3,188 research outputs found
The Global 21-cm Signal in the Context of the High-z Galaxy Luminosity Function
Motivated by recent progress in studies of the high- Universe, we build a
new model for the global 21-cm signal that is explicitly calibrated to
measurements of the galaxy luminosity function (LF) and further tuned to match
the Thomson scattering optical depth of the cosmic microwave background,
. Assuming that the galaxy population can be smoothly
extrapolated to higher redshifts, the recent decline in best-fit values of
and the inefficient heating induced by X-ray binaries (HMXBs; the
presumptive sources of the X-ray background at high-) imply that the
entirety of cosmic reionization and reheating occurs at redshifts . In contrast to past global 21-cm models, whose (
MHz) absorption features and strong mK emission features were driven
largely by the assumption of efficient early star-formation and X-ray heating,
our new fiducial model peaks in absorption at MHz at a depth of
mK and has a negligible emission component. As a result, a strong
emission signal would provide convincing evidence that HMXBs are not the only
drivers of cosmic reheating. Shallow absorption troughs should accompany strong
heating scenarios, but could also be caused by a low escape fraction of
Lyman-Werner photons. Generating signals with troughs at MHz
requires a floor in the star-formation efficiency in halos below , which is equivalent to steepening the faint-end of the galaxy LF.
These findings demonstrate that the global 21-cm signal is a powerful
complement to current and future galaxy surveys and efforts to better
understand the interstellar medium in high- galaxies.Comment: 17 pages, 9 figures, in pres
The Persistence of Population III Star Formation
We present a semi-analytic model of star formation in the early universe,
beginning with the first metal-free stars. By employing a completely
feedback-limited star formation prescription, stars form at maximum efficiency
until the self-consistently calculated feedback processes halt formation. We
account for a number of feedback processes including a meta-galactic
Lyman-Werner background, supernovae, photoionization, and chemical feedback.
Halos are evolved combining mass accretion rates found through abundance
matching with our feedback-limited star formation prescription, allowing for a
variety of Population III (Pop III) initial mass functions (IMFs). We find
that, for a number of models, massive Pop III star formation can continue on
until at least and potentially past at rates of around
to M yr Mpc, assuming these stars
form in isolation. At this point Lyman-Werner feedback pushes the minimum halo
mass for star formation above the atomic cooling threshold, cutting off the
formation of massive Pop III stars. We find that, in most models, Pop II and
Pop III star formation co-exist over cosmological time-scales, with the total
star formation rate density and resulting radiation background strongly
dominated by the former before Pop III star formation finally ends. These halos
form at most M of massive Pop III stars during this phase
and typically have absolute magnitudes in the range of to . We also briefly discuss how future observations from telescopes such as
JWST or WFIRST and 21-cm experiments may be able to constrain unknown
parameters in our model such as the IMF, star formation prescription, or the
physics of massive Pop III stars.Comment: 16 pages, 13 figures, submitted to MNRA
On the expected purity of photometric galaxy surveys targeting the Cosmic Dawn
Over the last three decades, photometric galaxy selection using the
Lyman-break technique has transformed our understanding of the high-z Universe,
providing large samples of galaxies at 3 < z < 8 with relatively small
contamination. With the advent of the James Webb Space Telescope, the
Lyman-break technique has now been extended to z ~ 17. However, the purity of
the resulting samples has not been tested. Here we use a simple model, built on
the robust foundation of the dark matter halo mass function, to show that the
expected level of contamination rises dramatically at z > 10, especially for
luminous galaxies, placing stringent requirements on the selection process. The
most luminous sources at z > 12 are likely at least ten thousand times rarer
than potential contaminants, so extensive spectroscopic followup campaigns may
be required to identify a small number of target sources.Comment: 6 pages, 2 figures, submitted to MNRA
Design of Reconfigurable Rim Scattering Reflectarrays for Null Steering in Reflector Antennas
Reflector antennas are susceptible to interference arriving through its
sidelobes. The design of a reconfigurable rim scattering reflectarray which can
be retrofitted to existing high gain reflector antennas giving the reflector
adaptive null steering capabilities is presented thereby giving the reflector
interference mitigation capabilities. The conformal reflectarray, placed along
the outer rim of the reflector antenna, contains reconfigurable unit cells
(RUC). The states of the RUCs allow the reflectarray to place a null in any
direction within the first few sidelobes of the reflector pattern. A 1-bit RUC
containing two pin diodes with less than 0.44dB loss at the operating frequency
is presented. It is found that the dielectric and switch losses do not
significantly affect the overall efficiency of the reflector as they are placed
only along the outer rim of the reflector which is weakly illuminated. Two
reflectarrays are designed. First, a L-band non-reconfigurable reflectarray
based on variable patch size for an 18m prime focus fed parabola is presented.
A full-wave simulation of this design serves to validate the design approach.
Subsequently, a reflectarray design using RUCs is presented. All necessary
formulations for complete reproducibility of results are provided in appendices
allowing for follow-on work
A minimalist feedback-regulated model for galaxy formation during the epoch of reionization
Near-infrared surveys have now determined the luminosity functions of galaxies at 6 ≲ z ≲ 8 to impressive precision and identified a number of candidates at even earlier times. Here, we develop a simple analytic model to describe these populations that allows physically motivated extrapolation to earlier times and fainter luminosities. We assume that galaxies grow through accretion on to dark matter haloes, which we model by matching haloes at fixed number density across redshift, and that stellar feedback limits the star formation rate. We allow for a variety of feedback mechanisms, including regulation through supernova energy and momentum from radiation pressure. We show that reasonable choices for the feedback parameters can fit the available galaxy data, which in turn substantially limits the range of plausible extrapolations of the luminosity function to earlier times and fainter luminosities: for example, the global star formation rate declines rapidly (by a factor of ∼20 from z = 6 to 15 in our fiducial model), but the bright galaxies accessible to observations decline even faster (by a factor ≳ 400 over the same range). Our framework helps us develop intuition for the range of expectations permitted by simple models of high-z galaxies that build on our understanding of ‘normal’ galaxy evolution. We also provide predictions for galaxy measurements by future facilities, including James Webb Space Telescope and Wide-Field Infrared Survey Telescope
Identity Implications of Relationship (Re)Definition Goals: An Analysis of Face Threats and Facework as Young Adults Initiate, Intensify, and Disengage from Romantic Relationships
DOI: 10.1177/0261927X08325746Identity implications theory (IIT) is applied to analyze how young adults manage identity concerns
associated with the goals of initiating, intensifying, and disengaging from romantic relationships.
Participants wrote their responses to one of six hypothetical romantic (re)definition scenarios, indicated
whether they actually would pursue the relational goal if their scenario were real, and rated degree of
threat to both parties' face. Responses were coded for positive and negative politeness strategies.
Participants in different relational goal conditions perceived different face threats, varied in their
likelihood of pursuing the relational goal, and employed different politeness strategies. Relationship
(re)definition goal also moderated associations between perceived face threats and goal pursuit as well
as politeness strategies. The findings show how multiple goal theories such as IIT can be applied to
situations where relational goals are primary as well as how, to varying degrees, identity concerns shape and constrain how young adults pursue relational (re)definition goals
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