37 research outputs found
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
SPICES II. Optical and Near-Infrared Identifications of Faint X-Ray Sources from Deep Chandra Observations of Lynx
We present our first results on field X-ray sources detected in a deep, 184.7
ks observation with the ACIS-I camera on Chandra. The observations target the
Lynx field of SPICES, and contains three known X-ray-emitting clusters out to
z=1.27. Not including the known clusters, in the 17'x17' ACIS-I field we detect
132 sources in the 0.5-2 keV (soft) X-ray band down to a limiting flux of
\~1.7e-16 erg/cm2/s and 111 sources in the 2-10 keV (hard) X-ray band down to a
limiting flux of ~1.3e-15 erg/cm2/s. The combined catalog contains a total of
153 sources, of which 42 are detected only in the soft band and 21 are detected
only in the hard band. Confirming previous Chandra results, we find that the
fainter sources have harder X-ray spectra, providing a consistent solution to
the long-standing `spectral paradox'. From deep optical and near-infrared
follow-up data, 77% of the X-ray sources have optical counterparts to I=24 and
71% of the X-ray sources have near-infrared counterparts to K=20. Four of the
24 sources in the near-IR field are associated with extremely red objects
(EROs; I-K>4). We have obtained spectroscopic redshifts with the Keck
telescopes of 18 of the Lynx Chandra sources. These sources comprise a mix of
broad-lined active galaxies, apparently normal galaxies, and two late-type
Galactic dwarfs. Intriguingly, one Galactic source is identified with an M7
dwarf exhibiting non-transient, hard X-ray emission. We review non-AGN
mechanisms to produce X-ray emission and discuss properties of the Lynx Chandra
sample in relation to other samples of X-ray and non-X-ray sources.Comment: 42 pages, 16 figures. Accepted for publication in the May 2002
Astronomical Journa
A Galaxy at z=5.34
We report the discovery of Lyman alpha emission from a galaxy at z=5.34, the
first object at z>5 with a spectroscopically confirmed redshift. The faint
continuum emission (m(8000A)=27 AB mag), relatively small rest-frame equivalent
width of the emission line (rest equivalent width of 95A), and limits on the
NV/Lya ratio suggest that this is a star-forming galaxy and not an AGN. The
star-formation rates implied by the UV continuum emission and the Lyman alpha
emission are (in the absence of dust extinction) fairly modest (about 6 Msun/yr
for q0=0.5, H0=50). The continuum luminosity is similar to that of sub-L*(1500)
star-forming galaxies at z~3, and the width of the Lyman alpha line yields an
upper limit to the mass of < 2.6 x 10^{10} Msun. The strong emission line
detected in this low-luminosity galaxy provides hope for the discovery of
higher luminosity primeval galaxies at redshifts z>5.Comment: 13 pages, 3 postscript figures; Accepted for publication in The
Astrophysical Journal Letter
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