University of Colorado Boulder

About: University of Colorado Boulder is a education organization based out in Boulder, Colorado, United States. It is known for research contribution in the topics: Population & Galaxy. The organization has 48794 authors who have published 115151 publications receiving 5387328 citations. The organization is also known as: CU Boulder & UCB.

Differential Galaxy Evolution in Cluster and Field Galaxies at z ≈ 0.3

Abstract: We measure spectral indexes for 1823 galaxies in the Canadian Network for Observational Cosmology 1 (CNOC1) sample of 15 X-ray luminous clusters at 0.18 5 A] but no [O II] emission [W0(O ) < 5 A], perhaps indicative of recently terminated star formation. The observed fraction of 4.4% ± 0.7% in the cluster sample is an overestimate due to a systematic effect that results from the large uncertainties on individual spectral index measurements. Corrected for this bias, we estimate that K+A galaxies make up only 2.1% ± 0.7% of the cluster sample and 0.1% ± 0.7% of the field. From the subsample of galaxies more luminous than Mr = -18.8 + 5 log h, which is statistically representative of a complete sample to this limit, the corrected fraction of K+A galaxies is 1.5% ± 0.8% in the cluster and 1.2% ± 0.8% in the field. Compared with the z ≈ 0.1 fraction of 0.30%, the fraction of K+A galaxies in the CNOC1 field sample is greater by perhaps a factor of 4, but with only 1 σ significance; no further evolution of this fraction is detectable over our redshift range. We compare our data with the results of PEGASE and GISSEL96 spectrophotometric models and conclude, from the relative fractions of red and blue galaxies with no [O II] λ3727 emission and strong Hδ absorption, that up to 1.9% ± 0.8% of the cluster population may have had its star formation recently truncated without a starburst. However, this is still not significantly greater than the fraction of such galaxies in the field, 3.1% ± 1.0%. Furthermore, we do not detect an excess of cluster galaxies that have unambiguously undergone starbursts within the last 1 Gyr. In fact, at 6.3% ± 2.1%, the A+em galaxies that Poggianti et al. have recently suggested are dusty starbursts are twice as common in the field as in the cluster environment. Our results imply that these cluster environments are not responsible for inducing starbursts; thus, the increase in cluster blue galaxy fraction with redshift may not be a strictly cluster-specific phenomenon. We suggest that the truncation of star formation in clusters may largely be a gradual process, perhaps due to the exhaustion of gas in the galactic disks over fairly long timescales; in this case differential evolution may result because field galaxies can refuel their disks with gas from extended halos, thus regenerating star formation, while cluster galaxies may not have such halos and so continue to evolve passively.

An absorption profile centred at 78 megahertz in the sky-averaged spectrum

Abstract: The 21-cm absorption profile is detected in the sky-averaged radio spectrum, but is much stronger than predicted, suggesting that the primordial gas might have been cooler than predicted. As the first stars heated hydrogen in the early Universe, the 21-cm hyperfine line—an astronomical standard that represents the spin-flip transition in the ground state of atomic hydrogen—was altered, causing the hydrogen gas to absorb photons from the microwave background. This should produce an observable absorption signal at frequencies of less than 200 megahertz (MHz). Judd Bowman and colleagues report the observation of an absorption profile centred at a frequency of 78 MHz that is about 19 MHz wide and 0.5 kelvin deep. The profile is generally in line with expectations, although it is deeper than predicted. An accompanying paper by Rennan Barkana suggests that baryons were interacting with cold dark-matter particles in the early Universe, cooling the gas more than had been expected. After stars formed in the early Universe, their ultraviolet light is expected, eventually, to have penetrated the primordial hydrogen gas and altered the excitation state of its 21-centimetre hyperfine line. This alteration would cause the gas to absorb photons from the cosmic microwave background, producing a spectral distortion that should be observable today at radio frequencies of less than 200 megahertz1. Here we report the detection of a flattened absorption profile in the sky-averaged radio spectrum, which is centred at a frequency of 78 megahertz and has a best-fitting full-width at half-maximum of 19 megahertz and an amplitude of 0.5 kelvin. The profile is largely consistent with expectations for the 21-centimetre signal induced by early stars; however, the best-fitting amplitude of the profile is more than a factor of two greater than the largest predictions2. This discrepancy suggests that either the primordial gas was much colder than expected or the background radiation temperature was hotter than expected. Astrophysical phenomena (such as radiation from stars and stellar remnants) are unlikely to account for this discrepancy; of the proposed extensions to the standard model of cosmology and particle physics, only cooling of the gas as a result of interactions between dark matter and baryons seems to explain the observed amplitude3. The low-frequency edge of the observed profile indicates that stars existed and had produced a background of Lyman-α photons by 180 million years after the Big Bang. The high-frequency edge indicates that the gas was heated to above the radiation temperature less than 100 million years later.

Reducing the racial achievement gap: a social-psychological intervention.

Abstract: Two randomized field experiments tested a social-psychological intervention designed to improve minority student performance and increase our understanding of how psychological threat mediates performance in chronically evaluative real-world environments. We expected that the risk of confirming a negative stereotype aimed at one's group could undermine academic performance in minority students by elevating their level of psychological threat. We tested whether such psychological threat could be lessened by having students reaffirm their sense of personal adequacy or "self-integrity." The intervention, a brief in-class writing assignment, significantly improved the grades of African American students and reduced the racial achievement gap by 40%. These results suggest that the racial achievement gap, a major social concern in the United States, could be ameliorated by the use of timely and targeted social-psychological interventions.

The Chicago face database: A free stimulus set of faces and norming data

Abstract: Researchers studying a range of psychological phenomena (e.g., theory of mind, emotion, stereotyping and prejudice, interpersonal attraction, etc.) sometimes employ photographs of people as stimuli. In this paper, we introduce the Chicago Face Database, a free resource consisting of 158 high-resolution, standardized photographs of Black and White males and females between the ages of 18 and 40 years and extensive data about these targets. In Study 1, we report pre-testing of these faces, which includes both subjective norming data and objective physical measurements of the images included in the database. In Study 2 we surveyed psychology researchers to assess the suitability of these targets for research purposes and explored factors that were associated with researchers' judgments of suitability. Instructions are outlined for those interested in obtaining access to the stimulus set and accompanying ratings and measures.

Twitter adoption and use in mass convergence and emergency events

Abstract: This paper offers a descriptive account of Twitter (a microblogging service) across four high-profile, mass convergence events - two emergency and two national security. We statistically examine how Twitter is being used surrounding these events, and compare and contrast how that behaviour is different from more general Twitter use. Our findings suggest that Twitter messages sent during these types of events contain more displays of information broadcasting and brokerage, and we observe that general Twitter use seems to have evolved over time to offer more of an information-sharing purpose. We also provide preliminary evidence that Twitter users who join during and in apparent relation to a mass convergence or emergency event are more likely to become long-term adopters of the technology.