Australia Telescope National Facility

Deep Observations with the Parkes 21-cm Multibeam System
M.J. Disney , P.J. Boyce , G.D. Banks, R.F. Minchin , A.E. Wright, PASA, 16 (1), in press.

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Why Go Deep?

It is worth remembering that HIPASS is a shallow survey, with only 450 sec integrations per point, and this may severely limit what we can find with the survey (Staveley-Smith et al. 1996). Therefore it was always intended that much deeper observations would be made of limited areas of sky, though it was never certain how long such integrations could be profitably carried on, i.e. for how long the signal-to-noise would continue to improve as $\sqrt{t}$ (Disney & Banks 1997). We here report, on behalf of the DEEP team, some results from our early experiments.

Reasons for going deep:

(a) To find intrinsically fainter dwarf galaxies. HIPASS (Banks et al. 1998) is capable of finding $\ge$ 10⁷ M $_{\odot}$ of HI in a dwarf at a distance of 3.5 Mpc. In view of the present controversy over the faint end of the Luminosity Function (Phillipps et al. 1998, Trentham 1997) it is desirable to go deeper, particularly in nearby groups.

(b) We should expect that Low Surface Brightness Objects will generally have low HI column densities - and this is born out by observations (Bothun, Impey and McGaugh 1997). However, system noise puts a limit to the lowest column density attainable by any radio telescope, irrespective of size, i.e.

$\begin{displaymath}N_{\rm HI} \ge 10^{18} T_{\rm sys} \sqrt{\Delta V (km/sec) /t_{obs}(sec)} \hspace{1cm} {\rm cm}^{-2}\end{displaymath}$

(1)

(e.g. Disney and Banks 1997) and N $_{\rm HI}$ and Surface Brightness $\mu_{\rm B}$ (in Blue mags per square arcsec) will be related through

$\begin{displaymath}N_{\rm HI} \sim 10^{20} (M_{\rm HI}/L_{\rm B}) 10^{0.4(27-\mu_{\rm B})}\end{displaymath}$

(2)

Thus the HIPASS survey would appear to have a column density limit (T_s $\simeq$ 23 K, $\Delta$ V $\simeq$ 150 km s^-1) of around 2 x 10¹⁹ cm^-2, which corresponds to a mean SB (if M $_{\rm HI}$ /L_B $\simeq$ 0.5) of 28 B $\mu$ over the whole hydrogen radius. If the HI radius is twice the optical (van Zee, Haynes and Giovanelli 1995) and if most of the light from the exponential disc comes from within 4 optical scalelengths then the central SB, $\mu_{o}$ (B), of a galaxy with 28 B $\mu$ as defined above will be about 24 B $\mu$ . Thus we cannot expect to find many LSBGs with HIPASS. We need to go 1-2 magnitudes fainter, i.e to integrate for 6 to 36 times as long.

(c) A rather different way to approach this is to ask what we would have to do to find optically invisible objects in HI:

To detect in HI: distance d<d_max (M $_{\rm HI}$ ) $\sim$ (M $_{\rm HI}$ /f_min)^1/2

Not to detect in optical: distance d>d_max(L $_{\rm B}$ ) $\sim$ (L $_{\rm B}$ /l_min)^1/2

where f_min and l_min are the minimum detectable HI and optical fluxes respectively. Combining these:
for an invisible HI detection

$\begin{displaymath} \frac{M_{\rm HI}}{L_{\rm B}} > \frac{f_{min}({\rm HI})}{l_{m... ...cm} {\rm (independant \hspace{3mm} of \hspace{3mm} distance)} \end{displaymath}$

(3)

Now we can calibrate (3) for the Multibeam system, using our recent HIPASS survey of the Cen-A group (d=3.5 Mpc) (Banks et al. 1998) where we found M_min=10⁷ M $_{\odot}$ , t_obs=450 sec and the faintest optical identification at m $_{\rm B}$ =19^m. We can thus construct a table (Table 1) of the minimum M $_{\rm HI}$ /L $_{\rm B}$ at which we could find invisible objects in different surveys.

Table 1: Minimum M $_{\rm HI}$ /L $_{\rm B}$ ratios for optically invisible galaxies to be detected in different Parkes Multibeam surveys

Survey		(M $_{\rm HI}$ ) $_{\odot}$ /(L $_{\rm B}$ ) $_{\odot}$
	Normal Galaxy	LSBG	VLSBG

HIPASS (450 sec)	25	2.5	0.6

5 x HIPASS	11	1	0.25

12.5 x HIPASS	7	0.7	0.2

25 x HIPASS	5	0.5	0.1

The reason for the 3 different columns in Table 1 is that Low SB Galaxies show only part of their light above the sky. Thus a LSBG in the table is assumed to show only 10%, the VLSBG only 2.5% whereas the normal galaxy is assumed to show all (not of course strictly true).

The table makes it clear that if you want to find objects with the Multibeam which are not on the Digital Sky Survey and yet have M $_{\rm HI}$ /L $_{\rm B}$ 's that are not too extreme ( $\le$ 1) you need to observe for at least 5 times longer than HIPASS.

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