| Names |
Eric B. Burgh, J. S. Gallagher, K. H. Nordsieck, J. W. Percival, M. P. Smith, D. O'Donoghue, D. A. Buckley, N. S. Loaring |
| Abstract |
We present a long-slit spectrum of the amorphous starbursting dwarf
galaxy NGC 1140 taken by the Robert Stobie Spectrograph of the Southern
African Large Telescope during its commissioning phase in November 2005.
A 1.2 arcsecond wide longslit oriented along the major axis in
conjunction with a volume phase holographic grating were used to produce
a 800 Angstrom wide spectrum, centered on H-alpha, with a resolving
power of 5500. Twelve minutes of data were obtained. Emission features
from H-alpha, [NII], [SII] and HeI were observed. The peak of the
emission features is observed to be 2 arcseconds north of the stellar
continuum, consistent with HST imaging of the galaxy, and a bright knot
of gas with a velocity offset of 100 km/s is detected about 18
arcseconds south of the continuum peak. The emission lines are
well-resolved with FWHM 80-100 km/s in the inner galaxy, typical of
intensely star-forming regions, while FWHM 100-140 km/s are seen in the
outer galaxy, suggesting a possible galactic wind. The complex velocity
field is consistent with the merger model for NGC 1140. The intensity
ratio of the [SII] doublet, 6717/6731 is 1.3, indicating a low electron
density, and thus modest thermal pressure despite the galaxy's starburst
status. The H-alpha/[SII] ratio, which is an indicator of the presence
of shocks,varies along the slit with a maximum of 8 at the peak of the
gaseous emission and dropping to nearly 2 at the edges of the galaxy,
with a value of 3 for the offset knot of gas. Thus shocks could play a
role in the outer galaxy, while the knot is more likely a star forming
region. We interpret our results in the context of a starburst induced
by the merger of two low mass galaxies, as previously discussed by
Hunter et al. (1994, ApJS, 91, 79). |