3. THE INTERSTELLAR MEDIUM IN RING GALAXIES

3.1. Atomic Hydrogen

L-S and Cartwheel have been mapped in HI (Figure 1). In such large systems 95% of the atomic ISM is concentrated in the rings, resulting in high HI surface densities: _HI = 30-120 M pc^-2. At the same time, their interiors are very gas poor, with _HI 2 M pc^-2. HI line-widths are typically narrow (_HI < 10 km s^-1). Kinematic analysis of the HI yields the ring's expansion speed (v_exp), and thus an estimate of the ring galaxy's age (R_ring / v_exp). From their measured radii and v_exp (53 ± 9 and 154 ± 10 km s^-1, respectively), the rings in Cartwheel and L-S are 250 and 140 Myrs old (Higdon 1996; Higdon et al. 2010). The SFR in ring galaxies correlates with their peak _HI, which explains why young systems (e.g., NGC 2793 with age 50 Myr) have such low SFR: they are still organizing their ISM into a dense ring.

3.2. Molecular Gas in the L-S Ring Galaxy

Stars form in cold molecular gas, so HI data can only tell part of the story. Ring galaxies are not very luminous in the rotational transitions of ¹²CO, a fact often attributed to reduced metallicities from snow-plowing outer disk gas into the ring (cf. Horrelou et al. 1995). However L-S's ring possesses solar abundances (Few et al. 1982), which together with its large angular size, made it an ideal target for the SEST (Higdon et al. 2010). We observed 16 positions in L-S in ¹²CO(J = 1-0) and ¹²CO(J = 2-1) transitions: 14 on the ring and one each centered on the nucleus and enclosed disk. Figure 2 shows CO detections in 9/14 ring positions, defining two molecular arcs in the ring's north and southwest. The latter coincides with the galaxy's peak _HI and _H.

Figure 2. H2 in L-S as traced by 12CO(J = 2-1) emission using the Swedish ESO Sub-millimeter Telescope (SEST). The circles represent the beam at 230 GHz (22 FWHM). H emission is shown in grey-scale (linear stretch), while contours show HI surface density. Molecular gas is clearly detected at P3-P7 (north) and P9-P12 (southwest), with broad and complicated line profiles as a rule. The disk and nucleus are not detected (Higdon et al. 2010).

L-S's ring is dominated by atomic rather than molecular gas. For a Galactic I_CO-N_H2, we find M_H2 / M_HI = 0.06 ± 0.01. Astonishingly, a typical dwarf galaxy has nearly as much H₂ as L-S's ring (Leroy et al. 2005). The molecular gas fraction (f_mol = M_H2 / (M_HI + M_H2)) varies considerably around the ring, and is lowest in the ring's southwest quadrant (f_mol 0.03 at P9-P11), where both _HI and _H peak.

The ¹²CO and HI line profiles in L-S's ring can be extremely broad (_gas = 250-400 km s^-1), with multiple velocity components or broad tails evident. It is not clear if this represents out-of-plane gas motions or caustics, though preliminary numerical models suggest the latter (J. Wallin, private communication).