309 research outputs found
New Star Forming Galaxies at z\approx 7 from WFC3 Imaging
The addition of Wide Field Camera 3 (WFC3) on the Hubble Space Telescope
(HST) has led to a dramatic increase in our ability to study the z>6 Universe.
The increase in the near-infrared (NIR) sensitivity of WFC3 over previous
instruments has enabled us to reach apparent magnitudes approaching 29 (AB).
This allows us to probe the rest-frame ultraviolet (UV) continuum, redshifted
into the NIR at . Taking advantage of the large optical depths at this
redshift, resulting in the Lyman-alpha break, we use a combination of WFC3
imaging and pre-existing Advanced Camera for Surveys (ACS) imaging to search
for z approx 7 over 4 fields. Our analysis reveals 29 new z approx 7 star
forming galaxy candidates in addition to 16 pre-existing candidates already
discovered in these fields. The improved statistics from our doubling of the
robust sample of z-drop candidates confirms the previously observed evolution
of the bright end of the luminosity function.Comment: 15 pages, accepted in MNRA
Optical spectroscopy of two overlapping, flux-density-limited samples of radio sources in the North Ecliptic Cap, selected at 38 MHz and 151 MHz
We present the results of optical spectroscopy of two flux-density-limited
samples of radio sources selected at frequencies of 38 and 151 MHz in the same
region around the North Ecliptic Cap, the 8C-NEC and 7C-III samples
respectively. Both samples are selected at flux density levels ~20 times
fainter than samples based on the 3C catalogue. They are amongst the first
low-frequency selected samples with no spectral or angular size selection for
which almost complete redshift information has been obtained and they will
therefore provide a valuable resource for understanding the cosmic evolution of
radio sources and their hosts and environments. The 151-MHz 7C-III sample is
selected to have S_151 >=0.5 Jy and is the more spectroscopically complete; out
of 54 radio sources fairly reliable redshifts have been obtained for 44
objects. The 8C sample has a flux limit of S_38 >=1.3 Jy and contains 58
sources of which 46 have fairly reliable redshifts. We discuss possible biases
in the observed redshift distribution, and some interesting individual objects.
Using the 8C-NEC and 7C-III samples in conjunction, we form the first sample
selected on low-frequency flux in the rest-frame of the source, rather than the
usual selection on flux density in the observed frame. This allows us to remove
the bias associated with an increasing rest-frame selection frequency with
redshift. We investigate the difference this selection makes to correlations of
radio source properties with redshift and luminosity. We show in particular
that flux-density-based selection leads to an overestimate of the steepness of
the correlation of radio source size with redshift. (abridged)Comment: 22 pages, 10 figures, accepted by MNRA
A star-forming galaxy at z= 5.78 in the Chandra Deep Field South
We report the discovery of a luminous z = 5.78 star-forming galaxy in the Chandra Deep Field South. This galaxy was selected as an âi-dropâ from the GOODS public survey imaging with the Hubble Space Telescope/Advanced Camera for Surveys (object 3 in the work of Stanway, Bunker & McMahon 2003). The large colour of (iâČâzâČ)AB = 1.6 indicated a spectral break consistent with the Lyman α forest absorption shortward of Lyman α at zâ 6. The galaxy is very compact (marginally resolved with ACS with a half-light radius of 0.08 arcsec, so rhl 5. Our spectroscopic redshift for this object confirms the validity of the iâČ-drop technique of Stanway et al. to select star-forming galaxies atzâ 6
Serendipitously Detected Galaxies in the Hubble Deep Field
We present a catalog of 74 galaxies detected serendipitously during a
campaign of spectroscopic observations of the Hubble Deep Field North (HDF) and
its environs. Among the identified objects are five candidate Ly-alpha emitters
at z > 5, a galaxy cluster at z = 0.85, and a Chandra source with a heretofore
undetermined redshift of z = 2.011. We report redshifts for 25 galaxies in the
central HDF, 13 of which had no prior published spectroscopic redshift. Of the
remaining 49 galaxies, 30 are located in the single-orbit HDF Flanking Fields.
We discuss the redshift distribution of the serendipitous sample, which
contains galaxies in the range 0.10 < z < 5.77 with a median redshift of z =
0.85, and we present strong evidence for redshift clustering. By comparing our
spectroscopic redshifts to optical/IR photometric studies of the HDF, we find
that photometric redshifts are in most cases capable of producing reasonable
predictions of galaxy redshifts. Finally, we estimate the line-of-sight
velocity dispersion and the corresponding mass and expected X-ray luminosity of
the galaxy cluster, we present strong arguments for interpreting the Chandra
source as an obscured AGN, and we discuss in detail the spectrum of one of the
candidate z > 5 Ly-alpha emitters.Comment: 18 pages, 9 figures, accepted for publication in the Astronomical
Journa
Discovery of a Color-Selected Quasar at z=5.50
We present observations of RD J030117+002025, a quasar at z=5.50 discovered
from deep, multi-color, ground-based observations covering 74 square arcmin.
This is the most distant quasar or AGN currently known. The object was targeted
as an R-band dropout, with R(AB)>26.3 (3-sigma limit in a 3 arcsec diameter
region), I(AB)=23.8, and z(AB)=23.4. The Keck/LRIS spectrum shows broad
Lyman-alpha/NV emission and sharp absorption decrements from the
highly-redshifted hydrogen forests. The fractional continuum depression due to
the Lyman-alpha forest is D(A)=0.90. RD J030117+002025 is the least luminous,
high-redshift quasar known (M(B)~-22.7).Comment: 9 pages, 2 figures; to appear in the The Astrophysical Journal
Letter
Spectroscopy of z ~ 7 candidate galaxies: using Lyman α to constrain the neutral fraction of hydrogen in the high-redshift universe
Following our previous spectroscopic observations of z > 7 galaxies with Gemini/Gemini Near Infra-Red Spectrograph (GNIRS) and Very Large Telescope (VLT)/XSHOOTER, which targeted a total of eight objects, we present here our results from a deeper and larger VLT/FOcal Reducer and Spectrograph (FORS2) spectroscopic sample of Wide Field Camera 3 selected z > 7 candidate galaxies. With our FORS2 setup we cover the 737â1070ânm wavelength range, enabling a search for Lyman α in the redshift range spanning 5.06â7.80. We target 22 z-band dropouts and find no evidence of Lyman α emission, with the exception of a tentative detection (<5Ï, which is our adopted criterion for a secure detection) for one object. The upper limits on Lyman α flux and the broad-band magnitudes are used to constrain the rest-frame equivalent widths for this line emission. We analyse our FORS2 observations in combination with our previous GNIRS and XSHOOTER observations, and suggest that a simple model where the fraction of high rest-frame equivalent width emitters follows the trend seen at z = 3-6.5 is inconsistent with our non-detections at z ⌠7.8 at the 96 per cent confidence level. This may indicate that a significant neutral HâI fraction in the intergalactic medium suppresses Lyman α, with an estimated neutral fraction ÏHIâŒ0.5, in agreement with other estimates
A Serendipitous Search for High-Redshift Lyman alpha Emission: Two Primeval Galaxy Candidates at z~3
In the course of our ongoing search for serendipitous high-redshift Lyman
alpha (LyA) Emissionin deep archival Keck spectra, we discovered two very high
equivalent width (W_{obs} ~ 450A, 2-sigma) LyA emission line candidates at z ~3
in a moderate dispersion (R~1200) spectrogram. Both lines have low velocity
dispersions (sigma_v ~ 60 km/s) and deconvolved radii r ~ 1 kpc (h = 0.5). We
argue that the lines are LyA, and are powered by stellar ionization. The
surface density of robust, high equivalent width LyA candidates is estimated to
be ~3 \pm 2 per arcmin^2 per unit redshift at z ~ 3, consistent with the
estimate of Cowie etal (1998). The LyA emission line source characteristics are
consistent with the galaxies undergoing their first burst of star formation,
ie, with being primeval. Source sizes and velocity dispersions are comparable
to the theoretical primeval galaxy model of Lin and Murray (1992) based on the
inside-out, self-similar collapse of an isothermal sphere. In this model, star
formation among field galaxies is a protracted process. Galaxies are thought to
be able to display high equivalent widths for only the first few x 10 Myr. This
time is short in relation to the difference in look back times between z=3 and
z=4, and implies that a substantial fraction of strong line-emitting galaxies
at z=3 were formed at redshifts z < 4. We discuss the significance of
high-equivalent width LyA-emitting galaxies in terms of the emerging picture of
the environment, and the specific characteristics of primeval galaxy formation
at high redshift.Comment: 17 pages, 3 figures, one table. To appear in the Astrophysical
Journa
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