1,721,347 research outputs found
Star formation quenching in massive galaxies
No full textUnderstanding how and why star formation turns off in massive galaxies is a major challenge for studies of galaxy evolution. Many theoretical explanations have been proposed, but a definitive consensus is yet to be reached
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Constraining the cosmic-ray ionization rate and spectrum with NIR spectroscopy of dense clouds. A testbed for JWST
Full text linkContext. Low-energy cosmic rays (CRs) control the thermo-chemical state and the coupling between gas and magnetic fields in dense molecular clouds. However, current estimates of the low-energy CR spectrum (E-2-1 GeV) and the associated CR ionization rate are highly uncertain. Aims. We apply, for the first time, a new method for constraining the CR ionization rate and the CR spectral shape using H2 rovibrational lines from cold molecular clouds. Methods. Using the MMIRS instrument on the MMT, we obtained deep near-infrared (NIR) spectra in six positions within four dense cores, namely, G150, G157, G163, G198, with column densities of NH2-1022 cm-2. Results. We derived 3ÏA? upper limits on the H2 (1-0)S(0) line (2.22 μm) brightness in the range I-=-5.9-ÃA - -10-8 to 1.2-ÃA - -10-7 erg cm-2 s-1 sr-1 for the different targets. Using both an analytic model and a numerical model of CR propagation, we convert these into upper limits on the CR ionization rate in the clouds-interior, ζ-=-1.5 to 3.6-ÃA - -10-16 s-1, and lower limits on the low-energy spectral slope of interstellar CR protons, α-=-0.97 to -0.79. We show that while MMT was unable to detect the H2 lines due to high atmospheric noise, JWST/NIRSpec will be able to efficiently detect the CR-excited H2 lines, making it the ideal method for constraining the otherwise elusive low-energy CRs and shedding light on the sources and propagation modes of CRs
Flame: A flexible data reduction pipeline for near-infrared and optical spectroscopy
No full textWe present flame, a pipeline for reducing spectroscopic observations obtained with multislit near-infrared and optical instruments. Because of its flexible design, flame can be easily applied to data obtained with a wide variety of spectrographs. The flexibility is due to a modular architecture, which allows changes and customizations to the pipeline, and relegates the instrument-specific parts to a single module. At the core of the data reduction is the transformation from observed pixel coordinates (x, y) to rectified coordinates (λ, γ). This transformation consists in the polynomial functions λ(x, y) and γ(x, y) that are derived from arc or sky emission lines and slit edge tracing, respectively. The use of 2D transformations allows one to wavelength calibrate and rectify the data using just one interpolation step. Furthermore, the γ(x, y) transformation includes also the spatial misalignment between frames, which can be measured from a reference star observed simultaneously with the science targets. The misalignment can then be fully corrected during the rectification, without having to further resample the data. Sky subtraction can be performed via nodding and/or modelling of the sky spectrum; the combination of the two methods typically yields the best results. We illustrate the pipeline by showing examples of data reduction for a near-infrared instrument (LUCI at the Large Binocular Telescope) and an optical one (LRIS at the Keck telescope)
The stellar chemical abundances of simulated massive galaxies at z = 2
Full text linkWe analyse the stellar abundances of massive galaxies (log M∗/M☉ > 10.5) at redshift, z = 2, in the IllustrisTNG simulation with the goal of guiding the interpretation of current and future observations, particularly from JWST. We find that the effective size, Re, of galaxies strongly affects the abundance measurements: both [Mg/H] and [Fe/H] are anticorrelated with Re, while the relative abundance [Mg/Fe] slightly increases with Re. The α enhancement as tracked by [Mg/Fe] traces the formation time-scale of a galaxy weakly, and mostly depends on Re. Aperture effects are important: measuring the stellar abundances within 1 kpc instead of within Re can make a large difference. These results are all due to a nearly universal, steeply declining stellar abundance profile that does not scale with galaxy size - Small galaxies appear metal-rich because their stars live in the inner part of the profile where abundances are high. The slope of this profile is mostly set by the gas-phase abundance profile and not substantially modified by stellar age gradients. The gas-phase abundance profile, in turn, is determined by the strong radial dependence of the gas fraction and star-formation efficiency. We develop a simple model to describe the chemical enrichment, in which each radial bin of a galaxy is treated as an independent closed-box system. This model reproduces the gas-phase abundance profile of simulated galaxies, but not the detailed distribution of their stellar abundances, for which gas and/or metal transport are likely needed
Quenching, Mergers, and Age Profiles for z = 2 Galaxies in IllustrisTNG
Full text linkUsing the IllustrisTNG cosmological galaxy formation simulations, we analyze the physical properties of young quiescent galaxies at z = 2 with stellar masses above 1010.5 M o ̇. This key population provides an unaltered probe into the evolution of galaxies from star-forming to quiescent, and has been recently targeted by several observational studies. Young quiescent galaxies in the simulations do not appear unusually compact, in tension with observations, but they show unique age profiles that are qualitatively consistent with the observed color gradients. In particular, more than half of the simulated young quiescent galaxies show positive age gradients due to recent intense central starbursts, which are triggered by significant mergers. Yet, there is a sizable population of recently quenched galaxies without significant mergers and with flat age profiles. Our results suggest that mergers play a fundamental role in structural transformation, but are not the only available pathway to quench a z = 2 galaxy
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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