The large-scale atomic and molecular gas in the Circinus galaxy

Abstract

We have used the Australia Telescope Compact Array (ATCA) and the Swedish–ESO Submillimetre Telescope (SEST) to map the large‐scale atomic and molecular gas in the nearby (4 Mpc) Circinus galaxy. The ATCA H i mosaic of Circinus exhibits the warps in position angle and inclination revealed in the single‐pointing image of Jones et al., both of which appear to settle beyond the inner 30 kpc which was previously imaged. The molecular gas has been mapped in both the CO J= 1 → 0 and J= 2 → 1 transitions down to a column density of (3σ), where we derive a total molecular gas mass of . Within a radius of 3 kpc, i.e. where CO was clearly detected, the molecular fraction climbs steeply from ≈ 0.7 to unity (where , cf. NH i= 1021 cm−2) with proximity to the nucleus. Our H i mosaic gives an atomic gas mass of MH i≈ 6 × 109 M⊙, which is 70 per cent of the fully mapped single dish value. Combining the atomic and molecular gas masses gives a total gas mass of , cf. the total dynamical mass of ≈ 3 × 1011 M⊙ within the inner 50 kpc of our mosaiced image. The total neutral gas mass to dynamical mass ratio is therefore 3 per cent, consistent with the SAS3 (Third Small Astronomy Satellite) classification of Circinus. The high (molecular) gas mass fraction of per cent found by Curran et al., only occurs close to the central ≈ 0.5 kpc and falls to ≲10 per cent within and outwith this region, allaying previous concerns regarding the validity of applying the Galactic conversion ratio to Circinus. The rotation curve, as traced by both the H i and CO, exhibits a steep dip at ≈ 1 kpc, the edge of the atomic/molecular ring, within which the starburst is occurring. We find the atomic and molecular gases to trace different kinematical features and believe that the fastest part (≳130 km s−1) of the subkpc ring consists overwhelmingly of molecular gas. Beyond the inner kpc, the velocity climbs to settle into a solid body rotation of ≈ 150 km s−1 at ≳10 kpc. Most of the starlight emanates from within this radius and so much of the dynamical mass, which remains climbing to the limit of our data (≳50 kpc), must be due to the dark matter halo.

Physical conditions of molecular gas in the Circinus galaxy Multi-JCO and Ci3PP0observations

“…For 12 CO J = 3 → 2 we get a position angle of 33.5 • , which is adopted in the following analysis to define the major axis of molecular gas emission. This result is also close to the position angle of 34 • derived from 12 CO J = 1 → 0 and 2 → 1 maps (Curran et al 2008). The fitted position angle of C i 1 → 0 A122, page 4 of 16 Table 3.…”

“…3.1.3 (see also Table 4). In the P-V diagram of 12 CO J = 3 → 2, the ridge of maximum intensity covers a velocity range of about 400 km s −1 , in accordance with the high inclination (i ∼ 65−78 • ; Curran et al 2008) of the galaxy, over a region of roughly ±10 . The rotation field in this area is characterized by a velocity gradient of d /dθ = 400 km s −1 /20 (∼20 km s −1 /arcsec), corresponding to d /dr ∼ 1.0 km s −1 pc −1 in the plane of the galaxy, when an inclination angle of 65 • is adopted (Freeman et al 1977).…”

“…Besides the isotopologues of CO lines (i.e., 13 CO, C 18 O), Curran et al (2001) contained rich molecular spectra, including lines from HCN, HNC, and HCO + , which indicate the presence of highly excited dense gas in the central region of Circinus. Furthermore, Circinus was also mapped in the J = 1 → 0, 2 → 1, and 3 → 2 transitions of CO (Curran et al 2001(Curran et al , 2008. With the deconvolved 12 CO J = 2 → 1 map, Johansson et al (1991) found a faceon molecular ring structure, which seems to be associated with the S-F ring shown in Hα (Marconi et al 1994).…”

The Australia Telescope Compact Array Broad-band Backend: description and first results★

“…Discovered relatively recently (Freeman et al 1977) and obscured by Galactic foreground stars and dust, it nevertheless remains a prominent observing target at a large range of frequencies. It harbours an active galactic nucleus (AGN) surrounded by a star‐forming ring, an extended stellar disc, double radio lobes (orthogonal to the disc) and an enormous H i gas envelope (Jones et al 1999; Curran, Koribalski & Bains 2008). Bauer et al (2008) discovered a supernova, named SN1996cr, located just 25 arcsec south of the AGN, adding to the continuing interest in this spectacular galaxy (see Fig.…”

Λcdm Satellites and H I Companions—the Arecibo Alfa Survey of NGC 2903

“…7 (105 kpc) after correcting for the beam, nearly twice the value measured from the VLA data. Thus, the H i extent of NGC 2903 is at least 3.2 times its optical diameter (Table 1) and ranks among the largest known (Matthews et al 2001;del Rio et al 2004;Spekkens & Giovanelli 2006;Oosterloo et al 2007;Curran et al 2008).…”