738 research outputs found
VLT/GIRAFFE spectroscopic observations of the metal-poor blue compact dwarf galaxy SBS 0335-052E
We present two-dimensional spectroscopy of the extremely metal-deficient blue
compact dwarf (BCD) galaxy SBS 0335-052E aiming to studyphysical conditions,
element abundances and kinematical properties of the ionised gas in this
galaxy. Observations were obtained in the spectral range 3620-9400A with the
imaging spectrograph GIRAFFE installed on the UT2 of the Very Large Telescope
(VLT). These observations are the first ones carried out so far with GIRAFFE in
the ARGUS mode which allows to obtain simultaneously 308 spectra covering a
11.4"x7.3" region. We produced images of SBS 0335-052E in the continuum and in
emission lines of different stages of excitation. We find that while the
maximum of emission in the majority of lines, including the strong lines Hbeta
4861A, Halpha 6563A, [OIII] 4363,5007A, [OII] 3726,3729A, coincides with the
youngest south-eastern star clusters 1 and 2, the emission of HeII 4686A line
is offset to the more evolved north-west clusters 4,5. This suggests that hard
ionising radiation responsible for the HeII 4686A emission is not related to
the most massive youngest stars, but rather is connected with fast radiative
shocks. This conclusion is supported by the kinematical properties of the
ionised gas from the different emission lines as the velocity dispersion in the
HeII 4686A line is systematically higher, by ~50%-100%, than that in other
lines. The variations of the emission line profiles suggest the presence of an
ionised gas outflow in the direction perpendicular to the galaxy disk.
(abridged)Comment: 21 pages, 11 figures, accepted for publication in Astronomy and
Astrophysic
HST observations of the blue compact dwarf SBS 0335-052: a probable young galaxy
We present HST WFPC2 V and I images and GHRS UV spectrophotometry of the
spectral regions around Ly and OI 1302 of the extremely metal-deficient
(Z~Zsun/41) blue compact dwarf (BCD) galaxy SBS 0335-052. All the star
formation in the BCD occurs in six super-star clusters (SSC) with ages =< 3-4
Myr. Dust is clearly present and mixed spatially with the SSCs. There is a
supershell of radius ~380 pc, delineating a large supernova cavity. The
instantaneous star formation rate is ~0.4 Msun yr^-1. Strong narrow Ly
emission is not observed. Rather there is low intensity broad (FWZI = 20 A)
Ly emission superposed on even broader Ly absorption by the HI
envelope. This broad low-intensity emission is probably caused by resonant
scattering of Ly photons. The BCD appears to be a young galaxy,
undergoing its very first burst of star formation. This conclusion is based on
the following evidence: 1) the underlying extended low-surface-brightness
component is very irregular and filamentary, suggesting that a significant part
of the emission comes from ionized gas; 2) it has very blue colors (-0.34 =<
(V-I) =< 0.16), consistent with gaseous emission colors; 3) the OI 1302
line is not detected in absorption in the GHRS spectrum, setting an upper limit
for N(O)/N(H) in the HI envelope of the BCD of more than 3000 times smaller
than the value in Orion.Comment: 20 pages and 6 Postscript figures. Submitted to Astrophysical Journa
A New Approach to Systematic Uncertainties and Self-Consistency in Helium Abundance Determinations
Tests of big bang nucleosynthesis and early universe cosmology require
precision measurements for helium abundance determinations. However, efforts to
determine the primordial helium abundance via observations of metal poor H II
regions have been limited by significant uncertainties. This work builds upon
previous work by providing an updated and extended program in evaluating these
uncertainties. Procedural consistency is achieved by integrating the hydrogen
based reddening correction with the helium based abundance calculation, i.e.,
all physical parameters are solved for simultaneously. We include new atomic
data for helium recombination and collisional emission based upon recent work
by Porter et al. and wavelength dependent corrections to underlying absorption
are investigated. The set of physical parameters has been expanded here to
include the effects of neutral hydrogen collisional emission. Because of a
degeneracy between the solutions for density and temperature, the precision of
the helium abundance determinations is limited. Also, at lower temperatures (T
\lesssim 13,000 K) the neutral hydrogen fraction is poorly constrained
resulting in a larger uncertainty in the helium abundances. Thus the derived
errors on the helium abundances for individual objects are larger than those
typical of previous studies. The updated emissivities and neutral hydrogen
correction generally raise the abundance. From a regression to zero
metallicity, we find Y_p as 0.2561 \pm 0.0108, in broad agreement with the WMAP
result. Tests with synthetic data show a potential for distinct improvement,
via removal of underlying absorption, using higher resolution spectra. A small
bias in the abundance determination can be reduced significantly and the
calculated helium abundance error can be reduced by \sim 25%.Comment: 51 pages, 13 figure
On ionisation effects and abundance ratios in damped Lyman-alpha systems
The similarity between observed velocity structures of Al III and singly
ionised species in damped Lyman-alpha systems (DLAs) suggests the presence of
ionised gas in the regions where most metal absorption lines are formed.
To explore the possible implications of ionisation effects we construct a
simplified two-region model for DLAs consisting of an ionisation bounded region
with an internal radiation field and a neutral region with a lower metal
content. Within this framework we find that ionisation effects are important.
If taken into account, the element abundance ratios in DLAs are quite
consistent with those observed in Milky Way stars and in metal-poor H II
regions in blue compact dwarf galaxies. In particular we cannot exclude the
same primary N origin in both DLAs and metal-poor galaxies. From our models no
dust depletion of heavy elements needs to be invoked; little depletion is
however not excluded.Comment: to appear in "Evolution of Galaxies. I. Observational clues", Eds.
J.M. Vilchez, G. Stasinska, Astrophysics and Space Science, in press. 5
pages, including 3 figure
X-ray binaries as the origin of nebular HeII emission in low-metallicity star-forming galaxies
The origin of nebular HeII emission, which is frequently observed in
low-metallicity (O/H) star-forming galaxies, remains largely an unsolved
question. Using the observed anticorrelation of the integrated X-ray luminosity
per unit of star formation rate () of an X-ray binary population
with metallicity and other empirical data from the well-studied galaxy I Zw 18,
we show that the observed HeII 4686 intensity and its trend with metallicity is
naturally reproduced if the bulk of He ionizing photons are emitted by the
X-ray sources. We also show that a combination of X-ray binary population
models with normal single and/or binary stellar models reproduces the observed
intensities and its dependency on metallicity and age. We
conclude that both empirical data and theoretical models suggest that high-mass
X-ray binaries are the main source of nebular HeII emission in low-metallicity
star-forming galaxies.Comment: 5 pages, 3 figures. Accepted for publication in A&A Letter
Lyman-alpha spectral properties of five newly discovered Lyman continuum emitters
We have recently reported the discovery of five low redshift Lyman continuum
(LyC) emitters (LCEs, hereafter) with absolute escape fractions fesc(LyC)
ranging from 6 to 13%, higher than previously found, and which more than
doubles the number of low redshift LCEs.We use these observations to test
theoretical predictions about a link between the characteristics of the
Lyman-alpha (Lya) line from galaxies and the escape of ionising photons. We
analyse the Lya spectra of eight LCEs of the local Universe observed with the
Cosmic Origins Spectrograph onboard the Hubble Space Telescope (our five
leakers and three galaxies from the litterature), and compare their strengths
and shapes to the theoretical criteria and comparison samples of local
galaxies: the Lyman Alpha Reference Survey, Lyman Break Analogs, Green Peas,
and the high-redshift strong LyC leaker Ion2. Our LCEs are found to be strong
Lya emitters, with high equivalent widths, EW(Lya)> 70 {\AA}, and large Lya
escape fractions, fesc(Lya) > 20%. The Lya profiles are all double-peaked with
a small peak separation, in agreement with our theoretical expectations. They
also have no underlying absorption at the Lya position. All these
characteristics are very different from the Lya properties of typical
star-forming galaxies of the local Universe. A subset of the comparison samples
(2-3 Green Pea galaxies) share these extreme values, indicating that they could
also be leaking. We also find a strong correlation between the star formation
rate surface density and the escape fraction of ionising photons, indicating
that the compactness of star-forming regions plays a role in shaping low column
density paths in the interstellar medium of LCEs. The Lya properties of LCEs
are peculiar: Lya can be used as a reliable tracer of LyC escape from galaxies,
in complement to other indirect diagnostics proposed in the literature.Comment: 11 pages, 10 figures, accepted for publication in A&
Heating of blue compact dwarf galaxies: gas distribution and photoionization by stars in I Zw 18
Photoionization models so far are unable to account for the high electron
temperature Te([O III]) implied by the line ratio [O III]4363A/[O III]5007A in
low-metallicity blue compact dwarf galaxies, casting doubts on the assumption
of photoionization by hot stars as the dominant source of heating of the gas in
these objects. Combinations of runs of the 1-D photoionization code NEBU are
used to explore alternative models for the giant H II region shell I Zw 18 NW.
Acceptable models are obtained, which represent schematically an incomplete
shell comprising radiation-bounded condensations embedded in a low-density
matter-bounded diffuse medium. The thermal pressure contrast between gas
components is about a factor 7. The diffuse phase can be in pressure balance
with the hot superbubble fed by mechanical energy from the inner massive star
cluster. The failure of previous modellings is ascribed to (1) the adoption of
an inadequate small-scale gas density distribution, which proves critical when
the collisional excitation of hydrogen contributes significantly to the cooling
of the gas, and possibly (2) a too restrictive implementation of Wolf-Rayet
stars in synthetic stellar cluster spectral energy distributions. A neutral gas
component heated by soft X-rays, whose power is less than 1% of the star
cluster luminosity and consistent with CHANDRA data, can explain the
low-ionization fine-structure lines detected by SPITZER. [O/Fe] is slightly
smaller in I Zw 18 NW than in Galactic Halo stars of similar metallicity and
[C/O] is correlatively large. Extra heating by, e.g., dissipation of mechanical
energy is not required to explain Te([O III]) in I Zw 18. Important
astrophysical developments are at stakes in the 5% uncertainty attached to [O
III] collision strengths.Comment: 20 pages, 8 figures, to be published in A&
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