98 research outputs found
Faintest Galaxy Morphologies from WFPC2 Imaging of the Hawaii Survey Fields
We present very deep WFPC2 images in the F814W filter of two Hawaii
Survey fields, SSA13 and SSA22. Using these data with previous ground-based
imaging and spectroscopy, we compare the colors, star-forming properties and
morphologies of the faintest galaxies with a reference sample of bright nearby
galaxies and analyze the changes in field galaxy morphology with magnitude. Our
principal result is the identification of a new morphological class of
``chain'' galaxies at the faintest magnitudes. Based on limited spectroscopy,
we tentatively conclude that these are linearly organized giant star-forming
regions at and, if this is correct, that these are large galaxies
in the process of formation.Comment: 18 pages + 1 table of text as 1 LaTeX file (uses aastex style macros:
aaspp.sty, flushrt.sty) plus 1 uuencoded compressed tar file of 12 PostScript
figures (Figs. 3-9, 16-17, and 21-23). The remaining gray-scale plots are
available by anonymous ftp at
ftp://hubble.ifa.hawaii.edu/pub/preprints/plates To appear in the October
1995 Astronomical Journa
Metal Enrichment and Ionization Balance in the Lyman Forest at
The recent discovery of carbon in close to half of the low neutral hydrogen
column density [N({\rm H~I}) > 3\ten{14}\cm2] Lyman forest clouds toward quasars has challenged the widely held view of this forest as a
chemically pristine population uniformly distributed in the intergalactic
medium, but has not eliminated the possibility that a primordial population
might be present as well. Using extremely high signal-to-noise observations of
a sample of quasars we now show that \ion{C}{4} can be found in 75% of clouds
with N({\rm H~I}) > 3\ten{14}\cm2 and more than 90% of those with N({\rm
H~I}) > 1.6\ten{15}\cm2. Clouds with N({\rm H~I}) > 10^{15}\cm2 show a
narrow range of ionization ratios, spanning less than an order of magnitude in
\ion{C}{4}/\ion{H}{1}, \ion{C}{2}/\ion{C}{4}, \ion{Si}{4}/\ion{C}{4} and
\ion{N}{5}/\ion{C}{4}, and their line widths require that they be photoionized
rather than collisionally ionized. This in turn implies that the systems have a
spread of less than an order of magnitude in both volume density and
metallicity. Carbon is seen to have a typical abundance of very approximately
of solar and Si/C about three times solar, so that the chemical
abundances of these clouds are very similar to those of Galactic halo stars.
\ion{Si}{4}/\ion{C}{4} decreases rapidly with redshift from high values () at , a circumstance which we interpret as a change in the
ionizing spectrum as the intergalactic medium becomes optically thin to He\
ionizing photons. Weak clustering is seen in the \ion{C}{4} systems for \Delta
v < 250\kms, which we argue provides an upper limit to the clustering of
\ion{H}{1} clouds. If the clouds are associated with galaxies, this requires a
rapid evolution in galaxy clustering between and .Comment: 31 pages plus 5 tables, 21 Postscript figures, Figures 1 and 2
available at http://www.ifa.hawaii.edu/faculty/acowie/igm_aj.html . To be
published in Astronomical Journa
Star Formation History since z = 1.5 as Inferred from Rest-Frame Ultaviolet Luminosity Density Evolution
We investigate the evolution of the universal rest-frame ultraviolet
luminosity density from z = 1.5 to the present. We analyze an extensive sample
of multicolor data (U', B, V = 24.5) plus spectroscopic redshifts from the
Hawaii Survey Fields and the Hubble Deep Field. Our multicolor data allow us to
select our sample in the rest-frame ultraviolet (2500 angstrom) over the entire
redshift range to z = 1.5. We conclude that the evolution in the luminosity
density is a function of the form (1+z)^{1.7\pm1.0} for a flat lambda cosmology
and (1+z)^{2.4\pm1.0} for an Einstein-de Sitter cosmology.Comment: 12 pages, 7 figs, 5 tables, submitted to A
An Extremely Luminous Galaxy at z=5.74
We report the discovery of an extremely luminous galaxy lying at a redshift
of z=5.74, SSA22-HCM1. The object was found in narrowband imaging of the SSA22
field using a 105 Angstrom bandpass filter centered at 8185 Angstroms during
the course of the Hawaii narrowband survey using LRIS on the 10 m Keck II
Telescope, and was identified by the equivalent width of the emission
W_lambda(observed)=175 Angstroms, flux = 1.7 x 10^{-17} erg cm^{-2} s^{-1}).
Comparison with broadband colors shows the presence of an extremely strong
break (> 4.2 at the 2 sigma level) between the Z band above the line, where the
AB magnitude is 25.5, and the R band below, where the object is no longer
visible at a 2 sigma upper limit of 27.1 (AB mags). These properties are only
consistent with this object's being a high-z Ly alpha emitter. A 10,800 s
spectrum obtained with LRIS yields a redshift of 5.74. The object is similar in
its continuum shape, line properties, and observed equivalent width to the
z=5.60 galaxy, HDF 4-473.0, as recently described by Weymann et al. (1998), but
is 2-3 times more luminous in the line and in the red continuum. For H_0 = 65
km s^{-1} Mpc^{-1} and q_0 = (0.02, 0.5) we would require star formation rates
of around (40, 7) solar masses per year to produce the UV continuum in the
absence of extinction.Comment: 5 pages, 4 figures, Latex with emulateapj style file; to appear in
the Astrophysical Journal (Letters
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