New observations and modelling of the Leo Ring

Rhys Taylor 1

  • 1 Astronomical Institute of the Czech Academy of Sciences, Prague


Prior to its tragic collapse in December 2020, the Arecibo telescope completed a new HI survey of the Leo Ring. The Ring is an enormous (~200 kpc diameter), nearby (11 Mpc) feature in the Leo Group, with a total mass exceeding 1E9 solar masses of HI. The structure of the ring, in particular its non-uniform density and complex kinematics (as well as its lack of clear connection to a parent galaxy), does not lend itself to any obvious explanations as to its origin. Much of the gas is optically dark, though several low surface brightness features (including UV emission) are visible within it. Our new survey extends the HI column density sensitivity limit down to 1.5E-17 cm^-2 (at 10 km/s velocity resolution with a 3.5' beam). This reveals several new features including extensions towards galaxies in the Leo Group that were not previously detected, as well as optically dark HI features outside the Ring itself. I will describe the details of these observations as well as efforts to model the formation of the Ring by various scenarios of galaxy-galaxy interactions. The two main proposals are a collisional origin and the disruption of a galaxy along an orbit. While the full details of the structure are very difficult to reproduce with only two interacting galaxies, a head-on collision between galaxies appears better able to reproduce the major structures and kinematics than a tidal disruption along an elliptical or circular orbit.

1. Observations

The Arecibo Galaxy Environment Survey is a deep HI survey covering 16 target areas to an rms of 0.7 mJy at 10 km/s velocity resolution (see Auld et al. 2006 for full details). With a 3.5' FWHM, the survey reaches a column density sensitivity of 1.5x1017 cm-2, enabling a search for low-mass extended gas features that can reveal new information on the origins of structures such as the Leo Ring. Observations of the Leo field cover 5x4 degrees and were carried out betwen 2012 and 2019.

AGES coverage regions (red and green) compared to ALFALFA (blue). For the present analysis we consider only the Leo field to a velocity < 1,600 km/s.

The Leo Ring was first fully mapped in Schneider 1989. It is an enormous, exceptional feature, 200 kpc across with an HI mass in excess of 109 M. Unlike many HI features, although found in a group it has no obvious parent galaxy. While most of the Ring is optically dark, some optical and UV emission has been detected in different areas (e.g. Schneider et al. 1989, Michel-Dansac et al. 2010, Mihos et al. 2018).

Previous imaging of the Leo Ring using the Westerbork interferometer, from Michel-Dansac et al. 2010.

2. The new map

The renzogram below shows the HI contours from AGES at 3.5 sigma (overlaid on an SDSS optical image), coloured according to velocity. Optically dark clouds - at least five are seen which are apparently distinct from the main Ring, see panel 4 - are highlighted with red circles. Regions apparently possessing more extended HI than seen in previous observations are indicated by arrows. Major galaxies are labelled.

While there are no new enormous structures detected by AGES, several smaller features are uncovered which may affect our interpretation of the Ring. As well as the discrete clouds, there are extended regions of HI not seen in earlier observations. In particular, the cloud near NGC 3384 and the spur towards M105 point to both of those galaxies likely having at least some interaction with the main body of the structure.

3. Leo in 3D

Understanding the Ring and other features in the Leo Group requires not only morphology but also kinematics. The movie below shows a volume render followed by isosurfaces at 3.5, 7.0, 14.0, and 28.0 sigma significance. The main Ring forms a largely planar structure in PV space, with some deviations.

The movie was rendered using a a developmental version of FRELLED, described in Taylor 2015. An interactive version can be seen here.

4. Optically dark blobs

Five clouds are detected in the survey region which are clearly distinct from the main body of the Ring. Three of these are between M96 and NGC 3351. Interestingly, a fourth object in this region has a clear optical counterpart, LeG13. It is unclear why this object should have stellar emission while the other clouds - depsite similar masses and kinematics - do not. One cloud is marginally extended while the others are unresolved by AGES. Whether the presumed interaction between M96 and NGC3351 relates to the formation of the main Ring remains to be determined.

The physical properties of all five clouds are quite similar : W50 (W20) ranges from 30 - 44 km/s (45 - 106 km/s), while HI mass varies from 2 - 9x106 M.

North of the four dark clouds between M96 and NGC 3351, another discrete cloud is detected which is harder to explain. It is substantially separated from the main Ring, at least 0.62 degrees (120 kpc) from the nearest point - it is hard to see what kind of interaction could produce a gigantic, extended Ring and also a single, small, distant, completely detached cloud (at the same line-of-sight velocity as the Ring). No other galaxies are present nearby which are obvious candidate parents. Although the velocity width is < 50 km/s, explaining the origin of this small cloud is not at all easy !

Left : stacked g,r,i images from the SDSS optical survey. Right : HI spectrum from AGES.

The fifth cloud, not shown here, is just east of the main Ring (see panel 2). This feature has similar properties to the others, but seems more likely to be directly connected to the main Ring itself due to its close proximity.

5. Modelling

Explaining all of the features of the Ring is a formiddable challenge. We are modelling a plethora of different scenarios, but so far none are satisfactory :

  1. — A galaxy-galaxy collision can explain the morphology of the main Ring, but has serious problems with the velocity structure (this is true for any ring in pure expansion)
  2. — Gas deposition from an orbiting galaxy could explain both the kinematics and morphology of the Ring, but there is no obvious physical stripping mechanism, nor is any optical parent candidate visible
  3. — A sufficiently complex rosette orbit can explain everything, but requires 11 Gyr of evolution ! (lots of fun, but not physical)

Example model, trying to fit as many features within the Ring as possible. Test particles are shown as filled dots, real features as larger open circles (both coloured according to velocity). In this example the particles are deposited along an orbit.

It is also unclear if the various features within and around the Ring were formed in a common event, or if we are seeing the result of several independent but simulataneous processes. We are currently exploring parameter space using test particle models; eventually, grid or SPH codes may be used for greater accuracy.

6. Acknowlegements

With thanks to the tireless efforts of Robert Minchin, Joachim Köppen, Jessica Rosenberg and Steve Schneider.