Australia Telescope National Facility

A new HI ring: LGG 138
David G. Barnes , PASA, 16 (1), in press.

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Observations and Basic Properties of the Ring

Observations of the 21 cm HI line in the LGG 138 group were acquired with the Australia Telescope (AT) Compact Array initially in a moderately compact configuration with a maximum baseline of 750 m in November 1996, and later in an additional 750 m configuration (March 1997) and three 1500 m configurations (August and September 1997) which sampled higher spatial frequencies on the sky. The AT correlator was programmed to record XX and YY correlations between all telescope pairings, over a bandwidth of 16 MHz. In this mode, each correlator spectrum has 512 equally-spaced channels. Consequently, the channel width was 31.25 kHz, corresponding to a velocity channel width of 6.6 km s^-1 in the rest frame of the observations. The total effective on-source integration time was 48 hr, and the combined data from the multiple array configurations gave excellent coverage of the uv plane. The data were calibrated, continuum-subtracted and imaged in MIRIAD, and for naturally weighted visibilities yielded channel maps with root mean square noise fluctuations of 0.75 mJy beam^-1, and a synthesised beam of dimensions

$70^{\prime\prime} \times 35^{\prime\prime}$ .

In addition to the HI data, optical images of the ring were acquired using the Mount Stromlo Observatory 40 inch telescope at Siding Spring Mountain in July 1997. Continuum images in the B, V and R bands were made using a

2048 x 2048 element, thinned, charge-coupled device camera with a pixel scale of

$0.6^{\prime\prime}$ pixel^-1 when mounted on the telescope. Dark frame subtraction, flat-fielding and calibration of the data was applied using standard procedures in IRAF.

The NGC 2292/2293 pair of galaxies was the principal target for the extended integration on LGG 138 obtained with the AT Compact Array. shows the distribution of neutral hydrogen gas relative to the V band optical emission of these two galaxies. shows the same distribution overlaid on a difference image representing B-R colour. It is clear that the HI is arranged in a large scale ring surrounding the pair, and furthermore, aligns exactly with a distinct colour break in the stellar populations in the south western arc of the ring.

**Figure 1:** Total HI column density contours of the LGG 138 ring, at levels of 1, 2, 3, 4 and , overlaid on a V band continuum image showing the three galaxies NGC 2293/2292/2295, listed in order from east to west. The optical velocities of the galaxies are indicated above the figure; intensity-weighted mean HI velocities are given below the figure for the kinematically-opposite edges of the ring.
$\begin{figure} \begin{center} \centerline{\psfig{figure=velocity.eps,width=5in}} \end{center}\end{figure}$

**Figure 2:** HI contours (at the same levels as in ) overlaid on a B-R colour image showing a distinct shelf in the stellar colours associated with the peak HI column density in the south western arc of the ring.
$\begin{figure} \begin{center} \centerline{\psfig{figure=atnfnews.eps,width=5in}} \end{center}\end{figure}$

The observations show the HI ring in LGG 138 to be in large scale rotation with a maximum projected speed of 200 km s^-1, or in expansion with a projected expansion speed of 200 km s^-1, or, as is more likely, a combination of both rotation and expansion. At an assumed distance of , the major and minor projected diameters of the ring are

$\sim30~h_{75}^{-1}$ kpc and

$\sim21~h_{75}^{-1}$ kpc respectively. The HI mass of the ring, under the usual assumptions, is --typical of the quantity of neutral hydrogen bound to an early-type spiral galaxy (Roberts & Haynes, 1994). The ATCA observations reach a $3\sigma$ column density sensitivity of per 13.2 km s^-1 channel, and with no detection of HI at this level directly associated with the optical galaxies, the HI masses of the encircled galaxies NGC 2292 and 2293 are constrained to be less than each for $10\sigma$ detections extending over 150 km s^-1. The spatial HI distribution is consistent with the ring actually being narrower than the synthesised beam, ie. the ring thickness

$\delta r \mathrel{\hbox{\rlap{\hbox{\lower2pt\hbox{$\sim$}}}\hbox{\raise2pt\hbox{$<$}}}}4$ kpc, or alternatively,

$\delta r / r \mathrel{\hbox{\rlap{\hbox{\lower2pt\hbox{$\sim$}}}\hbox{\raise2pt\hbox{$<$}}}}0.25$ , where r is the radius of the ring. For column densities above 10²⁰ cm^-2,

$\delta r/r \simeq 0.3$ , and thus the ring is believed to be radially thin.

Without knowing the true physical shape of the ring, it is difficult to determine the kinematics and hence dynamics of the NGC 2292/2293 system. However, one particularly simple model can be tested: if the ring is intrinsically circular, or nearly circular, then the projected eccentricity (e = 0.71) can be used to estimate the inclination angle, which is found to be of order 45 $^\circ$ . The radius of this circular ring is $\sim 15$ kpc, and the ring rotates with a maximum (de-projected) circular speed of 280 km s^-1. In these conditions, the rotation period of the ring is 330 Myr--only 50 per cent higher than that of the Sun around the centre of the Galaxy. The mass within the ring needed to support the rotation is . The total (Cousins) blue luminosities of NGC 2292 and 2293 are 11.8 and 11.7 (Lauberts & Valentijn, 1989), yielding a total integrated blue luminosity from the system of , and a total mass to blue luminosity ratio of only

$\sim6~M_\odot/L_\odot$ , if both galaxies are physically bound and the ring encircles both galaxies. If the ring is actually collapsing onto the galaxies, then the true mass to luminosity ratio is higher; conversely, if the ring is a tidal feature which is no longer bound to the NGC 2292/2293 pair, the mass to luminosity ratio will be lower than that calculated here.

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