinematics of the H i appears to be different from that of gas-dominated, almost dark galaxies of similar size.