The Optical Counterparts to Galaxies in the Cen A Group Discovered by HIPASS
Patricia M. Knezek, PASA, 16 (1), in press.

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Observations

HI Observations

The HIPASS, as mentioned above, is being conducted on the 64 m radio telescope at Parkes, Australia. It uses the Multibeam system, which comprises 13 separate beams (26 receivers). This system surveys 1024 velocity channels simultaneously as the telescope is scanned across the sky in declination at a rate of 1 degree/minute. For HIPASS the velocity coverage is -1200 to 12700 km ${\rm s}^{-1}$ . Scans are stored every 5 seconds, and the average system temperature is 23 K. In order to allow for rejection of time-dependent interference, the scanning pattern has been designed such that every source passes through several beams during a scan, and through several different scans separated by days or weeks of time. The effective total integration time per beam is 450 seconds, and the 5 $\sigma$ sensitivity is measured to be 70 mJy/channel/beam.

The channel spacing is 13.2 km ${\rm s}^{-1}$ per channel, but the spectra are smoothed on-line using a 25% Tukey filter, to reduce ``ringing'' caused by the strong Galactic signal entering through spectral sidelobes. This causes a loss in velocity resolution of 37%. Consequently velocity resolution is 18.2 km ${\rm s}^{-1}$ , or 35 km ${\rm s}^{-1}$ after Hanning smoothing. The on-line data reduction, including the bandpass correction, is described in Barnes et al. (1998), while the whole Multibeam system is described in Staveley-Smith et al. (1996). HIPASS started early in 1997, and the southern sky should be complete within 3 years.

Data cubes are generated from the observations by gridding the individual spectra with 4

$^{\prime}{\rm x}4^{\prime}$ pixels. A by-eye search of velocity space from 0 to 1000 km ${\rm s}^{-1}$ resulted in many detections below 200 km ${\rm s}^{-1}$ , most of which were assumed to be due to high velocity clouds within our own Galaxy (Wakker 1991). Thus we chose not to include those detections, with the exception of one source at v

$_{\rm hel}=122$ km ${\rm s}^{-1}$ , which has previously been identified as a member of the Cen A group (CEN 5, Côté et al. 1997). Including CEN 5, 28 clear detections were found in the search, of which 18 were previously known members. The three known members within our search area which were not detected by HIPASS are Centaurus A itself, which is strongly self-absorbing at 21 cm, and NGC 5206 and ESO 272-G025, neither of which were detected in HI by Côté et al. (1997) at a limit of

$< 5.5{\rm x}10^6$ M $_{\odot}$ . Thus we have recovered all of the known members with HI masses above our sensitivity limit, and in addition we have identified 10 new candidate group members.

The new candidate members of the Cen A group are found scattered through the known distribution of velocities for the group, as can be seen in Figure 1. For this figure, we have corrected the velocities of the galaxies to their velocity with respect to the Local Group, v $_{\rm LG}$ , following the prescription of Yahil et al. (1977). The filled squares represent previously identified galaxies within the HIPASS survey area, the stars represent previously identified galaxies outside the HIPASS survey area, and the open triangles represent new candidate group members identified through the HIPASS survey. The lack of new group members north of

$\delta=-30^{\circ}$ is artifical, since that region was not included within the original 600 square degrees surveyed in this study. The apparent cutoff in velocity at v

$_{\rm LG} \sim 400$ km s^-1, however, appears real. Scanning the data cubes to v

$_{\rm hel}=1000$ km ${\rm s}^{-1}$ should detect systems with v

$_{\rm LG} < 735$ km ${\rm s}^{-1}$ in the direction of the Cen A group. All of the newly detected galaxies had HI masses several times higher than the nominal sensitivity limit of the HIPASS survey, thus it is unlikely that candidate members were missed at higher velocities because their HI masses fell below the sensitivity limit due to their presumably being slightly farther away.

**Figure 1:** A graph of the velocities of the known Cen A group members, corrected relative to the velocity of the Local Group, versus declination.
$\begin{figure} \begin{center} \leavevmode \epsfysize=250pt \epsfbox{vlg.eps}\end{center}\end{figure}$

Optical Observations

B and R images were obtained of most of the candidate group members without previous photometry using either the 40-inch Swope telescope at Las Campanas Observatory (LCO) in Chile or the 40-inch telescope at Siding Springs Observatory (SSO) in Australia. The two primary observing runs were at LCO on 10-16 April 1997 and on 23-28 July 1997. Those galaxies observed in non-photometric conditions were calibrated with observations at SSO during January and April 1998, with a few additional calibrations using the Curtis Schmidt telescope at CTIO in April 1998. A

$2048{\rm x}2048$ chip was used, except during the January 1998 run at SSO, when a $800{\rm x}800$ chip was used. For the two primary observing runs, the Tek#1 CCD chip was used, which is a thinned, blue-sensitive CCD. The field of view was 24 $^{\prime }$ with the chip mounted at f7.5, and pixel size was 0.7^''. Typical total integrations times are 3600 seconds in B, and 1800 seconds in R. Only the B images have been used for the following analysis. For the galaxy HIPASS 1337-39, the blue magnitude has been estimated from the Digitized Sky Survey, and should be considered to be very uncertain.

Data processing proceeded in the usual manner. The data were overscan-subtracted, and then bias subtracted using a median filtered bias from each night. Primary flat fielding was done using sky flats obtained each evening. Several Landolt standard fields (Landolt 1992) were observed each night in the broadband filters. The atmospheric extinction was determined using the Landolt standards and found to be similar to the standard CTIO extinction coefficients. Figure 2 shows an example of the final B image of one of the newly identified galaxies, HIPASS1321-31.