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On October 28, 2001, PMAS was used to observe the blue compact dwarf galaxy SBS0335-052.
This object is the second most metal-poor known galaxy after I Zw18, and thus
an interesting target for spectrophotometric observations. Its oxygen abundance
is 77 times lower than solar. It is thought to contain 6 embedded star clusters with
a significant number of supermassive stars of around 100 solar masses. The intense far
UV radiation of those stars leads to high excitation ionization of the associated H II
regions, showing electron temperatures as high as 25000 K. The emission line spectrum
of this galaxy has been studied sufficiently well in the literature, providing an excellent
test case for the PMAS science verification observations.
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The figure above is a processed copy of the HST WFPC image
(1800sec V exposure), originally published by
Thuan et al. 1997, ApJ 477, 661.
It is rotated to match the conventional orientation (North up, East left),
and blurred to a seeing of about 1 arcsec FWHM. The size is roughly 10 by 10 arcsec
squared.
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The spectrum below was created from the PMAS datacube, which is the result from a total
observing time of 2x 1800sec. The seeing was somewhat inferior compared to the simulated image
(1.5" instead of 1"). A digital aperture was defined to coadd a total of 62 spatial elements,
each measuring 0.5 x 0.5 arcsec squared. This roughly elliptical aperture was adapted to the
elongated region of significant surface brightness along the NW direction.
The spectrum covers the wavelength range between 3700 and 5150 A. It shows a rich emission
line spectrum, including the Balmer series, the forbidden lines of [O III] 5007,4959
and [O III] 4363, [O II] 3727,3729, He I 4472, He II 4686, and many others.
It is in good agreement with the spectrum published by
Melnick et al. 1992, AA 253, 16.
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Apart from the total integrated spectrum, 3D spectroscopy offers the opportunity to study
extended objects in full 2-dimensional spatial resolution. The figure below shows the
comparison with the Thuan et al. WFPC2 image (rebinned to the PMAS sampling of 0.5")
with a map in He I 4472, which was derived from the PMAS datacube. The He I
surface brightness traces roughly the continuum distribution, with a faint, but significant
detection of the separate knot in the West.
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The figure below show more maps in the emission lines of [O III] 5007,
[O III] 4959, H_beta, and the fainter lines of He I 4472, He II 4686 :
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