159 research outputs found
Social Media in a Subjective-science Mode: The “Facebook Likes” Study Reconfigured with Self-reference
A Cambridge University study of more than 58,000 users of the popular social medium Facebook examined the extent to which the Facebook "Likes" button predicted behaviors and attributes of a diverse nature (IQ, sexual identity, political and popular-culture preferences, religious affiliation, and the like). Despite revealing several intriguing and statistically significant relationships, the research sheds scant light on the nature of the subjectivity at play. In a Q-methodological study of a sample of subjectively communicated responses to the Cambridge research, three versions of the subjective interface between the users of Facebook and the social medium are reported. Implications for studying the social-psychological aspects of social media from the methodological principle of self-reflection are discussed
A three solar cell system based on a self-supporting, transparent AlGaAs top solar cell
Development of a three solar cell stack can lead to practical efficiencies greater than 30 percent (1x,AM0). A theoretical efficiency limitation of 43.7 percent at AM0 and one sun is predicted by this model. Including expected losses, a practical system efficiency of 36.8 percent is anticipated. These calculations are based on a 1.93eV/1.43eV/0.89eV energy band gap combination. AlGaAs/GaAs/GaInAsP materials can be used with a six-terminal wiring configuration. The key issues for multijunction solar cells are the top and middle solar cell performance and the sub-bandgap transparency. AstroPower has developed a technique to fabricate AlGaAs solar cells on rugged, self-supporting, transparent AlGaAs substrates. Top solar cell efficiencies greater than 11 percent AM0 have been achieved. State-of-the-art GaAs or InP devices will be used for the middle solar cell. GaInAsP will be used to fabricate the bottom solar cell. This material is lattice-matched to InP and offers a wide range of bandgaps for optimization of the three solar cell stack. Liquid phase epitaxy is being used to grow the quaternary material. Initial solar cells have shown open-circuit voltages of 462 mV for a bandgap of 0.92eV. Design rules for the multijunction three solar cell stack are discussed. The progress in the development of the self-supporting AlGaAs top solar cell and the GaInAsP bottom solar cell is presented
Ly at Cosmic Dawn with a Simulated Roman Grism Deep Field
The slitless grism on the Nancy Grace Roman Space Telescope will enable deep
near-infrared spectroscopy over a wide field of view. We demonstrate Roman's
capability to detect Ly galaxies at using a multi-position-angle
(PA) observational strategy. We simulate Roman grism data using a realistic
foreground scene from the COSMOS field. We also input fake Ly galaxies
spanning redshift z=7.5-10.5 and a line-flux range of interest. We show how a
novel data cube search technique -- CUBGRISM -- originally developed for GALEX
can be applied to Roman grism data to produce a Ly flux-limited sample
without the need for continuum detections. We investigate the impact of
altering the number of independent PAs and exposure time. A deep Roman grism
survey with 25 PAs and a total exposure time of hrs can achieve Ly
line depths comparable to the deepest narrow-band surveys
(erg s). Assuming a null result,
where the opacity of the intergalactic medium (IGM) remains unchanged from
, this level of sensitivity will detect deg Ly
emitters from . A decline from this expected number density is the
signature of an increasing neutral hydrogen fraction and the onset of
reionization. Our simulations indicate that a deep Roman grism survey has the
ability to measure the timing and magnitude of this decline, allowing us to
infer the ionization state of the IGM and helping us to distinguish between
models of reionization.Comment: 11 pages, 7 figures, Submitted to Ap
Probing Patchy Reionization with the Void Probability Function of Lyman- Emitters
We probe what constraints for the global ionized hydrogen fraction the Void
Probability Function (VPF) clustering can give for the Lyman-Alpha Galaxies in
the Epoch of Reionization (LAGER) narrowband survey as a function of area.
Neutral hydrogen acts like a fog for Lyman-alpha emission, and measuring the
drop in the luminosity function of Lyman- emitters (LAEs) has been used
to constrain the ionization fraction in narrowband surveys. However, the
clustering of LAEs is independent from the luminosity function's inherent
evolution, and can offer additional constraints for reionization under
different models. The VPF measures how likely a given circle is to be empty. It
is a volume-averaged clustering statistic that traces the behavior of higher
order correlations, and its simplicity offers helpful frameworks for planning
surveys.
Using the \citet{Jensen2014} simulations of LAEs within various amount of
ionized intergalactic medium, we predict the behavior of the VPF in one
(301x150.5x30 Mpc), four (5.44 Mpc), or eight (1.1 Mpc) fields of LAGER imaging. We examine the VPF at 5 and 13
arcminutes, corresponding to the minimum scale implied by the LAE density and
the separation of the 2D VPF from random, and the maximum scale from the
8-field 15.5 deg LAGER area. We find that even a single DECam field of
LAGER (2-3 deg) could discriminate between mostly neutral vs. ionized.
Additionally, we find four fields allows the distinction between 30, 50, and 95
percent ionized; and that eight fields could even distinguish between 30, 50,
73, and 95 percent ionized.Comment: 14 pages, 5 figure
Searching for z~7.7 Lyman Alpha Emitters in the COSMOS Field with NEWFIRM
The study of Ly-alpha emission in the high-redshift universe is a useful
probe of the epoch of reionization, as the Ly-alpha line should be attenuated
by the intergalactic medium (IGM) at low to moderate neutral hydrogen
fractions. Here we present the results of a deep and wide imaging search for
Ly-alpha emitters in the COSMOS field. We have used two ultra-narrowband
filters (filter width of ~8-9 {\deg}A) on the NEWFIRM camera, installed on the
Mayall 4m telescope at Kitt Peak National Observatory, in order to isolate
Ly-alpha emitters at z = 7.7; such ultra-narrowband imaging searches have
proved to be excellent at detecting Ly-alpha emitters. We found 5-sigma
detections of four candidate Ly-alpha emitters in a survey volume of 2.8 x 10^4
Mpc^3 (total survey area ~760 arcmin^2). Each candidate has a line flux greater
than 8 x 10^-18 erg s^-1 cm^-2. Using these results to construct a luminosity
function and comparing to previously established Ly-alpha luminosity functions
at z = 5.7 and z = 6.5, we find no conclusive evidence for evolution of the
luminosity function between z = 5.7 and z = 7.7. Statistical Monte Carlo
simulations suggest that half of these candidates are real z = 7.7 targets, and
spectroscopic follow-up will be required to verify the redshift of these
candidates. However, our results are consistent with no strong evolution in the
neutral hydrogen fraction of the IGM between z = 5.7 and z = 7.7, even if only
one or two of the z = 7.7 candidates are spectroscopically confirmed.Comment: 29 pages, 5 figures, accepted to ApJ (12/11
How to Tell a Jet from a Balloon: A Proposed Test for Beaming in Gamma Ray Bursts
If gamma ray bursts are highly collimated, the energy requirements of each
event may be reduced by several (~ 4-6) orders of magnitude, and the event rate
increased correspondingly. Extreme conditions in gamma ray bursters lead to
highly relativistic motions (bulk Lorentz factors Gamma > 100). This results in
strong forward beaming of the emitted radiation in the observer's rest frame.
Thus, all information on gamma ray bursts comes from those ejecta emitted in a
narrow cone (opening angle 1/Gamma) pointing towards the observer. We are at
present ignorant of whether there are ejecta outside that cone or not.
The recent detection of longer wavelength transients following gamma ray
bursts allows an empirical test of whether gamma ray bursts are collimated jets
or spherical fireballs. The bulk Lorentz factor of the burst ejecta will
decrease with time after the event, as the ejecta sweep up the surrounding
medium. Thus, radiation from the ejecta is beamed into an ever increasing solid
angle as the burst remnant evolves. It follows that if gamma ray bursts are
highly collimated, many more optical and radio transients should be observed
without associated gamma rays than with them. Published supernova searches may
contain enough data to test the most extreme models of gamma ray beaming. We
close with a brief discussion of other possible consequences of beaming,
including its effect on the evolution of burst remnants.Comment: Original replaced with accepted refereed manuscript. 11 pages, uses
AASTeX 4.0 LaTeX macros. To be published in The Astrophysical Journal
Letters, vol. 487, p. L1 (20 September 1997
Infrared Emission from the Nearby Cool Core Cluster Abell 2597
We observed the brightest central galaxy (BCG) in the nearby (z=0.0821) cool
core galaxy cluster Abell 2597 with the IRAC and MIPS instruments on board the
Spitzer Space Telescope. The BCG was clearly detected in all Spitzer
bandpasses, including the 70 and 160 micron wavebands. We report aperture
photometry of the BCG. The spectral energy distribution exhibits a clear excess
in the FIR over a Rayleigh-Jeans stellar tail, indicating a star formation rate
of ~4-5 solar masses per year, consistent with the estimates from the UV and
its H-alpha luminosity. This large FIR luminosity is consistent with that of a
starburst or a Luminous Infrared Galaxy (LIRG), but together with a very
massive and old population of stars that dominate the energy output of the
galaxy. If the dust is at one temperature, the ratio of 70 to 160 micron fluxes
indicate that the dust emitting mid-IR in this source is somewhat hotter than
the dust emitting mid-IR in two BCGs at higher-redshift (z~0.2-0.3) and higher
FIR luminosities observed earlier by Spitzer, in clusters Abell 1835 and Zwicky
3146.Comment: Accepted at Ap
Using Spatial Distributions to Constrain Progenitors of Supernovae and Gamma Ray Bursts
We carry out a comprehensive theoretical examination of the relationship
between the spatial distribution of optical transients and the properties of
their progenitor stars. By constructing analytic models of star-forming
galaxies and the evolution of stellar populations within them, we are able to
place constraints on candidate progenitors for core-collapse supernovae (SNe),
long-duration gamma ray bursts, and supernovae Ia. In particular we first
construct models of spiral galaxies that reproduce observations of
core-collapse SNe, and we use these models to constrain the minimum mass for
SNe Ic progenitors to approximately 25 solar masses. Secondly, we lay out the
parameters of a dwarf irregular galaxy model, which we use to show that the
progenitors of long-duration gamma-ray bursts are likely to have masses above
approximately 43 solar masses. Finally, we introduce a new method for
constraining the time scale associated with SNe Ia and apply it to our spiral
galaxy models to show how observations can better be analyzed to discriminate
between the leading progenitor models for these objects.Comment: 18 pages, 19 figures, ApJ, in pres
A Link to the Past: Using Markov Chain Monte Carlo Fitting to Constrain Fundamental Parameters of High-Redshift Galaxies
We have a developed a new method for fitting spectral energy distributions
(SEDs) to identify and constrain the physical properties of high-redshift (4 <
z < 8) galaxies. Our approach uses an implementation of Bayesian based Markov
Chain Monte Carlo (PiMC^2) that allows us to compare observations to
arbitrarily complex models and to compute 95% credible intervals that provide
robust constraints for the model parameters. The work is presented in 2
sections. In the first, we test PiMC^2 using simulated SEDs to not only confirm
the recovery of the known inputs but to assess the limitations of the method
and identify potential hazards of SED fitting when applied specifically to high
redshift (z>4) galaxies. Our tests reveal five critical results: 1) the ability
to confidently constrain metallicity, population ages, and Av all require
photometric accuracy better than what is currently achievable (i.e. less than a
few percent); 2) the ability to confidently constrain stellar masses (within a
factor of two) can be achieved without the need for high-precision photometry;
3) the addition of IRAC photometry does not guarantee that tighter constraints
of the stellar masses and ages can be defined; 4) different assumptions about
the star formation history can lead to significant biases in mass and age
estimates; and 5) we are able to constrain stellar age and Av of objects that
are both young and relatively dust free. In the second part of the paper we
apply PiMC^2 to 17 4<z<8 objects, including the GRAPES Ly alpha sample (4<z<6),
supplemented by HST/WFC3 near-IR observations, and several broad band selected
z>6 galaxies. Using PiMC^2, we are able to constrain the stellar mass of these
objects and in some cases their stellar age and find no evidence that any of
these sources formed at a redshift much larger than z_f=8, a time when the
Universe was ~ 0.6 Gyr old.Comment: Submitted to ApJ (Full abstract, 47 pages, 17 figures, 7 tables
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