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Gas and Dark Maer in the Sculptor Group
Tobias Westmeier (ICRAR/UWA)
Observations
We have observed the nearest galaxies of the Sculptor group in neutral hydrogen (H) emission with the Australia Telescope Compact Array (ATCA) to Study the kinematics and dynamics of the galaxies , Search for extra-planar gas and "missing satellites".

NGC 300
Westmeier, Braun Koribalski 2011, MNRAS, 410, 2217 Main results: Extended, twisted H disc (Fig. 1), >1А in diameter. Compact inner gas disc aligned with stellar disc. Extended outer gas disc with different orientation. Rotation curve measured out to almost 20 kpc (Fig. 2). Maximum rotation velocity of 100 kms -1 at 10 kpc. Declining rotation curve across outer disc. Asymmetries in H disc suggest ram-pressure distortion of outer gas disc (Fig. 3 4).
Figure 1: Optical DSS image of NGC 300 with ATCA H contours overlaid. Figure 2: Rotation curve of NGC 300 (red data points). The grey Gravitational Pressure data points show Pgrav = gas(r) з |(r)/z|max the previous rotation curve derived by Puche et al. (1990) from VLA G as D i s c observations for comparison. DM Halo Figure 3: H column density profile along a line of constant right ascension through the centre of NGC 300. There is an almost perfectly exponential slope on the northern side of the galaxy, but a plateau followed by a steep drop on the southern side, suggesting ram-pressure interaction with the IGM of the Sculptor group. Ram Pressure Pram = IGM v 2 Figure 4: Ratio of gravitational pressure and ram pressure in the gas disc of NGC 300 as a function of radius for different IGM densities (in cm-3). Ram-pressure effects start to dominate near the outer edge of the disc.

Parameters
Array configurations: Covered area: Number of pointings: Integration time: Angular resolution: Velocity resolution: 5 H sensitivity: EW352 / EW367 2Аз2А 32 96 h per galaxy 90з180 ( 1 kpc) -1 4 kms 1019 cm-2 / 105 M

NGC 55
Westmeier et al., in prep. Main results: Extended, asymmetric H disc (Fig. 5). Asymmetries again suggestive of ram-pressure. Apparent "thickness" of the disc due to warping of the outer disc, not gas in the halo. We discovered a population of extra-planar gas clouds (Fig. 5 and 6) around NGC 55. Typical H masses of a few times 106 M. Six clouds presumably at distance of NGC 55, two clouds (6 and 7) most likely foreground from the Milky Way or Magellanic Stream (Fig. 7).
6 4 1 7 5 2 8 3

Summary Conclusions
Ram-pressure interaction does occur in groups (not just massive clusters) under reasonable assumptions on the density of the IGM and relative velocity of galaxies. We find strong asymmetries in the outer gas discs of NGC 55 and 300 suggesting the presence of ram pressure. We are currently working on numerical simulations (Bekki et al., in prep.) to support the observations and determine the density of the IGM. NGC 55 is surrounded by a population of extra-planar gas clouds or high-velocity clouds (HVCs). H masses comparable to H VCs around the Milky Way (e.g. complex C; Wakker et al. 2007; Thom et al. 2008). Origin of gas clouds around NGC 55 not yet clear, but similar detections near M31 (Thilker et al. 2004; Westmeier et al. 2005) suggest combination of different origins (tidal stripping, gaseous satellites, outflows, etc.). Future H surveys with ASKAP and MeerKAT will allow us to study the evolution and interaction of galaxies in the nearby universe in much greater detail: W laby (PIs: B. Koribalski, L. Staveley-Smith) al Mhongoose (PI: E. de Blok)

Figure 5: Optical DSS image of NGC 55 with ATCA H contours overlaid.

References
Figure 7: Line width vs. H mass of the extraplanar gas clouds near NGC 55. Diagonal lines indicate lines of constant virial-to-H mass ratio, = Mvir/MH. Six of the clouds are aligned near values of 50...100, suggesting a common origin. Two of the clouds (6 and 7 in Fig. 5) have very different mass ratios, suggesting that they are foreground objects.

Figure 6: H spectra of the 8 extra-planar gas clouds discovered near NGC 55. The numbers correspond to the ones shown in Fig. 5.



Puche, Carignan Bosma 1990, AJ, 100, 1468 Thilker et al. 2004, ApJ, 601, L39 Thom et al. 2008, ApJ, 684, 364 Wakker et al. 2007, ApJ, 670, L113 Westmeier, Braun Thilker 2005, AA, 436, 101 Westmeier, Braun Koribalski 2011, MNRAS, 410, 2217

Tobias Westmeier Research Assistant Professor tobias.westmeier[at]uwa.edu.au

ICRAR M468 The University of Western Australia 35 Stirling Highway Crawley WA 6009