10 research outputs found
Determining the extragalactic extinction law with SALT. II. Additional sample
We present new results from an on-going programme to study the dust
extragalactic extinction law in E/S0 galaxies with dust lanes with the Southern
African Large Telescope (SALT) during its performance-verification phase. The
wavelength dependence of the dust extinction for seven galaxies is derived in
six spectral bands ranging from the near-ultraviolet atmospheric cutoff to the
near-infrared. The derivation of an extinction law is performed by fitting
model galaxies to the unextinguished parts of the image in each spectral band,
and subtracting from these the actual images. We compare our results with the
derived extinction law in the Galaxy and find them to run parallel to the
Galactic extinction curve with a mean total-to-selective extinction value of
2.71+-0.43. We use total optical extinction values to estimate the dust mass
for each galaxy, compare these with dust masses derived from IRAS measurements,
and find them to range from 10^4 to 10^7 Solar masses. We study the case of the
well-known dust-lane galaxy NGC2685 for which HST/WFPC2 data is available to
test the dust distribution on different scales. Our results imply a scale-free
dust distribution across the dust lanes, at least within ~1 arcsec (~60 pc)
regions.Comment: 11 pages, 7 figures, 3 tables. Accepted for publication in MNRAS.
R-band contour maps and B-R colour-index maps are low-resolution versions of
those used in the MNRAS versio
Discovery of the first symbiotic star in NGC6822
We report the discovery of the first symbiotic star (V=21.6, K_S=15.8 mag) in
the Local Group dwarf irregular galaxy NGC6822. This star was identified during
a spectral survey of Ha emission-line objects using the Southern African Large
Telescope (SALT) during its performance-verification phase. The observed strong
emission lines of HI and HeII suggest a high electron density and T* < 130 000
K for the hot companion. The infrared colours allow us to classify this object
as an S-type symbiotic star, comprising a red giant losing mass to a compact
companion. The red giant is an AGB carbon star, and a semi-regular variable,
pulsating in the first overtone with a period of 142 days. Its bolometric
magnitude is M_bol=-4.4 mag.
We review what is known about the luminosities of extragalactic symbiotic
stars, showing that most, possibly all, contain AGB stars. We suggest that a
much larger fraction of Galactic symbiotic stars may contain AGB stars than was
previously realised.Comment: 6 pages, 4 figures, accepted to MNRA
Determining the extragalactic extinction law with SALT
We present CCD imaging observations of early-type galaxies with dark lanes
obtained with the Southern African Large Telescope (SALT) during its
performance-verification phase. We derive the extinction law by the
extragalactic dust in the dark lanes in the spectral range 1.11mu m^{-1} <
lambda^{-1} < 2.94 mu m^{-1} by fitting model galaxies to the unextinguished
parts of the image, and subtracting from these the actual images. We find that
the extinction curves run parallel to the Galactic extinction curve, which
implies that the properties of dust in the extragalactic enviroment are similar
to those of the Milky Way. The ratio of the total V band extinction to the
selective extinction between the V and B bands is derived for each galaxy with
an average of 2.82+-0.38, compared to a canonical value of 3.1 for the Milky
Way. The similar values imply that galaxies with well-defined dark lanes have
characteristic dust grain sizes similar to those of Galactic dust.Comment: 20 pages, 15 figures and 4 tables. Accepted for publication in MNRA