789 research outputs found
Starburst Intensity Limit of Galaxies at z~5-6
The peak star formation intensity in starburst galaxies does not vary
significantly from the local universe to redshift z~6. We arrive at this
conclusion through new surface brightness measurements of 47 starburst galaxies
at z~5-6, doubling the redshift range for such observations. These galaxies are
spectroscopically confirmed in the Hubble Ultra Deep Field (HUDF) through the
GRism ACS program for Extragalactic Science (GRAPES) project. The starburst
intensity limit for galaxies at z~5-6 agree with those at z~3-4 and z~0 to
within a factor of a few, after correcting for cosmological surface brightness
dimming and for dust. The most natural interpretation of this constancy over
cosmic time is that the same physical mechanisms limit starburst intensity at
all redshifts up to z~6 (be they galactic winds, gravitational instability, or
something else). We do see two trends with redshift: First, the UV spectral
slope of galaxies at z~5-6 is bluer than that of z~3 galaxies, suggesting an
increase in dust content over time. Second, the galaxy sizes from z~3 to z~6
scale approximately as the Hubble parameter 1/H(z). Thus, galaxies at z~6 are
high redshift starbursts, much like their local analogs except for slightly
bluer colors, smaller physical sizes, and correspondingly lower overall
luminosities. If we now assume a constant maximum star formation intensity, the
differences in observed surface brightness between z~0 and z~6 are consistent
with standard expanding cosmology and strongly inconsistent with tired light
model.Comment: Accepted for publication in ApJ (23 pages, 5 figures). Minor changes
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Development of software for analysing entry accelerometer data in preparation for the Beagle 2 mission to Mars: towards a publicly available toolkit
We have tested techniques for turning Beagle 2’s entry accelerometer data into a T(z) profile. We reproduced the PDS results for Pathfinder. The PDS trajectory for Pathfinder appears inconsistent with its entry state. Our code is available online
Hierarchical Character-Word Models for Language Identification
Social media messages' brevity and unconventional spelling pose a challenge
to language identification. We introduce a hierarchical model that learns
character and contextualized word-level representations for language
identification. Our method performs well against strong base- lines, and can
also reveal code-switching
Near-Infrared Survey of the GOODS-North Field: Search for Luminous Galaxy Candidates at z=>6.5
We present near-infrared (NIR; J & Ks) survey of the Great Observatories
Origins Deep Survey-North (GOODS-N) field. The publicly available imaging data
were obtained using the MOIRCS instrument on the 8.2m Subaru and the WIRCam
instrument on the 3.6m Canada-France-Hawaii Telescope (CFHT). These
observations fulfill a serious wavelength gap in the GOODS-N data - i.e., lack
of deep NIR observations. We combine the Subaru/MOIRCS and CFHT/WIRCam archival
data to generate deep J and Ks band images, covering the full GOODS-N field
(~169 sq. arcmin) to an AB magnitude limit of ~25 mag (3sigma). We applied
z'-band dropout color selection criteria, using the NIR data generated here. We
have identified two possible Lyman Break Galaxy (LBG) candidates at z\gtrsim6.5
with J\lesssim24.5. The first candidate is a likely LBG at z\sim6.5 based on a
weak spectral feature tentatively identified as Lyalpha line in the deep
Keck/DEIMOS spectrum, while the second candidate is a possible LBG at z\sim7
based on its photometric redshift. These z'-dropout objects, if confirmed, are
among the brightest such candidates found so far. At z\gtrsim6.5, their star
formation rate is estimated as 100-200 solar mass per year. If they continue to
form stars at this rate, they assemble a stellar mass of ~5x10^10 solar mass
after about 400 million years, becoming the progenitors of massive galaxies
observed at z\sim5. We study the implication of the z'-band dropout candidates
discovered here, in constraining the bright-end of the luminosity function and
understanding the nature of high redshift galaxies.Comment: ApJ in press, minor text/reference update
Galaxy Formation In The Reionization Epoch As Hinted By Wide Field Camera 3 Observations Of The Hubble Ultra Deep Field
We present a large sample of candidate galaxies at z~7--10, selected in the
HUDF using the new observations made by the HST/WFC3. Our sample is composed of
20 z-dropouts, 15 Y-dropouts, and 20 J-dropouts. The surface densities of the
z-dropouts are close to what predicted by earlier studies, however, those of
the Y- and J-dropouts are quite unexpected. While no Y- or J-dropouts have been
found at AB < 28.0 mag, their surface densities seem to increase sharply at
fainter levels. While some of these candidates seem to be close to foreground
galaxies and thus could possibly be gravitationally lensed, the overall surface
densities after excluding such cases are still much higher than what would be
expected if the luminosity function does not evolve from z~7 to 10. Motivated
by such steep increases, we tentatively propose a set of Schechter function
parameters to describe the LFs at z~8 and 10. As compared to their counterpart
at z~7, here L* decreases by ~ 6.5x and Phi* increases by 17--90x. Although
such parameters are not yet demanded by the existing observations, they are
allowed and seem to agree with the data better than other alternatives. If
these LFs are still valid beyond our current detection limit, this would imply
a sudden emergence of a large number of low-luminosity galaxies when looking
back in time to z~10, which, while seemingly exotic, would naturally fit in the
picture of the cosmic hydrogen reionization. These early galaxies could easily
account for the ionizing photon budget required by the reionization, and they
would imply that the global star formation rate density might start from a very
high value at z~10, rapidly reach the minimum at z~7, and start to rise again
towards z~6. In this scenario, the majority of the stellar mass that the
universe assembled through the reionization epoch seems still undetected by
current observations at z~6. [Abridged]Comment: accepted for publication in Research in Astronomy and Astrophysic
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