417 research outputs found
Deep spectroscopy of the low-metallicity blue compact dwarf galaxy SBS 0335-052
The results of deep long-slit spectroscopy of the extremely low-metallicity
blue compact dwarf (BCD) galaxy SBS 0335-052 are presented. Down to intensity
levels of 10^{-3 ... -4} of Hbeta, unprecedented for spectroscopy of
extra-galactic giant H II regions, we detect numerous weak permitted and
forbidden nebular lines in the brightest part of the galaxy. With varying
degrees of confidence, the detections include lines of high-ionization ions
like Fe^{4+} -Fe^{6+}, implying very hard ionizing radiation. Two broad
emission features, possibly from Wolf-Rayet stars, and stellar He II 4200
absorption are seen in the same region. The large spatial extent of He II 4686
emission (implying the presence of sufficient ionizing photons with energies
above 54 eV) and the spatial distribution of the electron temperature suggest
that at least some part of the hard radiation is associated with shocks.
Extended Halpha emission is detected over ~ 6 - 8 kpc, a much larger area than
in previous studies, suggesting that hot ionized gas is spread out far away
from the central ionizing clusters. This shows that nebular line and continuous
emission can significantly modify the colours of these extended regions and
must be taken into account in studies of the underlying stellar population.Comment: 5 pages, 3 Postscript figures, accepted for publication in the
Astronomy and Astrophysic
The He abundance in the metal-deficient blue compact dwarf galaxies Tol 1214-277 and Tol 65
We present high-quality Keck telescope spectroscopic observations of the two
metal-deficient blue compact dwarf (BCD) galaxies Tol 1214-277 and Tol 65.
These data are used to derive the heavy-element and helium abundances. We find
that the oxygen abundances in Tol 1214-277 and Tol 65 are the same,
12+logO/H=7.54+/-0.01, or Zsun/24, despite the different ionization conditions
in these galaxies. The nitrogen-to-oxygen abundance ratio in both galaxies is
logN/O=-1.64+/-0.02 and lies in the narrow range found for the other most
metal-deficient BCDs. We use the five strongest HeI emission lines 3889, 4471,
5876, 6678 and 7065, to correct self-consistently their intensities for
collisional and fluorescent enhancement mechanisms and to derive the He
abundance. Underlying stellar absorption is found to be important for the HeI
4471 emission line in both galaxies, being larger in Tol 65. The weighted He
mass fractions in Tol 1214-277 and Tol 65 are respectively Y=0.2458+/-0.0039
and 0.2410+/-0.0050 when the three HeI emission lines, 4471, 5876 and 6678, are
used, and are, respectively, 0.2466+/-0.0043 and 0.2463+/-0.0057 when the HeI
4471 emission line is excluded. These values are in very good agreement with
recent measurements of the He mass fraction in others of the most
metal-deficient BCDs by Izotov and coworkers. We find that the combined effect
of the systematic uncertainties due to the underlying HeI stellar absorption
lines, ionization and temperature structure of the HII region and collisional
excitation of the hydrogen emission lines is likely small, not exceeding ~2%
(the error is 2sigma). Our results support the validity of the standard big
bang model of nucleosynthesis.Comment: 22 pages, 3 Postscript figures, accepted for publication in the
Astrophysical Journa
KECK HIRES Spectroscopy of APM 08279+5255
With an optical R-band magnitude of 15.2, the recently discovered z=3.911 BAL
quasar APM 08279+5255 is an exceptionally bright high redshift source. Its
brightness has allowed us to acquire a high signal-to-noise ratio (~100), high
resolution (~6 km/s) spectrum using the HIRES echelle spectrograph on the 10-m
Keck I telescope. Given the quality of the data, these observations provide an
unprecedented view of associated and intervening absorption systems. Here we
announce the availability of this spectrum to the general astronomical
community and present a brief analysis of some of its main features.Comment: 21 pages including 5 figures. Accepted for publication by PAS
C, N, O Abundances in the Most Metal-Poor Damped Lyman alpha Systems
This study focuses on some of the most metal-poor damped Lyman alpha
absorbers known in the spectra of high redshift QSOs, using new and archival
observations obtained with UV-sensitive echelle spectrographs on the Keck and
VLT telescopes. The weakness and simple velocity structure of the absorption
lines in these systems allows us to measure the abundances of several elements,
and in particular those of C, N, and O, a group that is difficult to study in
DLAs of more typical metallicities. We find that when the oxygen abundance is
less than about 1/100 of solar, the C/O ratio in high redshift DLAs and
sub-DLAs matches that of halo stars of similar metallicity and shows higher
values than expected from galactic chemical evolution models based on
conventional stellar yields. Furthermore, there are indications that at these
low metallicities the N/O ratio may also be above simple expectations and may
exhibit a minimum value, as proposed by Centurion and her collaborators in
2003. Both results can be interpreted as evidence for enhanced production of C
and N by massive stars in the first few episodes of star formation, in our
Galaxy and in the distant proto-galaxies seen as QSO absorbers. The higher
stellar yields implied may have an origin in stellar rotation which promotes
mixing in the stars' interiors, as considered in some recent model
calculations. We briefly discuss the relevance of these results to current
ideas on the origin of metals in the intergalactic medium and the universality
of the stellar initial mass function.Comment: 17 pages, 9 Figures, Accepted for publication in Monthly Notices of
the Royal Astronomical Societ
The Radio Afterglow and the Host Galaxy of the X-Ray Rich GRB 981226
We report the discovery of a radio transient VLA 232937.2-235553, coincident
with the proposed X-ray afterglow for the gamma-ray burst GRB 981226. This GRB
has the highest ratio of X-ray to gamma-ray fluence of all the GRBs detected by
BeppoSAX so far and yet no corresponding optical transient was detected. The
radio light curve of VLA 232937.2-235553 is qualitatively similar to that of
several other radio afterglows. At the sub-arcsecond position provided by the
radio detection, optical imaging reveals an extended R=24.9 mag object, which
we identify as the host galaxy of GRB 981226. Afterglow models which invoke a
jet-like geometry for the outflow or require an ambient medium with a radial
density dependence, such as that produced by a wind from a massive star, are
both consistent with the radio data. Furthermore, we show that the observed
properties of the radio afterglow can explain the absence of an optical
transient without the need for large extinction local to the GRB.Comment: Accepted for publication in the Astrophysical Journal Letters.
Thirteen pages. Three Postscript figure
A spectroscopic study of component C and the extended emission around I Zw 18
Long-slit Keck II, 4m Kitt Peak, and 4.5m MMT spectrophotometric data are
used to investigate the stellar population and the evolutionary status of I Zw
18C, the faint C component of the nearby blue compact dwarf galaxy I Zw 18.
Hydrogen H and H emission lines are detected in the spectra of I
Zw 18C, implying that ionizing massive stars are present. High signal-to-noise
Keck II spectra of different regions in I Zw 18C reveal H, H
and higher order hydrogen lines in absorption. Several techniques are used to
constrain the age of the stellar population in I Zw 18C. Ages derived from two
different methods, one based on the equivalent widths of the H,
H emission lines and the other on H, H absorption lines
are consistent with a 15 Myr instantaneous burst model. We find that a small
extinction in the range = 0.20 -- 0.65 mag is needed to fit the observed
spectral energy distribution of I Zw 18C with that model. In the case of
constant star formation, all observed properties are consistent with stars
forming continuously between ~ 10 Myr and < 100 Myr ago. We use all available
observational constraints for I Zw 18C, including those obtained from Hubble
Space Telescope color-magnitude diagrams, to argue that the distance to I Zw 18
should be as high as ~ 15 Mpc. The deep spectra also reveal extended ionized
gas emission around I Zw 18. H emission is detected as far as 30" from
it. To a B surface brightness limit of ~ 27 mag arcsec we find no
observational evidence for extended stellar emission in the outermost regions,
at distances > 15" from I Zw 18.Comment: 38 pages, 11 Postscript figures, accepted for publication in the
Astrophysical Journa
The Metallicity of the Redshift 4.16 Quasar BR2248-1242
We estimate the metallicity in the broad emission-line region of the redshift
z=4.16 quasar, BR2248-1242, by comparing line ratios involving nitrogen to
theoretical predictions. BR2248-1242 has unusually narrow emission lines with
large equivalent widths, thus providing a rare opportunity to measure several
line-ratio abundance diagnostics. The combined diagnostics indicate a
metallicity of ~2 times solar. This result suggests that an episode of vigorous
star formation occurred near BR2248-1242 prior to the observed z=4.16 epoch.
The time available for this enrichment episode is only ~1.5 Gyr at z=4.16 (for
H_{0}=65 km s^-1 Mpc^-1, Omega_{m}=0.3 and Omega_Lambda ~< 1). This evidence
for high metallicities and rapid star formation is consistent with the expected
early-epoch evolution of dense galactic nuclei.Comment: 8 pages, 3 figures. Prepared in AAStex. Submitted to the
Astrophysical Journal Revised version: added 1 referenc
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