13 research outputs found
A Simple Prediction on the Surface Density of Galaxies at z ~ 6
Systematic surveys are being proposed to discover a significant number of
galaxies at z ~ 6, which is now suggested as the epoch when the reionization
era of the Universe ends. To plan such surveys, we need a reasonable
expectation of the surface density of high redshift galaxies at different flux
limits. Here we present a simple prediction of the surface density of 5.5 < z <
6.5 galaxies in the optical regime, extrapolating from what is already known
about galaxies at z ~ 3. This prediction is consistent with the results of
nearly all known searches for objects at z ~ 6, giving confidence that we may
use it to plan optimal combination of survey depth and sky coverage in
searching for such objects. We suggest that the most efficient strategy with
existing ground-based facilities is to do medium-depth (m_{AB}= 24.0 -- 24.5
mag), wide-field (a couple of square degrees) survey using a wide-field camera
at a 4m-class telescope. As the predicted surface density at this brightness
level is very sensitive to the value of L*, the result of such a survey can be
easily used to constrain the luminosity evolution from z ~ 3 to 6.Comment: Accepted for publication in the Astrophysical Journa
The Infrared Counterparts of the Optically Unidentified CDF-S 1Ms Sources
The Chandra Deep Field South (CDF-S) 1Ms exposure produced a catalog of 346
X-ray sources, of which 59 were not visible on the VLT/FORS1 and the
ESO-MPI/WFI deep R-band images to a limit of R_{vega}=26.1--26.7 mag. Using the
first release of the ESO VLT/ISAAC JHK_s data on the CDF-S, we identified six
of the twelve such objects that were within the coverage of these IR
observations. The VLT/FORS1 I-band data further confirms that five of these six
objects are undetected in the optical. The photometric properties of these six
counterparts are compared against those of the optically brighter counterparts
of Chandra sources in the same field. We found that the location of these
optically brighter Chandra sources in the near-IR color space was bifurcated,
with the color of one branch being consistent with that of E/S0 galaxies at 0
<= z <= 1.5 and the other branch being consistent with that of unreddened
AGN/QSOs at 0 <= z <= 3.5. The six counterparts that we identified seemed to
lie on the E/S0 branch and its extension, suggesting that these X-ray source
hosts are mostly luminous E/S0 galaxies (M_V ~ -20 mag in AB system) at 1 <= z
<= 2.5. On the other hand, some of them can also be explained by AGN/QSOs over
a wide redshift range (0 <= z <= 5) if a range of internal extinction (A_V=0--1
mag) is allowed. However, the later interpretation requires fine-tuning
extinction together with redshift for these objects individually. If they are
indeed AGN/QSOs, the most luminous of them is just barely qualified for being a
QSO. Finally, we point out that neither high-redshift (z > 5) star-forming
galaxies nor irregular galaxies at lower redshift can be a viable explanation
to the nature of these six counterparts.Comment: Accepted for publication in Ap
UV-Optical Pixel Maps of Face-On Spiral Galaxies -- Clues for Dynamics and Star Formation Histories
UV and optical images of the face-on spiral galaxies NGC 6753 and NGC 6782
reveal regions of strong on-going star formation that are associated with
structures traced by the old stellar populations. We make NUV--(NUV-I) pixel
color-magnitude diagrams (pCMDs) that reveal plumes of pixels with strongly
varying NUV surface brightness and nearly constant I surface brightness. The
plumes correspond to sharply bounded radial ranges, with (NUV-I) at a given NUV
surface brightness being bluer at larger radii. The plumes are parallel to the
reddening vector and simple model mixtures of young and old populations, thus
neither reddening nor the fraction of the young population can produce the
observed separation between the plumes. The images, radial surface-brightness,
and color plots indicate that the separate plumes are caused by sharp declines
in the surface densities of the old populations at radii corresponding to disk
resonances. The maximum surface brightness of the NUV light remains nearly
constant with radius, while the maximum I surface brightness declines sharply
with radius. An MUV image of NGC 6782 shows emission from the nuclear ring. The
distribution of points in an (MUV-NUV) vs. (NUV-I) pixel color-color diagram is
broadly consistent with the simple mixture model, but shows a residual trend
that the bluest pixels in (MUV-NUV) are the reddest pixels in (NUV-I). This may
be due to a combination of red continuum from late-type supergiants and [SIII]
emission lines associated with HII regions in active star-forming regions. We
have shown that pixel mapping is a powerful tool for studying the distribution
and strength of on-going star formation in galaxies. Deep, multi-color imaging
can extend this to studies of extinction, and the ages and metallicities of
composite stellar populations in nearby galaxies.Comment: LaTeX with AASTeX style file, 29 pages with 12 figures (some color,
some multi-part). Accepted for publication in The Astrophysical Journa
An HST Survey of the mid-UV Morphology of Nearby Galaxies
(Abbreviated) We present an imaging survey of 37 nearby galaxies observed
with HST/WFPC2 in the mid-UV F300W filter and in F814W. 11 galaxies were also
imaged in F255W. These galaxies were selected to be detectable with WFPC2 in
one orbit, and cover a wide range of Hubble types and inclinations. The mid-UV
spans the gap between our groundbased optical/NIR images and far-UV images
available from the Astro/UIT missions. Our first qualitative results are:
(1) Early-type galaxies show a significant decrease in surface brightness
going from the red to the mid-UV, and in some cases the presence of dust lanes.
Some galaxies would be classified different when viewed in the mid-UV, some
become dominated by a blue nuclear feature or point source.
(2) Half of the mid-type spiral and star-forming galaxies appear as a later
morphological type in the mid-UV, as Astro/UIT also found in the far-UV. Some-
times these differences are dramatic. The mid-UV images show a considerable
range in the scale and surface brightness of individual star-forming regions.
Almost all mid-type spirals have their small bulges bi-sected by a dust-lane.
(3) Most of the heterogeneous subset of late-type, irregular, peculiar, and
merging galaxies display F300W morphologies that are similar to those seen in
F814W, but with differences due to recognizable dust features absorbing the
bluer light, and due to UV-bright hot stars, star-clusters, and star-forming
ridges.
In the rest-frame mid-UV, early- to mid-type galaxies are more likely to be
misclassified as later types than vice versa. This morphological K-correction
explains only part of the excess faint blue galaxies seen in deep HST fields.Comment: 30 pages, LateX (AASTeX5.0), 2 figures and 3 tables included, mid-UV
atlas and pan-chromatic atlas provided as 63 JPG figures. Full resolution PS
version (~100Mb) available upon request. Accepted for publication in ApJ
Variation in Dube3a expression affects neurotransmission at the Drosophila neuromuscular junction
Changes in UBE3A expression levels in neurons can cause neurogenetic disorders ranging from Angelman syndrome (AS) (decreased levels) to autism (increased levels). Here we investigated the effects on neuronal function of varying UBE3A levels using the Drosophila neuromuscular junction as a model for both of these neurogenetic disorders. Stimulations that evoked excitatory junction potentials (EJPs) at 1 Hz intermittently failed to evoke EJPs at 15 Hz in a significantly higher proportion of Dube3a over-expressors using the pan neuronal GAL4 driver C155-GAL4 (C155-GAL4>UAS-Dube3a) relative to controls (C155>+ alone). However, in the Dube3a over-expressing larval neurons with no failures, there was no difference in EJP amplitude at the beginning of the train, or the rate of decrease in EJP amplitude over the course of the train compared to controls. In the absence of tetrodotoxin (TTX), spontaneous EJPs were observed in significantly more C155-GAL4>UAS-Dube3a larva compared to controls. In the presence of TTX, spontaneous and evoked EJPs were completely blocked and mEJP amplitude and frequency did not differ among genotypes. These data suggest that over-expression of wild type Dube3a, but not a ubiquitination defective Dube3a-C/A protein, compromises the ability of motor neuron axons to support closely spaced trains of action potentials, while at the same time increasing excitability. EJPs evoked at 15 Hz in the absence of Dube3a (Dube3a15b homozygous mutant larvae) decayed more rapidly over the course of 30 stimulations compared to w1118 controls, and Dube3a15b larval muscles had significantly more negative resting membrane potentials (RMP). However, these results could not be recapitulated using RNAi knockdown of Dube3a in muscle or neurons alone, suggesting more global developmental defects contribute to this phenotype. These data suggest that reduced UBE3A expression levels may cause global changes that affect RMP and neurotransmitter release from motorneurons at the neuromuscular junction. Similar affects of under- and over-expression of UBE3A on membrane potential and synaptic transmission may underlie the synaptic plasticity defects observed in both AS and autism