1,301 research outputs found
Altered intrinsic functional coupling between core neurocognitive networks in Parkinson\u27s disease
Parkinson3s disease (PD) is largely attributed to disruptions in the nigrostriatal dopamine system. These neurodegenerative changes may also have a more global effect on intrinsic brain organization at the cortical level. Functional brain connectivity between neurocognitive systems related to cognitive processing is critical for effective neural communication, and is disrupted across neurological disorders. Three core neurocognitive networks have been established as playing a critical role in the pathophysiology of many neurological disorders: the default-mode network (DMN), the salience network (SN), and the central executive network (CEN). In healthy adults, DMNâCEN interactions are anti-correlated while SNâCEN interactions are strongly positively correlated even at rest, when individuals are not engaging in any task. These intrinsic between-network interactions at rest are necessary for efficient suppression of the DMN and activation of the CEN during a range of cognitive tasks. To identify whether these network interactions are disrupted in individuals with PD, we used resting state functional magnetic resonance imaging (rsfMRI) to compare between-network connectivity between 24 PD participants and 20 age-matched controls (MC). In comparison to the MC, individuals with PD showed significantly less SNâCEN coupling and greater DMNâCEN coupling during rest. Disease severity, an index of striatal dysfunction, was related to reduced functional coupling between the striatum and SN. These results demonstrate that individuals with PD have a dysfunctional pattern of interaction between core neurocognitive networks compared to what is found in healthy individuals, and that interaction between the SN and the striatum is even more profoundly disrupted in those with greater disease severity
Spectroscopic Properties of the z=4.5 Lyman-alpha Emitters
We present Keck/LRIS optical spectra of 17 Lya-emitting galaxies and one
Lyman break galaxy at z=4.5 discovered in the Large Area Lyman Alpha (LALA)
survey. The survey has identified a sample of ~350 candidate Lya-emitting
galaxies at z=4.5 in a search volume of 1.5 x 10^6 comoving Mpc^3. We targeted
25 candidates for spectroscopy; hence, the 18 confirmations presented herein
suggest a selection reliability of 72%. The large equivalent widths (median
W(rest)~80 A) but narrow physical widths (v < 500 km/s) of the Lya emission
lines, along with the lack of accompanying high-ionization state emission
lines, suggest that these galaxies are young systems powered by star formation
rather than by AGN activity. Theoretical models of galaxy formation in the
primordial Universe suggest that a small fraction of Lya-emitting galaxies at
z=4.5 may still be nascent, metal-free objects. Indeed, we find with 90%
confidence that 3 to 5 of the confirmed sources show W(rest) > 240 A, exceeding
the maximum Lya equivalent width predicted for normal stellar populations.
Nonetheless, we find no evidence for HeII 1640 emission in either individual or
composite spectra, indicating that though these galaxies are young, they are
not truly primitive, Population III objects.Comment: 12 pages, Accepted to Ap
Spectroscopic Confirmation of Three Redshift 5.7 Lyman-alpha Emitters from the Large Area Lyman Alpha Survey
Narrow-band searches for Lyman alpha emission are an efficient way of
identifying star-forming galaxies at high redshifts. We present Keck telescope
spectra confirming redshifts z = 5.7 for three objects discovered in the Large
Area Lyman Alpha (LALA) survey at Kitt Peak National Observatory.
All three spectra show strong, narrow emission lines with the asymmetric
profile that is characteristically produced in high redshift Lyman alpha
emitters by preferential HI absorption in the blue wing of the line. These
objects are undetected in deep Bw, V, R, and 6600A narrow-band images from the
NOAO Deep Wide-Field Survey and from LALA, as expected from Lyman break and
Lyman alpha forest absorption at redshift z = 5.7. All three objects show large
equivalent widths (>= 150A in the rest-frame), suggesting at least one of the
following: a top-heavy initial mass function, very low stellar metallicity, or
the presence of an active nucleus. We consider the case for an active nucleus
to be weak in all three objects due to the limited width of the Lyman alpha
emission line (< 500 km/s) and the absence of any other indicator of quasar
activity.
The three confirmed high redshift objects were among four spectroscopically
observed targets drawn from the sample of 18 candidates presented by Rhoads and
Malhotra (2001). Thus, these spectra support the Lyman alpha emitter population
statistics from our earlier photometric study, which imply little evolution in
number density from z=5.7 to z=4.5 and provide strong evidence that the
reionization redshift is greater than 5.7.Comment: Submitted to AJ, June 2002. 15 pages, AASTe
The Impact of an Intense Cyclone on Short-Term Sea Ice Loss in a Fully Coupled Atmosphere-Ocean-Ice Model
AbstractArctic cyclones may be associated with periods of locally enhanced sea ice loss during the summer, and some studies have found that an intense cyclone in August 2012 resulted in a rapid sea ice retreat. This study uses a coupled atmosphereâoceanâice model (NavyâESPC) to explore the relationship between the 2012 cyclone and shortâterm sea ice melting. There are two mechanisms of cycloneâinduced melting in NavyâESPC: turbulent mixing of a warm layer located at 15â to 35âm depth increases bottom melting and warm air advection by the strong surface winds increases surface melting. Although the rate of sea ice melt is substantially increased in association with the cyclone, this effect is confined to a relatively small region and only lasts for a few days. There is no clear signature of the cyclone on the overall Arctic sea ice extent in NavyâESPC
A Galaxy at z = 6.545 and Constraints on the Epoch of Reionization
We report the discovery of a Lyman-alpha-emitting galaxy at redshift z=6.545
serendipitously identified in the course of spectroscopic follow-up of hard
X-ray sources on behalf of the Serendipitous Extragalactic X-Ray Source
Identification (SEXSI) survey. The line flux of the galaxy, 2.1e-17 erg/cm2/s,
is similar to line fluxes probed by narrow-band imaging surveys; the 5.2
square-arcminutes surveyed implies a surface density of z~6.5 Lyman-alpha
emitters somewhat higher than that inferred from narrow-band surveys. This
source marks the sixth Lyman-alpha-emitting galaxy identified at z~6.5, a
redshift putatively beyond the epoch of reionization when the damping wings of
the neutral hydrogen of the intergalactic medium is capable of severely
attenuating Lyman-alpha emission. By comparing the Lyman-alpha emitter
luminosity functions at z~5.7 and z~6.5, we infer that the intergalactic medium
may remain largely reionized from the local universe out to z~6.5.Comment: 16 pages, 4 figures; submitted to the Astrophysical Journa
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