Received 20th March 1998
Abstract.
We present a complete CO map of the magellanic type galaxy NGC 4449 and
compare the kinematics and morphology with the neutral and ionized atomic gas
component.
We detected several Giant Molecular Cloud Complexes (GMCCs) which are more
extended than the optical counterpart of the galaxy.
They are located inside peaks of H I and have smaller
velocity dispersions.
All GMCCs belong to the inner dynamical system, which is counterrotating with
respect to the giant H I-halo.
This leads to an interaction scenario in which the ongoing formation of GMCCs
and star-forming regions is triggered by collision with halo-gas.
1. Introduction
NGC 4449 is an irregular galaxy of type IBm at a distance of about
4 Mpc, and in terms of morphology and linear size comparable with the LMC.
One major difference between these galaxies is that NGC 4449 is surrounded
by a huge counter-rotating halo of neutral gas (Bajaja et al. 1994),
despite the absence of any nearby massive disk galaxy.
Deep Halpha images show many star-forming regions across
the bar and with a northern chain of H II regions
(see Fig. 1).
Additional supergiant shells, loops and filaments are obvious.
[Click here to see Fig. 1!]
2. Observations
During two observing runs at Pico Veleta in August 1996 and September 1997
we mapped the main body of NGC 4449 using the 30 m telescope in
12CO(1-0) with a grid spacing of 20".
Typical integration times were one hour per position (on+off) which yielded on
rms of 0.01-0.03 mK.
The pointing was generally better than 10" and was checked every two hours.
We subtracted linear baselines and created the final moment maps using the
GILDAS software.
3. Results
There are several GMCCs that are more extended than the optical counterpart
of the galaxy, which all belong to the inner dynamical system and are embedded
in H I surface density peaks with column densities
> 4·1021 cm s-1 (Hunter et al. 1998).
They have roughly the same peak velocity, but the velocity dispersion
of the integrated CO lines for each complex is smaller than that of
the corresponding H I.
This is consistent with the view that molecular clouds form in regions where
atomic hydrogen is shielded by dust from the interstellar radiation field.
The most intense peak of CO is located near the optical center of the galaxy,
but there is no simple correlation between CO, H I or
Halpha.
4. Kinematics of H I and CO
The distribution of the CO can be divided into two groups.
First there are two GMCCs near the northern chain of on-going starformation.
This is exactly the region where the two dynamical components of
H I belonging to the halo and the inner part of
the galaxy overlap.
The second group of GMCCs, including the most luminous one, is located near
the kinematical and optical center.
The H I data of Hunter et al. (1998) show a narrow filament
having a velocity gradient of 70 km s-1 within less than
400 pc projected distance that is connected to the outer halo gas by
a velocity bridge.
5. Interpretation
Despite the fact that CO is mostly hard to detect in dwarf galaxies,
NGC 4449 as an irregular dwarf galaxy in the upper mass range shows
clearly several GMCCs as fuel for the on-going and future star-formation.
As a long-term and large-scale trigger mechanism for the high star-formation
rate since 109 yr (Thronson et al. 1987), which may even go
on for some more 109 yr we propose an interaction scenario
of two dynamically different gaseous components in the transition zone between
counter-rotating disks.
The three-dimensional structure of both disks can be deduced from a model
in which the nearby dwarf galaxy DDO 125 forms the extended streamers
in the halo of NGC 4449 by tidal interaction (Hunter et al. 1998;
Theis & Kohle, this
volume).
The outstanding central region of NGC 4449 may experience recent infall
of high velocity gas, which is evident in the high-resolution
H I data.
References
- Bajaja E., Huchtmeier W.K., Klein U., 1994, A&A 285, 385
- Bomans D.J., Chu Y-H., Hopp U., 1997, AJ 113, 1678
- Hunter D.A., Wilcots E.M., van Woerden H., Gallagher S.S., Kohle S.,
1998, ApJ 495, L47
- Theis C., Kohle S., 1998,
these proceedings
- Thronson H.A. Jr., Hunter D.A., Telesco C.M., Decher R., Harper D.A.,
1987, ApJ 317, 180
Links (back/forward) to:
| First version: | 12th | August, | 1998
|
| Last update: | 08th | October, | 1998
|
Jochen M. Braun &
Tom Richtler
(E-Mail: jbraun|richtler@astro.uni-bonn.de)