Observations of the 21 cm Line from the Magellanic Clouds

Abstract

SummaryA survey has been made of the 21 cm line from neutral interstellar hydrogen in the Magellanic Clouds. These are the fu;st observations of this radiation from an extragalactic source.The observations show that neutral hydrogen extends well beyond the easily visible regions of each Cloud, the gas being in each case less concentrated towards the nucleus than are the bright stars. From the total radiation received, the masses of neutral hydrogen are calculated to be 6 X 10 8 and 4 X 10 8 solar masses for the Large and Small Cloud respectively. Since the Small Cloud is believed to contain much less dust, this means that the ratio of gas to dust is very different in the two systems.The measurement of Doppler frequency shifts has yielded extensive new information about radial velocities within the Clouds. The velocity distribution indicates that both Clouds are rotating. Detailed discussion of the velocities has been deferred for subsequent pUblication.

A Sky Survey of Neutral Hydrogen at l 21 cm. II. The Detailed Distribution of Low Velocity Gas

“…The only really satisfactory total impression of the distribution of neutral hydrogen over the celestial sphere appears to have been given by the method of presentation employed in the work on the Magellanic Clouds by Kerr, Hindman, and Robinson (1954). The Leiden Group (1957) published a number of papers in the first of which some 700 H-line profiles were given; in a following paper the completed interpretation appeared in terms of contours of number density per unit volume of neutral hydrogen atoms plotted on z and r coordinates (z = distance perpendicular to the galactic plane, r = distance from the Sun) for galactic longitudes 2t ° and 5° apart.…”

A Low Resolution Hydrogen-line Survey of the Magellanic System. II. Interpretation of Results

“…Provided the gas is optically thin, and therefore the line is not saturated, the number of hydrogen atoms, N, in a column extending from the observer to infinity is related to the integrated brightness, Bint., by the relation: N = 6·2 X 10 35 Bint., where N is expressed in atoms per cm 2 , and Bint. in W m-2 sterad-1 (Kerr, Hindman, and Robinson 1954).…”

“…(2) To check and extend the original 21-cm observations of the Clouds (Kerr, Hindman, and Robinson 1954). As shown in paper I, the use of the digital system produced an effective improvement in receiver sensitivity through integration of a number of successive profiles and through the use of more sophisticated calibration procedures.…”

A Low Resolution Hydrogen-line Survey of the Magellanic System. I. Observations and Digital Reduction Procedures

“…The previous contours produced by Kerr, Hindman, and Robinson (1954) are very similar, particularly when allowance is made for a difference of aerial resolution (2~2 compared with r5). There was also a considerable improvement in receiver sensitivity, and this, coupled with the effect of digital integration, has raised the sensitivity in the low density region between the Clouds to the point where the gas now appears to form a continuous bridge.…”