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Astron. Astrophys. 334, 659-669 (1998)

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Complex H: a case of HVC-galaxy collision? *

R. Morras *** 1, E. Bajaja ** 1 and E.M. Arnal ** *** 1

Instituto Argentino de Radioastronomía, C.C. 5, 1894 Villa Elisa (Prov. de Bs.As.), Argentina

Received 16 June 1997 / Accepted 16 February 1998


This work has been undertaken with the intention of studying the interaction between HI high velocity clouds (HVCs) and the galactic HI. For this purpose we have selected the HVC complex known as Complex H, which includes HVC131+1-200, because its low galactic latitude and its size make such interaction very likely. Furthermore, the low latitude lessens the uncertainty about its vertical height above the plane. In order to verify the existence of such interaction, we studied the available observations of the complex and we observed, in the HI 21 cm line, a region centered on HVC131+1-200 with the 100 m Effelsberg telescope. Our map for the distribution of the high velocity HI shows, within the observed region, four peaks, two of them not seen before, and, in some places, steep gradients in the column densities. The latter suggests the existence of shocks which might represent the effects of the interaction on the high velocity cloud. We studied the distribution of the HI at low and intermediate velocities, using the existing surveys of galactic HI covering the region of Complex H, and we found a hole, approximately constant in size and position, within the velocity range of -109 to -98 [FORMULA]. Determining kinematical distances, the hole appears to be at about 22 kpc from the galactic center and 15.4 kpc from the Sun. We present arguments that suggest that this hole and Complex H are the results of the collision of a HVC with the galactic HI in a warped region of the Galaxy.

Key words: ISM: HVC Complex H – ISM: clouds; kinematics and dynamics; bubbles – radio lines: ISM

* Based on observations made with the 100 m telescope of the Max-Planck-Institut für Radioastronomy in Effelsberg, Germany
** Member of the Carrera del Investigador Científico of the Consejo Nacional de Investigaciones Científicas y Técnicas of Argentina
*** Member of the Facultad de Ciencias Astronómicas y Geofísicas of the Universidad Nacional de La Plata

Send offprint requests to: R. Morras

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© European Southern Observatory (ESO) 1998

Online publication: May 15, 1998