Star Formation in the HI Cloud Associated with the NE Radio Lobe of NGC5128 (Cen A)

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Session 109 -- Extragalactic Radio Sources, Jets
Display presentation, Thursday, 12, 1995, 9:20am - 6:30pm

[109.10] Star Formation in the HI Cloud Associated with the NE Radio Lobe of NGC5128 (Cen A)

J.A. Graham (Carnegie-DTM)

A new perspective of the nearest bright radio galaxy NGC5128 (CenA) is revealed by the breakthrough discovery of neutral hydrogen associated with the diffuse optical shells (Schiminovich et al ApJL 423 L101). NE of the galaxy, in the direction of the radio lobes, loose chains of blue stars are observed along the border of one of these clouds. Some of these stars are apparently clumped as small clusters and excite 3 compact HII regions whose velocities (+299,+267, +337 km/sec h-c) correspond very closely to those of the HI gas in the area. Distinct from these HII regions, is more diffuse, filamentary gas which extends much further along the radio lobe axis (Graham and Price ApJ 247,813) and which appears to be excited by beamed continuum radiation from the nucleus (Morganti et al MNRAS 249,91) or by shocks associated with a high velocity jet (Sutherland et al ApJ 414,510). The velocity spread of this gas (ranging from +65 to +500 km/sec) is much larger than that of the compact HII regions associated with the blue stars. Two of the brightest of the approximately 100 identifiable blue stars are at V magnitudes 20.0 and 21.0. At a distance of 3.5 Mpc, these correspond to absolute visual magnitudes of $-$7.7 and $-$6.7; i.e. young supergiant stars no older than a few million years. Stars as massive as this are predestined to become supernovae and will contribute to the dispersion of the HI cloud. The blue stars are only seen in the HI found in the direction of the NE radio lobes and we conclude that star formation under admittedly very unusual circumstances is taking place probably as the result of shock compression arising from the radio jet. Plans for a comprehensive photometric study of this stellar component are discussed.

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