CO J = 10 observations of the Circinus galaxy using the Mopra 22-m radio-telescope

Discussion

Molecular mass and densities in the Circinus galaxy

We have only sparsely sampled the CO distribution of the Circinus galaxy. However, we can predict some global characteristics of the molecular gas if we make assumptions about the gas distribution in the galaxy. Based on CO observations of other edge-on spirals (Dahlem et al, 1993; Adler et al 1991) we have assumed that the shape of the intensity distribution of integrated CO (1-0) approximates that of the radio continuum emission. That is, the CO (1-0) profile is probably also dominated by a strong, gaussian like nucleus sitting on a weak, extended disk. The integrated CO emission at the centre of the Circinus galaxy is 145 K kms . Assuming the emission covers a solid angle ( sr) similar to that observed in the radio continuum as seen in other galaxies (Adler et al, 1991), and using a beam convolved FWHM value of 53 (see section 3), we estimate the total integrated CO emission from the Circinus galaxy to be I = 210 50 K kms and a mean integrated emission per beam area of 14 5 K kms . We can use this latter value to estimate the H column density in the galaxy if we assume the standard CO line intensity to H column density conversion applies (Strong et al, 1988),

We calculate a mean H column density of 3.2 0.8 10 cm. Assuming a distance of 4 Mpc, this yields a molecular gas mas of M(H)=7.5 4.1 10 M. Note that this is a tentative value as we are making a global estimate based solely on 5 data points in the nuclear bulge and have no information about the extended disk component. For comparison, the HI gas mass of the Circinus galaxy is 8 3 10 M (Freeman et al, 1977) and so the ratio of molecular to atomic gas is around 0.1. This is a value typical for late type spiral galaxies (Young & Scoville 1991).

Star formation

The total far-infrared luminosity of a galaxy, L is believed to represent the recent star formation rate (Helou 1986). The value of L for the Circinus galaxy is 1.2 L (Moorwood & Glass 1984). We note the relation derived by Thronson & Telesco (1986) which states that the star forming rate (SFR) and the L are related by

The factor F takes account of the diffuse cloud component which is not reponsible for star forming (eg: Helou 1986). Using F = 1, we derive a value of SFR = 2.52 M. This assumes that the total observed FIR luminosity is a result of recent star formation, however the coefficient is probably around 0.7 (eg: Dahlem et al, 1993). Nonetheless, this SFR is above average for normal spiral galaxies (Devereux & Young 1991).

The star formation efficiency (SFE) is defined as the star formation rate per unit gas mass. Young and Scoville (1991) provided a measure of this by taking the ratio of the infra-red luminosity to the H mass. For the Circinus galaxy, this ratio is 16 9 LM . This value is higher than for normal spiral galaxies (Devereux & Young 1991; Sanders 1990) or similarly active starburst galaxies like NGC 4945 (Dahlem 1993). However, note that estimates of SFE based on using global values of far infra-red luminosity and molecular mass should be interpreted with caution, especially when investigating galaxies undergoing starbursts.

The kinematics of the nuclear CO

The velocity dispersion at position 1 is similar to that seen in recently obtained HI data (Jones et al, in preparation) and is indicative of rapid rotation of the central molecular gas. The simplest interpretation of the spectrum is that a solid body nuclear bulge, travelling with a systemic velocity of about 430 kms , is rotating with a velocity of about 200 cosec(i) kms , where i is the inclination of the inner molecular disk. It is currently believed that the inner disk is inclined somewhat differently to the extended disk (i = 65 ) and is probably closer to edge-on (Jones et al, in preparation). Previous studies (Harnett et al, 1990) have indicated that a rotating gas ring exists at the centre of the Circinus galaxy. Our results have insufficient resolution to determine if a corresponding CO ring exists at the centre of the bulge.

Recently, Elmouttie et al (1995) discovered the existence of a polarised radio morphology in the lobe regions. We believe these lobes may represent outflow features emanating from a central source, and recent studies (Oliva et al, 1994; Marconi et al, 1994; Elmouttie et al, in preparation) indicate this source is probably a compact Seyfert nucleus. However, the high values derived for the SFR and SFE may imply that the unusual radio morphology is also a product of a nuclear starburst. Such a starburst driven wind has been seen in other galaxies, most notable being NGC 3079 (Duric & Seaquist 1988).

The extent of the outflow features as determined from the radio study is approximately 2.5 kpc. There is a possibility that the anomalous CO emission detected at point 4 is related to this outflow mechanism, implying outflow (deprojected) velocities (around 320 kms ). This is consistent with those seen in other spiral galaxies with outflow features (Irwin & Sofue 1992). We emphasise, however, that more data is required to confirm the result.

© Copyright Astronomical Society of Australia 1997