HI Gas Distribution of the Circinus Galaxy

An extract from Koribalski (1996), published in the The Minnesota Lectures on Extragalactic Neutral Hydrogen, ASP Conf. Series Vol. 106, ed. Evan Skillman, p.238).

Circinus is a nearby spiral galaxy only 4 degrees below the Galactic plane. It was found while inspecting a Schmidt plate and shortly afterwards observed in HI with the Parkes 64-m telescope. Freeman et al. (1977) measured a half-width of at least 32 arcmin x 15 arcmin for the HI extent of Circinus, much larger than the optical diameter (about 10 arcmin) of the galaxy. Jones et al. (1996, in prep.) and Koribalski & Whiteoak (1996) only recently mapped Circinus with several configurations of the ATCA; a few preliminary results are presented here.

Figure 1 shows part of the overall HI distribution, clearly outlining the two spiral arms. The elongated central disk (length about 5 arcmin) indicates a bar, and the central `hole' is caused by HI absorption against the bright central region of Circinus. The nuclear activity is caused by star formation, a Seyfert 2 nucleus and giant radio lobes (perhaps associated with outflow), very similar to those in NGC 3079. Harnett et al. (1990) observed Circinus with Parkes at 1665 and 1667 MHz and found strong OH absorption lines over a velocity range of nearly 200 km/s. High-resolution ATCA measurements reveal HI absorption over about the same velocity range (Fig. 2). The data show a shift of the line over the face of the continuum, indicating a fast-rotating nuclear ring (Koribalski & Whiteoak 1996). This is supported by Halpha images taken with AAT Taurus Fabry-Perot interferometer (see Fig. 3) and with the ESO New Technology Telescope (Marconi et al. 1994, their Fig. 4) showing a broken ring or spiral arms at about 200 pc from the nucleus. Marconi et al. also found an OIII emission cone toward the NW indicating highly ionized gas flowing out from the nuclear region (their Figures 3 and 10).

Figure 2: HI absorption spectrum toward the nuclear region of the Circinus galaxy. These data were taken with the largest array of the ATCA (Koribalski & Whiteoak 1996).

Figure 3: Halpha emission from the nuclear region of the Circinus galaxy. This is a very preliminary image taken with the TAURUS-2 Fabry-Perot instrument at the Anglo-Australian Telescope (courtesy: B. Koribalski, K. Taylor, J. Whiteoak). The angular resolution is 1.5".