496 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
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
The Distribution of Column Densities and b Values in the Lyman-Alpha Forest
We describe the properties of the Ly forest in the column density
range \nhi \geq 2\times10^{12} cm based on 1056 lines in the
wavelength range 4300--5100\AA measured in extremely high S/N,
spectra of four quasars. The column density distribution is well described by a
-1.5 power law to cm, below which limit confusion
becomes too severe to measure a spectrum of individual clouds. The distribution
of values shows a well-defined lower envelope with a cutoff at b=20\kms
corresponding to a cloud temperature of 24,000 K. There is only a very small
fraction (less than 1%) of narrow line clouds which cannot be identified with
metal-lines. From modeling the Ly absorption lines as complexes of
clouds each with thermal broadening corresponding to we find the
distribution can be understood if there is a mean of 3.25 clouds per absorption
line with a spread in velocity centroids characterized by a dispersion of
10.75\kms.Comment: 17 pages + 3 tables of text as 1 LaTeX file (uses aastex version 4
style macros: aaspp4.sty, flushrt) plus 1 uuencoded compressed tar file of 7
PostScript figures. Appendix tables and figures [complete spectra and line
lists (~1.6 Mb uncompressed)] are available by anonymous ftp at
ftp://hubble.ifa.hawaii.edu/pub/preprints. To appear in the October 1995
Astronomical Journa
The Density of Lyman-alpha Emitters at Very High Redshift
We describe narrowband and spectroscopic searches for emission-line star
forming galaxies in the redshift range 3 to 6 with the 10 m Keck II Telescope.
These searches yield a substantial population of objects with only a single
strong (equivalent width >> 100 Angstrom) emission line, lying in the 4000 -
10,000 Angstrom range. Spectra of the objects found in narrowband-selected
samples at lambda ~5390 Angstroms and ~6741 Angstroms show that these very high
equivalent width emission lines are generally redshifted Lyman alpha 1216
Angstrom at z~3.4 and 4.5. The density of these emitters above the 5 sigma
detection limit of 1.5 e-17 ergs/cm^2/s is roughly 15,000 per square degree per
unit redshift interval at both z~3.4 and 4.5. A complementary deeper (1 sigma
\~1.0 e-18 ergs/cm^2/s) slit spectroscopic search covering a wide redshift
range but a more limited spatial area (200 square arcminutes) shows such
objects can be found over the redshift range 3 to 6, with the currently highest
redshift detected being at z=5.64. The Lyman alpha flux distribution can be
used to estimate a minimum star formation rate in the absence of reddening of
roughly 0.01 solar masses/Mpc^3/year (H_0 = 65 km/s/Mpc and q_0 = 0.5).
Corrections for reddening are likely to be no larger than a factor of two,
since observed equivalent widths are close to the maximum values obtainable
from ionization by a massive star population. Within the still significant
uncertainties, the star formation rate from the Lyman alpha-selected sample is
comparable to that of the color-break-selected samples at z~3, but may
represent an increasing fraction of the total rates at higher redshifts. This
higher-z population can be readily studied with large ground-based telescopes.Comment: 7 pages, 5 encapsulated figures; aastex, emulateapj, psfig and lscape
style files. Separate gif files for 2 gray-scale images also available at
http://www.ifa.hawaii.edu/faculty/hu/emitters.html . Added discussion of
foreground contaminants. Updated discussion of comparison with external
surveys (Sec. 5 and Fig. 5). Note: continuum break strength limits (Fig. 3
caption) are correct here -- published ApJL text has a sign erro
Hawaii quasar and T dwarf survey. I. Method and discovery of faint field ultracool dwarfs
The Hawaii Quasar and T dwarf survey (HQT Survey) is a wide-field, red optical survey carried out with the
Suprime-Cam mosaic CCD camera on the 8.2 m Subaru telescope. The HQT survey is designed to search for
low-luminosity (M_(AB1450) 5.7) as well as T dwarfs, both of which are selected
by their very red I â z' colors. We use an optical narrowband filter NB816 to break a well-known I â z' color degeneracy between high-z quasars and foreground M and L dwarfs, which are more numerous than quasars.
This paper is the first in a series of papers from the HQT survey and we report on the discovery of six faint
(19 †J †20) ultracool dwarfs found over a ~9.3 deg^2 area with a limiting magnitude of z'_(AB) †23.3. These
dwarfs were confirmed by near-IR imaging and/or spectroscopy conducted at various facilities on Mauna Kea.
With estimated distances of 60â170 pc, these are among the most distant spectroscopically confirmed field brown
dwarfs to date. Limits on the proper motions of these ultracool dwarfs suggest that they are old members of the
Galactic disk, though future follow-up observations are necessary to minimize errors. Our finding rate of ultracool
dwarfs is within model predictions of Liu et al. However, the large brightening amplitude (~1 mag) previously
reported for the L/T transition objects appears to overpredict the numbers. We also examine how the survey field latitude affects the survey sensitivity to the vertical scale height of ultracool dwarfs
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