Results from the IAR HI southern sky survey: The Magellanic Complex

  • Description

The 21-cm HI line survey of the southern sky, which is being made with the IAR radiotelescope and which is almost complete, has been done with a previous agreement with members of the Leiden Observatory about the observational parameters to be used. The idea was that this survey, together with the one to be made from the northern hemisphere, from Dwingeloo, should provide to the astronomical community a whole-sky HI survey with uniform parameters. The Dwingeloo radiotelescope consists of a dish with a diameter of 25 m, fully steerable, and a 1024 channels digital correlator. At the IAR we have a 30 m dish, which can point only to declinations below -10°, and a 1008 channels digital correlator. With so similar basic equipment it was rather easy to agree about the common parameters. Only the sky coverage had to be different in view of the limited movement of the IAR dish. Finally it was agreed that the surveys would be done with a velocity resolution of 1 km/s (the angular resolution is fixed by the size of the dishes to 0.5°) covering the velocity range of –500 to +500 km/s. The observation would be made on a 0.5° x 0.5° (real) grid with one axis at constant latitude. The Dwingeloo survey would be extended to a declination of -30° and the IAR survey to -25°, allowing a region of 5° for comparison and calibration.

We present here the results obtained with the IAR survey, in the present state of the data, oriented to show the main components of the Magellanic Complex. This means to show the HI distribution and velocities over most of the southern hemisphere, and probably more. Most of the components are present in our survey but there are extensions that require the use of the northern survey which has been already finished and published (Hartmann and Burton 1997). We call Magellanic Complex the system consisting of the Clouds themselves, the Bridge between them, the Halo and the Magellanic Stream. Except for the Clouds and the Bridge, the determination of the HI that belongs to each component is not easy in the absence of distance indicators.

As usual with HI, for studying these objects, we use as observational parameters the velocity profile area, which is proportional to the column density, and its mean velocity along the line of sight. There is a third parameter, the width of the profile, which provides the velocity dispersion. This has not yet been done because this study requires to separate the velocity components. For this reason also the mean velocity has to be considered as a rough estimate of the global velocities in the objects.

The graphics in this Poster use two kinds of projections, Aitoff or spherical, depending on the scale and the position of the features wanted to show. The purpose is mainly to show the amount and complexity of the information that requires to be processed for its thorough study. The Magellanic Clouds and their surroundings have been object of numerous observations and studies so it is difficult to foresee that this survey will provide new results, even if it is not discarded, but the main value of these observations is their uniformity on a large scale which permits to study the eventual extensions of the connections and streams derived from the Clouds on a global scale

  • The Magellanic Clouds (MCs)  (Mostly from Westerlund 1997)
  • Physical parameters
LMC SMC
Type SB(s)m SB(s)mp
Galactic Coordinates 280.5°, -32.9° 302.08°, -44.3°
Equat. Coordinates (1950) 05 h 24 m, -69° 50’ 00 h 51 m, -73° 06’
Heliocentric Velocity 275 km s-1 148 km s-1
Distance ~ 50 kpc ~ 60 kpc
B (integrated) 0.9 mag 2.9 mag
B-V (integrated) 0.5 mag 0.4 mag
B (central) 21,2 mag/(arc sec)2 21.4 mag/(arc sec)2
Total mass 2 x1010 Mo 2 x 109 Mo
HI (21 cm) mass 7 x 108 Mo 6.5 x 108 Mo
H2(from CO) mass 1.4 x 108 Mo 3 x 108 Mo
  • Description of the Clouds
LMC SMC
Disk Disk consisting of intermediate-age and young (stars and gas) population.
Many HII regions.
Extended Ha emission
Several molecular clouds.
If there is a disk, it must be along the line of sight.
Few large HII regions
Extended Ha emission
Few molecular clouds.
Bar Red stars, HII and star forming regions. Blue stars predominant and scatter of red stars.
Halo Optically thin thermal plasma ~2 x 106 K to 2 x 107 K Optically thin thermal plasma
~106 K
  • The bridge or InterCloud Region (ICR)

Is the HI between the two Clouds. There are two main components, at +214 and +238 km/s. The region was discovered in 1964 by Hindman et al. But only in 1980 could Kunkel identify a group of young stars associated to this gas in the tip of the optical Wing of the SMC. In 1992, Grondin et al. Identified six stellar associations in the Bridge and there may be up to 18 blue globular clusters in the Wing. The age of the clusters in the Bridge is £16 106 years.

  • The magellanic stream (MS)

The MS is a narrow band of HI along a circle 7° from and parallel to a great circle passing through the south galactic pole and cutting the galactic plane at l = 280°. It was detected by Wannier et al. (1972). If the MS lies on a great circle then the parallax effect would put it at the distance of the MCs. The MS extends at least 100° from the ICR as a continuous filament. The radial velocities vary from 0 km/s (at the galactic center) to 200 km/s (GSR) along six HI concentrations (MSI to MSVI) (Mathewson et al. 1987). The HI column desity decreases, from2 x 1020 at cm-2, close to the Clouds, to 1 x 1019 at cm-2 on the farthest point. Neither stars nor dust have been found in the MS. There is also no IRAS 100m assciated with it.

There are several models for the interp0retation of the origin of the MS. All of them, necessarily, are base on the interaction betwseen the Clouds and between them and the Galaxy. The differences reside in the origin of the Clouds themselves, in the evolution of their orbits and in the type of interaction taht originated the Stream.

  • References
    • Grondin L., Demers S.,Kunkel W.E., 1992, AJ 103, 1234
    • Hartmann D., Burton W. B., 1997, “Atlas of Galactic Neutral Hydrogen”. Cambridge University Press.
    •  Hindman J.V., 1964, IAU/URSI Symp. No. 20, p. 255
    • Kunkel W.E., 1980, IAU Symp. No. 85, p. 353
    • Mathewson D.S., Wayte S.R., Ford V.L., Ruan K., 1987. Proc. Astron. Soc. Aust. 7, 19
    • Westerlund B., 1997, en “The Magellanic Clouds”. Cambridge University Press
  • Author:
    • Esteban BAJAJA