Imaging the nuclear environment of NGC 1365 with the Hubble Space Telescope

Abstract

The region surrounding the active nucleus of the barred spiral galaxy NGC 1365 is observed in the [Oiii] lambda 5007 line and neighbouring continuum using the Faint Object Camera (FOC) aboard the Hubble Space Telescope (HST). In the continuum light numerous bright ``super star clusters'' (SSCs) are seen in the nuclear region. They tend to fall on an elongated ring around the nucleus and contribute about 20 % of the total continuum flux in this wavelength regime. Without applying any extinction correction the brightest SSCs have an absolute luminosity M_B=-14fm1 +/- 0fm3 and are very compact with radii R la 3 pc. Complementary ground-based spectroscopy gives an extinction estimate A_B = 2fm5 +/- 0fm5 towards these regions, indicating a true luminosity M_B = -16fm6 +/- 0fm6 . The bright compact radio source NGC 1365:A is found to coincide spatially with one of the SSCs. We conclude that it is a ``radio supernova''. The HST observations resolve the inner structure of the conical outflow previously seen in the [Oiii] lambda 5007 line in ground-based observations, and reveal a complicated structure of individual emission-line clouds, some of which gather in larger agglomerations. The total luminosity in the [Oiii] line amounts to L_[OIII] =~ 3.7x 10(40) erg s(-1) where about 40 % is emitted by the clouds. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555, and observations at the European Southern Observatory (ESO), La Silla, Chile.