Australia Telescope National Facility

The HI Mass Function from HIPASS
V. Kilborn ,
R. L. Webster ,
L. Staveley-Smith ,
, PASA, 16 (1), in press.

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Survey Parameters and Data Selection

HIPASS is being conducted at the 64m Parkes Radio-Telescope in NSW, Australia. The survey uses the multibeam receiver, which has individual beam sizes on the sky of 14'. The sky is actively scanned in

3^o x 8^o strips, and it will be scanned 5 times to reach the final survey sensitivity. HIPASS will survey the southern sky between -1200 km ${\rm s}^{-1}$ and 12700 km ${\rm s}^{-1}$ , with a velocity resolution of 18 km ${\rm s}^{-1}$ . The RMS noise in the final data cubes is $\sim$ 13 mJy per beam. Bandpass subtraction is undertaken in real-time at the telescope (Barnes et al. 1998), and the data is gridded at a later stage into

$8^{\circ} \times 8^{\circ}$ cubes, with a pixel size of 4' x 4' and velocity spacing of 13.2 km s^-1. There will be 388 data cubes in the finished southern sky survey. The $3 \sigma$ HI mass limit for the survey is

$\sim 1.4 \times 10^6 D^2_{\rm Mpc} M_{\odot}$ , and the 3- $\sigma$ HI column density limit is

$\sim 7 \times 10^{17}$ cm^-2 per velocity channel. A follow-up observation program is taking place at the Australia Telescope Compact Array (ATCA) to obtain better positional accuracy of interesting sources, and optical follow-up is undertaken at the Siding Spring and CTIO 40 inch telescopes. These observations are vital to the survey, as they will confirm marginal detections and provide greater resolution for interesting objects such as intergalactic HI clouds. We have selected our sample of galaxies from the declination region

$-62^{\circ} > \delta > -70^{\circ}$ and

$\delta < -78^{\circ}$ , through the full range in Right Ascension. This region has been scanned to the full multibeam sensitivity of 5 scans, and represents 0.572 steradians in area (7% of the total HIPASS survey area). The galaxies were detected by eye in the data cubes; in total, 304 galaxies were found in this region. Of the sample, 15% are previously uncatalogued and 30% are new detections in HI. There are no confirmed detections of HI clouds without optical counterparts in this sample. To construct a complete sample, a cut-off was made at the integrated flux value of 4 Jy km ${\rm s}^{-1}$ . This was chosen as a conservative limit, because the detection limits for the HIPASS data have not been explored fully at this stage. By choosing such a conservative cut-off, the detections with the lowest peak flux and velocity width are omitted from the sample, and we are limited to characterising the HIMF above a mass of

$M_{HI} \sim 10^7 M_{\odot}$ . This means that once again the low mass end of the HIMF will be determined on the data from just a couple of galaxies. A plot showing the theoretical mass cut-off versus the masses in the sample is shown in Figure 1, which shows that the sample seems to be complete to the chosen cut-off. With this integrated flux limit imposed, there were 263 galaxies left in the sample. A histogram showing the velocity distribution of the sample can be seen in Figure 2 - we have detected galaxies between 400 km ${\rm s}^{-1}$ and 10,000 km ${\rm s}^{-1}$ in this sample. The mass distribution for the sample can be seen in Figure 3. While the number statistics have been increased in the higher mass bins, the two lowest bins combined have just 3 galaxies.

**Figure 2:** Velocity distribution for the full sample.
$\begin{figure} \begin{center} \centerline{\psfig{file=kilborn2.epsi,height=10cm,width=12cm,angle=-90}} \end{center} \end{figure}$

Next Section: Results
Title/Abstract Page: The HI Mass Function
Previous Section: Introduction

Contents Page: Volume 16, Number 1

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