Transcript NGC 1569 A Dwarf Galaxy with a Giant Starburst Stefanie Mhhle1,2, Uli Klein1, Susanne Hhttemeister3, Eric M.

NGC 1569
A Dwarf Galaxy with a Giant Starburst
Stefanie Mhhle1,2, Uli Klein1, Susanne Hhttemeister3, Eric M. Wilcots4
1University
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
1 arcmin
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).
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
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.
highly disturbed in the central kpc of the galaxy with evidence for gas being pushed out of
the core to larger radii 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
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).
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
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.
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.
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
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
HI arm
Mhhle, S., Klein, U., Wilcots, E.M., &
Hhttemeister, S. 2004, AJ (subm.)
Stil, J. & Israel, F.P. 1998, A&A 337, 64
NGC1569-HI
stream
bridge
Taylor, C.L., Hhttemeister, S., Klein, U.,
& Greve, A. 1999, A&A 349, 424
Vallenari, A., & Bomans, D.J. 1996, A&A
313, 713
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.
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