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Дата изменения: Tue Nov 24 21:32:27 1998 Дата индексирования: Sat Sep 11 22:12:36 2010 Кодировка: Поисковые слова: saturn's moon |
The Keck II telescope optical spectra of SBS 0335-052W were obtained on
1997 November 10 with Low Resolution Imaging Spectrometer, using the
600g/mm grating which provides a dispersion 1.28 Å pixel-1 and a
spectral resolution of about 4 Å in first order. The total spectral
range covered was 4100-6800 Å. The slit was
1
180 and oriented in direction of the SBS 0335-052W
major axis with position angle P.A. = -80. No binning
along the spatial axis has been done, yielding a spatial sampling of
02 pixel-1. The total exposure time was 90 min, broken to three
exposures of 30 min. The seeing during the observations was around 06
and the galaxy was observed at the airmass of 1.25.
The standard star GD 50
was observed for absolute flux calibration. The spectrum of a He-Ne-Ar
comparison lamp was obtained after observation to facilitate wavelength
calibration.
The data reduction was carried out at the NOAO headquarters in Tucson using
the IRAF6 software package. Procedures included bias subtraction,
cosmic-ray removal and flat-field correction using exposures of a
quartz incandescent lamp. After wavelength mapping and night sky background
subtraction each frame was corrected for atmospheric extinction and flux
calibrated. The one-dimensional spectra were extracted from each
flux-calibrated frame using the APALL routine. The extracted spectra were then
co-added, not combined. The cosmic rays hits have been removed manually.
To derive the sensitivity curves, we have fitted the observed spectral energy
distributions of the standard stars with a high-order polynomial.
The MMT spectrum of brightest 1
2 region of
SBS 0335-052W is shown in Figure 1. Two-dimensional spectrum obtained
with Keck II telescope along the major axis of SBS 0335-052W shows the presence
of three emitting regions with the brightest region at the NW side of the
galaxy and two fainter SE regions. One-dimensional spectra of each
region in apertures 1
2 each are shown in Figure 2.
In Figure 3 we show the spatial distribution of the observed flux and
equivalent width of H
emission line. The location of regions with
spectra presented in Figure 2 is shown by vertical lines.
While in the MMT spectrum the [O III]
4363 emission line is barely detected and is blended
with strong H
emission line, it is evidently present
in Keck II telescope spectra of two regions allowing the reliable determination
of electron temperature and element abundances.
In Table 1, the observed line intensities and intensities corrected for interstellar
extinction for SBS 0335-052W are shown along with extinction
coefficient C(H), observed flux of the H
emission line, the equivalent width EW(H
)
and equivalent width of absorption Balmer
hydrogen lines EW(abs). For the lines which have not been detected we show
1
intensity upper limits.
To correct for extinction we used the reddening law for
our Galaxy by Whitford (1958). The errors of the line intensities listed in
Table 1 take into account the noise statistics in the continuum
which implicitly includes the uncertainties of data reduction ( flat-field
correction, sky subtraction), the errors
in placing the continuum and fitting the line profiles with gaussians. These
errors have been propagated to calculate element abundances. Skillman et al.
(1994) have discussed the uncertainties in the measurements of the line
intensities, including different steps of data reduction. However, as it is
difficult to take into account all possible uncertainties which may enter
during the data reduction process ( Skillman et al. (1994) have taken
into account the uncertainties in the flat-field
correction, night sky subtraction and flux calibration, but they do not
take into account the uncertainties introduced by COMBINE-CRREJECT routine
and uncertainties introduced by the extraction of
one-dimensional spectra from two-dimensional data due to tilts, distortions
and not perfect focusing of telescope and spectrograph ) the best way to
estimate real uncertainties is by cross-comparing data taken with different
telescopes.
The line intensities in the brightest knot derived from the MMT and the Keck
telescope observations are in general agreement implying the correctness of
the measurements despite that the orientation of slits was different.
To derive the element abundances in
SBS 0335-052W we adopted a two-zone photoionized H II region model (Stasinska
1990). The electron temperature, Te(O III), is derived from the
[O III]4363/(
4959 + 5007) line intensity ratio using
five-level atom model, and the electron
temperature Te(O II) from the relation between Te(O II) and Te(O III)
fitted by Izotov, Thuan & Lipovetsky (1994, hereafter ITL94)
for photoionized H II models
by Stasinska (1990). The electron temperature for S III ion is obtained using
prescriptions by Garnett (1992).
The electron number density Ne is derived from the
[S II]
6717, 6731 lines.
The values of Te(O III), Te(O II), Te(S III)
and Ne(S II) are shown in Table 2.
The ionic and total abundances are derived from Keck II telescope spectra
as described by ITL94 and by Izotov, Thuan & Lipovetsky (1997, hereafter
ITL97) and are
shown in Table 2. However, due to the fact that these spectra cover a
small spectral
range, we use the intensities of the [O II]
3727, [Ne III]
3868
and [S II]
6717, 6731 emission lines in the MMT spectrum, scaled to
Keck II telescope spectra, to derive oxygen, neon and sulfur abundances.
The oxygen abundance in both of the brightest regions of SBS 0335-052W is
0.1-0.2 dex smaller than in SBS 0335-052 (Izotov et al. 1997) and is
the same as that in I Zw 18 (SK93). The helium abundance
derived from He I
4471 and
5876 emission line intensities
is in agreement with He abundances in brighter blue compact galaxies
(ITL94, ITL97). The abundances of other elements are close to mean values
for extremely metal-deficient galaxies (Thuan, Izotov
& Lipovetsky 1995; Izotov et al. 1997; Izotov & Thuan 1998a).