References: Hunter, D.A., Hawley, W.N., & Gallagher, J.S. 1993, AJ 106, 179

1. 1 arcmin Figure 2: Ha emission from NGC 1569 (Hunter et al. 1993). Note the numerous filaments extending far into the halo and the prominent Ha arm in the west. Figure 1: Composite image of NGC 1569, red: Ha emission, green: X-ray emission, blue: stars, white contours: HI column density (Martin et al. 2002). Figure 3: High-resolution HI column density map of NGC 1569. The prominent HI funnel coincides with the location of two very bright super-star clusters (arrow). Figure 4: Fully sampled CO(2-1) map (12” resolution). The letters denote the position of the super-star clusters, while the numbers show the location of the identified GMAs. NGC 1569A Dwarf Galaxy with a Giant StarburstStefanie Mhhle1,2, Uli Klein1, Susanne Hhttemeister3, Eric M. Wilcots41University of Bonn, 2University of Toronto, 3University of Bochum, 4University of Wisconsin (Madison) The Starburst in NGC 1569 The nearby (D = 2.0 Mpc) gas-rich dwarf galaxy NGC 1569 has experienced a tremendous starburst with star formation rates of up to 0.5 Msun/yr without evidence for extended quiescent periods (>10 Myr) over the last 100 Myr (Vallenari & Bomans 1996). A strong metal-rich galactic wind traced by extended X-ray emission (Figure 1) and numerous Ha filaments (Figure 2) bear witness to the deep impact of the starburst on the ionized gas of the galaxy. Radio polarization measurements show that magnetic field lines have been dragged out into the halo along with the hot outflow (Mhhle 2003). At the position of the prominent western Ha arm, the X-ray and radio observations suggest a region of shocked gas (Martin et al. 2002, Mhhle 2003). Consequences for the neutral ISM We have studied the distribution and kinematics of the neutral atomic hydrogen in NGC 1569 using a high-resolution HI data cube complemented with single-dish observations. The velocity structure is highly disturbed in thecentral kpc of the galaxy with evidence for gas being pushed out of the core to largerradii indicating that the neutral atomic gas may be located in a torus viewed at high inclination (Mhhle et al. 2004). X-ray colors suggest that the northern part of the HI distribution is the one closer to us (Martin et al. 2002). A pronounced funnel that is probably located in the southern half of the torus (Figure 3) coincides with two very bright super-star clusters (SSCs), whose winds and supernovae may have blown the neutral gas away and caused the formation of the surrounding horse-shoe structure of HII regions. Our single-dish CO(2-1) map of the region near the super-star clusters (Figure 4) shows a depression in the integrated line intensity near the SSCs, which is framed in the east and in the west by strong CO emission from giant molecular associations (GMAs). The western GMA consists of two components at v = -72 km/s (GMA 1a) and v = - 90 km/s (GMA 1b), respectively. Their positions and central velocities agree very well with those of the giant molecular clouds that have been detected with the Plateau de Bure Interferometer (Taylor et al. 1999). A comparison of the total flux in our single-dish map with that of the interferometric observations suggests that up to 80% of the CO flux may originate from diffuse gas. The position angle of the (diffuse) CO distribution is very similar to that of the HI ridge and the three GMAs coincide with the high-column density rim of the HI funnel, which suggests a common origin for both the funnel and the CO depression. A one-component non-LTE analysis of several 12CO and 13CO transition lines at the position of GMA 1a yields a moderate average gas density of n(H2) ~ 102.9…3.3 cm-3 and a high average kinetic temperature of Tkin > 110 K (Mhhle et al. 2002). A Possible Trigger of the Starburst Stil & Israel (1998) first reported the detection of HI gas in the halo of NGC 1569 (“NGC1569-HI” and a “bridge”), which seemed to connect to the eastern part of the galaxy’s HI disk. Our sensitive HI data cube not only confirms this detection, but shows additional very low-column density HI emission in an arc that connects the bridge spatially and kinematically with the peculiar HI arm in the west of NGC 1569 (Figure 5). Given the isolated position of NGC 1569 and the low probability of this structure being a chance alignment of Galactic foreground emission with the halo gas of the dwarf galaxy, we propose that the extended halo feature is the tidally disrupted remnant of an intergalactic HI cloud that is in the process of merging with NGC 1569. This scenario can readily explain the observed distribution and kinematics of the halo structure as well as the apparent retrograde rotation of the HI arm with respect to the HI disk and the evidence for several kinematic HI components (Mhhle et al. 2004) and shocked gas near the Ha arm (impact zone). The merger may not only have triggered the starburst in NGC 1569, but also replenish the gas reservoir of the dwarf galaxy, enabling it to form stars at the observed exceptionally high rate. References: Hunter, D.A., Hawley, W.N., & Gallagher, J.S. 1993, AJ 106, 179 Martin, C.L., Kobulnicky, H.A., & Heckman, T.M. 2002, ApJ 574, 663 Mhhle, S., Hhttemeister, S., Klein, U., & Wilcots, E.M. 2002, ApSS 281, 327 Mhhle, S. 2003, PhD thesis, Univ. Bonn Mhhle, S., Klein, U., Wilcots, E.M., & Hhttemeister, S. 2004, AJ (subm.) Stil, J. & Israel, F.P. 1998, A&A 337, 64 Taylor, C.L., Hhttemeister, S., Klein, U., & Greve, A. 1999, A&A 349, 424 Vallenari, A., & Bomans, D.J. 1996, A&A 313, 713 Contact: muehle@astro.utoronto.ca HI arm stream NGC1569-HI bridge Figure 5: Velocity map of our high-sensitivity HI data cube. NGC1569-HI and the bridge are connected spatially and kinematically with the peculiar HI arm by a stream of low-column density HI gas.