The Multibeam Advantage

Table 1: HIPASS survey parameters versus those of B85 and VESSS.

Parameter	HIPASS	B85	VESSS
Telescope	Parkes 64m	IAR 30m	IAR 30m
Sky
HPBW
Grid
v (km s-1)	26.4	16	8
TB(K) ()	0.045 (5 scans)	0.125
	0.1 (1 scan)
NHI(cm-2) ()	1.6 x 1018 (5 scans)	4.6 x 1018
(v = 20 km s-1)	3.6 x 1018 (1 scan)

Figure 1: Column density maps of HIPASS data (top plot) and data from the Bajaja et al (1985) survey (bottom plot). The maps cover the same deg² region of sky, centred upon the Extreme Positive Population of HVCs (adopting the nomenclature of Wakker & van Woerden 1991). The velocity range included here is

v_lsr=170 - 400 km s^-1. The star symbol represents the position of the Seyfert 1, ESO 265-G23 (see text).

The key parameters of HIPASS are contrasted with the B85 and VESSS southern surveys in Table 1. The primary advantages of HIPASS are its superior spatial resolution and dense spatial sampling. If HIPASS's active scanning method was put on a point-and-shoot grid it would correspond to a grid spacing of with a

beam. The HIPASS survey zone also covers almost twice as much of the sky as the Argentinian surveys, providing desired consistency checks with the northern hemisphere HVC surveys. Some of the differences alluded to in Table 1 are best expressed in a picture. Figure 1 shows the same region of sky as viewed by HIPASS and B85. Clearly, the gross properties are visible in the B85 data, but just as clear is the startling improvement provided by the HIPASS data. The low resolution and sparse sampling of B85 tends to merge many individual features into a single, amorphous blob; the complex structure of the clouds only becomes apparent when examining the HIPASS data.

Figure 2: Peak intensity map of positive velocity HVCs (v_lsr=85 - 400 km s^-1). The solid boxes represent the positions of all (i.e., 7) clouds in the Wakker & van Woerden (1991) compilation.

We are already in a position to speculate on the expected increase in the number of catalogued, isolated, HVCs and CHVCs. Based on a visual inspection of the data in hand, we expect this number to increase by a factor of five. This is not to say that the integrated HI flux or mass will increase by a factor of 5, but that the number (especially of CHVCs) is expected to increase dramatically. These results will greatly affect HVC catalog statistics. What previously appeared to be a lack of positive velocity clouds, may simply have been the inability of previous surveys to resolve the individual clumps. Figure 2 offers further evidence to this effect; the boxes therein mark the positions of all previously catalogued clouds in this area (from Wakker & van Woerden 1991). Again, the large population of individual HVCs missed by B85 (and thus the Wakker & van Woerden compilation) is clearly evident. We will return to Figure 2 shortly.

Just as important as the discovery of enormous numbers of new clouds, is the parallel increase in positional accuracy of existing (and new) HVCs. This has important implications for optical and UV follow-up programs to determine HVC distances and metallicities. Both programs hinge upon knowledge of an HVC's intervening HI column density along the line of sight to a suitable stellar or extragalactic probe. Basing these programs upon the older, low resolution data is risky, but was in the past, a necessity. As alluded to in Figure 1, HIPASS will improve this situation dramatically. Looking at the B85 data, the background Seyfert galaxy ESO 265-G23 was thought to be aligned with the extreme positive HVC shown; the HIPASS data shows that it is actually situated in what appears to be an HI-free region. Ultimately, even higher-resolution synthesis observations are needed (a program we are pursuing at the ATCA), but obviously HIPASS offers a superior base to build upon when prioritising candidate background probes. Similar arguments can be made when drawing up target lists for follow-up emission-line studies, a program we are pursuing with J. Bland-Hawthorn at the AAT and WHT (see Bland-Hawthorn et al. 1998).

Figure 3: Peak intensity map of a 2400 deg² region centred upon the South Celestial Pole, including the region shown in Figure 2. The velocity range encompassed here is

v_lsr=85 - 400 km s^-1. The Large and Small Magellanic Clouds, the Magellanic Bridge and the beginning of the Magellanic Stream are shown at the top of the figure. The Galactic Plane, which extends out to 120 km s^-1 in the direction

(Burton 1988), is seen at the bottom of the figure.

Another advantageous feature of HIPASS is its unbiased sky coverage. The displayed population of positive HVCs in Figure 2 look fairly unremarkable when framed within a deg² region. It is only upon expanding one's view by an order of magnitude that their true nature can be appreciated. The sheer size of the HIPASS dataset makes such ``big picture'' views easily attainable. Figure 3 takes the clouds of Figure 2, and places them into the context of a 2400 deg² area of sky. Only now does this population of HVCs' relationship to the Magellanic System become apparent. Their link to the Magellanic Clouds becomes even more evident in the individual channel maps. Figure 4 shows the channel centred upon

km s^-1 and depicts the feature's spatial continuity for over , starting from a region in the Magellanic Bridge.

Figure 4: A detailed view of the Leading Arm at the channel centred upon

km s^-1. Contours are from 10 - 90% of the brightness temperature maximum (T K). The link between the Magellanic System and the strong emission features at

and

, is not visible in the earlier data of Mathewson & Ford (1984) (their figure 2) or Morras (1982), due to their sparse spatial sampling. The total HI mass of the Arm is

M. See Putman et al. (1998) for further details.

This continuous feature on the leading side of the Magellanic Clouds, highlighted by Figure 4, is a natural prediction of tidal models which simulate the dynamical interaction of the Clouds with the Milky Way (e.g. Gardiner & Noguchi 1996). Our discovery of this Leading Arm was reported in Putman et al. (1998); parts of the feature are apparent in the old Mathewson & Ford (1984) data, but it was only the advent of the HIPASS reductions which allowed us to demonstrate the spatial and kinematic continuity of this feature. The lack of continuity had been used by proponents of ram pressure Magellanic Stream formation models to claim the tidal model was excluded (e.g. Moore & Davis 1994); on the contrary, our finding of a continuous Leading Arm which emanates from the Magellanic System has allowed us to eliminate virtually all pure ram pressure models.

© Copyright Astronomical Society of Australia 1997