Complex Structure of the Superwind-Driven Nebula around the Ultraluminous Infrared Galaxy Arp 220

Youichi Ohyama (NAOJ-Subaru), Yoshiaki Taniguchi (Tohoku Univ), J. E. Hibbard (NRAO), William D. Vacca (MPIfE), Michitoshi Yoshida (NAOJ-OAO), Nobunari Kashikawa (NAOJ).

In preparation

We present results of new optical narrow-band imaging and long-slit spectroscopy of a superwind-driven nebula around an archetypical ultraluminous infrared galaxy, Arp 220. Although the nebula is known to show a simple "figure-8" shaped morphology and is often recognized as a pair of twin bubbles, we find that the nebula has rather complex properties than expected from its simple appearance. In Ha+[NII] image, we identify two more pairs of smaller shell-like morphological features within the twin bubbles previously known. By analyzing the kinematical structure along the major axis of the nebula, we find that the inner region between 6" ~ SE and 2" ~ NW shows a broad emission-line component whose full width at half maximum (FWHM) varies from ~500 km/s to 1200 km/s as well as the narrow one with FWHM ~250 km/s. On one hand, a double-peaked narrow profile is found at 6-9" SE, ~12" SE, and 11" NW and a single narrow blue-shifted profile is found in the remaining parts. These complex morphological and kinematical properties are difficult to be interpreted simply as the effect of the expanding twin bubbles. Rather, a model of overlapping multiple bubbles each of which has different size and kinematical structure seems to be more preferred since each shell-like structure seems to have its own kinematical characteristics. It is therefore suggested that Arp 220 experienced multiple starburst events in its central region.

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The Cold-Gas superwind of Arp 220 Traced by NaD5890 Line

Youichi Ohyama (NAOJ-Subaru), Yoshiaki Taniguchi (Tohoku Univ), William D. Vacca (MPIfE), J. E. Hibbard (NRAO).

In preparation

Arp 220 is a prototypical ultraluminous infrared galaxy with evidence for a large-scale shock-heated nebula driven by superwind activity. We present results of our new long-slit optical spectroscopy on this galaxy. We analyze the kinematic properties of both the Halpha and NII (6548A, 6583A) emission lines and the Na D (5890A) absorption line along the axis of the superwind. We find that the velocities traced by all three lines are quite similar, with similar asymmetric and/or double-peaked line profiles. All lines show that the outer regions (r > 1.8 kpc) are blue-shifted by > -300 km/s. Since both Halpha and NII emission are known to come from the shock-heated ionized gas associated with the superwind, the line similarities indicate that the neutral interstellar matter giving rise to the Na D absorption is also entrained within the superwind outflow. We also detect blue-shifted Na D in emission from outer NW regions, suggesting that this gas is excited by the shock between the expanding interstellar gas and the ambient matter. The mass of the interstellar HI gas associated with the superwind is estimated to be ~1E9 Msun, which is roughly comparable to that of the nuclear molecular gas disk, and is even higher than that of the hot ionized gas (HII) within the superwind. The energy required to blow out the HI gas (~3E57 ergs) could be supplied by 1E8 Type II supernovae.

R data obtained from the UH 88" in 6/95
Keck LRIS Spectra obtained from Keck II with W. D. Vacca in 4/97
Ha+[NII] imaging from Subaru by Y. Ohyama in 2/00
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