The Origin of the LGG 138 Ring

Merging and Shocking in the NGC 2292/2293 Pair

It is tempting to conclude from the optical data alone that the galaxies NGC 2292 and 2293 are interacting, and possibly merging to form a luminous elliptical galaxy over a period of order one Gyr. Indeed, Corwin et al. (1985) list these two galaxies as an interacting pair, with plumes and loops visible in NGC 2292, and a knotty corona with dust lanes in NGC 2293. Their close proximity on the sky within a common luminous envelope, and their nearly equal--and bright--magnitudes are highly suggestive of a collisional merger that is generating the tidal features visible in a high-contrast Digitized Sky Survey (DSS) image of the pair. The ring of neutral hydrogen enclosing both optical bulges lends further weight to the idea that the association is physically real. Finally, the exact coincidence of the neutral gas ring with the break in optical colour of the system in the southwest quadrant of the ring, is strongly suggestive of gas compression and shocking at the radius of the ring, leading to distinctly different stellar populations either side of the shock. The slightly bluer population of stars seen to the outer side of the ring is consistent with star formation occurring at the leading edge of an expanding ring density wave.

Expansion and cooling of a gas envelope or ring, and subsequent compression via shocking, are normal processes during mergers. Spiral galaxy mergers are known to convert a large fraction of the interstellar gas in the participant galaxies into new stars during intensive star formation periods. For example, the Cartwheel ring galaxy shows a strong anti-correlation between HI and H $\alpha$ surface densities in the outer ring, leading Higdon (1996) to conclude that the depression of HI in parts of the ring has been caused by the consumption of the gas supply by star formation, which is occurring on the leading edge of a gaseous ring density wave. In light of this observation, it is interesting to note that the southwestern edge of the LGG 138 ring which exhibits the optical colour break coincides with the weakest HI emission detected from the gas ring. The Cartwheel ring galaxy is generally believed to have its unusual structure as a result of a past collision with a gas-rich galaxy. Milder star-bursts are seen in a number of systems where the interaction is drawn out over a long period of time. For example, the interacting disk-disk system NGC 520 appears to have recently undergone mild star formation throughout both bulges and one of the disks (Stanford & Balcells, 1990; Stanford, 1991).

If the galaxies are bound members of the LGG 138 group, as seems likely, then their radial velocity difference can be attributed to peculiar motions within the group. In this case, for NGC 2292 and 2293 to form a bound pair, their physical separation (r) must be constrained to be not much more than what is seen projected on the sky:

$\begin{displaymath}r < {2 G M \over v_{\rm escape}^2} = 20~h_{75}^{-1}~{\rm kpc},\end{displaymath}$

where M is the mass interior to r for, say, NGC 2293, which has as an upper limit equal to the mass interior to the ring (), and the escape velocity

$v_{\rm escape}$ is set to 340 km s^-1. This maximum separation is on the order of the radius of the ring, so if NGC 2292 and 2293 form a bound pair, they are indeed physically nearby each other. It is difficult to imagine how the HI ring could survive in such a system where the three orbits (of the two galaxies and the ring) are so similar in scale, and probably inclination. That is, over the period of a few orbits, the HI ring would be distributed into a common halo, or at the very least dramatically disturbed by the galaxies approaching and perhaps crossing the ring. Future numerical modelling would be worthwhile to establish the geometric structure of the system. Combined with the normal appearance of the galaxy nuclei, and the lack of X-ray or radio continuum emission from the nuclei of the galaxies, it is probably unlikely that the two galaxies NGC 2292 and 2293 are currently merging.

Conversely, if the galaxies are not bound members of the LGG 138 group, and some component of their differential velocity is due to smooth Hubble flow, the natural conclusion is that NGC 2292 is physically behind NGC 2293 by $\sim 5$ Mpc, and it is merely by chance that NGC 2292 is projected within the HI ring. An interesting but difficult measurement to make to shed light on the front-to-back ordering of the galaxies would be to look for absorption in the NGC 2293 system of a spectral line emitted at the red-shift of NGC 2292, or vice versa. The presence of such an absorption feature would immediately determine the ordering of the two galaxies. Undoubtedly it would be fascinating if NGC 2292 was found to be in front of NGC 2293, for then they must certainly both be members of the LGG 138 group, and may well collide in the next $\sim 10$ Gyr.

One final remark worth making is that the original survey which lead to the discovery of the LGG 138 ring was partly motivated by the striking images of shells in giant field ellipticals reported by Malin & Hadley (1997). It is conceivable that if the galaxies NGC 2292 and 2293 are in fact merging, then their product may well be a field elliptical similar to the Malin & Hadley (1997) examples. Further optical imaging, including ``stacking'' of sky survey plates, may be worthwhile to pursue this possibility.

Non-merging Scenario

A simpler explanation for the HI ring encircling the NGC 2292/2293 pair is that one of the galaxies was formed with a primordial HI ring of the type seen in a number of S0/S0a spirals (van Woerden et al., 1983), and the other galaxy is not physically nearby the ring galaxy, but is projected within the ring because of the line of sight. This model will be referred to as the ``non-merging'' scenario, but does not preclude any interaction with the more distant NGC 2295 galaxy. The two main arguments in favour of this scenario are that the radial velocities of the two galaxies differ substantially, ie. by $\sim 340$ km s^-1; and that the ring appears to be connected and circumferentially smooth, with the exception of the bulk concentration to the southeast, coincident with the faint luminous arm or tidal tail. The most likely galaxy to be associated with the HI ring is NGC 2293, since it appears to be physically closer to the projected centre of the ring, is considerably closer in central radial velocity to the ring, and is mildly more spiral-like than NGC 2292--see . The parameters of the ring--viz. a physical radius of order , a circular speed of 280 km s^-1 and an orbital period of $\sim 0.3$ Gyr--are typical of those for the gas rings in the S0 (lenticular) galaxies observed by van Woerden et al. (1983). Furthermore, most of the rings found by van Woerden et al. (1983) show similar peak column densities ( $\sim10^{21}$ cm^-2) to that in the LGG 138 ring, and lack central concentrations of HI.

The two principle models for the formation of a ring around a single galaxy are: an extended and warped primordial gas disk that fails to collapse completely, leaving a system with inner and outer HI disks, possibly having substantially different inclination angles; and the accretion of gas from close interactions or glancing collisions with smaller gas-rich galaxies. For the former, the inner disk is depleted by star formation, while the outer disk barely reaches the critical threshold to form stars--in cases where it does reach critical density over a large part of the ring, the resultant galaxy may become polar ring-like. However, it is difficult to imagine how the colour break seen at the position of the HI ring could arise without external intervention by an intruder galaxy or potential, and a gas accretion model is more able to explain the uneven structure of typical HI rings via the capture and tidal stretching of gas-rich dwarf galaxies. Formation of a ring via gas accretion is discussed below. ``Broken'' rings may survive for up to a few orbital periods, say $\sim 1$ Gyr, before losing their structure through friction and tidal action to become smooth in appearance.

If the non-merging model is correct for the LGG 138 ring, then the mass to light ratio estimated in §2 should be revised accordingly. The rotation speed of the ring implies a total mass within the ring of , and NGC 2293 has a total blue magnitude of 11.7. Thus the total mass to blue luminosity ratio within the ring is

$11~M_\odot/L_\odot$ , provided NGC 2292 is not part of the system, and is instead a background galaxy projected to lie within the ring. This is within the normal range for S0/S0a galaxies (Roberts & Haynes, 1994).

A Gas-sweeping Collision with NGC 2295

It appears that NGC 2292 and 2293 are not merging, and probably not physically related, except that they may be bound members of the LGG 138 group, but not bound to each other. Whilst the ring of HI may then be a primordial feature associated with the formation of NGC 2293, it is possible that the ring has other origins. The nearby galaxy NGC 2295 has a heliocentric radial velocity of $1823 \pm 60$ km s^-1 (Bottinelli et al., 1992), and is projected at a distance of $\sim 30$ kpc from the NGC 2292/2293 pair. This galaxy was not detected in HI emission, yet is a 13.6 mag Sab spiral well within the spatial and spectral field of view of the ATCA. The absence of gas in NGC 2295 is highly suggestive that the gas has been removed from the galaxy in some way. Corwin et al. (1985) report the detection of a very faint plume to NGC 2292/2293 from NGC 2295. Has NGC 2295 swept past NGC 2293 and been stripped of its gas disk and outer stellar population?

One way to help determine if this is the case would be to compare the sense of rotation of the HI ring with that of the nucleus of the central galaxy. If the angular momentum vectors of the HI ring and the bulge and disk stellar populations are directed away from each other, then the case for a past gas stripping event is strengthened.¹ This is because there is no known mechanism by which the gas and stellar components of an isolated spiral that formed normally and has had no interaction with other galaxies can rotate in opposite directions. Recent studies (eg. Kuijken et al., 1996; Bertola et al., 1992) have shown that at least 10 per cent of S0 galaxies have disks which counter-rotate relative to the stellar components; and it is generally thought that these gas disks result from the capture and tidal stretching of gas-rich dwarf galaxies by massive early type galaxies. NGC 3626 is a classic example of such a system, and Ciri et al. (1995) suggest that any primordial gas remaining in the galaxy, rotating with the stellar component, will cause the counter-rotating gas to fall towards the centre of the galaxy over a short period of time. The HI ring in IC 2006 also appears to rotate in the opposite sense to the stellar population (Schweizer et al., 1989).

The projected displacement of NGC 2295 from the centre of the HI ring implies a most likely true separation of order

$\sim50~h_{75}^{-1}$ kpc. This distance is consistent with what is expected for a gas-sweeping collision. Moore et al. (1996) show that by the time the response of a galaxy disk to typical close encounters--specifically, the harassment of a large disk by smaller orbiting galaxies--is visible, the perturbing galaxy is usually

$\mathrel{\hbox{\rlap{\hbox{\lower2pt\hbox{$\sim$}}}\hbox{\raise2pt\hbox{$>$}}}}100$ kpc distant from the disk galaxy. Furthermore, the simulations of Moore et al. (1996) show the gas distribution to commonly form ring structures and tidal tails in the resultant system. The radial velocity of NGC 2295 relative to the ring is 220 km s^-1, very similar to the maximum observed circular velocity of gas in the ring. Assuming a bound spherical system, this velocity corresponds to a physical relative velocity of $\sim380$ km s^-1, leading to the conclusion that if NGC 2295 has recently experienced a close encounter with NGC 2293, the interaction was on the order of 100 Myr ago.

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Contents Page: Volume 16, Number 1

ASKAP

Public

Australia Telescope National Facility

The Origin of the LGG 138 Ring

Merging and Shocking in the NGC 2292/2293 Pair

Non-merging Scenario

A Gas-sweeping Collision with NGC 2295