278 research outputs found
Two extremely metal-poor emission-line galaxies in the Sloan Digital Sky Survey
We present spectroscopic observations with the 3.6m ESO telescope of two
emission-line galaxies, J2104-0035 and J0113+0052, selected from the Data
Release 4 (DR4) of the Sloan Digital Sky Survey (SDSS). From our data we
determine the oxygen abundance of these systems to be respectively 12+logO/H =
7.26+/-0.03 and 7.17+/-0.09, making them the two most metal-deficient galaxies
found thus far in the SDSS and placing them among the five most metal-deficient
emission-line galaxies ever discovered. Their oxygen abundances are close to
those of the two most metal-deficient emission-line galaxies known,
SBS0335-052W with 12+logO/H = 7.12+/-0.03 and I Zw 18 with 12+logO/H =
7.17+/-0.01.Comment: 5 pages, 3 figures. Accepted for publication in Astronomy and
Astrophysic
The blue compact dwarf galaxy I Zw 18: a comparative study of its low-surface-brightness component
(abridged): Using HST and ground-based optical and NIR imaging data, we
investigate whether the blue compact dwarf (BCD) galaxy I Zw 18 possesses an
extended low-surface-brightness (LSB) old stellar population underlying its
star-forming (SF) regions. We show that the exponential intensity decrease
observed in the filamentary LSB envelope of the BCD out to 18 arcsec (1.3 kpc
at a distance of 15 Mpc) is not due to an evolved stellar disc, but rather due
to extended ionized gas emission. Broad-band images reveal, after subtraction
of nebular line emission, a compact stellar LSB component extending slightly
beyond the SF regions. This stellar host, being blue over a radius range of 5
exponential scale lengths and showing little colour contrast to the SF
component, differs strikingly from the red LSB host of standard BCDs. This
fact, in connection with the blue colors of component I Zw 18 C, suggests that
most of the stellar mass in I Zw 18 has formed within the last 0.5 Gyr.
Furthermore, we show that the exponential intensity fall-off in the filamentary
ionized envelope of I Zw 18 is not particular to this system but a common
property of the ionized halo of many SF dwarf galaxies on galactocentric
distances of several kpc. In the absence of an appreciable underlying stellar
background, extended ionized gas emission dominates in the periphery of I Zw
18, mimicking an exponential stellar disc on optical surface brightness
profiles.Comment: 24 pages, 16 figures; accepted for publication in Astronomy &
Astrophysics; higher resolution images available at
http://alpha.uni-sw.gwdg.de/~papade/IZw18
New southern blue compact dwarf galaxies in the 2dF Galaxy Redshift Survey
Aiming to find new extremely metal-deficient star-forming galaxies we
extracted from the Two-Degree Field Galaxy Redshift Survey (2dFGRS) 100K Data
Release 14 emission-line galaxies with relatively strong [OIII] 4363 emission.
Spectroscopic and photometric studies of this sample and, in addition, of 7
Tololo and 2 UM galaxies were performed on the basis of observations with the
ESO 3.6m telescope. All sample galaxies qualify with respect to their
photometric and spectroscopic properties as blue compact dwarf (BCD) galaxies.
Additionally, they show a good overlap with a comparison sample of 100
well-studied emission-line galaxies on the 12+log(O/H) vs. log(Ne/O), log(Ar/O)
and log(Fe/O) planes. From the analysis of the 2dFGRS subsample we report the
discovery of two new extremely metal-deficient BCDs with an oxygen abundance
12+log(O/H) < 7.6 and of another seven galaxies with 12+log(O/H) < 7.8.
Furthermore, we confirm previous oxygen abundance determinations for the BCDs
Tol 1304-353, Tol 2146-391, UM 559 and UM 570 to be 12+log(O/H) < 7.8.Comment: 26 pages, 65 figures, 5 tables, uses psfig.sty, Accepted for
publication in Astronomy and Astrophysics. The paper with high-resolution
figures can be downloaded at
http://www.uni-sw.gwdg.de/~papade/Publications/Papaderos2006_2dF.pd
Oxygen abundance variations in the system of the two blue compact dwarf galaxies SBS 0335-052E and SBS 0335-052W
We present 3.6m ESO telescope spectroscopic observations of the system of the
two blue compact dwarf galaxies SBS 0335-052W and SBS 0335-052E. The oxygen
abundance in SBS 0335-052W is 12 + log O/H = 7.13 +/- 0.08, confirming that
this galaxy is the most metal-deficient emission-line galaxy known. We find
that the oxygen abundance in SBS 0335-052E varies from region to region in the
range from 7.20 to 7.31, suggesting the presence of an abundance gradient over
a spatial scale of 1 kpc. Signatures of early carbon-type Wolf-Rayet stars are
detected in cluster 3 of SBS 0335-052E, corresponding to the emission of three
to eighteen WC4 stars, depending on the adopted luminosity of a single WC4 star
in the CIV 4658 emission line.Comment: 6 pages, 5 figures, uses psfig.sty, Accepted for publication in
Astronomy and Astrophysic
Spectroscopic and photometric studies of low-metallicity star-forming dwarf galaxies. I. SBS 1129+576
Spectroscopy and V,I CCD photometry of the dwarf irregular galaxy SBS
1129+576 are presented for the first time. The CCD images reveal a chain of
compact H II regions within the elongated low-surface-brightness (LSB)
component of the galaxy. Star formation takes place mainly in two
high-surface-brightness H II regions. The mean (V-I) colour of the LSB
component in the surface brightness interval between 23 and 26 mag/sq.arcsec is
relatively blue ~0.56+/-0.03 mag, as compared to the (V-I)~0.9-1.0 for the
majority of known dwarf irregular and blue compact dwarf (BCD) galaxies.
Spectroscopy shows that the galaxy is among the most metal-deficient galaxies
with an oxygen abundance 12+log(O/H)= 7.36+/-0.10 in the brightest H II region
and 7.48+/-0.12 in the second brightest H II region, or 1/36 and 1/28 of the
solar value, respectively. Hbeta and Halpha emission lines and Hdelta and
Hgamma absorption lines are detected in a large part of the LSB component. We
use two extinction-insensitive methods based on the equivalent widths of (1)
emission and (2) absorption Balmer lines to put constraints on the age of the
stellar populations in the galaxy. In addition, we use two extinction-dependent
methods based on (3) the spectral energy distribution (SED) and (4) the (V-I)
colour. The observed properties of the LSB component can be reproduced by a
stellar population forming continuously since 10 Gyr ago, provided that the
star formation rate has increased during the last 100 Myr by a factor of 6 to
50 and no extinction is present. However, the observational properties of the
LSB component can be reproduced equally well by continuous star formation which
started not earlier than 100 Myr ago and stopped at 5 Myr, if some extinction
is assumed.(Abridged)Comment: 17 pages, 12 figures, Accepted for publication in A&
Discovery of the high--ionization emission line [Ne V] 3426 in the blue compact dwarf galaxy Tol 1214-277
The discovery of the high-ionization [Ne V] 3426A emission line in the
spectrum of the blue compact dwarf (BCD) galaxy Tol 1214-277 is reported. The
detection of this line implies the presence of intense ionizing X-ray emission
with a luminosity in the range 10^39-10^40 erg/s. Such a high X-ray luminosity
cannot be reproduced by models of massive stellar populations. Other
mechanisms, such as fast shocks or accretion of gas in high-mass X-ray binaries
need to be invoked to account for the high intensity of the [Ne V] 3426A
emission line.Comment: 4 pages, 1 figure, Accepted for publication in A&
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