40 research outputs found
Factors that affect the degree of axonal regeneration following spinal cord transection in larval lamprey
Abstract only availableIn all vertebrate animals, reticulospinal (RS) neurons in a locomotor “command” systems in the brain send neural processes to the spinal cord to activate “central pattern generators” (CPGs) and initiate locomotor behavior. Unlike higher vertebrates, such as birds and mammals, lower vertebrates such as lamprey can behaviorally recover following spinal cord injury because axons from brain neurons regenerate and reconnect with neurons in the spinal CPGs. Although the lamprey can behaviorally recover following spinal cord transections, most injured axons regenerate for relatively short distances below the lesion. Thus, axonal regeneration is incomplete, possibly because regenerating axons make synapses just below the lesion, and these synapses might provide factors that suppress further regeneration. We hypothesize that following spinal cord transection, the degree to which axons of RS neurons are stimulated to regenerate is determined, in part, by the numbers of synapses these neurons have made, either above or below the lesion. Preliminary results indicate that rostral spinal cord transections are a “strong” stimulus for regeneration, while caudal spinal cord transections are a relatively “weak” stimulus. The purpose of the present study was to determine the degree to which the above “strong” or “weak” axonal regeneration could initiate locomotor activity below the lesion. Spinal cord transections were made at 10% body length (BL, normalized distance from the head; “rostral” lesion) or 50% BL (“caudal” lesion) in larval lamprey. After various recovery times, muscle activity (EMGs) was recorded below each of the two types of lesions to determine the degree of recovery of locomotor activity. First, 2 weeks after rostral spinal cord transections, animals could produce weak swimming movements, and muscle activity was present just below the lesion. With increasing recovery times muscle activity could be recorded at progressively lower levels of the body. Second, 2 weeks after caudal spinal cord transections, muscle activity was present above the lesion, but did not appear just below the lesion until ~8 weeks after the lesion. The results indicate that following rostral (caudal) spinal cord lesions, axonal regeneration from RS neurons is robust (weak), and locomotor behavior recovers relatively quickly (slowly). These results support our hypothesis and suggest that factors associated with the number of synapses are involved in controlling regeneration. Identification of these factors might be used to modulate axonal regeneration following spinal cord injury, especially in animals where regeneration is limited, such as humans.NSF-REU Biology & Biochemistr
A CANDELS WFC3 Grism Study of Emission-Line Galaxies at z~2: A Mix of Nuclear Activity and Low-Metallicity Star Formation
We present Hubble Space Telescope Wide Field Camera 3 slitless grism
spectroscopy of 28 emission-line galaxies at z~2, in the GOODS-S region of the
Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS). The
high sensitivity of these grism observations, with 1-sigma detections of
emission lines to f > 2.5x10^{-18} erg/s/cm^2, means that the galaxies in the
sample are typically ~7 times less massive (median M_* = 10^{9.5} M_sun) than
previously studied z~2 emission-line galaxies. Despite their lower mass, the
galaxies have OIII/Hb ratios which are very similar to previously studied z~2
galaxies and much higher than the typical emission-line ratios of local
galaxies. The WFC3 grism allows for unique studies of spatial gradients in
emission lines, and we stack the two-dimensional spectra of the galaxies for
this purpose. In the stacked data the OIII emission line is more spatially
concentrated than the Hb emission line with 98.1 confidence. We additionally
stack the X-ray data (all sources are individually undetected), and find that
the average L(OIII)/L(0.5-10 keV) ratio is intermediate between typical z~0
obscured active galaxies and star-forming galaxies. Together the compactness of
the stacked OIII spatial profile and the stacked X-ray data suggest that at
least some of these low-mass, low-metallicity galaxies harbor weak active
galactic nuclei.Comment: ApJ accepted. 8 pages, 6 figure
The Landsat Data Continuity Mission Operational Land Imager (OLI) Sensor
The Landsat Data Continuity Mission (LDCM) is being developed by NASA and USGS and is currently planned for launch in January 2013 [1]. Once on-orbit and checked out, it will be operated by USGS and officially named Landsat-8. Two sensors will be on LDCM: the Operational Land Imager (OLI), which has been built and delivered by Ball Aerospace & Technology Corp (BATC) and the Thermal Infrared Sensor (TIRS)[2], currently being built and tested at Goddard Space Flight Center (GSFC) with a planned delivery of Winter 2012. The OLI covers the Visible, Near-IR (NIR) and Short-Wave Infrared (SWIR) parts of the spectrum; TIRS covers the Thermal Infrared (TIR). This paper discusses only the OLI instrument and its pre-launch characterization; a companion paper covers TIRS
AEGIS: Demographics of X-ray and Optically Selected AGNs
We develop a new diagnostic method to classify galaxies into AGN hosts,
star-forming galaxies, and absorption-dominated galaxies by combining the [O
III]/Hbeta ratio with rest-frame U-B color. This can be used to robustly select
AGNs in galaxy samples at intermediate redshifts (z<1). We compare the result
of this optical AGN selection with X-ray selection using a sample of 3150
galaxies with 0.3<z<0.8 and I_AB<22, selected from the DEEP2 Galaxy Redshift
Survey and the All-wavelength Extended Groth Strip International Survey
(AEGIS). Among the 146 X-ray sources in this sample, 58% are classified
optically as emission-line AGNs, the rest as star-forming galaxies or
absorption-dominated galaxies. The latter are also known as "X-ray bright,
optically normal galaxies" (XBONGs). Analysis of the relationship between
optical emission lines and X-ray properties shows that the completeness of
optical AGN selection suffers from dependence on the star formation rate and
the quality of observed spectra. It also shows that XBONGs do not appear to be
a physically distinct population from other X-ray detected, emission-line AGNs.
On the other hand, X-ray AGN selection also has strong bias. About 2/3 of all
emission-line AGNs at L_bol>10^44 erg/s in our sample are not detected in our
200 ks Chandra images, most likely due to moderate or heavy absorption by gas
near the AGN. The 2--7 keV detection rate of Seyfert 2s at z~0.6 suggests that
their column density distribution and Compton-thick fraction are similar to
that of local Seyferts. Multiple sample selection techniques are needed to
obtain as complete a sample as possible.Comment: 24 pages, 14 figures, submitted to ApJ. Version 2 matches the ApJ
accepted version. Sec 3 was reorganized and partly rewritten with one
additional figure (Fig.3
CANDELS: The progenitors of compact quiescent galaxies at z~2
We combine high-resolution HST/WFC3 images with multi-wavelength photometry
to track the evolution of structure and activity of massive (log(M*) > 10)
galaxies at redshifts z = 1.4 - 3 in two fields of the Cosmic Assembly
Near-infrared Deep Extragalactic Legacy Survey (CANDELS). We detect compact,
star-forming galaxies (cSFGs) whose number densities, masses, sizes, and star
formation rates qualify them as likely progenitors of compact, quiescent,
massive galaxies (cQGs) at z = 1.5 - 3. At z > 2 most cSFGs have specific
star-formation rates (sSFR = 10^-9 yr^-1) half that of typical, massive SFGs at
the same epoch, and host X-ray luminous AGN 30 times (~30%) more frequently.
These properties suggest that cSFGs are formed by gas-rich processes (mergers
or disk-instabilities) that induce a compact starburst and feed an AGN, which,
in turn, quench the star formation on dynamical timescales (few 10^8 yr). The
cSFGs are continuously being formed at z = 2 - 3 and fade to cQGs by z = 1.5.
After this epoch, cSFGs are rare, thereby truncating the formation of new cQGs.
Meanwhile, down to z = 1, existing cQGs continue to enlarge to match local QGs
in size, while less-gas-rich mergers and other secular mechanisms shepherd
(larger) SFGs as later arrivals to the red sequence. In summary, we propose two
evolutionary scenarios of QG formation: an early (z > 2), fast-formation path
of rapidly-quenched cSFGs that evolve into cQGs that later enlarge within the
quiescent phase, and a slow, late-arrival (z < 2) path for SFGs to form QGs
without passing through a compact state.Comment: Submitted to the Astrophysical Journal Letters, 6 pages, 4 figure
PTF10iya: A short-lived, luminous flare from the nuclear region of a star-forming galaxy
We present the discovery and characterisation of PTF10iya, a short-lived (dt
~ 10 d, with an optical decay rate of ~ 0.3 mag per d), luminous (M_g ~ -21
mag) transient source found by the Palomar Transient Factory. The
ultraviolet/optical spectral energy distribution is reasonably well fit by a
blackbody with T ~ 1-2 x 10^4 K and peak bolometric luminosity L_BB ~ 1-5 x
10^44 erg per s (depending on the details of the extinction correction). A
comparable amount of energy is radiated in the X-ray band that appears to
result from a distinct physical process. The location of PTF10iya is consistent
with the nucleus of a star-forming galaxy (z = 0.22405 +/- 0.00006) to within
350 mas (99.7 per cent confidence radius), or a projected distance of less than
1.2 kpc. At first glance, these properties appear reminiscent of the
characteristic "big blue bump" seen in the near-ultraviolet spectra of many
active galactic nuclei (AGNs). However, emission-line diagnostics of the host
galaxy, along with a historical light curve extending back to 2007, show no
evidence for AGN-like activity. We therefore consider whether the tidal
disruption of a star by an otherwise quiescent supermassive black hole may
account for our observations. Though with limited temporal information,
PTF10iya appears broadly consistent with the predictions for the early
"super-Eddington" phase of a solar-type star disrupted by a ~ 10^7 M_sun black
hole. Regardless of the precise physical origin of the accreting material, the
large luminosity and short duration suggest that otherwise quiescent galaxies
can transition extremely rapidly to radiate near the Eddington limit; many such
outbursts may have been missed by previous surveys lacking sufficient cadence.Comment: 18 pages, 8 figures; revised following referee's comment
AEGIS: The Clustering of X-ray AGN Relative to Galaxies at z~1
We measure the clustering of non-quasar X-ray AGN at z=0.7-1.4 in the AEGIS
field. Using the cross-correlation of 113 Chandra-selected AGN, with a median
log L_X=42.8 erg s^-1, with ~5,000 DEEP2 galaxies, we find that the X-ray AGN
are fit by a power law with a clustering scale length of r_0=5.95 +/-0.90 h^-1
Mpc and slope gamma=1.66 +/-0.22. X-ray AGN have a similar clustering amplitude
as red, quiescent and `green' transition galaxies at z~1 and are significantly
more clustered than blue, star-forming galaxies. The X-ray AGN clustering
strength is primarily determined by the host galaxy color; AGN in red host
galaxies are significantly more clustered than AGN in blue host galaxies, with
a relative bias that is similar to that of red to blue DEEP2 galaxies. We
detect no dependence of clustering on optical brightness, X-ray luminosity, or
hardness ratio within the ranges probed here. We find evidence for galaxies
hosting X-ray AGN to be more clustered than a sample of galaxies with matching
joint optical color and magnitude distributions. This implies that galaxies
hosting X-ray AGN are more likely to reside in groups and more massive dark
matter halos than galaxies of the same color and luminosity without an X-ray
AGN. In comparison to optically-selected quasars in the DEEP2 fields, we find
that X-ray AGN at z~1 are more clustered than optically-selected quasars (with
a 2.6-sigma significance) and therefore likely reside in more massive dark
matter halos. Our results are consistent with galaxies undergoing a quasar
phase while in the blue cloud before settling on the red sequence with a
lower-luminosity X-ray AGN, if they are similar objects at different
evolutionary stages.Comment: 18 pages, accepted to ApJ, minor changes made to match accepted
version, Figure 1 adde
The CANDELS/SHARDS multiwavelength catalog in GOODS-N : photometry, photometric redshifts, stellar masses, emission-line fluxes, and star formation rates
We present a WFC3 F160W (H-band) selected catalog in the CANDELS/GOODS-N field containing photometry from the ultraviolet (UV) to the far-infrared (IR), photometric redshifts, and stellar parameters derived from the analysis of the multiwavelength data. The catalog contains 35,445 sources over the 171 arcmin(2) of the CANDELS F160W mosaic. The 5 sigma detection limits (within an aperture of radius 0 ''.17) of the mosaic range between H = 27.8, 28.2, and 28.7 in the wide, intermediate, and deep regions, which span approximately 50%, 15%, and 35% of the total area. The multiwavelength photometry includes broadband data from the UV (U band from KPNO and LBC), optical (HST/ACS F435W, F606W, F775W, F814W, and F850LP), near-to-mid IR (HST/WFC3 F105W, F125W, F140W, and F160W; Subaru/MOIRCS Ks; CFHT/Megacam K; and Spitzer/IRAC 3.6, 4.5, 5.8, and 8.0 mu m), and far-IR (Spitzer/MIPS 24 mu m, HERSCHEL/PACS 100 and 160 mu m, SPIRE 250, 350 and 500 mu m) observations. In addition, the catalog also includes optical medium-band data (R similar to 50) in 25 consecutive bands, lambda = 500-950 nm, from the SHARDS survey and WFC3 IR spectroscopic observations with the G102 and G141 grisms (R similar to 210 and 130). The use of higher spectral resolution data to estimate photometric redshifts provides very high, and nearly uniform, precision from z = 0-2.5. The comparison to 1485 good-quality spectroscopic redshifts up to z similar to 3 yields Delta z/(1 + z(spec)) = 0.0032 and an outlier fraction of eta = 4.3%. In addition to the multiband photometry, we release value-added catalogs with emission-line fluxes, stellar masses, dust attenuations, UV- and IR-based star formation rates, and rest-frame colors
CANDELS: The Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey
The Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS)
is designed to document the first third of galactic evolution, over the
approximate redshift (z) range 8--1.5. It will image >250,000 distant galaxies
using three separate cameras on the Hubble Space Telescope, from the
mid-ultraviolet to the near-infrared, and will find and measure Type Ia
supernovae at z>1.5 to test their accuracy as standardizable candles for
cosmology. Five premier multi-wavelength sky regions are selected, each with
extensive ancillary data. The use of five widely separated fields mitigates
cosmic variance and yields statistically robust and complete samples of
galaxies down to a stellar mass of 10^9 M_\odot to z \approx 2, reaching the
knee of the ultraviolet luminosity function (UVLF) of galaxies to z \approx 8.
The survey covers approximately 800 arcmin^2 and is divided into two parts. The
CANDELS/Deep survey (5\sigma\ point-source limit H=27.7 mag) covers \sim 125
arcmin^2 within GOODS-N and GOODS-S. The CANDELS/Wide survey includes GOODS and
three additional fields (EGS, COSMOS, and UDS) and covers the full area to a
5\sigma\ point-source limit of H \gtrsim 27.0 mag. Together with the Hubble
Ultra Deep Fields, the strategy creates a three-tiered "wedding cake" approach
that has proven efficient for extragalactic surveys. Data from the survey are
nonproprietary and are useful for a wide variety of science investigations. In
this paper, we describe the basic motivations for the survey, the CANDELS team
science goals and the resulting observational requirements, the field selection
and geometry, and the observing design. The Hubble data processing and products
are described in a companion paper.Comment: Submitted to Astrophysical Journal Supplement Series; Revised
version, subsequent to referee repor
CANDELS: The Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey - The Hubble Space Telescope Observations, Imaging Data Products and Mosaics
This paper describes the Hubble Space Telescope imaging data products and
data reduction procedures for the Cosmic Assembly Near-IR Deep Extragalactic
Legacy Survey (CANDELS). This survey is designed to document the evolution of
galaxies and black holes at , and to study Type Ia SNe beyond
. Five premier multi-wavelength sky regions are selected, each with
extensive multiwavelength observations. The primary CANDELS data consist of
imaging obtained in the Wide Field Camera 3 / infrared channel (WFC3/IR) and
UVIS channel, along with the Advanced Camera for Surveys (ACS). The
CANDELS/Deep survey covers \sim125 square arcminutes within GOODS-N and
GOODS-S, while the remainder consists of the CANDELS/Wide survey, achieving a
total of \sim800 square arcminutes across GOODS and three additional fields
(EGS, COSMOS, and UDS). We summarize the observational aspects of the survey as
motivated by the scientific goals and present a detailed description of the
data reduction procedures and products from the survey. Our data reduction
methods utilize the most up to date calibration files and image combination
procedures. We have paid special attention to correcting a range of
instrumental effects, including CTE degradation for ACS, removal of electronic
bias-striping present in ACS data after SM4, and persistence effects and other
artifacts in WFC3/IR. For each field, we release mosaics for individual epochs
and eventual mosaics containing data from all epochs combined, to facilitate
photometric variability studies and the deepest possible photometry. A more
detailed overview of the science goals and observational design of the survey
are presented in a companion paper.Comment: 39 pages, 25 figure